Keywords: ammonia, laminar flame speed, flame thickness, chemiluminescence, excited species, flameacoustic interaction, local stretch

Alka KARAN Etude et analyse de la dynamique de flammes prémélangées d'un carburant zéro carbone -ammoniac Résumé : Dans le cadre de la recherche de carburants alternatifs pour atténuer la pollution, le réchauffement climatique, l'épuisement des ressources, etc., l'ammoniac est apparu comme l'un des principaux candidats parmi les autres carburants verts. L'objectif de cette thèse est de fournir des données expérimentales et des analyses qui soient utiles à la fois à l'industrie et à la communauté scientifique. Ce travail a été divisé en 3 parties pour atteindre les objectifs fixés. La première partie est dédiée à fournir des vitesses de flamme laminaire, qui est un paramètre de dimensionnement important pour les chambres de combustion. Cette section met également en évidence la chimie impliquée et les différents facteurs qui l'influencent, expliquant certains comportements atypique comme l'indépendance vis-à-vis de la pression. La deuxième partie est une tentative de compréhension de la structure interne des flammes d'ammoniac. Il est important de comprendre la réponse des différentes espèces aux changements de paramètres. Cela permet de mieux paramétrer la combustion en fonction des besoins. L'épaisseur de la flamme d'ammoniac a été calculé. Cette épaisseur influe sur le calcul de la vitesse de flamme. De plus, la courbure impacte les différentes espèces excitées, ce qui modifie la position du taux de libération de chaleur. La dernière partie de cette thèse est une étude approfondie de la deuxième partie. La concentration de certaines des espèces excitées, qui est utile pour s'adapter à tout schéma cinétique, a été donnée. Il est connu que les flammes turbulentes permettent d'améliorer certaines propriétés souhaitables. Cependant, les flammes turbulentes sont très complexes et ne sont pas faciles à analyser. La première étape est d'étudier une flamme perturbée. Ainsi, des interactions flamme-acoustique ont été réalisées pour comprendre l'influence de la courbure sur les flammes d'ammoniac. Cela conclut ce travail, ouvrant la voie à de nouvelles directions dans le monde de la combustion de l'ammoniac.

Mots clés : ammoniac, vitesse de la flamme laminaire, épaisseur de la flamme, chimiluminescence, espèces excitées, interaction flamme-acoustique, étirement local

Study and analysis of premixed flame dynamics of zero carbon fuel -ammonia

Abstract : In the quest for alternative fuels to mitigate crises like pollution, global warming, depletion of resources, etc, ammonia has emerged as one of the top contenders amongst other green fuels. The goal of this thesis is to provide experimental data and analyses that are useful both to the industry and the scientific community. This work has been divided into 3 parts to meet the set objectives. The first part is dedicated to providing laminar flame speeds, which is an important sizing parameter, at conditions that are encountered in combustors. This section also highlights the chemistry involved and various factors influencing it; explaining some unique behaviour like pressure independency. The second part is an attempt to understand the inner structure of ammonia flames. It is important to understand the response of different species to variations introduced into the parameters which may help enhance various desirable properties. The thickness of ammonia flame and its impact while calculating flame speeds have been pointed out. The effect of curvature on different excited species and the position of the heat release rate has been determined. The final part of this thesis is an extension of the second part. The concentration of some of the excited species is useful in accommodating it in any kinetic scheme and has been given here. It is known that turbulent flames help to improve certain desirable properties. However, turbulent flames are very complex and not easy to analyse. The most simple step would be to study a perturbed flame and so, tests on flame-acoustic interactions have been performed to understand the influence of curvature for ammonia flames. This study is concluded with insights paving paths to new directions in the world of ammonia combustion.

LE SOMMAIRE

L'idée d'utiliser l'ammoniac comme combustible a reçu une attention particulière ces dernières années. En eet l'ammoniac est l'un des combustibles les plus étudiés par les chercheurs en combustion au cours des 5 ans écoulés.

Mais comme toute nouvelle idée proposée, il y a toujours un débat entre les chercheurs ; certains se portant garants de l'idée en raison de son côté décarboné et de ses autres avantages, tandis que d'autres soulignent largement les inconvénients de l'utilisation de l'ammoniac. Formulé chimiquement sous le nom de NH 3 , l'ammoniac est bien connu du grand public pour son utilisation comme réfrigérant et comme engrais. L'augmentation constante des émissions de carbone et l'épuisement rapide des combustibles fossiles suscitent des inquiétudes alarmantes et appellent à la recherche de combustibles verts alternatifs.

Plusieurs tentatives sont faites pour réduire ces émissions nocives qui provoquent le réchauement de la planète et le changement climatique. De nombreux pays se sont engagés à réduire leur empreinte carbone et à recourir à des sources d'énergie vertes. Ils ont également signé des conventions telles que le protocole de Kyoto et l'accord de Paris et ont participé activement à des sommets mondiaux. L'hydrogène est un excellent exemple de carburant décarbone qui a été largement accepté comme solution aux problèmes actuels. Toutefois, le stockage et le transport de l'hydrogène présentent toujours un risque en raison de sa nature explosive. Une autre solution proposée est l'utilisation de batteries électriques, même si l'impact environnemental et l'ecacité de ces batteries restent contestés. Les études sur la combustion métallique et les biocarburants sont toujours en cours. Le stockage de l'énergie provenant de sources d'énergie renouvelables est toujours un problème majeur. Les recherches sur l'ammoniac montrent qu'à l'exception de quelques obstacles, l'ammoniac en tant que carburant est a un avenir prometteur. Au l des ans, son utilisation en tant que combustible a été viable avec des performances comparables à celles de ses homologues conventionnels. Si l'on compare les tendances de la recherche sur l'ammoniac, l'hydrogène et le méthane, on constate que l'ammoniac a suscité un grand intérêt au cours de la dernière décennie et qu'il devrait connaître une croissance exponentielle au cours de la prochaine décennie. Du point de vue de la recherche, il reste beaucoup de questions à résoudre pour comprendre le comportement des ammes d'ammoniac. i ii L'utilisation de l'ammoniac dans les moteurs à allumage commandé et les turbines à gaz est bien établie. Cependant, les données nécessaires au dimensionnement des moteurs dans les conditions rencontrées dans ces chambres de combustion font défaut. Le premier objectif de ce travail de thèse est de trouver des données sur la vitesse de la amme dans ces conditions. En outre, on a constaté qu'il y avait peu d'études sur la structure interne de la amme. Si l'on veut savoir comment améliorer le processus de combustion, il est important de comprendre les paramètres qui aectent la zone de réaction. Ainsi, le second objectif est de comprendre certaines propriétés de la amme d'ammoniac. Il a été noté qu'il n'y a pas eu de tentative pour fournir la concentration de certaines espèces excitées. Une analyse qualitative et quantitative a été faite pour diverses de ces espèces an de déterminer leurs concentrations . Il est connu que les systèmes industriels mettent en jeu des écoulements réactifs turbulents.

Cependant, les ammes turbulentes sont extrêmement complexes par nature.

La première étape, et la plus fondamentale, consiste à étudier une amme laminaire perturbée. Bien que les ammes laminaires perturbées soient diérentes des ammes turbulentes, elles peuvent donner un aperçu de la façon dont les ammes réagissent lorsqu'elles sont soumises à une perturbation turbulente.

L'objectif nal de cette thèse est étudier les interactions amme-acoustique.

Pour répondre aux objectifs xés, cette thèse a été divisée en 3 parties : La partie I est consacrée à l'obtention des vitesses de amme et à l'évaluation des schémas cinétiques présents dans la littérature dans des conditions élevées pour des mélanges pauvres, quasi st÷chiométriques et riches. L'objectif de la partie II est de comprendre les diérentes propriétés réactives ainsi que la structure interne de la amme prémélangée étirée à diérents richesses. La partie III a deux aspects importants. Le premier aspect est de fournir analyses qualitatives et quantitatives de certaines des espèces excitées. Le second aspect est de comprendre l'eet de la perturbation sur la structure interne de la amme à diérents richesses. Chaque partie est divisée en chapitres et inclue une brève introduction et une conclusion. Cette brève introduction vise à fournir l'objectif de l'étude et l'essentiel de chaque chapitre et la conclusion donne un aperçu de l'étude. Cette thèse se termine enn par un résumé concluant et une note de réexion sur la portée future de ce projet.

Partie I : Vitesses de ammes laminaires à haute pression et haute température Comme indiqué précédemment, on sait que l'utilisation de l'ammoniac dans les moteurs à allumage commandé et les turbines à gaz est favorable en raison de ses propriétés antidétonantes dues à son indice d'octane élevé. La pression rencontrée dans ces chambres de combustion est de l'ordre de 20 bar à faible charge et de 70-100 bar à pleine charge. La vitesse de propagation d'une amme laminaire de prémélange est l'un des principaux paramètres de dimensionnement d'un moteur. Il y a un besoin de données de vitesse de amme dans iii des conditions élevées. L'objectif de cette partie est de fournir des données de vitesse de amme pour des ammes d'oxy-ammoniac jusqu'à une pression de 30 bar et une température de 585 K et d'évaluer les schémas cinétiques récents disponible dans la littérature pour ces conditions.

Le Chapitre 1 présente la littérature disponible pour l'évaluation de la vitesse de la amme et du schéma cinétique. La plupart des expériences sont réalisées dans des conditions atmosphériques. Dans des conditions élevées, il n'y a pas susamment de données. On a également constaté que la technique la plus couranment utilisée pour réaliser les expériences était la chambre sphérique fermée. Les dernières données disponibles visent à combiner l'ammoniac avec d'autres combustibles non conventionnels comme le Dimethyl éther (DME), l'éthanol et le méthanol. Lors de l'analyse des diérents schémas cinétiques, il a été constaté que Okafor [START_REF] Okafor | Measurement and modelling of the laminar burning velocity of methane-ammonia-air ames at high pressures using a reduced reaction mechanism[END_REF] a donné les résultats les plus conformes pour les mélanges pauvres et que Klippenstein [START_REF] Klippenstein | Theory and modeling of relevance to prompt-NO formation at high pressure, Combust[END_REF] et Zhang [START_REF] Zhang | Assessing the predictions of a NO x kinetic mechanism on recent hydrogen and syngas experimental data[END_REF] ont donné les meilleurs résultats pour les mélanges riches en carburant. Konnov [4] et Mathieu [START_REF] Mathieu | Experimental and modeling study on the high-temperature oxidation of ammonia and related NO x chemistry[END_REF] semblent surestimer et sous-estimer respectivement les vitesses de amme par rapport aux données expérimentales [START_REF] Kobayashi | Science and technology of ammonia combustion[END_REF]. Du point de vue des analyses de sensibilité [START_REF] Da Rocha | Chemical kinetic modelling of ammonia/hydrogen/air ignition, premixed ame propagation and NO emission[END_REF], H + O 2 --⇀ ↽--O + OH est la réaction la plus dominante, sa sensibilité à la vitesse de la amme étant d'environ 0,5.

Le Chapitre 2 se concentre sur la méthodologie utilisée pour atteindre cet objectif. Les expériences sont réalisées dans l'installation OPTIPRIME qui consiste en une chambre de combustion sphérique à volume constant avec une vue optique à 360 degrés. Ce large champ de visibilité permet de visualiser la amme depuis son point d'allumage jusqu'à ce qu'elle atteigne la paroi.

Cette chambre est placée à l'intérieur d'un four, ce qui permet d'eectuer des tests à une température initiale supérieure à la température ambiante. Cette chambre est équipée d'un thermocouple de type K et de 2 transducteurs de pression. Une caméra CMOS est utilisée pour enregistrer l'évolution du rayon de amme. L'évolution de la pression et du rayon sont combinées pour évaluer la vitesse de la amme. Un modèle de perte de chaleur est utilisé pour limiter les valeurs maximales de température et pression à considéré . Pour éviter les eets d'étirement qui aectent la vitesse de amme, le front de amme est considéré lorsque la pression dans la chambre atteint deux fois la pression initiale. Après avoir eectué une recherche bibliographique, 9 schémas cinétiques récents pour la combustion de l'ammoniac ont été choisis pour cette étude et sont présentés dans le Tableau 2.1. Des tests ont été réalisés pour un mélange ammoniacoxygène à 3 richesses diérentes : 0,8, 1,1 et 1,3 à une température initiale de 300 K. Pour les mélanges riches, la pression initiale était comprise entre 1 bar et 4 bar alors que pour le mélange pauvre, elle était à 1 bar et 2 bar. Le mélange comburant est composé à 30% d'oxygène et les 70 % restants constituent le diluant pour toutes les conditions. Le diluant est nécessaire car ces ammes sont assez instables par nature, les instabilités hydrodynamiques et thermodiusives étant favorisées dans ces conditions de température et de pression.

Les conditions initiales utilisées sont représentées dans le Tableau 2.2.

Le Chapitre 3 se concentre sur les vitesses de amme et les analyses de sensibilité pour les diérentes conditions d'essai an de mettre en évidence les réactions prédominantes dans les diérents schémas cinétiques. Pour toutes les conditions, les mécanismes de Nakamura [START_REF] Nakamura | Combustion and ignition characteristics of ammonia/air mixtures in a micro ow reactor with a controlled temperature prole[END_REF] et Stagni [START_REF] Stagni | An experimental, theoretical and kineticmodeling study of the gas-phase oxidation of ammonia[END_REF] se situaient dans la gamme expérimentale. Des analyses de sensibilité ont été eectuées sur quelques mécanismes pour comprendre le comportement et mettre en évidence iv les réactions clés. On a vu que la réaction la plus dominante, quel que soit le mécanisme et les conditions initiales, est O 2 + H --⇀ ↽--O + OH. Il est intéressant de noter que même si la vitesse de amme prédite par Nakamura et Stagni est assez proche, les 10 réactions les plus prédominantes ne sont pas exactement les mêmes. Les deux réactions communes en dehors de la réaction dominante sont : HNO + H --⇀ ↽--NO + H 2 et NH 2 + NO --⇀ ↽--NNH + OH. De plus, d'après analyses de sensibilité, il a été constaté que la vitesse de la amme n'est pas très sensible à la pression.

Le Chapitre 4 se penche sur la propriété d'indépendance à la pression des ammes d'ammoniac. Une liste d'études de cas, dissociant la température et la pression, est réalisée pour trois mécanismes diérents : Shrestha [START_REF] Shrestha | Detailed kinetic mechanism for the oxidation of ammonia including the formation and reduction of nitrogen oxides[END_REF], Nakamura et Stagni pour trois richesses diérentes. Ces études de cas permettent de comprendre les eets individuels sur la vitesse de la amme pour diérents mécanismes. Il a été constaté l'importance de la température sur la vitesse de amme contrairement à la pression qui joue un rôle moindre, particulièrement à haute pression. Ces observations restent valables indépendamment de la richesse et du mécanisme choisi. L'impact maximum de la pression est remarqué entre 1-5 bar. Une autre étude de cas a été réalisée pour comprendre les eets de la pression et de la température lorsque le gaz diluant est changé. Des conclusions similaires ont été tirées. En outre, il a été constaté que les mécanismes de Shrestha étaient les moins aectés par le changement de la composition du gaz diluant. Enn, les facteurs exponentiels α et β de l'équation 4.1 qui renseignent sur les dépendances à la température et à la pression,sont calculés.

Il a été remarqué que la valeur de β était beaucoup plus faible que la plupart des combustibles. Ces facteurs exponentiels ont également été calculés pour les conditions d'essai données.

Pour conclure, des vitesses de amme expérimentales pour la combustion de l'ammoniac jusqu'à une pression de 30 bars et une température de 585 K ont été fournies. L'évaluation des schémas cinétiques choisis dans les mêmes conditions d'essai a été eectuée. La propriété d'indépendance vis-à-vis de la pression des vitesses de amme de l'ammoniac, en particulier à haute pression, suggère que l'ammoniac est un combustible idéal pour les moteurs à allumage commandé et les turbines à gaz, puisque les oscillations de pression auraient des eets moindre sur la vitesse de amme. Enn, à partir des données générées, une carte 3D de la vitesse de amme en fonction de la pression et de la température utilisant une adaptation proposée par [START_REF] Hu | Laminar ame speeds and ignition delay times of methaneair mixtures at elevated temperatures and pressures[END_REF] été réalisée.

Partie II: Structure interne de la amme prémélangée étirée Dans la partie précédente, on a vu que les vitesses de amme ammoniac/oxygène dans des conditions de température et pression élevées sont comparables à celles des combustibles classiques. Cependant, on sait que les vitesses de amme ammoniac/air dans des conditions atmosphériques sont assez faibles.

Il est important de connaître la structure interne de la amme pour mieux comprendre le processus de combustion et, par conséquent, pour comprendre l'inuence des diérents paramètres an d'améliorer ce processus. La partie II donne un aperçu de la structure des ammes étirées et prémélangées ammoniac/air aux conditions atmosphériques. Les paramètres étudiés sont l'inuence v de la richesse et de la courbure sur l'épaisseur et la réactivité de la amme. Cela a été réalisé expérimentalement en étudiant la signature par chimiluminescence de certaines espèces.

Le Chapitre 5 donne une vue d'ensemble des études de la littérature sur la vitesse des ammes d'ammoniac dans l'air aux conditions atmosphériques et sur les diérents diagnostics utilisés. Un résumé sur la signature par chimiluminescence des ammes d'ammoniac, sur les eets d'étirement, et sur la structure de la amme est également fourni. Il existe plusieurs méthodes pour déterminer expérimentalement la vitesse de la amme. La méthode la plus couramment utilisée est l'utilisation d'une chambre sphérique fermée, comme OPTIPRIME (cf. chapitre 2). Selon le but de l'expérience, la technique la plus appropriée est choisi pour réaliser l'expérience. Les techniques expérimentales trouvées dans la littérature pour la combustion de l'ammoniac sont : la chambre à pression constante, la chambre à volume constant, le brûleur Bunsen, le brûleur à ux thermique et la méthode du ux stagnant. Ici, les expériences sont réalisées sur le brûleur Bunsen avec des diagnostics comme la PIV (Particle image velocimetry) et la chimiluminescence. À partir des spectres d'émission des ammes d'ammoniac et d'air, NH* et NH 2 * ont été reconnus comme des espèces intéressantes à étudier. La chimiluminescence de diverses espèces [START_REF] Vigueras-Zúñiga | Methane/Ammonia Radical Formation during High Temperature Reactions in Swirl Burners[END_REF] a été utilisée pour étudier la formation de radicaux dans des réactions à haute température pour des mélanges NH 3 /CH 4 dans des écoulements tourbillonnaires, à la fois expérimentalement et numériquement, et également pour déterminer l'emplacement du taux de dégagement de chaleur le plus élevé.

À notre connaissance, aucune information sur le prol du taux de libération de chaleur (HRR :heat release rate) et les eets de courbure sur les espèces via la Stephane Bostyn, Dr. Veronica Belandria and Dr. Mahmoud Idir for the friendly chats. I would also like to thank Prof. Guillaume Legros and Dr.

Touk Boushaki for their help. To the Team OC -during these three years, the entire team including the interns and the permanent sta along with the Ph.D. students and the postdocs have always considered me one as their own.

I will always be grateful to this entire team. I have had the pleasure to spend wonderful moments with all of you. Thank you Dr. Nabiha Chaumeix for the interesting conversations and for the fun times with the team.

When Laminar ame speeds at p 0 =4 bar, T 0 =302.3K, φ = 1.3, 21%

Ar and 49% He.The experimental ame speeds have been thickened in yellow to account for the uncertainties incurred. The y-axis represents the ame speed whereas the x-axis is split as temperature and pressure. This is done as the ame speed is a function of both pressure and temperature which evolves isotropically during the combustion process. The orange colour represents Klippenstein [START_REF] Klippenstein | Theory and modeling of relevance to prompt-NO formation at high pressure, Combust[END_REF], violet represents Zhang (2017) [START_REF] Zhang | Assessing the predictions of a NO x kinetic mechanism on recent hydrogen and syngas experimental data[END_REF], blue represents Dagaut [START_REF] Dagaut | The oxidation of hydrogen cyanide and related chemistry[END_REF], brown represents Otomo [START_REF] Otomo | Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion[END_REF], pink represents Mathieu [START_REF] Mathieu | Experimental and modeling study on the high-temperature oxidation of ammonia and related NO x chemistry[END_REF], cyan represents Modied Okafor [START_REF] Okafor | Measurement and modelling of the laminar burning velocity of methane-ammonia-air ames at high pressures using a reduced reaction mechanism[END_REF], green represents Shrestha [START_REF] Shrestha | Detailed kinetic mechanism for the oxidation of ammonia including the formation and reduction of nitrogen oxides[END_REF], red represents Stagni [START_REF] Stagni | An experimental, theoretical and kineticmodeling study of the gas-phase oxidation of ammonia[END_REF] and black represents Nakamura [START_REF] Nakamura | Combustion and ignition characteristics of ammonia/air mixtures in a micro ow reactor with a controlled temperature prole[END_REF] . . . . . . . . . . . . . . . . . . . . . . . . 24

3.2

Laminar ame speeds at p 0 =1 bar, T 0 =296.8K, φ = 1. Beta trends from isotherms at T=500 K calculated using p 0 =1 bar, T 0 = 500 K and the corresponding ame speed depending on the equivalence ratio and the diluent mixture. φ=0.

4.6

Experimental ame speeds of NH 3 /O 2 /He/Ar mixtures at ϕ = 1.1, T 0 = 300 K, p 0 = 1 (blue), 2 (red), and 3 bar (green). . . A.1 Laminar ame speeds at p 0 =2 bar, φ = 0.8, 49% Ar and 21% He A.2 Laminar ame speeds at p 0 =1 bar, φ = 1.1, 49% Ar and 21% He A.3 Laminar ame speeds at p 0 =2 bar, φ = 1.1, 49% Ar and 21% He A. [START_REF] Konnov | Implementation of the NCN pathway of prompt-NO formation in the detailed reaction mechanism[END_REF] Laminar ame speeds at p 0 =2 bar, φ = 1.3, 49% Ar and 21% He A.5 Laminar ame speeds at p 0 =1 bar, φ = 1.1, 49% He and 21% Ar A.6 Laminar ame speeds at p 0 =2 bar, φ = 1.1, 49% He and 21% Ar A.7 Laminar ame speeds at p 0 =3 bar, φ = 1.1, 49% He and 21% Ar A.8 Laminar ame speeds at p 0 =2 bar, φ = 1.3, 49% He and 21% Ar A.9 Laminar ame speeds at p 0 =4 bar, φ = 1.3, 49% He and 21% Ar A.10 Laminar ame speeds at p 0 =4 bar, φ = 1.1, 70% He . . . . . . A.11 Laminar ame speeds at p 0 =4 bar, φ = 1.3, 70% He . . . . . . A.12 Sensitivity analyses at p 0 =2 bar, φ = 0.8, 49% Ar and 21% He A.13 Sensitivity analyses at p 0 =2 bar, φ = 1.1, 49% Ar and 21% He A.14 Sensitivity analyses at p 0 =1 bar, φ = 1.3, 49% Ar and 21% He A.15 Sensitivity analyses at p 0 =2 bar, φ = 1.3, 49% Ar and 21% He A.16 Sensitivity analyses at p 0 =1 bar, φ = 1.1, 49% He and 21% Ar A.17 Sensitivity analyses at p 0 =2 bar, φ = 1.1, 49% He and 21% Ar A.18 Sensitivity analyses at p 0 =3 bar, φ = 1.3, 49% He and 21% Ar A.19 Sensitivity analyses at p 0 =3 bar, φ = 1.3, 49% He and 21% Ar

LIST OF TABLES

1.1 Experimental literature available on ammonia combustion . . .

2.1

Chemical kinetic schemes used in this study . . . . . . . . . . .

2.2

Initial conditions for the test cases along with the adiabatic ame temperature (AFT) . . . . . . . . . . . . . . . . . . . . . . . . .

3.1

Initial pressure and equivalence ratio with the range of pressure and temperature for which the ame speed has been measured.

The initial temperature, T 0 , was maintained at 300K . . . . . .

GENERAL INTRODUCTION

Today, in 2022, the idea of using ammonia as a fuel has received worldwide attention and is one of the most sought fuels by combustion researchers. But like any new idea proposed faces a dispute, there is still a debate among the researchers, some vouching for the use of ammonia as a fuel due to its non-zero carbon along with some other desirable properties while others emphasizing largely on the downside of using ammonia. Chemically formulated as NH 3 , ammonia is well known to the common man for its use as a refrigerant and also as a fertilizer. It was rst used as a refrigerant in the 1850s in France and was applied in the United States in the 1860s for articial ice production [START_REF] Das | Ammonia -Molecule of the Month June 2013[END_REF].

Ammonia is produced on a large scale by the well-known Haber-Bosch process which was originally developed by Fritz Haber in Germany to use ammonia as fertilizers, later joined by Carl Bosch to scale up the production during World War I (WWI) to use nitrates as a source of ammunition [START_REF][END_REF]. Using ammonia as a fuel for internal combustion (IC) engines dates back to the early 1800s and saw a surge in its use as an alternative fuel during World War II (WWII) when oil stockpiles became low [START_REF]What's the verdict on ammonia as fuel or as hydrogen carrier?[END_REF]. In recent times, there is an alarming concern with the ever increase in carbon emissions and the rapid depletion of fossil fuels calling for the need for alternative green fuels. Several attempts are being made to curb these harmful emissions which cause global warming further leading to climate change. Many countries have pledged to reduce their carbon footprint and resort to green sources of energy and have also signed conventions like the Kyoto protocol [START_REF]The Kyoto Protocol in a Global Perspective[END_REF] and the Paris agreement [START_REF]Paris Agreement[END_REF] and have been actively participating in global summits. Hydrogen is an excellent example of a green fuel that has been accepted widely as a solution to present-day problems. However, there is still a risk of storing and transporting hydrogen due to its explosive nature. Another solution that is proposed is to use electric batteries but the safe disposal of these batteries without impacting the environment still needs to be addressed. Studies on metallic combustion and biofuels are still in progress. The storage of energy from renewable sources of energy is still a major issue. Research on ammonia shows that on overcoming a few obstacles, ammonia as a fuel is feasible and has a promising future.

The Gazamo process (patented by J.L. Restieau and E. Kroch) was the rst application of using ammonia as a fuel in a large-scale road test in 1941-age of diesel, public transportation had come to a halt. The engineers had adapted about 100 buses to run on liquid ammonia, pumped into tanks on the buses' roofs. The buses travelled over 100,000 km without a single accident or spill [26]. During the 1960s, eorts were made by the US military forces to study alternative fuels including ammonia [START_REF] Cornelius | Ammonia as an Engine Fuel[END_REF]. NASA developed X-15, the fastest aircraft ever built that used the XLR99 engine fuelled by liquid ammonia had set outstanding records [START_REF]The Best Film Archives, X-15 Rocket Plane | The World's Fastest Airplane[END_REF]. The use of ammonia in gas turbines [START_REF] Valera-Medina | Preliminary study on lean premixed combustion of ammonia-hydrogen for swirling gas turbine combustors[END_REF] and especially in spark-ignition (SI) engines [START_REF] Frigo | Analysis of the behaviour of a 4-stroke Si engine fuelled with ammonia and hydrogen[END_REF] has already been tested. In

2008, a diesel-fuelled 2007 Dodge Truck and a 2008 gasoline-fuelled Ford Crown

Victoria was converted to run on NH 3 [25]. Zamrescu and Dincer [31] have shown that ammonia is the cheapest fuel per 100 km driving range and is costeective when compared to other fuels like hydrogen. The economic statistics report by the ARPA-E, U.S. Department of Energy [START_REF]research projects agency-energy[END_REF] shows that ammonia is the most feasible form of energy delivery option for transportation when compared to both hydrogen and gasoline. This non-carbon fuel has also been of interest to power up marine engines. Germany's MAN Energy Solutions and Korean shipbuilder Samsung Heavy Industries are part of an initiative to develop the rst ammonia-fuelled oil tanker by 2024 [START_REF]Why the Shipping Industry Is Betting Big on Ammonia -IEEE Spectrum[END_REF].

Although the rst use of ammonia combustion in the last century was in public transportation in Belgium, the rst publication available in the database today is by Egerton [START_REF] Egerton | The Ammonia Flame[END_REF] and Strutt [START_REF] Strutt | [END_REF] in 1912 where they inspect an ammonia ame. Bodenstein [START_REF] Bodenstein | The Mechanism of the Catalytic Combustion of[END_REF] traced out the intermediate products both experimentally and theoretically. One of the rst studies of this century published on ammonia combustion in an engine is by Peter Van Blarigan [START_REF] Blarigan | 1advanced Internal Combustion Engine Research[END_REF] where he studies the use of several alternative fuels in an engine. Konnov and Ruyck [START_REF] Konnov | Kinetic Modeling of the Thermal Decomposition of[END_REF] provided improved kinetic modelling for ammonia decomposition with corrected sensitivity coecients and rate constants. Most of the studies concluded that ammonia combustion has low ame speeds leading to an early blow-o. Interest began to spark in studying blended fuels. One of the rst thesis published this century on ammonia/methane combustion is by Mann [START_REF] Mann | Premixed ammonia-methane-air combustion[END_REF] where he studied the use of ammonia as an additive in methane/air ames. It was concluded that the NOx emissions were quite high. CFD studies in various domains started becoming popular and one of the rst simulation studies of this century dedicated to ammonia combustion was a detailed DNS on ammonia/methane combustion provided by Grcar et al [START_REF] Grcar | Eects of mixing on ammonia oxidation in combustion environments at intermediate temperatures[END_REF] in 2004, where they used the kinetic scheme of Glarborg [START_REF] Glarborg | Fuel nitrogen conversion in solid fuel red systems 29 89113[END_REF]. Overall the results in ammonia combustion started to show that this fuel has good potential but it was important to address the emissions from ammonia itself. There are several studies on NOx emissions and on how to reduce them [4244]. It is known that ammonia is hazardous when released into the atmosphere due to its high toxicity. From the investigations made by Westlye et al., [START_REF] Westlye | Experimental investigation of nitrogen based emissions from an ammonia fueled SI-engine[END_REF] on a spark-ignition engine, it was concluded that there may be unburned ammonia emissions along with intermediary oxides of nitrogen which are harmful to the environment. To reduce the impact of these emissions, a selective catalytic reduction (SCR) was developed to promote complete combustion and also to ensure that exhaust emissions do not pollute the environment. Attempts are being made to make the manufacturing process of ammonia carbon-free. Green ammonia is made with hydrogen that comes from water electrolysis powered by alternative energy. A massive project in Saudi Arabia aims to make more than 1 million metric tons of ammonia per g/mol and is similar in structure. Although methane ames are associated with higher speeds, ammonia ames have a higher-octane rating. Researchers often compare and contrast the behaviour of these two ames. As research in blend studies depict an easy solution to overcome the undesirable aspects, ammonia is often combined with hydrogen or methane to increase the overall performance like the ame speeds, ammability limits, and power output [START_REF] Lhuillier | EXPERIMEN-TAL STUDY ON NH3/H2/AIR COMBUSTION IN SPARK-IGNITION EN-GINE CONDITIONS 13[END_REF][START_REF] Han | Experimental and kinetic modeling study of laminar burning velocities of NH3/air, NH3/H2/air[END_REF]. There are also a large number of studies on blending ammonia with oxygenated fuels or oxygenated air which results in increasing the performance as well. Since ammonia can be easily stored and transported and by decomposing one mole of ammonia, one can produce 3 moles of hydrogen; studies show that it can be used as a hydrogen carrier [START_REF] Cechetto | H2 production via ammonia decomposition in a catalytic membrane reactor[END_REF][START_REF] Lamb | Ammonia for hydrogen storage; A review of catalytic ammonia decomposition and hydrogen separation and[END_REF] by simply cracking ammonia. It can also be used as fuel cells as shown in some studies [5153].

From preliminary research in the literature available back in 2019 and on reading reports from various countries/continents including Japan, China and, Europe, it was seen that ammonia could be the main source of energy of tomorrow if the major problems pertaining to the use of ammonia is well addressed.

This meant that there is a large scope of work that needs to be done to put it into use. From a scientic point of view, ammonia has denitely become a trendsetter and the new talk amongst combustion researchers who are in the quest for a capable green replacement for conventional fuels. decade stands at a massive 220000. On comparing the three fuels, it can be seen that a large amount of work is dedicated to hydrogen combustion followed by methane, and nally ammonia combustion. However, the trend over the decades shows that publications in ammonia combustion are increasing much faster than methane combustion but still lesser than hydrogen combustion.

This is an indicator of the dichotomy among combustion researchers. As an aspiring researcher myself with similar goals, it was agreed upon by my team to contribute to the ammonia research. There is a lot of literature on ammonia blended fuels. In order to understand the behaviour of blended fuels, one must have sucient knowledge of the properties of the individual fuels. The main priority of this particular work is to add to the pure ammonia studies.

The use of ammonia in SI engines and gas turbines has been well-established due to its high octane number and anti-knock characteristics. However, there is a dearth of data needed for engine sizing at the conditions encountered in these combustors. Finding ame speed data under these conditions was the rst goal. Further, it was seen that there were not a lot of studies on the inner structure of the ame. If one needs to know how to enhance the combustion process, it is important to understand the parameters aecting the ame. And so, the second goal is to understand certain properties of ammonia ames. It was noted that there was no previous attempt to provide the concentration of some of the excited species. A qualitative and quantitative analysis was done for various excited species. It is known that turbulent ames enhance mixing thereby promoting the combustion process. It is also known that turbulent ames are extremely complex in nature. The rst and perhaps, the most basic step is to study a perturbed laminar ame. Though a perturbed laminar ame is quite far from turbulent ames, it can help to give an insight into how ames respond when subjected to a perturbation. Therefore, establishing the nal objective of this thesis: to study ame-acoustic interactions.

To meet the objectives set, this thesis has been divided into 3 parts: Laminar ame speeds at high pressure and temperature

INTRODUCTION

It is known that using ammonia in SI engines [START_REF] Frigo | Analysis of the behaviour of a 4-stroke Si engine fuelled with ammonia and hydrogen[END_REF] and gas turbines [START_REF] Valera-Medina | Preliminary study on lean premixed combustion of ammonia-hydrogen for swirling gas turbine combustors[END_REF] and is favourable in SI engines due to its anti-knock properties owing to its high-octane number. The pressure encountered in these combustors is in the range of 20 bar at light loads and 70-100 bar at full load. Flame speed is one of the important sizing parameters of an engine. And so, there is a need of ame speed data at elevated conditions. The objective of this part is to provide ame speed data for oxy-ammonia ames up to a pressure of 30 bar and a temperature of 585 K and to evaluate the recent kinetic schemes in the literature for these conditions [START_REF] Karan | Experimental study and numerical validation of oxy-ammonia combustion at elevated temperatures and pressures[END_REF].

Chapter 1 discusses the literature available for the ame speed of pure ammonia and ammonia blended with other fuels along with a brief assessment of the kinetic schemes available in the literature. To conclude a 3D evolution of ame speed with pressure and temperature along with a t model optimised to t the experimental data has been provided. This 3D map and the t equation enable us to provide ame speed for any Introduction pressure and temperature within the given pressure and temperature range and the initial conditions used in this study.

CHAPTER 1 CHAPTER 1

Ammonia combustion has indeed become a vast topic with a lot of work done especially in the past few years as pointed out in the statistical analysis in the section General Introduction. This chapter aims to give a brief literature review on laminar ame speed data obtained experimentally and numerically.

Literature review on experimental ame speed data in ammonia combustion

One of the rst reports on experimental ammonia ame speed was by Andrews and Gray [START_REF] Andrews | Combustion of Ammonia Supported by Oxygen, Nitrous Oxide or Nitric Oxide: Laminar Flame Propagation at Low Pressures in Binary Mixtures[END_REF]. They measured ame speeds for ammonia in oxygen, a mixture of ammonia and oxygen with dierent diluents like argon, helium and nitrogen for an equivalence ratio ranging between 0.18 and 1.13 at 333 K and a pressure ranging between 0.005-0.009 MPa in a closed vessel with schlieren photography. Dierent techniques are used to determine the ame speed to achieve dierent ranges of conditions. The most common technique that was used was a closed spherical vessel.

Despite the numerous properties which make ammonia a desirable fuel, there are a few added disadvantages in addition to the low ammability limit and the low LHV. Ammonia is well known to be hazardous when released into the atmosphere due to its high toxicity. The value of ame speed for an ammonia-air mixture at ambient atmospheric conditions has been reported to be as low as 7 cm/s at an equivalence ratio, φ = 1.1. From a combustion performance point of view, this low speed of ammonia ames [START_REF] Han | Experimental and kinetic modeling study of laminar burning velocities of NH 3 /air, NH 3 /H 2 /air, NH 3 /CO/air and NH 3 /CH 4 /air premixed ames[END_REF][START_REF] Hayakawa | Laminar burning velocity and markstein length of ammonia/air premixed ames at various pressures[END_REF] leads to an early blow-o and a diculty in the ignition. Ammonia is often combined with hydrogen or methane to increase the overall performance like the ame speeds, ammability limits and power output. Literature is available on dierent conditions for the combined fuel mixture [START_REF] Lhuillier | Experimental study on NH 3 /H 2 /air combustion in spark-ignition engine conditions, 11 th Mediterranean Combustion Symposium[END_REF][START_REF] Mørch | Ammonia/hydrogen mixtures in an SI-engine: Engine performance and analysis of a proposed fuel system[END_REF]. Data at high pressure and high temperature is available for ammonia mixed with hydrogen or methane [6064]. On one hand, ammonia is blended with other fuels and on the other hand, ammonia is also burnt with oxygenated air mixtures [START_REF] Xia | Turbulent burning velocity of ammonia/oxygen/nitrogen premixed ame in O2-enriched air condition 268[END_REF] For pure ammonia combustion, analyses have been made for a narrow range of pressure and temperature conditions for dierent equivalence ratios [START_REF] Hayakawa | Laminar burning velocity and markstein length of ammonia/air premixed ames at various pressures[END_REF].

Most of the data are obtained at ambient conditions. The newest data available aims at combining ammonia with other non-conventional fuels like Dimethyl ether (DME), ethanol and even, methanol. Table 1.1 gives a synopsis of the experimental literature available in ammonia combustion.

Despite the abundant data, when it comes to high pressure and temperature conditions it is seen that the maximum ame speed for ammonia combustion available in the literature is much lesser than what is encountered in gas turbines and engines.

From the ame speed data obtained by combining ammonia with either other fuels or by using oxidizers that contain more oxygen than that of air, the ame speed, ammability limit and energy release have been signicantly

improved. Some of the unique properties like the lack of change in values of burned Markstien length at 0.3 MPa and 0.5 MPa have been recently pointed out [START_REF] Ichikawa | Laminar burning velocity and Markstein length of ammonia/hydrogen/air premixed ames at elevated[END_REF].

Literature review on kinetic schemes in ammonia combustion

There are several kinetic schemes developed for ammonia combustion which are available in the literature. There are assessments on various aspects like predictions of NOx emissions, ignition delay time etc. Nevertheless, the focus of this section is purely on evaluating the performance of laminar ame speed prediction and sensitivity analyses. Despite the focus being on laminar ame speed assessment and sensitivity analyses, this is not the only criteria to evaluate to rate the performance of any kinetic scheme.

On performing a literature review [START_REF] Da Rocha | Chemical kinetic modelling of ammonia/hydrogen/air ignition, premixed ame propagation and NO emission[END_REF][START_REF] Wang | Measurement of oxyammonia laminar burning velocity at normal and elevated temperatures[END_REF], it was understood that Okafor [START_REF] Okafor | Measurement and modelling of the laminar burning velocity of methane-ammonia-air ames at high pressures using a reduced reaction mechanism[END_REF] gave the most congruent results for lean mixtures and Klippenstein [START_REF] Klippenstein | Theory and modeling of relevance to prompt-NO formation at high pressure, Combust[END_REF] and

Zhang [START_REF] Zhang | Assessing the predictions of a NO x kinetic mechanism on recent hydrogen and syngas experimental data[END_REF] gave the best results for the fuel-rich mixtures. Konnov [START_REF] Konnov | Implementation of the NCN pathway of prompt-NO formation in the detailed reaction mechanism[END_REF] and Mathieu [START_REF] Mathieu | Experimental and modeling study on the high-temperature oxidation of ammonia and related NO x chemistry[END_REF] seem to over-estimate and under-estimate the ame speeds respectively when compared to the experimental data [START_REF] Kobayashi | Science and technology of ammonia combustion[END_REF]. Goldmann and Dinkelacker [START_REF] Goldmann | Approximation of laminar ame characteristics on premixed ammonia/hydrogen/nitrogen/air mixtures at elevated temperatures and pressures[END_REF] analyse dierent schemes and concluded that Mathieu's mechanism tends to underpredict whereas Tian's [START_REF] Tian | A proposed mechanism for the ammonia-LCFA synergetic co-inhibition eect on anaerobic digestion process 349[END_REF] and Konnov's mechanism tend to overpredict the laminar ame speed of ammonia/air mixtures. Xiao and Valera-Medina [START_REF] Xiao | Chemical Kinetic Mechanism Study on Premixed Combustion of Ammonia/Hydrogen Fuels for Gas Turbine Use[END_REF] compared 12 of the latest schemes for ammonia/hydrogen mixtures and concluded that Mathieu gave the best agreement with their experimental data.

The mechanisms of Konnov, Miller [START_REF] Miller | Kinetic Modeling of the Oxidation of Ammonia in Flames[END_REF], Duynslaegher [START_REF] Duynslaegher | Flame structure studies of premixed ammonia/hydrogen/oxygen/argon ames: Experimental and numerical investigation[END_REF] overpredicted the values whereas Lindsted [START_REF] Lindstedt | Detailed Kinetic Modelling of Chemistry and Temperature Eects on Ammonia Oxidation[END_REF] overpredicted only for the lean conditions and were well within the range for rich conditions. On the other, the scheme of Abo Akademi (AA) [START_REF] Norström | Comparisons of the Validity of Dierent Simplied NH 3 Oxidation Mechanisms for Combustion of Biomass[END_REF] underpredicted for lean mixtures and was within the experimental range for rich mixtures. Dagaut [START_REF] Dagaut | The oxidation of hydrogen cyanide and related chemistry[END_REF] , Klippenstein [START_REF] Klippenstein | Theory and modeling of relevance to prompt-NO formation at high pressure, Combust[END_REF] , Tian and Mendiara [START_REF] Mendiara | Ammonia chemistry in oxy-fuel combustion of methane[END_REF] slightly underestimated the ame speed values. GRI 3.0 [START_REF] Gri-Mech | Gas Research Institute[END_REF] and Mevel [START_REF] Mével | Hydrogennitrous oxide delay times: Shock tube experimental study and kinetic modelling[END_REF] gave ame speed data that was lower than the experimental range by over 50%.

From the perspective of sensitivity analyses [START_REF] Da Rocha | Chemical kinetic modelling of ammonia/hydrogen/air ignition, premixed ame propagation and NO emission[END_REF], H + O 2 show the reaction NH2+NO <=> NNH+OH as one of the most important with sensitivity values higher than 0.1.

Wang et al [START_REF] Wang | Measurement of oxyammonia laminar burning velocity at normal and elevated temperatures[END_REF] in their work also mention that H + O 2 --⇀ ↽--O + OH is the most dominant reaction with the most important rate-limiting branching reaction for laminar burning velocity predictions. They noted that H 2 + O --⇀ ↽-- H + OH is active when the H 2 concentration is high at rich mixtures, but its sensitivity is not notable in the scheme of Okafor. In contrast, the sensitivity of reactions in the HNO, and NHx sub-mech presented higher positive values at lean mixtures. Two reactions NO + H + M --⇀ ↽--HNO + M , NH 2 + NO --⇀ ↽-- Zhang (2017). Similarly, two reactions in the N 2 H x block N 2 H 2 + M --⇀ ↽--NNH + H + M, N 2 H 2 + H --⇀ ↽--NNH + H 2 also gives large sensitivity, but they were not notable in Okafor. It implies that the discrepancy of Okafor in the rich conditions may due to the incompleteness of N-2-related reactions.

2 + NH --⇀ ↽--N 2 H 2 + H, NH 2 + O --⇀ ↽--HNO + H are notable in
From Xiao and Valera-Medina's work [START_REF] Xiao | Chemical Kinetic Mechanism Study on Premixed Combustion of Ammonia/Hydrogen Fuels for Gas Turbine Use[END_REF], it was concluded that H + O 2 --⇀ ↽--O + OH and H + O 2 ( + M) --⇀ ↽--HO 2 ( + M) have the largest impacts on the predictions of ame speed in Miller and Mathieu with these reactions playing a larger role in the scheme of Miller. For rich conditions, H + O 2 --⇀ ↽--O + OH has the largest impacts on the predictions of ame speed in both mechanisms while the most inhibiting reactions in the two mechanisms are dierent,

N + OH --⇀ ↽--H + NO in Mathieu mechanism and H + O 2 ( + M) --⇀ ↽--H + O 2 ( + M) in Miller mechanism.
Overall, depending on the mechanism, dierent reactions are highlighted as the most important set of reactions for the same initial conditions. As the initial conditions change, the important set of reactions that plays a major role too changes. CHAPTER 2

CHAPTER 2

This chapter is dedicated to describing the experimental method and the simulations that were carried out using Chemkin-Pro. The test conditions and the chosen kinetic schemes are also presented here.

Methodology

There are several methods to experimentally calculate ame speeds. The method to be used depends on the targeted conditions. For example, the stagnation/counterow method and Bunsen burners are used for low-pressure and temperature conditions. Spherical ame propagating in constant pressure chambers and constant volume chambers are the most common methods used for elevated conditions. Constant volume chambers can cover a wider range of high-pressure and temperature conditions. Hence, it has been deployed as the experimental apparatus here. Also, since the idea is to nd ame speed for SI engines and gas turbines which under ideal conditions follow the Otto cycle where two strokes happen at constant volume conditions, it is useful to investigate ame speeds using the constant volume method.

To validate the kinetic schemes available at these conditions, CHEMKIN a software tool originally developed at Sandia National Laboratories for solving complex chemical kinetics problems is used. Chemkin-Pro 2009 version is used to obtain the results here. Three input les are used to carry out the simulations.

(i) an input le containing the elements, species and reactions, (ii) a thermodata le containing data required to compute dierent thermal quantities like enthalpy, entropy and, heat capacity for a given temperature range, (iii) and, a transport data le containing the diusion and collision coecients.

2.1. METHODOLOGY

Experimental set-up description

The experiments are performed in the full OPTIcal access Perfectly spheRical combustIon chaMber (OPTIPRIME) facility of ICARE-CNRS, Orléans,

France. The salient features of this new chamber are a perfect sphericity of the inner chamber surface and a full visualization of the ame front propagation from the central ignition to the walls. The use of a 360°fused silica ring enables a complete ame visualization. The inner and outer surfaces of this fused silica ring are machined to ensure perfect internal and external spherical proles.

The outer diameter of the crown is 144 mm with a quartz thickness of 11 mm.

This thickness was calculated such that it holds at a pressure greater than 100 bars. OPTIPRIME consists of the assembly of two stainless steel half shells with a 360°fused silica ring with spherical inner and outer surfaces in-between as illustrated in Figure 2.1. The internal volume is scrupulously spherical, and its radius is R 0 = 60.85 mm (i.e. 0.943 litres in volume). There are several important advantages to smaller combustion bombs. The eects of radiation and hydrodynamic instabilities are less pronounced at small ame radii. One drawback of working with small ames is that they are more strongly aected by stretch.

To minimize dead volumes, the diameters of the inlet (1/16 inch) and exhaust (1/8 inch) tubing were reduced and their end is ushed with the inner surface of the chamber. The exhaust tubing is larger to facilitate the vacuum procedure after each test. Two pressure sensors (AVL GU21D), with high sensitivity and high frequency, are arranged diametrically opposite in the chamber.

The sensitive surface of the sensors is ushed. The sensitivity and frequency of the sensor are 35 pC/bar and 85 kHz, respectively. The linearity of the sensor is ± 0.3 % of the full-scale range. A type-K thermocouple with a stainless steel sheath is introduced into the chamber to control the initial temperature of the unburnt gas. As shown in Figure 2.1, the combustion chamber is placed in the centre of a furnace with a volume of 32 L, having a power of 1050 W and is capable of operating over a temperature range of 10 °C to 300 °C with time stability lower than 0.2 °C. A complete immersion of the chamber has the major advantage of avoiding temperature gradients either on the enclosure or in unburnt gas. The maximum initial temperature (200 °C) is limited by the thermal resistance of the pressure sensor cables as the whole system is in the furnace. In the context of this study, the furnace was not used as the experiments were performed at room temperature.

To ensure perfect mixture repeatability, a high-pressure buer tank (3.78 L) is lled with the components of the desired mixture following Dalton's law of partial pressures. The relatively large volume of the buer tank, compared to the chamber volume (i.e. less than 1 L), allows for carrying out successive shots tests to ensure repeatability with minimum errors. The detailed experiment set-up description is provided in [START_REF] Halter | Development of an optically accessible apparatus to characterize the evolution of spherically expanding ames under constant volume conditions[END_REF].

Radius and pressure evolution

Flame propagation was observed from its ignition point until it hit the chamber walls through the 360°fused silica ring. The accurate ame front position determination using chemiluminescence through the ring was assessed in [START_REF] Halter | Development of an optically accessible apparatus to characterize the evolution of spherically expanding ames under constant volume conditions[END_REF]. A CMOS camera (PHANTOM V1611) placed in front of the chamber at the ignition deposit. When the ame is close to the chamber walls, the pressure evolution is aected by heat losses which prevent it from reaching the theoretical end pressure. The minimal ame radius detection is limited to 10 mm to avoid any disturbance from the ignition. Both isobaric and isochoric measurements can be taken from this setup. However, in the present study, only the isochoric measurements are of interest. The isochoric method has the advantage to aord a large quantity of ame speed data for elevated pressure and temperature conditions. This method had been initially limited, to 2 < p/p0 < 5. The maximum pressure/radius to be considered for the ame speed evaluation should correspond to the adiabatic process. As explained in Omari and Tartakovsky [START_REF] Omari | Measurement of the laminar burning velocity using the conned and unconned spherical ame methods A comparative analysis[END_REF] and Burrell et al. [START_REF] Burrell | Eects of stretch and thermal radiation on diuoromethane/air burning velocity measurements in constant volume spherically expanding ames[END_REF] , the time derivative of the pressure trace is a good indicator of the heat losses to the wall. In [START_REF] Halter | Development of an optically accessible apparatus to characterize the evolution of spherically expanding ames under constant volume conditions[END_REF] , a criterion of 90% of max ( dp/ dt ) was introduced to dene the maximum experimental pressure to be considered in order to ensure that no wall eect can occur during the propagation investigated. The detection of instabilities by direct visualization or chemiluminescence was illustrated in [START_REF] Halter | Development of an optically accessible apparatus to characterize the evolution of spherically expanding ames under constant volume conditions[END_REF] , 23] and the reader is referred to [START_REF] Halter | Development of an optically accessible apparatus to characterize the evolution of spherically expanding ames under constant volume conditions[END_REF] . This method may help to detect the occurrence of instabilities for weakly stretched ames. For unstable ames, the curves exhibit a slight change of slope and the position of max ( dp/ dt ) is advanced.

This information may be useful to detect the occurrence of instabilities in the absence of optical access. With a criterion of 90% of max ( dp/dt ) , the maximal pressure to consider is initially estimated to be at least ve times the initial pressure (5 p0 ) for all stable conditions. The position of max ( dp/dt

) occurs for slightly lower p/p0 when pressure is increased, keeping the initial temperature constant (i.e. T 0 = 300 K). This would suggest that heat losses to the wall are more pronounced as initial pressure increases, which can be explained by the slowing down of the ame front and so an increase of the propagation and heat exchange times. More details associated to the use of this method for the experiments performed is described in [START_REF] Halter | Laminar ame speed determination at high pressure and temperature conditions for kinetic schemes assessment[END_REF].

Heat loss model

In [START_REF] Mouze-Mornettas | Eect of wall heat transfer on the dynamics of premixed spherical expanding ames[END_REF] various criteria to determine the limit of high pressure at which heat losses occur including the previously proposed 90% of max ( dp/dt ) have been compared. The newly proposed criterion [START_REF] Mouze-Mornettas | Eect of wall heat transfer on the dynamics of premixed spherical expanding ames[END_REF] is dened as the minimum of δ ph . This denition relies on a constant value of A (the ratio between the preheating and the reaction zone) and can be numerically assessed as δ ph over δ f where δ ph is dened between T u + 1% and T ignition , the latter being dicult to evaluate. The temperature corresponding to the position where 5% of the unburnt fuel mass fraction is consumed was arbitrarily considered as T ignition .

Over evaluating a large range of conditions, A was determined to be around 5.

Upon comparison with other criteria, the authors arm that the minimum of (δ ph ) is a well-optimized criterion for the extraction of high-pressure S u and has been validated against the DNS calculations. For ame speed evaluation, this criterion is chosen as the upper-end limit on pressure. The results presented here have been evaluated using this criterion. For more information on this model, the reader is referred to [START_REF] Mouze-Mornettas | Eect of wall heat transfer on the dynamics of premixed spherical expanding ames[END_REF].

2.1. METHODOLOGY

Flame speed evaluation

OPTIPRIME is designed to aord several important pieces of information on ame dynamics like:

(i) information on the occurrence of instabilities on the ame surface (ii) the laminar ame speed and the burned gas Markstein length can be evaluated during the initial isobaric propagation (iii) the knowledge of the pressure and the ame radius traces allows evaluating the ame speed evolution for quite a large range of pressure and temperature conditions during the isentropic compression.

For the given scope, only the variation of pressure and temperature under isochoric conditions is of interest. It is assumed that pressure and temperature vary isentropically. Keeping the same pressure range for all conditions allows quasi-identical fresh gas temperature variations as the heat capacity ratio of the unburnt gas remains almost unchanged. The isentropic compression induces an absolute temperature variation of 100 K in the considered pressure range whereas the absolute pressure variation is proportional to the initial pressure.

These dierent absolute variations explain the dierent trends observed for dierent initial pressure conditions. The constant volume method proposed by

Lewis and von Elbe [START_REF] Lewis | Determination of the Speed of Flames and the Temperature Distribution in a Spherical Bomb from Time-Pressure Explosion Records[END_REF] uses the unsteady pressure-time history during the ame propagation event. Conversion of the reactants to hot products across the ame front results in a rapid pressure rise and a corresponding temperature rise in unburnt and burnt gas. The constant volume technique seeks to relate the instantaneous ame speed to the pressure history. In other words, in a single test, ame speeds can be obtained for a range of higher pressures and temperatures. Recently, Hinton et al. [START_REF] Hinton | Laminar burning velocity measurements in constant volume vessels Reconciliation of ame front imaging and pressure rise methods[END_REF] pointed out the fact that the method using the pressure rise requires a more complex analysis, but has the advantage that a single experiment generates data across a range of linked temperatures and pressures [START_REF] Burrell | Eects of stretch and thermal radiation on diuoromethane/air burning velocity measurements in constant volume spherically expanding ames[END_REF]. The pressure and temperature rise also meaning that data can be obtained for engine-like conditions. Equations for this method have been the subject of several pioneered publications [START_REF] Bradley | Mathematical solutions for explosions in spherical vessels[END_REF] . Based on the following assumptions: the pressure (p) is spatially uniform; the constituents of the burnt and unburnt gas behave as ideal gases, the following expression for the ame speed (S u ) was derived:

S u = dR f dt - (R 3 0 -R 3 f ) 3γ u R 2 f p 1 ρ u dρ u dt (2.1)
where R f and R 0 represent the ame radius and the inner chamber radius respectively, p is the pressure in the chamber and γ u is the heat capacity ratio of unburnt gas.

Experimental evaluation of the compression term

( 1 ρ u dρ u dt ) (2.2)
is possible but delicate [START_REF] Lefebvre | Formalism for spatially averaged consumption speed considering spherically expanding ame conguration 173[END_REF] . Alternatively, assuming that the unburnt gas is isentropically compressed yields:

S u = dR f dt - (R 3 0 -R 3 f ) 3γ u R 2 f p dp dt (2.3)

CHEMICAL KINETIC SCHEMES

where R f and R 0 represent the ame radius and the inner chamber radius respectively, p is the pressure in the chamber and γ u is the heat capacity ratio of unburnt gas. To evaluate the ame speed, both the pressure and the ame radius need to be determined simultaneously over a suciently long period to enable the computation of fairly accurate derivatives. To overcome the lack of optical access in conventional isobaric combustion chambers, a relation linking the ame radius to the pressure is usually used: unburnt gas is isentropically compressed yielding:

R f R o = (1 -(1 -x)( p o p ) ( 1/γ u )) ( 1/3) (2.4)
with p0 the initial pressure in the chamber and x = m b /m the burnt gas mass fraction (with m the total mass and m b the burnt gas mass). Similar to the ame radius, the burnt gas mass fraction is not accessible experimentally. The x = f(p) relation is either determined via numerical modelling [START_REF] Bradley | Mathematical solutions for explosions in spherical vessels[END_REF][START_REF] Moghaddas | Laminar burning speed measurement of premixed n-decane/air mixtures using spherically expanding ames at high temperatures and pressures[END_REF] or via simplied assumptions. The simple linear relation initially derived by Lewis and Von Elbe [START_REF] Lewis | Determination of the Speed of Flames and the Temperature Distribution in a Spherical Bomb from Time-Pressure Explosion Records[END_REF] is still widely used:

x = p -p o p e -p o (2.5)
with p e the theoretical end-pressure, which should be evaluated using ther- mochemical calculations. Its value signicantly aects the ame speed determination and may be inuenced by dissociation [START_REF] Luijten | Impact of dissociation and end pressure on determination of laminar burning velocities in constant volume combustion[END_REF] . However, in this case, thanks to the wide optical access, instantaneous pressure and radius data can be obtained and may be directly used in equation 2.3 to evaluate the ame speed. More information on the methodology and validation is provided in [START_REF] Halter | Development of an optically accessible apparatus to characterize the evolution of spherically expanding ames under constant volume conditions[END_REF][START_REF] Halter | Laminar ame speed determination at high pressure and temperature conditions for kinetic schemes assessment[END_REF][START_REF] Burrell | Eects of stretch and thermal radiation on diuoromethane/air burning velocity measurements in constant volume spherically expanding ames[END_REF]107109].

Chemical kinetic schemes

On performing a literature survey, 9 recent kinetic schemes for ammonia combustion have been chosen for this study and are presented in Table 2.1.

The ame speed was calculated in Chemkin-Pro for a pressure-temperature pair which evolves isentropically mimicking the experimental combustion process.

Among the 9 chosen mechanisms, it was seen that 2 mechanisms-Nakamura

and Hasegawa [START_REF] Nakamura | Combustion and ignition characteristics of ammonia/air mixtures in a micro ow reactor with a controlled temperature prole[END_REF] and Stagni et al. [START_REF] Stagni | An experimental, theoretical and kineticmodeling study of the gas-phase oxidation of ammonia[END_REF] predicted and captured the experimental results at all conditions better than the other mechanisms. Although all the chosen mechanisms could aptly produce the trend, i.e. the ame speed increases with an isentropic increase of temperature and pressure for all the test conditions; there is a variation in the ame speed values. To understand this and to highlight the role of the key reactions, sensitivity analyses were performed. In addition to these 9 mechanisms, three latest mechanisms were added and it was seen that three mechanims didn't predict ame speeds close to the experimental ame speeds.

Test conditions

The tests were performed for a mixture of ammonia-oxygen at 3 dierent equivalent ratios: 0.8, 1.1 and 1.3 at an initial temperature of 300 K. For the 2.3. TEST CONDITIONS * Okafor scheme was modied to add helium using the third body coecients of argon.

rich mixtures, the initial pressure ranged between 1 bar and 4 bar whereas, for the lean mixture, it was at 1 bar and 2 bar. 30% of the oxidiser mixture consists of oxygen and the remaining 70% is the diluent for all the conditions. The diluent is required because these ames are quite unstable in nature as both hydrodynamic and thermo-diusive instabilities are favoured in these temperature and pressure conditions. The Lewis number of ammonia ames is close to 1 and so, in order to increase the Lewis number, there is a need of adding bath gases with high thermal diusivity. A combination of argon and helium was used to stabilise the ame. The ratio of the diluent was chosen such that the mixture could be readily ignited and the ame sustains till it hits the wall.

It is dicult to ignite mixtures with a high helium content at low pressures as helium possesses a high thermal diusivity. In the case of high-pressure conditions, the occurrence of instabilities is promoted and so, a large percentage of helium is required to stabilise the ame. The initial conditions used are represented in Table 2.2. It is true that from an application point of view, ammonia-air ames are more relevant but these high pressure and temperature conditions cannot be easily attained by using nitrogen as the diluent. One of the ideas is also to challenge the kinetic schemes at these elevated conditions and in doing so, the ame speeds for dierent conditions can be predicted by using the best performing kinetic scheme. The adiabatic ame temperature has been calculated and reported in

Experimental and numerical ame speeds

As mentioned in the previous chapter, the experiments were performed in the isochoric condition. The isochoric method helps to retrieve a wide band of ame speed data at elevated conditions [START_REF] Halter | Laminar ame speed determination at high pressure and temperature conditions for kinetic schemes assessment[END_REF][START_REF] Burrell | Eects of stretch and thermal radiation on diuoromethane/air burning velocity measurements in constant volume spherically expanding ames[END_REF]107109]. This range has been chosen such that the stretch eects that occur in the initial phase and the heat losses endured when the ame front hits the wall do not aect the propagation of the ame front and hence, the ame speed. Table 3.1 represents the range of pressure and temperature for which the ame speeds have been determined.

It is assumed that the temperature of fresh gases evolves isentropically while the ame front propagates. A relative accuracy of less than 0.5% on the ame radius was propagated on the ame speed leading to a maximal error lower than 5% [START_REF] Halter | Laminar ame speed determination at high pressure and temperature conditions for kinetic schemes assessment[END_REF]. The experiment traces have been widened to account for this uncertainty. An average of 6 shots were performed for each case to check for repeatability.

The experimental ame speeds and the ame speeds from the chosen mechanisms were calculated and plotted for all the conditions given in Table 2.2.

A very similar trend for all the given conditions was noted. However, most brown represents Otomo [START_REF] Otomo | Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion[END_REF], pink represents Mathieu [START_REF] Mathieu | Experimental and modeling study on the high-temperature oxidation of ammonia and related NO x chemistry[END_REF], cyan represents Modied Okafor [START_REF] Okafor | Measurement and modelling of the laminar burning velocity of methane-ammonia-air ames at high pressures using a reduced reaction mechanism[END_REF], green represents Shrestha [START_REF] Shrestha | Detailed kinetic mechanism for the oxidation of ammonia including the formation and reduction of nitrogen oxides[END_REF], red represents Stagni [START_REF] Stagni | An experimental, theoretical and kineticmodeling study of the gas-phase oxidation of ammonia[END_REF] and black represents Nakamura [START_REF] Nakamura | Combustion and ignition characteristics of ammonia/air mixtures in a micro ow reactor with a controlled temperature prole[END_REF] observations. The chosen conditions help to demonstrate a fuel-rich versus fuel-lean mixture at similar initial conditions and a low-pressure versus highpressure condition at the same equivalence ratio. φ=1.3, P 0 =1 bar and T 0 =296.8 K and Figure 3.3 represents the ame speed at φ=0.8, p 0 =1 bar and T 0 =296.2 K. Both the cases use the same diluent mixture: 49% Ar and 21% He. The ame speeds of the lean fuel-air ratio case, φ=0.8 higher than that of the rich case, φ=1.3. The y-axis represents the ame speed whereas the x-axis is split as temperature and pressure. This is done as the ame speed is a function of both pressure and temperature which evolves isotropically and isentropically during the combustion process.

On analysing all the cases, it was concluded that both Nakamura

Sensitivity analyses

Sensitivity analyses were performed on a few mechanisms to understand the behaviour and to highlight the key reactions. It was seen that the most dominant reaction irrespective of the mechanism and the initial conditions are O 2 + H --⇀ ↽--O + OH. It is interesting to note that the sensitivity coecient on the ow rate for the reactions for all the schemes is quite dierent even though the rate constant, k is quite similar. On comparing the rate constants of this reaction O 2 + H --⇀ ↽--O + OH for Klippenstein and Nakamura and the ame speeds it can be conrmed that the discrepancy in the magnitude of ame speeds does not arise from the small dierence in values of the k. the sensitivity results at φ = 1.3 for a p 0 =2 bar with a diluent mixture of 21% Ar and 49% He and for a p 0 =4 bar with a diluent mixture of 70% He respectively.

The analyses illustrate the use of dierent equivalence ratios, initial pressures and diluent mixtures. The sensitivity analyses for the remaining conditions are provided under the annexe section: A.

It is interesting to note that even though the ame speed predicted by Nakamura and Stagni is quite close, the top 10 important reactions are not exactly the same. The two common reactions apart from the dominant reaction are:

HNO + H --⇀ ↽--NO + H 2 and NH 2 + NO --⇀ ↽--NNH + OH. It is noticed that the reaction H 2 + O --⇀ ↽--H + OH exists within the top 10 important reactions in all cases for both mechanisms except for the lean cases of Nakamura. The rich cases for both the mechanisms witnessed another 2 common reactions: NH 2 + H --⇀ ↽--NH + H 2 and NH 2 + O --⇀ ↽--HNO + H. A standard set of reactions is noted for each equivalence ratio apart from the above-stated common reactions irrespective of the initial pressure except for H + O 2 + (M) --⇀ ↽--HO 2 + (M) which is found only for those with an initial pressure of 2 bar or more.

However, none of the stated reactions plays a role as signicant as the O 2 + H --⇀ ↽--O + OH reaction does. Also, from the sensitivity analysis charts it can be seen that the ame speed is not highly sensitive to pressure. The sensitivity analyses show that these mechanisms predict a weak dependence of ame speed on pressure. On comparing with methane ames [START_REF] Halter | Laminar ame speed determination at high pressure and temperature conditions for kinetic schemes assessment[END_REF], the recombination reaction, CH 3 + H --⇀ ↽--CH 4 is one of the main reactions which becomes more sensitive at a higher pressure and equivalence ratio. The equivalent recombination reaction in ammonia ames, NH 2 + H --⇀ ↽--NH 3 is not as sensitive as it is for methane ames. Indeed, the mass burning rate is a parameter that indi- pressure initial condition, it can be seen that the ame speed variation over a pressure dierence of 30 and 6 bar are translated as a variation of about 180 K in the temperature scale. In other words, the ame speed change is much higher when the temperature changes by a small amount as compared to a variation in pressure. The temperature increase is higher at the beginning of the compression and this eect decreases at higher pressure. All of these suggest that the combustion of ammonia is more temperature-sensitive rather than pressure-sensitive, thereby implying that the mechanisms may be temperature-driven and not pressure-driven. In order to understand the working of these mechanisms further study has been made. Chapter 4 gives a more detailed study of this aspect.

CHAPTER 4 CHAPTER 4

In order to understand the working of these mechanisms further study has been done. This chapter focuses on some case studies to de-couple the pressure and temperature to understand the individual eects on ame speed. A comparison among the three chosen mechanisms: Shrestha, Nakamura and Stagni is also done to see how each of these mechanisms behaves. Similar studies were done for dierent bath gases to see their inuence. As for the case of ame speed variation over the temperature, it is seen that the ame speed increases with temperature and the change is prominent at a higher temperature. This essentially proves that the mechanisms are indeed driven by temperature. On evaluating the performance of the 3 mechanisms, it can be said that the temperature eect is the same in the 3 mechanisms.

Isotherms and isobars

Bath gases eects

The eect of the bath gases is studied and is displayed in gures 4.4a, 4.4b, 4.3a and 4.3b. Isotherms at T=300 K and 500 K and isobars at p=1 bar and 3 bar gave similar results. Isothermal plots at T=300 K and T=500 K

Alpha and beta

The pressure and temperature dependencies may be expressed as given in the equation 2.3 using a reference ame speed and the corresponding conditions if the exponential factors α and β are known.

S u S o u = ( T u T o u ) α ( P u P o u ) β (4.1)
Using the isothermal and isobaric curves, the values for α and β are estimated. It is observed that α varies linearly with the temperature for all the 3 mechanisms and the value ranges between 1.5 and 2. This range of value is quite common to most of the fuels [START_REF] Konnov | A comprehensive review of measurements and data analysis of laminar burning velocities for various fuel+air mixtures[END_REF]. β variation for Nakamura and Stagni is quadratic whereas it is almost a constant for Shrestha as seen in Figure 4.5.

Beta trends represented in Figure 4.5 are calculated using p 0 =1 bar, T 0 = 500 K and the corresponding ame speed depending on the equivalence ratio and the diluent mixture. The values of β lie between -0.04 and -0.17 which is much lower than most of the fuels [START_REF] Konnov | A comprehensive review of measurements and data analysis of laminar burning velocities for various fuel+air mixtures[END_REF]. The low value of beta results from pressure-4.2. ALPHA AND BETA VALUES FOR THE EXPERIMENT AND NUMERICAL FLAME SPEEDS independent behaviour. It is worth emphasising that the pressure independence increases with an increase in pressure.

Alpha and beta values for the experiment and numerical ame speeds

From the experimental test conditions, it is possible to have at least two conditions with the same equivalence ratio, initial temperature and diluent mixture but with a dierent initial pressure giving rise to two (p, T) pairs. A certain range of pressure is common to both conditions but the corresponding temperatures are dierent due to the initial conditions. The ame speeds can be obtained for two dierent temperatures at an isobaric condition. Considering one of the ame speed curves (here, the ame speed corresponding to the initial condition with the lower pressure) as the reference curve, an estimation of α can be made using equation 4.1 and can be optimised to get the most precise value. A similar process can be used to estimate the β values from ame speeds corresponding to two dierent pressures at isothermal conditions.

The values of α and β from Nakamura and Stagni lie within the experiment range. The values of β range between -0.07 to -0.21 and the values of α range between 1.5 and 1.92 depending on the initial conditions. Both α and β are not constant values, therefore an estimation of the ame speed using equation 2.3 can be made by interpolating the values within the given range for the given set of conditions. It can be intuitively said that having a low β value is advantageous as pressure oscillations would have fewer eects on ame stabilization in systems like gas turbines.

CONCLUSION

Experiments to measure ame speeds corresponding to the conditions encountered in spark-ignition engines and gas turbines have been performed. The study was done for 3 dierent equivalence ratios: 0.8, 1.1 and 1.3 at an initial temperature of 300 K and an initial pressure ranging from 1 to 4 bar. The data available in the literature gives the ame speeds for a maximum pressure of 10 bar and a temperature of 473 K. From this study, the laminar ame speeds for a pressure of 30 bar and a temperature of 585 K have been obtained. These tests were performed under constant volume conditions which is the most ecient way to obtain a wide range of data for elevated conditions. A literature study had been made to select the most recent kinetic schemes for ammonia combustion. These schemes have been evaluated for the same conditions as that of the experiments. It was seen that the chosen schemes could produce the experimental trend-an increase in ame speed with an isotropic increase of pressure and temperature for all the conditions. On performing sensitivity analyses, it was understood that the most dominant reaction is O 2 + H --⇀ ↽--O + OH. Despite the high sensitivity of this reaction and the rate constant of this reaction being similar for all mechanisms, only 4 mechanisms: Shrestha, Nakamura, Modied Okafor and Stagni could predict the ame speeds with the ± 5 % error bar of the experimental ame speeds. The recombination reaction of the ammonia ames is pressure insensitive. In order to understand the working of the mechanisms, various simulations were performed at isobaric and isothermal conditions. The results of these simulations prove that the mechanisms are temperature-driven and not pressure-driven as the change in ame speed for a small increase in temperature is much higher than the change in ame speed obtained from the same increase in pressure.

The maximum inuence of pressure is seen only from 1-5 bar after which the ame speeds do not vary a lot. The increase in the ame speed is attributed only to the increase of temperature which promotes chemical reactivity. The impact of adding the bath gases has been noted for some of the mechanisms.

The change in the ame speed response to pressure for Shrestha seems to have a lesser eect from the bath gas composition whereas Stagni and Nakamura seem to be inuenced by the constituents of the bath gases.

A range of exponential factors α and β required to calculate the ame Conclusion speeds at a given pressure and temperature using a reference condition was determined. It is interesting to note that β, the exponential factor for the pressure is much smaller than most of the fuels emphasising the pressure independency behaviour. This implies that ammonia is an ideal fuel for sparkignition engines and gas turbines since pressure oscillations would have limited eects on ame speed.

Finally, from the data generated, a 3D map of ame speed as a function of pressure and temperature using a t proposed by [START_REF] Hu | Laminar ame speeds and ignition delay times of methaneair mixtures at elevated temperatures and pressures[END_REF] can be done. As an example Figure 4.6 3 represents the ame speeds at ϕ = 1.1, diluent mixture of 49% He and 21% Ar, T 0 = 300 K, p 0 = 1, 2, and 3 bar as a function of p and T. The experimental data are the blue, red, and green thick lines in the volume. The blue, red, and green thin lines on the lower plane are the projections of these experimental data on the (p, T) plane representing the isentropic evolution of pressure and temperature starting from each initial condition. The idea of using this map is that by interpolating data, ame speeds can be obtained for any pressure and temperature for the given gas mixtures without the need of performing new experiments. Figure 4.6: Experimental ame speeds of NH 3 /O 2 /He/Ar mixtures at ϕ = 1.1, T 0 = 300 K, p 0 = 1 (blue), 2 (red), and 3 bar (green).

Part II Inner structure of a stretched premixed ame

INTRODUCTION

From the previous part, it was seen that ammonia/oxygen ame speeds at elevated conditions are on par with conventional fuels. However, it is known that ammonia/air ame speeds at atmospheric conditions are quite low. It is important to know the inner structure of the ame to understand the combustion process better and therefore, to comprehend the inuence of various parameters to nally enhance the process. Ammonia/air ames are known to be much thicker than hydrocarbon ames [START_REF] Kobayashi | [END_REF]. Part II is an attempt to give an insight to the structure of stretched premixed ammonia/air ames at atmospheric conditions. The parameters that were focussed on to study were the inuence of both the equivalence ratio and the curvature on the thickness and reactivity of the ame. This was experimentally achieved by studying the chemiluminescence signature of the important species.

Chapter 5 consists of a brief discussion on the dierent experimental setups along with the diagnostics used to determine the ame speed for ammonia/air combustion at atmospheric conditions. A glimpse into the literature describing the structure of ammonia ames has also been provided. A review of the chemiluminescence and the curvature eect has been done as well.

Chapter 6 presents a description of the experimental set-up used here.

This chapter also discusses the dierent techniques used here in order to full the set objectives.

Chapter 7 emphasizes the ame thickness and the evaluation of ame speed over a range of equivalence ratios. A comparison of the obtained ame speed with the existing literature data has been provided.

Chapter 8 pinpoints the curvature eects on dierent species emitted from the ame. A study of the inner structure of the ame for a slightly lean mixture, near stoichiometric conditions and a rich mixture, have been presented here.

The chemiluminescence signature of these ames provides a piece of important information on the HRR which has been well noted.

To conclude, a global summary highlighting the importance of the ame thickness and evaluation of other parameters aected by the thickness like ame speed has been provided. Also, the eect of curvature on the chemical reactivity of ammonia/air ames for a given range of equivalence ratios has been summarised.

CHAPTER 5 CHAPTER 5

As mentioned in the introduction, it is important to study the ame structure and other characteristics to understand the combustion process better and to enhance the performance in energy-based applications. Determining the ame speed is important as it is one of the basic parameters which is used for assessing the characteristics of fuel and engine sizing. It is known that ammonia-air ames have very low ame speed values as reported in the literature. Flame speed is generally evaluated for an isotherm that is as close as possible to the fresh gas's temperature. The error associated with nding the values of the ame speed by not choosing the right isotherm can be large which can lead to huge errors in the assessment of other parameters based on the fresh gas isotherm. The laminar ame speed, S L , is dened as the propagation speed relative to the unburnt mixture of a steady, laminar, 1D, planar, stretch-free, and adiabatic ame. Hence, the required isotherm is the one at the fresh gas surface for which dening the ame thickness is needed.

Experiments and optical diagnostics for ame speeds in ammonia combustion

There are several methods to experimentally determine ame speed. The most common method is the closed spherical vessel. Figure 5.1 shows the dierent experimental methods that are found in literature along with their pressure and temperature range. Depending on the objective of the experiment, the apt technique is used to carry out the experiment. For example, in Part I, the constant volume chamber was used as the target was to hit high temperature and pressure conditions. The constant pressure chamber is used to determine ame speeds at a constant high pressure. For both these methods, the detection of the ame front can be done by following the chemiluminescent part of the ame or by using the Schlieren method. Stagnation ow is used for a lower pressure and temperature range. Sampling probes can be used to determine the temperature at dierent distances. The heat ux burner is used for a narrower range of pressure and temperature. Thermocouples are used to measure the temperature dierence on dierent parts of the plate. Using this temperature gradient, ame speeds can be found. The externally heated channel method is used for a wide range of temperature conditions. Flame speeds are computed using the temperature and the ow. The Bunsen burner is used for a slightly lower temperature range but for a higher pressure range. PIV can be used to trace particles, and so the ow velocity can be obtained.

CHEMILUMINESCENCE SIGNATURE OF AMMONIA FLAMES

Chemiluminescence signature of ammonia ames

Chemiluminescence is the emission of light, usually in the visible or nearinfrared spectral region, as a result of an excited electronic molecular state, formed in a chemical reaction, returning to the ground state. It is often observed during the combustion process when species get excited and return to the ground state often giving a hue to the ames. In combustion, chemiluminescence is an important diagnostic tool which is often deployed to give the heat release rate position. Another reason, why these excited species are of interest from the heat release rate point of view is because it can be directly used in simulations which implement kinetic models that are based on reactions of several excited and non-excited species. Chemiluminescence can be used to study ame properties like equivalence ratio, temperature and concentration of the species. Bedard et al. [START_REF] Bedard | Chemiluminescence as a diagnostic in studying combustion instability in a practical combustor 213[END_REF] in their paper give an extended overview of the use of chemiluminescence as a diagnostic tool in combustion.

STUDIES OF STRETCH EFFECTS IN AMMONIA FLAMES

From the emission spectra of ammonia-air ames, NH* and NH 2 * have been recognized as species of interest to study. Extensive work on the chemiluminescence of species in ammonia/air and other blended fuels [115117] can be found in the literature. Pugh et al. [START_REF] Pugh | An investigation of ammonia primary ame combustor concepts for emissions reduction with OH*, NH2* and NH* chemiluminescence at elevated conditions[END_REF] used chemiluminescence in both NH 3 /air and NH 3 /H 2 /air to quantify NO in a combustor. Zhu et al. [119] obtained OH* to quantify NO in NH 3 /H 2 /air ames. NH 2 * chemiluminescence has also been used to study NO emissions in plasma-assisted ammonia combustion [START_REF] Choe | Plasma assisted ammonia combustion: Simultaneous NOx reduction and ame enhancement[END_REF].

Wiseman et al. [START_REF] Wiseman | A comparison of the blow-out behavior of turbulent premixed ammonia/hydrogen/nitrogen-air and methaneair[END_REF] showed that OH* can be used to compare the ame topology before the blowout. Chemiluminescence from various species [START_REF] Vigueras-Zúñiga | Methane/Ammonia Radical Formation during High Temperature Reactions in Swirl Burners[END_REF] was used to study radical formation in high-temperature reactions for NH 3 /CH 4 blends in swirl ows both experimentally and numerically and also to determine the location of the highest heat release rate. Liu et al. [START_REF] Liu | The characteristics of ame propagation in ammonia/oxygen mixtures 363[END_REF] also used chemiluminescence to characterize the heat release rate. The chemiluminescence of selected species may be essential to characterize the ame stability and heat release.

To our knowledge, no information on HRR and the curvature eect on species via chemiluminescence is available for ammonia/air ames.

Studies of stretch eects in ammonia ames

Stretch eects can be classied into two components: strain rates and curvature eects [START_REF] Thiesset | Isolating strain and curvature eects in premixed ame/vortex interactions[END_REF]. It is important to know these eects on the ame as this changes the ame surface area and hence enhances or de-enhances reactivity depending on the stretch being negative or positive. Also, ame properties like ame speeds are not the same for stretched and unstretched ames. Hence, it is important to characterize these eects. Often quantities like Markstein length and Markstein number are used to parametrize stretch eects.

DNS studies [START_REF] Netzer | Curvature eects on NO formation in wrinkled laminar ammonia/hydrogen/nitrogen-air premixed ames 232[END_REF] were performed to study the curvature eects on NO formation in NH 3 /H 2 /N 2 /air ames. There is a vast study on stretch eects and strain rates. Studies on extinction stretch rate in both premixed and nonpremixed NH 3 /CH 4 ames were done experimentally using OH and NO PLIF and numerically using four of the mechanisms existing in literature by Colson et al. [START_REF] Colson | Experimental and Numerical Study of NH 3 /CH 4 Counterow Premixed and Non-premixed Flames for Various NH 3[END_REF]. Xiao et al. [START_REF] Xiao | Study on counterow premixed ames using high concentration ammonia mixed with methane 275[END_REF] studied strain rates for ammonia/methane ames and reported that the strain rate at extinction enhanced upon the addition of methane in the fuel mixtures. Lee et al [START_REF] Lee | Studies on properties of laminar premixed hydrogen-added ammonia/air ames for hydrogen production[END_REF] in their work use the Markstein number to quantify the stretch eect in hydrogen-added ammonia air ames.

They reported that the general characteristics of unstable behaviour (Ma < 0) at fuel-lean conditions and stable behaviour (Ma > 0) at fuel-rich conditions are consistent with conventional explanations of ame stability. In contrast to the consistent increase in S L with hydrogen substitution, the dependency of Ma on the nature and extent of substitution varies substantially.

Literature on ammonia ame structure

Studying the ame structure gives an insight into the dierent species that exist which gives information on reactivity. Understanding the position of these species and how these species react upon a perturbation gives direct knowledge of its inuence on reactivity. Hence, it is important to understand the structure of the ame to enhance its properties of the ame to promote combustion.

CHAPTER 6

CHAPTER 6

This chapter describes the experimental setup and the diagnostics used for the dierent measurements that have been carried out.

6.1 The experimental set-up studies [START_REF] Pugh | An investigation of ammonia primary ame combustor concepts for emissions reduction with OH*, NH2* and NH* chemiluminescence at elevated conditions[END_REF][START_REF] Hayakawa | NO formation/reduction mechanisms of ammonia/air premixed ames at various equivalence ratios and pressures 2 14004021400402[END_REF] show the use of pilot ame to stabilise the main ame. Here, a close to a stoichiometric mixture of methane/air is used as the pilot ame.

The ow of both methane and air are controlled by rotameters whose ow are set to 0.01 L/min and 0.1 L/min. The ow of ammonia and air are controlled by Brooks mass ow controllers. The mass ow controller for air ranged between 0-30 NL/min whereas for ammonia it ranged between 0-3 NL/min. The mass ow controllers required for ammonia are dierent from the ones used by 6.1. THE EXPERIMENTAL SET-UP conventional fuels as ammonia is highly incompatible with some materials like brass, copper etc. These mass ow meters are operated using software called Supervision V12. Special pipes that are compatible with ammonia were used for the feed. This helped to eliminate the problem of corrosion of pipes. Each of the ammonia pipes was linked to a pipe lled with nitrogen gas to ush out the ammonia remains in the pipe after every experiment thereby, elongating the lifespan of these pipes. The burner was placed inside a cabin and exhaust was used to evacuate unburnt ammonia from this cabin. The mean velocity of the fresh gases at the exit of the burner ranged from 0.18 to 0.2 m/s. The fresh gas mixture is injected through the sides of the burner which further ows through a 5 mm thick aluminium grid to obtain a laminar ow. This reactive ow is ignited at the exit of the burner by the presence of the pilot ame. The pilot ame emits a power of 55 W which is required to stabilize the ammonia/air ame. The power emitted by the main ame is between 83 and 117 W.

Although, the power emitted by the pilot ame in the present conguration is about half of what is emitted by the main ame, it can be safe to say that this power has no direct inuence on the characteristics of the main ame as tests were done where the ow of the pilot ame was changed, therefore the power of the pilot ame was varied. It was seen that changing the power of the pilot ame did not result in a change in any of the behaviour of the parameters used in this study. Moreover, these studies are carried out away from the base of the main ame. The heat losses to the burner are not considered for the same reason. The radiation eects may occur in the burnt gases from the water vapour. For this study, these eects are not of concern as the region of interest in restricted to the initial part of the reaction zone which is away from the burnt gases.

It was seen that the ame was subjugated to a buoyancy-driven ickering.

There are literature sources [START_REF] Wiseman | Measurements from ame chemiluminescence tomography of forced laminar premixed propane ames[END_REF] that show these eects can be reduced by using a chimney. A 150 mm high cylinder with an outer diameter of 120 mm and thickness of 1 mm was mounted at a height of 25 mm from the burner exit which acted as a chimney and helped to reduce the entrainment of air. Figure 6.2a and Figure 6.2b depict the buoyancy-driven ame ickering and Figure 6.2c shows that using a chimney helps to reduce this eect.

The PIV set-up

The ame speed was determined using a PIV setup. The airow is fed through a seeding system, consisting of an atomizer that produces droplets of respectively. Since the ow velocity was very low, every tenth image was chosen to track a droplet and hence, to determine the velocity. The PIV images were mainly used to determine the isotherm close to the fresh gas which is needed to evaluate parameters like ame speed and ame thickness whose denitions depend on the fresh gas isotherm. Figure 6.3 represents the schematic diagram of the set-up.

The chemiluminescence set-up

On studying the emission spectra of ammonia/air ames at stoichiometry, 4 species: OH*, NO*, NH* and NH 2 * were identied as species of interest.

The orange hue of the ame is attributed to the dierent peaks at about 600 nm. All of these peaks correspond to NH 2 *. Figure 6.4 represents the non- calibrated emission spectra of ammonia/air at stoichiometry. The emission spectrum from the pilot ame has been subtracted from the raw spectrum of the ammonia-air ame and has been represented here. Several of the peaks have been identied using the emission spectra of ammonia ames which were found in the literature [START_REF] Gaydon | The Spectroscopy of Flames[END_REF]. For the current study, NH* and NH 2 * have been chosen as the species of interest as these two species have not been well studied.

The inner structure of the ame was determined using chemiluminescence.

The lters used for NH* and NH 2 * are centred at wavelengths 337 nm and 632 nm respectively with a diameter of 50 mm and a bandpass width of 10 nm. These lters were manufactured by Edmund Optics. The lter is placed in front of the high-speed IRO intensier which is placed in front of the CMOS camera. The intensier is used to have maximum sensitivity and to have good image quality. Two dierent objective lenses were used: a UV lens for NH* and a visible range spectrum lens for NH 2 *. An instantaneous shot is used for the study as it was seen that an average ame perturbation of over 200 images is PIV set-up. This image was captured using NIKON D850 with a 180 mm lens and an exposure time set to 250 ms. It can be seen that this ame is much thicker than most hydrocarbon ames. The DEHS droplets are illuminated by the laser. When these droplets reach the 525 K isotherm, they evaporate. Thus, fresh gases are in the inner cone (in green) and the isotherm 525 K appears in Figure 7.2 as the limit between the green and the orange zones. The outermost surface of the ame is referred to as the outer cone in Figure 7.2 and is the limit between the orange and the black zones. The ame thickness, δ, can be dened as the zone between the inner and the outer cone (between the green and the black zones in Figure 7.2). The axes in Figure 7.2 represent the position normalized over the burner radius with the centre of the x-axis pinned at the centre of the burner and the zero of the y-axis at the exit of the burner. The eect of DEHS droplets has been tested and it was seen that the evaporation of these droplets didn't interfere with the parameters used in this study.

Flame speed determination

The most commonly used method to evaluate ame speed for ammonia combustion available in the literature is by using a closed spherical vessel with Schlieren [START_REF] Hayakawa | Laminar burning velocity and Markstein length of ammonia/air premixed ames at various pressures 159[END_REF][START_REF] Takizawa | Burning velocity measurements of nitrogen-containing compounds[END_REF][START_REF] Pfahl | Flammability limits, ignition energy, and ame speeds in H2CH4NH3N2OO2N2 mixtures[END_REF] and by vertical tube with direct imaging [START_REF] Jabbour | Burning Velocity and Refrigerant Flammability Classication[END_REF][START_REF] Zakaznov | Determination of normal ame velocity and critical diameter of ame extinction in ammonia-air mixture 14 710713[END_REF] Han et al. [START_REF] Han | Experimental and kinetic modeling study of laminar burning velocities of NH3/air, NH3/H2/air[END_REF] used the heat-ux burner method to provide the ame speed data whereas Ronney et al. [START_REF] Ronney | Eect of Chemistry and Transport Properties on Near-Limit Flames at[END_REF] provided the ame speed at microgravity in a closed Ref. [START_REF] Pfahl | Flammability limits, ignition energy, and ame speeds in H2CH4NH3N2OO2N2 mixtures[END_REF] and Ref. [START_REF] Hayakawa | Laminar burning velocity and Markstein length of ammonia/air premixed ames at various pressures 159[END_REF] respectively by omitting the surface area in the mass continuity equation. While considering the density of the burnt gases for this equation it is assumed that density remains homogeneous which may be true for thin ames. Tazikawa et al. used equation 1 in Ref. [START_REF] Takizawa | Burning velocity measurements of nitrogen-containing compounds[END_REF] by dening the burnt mass fraction which also depends on the radius chosen. When measurements are done using a thermocouple, the distance chosen for the measurement is directly linked to the isotherm which is used to calculate the ame speed. In the case of the vertical tube method, the outermost visible layer (equivalent to the OC as shown in Figure 7.2) is chosen as the ame front.

The laminar ame speed, S L , in the case of the Bunsen burner is obtained by applying conservation of mass at the exit of the burner to the inner surface and can be mathematically formulated as:

A b .U f g.ρ f g = A u .S L .ρ f g (7.1)
where U fg is the fresh gases velocity at the burner outlet, A b is the exit area 7.2. FLAME SPEED DETERMINATION of the burner, ρ fg is the density of the fresh gases and A u is the area of the unburnt gas surface. The displacement speed, S d , is dened as the propagation speed of a ame relative to the fresh gas ow and is given as:

S d = (u -w).n (7.2)
where u is the uid velocity, w is the absolute speed of the ame in a xed laboratory and n is the ame normal vector oriented towards the fresh gases.

Here, w = 0.

As per the denition of the ame speed, the isotherm to be chosen corresponds to the inner cone. The laminar ame speed, S L , and the displacement speed, S d , are computed using Equation 7.1 and Equation 7. given in the literature, it is seen that the ame speeds reported in the literature are closer to the ame speed obtained at the outer cone; far from the fresh gas isotherm. It can also be seen that the displacement speed evaluated at the inner cone which is represented by the black diamond in Figure 7.3 is in congruence with the laminar ame speed at the same surface.

These obtained experimental ame speeds were compared with the ame speeds obtained from simulating the previously chosen 2 kinetic schemes (from Part I) and it was seen that both the schemes predicted the ame speed within the error bars of the experimental ame speed.

CURVATURE EFFECTS ON THE SPECIES

Experiments were performed for equivalence ratios between 0.9 and 1.4. As mentioned in [START_REF] Thiesset | Isolating strain and curvature eects in premixed ame/vortex interactions[END_REF], stretch eects, K can be decomposed into two components: strain rate,K T and curvature eect, K C . Curvature eect,K C is dened as 2.S d .K m where K m is simply the average of two principal curvature: k1 and k2.

k2 is the curvature in the circumferential direction whereas k1 is the curvature in the plane. When the ame contour is expressed in terms of cylindrical coordinates with h in the streamwise direction and r in the radial direction. k1 and k2 are mathematically represented as:

k1 = ṙḧ -r ḣ (8.1) k2 = ḣ r (8.2)
where the dot signies with respect to the curvilinear coordinate s which is expressed as:

τ 0 ∂r ∂ϵ 2 - ∂h ∂ϵ 2 dϵ (8.3)
Since the ow velocity is low, it can be assumed that the strain eects are negligible and also, the focus here is on the curvature eects. Figure8.1 is a schematic representation of k1 and k2 in the ame contour. Essentially, k1 is along the contour and k2 is dened along the circumferential direction varying as the radius of the curvature changes with the ame height. Negative curvature is dened for a concave ame in the direction of the burnt gas and positive curvature is dened for a convex ame in the direction of the burnt gas. From the chemiluminescence of NH* and NH 2 *, it is seen that the richer the ame, the less prominent is the curvature eect. For all conditions, except for φ = 1.1 and 1.2, it was noticed in the Abel inversion transformed images that the negative curvature at the tip of the ame does not enhance the reactivity as the average intensity remained almost constant all along the species front. for φ= 1, 1.2, and 1.4. In these raw images, these eects are not visible simply because we see a 2D projection of the ame which means we see integrated information of the species. The chemiluminescence images of all species at all equivalence ratios can be seen in Annexe B. Since, the Lewis number increases with equivalence ratio and is close to unity [START_REF] Ichimura | Extinction limits of an ammonia/air ame propagating in a turbulent eld[END_REF], the eect of the negative curvature is not well pronounced at the tip of the ame, although a slightly higher eect is expected for richer ames which explains why φ= 1.1 and 1.2 witnesses a slight enhancement of reactivity at the ame tip. Irrespective of the excited species and the equivalence ratio of the mixture, it was observed that the species thickness continues to increase and begins to decrease when subjugated to the negative curve (indicated by k1) near the tip of the ame (in Figure 8.2 c and d, it is seen at x/r almost equal to -0.3). The magnitude of k2, on the other hand, increases when moving from the base of the ame to the tip of the ame. This goes to show that the mean curvature eect was simply seen as a change in thickness of the species produced if not directly the concentration of species in a given region as the average intensity remains the same (as seen in Figure 8.2). The concentration of the species was spread along a larger region where k1 was positive which was encountered near the tip of the burner for lean and near stoichiometric conditions and was concentrated over a smaller region where k1 was negative (seen at the tip of the ame). At the tip of the ame, the contribution of k2 is higher which gives an overall higher negative curvature eect. k1 is relatively smaller than k2 and is almost 0 until a distance of 0.3 times the burner radius. The stretch rate corresponding to the ame propagative/curved character, as presented in [START_REF] Thiesset | Isolating strain and curvature eects in premixed ame/vortex interactions[END_REF], Kc is proportional to the product of the mean curvature of k1 and k2 and the displacement speed.

For ammonia-air ames, both the mean curvature and S d are small. On performing a species-wise comparison, it was seen that NH 2 * undergoes a slightly higher mean curvature eect when compared to NH*. It was also seen that the NH 2 * production was higher than that of NH* except for near the base of the ame as seen in Figure 8.2 b and Figure 8.3 a, b, and c and the thickness and the reactivity at this region being maximum (seen in Figure 8.2 c and b respectively). It was interesting to note that the NH* production was the highest at the tip of the burner and there was no production of NH 2 * in this region. Since the positive curvature at the burner exit was prominent only for lean and near stoichiometric mixture conditions, the inuence of the curvature leading to higher reactivity may be ruled out as the increase in production of NH* was seen unanimously for all equivalence ratios. It was initially thought that the heat from the pilot ame promoted the direct production of NH*.

Nevertheless, it was conrmed that the pilot ame did not trigger thermal decomposition to give NH as if it were the case; the production of NH 2 would also be high. A simulation performed in Chemkin-Pro veried that it was not thermal decomposition. This is done under the assumption that the reaction pathway to NH is through the decomposition of NH 2 . There may exist a reaction pathway to produce NH directly without the formation of NH 2 and presence of other species whose wavelength is within the bandpass width of the lter used as if this were to be the case, the presence of other species would be detected throughout the ame and not just at the base of the ame. The production of NH* may also be attributed to photodecomposition [133] or nonequilibrium process [START_REF] Agostino | Kinetic and spectroscopic analysis of NH3 decomposition under R.F. Plasma at moderate pressures 1[END_REF] where d'Agostino et al. employ a direct proportional relating the excited species to the non-excited species in NH 3 decomposition under radio frequency plasma at moderate pressures. Another possibility could be that since NH has weaker bonds than NH 2 and since NH is produced closer to the outer cone, it is in direct contact with the pilot ame whose temperature is about 2200 K which results in an increase of collisions resulting in a brighter zone. Further investigation needs to be done to understand this observation.

Flame structure

To study the ame structure, PIV, chemiluminescence and Chemkin simulations were used. Since, the Mie scattering images give the position of the fresh gas isotherm, i.e. the 525 K isotherm it was used as the reference. The The thickened plot for each of the species represents the standard deviation obtained for the excited species. From this fresh gas isotherm along the normal of the front, the relative position of the excited species can be obtained. To compare the stretched experimentally obtained ame with a Chemkin simulation of a non-stretched ame, a position along the experimentally obtained ame should be chosen such that, the stretch rates are negligible or quite small.

For φ = 0.9, 1.1, and 1.4, at a height of about 6 mm (y/r slightly less than 1) from the base of the ame where k1 is almost zero and k2 is near -0.2 1/mm is considered as the curvature eect at this point is relatively less and may be compared to a 1D freely propagating unstretched ame. Chemkin simulations for the kinetic scheme of Stagni were performed to make this comparison. The temperature and the heat release rate (HRR) prole along with the concentration of the non-excited species can be when the excited species along the normal of the ame front and the non-excited species along the 1D domain are tted into a Gaussian curve t with a 95% condence interval. It can be seen in the gures presented here that the species are overlapping with each other.

For easy visualization, each of the species has been plotted as a quarter bar in order to minimize the overlapping between species which otherwise would have resulted in diculty to understand the gure. The thickness of the ame front for each case is computed using the maximum gradient of temperature and is

CONCLUSION

In this part, an investigation was made to understand the inner structure of the ame. The important species were identied. The thickness of the ame In this part, an investigation was made to understand the inner structure of the ame. The important species were identied. The thickness of the ame and the position of these species within the ame have been depicted. Both experiments and simulations were used to correlate excited and non-excited species which were quite important to understand the production of these excited species. A preliminary study on the stretch eects on these species and therefore, its impact on reactivity for dierent equivalence ratios was performed.

The non-negligible thickness of atmospheric ammonia/air ames has been highlighted which implies that the thin ame approximation cannot be used.

The dependence of the ame speed on the isotherm and the dierence in the ame speed by choosing the fresh gas surface and the outer surface of the ame has been noted. To remain consistent with the theoretical denition of the ame speed, the isotherm to be chosen has to correspond to that of the fresh gases. The laminar ame speed for ammonia-air ame on a Bunsen burner at atmospheric conditions for equivalence ratios, φ = 1-1.4 by choosing the right surface has been evaluated. Evaluating the ame speed for the case of φ = 0.9

has not been presented here owing to its large error bars.

The curvature eects on both NH* and NH 2 * for φ = 0.9-1.4 have been studied. It was seen that the curvature indirectly aects the production of species by modifying the thickness of the species. When k1>0, the thickness of the species is more than when compared to k1<0. The curvature eects are stronger for lean ames. Since the ow is of low velocity, the strain eects can be neglected. On comparing the species from the experiment at a height where the curvature eects are relatively lower, to a 1D freely propagating ame, the structure of the ammonia-air ame at atmospheric conditions for dierent equivalence ratios has been studied. It was found that both NH* and NH 2 * were produced close to their counterpart non-excited species. The position of the maximum HRR can be determined experimentally from the position of these two excited species which is a useful indicator for obtaining information on ame stability in combustors.

Part III

Chemical and dynamical analyses of a premixed ame INTRODUCTION

Part II focussed on the inner structure of a ame on a Bunsen burner using diagnostics like chemiluminescence. It was seen that there are no associated excited species accommodated in the chemical kinetics. It is interesting to add these species especially when these species can be used to obtain direct information on the position of HRR as seen in the previous part. The rst step of Part III is to take it one step further by providing the concentration of certain excited species for the same conguration which is essential to add the excited species in the kinetic schemes. This step is especially useful when running simulations for dierent geometries and congurations.

It is known that turbulence enhances reactivity. But, turbulence is very complex in nature and capturing all phenomena is hard when they are coupled.

So, the very rudimentary step is to understand a perturbed laminar ame.

In this spirit, ame-acoustic interactions are studied. The objective of this section is to study the inuence of curvature on the species. Upon surveying the literature, it was seen that there was no previous experimental work done on ame-acoustic interactions for ammonia ames.

Chapter 9 is a summary of the literature on the use of dierent excited species and the eect of perturbations in ame.

Chapter 10 is an overview of the experimental set-up used here for both determining the concentration of species and for ame-acoustic interactions.

Chapter 11 gives a detailed study of the procedure used to calibrate and nd the concentration of the species. A quantitative and qualitative analysis of the emitted excited species is provided as well.

Chapter 12 sums up the dierent behaviours observed and attempts to give an insight into ame response to acoustic perturbations, especially with respect to curvature eects on the species. This last part is concluded with interesting observations and results which give an answer to some of the unknowns, opening paths to new directions.

CHAPTER 9 CHAPTER 9

This chapter is dedicated to taking a quick peek into the literature to know the previous work that has been done on excited species from the chemiluminescence of ammonia ames and to understand the work done in perturbed laminar ammonia ames.

Quantitative and qualitative analysis of chemiluminescence of ammonia ames

Chemiluminescence in ammonia ames has been widely studied. A detailed review of chemiluminescence has been provided in Chapter 5. From these studies, NH 2 * and OH* have been classied as species of importance as these species are indicators of certain parameters of interest. From [START_REF] Mashruk | Numerical Analysis on the Evolution of NH 2 in Ammonia/hydrogen Swirling Flames and Detailed Sensitivity Analysis under Elevated Conditions[END_REF], it can be seen that NH 2 is directly linked to NOx formations. For these reasons, it is interesting to study these excited species. However, in the literature, only a qualitative analysis has been provided [START_REF] Zhu | Chemiluminescence signature of premixed ammonia-methane-air ames 231[END_REF][START_REF] Pugh | An investigation of ammonia primary ame combustor concepts for emissions reduction with OH*, NH2* and NH* chemiluminescence at elevated conditions[END_REF]. There has been no previous attempt to do quantitative analysis on these species.

Zhu et al. [START_REF] Zhu | Chemiluminescence signature of premixed ammonia-methane-air ames 231[END_REF] in their work perform analysis on species like OH*, NH*, CN*, CH*, and CO 2 *. The chemiluminescence intensity of these excited species is provided as a function of strain rate. The chemiluminescence intensity ratios are also provided as a function of the equivalence ratio. The objective of their work was to use their database to assess the feasibility of chemiluminescencebased sensors applied to ammonia-methane-air ames. Various ratios of chemiluminescence intensity were identied as potential surrogates for equivalence ratio and/or ammonia fuel fraction. From the results of Pugh et al. [START_REF] Pugh | An investigation of ammonia primary ame combustor concepts for emissions reduction with OH*, NH2* and NH* chemiluminescence at elevated conditions[END_REF], it was concluded that NH 2 * intensity is a good marker for NO consumption in rich NH 3 ames. 9.2. PERTURBED LAMINAR AMMONIA FLAMES

Perturbed laminar ammonia ames

There are several ways of studying the eect of stretch on ames. Some of the ways include studying swirl ows, ame-vortex interaction and even turbulent ames. Turbulent ames are quite complex in nature and so an easier way to assess the stretch eect is to study perturbed laminar ames.

Studies on perturbed laminar ames including ame-acoustic interactions are quite common in hydrocarbon fuels. The idea here is to perform similar studies for ammonia ames to understand the curvature eects on the reactivity of these ames.

For ammonia ames, there exist several studies on swirl ows [START_REF] Hussein | Ammonia-hydrogen combustion in a swirl burner with reduction of NOx emissions[END_REF][START_REF] Xiao | 3D Simulation of Ammonia Combustion in a Lean Premixed Swirl[END_REF] and turbulent ows [START_REF] Lhuillier | Experimental investigation on ammonia combustion behavior in a spark-ignition engine by means of laminar and turbulent expanding[END_REF][START_REF] Fan | Flame structure and burning velocity of ammonia/air turbulent premixed ames at high Karlovitz number conditions[END_REF]. There is hardly any experimental work done on studying perturbed laminar ammonia ames. However, there are some numerical simulations performed. Netzer et al. [START_REF] Netzer | Curvature eects on NO formation in wrinkled laminar ammonia/hydrogen/nitrogen-air premixed ames 232[END_REF] studied the curvature eects on NO formation and emphasized the diusion eects of lighter species and its eect on local curvature. From the work done by Wei et al. [START_REF] Wei | Large eddy simulation on ame topologies and the blow-o characteristics of ammonia/air ame in a model gas turbine combustor 298[END_REF], it can be concluded that the PDF of HRR indicated that the HRR was more likely to occur at slightly negatively curved ames. The present work is one of the rst studies of its kind.

CHAPTER 10

CHAPTER 10

This chapter describes the two experimental setups that were used for the study: chemiluminescence for Bunsen ames and the acoustic perturbation setup used for the ame-acoustic interactions.

10.1 The chemiluminescence set-up To study the ame acoustic interactions, a speaker is placed below the burner as shown in Figure 10.2. The speaker that was used is NSW 3-193-8A

of Aura. The range of frequency of the speaker is 50-100 Hz and its operating power is at 20 W. This speaker is connected to an amplier which amplies the signal coming from a wave generator. The amplitude and the frequency can both be controlled here. Also, the type of wave needed to be generated can also be monitored here. For this study, only sinusoidal waves that start from 0V to the desired amplitude are used.

The optical diagnostic used to study FAI is chemiluminescence. As demonstrated, from the previous part both NH* and NH 2 * are good indicators of HRR and NH 2 * has a higher signal than NH*. As a rst step using, NH 2 * alone as an indicator of the position of HRR is a good start. So the species of interest for this study is only NH 2 *.

One of the ways to increase the signal-to-noise ratio is to increase the exposure time of the camera. However, when studying perturbed ames, there is a compromise between getting an image with a high signal-to-noise ratio and getting a non-blurred image caused due to accumulation of perturbed ames data over a large amount of time due to the high exposure time. In doing a brief study, it was seen that the maximum exposure time that can be set without having blurred images is 1 ms. The signal-to-noise ratio was not high enough to post-treat these data. So, tests were done on a non-perturbed ame (a) NH 2 * lter with the visible range objective lens.

(b) Without the lter with the same conditions as before. with the NH 2 * lter and without this lter, both images were captured using the visible range objective lens. As seen in gure 10.3a and gure 10.3b, the only dierence in these two images is the intensities. Also, from the emission spectra, it is known that most of the peaks in the visible range belong to the NH 2 * band. It is safe to say that the images captured without the NH 2 * lter indicate collective information for NH 2 * species at a dierent wavelength in the visible range and so once again, indeed HRR is still being monitored by tracking the front obtained upon using Inverse Abel transform. The conversion factor is 1.67 x 10 -2 mm/pix.

CHAPTER 11

CHAPTER 11

The objective of this chapter is to give a meticulous description of the process involved in determining the concentration of the excited species from the chemiluminescence. The concentrations provided here do not account for the quenching eect and are for stretched ames. These concentrations that are provided are just related to the spontaneous emissions of the excited species and are proportional to the total population of the excited species. The range of the curvature undergone by these species was provided in the previous part. The idea of determining the concentration of these species is to accommodate these species in kinetic schemes that can be used in numerical simulation. Indeed the entire population of the excited species are related to collisions, chemical reactions that produce and consume these excited species and spontaneous emission of these excited species. Only the spontaneous emission of the species is measured here. The related Einstein emissions coecients are unknown and so the concentrations that are provided are estimated concentrations. However, there are hardly any attempts to determine an estimated concentration for the excited species in ammonia ames. Including NH 2 * is of special interest as it can be used to monitor quantities like the position of maximum HRR. In order to determine the concentration of these species, rst calibration is required to understand how each element of the chemiluminescence set-up responds. For this step, a 95% homogeneous LED lamp of dimension 172 x 132 mm 2 is placed at the burner. The next step is to repeat the same process with a light source whose irradiation has already been determined. For this step, a quartz tungsten halogen lamp of power 45 W and current 6.5 A is used. The nal step is to convert the pixel intensity as seen by the camera to concentration.

Step 1: Optics calibration

As mentioned before, to do a quantitative analysis, calibration is required to know the intensity of the images as a result of the camera response to the emission of the light source. The rst step is to study the camera and the optic response of the system. Before studying this response, a region of interest To study the camera response and the lter response, an LED lamp is placed at the burner's position. An image of the LED lamp is captured with and without the lter and is provided in Annexe C. On choosing a row passing through the centre of these images, both the intensities are normalized against its maximum value and are plotted as a function of distance. On comparing the intensity proles, it is seen that the maximum intensity is observed at the centre of the lter. Near the periphery, the transmissivity is lesser than that of the centre. Since these curves resemble a gaussian-like curve, both curves are tted with the appropriate gaussian curve. Upon the comparison of these intensities for both cases, it is seen that the dierence in normalised intensities is only 2% and may be neglected. Figure 11.1 represent these curves. The blue and the green lines represent the raw intensity for the case without and with the lter on respectively whereas the red and the black lines represent the gaussian curve corresponding to the raw data.

To understand the camera response, rst, the potentiometer of the lamp was set to a xed value and the exposure time of the camera was increased. Next, the exposure time was set to a constant and the potentiometer of the lamp was varied. Both congurations were done with the lter mounted onto the camera and without the lter. Since the lter response was already monitored.

Step 3: Correlation between concentration and intensity of the species

This step focuses on correlating the intensity as seen by the camera from an emission of the species to the concentration of the species. From [START_REF] Zhao | Experimental and numerical study of OH* chemiluminescence in hydrogen diusion ames 197[END_REF] and [START_REF] Liu | Quantitative Measurement of OH* and CH* Chemiluminescence in Jet Diusion Flames[END_REF],

the ratio of the intensity of the inverse Abel-transformed image of the species to the intensity of the tungsten lamp image can be directly related to the ratio of the number of photons emitted for each case. The equation is given as: A second equation correlating the ratio of photons to the concentration of the species is also provided and given:

I OH * ,Abel I c = N OH * N c (
N OH * N c = 1 4π [OH * ]N A η 3 08nmτ ∆t 315 λ=305 Sc(λ) hν η λ dλτ ∆t (11.2)
where N A is Avogadro's number, η is the eciency of the lter,τ is the eciency of the optics, ∆t is the exposure time, h is Planck's constant, ν is the frequency and S c is the irradiation of the lamp within the wavelength of the OH* lter bandwidth.

On equating equations 11.1 and 11.2, the equation can be written as:

I species,Abel I c = 1 4π [species]N A η λ τ 1 ∆t 1 b λ=a Sc(λ) hν η λ dλτ 2 ∆t 2 (11.3)
where 1 denotes the case of chemiluminescence and 2 denotes the case of the tungsten lamp. a and b denote the bandwidth wavelength of the lter used.

In equation 11.3, the eciency of the lter used here does not change within its given bandwidth and so, it can be taken out of the integral and can be cancelled with the numerator. The exposure time used for both cases is not the same and the optics' eciency isn't the same either as for the case of the tungsten lamp, neutral density lters are used. In equation 11.2, S c is expressed in terms of W/m 2 µ m sr.

11. [START_REF] Konnov | Implementation of the NCN pathway of prompt-NO formation in the detailed reaction mechanism[END_REF] Step 4: Adapting the given irradiation values to the experiment set-up

The value of irradiation of the tungsten lamp as a function of wavelength starting from 300 nm in steps of 5 nm was provided as a document. These values were calculated at a position 50 cm away from the lamp. The unit of irradiation provided in this document is in terms of W/m 2 nm. The distance between the burner and the lter in the experimental setup is 32 cm. This means that the irradiation needs to be calculated at 32 cm. To compute the irradiation at this distance of interest, the value of irradiation at 50 cm is multiplied by the area of the sphere at 50 cm to get the value of the total irradiation and is then divided by the area of the sphere at 32 cm. Finally, to express irradiation in terms of W/m 2 µ m sr, this new irradiation term is divided by the solid angle. Solid angle is dened as the area of the lter divided by the square of the distance of interest i.e. 32 cm. On plugging in these values in equation 11.3, the concentration of the species can be determined.

Quantitative and qualitative analyses of species

The species of interest are OH*, NO* and NH 2 *. NH* has not been used for this study owing to the low signal. The diameter of both OH* and NO* lters is 25 mm. The bandwidth of OH* is 10 nm. The lter used for NO* is a short pass cut o at 300 nm. All wavelengths below 300 nm can pass through the lter. Since the peak of other species below 300 nm is lower than that of 11.5. QUANTITATIVE AND QUALITATIVE ANALYSES OF SPECIES NO*, it can be assumed that most of the intensity corresponds to the emission of NO*. However, this lter is not very ecient as it can also pass light for a wavelength of more than 600 nm. As mentioned in the previous section, the value of the irradiation in the document provided is given from 300 nm, hence, it is not possible to determine the concentration of NO*. For these reasons, a qualitative analysis is done for NO* and a quantitative analysis is done for the other two species. To compare the behaviour of each of the excited species for dierent equivalence ratios, a distance has been chosen (about 6 mm from the base of the ame) such that curvature eects are small and comparable for the dierent cases. The concentrations were plotted as a function of distance along the normal to the front from the fresh gases to the burnt gases. For NH 2 *, each of these concentrations was tted using a gaussian t curve and the peaks have been realigned to facilitate the comparison and are presented as Figure ??. On the other hand for OH*, the signal-to-noise ratio was quite low and so tting these curves with gaussian curves would render wrong information and so raw 11.5. QUANTITATIVE AND QUALITATIVE ANALYSES OF SPECIES signals have been provided here in Figure 11.7. For OH*, unfortunately, a lot cannot be said but it can be seen that the intensity for OH* is lower than that of NH 2 *. The oh* fronts are thinner and do not increase as a function of equivalence ratios as is the case for NH 2 *. On the other hand for NH 2 *, it can be seen that the peak increases from 0.9 to 1.2 or 1. Finally, to facilitate the user, the important steps have been jotted down as points to follow and are given below:

(i) Fixing ROI in the domain.

(ii) Using the lamp as the source; recording an image with the desire lter on.

(iii) Recording an image at the same conditions without the lter on.

(iv) Comparison of these two images to know the error associated with the intensity.

CHAPTER 12 CHAPTER 12

This chapter deals with studies on ame-acoustic interactions of premixed ammonia air ames. The focus of interest here is to understand the inuence on reactivity from the curvature eect generated by acoustic perturbations.

Flame feedback due to acoustic perturbation

As mentioned in Chapter 9, the ame-acoustic interaction study is indeed a study of the ame feedback to the convective and the forced acoustic perturbation. In [START_REF] Birbaud | Upstream ow dynamics of a laminar premixed conical ame submitted to acoustic modulations[END_REF], Birbaud et al. used a critical Strouhal number, St c to divide the ow into three regimes.

For St < St c , the ow is convective in nature, For St = St c , the ow is of mixed type, For St > St c , the ow is dominated by acoustics, St c is ame geometry dependent and is expressed as S L / U o where S L is the laminar ame speed and U o is the convective ow velocity which can be measured from the PIV. Strouhal number, St is dened as:

St = f D U o (12.1)
where f is the forcing frequency, D is the diameter of the burner exit, and U o is the convective ow velocity. The acoustic perturbation itself is controlled by two parameters: A, the forcing amplitude and f, the forcing frequency. On changing the amplitude with keeping other parameters constant, it was seen that the more the amplitude more the ame elongation and compression, subsequently leading to pockets of fresh gas as shown in Figure 12.1a. Figure 12.1b was of lower amplitude and doesn't create a pocket of fresh gas. On increasing the frequency, dierent ame response modes were observed until a certain value, beyond which on increasing the frequency there is no change in the ame response. It was also seen that weaker ames are more easily perturbed i.e. the ame response was quicker for lower values of amplitude and frequency but on the other hand, the richer ames were not as responsive and began to show some changes only for a higher amplitude and frequency. Since, the objective of this study is solely on understanding the curvature eects on the reactivity, changing the frequency alone to obtain dierent excitation modes was of interest. From the Strouhal number and from understanding the eect of the equivalence ratio and the 90 12.2. CURVATURE EFFECTS ON THE REACTIVITY two parameters of the forced perturbation, it was decided that only one of the case studies was to be further pursued: The forcing frequency was set to 40

Hz, the forcing amplitude was set to 0.5 V, the amplier was set to 4 and, the equivalence ratio ranged between 1-1.4. As explained in Chapter 10, the species of interest is NH 2 * and the exposure time of the camera was set to 1 ms.

Curvature eects on the reactivity

To study the curvature eect for the given test condition, it was ensured that a similar ame response mode was captured i.e. a similar ame shape was captured for dierent equivalence ratios. This was done to ensure that uniform parameters are maintained for all conditions. Initially, the mean curvature was obtained to quantify the curvature eect but it was seen that the ame response was too complex and so, the two principal curvatures k1 and k2 are used. Four dierent parameters: k1, k2, thickness along the normal to the front, and maximum intensity along the normal to the front were all plotted against the ame contours. The change in thickness of the front is an indirect eect of the change in reactivity. To understand the complete response of reactivity, the integrated intensity along the normal must be monitored. Once again, plotting curvature against integrated intensity was too complex and so this parameter was further divided into two basic parameters: thickness and maximum intensity.

After careful examination, it was seen that the eect of k1 and thickness along the contour seemed to be correlated but when it came to understanding the maximum intensity and k2, it was seen that their eect for all equivalence ratios was not very apparent. The case studies for k2 and maximum intensity In the case of φ=1.0, Le=1 [START_REF] Ichimura | Extinction limits of an ammonia/air ame propagating in a turbulent eld[END_REF] which means that both thermal and molecular diusion plays an equally important role. It can be seen that the thickness does not vary a lot along the contour of the front. The eect of k1 on the thickness is not very evident in this case. Weak ames are more subjugated to curvature when compared to richer ames. It seems that the relatively higher magnitude of the curvature does not have a major impact on the thickness of these ames insinuating that rather than having an impact of curvature on the front, the thermo-diusive eects play a larger role. Here, since both eects play an equally important role, the thickness seems to be uniform all along the contour. . This implies that the thermal diusion from the burnt gas towards the fresh gas happens at a slightly higher pace than the molecular diusion of fresh gases into the burnt gases. In this case, a slight variation of thickness along the contour can be seen. The front gets thinner when a negative k1 is encountered and gets thicker when a positive k1 is encountered. In the case of, φ=1.2, Le increases slightly [START_REF] Ichimura | Extinction limits of an ammonia/air ame propagating in a turbulent eld[END_REF]. With an increase in equivalence ratio while almost maintaining the value of Le, shows a more prominent dierence in the thickness vs curvature trend. The transition from a negative k1 to a positive k1 with thickness changing can be seen in Figure 12.4b. On comparing with φ=1.1, it can be conrmed that the Lewis number is not the only parameter that controls the ame response. From Figure 12.4b, it can be seen that for the same values of k1, the thickness value are not the same.

These two points are located at two dierent positions along the contour. For the point further upstream, the value of thickness was higher implying that it is not just k1 that inuences thickness but other parameters change from the base of the ame to the tip of the ame, thereby inuencing the reactivity and 12.2. CURVATURE EFFECTS ON THE REACTIVITY so resulting in aecting the thickness. This is further discussed at the end of the case studies. For φ=1.3 with Le=1.09 [START_REF] Ichimura | Extinction limits of an ammonia/air ame propagating in a turbulent eld[END_REF], the trend of thickness vs k1 is quite similar to φ=1.2. However, the variation of thickness is much higher than that of φ=1.2 and the magnitude of k1 is lower than φ=1.2. As seen in the case of φ=1.2, the transition of thickness as the curvature changes can be seen in Figure 12.5b. For φ=1.4, the value of the Lewis number is slightly higher than 1. The range of the values of k1 for φ=1.4 and φ=1.3 is the same but the variation in thickness for φ=1.4 is higher than φ=1.3 implying that change of thickness as a function of k1 is more prominent for the very rich ames. As the ames get thicker, it is important to note that the curvature calculated at the outer contour (as done here) may not be the same as the values that will be obtained on computing in the inner contour. For consistency, only the outer contours have been used for all the cases. From all of these cases, it is quite apparent that irrespective of the condition, a negative k1 results in the reduction of thickness and a positive k1 manifests as an increased thickness. In Annexe C, both k1 vs contour of the ame and thickness vs contour of the ame for each of the cases have been provided. As noted for some of the cases, it also seems that the intensity is higher for the positive k1 implying that the reactivity is enhanced. To test this, images were captured using a high-speed CMOS camera (PHANTOM V1611). Figure 12.7 shows the dierent ame responses for the case of φ =1.1. Upon inspecting every case, it was concluded that indeed the intensity remains more or less the same along the sides of the ame irrespective of the direction of curvature.

Figure 12.7 represents this. On examining each pixel along the front, it can be seen that there is a slight increase in intensity for positive curvatures when compared to the negative curvature but this increase is so small that it is negligible. However, the thickening eect is inuenced by the direction of the curvature. This was further veried by capturing a dierent ame shape by using the KURO camera. For all cases, it was seen that the intensity decreases when moving away from the ame base. This phenomenon was captured in the case of φ = 1.2 where on plotting k1 vs thickness (Figure 12.4b) the thickness was dierent for the same k1 at two dierent positions along the contour. As mentioned before thickness is also an indicator of reactivity. The reason for the enhanced intensity on the sides closer to the base of the ame and decreased intensity as one moves away from the ame base is attributed to the diusionconvective velocities. The convective velocity is about 25 times more than the diusion velocity of ammonia into the air at the exit of the burner. The diusion velocity of ammonia into the air was calculated using Fick's law. It is understood that ammonia decomposes into lighter species whose diusion velocity is much higher than the convective velocity. The preferential diusion of H 2 and other species towards the sides play a major role in enhancing the local reactivity. For this reason, the intensity or reactivity is higher towards the sides of the ame closer to the base compared to the tip of the ame.

The eect of preferential diusion on the reactivity of curved ames has been highlighted by Mizomoto et al. [START_REF] Mizomoto | Eects of preferential diusion on the burning intensity of curved ames[END_REF]. A Chemkin simulation was performed using the mechanisms of Stagni et al. and it was seen that indeed there is a decomposition of ammonia into H 2 and is shown in Figure ??. On running a reaction pathway analysis and on looking into the rate of production chart, it was seen that the dominant reaction through which ammonia decomposition takes place is NH 3 + H --⇀ ↽--NH 2 + H 2 . The Damköhler number, Da is dened as the ratio of the ow time scales to the chemical time scales. The chemical times scale can be obtained by dividing the thickness of the ame by the laminar ame speed. The Damköhler number for these case studies was slightly more than 1, implying that the time of the forced perturbation on the ow was slightly more than the chemistry time scale and so, the perturbation was a little slow to interact with the ame front. However, it is important to emphasize that the time scales in both cases are still comparable and the eect of the perturbation on the ow is seen as a local change of thickness with respect to the orientation of k1 along with the Lewis number of the ame determining a major role in the ame response. It may also be concluded that for the studied cases, the preferential diusion eects play a large role in any changes of reactivity and as far as curvature eects are concerned, the eect of k1 can be seen as a change of thickness. Studies [START_REF] Netzer | Curvature eects on NO formation in wrinkled laminar ammonia/hydrogen/nitrogen-air premixed ames 232[END_REF] show that for a hydrogen-enriched ammonia ame, there is a change in the rate of preferential diusion as a function of the orientation of the curvature. These eects were not observed for the given case study here.

CURVATURE EFFECTS ON THE REACTIVITY

CONCLUSION

This part is dedicated to study the behaviour of dierent excited species that are quite vital to understand certain behaviours of ammonia ames. The rst objective of this section was to determine the concentration of some of these emitted excited species. No previous attempt was seen to determine the concentrations of these emitted excited species for at least ammonia-air ames.

From the previous part, it was seen that NH 2 * is a good indicator of the position of the maximum HRR. Since the excited species have not been accommodated in kinetic schemes due to lack of sucient data, the concentration of the species like OH* and NH 2 * have been provided here as a rst step to achieve this task. These concentrations do not account for the quenching eect and are dependent on the stretch. It was seen from the previous section that the curvature rates are quite low. Qualitative analysis on NO* has also been provided. A detailed procedure highlighting the important steps to nd the concentration has been given.

For the ame-acoustic interactions, NH 2 * was monitored as this species would give direct information on the heat release rate. However, NH 2 * lter was not used due to the amount of signal received by the camera. Instead, a visible range lens without any lter was used to obtain collective information of NH 2 * bands in the VR spectrum. From a preliminary analysis, perturbation conditions were chosen such that St > St c , so that the ow is dominated by the acoustic perturbations. The impact of the amplitude and the frequency on the ame response was studied. On pursuing a case study for dierent equivalence ratios, it was seen that the principle curvature k1 had a direct eect which was manifested as a change in the thickness of the species front. On studying the trends displayed by k2 and the maximum intensity, nothing conclusive was deducible. As far as reactivity enhancement is concerned, it was seen that for the given case, the convective-diusion velocities played a major role. It was understood that ammonia decomposes into lighter species which diuse faster towards the sides of the ames, enhancing the reactivity and decreasing this enhancement towards the ame tip. This was conrmed by performing a simulation in Chemkin-Pro. It was seen that ammonia decomposed into hydrogen.

Since the Da was slightly more than 1, the time scale of the perturbation was slightly longer than the chemistry time scale. It is important to emphasize Conclusion that the time scales in both cases are still comparable and the eect of the perturbation on the ow is seen as a local change of thickness with respect to the orientation of k1 along with the Lewis number of the ame determining a major role in the ame response. In the given case study, it can be said that the preferential diusion eects play a dominant role in changes in local reactivity although, no change in the rate of preferential diusion was observed as a function of curvature. The eect of k1 is simply seen as a change in the thickness of the species front. Ammonia combustion has indeed sparked interest all around the world as a replacement for conventional fuels. It has intrigued the scientic community in the last few years as there are a lot of phenomena pertaining to ammonia combustion that has not been fully understood. The motive of this thesis is to pick a few of these unknowns and make an attempt to understand the behaviour of these ames. The primary focus of this study is on the chemical properties of premixed, pure ammonia laminar ames. The ame speeds for dierent initial conditions, the inner structure of the ame, the behaviour of perturbed laminar ames, and the response of the ame to change in parameters for each of the cases have been thoroughly investigated. The major part of this work is experimental in nature and has been complemented with 1D numerical studies using Chemkin-Pro. Two dierent experimental set-ups: the constant volume combustion chamber and a Bunsen burner were used. One of the main goals was to provide experimental data that can be used to improve the existing kinetic schemes in ammonia combustion which is used in running numerical simulations. This thesis has been divided into three parts to carefully evaluate and achieve the set objectives. Each part has aorded new insights into ammonia combustion.

The rst part is dedicated to rendering ame speeds at high pressure and temperature conditions. The conditions that were targeted were much higher than the range found in the literature then. These conditions are tested with the idea of mimicking high-pressure and temperature conditions like those that are found in engines and gas combustors. In this study, the ame speeds have been reported for up to 30 bar and 585 K. The experiments were performed using premixed oxygen-ammonia mixtures with diluents for three dierent equivalent ratios: 0.8, 1.1, and 1.3 with dierent initial pressures. This work was one of the rst of its kind to determine the pressure-independency of ame speeds and that ame speeds are temperature driven. A range of values of the exponential factors α and β was provided showing that the pressure exponential factor β was much lower than most of the known fuels. Flames that lack pressure dependency are desirable in gas combustors as these ames are not easily destabilized by the constant change of conditions in the combustion chamber. A test on the dependency of bath gases was also done along with the evaluation of General Conclusion the performance of some of the recent kinetic schemes in ammonia combustion.

These experiments were performed in a relatively new set-up: OPTIPRIME at ICARE, CNRS, Orléans. It is a constant-volume combustion chamber which has a 360-degree view. This set-up indeed has some marvellous engineering done to obtain the smooth spherical fused silica ring. A high-speed camera was used to record these images.

The second part explored the structure of the ame. The diagnostics included PIV and chemiluminescence. A high-speed camera with an IRO intensier was used to record these images. These tests were performed using premixed ammonia-air mixtures for an equivalence ratio ranging between 0.9-1.4 at atmospheric conditions. The highlights of this part include determining the excited species that can give direct information on the position of the maximum HRR and the non-applicability of thin ame approximations for ammonia-air ames. NH* and NH 2 * have been found as indicators of the maximum position of HRR for ammonia-air ames. An assessment of ame speed calculation has also been provided. The structure of the ame for dierent equivalence ratios has also been described. Finally, the response of species to dierent equivalence ratios and curvature was noted. These experiments were performed under atmospheric conditions.

The last part involves an attempt to provide the concentration of some of these excited species. A quantitative analysis of species emitted from ammonia ames has not been given in the literature and for the same reason, these species have not been accommodated in kinetic schemes. The addition of the species to the kinetic schemes has not been done here but it is a work in progress and the core of a future publication. No previous attempts to study ame-acoustic interactions for ammonia ames have been recorded in the literature. The objective of the ame-acoustic interaction studies is to understand the curvature eect on reactivity. Monitoring mean curvature and integrated intensity along the normal (parameter to quantify reactivity) was a complicated task. And so, the mean curvature was split into two principal curvatures and integrated intensity was split into the thickness and maximum intensity along the normal to the front. For the given case study, it was concluded that even though the ow was dominated by the acoustic perturbation, the time scale to study the curvature eect on the reactivity due to the perturbation was slightly longer than the chemistry time scale. The preferential diusion of the lighter species playing a more dominant role has been noted. The acoustic perturbation on the ow along with the Lewis number resulted in dierent ame response and the changes in reactivity was simply seen as a change in the thickness of the species front. This change of thickness due to curvature was attributed to the principle curvature, k1 along the contour of the front. The impact of the curvature eect on the local displacement and the consumption speed has not been done here but it is of signicant interest and can give some deeper insights.

The second and the third parts were performed with the same test conditions on the same test bench which was set up specically to cater to the use of ammonia. Pipes, mass owmeters, and other connections that were compatible with ammonia were used to minimise ammonia-related corrosion and other hazards. A Bunsen burner with a pilot ame was used to perform these experiments at ambient conditions. The pilot ame of the methane-air mixture at stoichiometry was required to stabilise the weak ammonia ame. A sensitive camera was used to record these images.

There is still a vast amount of work that needs to be done to ll the gaps in ammonia combustion. Numerical simulations using LES or DNS can be performed at the same experimental conditions to complement the experimental data. The use of these experimental data to improve or update the existing kinetic schemes is interesting as the experiments performed here are at unique conditions when compared to the existing literature data. As mentioned before, the concentrations of the excited species can be used to accommodate these species in the kinetic schemes as these species can give direct information on some parameters of interest. For studies on ame-acoustic interactions, nding the local displacement speeds and the consumption speeds and the impact of curvature on it is quite important. Also, targetting ows with Da1, may be interesting to have sucient time for the perturbation to interact with the reaction zone. These conditions may give deeper insights into the curvature eects on the reactivity. For the given experimental test bench, these conditions are not easy to achieve owing to the low ow velocities which result in a limitation in exploring dierent perturbation frequencies as one might reach a certain frequency beyond which increasing the forcing frequency does not change the ame response. This value of frequency is dependent on the initial ow conditions.

Overall, the performance of ammonia combustion can be drastically improved to t within the criterion of a feasible fuel. The ame speeds can be signicantly increased by the addition of excess oxygen as done in the rst part here or by blending with other fuels. A signicant amount of the research work available is on blended fuels and this is done with the idea of taking advantage of the desirable properties of two or more fuels. Studies show that the reactivity enhancement due to blended fuels especially hydrogen can be benecial. Transporting and storing ammonia is easier when compared to other zero-carbon fuels of interest like hydrogen. Also, ammonia can be produced on large scales using the Haber-Bosch process. Research shows that cracking ammonia to blend ammonia and hydrogen may be one of the most sustainable ways to solve the problems that come with hydrogen and at the same time make use of the blended properties. The toxic nature of ammonia may be one of the reasons why ammonia is not user-friendly however, as mentioned in the introduction of this thesis, SCRs have been developed to convert ammonia directly into nitrogen in case of leaks. With a little more research and technology to overcome some of the other limitations, ammonia can be deployed as one of the top sources of energy of tomorrow. Indeed, ammonia shows the potential to solve the global energy crisis. From this analysis, it is seen that irresepctive of the temperature or the pressure, there is only one dominant reaction.

Flame-acoustic interactions

The dierent parameters used to characterise the reactvity with the curvature for dierent equivalence ratio is provided here. Each of the parameters is plotted along the contour of the front.

Phi=1.0 Here, the chosen points are at around midway of the total ame height.

There is a small change of thickness and k1. The maximum intensity tnds to continuously increase with k2 continuously decreasing.

Phi=1.2 In this case, the variation of k1 and thickness seem to go hand in hand. The trend of k2 cotinues to decrease with height whereas the maximum intensity continues to take the bell-shaped prole implying once again that the maximum reactivity is most likely near half of the total height of the ame. This could be explained with the preferential diusion eect of lighter species over the convective veocit of the ow.

Phi=1.3 The maximum variation in thickness is seen in this case. The prole of maximum intensity slightly diers from the other as there is a slight decrease followed by an increase with a subsequent decrease along the contour. This could be due to stronger diusion eects near the base of the ame.

  (i) Partie I : Vitesses de ammes laminaires à haute pression et température, (ii) Partie II : Structure interne de la amme prémélangée étirée et, (iii) Partie III : Analyses chimiques et dynamiques de la amme prémélangée.

  chimiluminescence n'est disponible pour les ammes ammoniac/air. Le travail eectué sur les eets d'étirement dans la amme a montré qu'il est important de les connaître car ils modient la surface de la amme et, par conséquent, augmentent ou diminuent la réactivité selon que l'étirement est négatif ou positif. Enn, l'étude de la structure de la amme donne un aperçu des diérentes espèces qui coexistent. Comprendre la position de ces espèces et la façon dont elles réagissent à une perturbation permet de connaître directement leur inuence sur la réactivité. Il est donc important de comprendre la structure de la amme pour améliorer les propriétés de la amme et favoriser la combustion.Le Chapitre 6 est consacré à la description du dispositif expérimental.Ces expériences sont réalisées aux conditions atmosphériques sur un brûleur Bunsen. Une amme pilote de méthane-air à la st÷chiométrie a été utilisée pour stabiliser la amme principale. Des tests ont été eectués pour s'assurer qu'il n'y avait pas d'eet direct de la amme pilote sur les mesures calculées sur la amme principale. De plus, la région d'intérêt était éloignée de la base de la amme. Les pertes de chaleur vers le brûleur ne sont pas considérées pour la même raison. Des eets de rayonnement peuvent se produire dans les gaz brûlés à partir de la vapeur d'eau. Pour cette étude, ces eets ne sont pas considéres car la région d'intérêt est limitée à la partie amont de la zone de réaction qui est éloignée des gaz brûlés. La vitesse moyenne des gaz frais à la sortie du brûleur était comprise entre 0,18 et 0,2 m/s. Pour le montage de PIV, le DEHS (Diethyl-hexyl-sebacate) qui s'évapore à 525 K a été utilisé pour ensemencer les gaz frais avec des gouttelettes. Le dispositif de PIV consiste en une source laser dirigée et mise syr la amme. La source lumineuse utilisée est un laser continu Coherent Verdi G20 de 532 nm. La puissance du laser est de 5 W. Le faisceau est vi rééchie sur le miroir à angle droit en traversant une lentille cylindrique puis une lentille sphérique de longueur focale f = 750 mm fournissant une nappe laser perpendiculaire à la amme. Un diaphragme est utilisé pour contrôler la taille de cette nappe laser an de s'assurer que seule la amme est illuminée par le laser. Une caméra CMOS haute vitesse (PHANTOM V1611) est utilisée pour capturer les images et un code écrit en Python est utilisé pour suivre les particules et fournir une carte du champ de vitesse. La structure interne de la amme a été déterminée par chimiluminescence. Les ltres utilisés pour NH* et NH 2 * sont centrés sur les longueurs d'onde de 337 nm et 632 nm respectivement avec un diamètre de 50 mm et une largeur de bande passante de 10 nm. Ces ltres ont été achetés chez Edmund Optics. Le ltre est placé devant l'intensicateur IRO à haute vitesse qui est placé devant la caméra CMOS. L'intensicateur est utilisé pour avoir une sensibilité maximale et une bonne qualité d'image. Deux objectifs diérents ont été utilisés : un objectif UV pour NH* et un objectif à spectre visible pour NH 2 *. Enn, pour étudier la structure de la amme, des simulations Chemkin 1D ont été réalisées pour donner le prol de température, le prol HRR et la fraction molaire des espèces non excitées.Le Chapitre 7 met l'accent sur l'épaisseur de la amme et explique pourquoi il est important de choisir la bonne isotherme pour calculer des paramètres tels que la vitesse de amme. Une comparaison avec la littérature montre que la valeur de la vitesse de amme change si la bonne isotherme n'est pas choisie.Les résultats concluent que les approximations de amme mince ne sont pas valables pour le cas des ammes ammoniac-air.Le Chapitre 8 se concentre sur les eets de la courbure sur les espèces et la structure de la amme. Deux courbures principales k1 et k2 sont introduites. k2 est la courbure dans la direction circonférentielle tandis que k1 est la courbure dans le plan. D'après la chimiluminescence de NH* et NH 2 *, on constate que plus la amme est riche, moins l'eet de courbure est important. Puisque le nombre de Lewis augmente avec la richesse et est proche de l'unité[START_REF] Ichimura | Extinction limits of an ammonia/air ame propagating in a turbulent eld[END_REF], l'eet de la courbure négative n'est pas bien prononcé à la pointe de la amme, bien qu'un eet légèrement plus élevé soit attendu pour les ammes plus riches. Indépendamment de l'espèce excitée et de la richesse du mélange, il a été observé que l'épaisseur de l'espèce continue à augmenter et commence à diminuer à sommet de la amme (courbature négative indiquée par k1). k2, par contre, augmente lorsqu'on se déplace de la base de la amme vers la pointe de la amme. Cela montre que l'eet de courbure moyenne est simplement considéré comme un changement d'épaisseur des espèces produites, et non directement comme la concentration des espèces dans une région donnée, puisque l'intensité moyenne reste la même. En eectuant une comparaison par espèce, on constate que NH 2 * subit un eet de courbure moyen légèrement supérieur à celui de NH*. La amme, à une hauteur d'environ 6 mm de sa base, a des coecient de courbure faibles : k1 quasiment nul et k2 proche de -0. 2mm -1 . A cette hauteur, la amme peut donc être comparée à une amme non étirée à propagation libre 1D. Des simulations Chemkin avec le schéma cinétique de Stagni ont été réalisées pour eectuer des comparaisons. La température, le HRR ainsi que la concentration des espèces non excitées correspondent à une gaussienne (à 95%) et sont tracés le long du domaine 1D. NH 2 * est produit à une température plus basse que NH*. De plus, NH* et NH 2 * sont assez proches de leurs espèces non excitées correspondantes. Un mécanisme de for-vii mation des espèces excitées pourrait être incorporé dans n'importe lequel des schémas cinétiques. Le HRR maximum se produit entre NH* et NH 2 * pour chacun des cas. Par conséquent, le produit de NH* et de NH 2 * est un bon in- dicateur pour déterminer le HRR maximal de manière expérimentale. Il s'agit en eet d'un moyen vraiment ecace pour déterminer la position du HRR. Partie III : Analyses chimiques et dynamiques de la amme prémélangée La deuxième partie s'est concentrée sur la structure interne d'une amme sur un brûleur Bunsen en utilisant des diagnostics comme la chimiluminescence. Il a été constaté qu'aucune espèce excitée associée n'est prise en compte dans la cinétique chimique. Il est intéressant d'ajouter ces espèces surtout lorsque ces espèces peuvent être utilisées pour obtenir des informations directes sur la position du HRR comme vu dans la partie précédente. La première étape de la partie III consiste à aller plus loin en fournissant la concentration de certaines espèces excitées pour la même conguration, ce qui est nécessaire pour ajouter les espèces excitées dans les schémas cinétiques. Cette étape est particulièrement utile lors de l'exécution de simulations pour diérentes géométries et congurations. Il est connu que la turbulence améliore la réactivité. Mais la turbulence est de nature très complexe et il est dicile de saisir tous les phénomènes lorsqu'ils sont couplés. Ainsi, une étape très rudimentaire consiste à comprendre une amme laminaire perturbée. C'est dans cet esprit que les interactions amme-acoustique sont étudiées. L'objectif de cette section est d'étudier l'inuence de la courbure sur les espèces. En parcourant la littérature, aucun travail expérimental antérieur sur les interactions ammeacoustique pour les ammes d'ammoniac n'a été trouvé. Le Chapitre 9 présente une brève revue de la littérature relative à l'analyse quantitative et qualitative de la chimiluminescence dans la combustion de l'ammoniac. Bien qu'il existe des études sur l'analyse qualitative, il n'y a pas d'études sur l'analyse quantitative de la combustion de l'ammoniac, ce qui fait du présent travail l'une des premières tentatives de quantier les espèces excitées à partir de la chimiluminescence. En ce qui concerne les études sur les ammes perturbées, il existe une littérature abondante sur les écoulements tourbillonnaires et turbulents, mais il n'y a pratiquement aucun travail expérimental sur les ammes laminaires perturbées. Quelques simulations numériques ont été réalisées pour étudier les eets de la courbure sur les espèces. Le travail actuel est la première étude expérimentale faite sur les interactions amme-acoustique pour la combustion d'ammoniac prémélangé. Le Chapitre 10 présente l'utilisation d'une caméra à faible vitesse (KURO sCMOS rétroéclairé ) qui a une meilleur résolution spatiale et une meilleur résolution quantitative (bit par pixel). La caméra est plus sensible et l'intensicateur IRO peut donc être retiré. La qualité des images obtenues à partir de cette caméra est bien meilleure car le rapport signal/bruit est assez élevé. Une fois encore, le diagnostic d'intérêt reste la chimiluminescence. Trois espèces diérentes sont étudiées: NH 2 *, OH*, et NO*. Le montage utilisé pour l'interaction amme-acoustique a également été décrit. Un haut-parleur est xé à la base du brûleur et ce dernier est connecté à un amplicateur qui est, lui-même, connecté à un générateur d'ondes. Le générateur d'ondes est utilisé pour contrôler viii la forme d'onde, l'amplitude et la fréquence. Seules les ondes sinusoïdales ont été utilisées. Pour cette étude, seul NH 2 * est observé car cette espèce est un bon indicateur de la position du HRR. Pour les ammes dynamiques, un compromis entre l'obtention d'une image nette et une image de haute qualité doit être fait. Pour augmenter le rapport signal/bruit, le temps d'exposition de la caméra est augmenté mais lorsqu'il s'agit de capturer des ammes dynamiques, il y a une limite pour éviter d'obtenir des images oues. Des tests ont été effectués pour des ammes stationnaires avec le ltre NH 2 * et sans le ltre en utilisant toujours l'objectif visible range (VR). D'après les spectres d'émission des ammes d'ammoniac/air, on sait que la région visible est constituée d'une large bande de pics NH 2 *. Par conséquent, sans utiliser le ltre NH 2 * et en utilisant uniquement l'objectif VR, NH 2 * est toujours visualisé. Le Chapitre 11 est consacré à la réalisation d'une analyse quantitative et qualitative de certaines des espèces importantes dans les ammes d'ammoniac. Aucune tentative antérieure n'a été faite pour déterminer la concentration de NH 2 * et de OH* à partir de la chimiluminescence dans les ammes d'ammoniac. L'eet d'extinction n'a pas été pris en compte et les concentrations fournies sont pour des ammes étirées. Dans la partie précédente, on a vu que ces eets d'étirement sont assez faibles. Les concentrations ont été déterminées dans le but d'aider à prendre en compte les espèces excitées dans n'importe quel schéma cinétique pour les ammes d'ammoniac. Pour les ammes d'ammoniac, ces espèces excitées doivent encore être ajoutées. L'ajout d'espèces comme le NH 2 * peut être important car son suivi dans les simulations numériques peut fournir des informations directes sur le HRR. Ce chapitre détaille le processus de détermination de la concentration des espèces. Un processus en quatre étapes impliquant la calibration de l'optique, la calibration de la lampe tungstène, la tentative de corréler l'intensité vue par la caméra à la concentration de l'espèce et enn l'adaptation de la méthode au dispositif expérimental donné a été méticuleusement décrit ici. Enn, une analyse qualitative de NO* a également été proposée. Le Chapitre 12 traite des études sur les interactions amme-acoustique des ammes d'ammoniac-air prémélangées. Il s'agit ici de comprendre l'inuence sur la réactivité de l'eet de courbure généré par les perturbations acoustiques. Dans un premier temps, le nombre de Strouhal critique a été déni pour la conguration donnée. D'après [14], on sait que pour avoir un écoulement dominé par l'acoustique, le nombre de Strouhal de l'écoulement doit être supérieur au nombre de Strouhal critique. Sur cette base, une fréquence a été choisie pour les études ultérieures. En étudiant la réponse de la amme au changement d'amplitude et de fréquence, il a été remarqué que lorsque l'amplitude augmente, l'eet d'élongation et de compression sur la amme est plus fort et que lorsque la fréquence augmente, diérents modes peuvent être obtenus jusqu'à une certaine valeur de fréquence au-delà de laquelle toute augmentation de fréquence ne change pas la réponse de la amme. Une fréquence de forçage de 40 Hz et une amplitude de 0,5 V pour des richesses allant de 1 à 1,4 sont étudiées ici. Initialement, la courbure moyenne a été obtenue pour quantier l'eet de courbure mais il a été constaté que la réponse de la amme était trop complexe et donc, les deux courbures principales k1 et k2 sont utilisées. Quatre paramètres diérents : k1, k2, l'épaisseur le long de la normale au front, et l'intensité maximale le long de la normale au front ont tous été tracés en fonction des contours de la amme. La modication de l'épaisseur du front est un ix eet indirect de la modication de la réactivité. Pour comprendre la réponse complète de la réactivité, l'intensité intégrée le long de la normale doit être surveillée. Une fois de plus, le tracé de la courbure en fonction de l'intensité intégrée était trop complexe et ce paramètre a donc été divisé en deux paramètres de base : l'épaisseur et l'intensité maximale. Après un examen attentif, on a constaté que l'eet de k1 et de l'épaisseur le long du contour semblait être corrélé, mais l'intensité maximale et k2, n'ont montré aucune correlation. En évaluant chaque cas, on a vu que l'intensité de la amme diminuait de la base à la pointe de la amme, quelle que soit la valeur de la courbure. Cependant, la réduction de l'intensité à la pointe par rapport à la base de la amme était plus apparente pour Le >1. Il semble que les vitesses de convection-diusion jouent un rôle majeur. En calculant le nombre de Damköhler, Da, on a vu que cette valeur était légèrement supérieure à 1, ce qui indique que les deux échelles de temps sont comparables. La vitesse de convection était environ 25 fois supérieure à la vitesse de diusion de l'ammoniac dans l'air aux conditions ambiantes. Ceci indique que ce n'est pas l'ammoniac qui diuse directement sur les côtés des ammes mais l'ammoniac lui-même qui se décompose en espèces plus légères qui diusent plus rapidement que le ux convectif. La diusion préférentielle de H 2 et d'autres espèces vers les côtés joue un rôle majeur dans l'amélioration de la réactivité locale. En conclusion, la combustion d'ammoniac a suscité un intérêt dans le monde entier en tant que substitut aux combustibles conventionnels. Elle a intrigué la communauté scientique au cours des dernières années, car de nombreux phénomènes liés à la combustion de l'ammoniac n'ont pas été entièrement compris. Le but de cette thèse est d'identier quelques-unes de ces inconnues et de tenter de comprendre le comportement de ces ammes. L'objectif principal de cette étude est d'étudier les propriétés chimiques des ammes laminaires d'ammoniac pur prémélangé. Les vitesses de amme pour diérentes conditions initiales, la structure interne de la amme, le comportement des ammes laminaires perturbées et la réponse de la amme aux changements de paramètres pour chacun des cas ont été minutieusement étudiés. La majeure partie de ce travail est de nature expérimentale et a été complétée par des études numériques 1D utilisant Chemkin-Pro. Deux montages expérimentaux diérents : une chambre de combustion à volume constant et un brûleur Bunsen ont été utilisés. L'un des principaux objectifs était de fournir des données expérimentales qui peuvent être utilisées pour améliorer les schémas cinétiques existants dans la combustion de l'ammoniac, qui sont également utilisés dans les simulations numériques. Cette thèse a été divisée en trois parties pour évaluer et atteindre les objectifs xés. Chaque partie a apporté de nouvelles connaissances sur la combustion de l'ammoniac. Il y a encore beaucoup de travail à faire pour combler les lacunes de la combustion de l'ammoniac. Des simulations numériques utilisant LES ou DNS peuvent être réalisées dans les mêmes conditions expérimentales pour compléter les données expérimentales. L'utilisation de ces données expérimentales pour améliorer ou mettre à jour les schémas cinétiques existants est intéressante car les expériences réalisées ici sont dans des conditions uniques par rapport aux données de la littérature existante. Comme mentionné précédemment, les concentrations des espèces excitées peuvent être utilisées pour intégrer ces espèces dans les schémas cinétiques, car elles peuvent fournir des informations sur certains paramètres d'intérêt. Pour les études sur les interactions amme-acoustique, trouver les vitesses de déplacement locales x et les vitesses de consommation et l'impact de la courbure sur celles-ci est très important. De même, en ciblant les ux avec Da<1, il peut être intéressant de voir si la courbure a un eet sur l'intensité. Avec un peu plus de recherche et de technologie pour surmonter certaines des autres limitations, l'ammoniac peut être déployé comme l'une des principales sources d'énergie de demain. En eet, l'ammoniac présente le potentiel nécessaire pour résoudre la crise énergétique mondiale. xii ACKNOWLEGEMENTS

8 . 1 2 NH 2 *. 3 62 8. 6 Flame 63 10. 1 73 10. 2 74 10. 3 75 11. 1 80 11. 5 83 11. 7 84 11. 8 85 12. 1

 81223626631732743751805837848851 Representation of the principal curvatures k1 and k2 in the ame contour. k1 is along the contour and k2 is along the circumferential direction. Negative curvature is dened for a concave ame in the direction of the burnt gas and a positive curvature is dened for a convex ame in the direction of the burnt gas. . and NH* signal in an atmospheric NH 3 /air ame at φ = 1.4, (a) Abel inversion transformed image of NH 2 *, (b) Abel inversion transformed image of NH* with the colour bar representing the pixel intensity, (c) The thickness of the species (NH 2 * in black and NH* in violet) evaluated in the direction of the normal of the front vs. normalized position, (d) k1 (blue) and k2 (red) of NH 2 * vs. normalized position. . . . . . . . . . . . . . 59 8Chemiluminescence images of NH* at (a) φ=0.9, (b) φ=1.1, (c) φ=1.3, and NH 2 * at (d) φ=1, (e) φ=1.2, (f ) φ=1.4. The expo- sure time of these images is not the same. . . . . . . . . . . . 60 8.4 Results obtained from the experiments . . . . . . . . . . . . . . 61 8.5 Results obtained from the simulations . . . . . . . . . . . . . . structure at (a) φ=0.9, (b) φ=1.1, (c) φ=1.4. Temperature (solid black line) and the normalized HRR (dashed black line) are plotted on the left and the right y-axis respectively. The green line represents the fresh gas isotherm. The distance along the x-axis represents the 1D domain. Each of the species has been thickened to a width corresponding to the standard deviation of the gaussian-t curve resulting in overlapping of the species. For easy visualization, each of the species has been plotted as a quarter bar in order to minimize the overlapping between species which otherwise would have resulted in diculty to understand the gure. . . . . . . . . . . . . . . . . . . . . . Schematic diagram of the chemiluminescence set-up. . . . . . . Schematic diagram of the chemiluminescence set-up. . . . . . . Raw images of the ame to note the dierences between using a NH 2 * lter and not. . . . . . . . . . . . . . . . . . . . . . . . . On comparing the normalized intensities along the centre of the images captured, it can be seen that by using the lter, the incoming intensity is reduced by less than 2%. The ROI is in betweeen 0.1 and 0.8 on the x-axis and is marked in orange. . . 78 11.2 Intensity vs potentiometer of LED lamp at an exposure time of 1ms with the NH 2 * lter mounted on the camera. . . . . . . . 79 11.3 Intensity vs exposure time of camera with the potentiometer of the lamp set at k=6 with the NH 2 * lter mounted on the camera. 79 11.4 Transmission of the neutral desnity lters as a function of wavelength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intensity vs exposure time of camera with the ND lters and NH 2 * lter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 11.6 Concentration of NH 2 * for φ = 0.9-1.4 with the colour bar rep- resenting the concentration in mol/m 3 . . . . . . . . . . . . . . . Concentration of OH* for φ = 0.9-1.4 with the colour bar representing the concentration in mol/m 3 . . . . . . . . . . . . . . . Qualitative analysis on NO* for φ = 0.9-1.4. . . . . . . . . . . . The eect of amplitude on ames creating pockets of fresh gases for the same frequency (30 Hz) and equivalence ratio (1.1). . . LIST OF FIGURES xxv 12.3 FAI for phi=1.1, A=0.5 V and f=40 Hz . . . . . . . . . . . . . 12.4 FAI for phi=1.2, A=0.5 V and f=40 Hz . . . . . . . . . . . . . 12.5 FAI for phi=1.3, A=0.5 V and f=40 Hz . . . . . . . . . . . . . 12.6 FAI for phi=1.4, A=0.5 V and f=40 Hz . . . . . . . . . . . . . 12.7 Flame response at dierent instants at φ =1.1, A=0.5 V, f=40 Hz. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A. 20 1 and 1 . 2 .

 20112 Sensitivity analyses for isotherms at T=300 K over a pressure range of 1-20 bar for φ = 1.3, diluent mixture of 21% He and 49% Ar indicating that the dominant reaction is O2+H<=>O+OH and the recombination reaction of ammonia is relatively not sensitive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.21 Sensitivity analyses for isobars at p= 3 bar over a temperature range of 300-700 K for φ = 1.3, diluent mixture of 21% He and 49% Ar indicating that the dominant reaction is O 2 + H and the recombination reaction of ammonia is relatively not sensitive although the magnitude of sensitivity to the ame speed is not the same for isobaric and isothermal cases. . . . . . . . . . . . . B.1 Chemiluminescence images of NH 2 * at φ = (a) 0.9, (b) 1, (c) 1.1, (d) 1.2, (e) 1.3, (f ) 1.4. . . . . . . . . . . . . . . . . . . . . B.2 Abel inversion transformed images of NH 2 * at φ = (a) 0.9, (b) 1, (c) 1.1, (d) 1.2, (e) 1.3, (f ) 1.4 depicting the positive curvature at the base of the ame for weak and stoichiometric conditions and the enhanced reactivity at the tip of the ame for φ = 1.The x and y axis represent the pixel and the color bar indicates the intensity of the pixel. . . . . . . . . . . . . . . . . B.3 k1 and k2 at φ = 0.9. . . . . . . . . . . . . . . . . . . . . . . . B.4 k1 and k2 at φ = 1.1. . . . . . . . . . . . . . . . . . . . . . . . B.5 Chemiluminescence images of NH* at φ = (a) 0.9, (b) 1, (c) 1.1, (d) 1.2, (e) 1.3, (f ) 1.4. . . . . . . . . . . . . . . . . . . . . . . xxvi LIST OF FIGURES B.6 Abel inversion transformed images of NH* at φ = (a) 0.9, (b) 1, (c) 1.1, (d) 1.2, (e) 1.3, (f ) 1.4 depicting the positive curvature at the base of the ame for weak and stoichiometric conditions and the enhanced reactivity at the tip of the ame for φ= 1.1 and 1.2 with strong reactivity near the burner for all the cases. The x and y axis represent the pixel and the color bar indicates the intensity of the pixel. . . . . . . . . . . . . . . . . . . . . . B.7 k1 and k2 at φ = 0.9. . . . . . . . . . . . . . . . . . . . . . . . B.8 k1 and k2 at φ = 1.1. . . . . . . . . . . . . . . . . . . . . . . . B.9 k1 and k2 at φ = 1.4. . . . . . . . . . . . . . . . . . . . . . . . C.1 Image captured at an exposure time of 1 ms with the ROI. . . C.2 Image captured at an exposure time of 1 ms, potentiometer of LED lamp set at k=6 with the lter. . . . . . . . . . . . . . . . C.3 Image captured at an exposure time of 1 ms, potentiometer of LED lamp set at k=6 without the lter. . . . . . . . . . . . . . C.4 Thickness, maximum intensity, k1 and k2 as a function of contour at φ=1.0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.5 Thickness, maximum intensity, k1 and k2 as a function of contour at φ=1.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.6 Thickness, maximum intensity, k1 and k2 as a function of contour at φ=1.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.7 Thickness, maximum intensity, k1 and k2 as a function of contour at φ=1.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . C.8 Thickness, maximum intensity, k1 and k2 as a function of contour at φ=1.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12. 1

 1 Abbreviations

Figure 1 :

 1 Figure 1: Comparison of the trend for ammonia (blue), methane (orange), and hydrogen (grey) for almost two centuries. The number of publications is represented in the logarithmic scale

Figure 1

 1 represents a bar graph to show an approximated number of papers published every decade on ammonia combustion (blue), methane combustion (orange), and, hydrogen combustion (grey). The number of publications is on a logarithmic scale. The numbers on top of the bar represent the approximated number of publications on ammonia combustion and it can be seen that it increases expo-General Introduction nentially signifying the strong research interest. The estimated value for this

  (i) Part I: Laminar ame speeds at high pressure and temperature, (ii) Part II : Inner structure of a stretched premixed ame and, (iii) Part III : Chemical and dynamical analyses of a premixed ame. Part I is dedicated to obtaining ame speeds and evaluating some of the kinetic schemes present in the literature at elevated conditions for lean, near stoichiometric, and, rich mixtures. The goal of Part II is to understand dierent reactive properties as well as the inner structure of the stretched premixed ame at dierent equivalence ratios. Part III has two important aspects. The rst aspect is to provide qualitative and quantitative analyses of some of the excited species. The second aspect is to understand the eect of the perturbation on the inner structure of the ame at dierent equivalence ratios. Each part is further divided into chapters with a brief introduction and a conclusion. This brief introduction aims to provide the goal of the study and the gist of each chapter and the conclusion gives an overview of the study. This thesis nally ends with a conclusive summary and a note probing into the future scope of this project.Part I

Chapter 2

 2 schemes. An eort has been made to understand the reactions involved with pressure change through sensitivity analyses. The important reactions have been marked out and the behaviour of the ame speed with the pressure and temperature change has been noted.Chapter 4 highlights the second part of the results. The eect of temperature and pressure on ame speeds has been isolated and studied to understand the behaviour noted in Chapter 3. The eect of diluents on the ame speed over the chosen range of pressure and temperature has also been examined. A range of exponential factors α and β in the ame speed model has been provided as well marking the importance of pressure and temperature eects on ammonia ame speeds.

Figure 2 . 1 :

 21 Figure 2.1: Sketch of the spherical combustion chamber -OPTIPRIME (left)and the furnace (right)[START_REF] Halter | Development of an optically accessible apparatus to characterize the evolution of spherically expanding ames under constant volume conditions[END_REF].

Figure 2 . 2 :

 22 Figure 2.2: Time-based evolution of ame radius as the ame front propagates from its ignition to the walls. The mixture used here is methane-air at φ=1.3, p0=1 bar and T0=300K [16].

Figure 2 . 3 :

 23 Figure 2.3: Pressure and radius evolution with regions of stretch eects and heat losses (regions where the data cannot be retrieved) and isochoric conditions(region for which data is presented here)[START_REF] Mouze-Mornettas | Eect of wall heat transfer on the dynamics of premixed spherical expanding ames[END_REF].

  of the schemes failed to give the ame speed value within the experimental range. Generally, it was seen that the schemes of Klippenstein, Zhang (2017) and Dagaut overestimate the ame speeds when compared to the experimental ame speed whereas Mathieu and Otomo under-estimate the ame speed. The other 4 mechanisms more or less fall within ± 5 % error bar of the experimental results. Out of the 14 cases, 3 of them are represented here in Figures 3.1, 3.2 and 3.3. The 3 chosen conditions represent a fuel-lean condition, a fuelrich condition and a high-pressure condition to illustrate the above-mentioned 3.1. EXPERIMENTAL AND NUMERICAL FLAME SPEEDS

Figure 3 . 1 :

 31 Figure 3.1: Laminar ame speeds at p 0 =4 bar, T 0 =302.3K, φ = 1.3, 21%Ar and 49% He.The experimental ame speeds have been thickened in yellow to account for the uncertainties incurred. The y-axis represents the ame speed whereas the x-axis is split as temperature and pressure. This is done as the ame speed is a function of both pressure and temperature which evolves isotropically during the combustion process. The orange colour represents Klippenstein[START_REF] Klippenstein | Theory and modeling of relevance to prompt-NO formation at high pressure, Combust[END_REF], violet represents Zhang (2017)[START_REF] Zhang | Assessing the predictions of a NO x kinetic mechanism on recent hydrogen and syngas experimental data[END_REF], blue represents Dagaut[START_REF] Dagaut | The oxidation of hydrogen cyanide and related chemistry[END_REF],

Figure 3 . 2 :Figure 3 . 3 :Figure 3 . 1

 323331 Figure 3.2: Laminar ame speeds at p 0 =1 bar, T 0 =296.8K, φ = 1.3, 49% Ar and 21% He

  and Stagni can predict the ame speed closest to the experimental values. It was seen that Mathieu and Otomo underestimated the values in all cases and on the other hand, Dagaut, Klippenstein and Zhang overestimated the values for all the cases. Modied Okafor predicted the values well within the experimental range for low pressure and rich conditions, whereas, for the other cases, it overestimated the values as well. A brief assessment was made on the specic rate constant, k used by the dierent schemes and it was seen that they didn't superimpose perfectly on each other probably explaining the dierence in the performance. Figure 3.4 shows a comparison of k of NH 3 + NH --⇀ ↽--NH 2 + H 2 for two of the schemes-Klippenstein (which over predicts the ame speed) and Nakamura (which is within the experimental ame speed). From the work done by Alturai et al. [112], it was seen that on updating the mechanisms with the ammonia pyrolysis equations, there is a signicant improvement in achieving ame speeds closer to their experimental reference. The experimental ame speeds along with the ame speeds from Nakamura and Stagni for all the cases are provided in Annex A. Since the study mainly focuses on 3 mechanisms, the following legend is maintained for convenience: red represents Stagni, black represents Nakamura and green represents Shrestha.

  The vast dierence in the ame speeds for dierent mechanisms may be attributed to the global summation eect of those reactions that do not fall under the top 10 important reactions. Since Nakamura and Stagni estimate the ame speeds closer to the experimental results, a deeper study has been made on them.

Figures 3 .Figure 3 . 4 :

 334 Figures 3.5a, 3.5b and gures 3.6a, 3.6b depict the sensitivity analyses for Nakamura and Stagni mechanisms at φ = 0.8 and φ = 1.1 for an initial pressure of 1 bar and a diluent mixture of 49% Ar and 21% He respectively. Figures 3.7a, 3.7b represent the sensitivity analyses for Nakamura and Stagni mechanisms respectively at φ = 1.1, p 0 =3 bar with a diluent mixture of 21% Ar and 49%

Figure 3 . 5 :

 35 Figure 3.5: Sensitivity analyses at p 0 =1 bar, φ = 0.8, 49% Ar and 21% He with the minimum pressure at 1 bar (dark blue) and the maximum at 8 bar (light blue)

Figure 3 . 6 :

 36 Figure 3.6: Sensitivity analyses at p 0 =1 bar, φ = 1.1, 49% Ar and 21% He with the minimum pressure at 1 bar (dark blue) and the maximum at 8 bar (light blue)

Figure 3 . 7 :

 37 Figure 3.7: Sensitivity analyses at p 0 =3 bar, φ = 1.1, 21% Ar and 49% He with the minimum pressure at 3 bar (dark blue) and the maximum at 24 bar (light blue)

Figure 3 . 8 :

 38 Figure 3.8: Sensitivity analyses at p 0 =4 bar, φ = 1.1, 70% He with the mini- mum pressure at 4 bar (dark blue) and the maximum at 36 bar (light blue)

Figure 3 . 9 :(

 39 Figure 3.9: Sensitivity analyses at p 0 =2 bar, φ = 1.3, 21% Ar and 49% He with the minimum pressure at 2 bar (dark blue) and the maximum at 15 bar (light blue)

Figure 3 . 10 : 29 3. 2 .Figures 3 . 1

 31029231 Figure 3.10: Sensitivity analyses at p 0 =4 bar, φ = 1.3, 70% He with the minimum pressure at 4 bar (dark blue) and the maximum at 36 bar (light blue)

4. 1 . 1

 11 Temperature and pressure eects

Figure 4 .

 4 Figure 4.1a depicts the variation of ame speed overpressure at T=300 K and Figure 4.1b over temperature at p=1 bar for 3 selected mechanisms: Nakamura, Stagni and Shrestha for φ = 0.8, 1.1 and 1.3 at a diluent mixture of 49% Ar and 21% He. Isotherms were plotted at T=300 K and at T=500 K and isobars were plotted for p=1 bar and p=3 bar. Similar results were obtained for both conditions. Isotherms at T=500 K and isobars at p=3 bar are shown in Figure 4.2a and Figure 4.2b. It is important to note that the ame speed axis range for the isobars is 4 times the range of the isotherms. It can be seen that as the pressure increases, the ame speed does not vary much. The drastic change in the ame speed is between 1-5 bar for all the mechanisms.

Figure 4 .

 4 Figure 4.1a and Figure 4.2a suggest that the pressure eect is slightly more important in Stagni when compared to the other 2 mechanisms and the least in Shrestha.

Figure 4 . 1 :

 41 Figure 4.1: Comparison study of Nakamura, Stagni and Shrestha for φ = 0.8, 1.1 and 1.3 at a diluent mixture of 49% Ar and 21% He with isothermal plots at T=300 K and isobaric plots at p=1 bar, circles represent φ = 0.8, squares represent φ = 1.1 and triangles represent φ = 1.3 whereas the colours green, black and red represent the mechanisms of Shrestha, Nakamura and Stagni respectively.

Figure 4 . 2 :

 42 Figure 4.2: Comparison study of Nakamura, Stagni and Shrestha for φ = 0.8, 1.1 and 1.3 at a diluent mixture of 49% Ar and 21% He with isothermal plots at T=500 K and isobaric plots at p=3 bar, circles represent φ = 0.8, squares represent φ = 1.1 and triangles represent φ = 1.3 whereas the colours green, black and red represent the mechanisms of Shrestha, Nakamura and Stagni respectively.

Figure 4 . 3 :

 43 Figure 4.3: Comparison study of Nakamura, Stagni and Shrestha for φ = 1.1 for dierent bath gases compositions, circles represent 70% He with isothermal plots at T=300 K and isobaric plots at p=1 bar, crosses represent 49% He and 21% Ar and diamonds represent 21% He and 49% Ar whereas the colours green, black and red represent the mechanisms of Shrestha, Nakamura and Stagni respectively.

  (a) Isothermal plots at T=500 K (b) Isobaric plots at p=3 bar

Figure 4 . 4 :

 44 Figure 4.4: Comparison study of Nakamura, Stagni and Shrestha for φ = 1.1 for dierent bath gases compositions, circles represent 70% He with isothermal plots at T=500 K and isobaric plots at p=3 bar, crosses represent 49% He and 21% Ar and diamonds represent 21% He and 49% Ar whereas the colours green, black and red represent the mechanisms of Shrestha, Nakamura and Stagni respectively.

4. 1 .Figure 4 . 5 :

 145 Figure 4.5: Beta trends from isotherms at T=500 K calculated using p 0 =1 bar, T 0 = 500 K and the corresponding ame speed depending on the equivalence ratio and the diluent mixture. φ=0.8, 21% He 49% Ar is represented by dashdot, φ=1.3, 21% He 49% Ar by a dash, φ=1.1, 21% He 49% Ar by round dot, φ=1.1, 49% He 21% Ar by solid and φ=1.1, 70% He by long dash-dot-dot and the colours black, red and green represent Nakamura, Stagni and Shrestha respectively.

Figure 5 . 1 :

 51 Figure 5.1: Dierent experimental techniques used to study ames along with the pressure and temperature range.

Figure 6 . 1 :

 61 Figure 6.1: Sketch of the Bunsen burner set-up.

Figure 6 . 2 :

 62 Figure 6.2: Images of buoyancy-driven ame ickering before and after using the chimney.

Figure 6 . 3 :

 63 Figure 6.3: Schematic diagram of the experimental set-up along with the PIV set-up. CL: cylindrical lens, SL: spherical lens, DM: dichroic mirror, RM: rotameter, MF: mass owmeter

6. 1 .Figure 6 . 4 :

 164 Figure 6.4: Emission spectra for ammonia/air ames at stoichiometry identifying important species. The intensity has not been calibrated. The background illumination and the pilot ame emissions have been subtracted. The emission peaks are not calibrated.

Figure 8 .

 8 3 represents the schematic diagram of the chemiluminescence set-up.

Figure 6 . 5 :

 65 Figure 6.5: Schematic diagram of the experimental set-up for the chemiluminescence study.

Figure 7 . 2 :

 72 Figure 7.2: Ammonia-air ame at φ = 1.1 with PIV depicting the thick ame. The inner cone and the outer cone represent the fresh gas surface and the outer ame surface respectively. The camera used to capture this image is NIKON D850 and the exposure time has been set to 250 ms. The x and y axes have been normalized over the radius of the burner axes.

7. 2 .

 2 FLAME SPEED DETERMINATION spherical vessel with thermocouples.

Figure 7 . 3 :

 73 Figure 7.3: Comparison of ammonia-air laminar ame speed at atmospheric conditions available in the literature for dierent φ . OC (black square) and IC (black circle) represent the ame speed at the outer cone and inner cone and Sd represented by a black diamond is the displacement ame speed.

  2. The dierence in the ame speed computed at the outer cone (OC) and the inner cone (IC) represented by black squares and circles respectively in Figure g:amespeedis around 2-3 cm/s. As shown in Figure g:amespeed, on comparing with values

Figure 8 .

 8 Figure 8.2 a and b represent the inverse Abel transformed image with the pixel intensity represented in the colour bar axis for NH 2 * and NH* respectively, Figure 8.2 c shows the thickness of the species (NH 2 * in black and NH* in violet), and Figure 8.2 d displays k1 and k2 for NH 2 *, all at φ = 1.4. k1 represents the principal curvature along the ame (shown in blue in Figure 8.2 d) and k2 represents the principal curvature in the circumferential direction (shown in red in Figure 8.2 d).The curvature for NH 2 * is only presented here to be legible to the reader. The range and the trend of both k1 and k2 for NH* are similar to NH 2 * (NH 2 * is aected slightly higher than NH*). The x-axes of the plots represent the position normalized by the burner radius with the zero of the x-axes at the centre of the burner. The y-axis in Figure8.2 a and b represents the position along the height normalized over the burner radius with the zero of the y-axis marking the exit of the burner. The thickness of the species was calculated as the full width at half maximum (FWHM) of the gaussian curve with a 95% condence interval obtained when moving along the normal to front. The Abel inversion transformed images for all species at all equivalence ratio is provided in Annexe B. k1 and k2 for both NH* and NH 2 * are also provided in the Annex B.

Figure 8 . 3 Figure 8 . 2 :

 8382 Figure 8.3 a, b, and c show chemiluminescence of NH* for φ= 0.9, 1.1 and 1.3

8. 1 .Figure 8 . 3 :

 183 Figure 8.3: Chemiluminescence images of NH* at (a) φ=0.9, (b) φ=1.1, (c) φ=1.3, and NH 2 * at (d) φ=1, (e) φ=1.2, (f ) φ=1.4. The exposure time of these images is not the same.

  (a) The schematic diagram of superimposition of PIV and chemiluminescence results (b) The schematic representation of the experimental results in a 1D domain

Figure 8 . 4 :

 84 Figure 8.4: Results obtained from the experiments

  species from the chemiluminescence image were superimposed upon the Mie scattering results which are shown as a schematic diagram in Figure 8.4a . Upon moving along the normal of the front, the species as a function of dis-(a) Temperature vs distance at φ=1.1 (b) Mole fractions and HRR as a function of distance at φ=1.1

Figure 8 . 5 :

 85 Figure 8.5: Results obtained from the simulations

Figure 10 . 1 :Figure 10 . 2 :

 101102 Figure 10.1: Schematic diagram of the chemiluminescence set-up.

Figure 10 . 3 :

 103 Figure 10.3: Raw images of the ame to note the dierences between using a NH 2 * lter and not.

11. 1 .

 1 STEP 1: OPTICS CALIBRATION needs to be xed. To x the ROI, an image of the ammonia/air ame with the maximum possible height amongst the cases of interest (here at φ=1.4) was captured. This xed ROI is given in Annexe C.

Figure 11 . 1 :

 111 Figure 11.1: On comparing the normalized intensities along the centre of the images captured, it can be seen that by using the lter, the incoming intensity is reduced by less than 2%. The ROI is in between 0.1 and 0.8 on the x-axis and is marked in orange.

Figure 11 . 2 :

 112 Figure 11.2: Intensity vs potentiometer of LED lamp at an exposure time of 1ms with the NH 2 * lter mounted on the camera.

Figure 11 . 3 :Figure 11 . 4 :Figure 11 . 4 .

 113114114 Figure 11.3: Intensity vs exposure time of camera with the potentiometer of the lamp set at k=6 with the NH 2 * lter mounted on the camera.

Figure 11 . 5 :

 115 Figure 11.5: Intensity vs exposure time of camera with the ND lters and NH 2 * lter.

11 . 1 )

 111 where the subscript c denotes calibration lamp (here tungsten lamp), I denotes the intensity of pixel as recorded by the camera and N is the number of photons 11.4. STEP 4: ADAPTING THE GIVEN IRRADIATION VALUES TO THE EXPERIMENT SET-UP emitted.

  Figures 11.6 and 11.7 represent the concentration of species of NH 2 * and OH* with the values given in the colour bar in terms of mol/m 3 and Figure 11.8 represents the qualitative analysis of NO* with the colour bar that represents intensity. It is important to remember that these concentrations do not account for the quenching and are dependent on the stretch (even though the ame stretch is quite low).

Figure 11 . 6 :Figure 11 . 7 :Figure 11 . 8 :

 116117118 Figure 11.6: Concentration of NH 2 * for φ = 0.9-1.4 with the colour bar repre- senting the concentration in mol/m 3 .

  3 and then begins to decrease whereas the width increases as the equivalence ratio increases. On nding the position of the peak with respect to the 525 K isotherm reference line, it was seen that the peaks appeared closest to the reference line for the fastest ame ( φ = 1.1) and it appeared the farthest for the slowest ame (φ =1.4).

Figure 11 . 9 :

 119 Figure 11.9: Comparison of NH 2 * peaks for dierent equivalence ratios.

( v )

 v Figure 11.10: Comparison of OH* peaks for dierent equivalence ratios.

Figure 12 . 1 :

 121 Figure 12.1: The eect of amplitude on ames creating pockets of fresh gases for the same frequency (30 Hz) and equivalence ratio (1.1).

Figure 12 . 2 :

 122 Figure 12.2: FAI for phi=1, A=0.5 V and f=40 Hz

92 12. 2 .Figure 12 . 3 :

 922123 Figure 12.3: FAI for phi=1.1, A=0.5 V and f=40 Hz

93 12. 2 .Figure 12 . 4 :

 932124 Figure 12.4: FAI for phi=1.2, A=0.5 V and f=40 Hz

12. 2 . 4 Figure 12 . 5 :

 24125 Figure 12.5: FAI for phi=1.3, A=0.5 V and f=40 Hz

12. 2 .

 2 Figure 12.6: FAI for phi=1.4, A=0.5 V and f=40 Hz

12. 2 .Figure 12 . 7 :

 2127 Figure 12.7: Simulation using Stagni et al. to indicate the decompisition of ammonia to produce H 2 conrming the possibility of preferential diusion

Figure 12 . 8 :

 128 Figure 12.8: Flame response at dierent instants at φ =1.1, A=0.5 V, f=40 Hz.

Figure A. 2 :

 2 Figure A.2: Laminar ame speeds at p 0 =1 bar, φ = 1.1, 49% Ar and 21% He

Figure A. 3 :

 3 Figure A.3: Laminar ame speeds at p 0 =2 bar, φ = 1.1, 49% Ar and 21% He

Figure A. 4 :

 4 Figure A.4: Laminar ame speeds at p 0 =2 bar, φ = 1.3, 49% Ar and 21% He

Figure A. 5 :

 5 Figure A.5: Laminar ame speeds at p 0 =1 bar, φ = 1.1, 49% He and 21% Ar

Figure A. 6 :

 6 Figure A.6: Laminar ame speeds at p 0 =2 bar, φ = 1.1, 49% He and 21% Ar

Figure A. 7 :

 7 Figure A.7: Laminar ame speeds at p 0 =3 bar, φ = 1.1, 49% He and 21% Ar

Figure A. 8 :

 8 Figure A.8: Laminar ame speeds at p 0 =2 bar, φ = 1.3, 49% He and 21% Ar

Figure A. 9 :

 9 Figure A.9: Laminar ame speeds at p 0 =4 bar, φ = 1.3, 49% He and 21% Ar

Figure A. 10 :

 10 Figure A.10: Laminar ame speeds at p 0 =4 bar, φ = 1.1, 70% He

Figure A. 11 :

 11 Figure A.11: Laminar ame speeds at p 0 =4 bar, φ = 1.3, 70% He

Figure A. 12 :

 12 Figure A.12: Sensitivity analyses at p 0 =2 bar, φ = 0.8, 49% Ar and 21% He

Figure A. 13 :

 13 Figure A.13: Sensitivity analyses at p 0 =2 bar, φ = 1.1, 49% Ar and 21% He

Figure A. 14 :

 14 Figure A.14: Sensitivity analyses at p 0 =1 bar, φ = 1.3, 49% Ar and 21% He

Figure A. 15 :

 15 Figure A.15: Sensitivity analyses at p 0 =2 bar, φ = 1.3, 49% Ar and 21% He

Figure A. 16 :

 16 Figure A.16: Sensitivity analyses at p 0 =1 bar, φ = 1.1, 49% He and 21% Ar

Figure A. 17 :

 17 Figure A.17: Sensitivity analyses at p 0 =2 bar, φ = 1.1, 49% He and 21% Ar

Figure A. 18 :

 18 Figure A.18: Sensitivity analyses at p 0 =3 bar, φ = 1.3, 49% He and 21% Ar

Figure A. 19 :

 19 Figure A.19: Sensitivity analyses at p 0 =3 bar, φ = 1.3, 49% He and 21% Ar

Figure A. 20 :

 20 Figure A.20: Sensitivity analyses for isotherms at T=300 K over a pressure range of 1-20 bar for φ = 1.3, diluent mixture of 21% He and 49% Ar indicating that the dominant reaction is O2+H<=>O+OH and the recombination reaction of ammonia is relatively not sensitive.

Figure A. 21 :

 21 Figure A.21: Sensitivity analyses for isobars at p= 3 bar over a temperature range of 300-700 K for φ = 1.3, diluent mixture of 21% He and 49% Ar indicating that the dominant reaction is O 2 + H and the recombination reaction of ammonia is relatively not sensitive although the magnitude of sensitivity to the ame speed is not the same for isobaric and isothermal cases.

Figure B. 7 ,

 7 Figure B.7, Figure B.8 and Figure B.9 represents k1 and k2 for NH* at φ= 0.9, 1.1 and 1.4. NH* undergoes a higher eect than NH 2 *.

Figure B. 4 :

 4 Figure B.4: k1 and k2 at φ = 1.1.

Figure B. 6 :

 6 Figure B.6: Abel inversion transformed images of NH* at φ = (a) 0.9, (b) 1, (c) 1.1, (d) 1.2, (e) 1.3, (f ) 1.4 depicting the positive curvature at the base of the ame for weak and stoichiometric conditions and the enhanced reactivity at the tip of the ame for φ= 1.1 and 1.2 with strong reactivity near the burner for all the cases. The x and y axis represent the pixel and the color bar indicates the intensity of the pixel.

Figure B. 7 :

 7 Figure B.7: k1 and k2 at φ = 0.9.

Figure B. 8 :

 8 Figure B.8: k1 and k2 at φ = 1.1.

Figure C. 2 :

 2 Figure C.2: Image captured at an exposure time of 1 ms, potentiometer of LED lamp set at k=6 with the lter.

Figure C. 3 :

 3 Figure C.3: Image captured at an exposure time of 1 ms, potentiometer of LED lamp set at k=6 without the lter.

Figure C. 4 : 1 Figure C. 5 :

 415 Figure C.4: Thickness, maximum intensity, k1 and k2 as a function of contour at φ=1.0

Figure C. 6 :

 6 Figure C.6: Thickness, maximum intensity, k1 and k2 as a function of contour at φ=1.2

Figure C. 7 : 4 Figure C. 8 :

 748 Figure C.7: Thickness, maximum intensity, k1 and k2 as a function of contour at φ=1.3

  I rst came to ICARE, I was struggling a lot with French and with being new to the place. I would like to thank Dr. Maxence Laillau, Dr.

	xviii CONTENTS xx	xv CONTENTS xix CONTENTS
	rendered support, my entire family and my parents who have played a huge Conclusion 37 Chapter 11
	role in helping me to chase my dreams. My parents have always stood by all 11.1 Step 1: Optics calibration . . . . . . . . . . . . . . . . . . . . .
	11.2 Step 2: Tungsten lamp calibration . . . . . . . . . . . . . . . . my decisions and have believed in me even when I haven't. None of this would have been possible if not for them. I believe that this entire journey was not II Inner structure of a stretched premixed ame 11.3 Step 3: Correlation between concentration and intensity of the 39 species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
	only just about achieving the target but also, was about meeting new people, learning new things, maintaining a work-life balance and nally with a huge Introduction 11.4 Step 4: Adapting the given irradiation values to the experiment 41 set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
	focus on holistic development. I have acquired skills and learnt a lot about 5 Chapter 5 43 11.5 Quantitative and qualitative analyses of species . . . . . . . . .
	myself through this process. Enn, je me dois d'écrire quelques mots en français pour vous faire savoir que vos eorts n'ont pas été vains. Merci à tous ceux qui ont participé à cette Chapter 5 43 5.1 Experiments and optical diagnostics for ame speeds in ammonia 12 Chapter 12 Chapter 12 LIST OF FIGURES
	combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1 Flame feedback due to acoustic perturbation . . . . . . . . . . expérience! 5.2 Chemiluminescence signature of ammonia ames . . . . . . . . 12.2 Curvature eects on the reactivity . . . . . . . . . . . . . . . .	43 44
	5.3	Studies of stretch eects in ammonia ames . . . . . . . . . . . 12.2.1 Phi = 1.0 . . . . . . . . . . . . . . . . . . . . . . . . . .	45
	5.4	Literature on ammonia ame structure . . . . . . . . . . . . . . 12.2.2 Phi = 1.1 . . . . . . . . . . . . . . . . . . . . . . . . . .	45
		12.2.3 Phi = 1.2 . . . . . . . . . . . . . . . . . . . . . . . . . .
	6 Chapter 6 12.2.4 Phi = 1.3 . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.5 Phi = 1.4 . . . . . . . . . . . . . . . . . . . . . . . . . .	47
	Chapter 6 6.1 The experimental set-up . . . . . . . . . . . . . . . . . . . . . . Conclusion 1 Comparison of the trend for ammonia (blue), methane (orange), 6.1.1 The PIV set-up . . . . . . . . . . . . . . . . . . . . . . . General Conclusion and hydrogen (grey) for almost two centuries. The number of	47 47 48
	Nicolas Rembaut, Dr. Hajar Zaidaoui, and Hugo Chouraqui for clearing my 6.1.2 The chemiluminescence set-up . . . . . . . . . . . . . . 50 6.2 1D simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 publications is represented in the logarithmic scale . . . . . . . 3 Communications french doubts and for the interesting chats. I had the pleasure of meeting some amazing people like Dr. Hajar El Othmani, Lydia Boualem, and Ahlem Ghaabi with whom I had some fun times. I am lucky to have met these colleagues who nally ended up being good friends to me: Dr. Anthony Roque, Dr. Sharath Nagaraju, Dr. Outmane Mghanen, Dr. Miles Burnett, Laura Vastier, Jean-Noel Semaan and Ronan Pelé. You all have always been there for me. Cheers to all the fun we have ever had. I don't know what would I have done if I hadn't meet these three people: Dr. Alaa Hamadi, Leticia Piton and, Fabian 7 Chapter 7 A Laminar ame speeds and sensitivity analyses 2.1 Sketch of the spherical combustion chamber -OPTIPRIME (left) 53 A.1 Flame speed data for dierent test conditions . . . . . . . . . . and the furnace (right) [15]. . . . . . . . . . . . . . . . . . . . . 15 Introduction 1 Chapter 1 Chapter 1 1.1 Chapter 7 A.2 Sensitivity analyses for dierent test conditions . . . . . . . . . A.3 Sensitivity analysis at isothermal and isobaric conditions for Stagni115 2.2 Time-based evolution of ame radius as the ame front propa-53 7.1 Flame thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . gates from its ignition to the walls. The mixture used here is 53 7.2 Flame speed determination . . . . . . . . . . . . . . . . . . . . 54 methane-air at φ=1.3, p0=1 bar and T0=300K [16]. . . . . . . 15 B Inner structure of stretched premixed ame B.1 NH 2 * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Pressure and radius evolution with regions of stretch eects and Literature review on experimental ame speed data in ammonia combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.2 NH* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . heat losses (regions where the data cannot be retrieved) and
	Cano. They have become family and have shown me compassion like I have 1.2 Literature review on kinetic schemes in ammonia combustion . C Chemical and dynamical analyses of a premixed ame isochoric conditions (region for which data is presented here)
	always been a part of them. It is amazing how we come from dierent parts of [17]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2 Chapter 2 the world, cross paths in the pursuit of our dreams and end up sharing a bond as we do. It was fun to travel and discover many unknown places and try out Chapter 2 Bibliography 3.1
	new adventures together. 2.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . .
	My team CT have always been very supportive and has always helped 2.1.1 Experimental set-up description . . . . . . . . . . . . . .
	2.1.2 me with everything I have ever needed. I am glad to have had some great Radius and pressure evolution . . . . . . . . . . . . . . . 2.1.3 Heat loss model . . . . . . . . . . . . . . . . . . . . . . . conversations and some help from all the interns we have seen in the last three 2.1.4 Flame speed evaluation . . . . . . . . . . . . . . . . . .
	years, Dr. Marco Di Lorenzo, Deniz Kaya, Valentin Glasziou, Antoine Mouze-2.2 Chemical kinetic schemes . . . . . . . . . . . . . . . . . . . . .
	Mornettas, and Samuel Jeanjean. I am happy to have spent some time and 2.3 Test conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction 69
	learn a lot from Deniz Kaya. I am also glad to have had fun conversations with Valentin Glasziou. Antoine Mouze-Mornettas -Thanks for all that fun and 3 Chapter 3 9 Chapter 9 71
	crazy conversations and for placing bets on things others wouldn't. The bets Chapter 3 3.1 Experimental and numerical ame speeds . . . . . . . . . . . . Chapter 9 71 are still on! I do not think words would do justice to what Samuel Jeanjean 3.2 Sensitivity analyses . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Quantitative and qualitative analysis of chemiluminescence of has done for me. Samuel and I started our thesis together and shared our ammonia ames . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
	oce. We have become very close friends over the years and have had crazy 9.2 Perturbed laminar ammonia ames . . . . . . . . . . . . . . . . 72 4 Chapter 4 conversations, played silly games and pranked each other incessantly. You have always helped me whenever I needed it, stood by me when I needed support, Chapter 4 10 Chapter 10 73
	taught me French, introduced me to French culture and whatnot! You have 4.1 Isotherms and isobars . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Temperature and pressure eects . . . . . . . . . . . . . Chapter 10 73 made work fun and you have played an important role to achieve my goal. I 4.1.2 10.1 The chemiluminescence set-up . . . . . . . . . . . . . . . . . . . 73 Bath gases eects . . . . . . . . . . . . . . . . . . . . . . am sure my path would have been completely dierent if not for you. And so, 4.1.3 Alpha and beta . . . . . . . . . . . . . . . . . . . . . . . 10.2 The ame-acoustic interaction set-up . . . . . . . . . . . . . . . 74
	I am thankful to have met you. And last but not the least, I would like to thank all my friends who have 4.2 Alpha and beta values for the experiment and numerical ame speeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Chapter 11 77
		xvii

  --⇀ ↽--O + OH is the most dominant reaction with its sensitivity to ame speed value being around 0.5. The mechanisms of Mendiara and Glarborg, Okafor et al., Otomo 1.2. LITERATURE REVIEW ON KINETIC SCHEMES IN AMMONIA COMBUSTION et al. and Mathieu and Petersen, which predict better the experimental data, show the reaction H + NO + M --⇀ ↽--HNO + M as the second most important in the promotion of ame speed in ammonia-air mixtures, with sensitivity values between 0.1 and 0.2. The mechanisms that overpredict the ame speeds have this reaction as a lesser important one. The mechanisms of Konnov and Dagaut et al do not list this reaction among the ten most important, and the mechanism of Glarborg et al. provides a sensitivity value of less than 0.1 for the same reaction. The mechanisms of Konnov, Klippenstein et al. and Glarborg et al.

Table 2 .

 2 1: Chemical kinetic schemes used in this study

	Kinetic Schemes	Species	Reactions	Reference
	Klippenstein et al.	33	211	[2]
	Dagaut et al.	42	250	[18]
	Zhang et al. (2017)	37	229	[3]
	Shrestha et al.	34	264	[10]
	Nakamura et al.	38	232	[8]
	Modied Okafor*	60	359	[1]
	Otomo et al.	32	213	[19]
	Stagni et al.	31	203	[9]
	Mathieu and Petersen	55	278	[5]
	Shrestha et al.	125	1099	[72]
	KAUST	38	263	[110]
	Gotama et al.	32	165	[111]

Table 2 .

 2 2. At this equilibrium condition,it is seen that the dissociation reaction of H 2 O to OH is favoured.

	20

Both Chapters 3 and 4 describe the results obtained. Chapter 3 focuses on ame speeds for the dierent test conditions and the sensitivity analyses for the same conditions to highlight the important reactions in dierent kinetic schemes.

Table 3 .

 3 1: Initial pressure and equivalence ratio with the range of pressure and temperature for which the ame speed has been measured. The initial

	temperature, T 0 , was maintained at 300K
	φ	P 0 (bar)	P range (bar)	T range (K)
	0.8	1	2-6	369-502
	0.8	2	4-16	374-555
	1.1	1	2-9	371-572
	1.1	2	4-16	371-557
	1.1	3	6-30	372-584
	1.1	4	8-37	371-575
	1.3	1	2-8	370-544
	1.3	2	4-17	370-555
	1.3	3	6-26	370-557
	1.3	4	8-36	369-563

Table 12

 12

	.1 shows the Strouhal number with the respective subscript rep-
	resenting the forcing frequency with the critical Strouhal number for dierent
	equivalence ratios. Here, only St 40 i.e. at 40 Hz is further explored as the
	Strouhal number at this frequency indicates that the acoustic perturbations
	are quite strong and dominate the ow very well which is the required kind of
	ow for this study.

12.2 FAI for phi=1, A=0.5 V and f=40 Hz . . . . . . . . . . . . . .

10.2. THE FLAME-ACOUSTIC INTERACTION SET-UP

LITERATURE ON AMMONIA FLAME STRUCTURE

Rocha et al. [START_REF] Rocha | Structure and Laminar Flame Speed of an Ammonia/Methane/Air Premixed Flame under Varying Pressure and Equivalence Ratio[END_REF] gave a joint experimental and DNS study on premixed NH 3 /CH 4 ames to characterize the structure of the blended ame accommo- dating the local ame stretch. NH and NO proles were measured using PLIF.

The molecular beam mass spectroscopy was used by Duynslaegher et al. [START_REF] Duynslaegher | Flame structure studies of premixed ammonia/hydrogen/oxygen/argon ames: Experimental and numerical investigation[END_REF] to study the structure of NH 3 /H 2 /Ar/O 2 ames along with the stretch eects.

Numerical simulations were also performed to compare with their experimental study. Premixed NH 3 /air ame structure was investigated [127] using LIF to obtain proles of OH, NH, and NO which were used to evaluate four kinetic models. Wolfhard and Parker [START_REF] Wolfhard | A Spectroscopic Investigation into the Structure of Diusion Flames[END_REF] carried out a spectroscopic investigation into the structure of at diusion ammonia-oxygen ame. Xiao et al. [START_REF] Xiao | Study on counterow premixed ames using high concentration ammonia mixed with methane 275[END_REF] studied the structure of ammonia/methane ame in a stagnation counter setup and analysed various proles like emissions of species to understand the reactions and the chemistry behind it. Lee et al [START_REF] Lee | Studies on properties of laminar premixed hydrogen-added ammonia/air ames for hydrogen production[END_REF] did a similar study with hydrogen-blended ammonia ames.

There are hardly any studies on the inner structure of ammonia-air ames using chemiluminescence. The work presented here comprises the use of chemiluminescence to study the curvature eect of dierent excited species and to understand the structure of ammonia-air ames at atmospheric conditions along with ame speeds at the same conditions for dierent equivalence ratios on a Bunsen burner.

CHAPTER 7 CHAPTER 7

This chapter discusses the results obtained using the PIV setup. The importance of the ame thickness and ame speed evaluated on the fresh gas isotherm has been provided here. Ammonia ames are known to be thicker than most ames [START_REF] Kobayashi | [END_REF]. One of the denitions of ame thickness is the distance from where the extended tangent meets the unburned fuel temperature, and the combustion products temperature. Figure 7.1 shows the ame thickness in a temperature vs distance prole. The line on the left indicates the fresh gas isotherm after which is the reaction zone, followed by the burnt gas isotherm. At atmospheric conditions, the temperature of fresh gas isotherm is about 500 K. This chapter describes the analyses done on the species using the chemiluminescence setup. Inverse able transformations were performed on the raw images using the Basex method to convert the integrated 3D raw image information onto a 2D plane. Negative curvature is dened for a concave ame in the direction of the burnt gas and a positive curvature is dened for a convex ame in the direction of the burnt gas.

Flame thickness

Curvature eects on the species

FLAME STRUCTURE

found to be 2.2 mm, 1.7 mm, and 2.1 mm for φ = 0.9, 1.1, and 1.4 respectively. From Figure 8.6, it can be seen that NH 2 * is produced at a lower tempera- ture than NH*. Also, both NH* and NH 2 * are quite close to their corresponding non-excited species. The maximum HRR occurs between NH* and NH 2 * for each of the cases. Therefore, the product of NH* and NH 2 * is a good indica- tor of determining the maximum HRR experimentally. This is indeed, truly a cost-eective way of determining the HRR position.

COMMUNICATIONS ASSOCIATED TO THIS WORK

Journal articles

A. Karan

APPENDIX C CHEMICAL AND DYNAMICAL ANALYSES OF A PREMIXED FLAME

The xed ROI is represented in gure C.1 with the longest ame possible ( at φ=1.4) The images of LED lamp captured with the lter and without the lter.