Keywords: Droplet, vaporization, alcohols, d 2 -law, water vapour, average and instantaneous vaporization rate, autoignition delay time, kinetics mechanism iv

I would like to thank Prof. Luis Le Moyne, the Director of ISAT as

INTRODUCTION

General introduction

For ages, population of the world has depended on the fossil fuels as the main resources of energy for many industries including the automotive. The reality that the fossil fuels will become extinct in near future has developed significant and extensive researches for alternative fuels. Environmental concerns also have led to the increased interest in the future use of these alternative fuels. This is where biofuels came into the picture; they have potential as they are derived from renewable sources, they are environmental friendly as in reducing the pollutant emissions and rather inexpensive means in terms of reducing the dependency on fossil fuels.

The term biofuels is usually defined and used to refer to that of alternative and substitutes for petrol, diesel or aircraft fuels that are based from fossil.

There are so many reasons for biofuels to have imminently been alternative to fossil fuels.

The biofuels that are manufactured from biomass are identified to be renewable, biodegrable, oxygenated and significantly produce much less pollutant emissions such as particulate matter (PM), unburned hydrocarbon (HC) and carbon monoxide (CO). They are also having prospective to reduce the production of carbon dioxide (CO 2 ). This is primarily based on the fact that plants which are used for biofuels production absorb CO 2 during their growth which is then released during the biofuels combustion.

Nonetheless, the study of the gasification of a biofuel droplet via vaporization is still needed and necessary to give better understanding and modelling of complex spray and mixture formation issues. The behaviour of biofuel droplets especially in atomization and spray is still unknown and unpredictable. The proper characterization of this behaviour is necessary as it involves heat, mass and momentum transfer processes in gas and liquid phase and their coupling at the droplet interface.

Objectives of study

The main objective of this study is to investigate the vaporization behaviour of an isolated droplet and its characteristics which affects the atomization of sprays in conventional internal combustion engines. A detailed description of the vaporization of an isolated droplet has been realized in this experimental study aimed at investigating the low molecular weight alcohols; ethanol and 1-propanol and the effects of their miscibility with water property will be further examined. The characterization of the vaporization phenomenon is necessary for this liquid fuel to design efficient injection systems for internal combustion engines, for propulsion and power generation. An extensive theoretical calculation and evaluation is also carried out to validate experimental results. Experimental studies of autoignition behaviour of alcohols at various initial temperatures and at atmospheric pressure are also accomplished using a Rapid Compression Machine (RCM), courtesy of Centre of Combustion Chemistry, National University of Ireland, Galway. To better assess the effect of the presence of oxygenated compound in alcohol fuels especially on the engine efficiency and the pollutants formation, it is very imperative to well validate a developed kinetic models for the combustion and oxidation of the oxygenated component of biofuels. Well-known detailed kinetics combustion analysis from literature is carried out to predict and validate the experimental results. The effects of different molecular weight alcohol on autoignition behaviour have been thoroughly studied and the impact of water addition to ethanol oxidation has been assessed by sensitivity analysis.

Challenges

The biggest challenge in this study is to overcome all the unworthiness of utilisation of alcohols as a reliable alternative fuel especially in automotive transportation. Many researchers found that alcohols are still unpractical to be explored as the definite alternative fuel to replace the current conventional fossil fuels such as gasoline and diesel. However for many decades, researches are trying to keep on working in detailing and improving the goodness and reliability of alcohols properties. The unpredictable and volatile behaviours of alcohols made the whole studies even much more challenging.

Thesis Layout

This thesis has been divided into seven chapters. After a brief overview of background and introduction in chapter 1, chapter 2 mainly consists of literature review and bibliography analysis related to biofuels generally and alcohols particularly. The development of isolated droplet vaporization experimental works on alcohols particularly is also included in this chapter. In addition, a description of the background history of autoignition characteristics of alcohols is also incorporated in this chapter. Chapter 3 gives the insights of the experimental set-up for isolated droplet vaporization study on ethanol and 1-propanol. The properties of ethanol and 1-propanol are provided and all parameters and variables are defined. The postprocessing details are included to give crystal clear view on the overall processes. Autoignition experimental set-up is also discussed and explained in great details. Chapter 4 describes in greater length the theoretical calculation of droplet vaporization that includes all related physical and thermodynamic properties. Chapter 5 reports the results of the experimental works for the isolated droplet of ethanol and 1-propanol vaporization at various ambient temperature ranges from 293K to 973K, which is necessary to gain both some physical insights and to give accessible range for theoretical evaluation while the furnace pressure is main-tained atmospheric. Meanwhile in Chapter 6, detailed kinetic model from literature to predict the autoignition behaviour of ethanol and 1-propanol is further exploited. Autoignition experimental results from Rapid Compression Machine (RCM) of ethanol, 1-propanol and ethanol/water mixture are presented. Sensitivity analysis that consisted a set of reactions of potential species is also carried out for both alcohols and ethanol/water mixture to discuss for further comprehension on the kinetic impact of water on ethanol oxidation. Conclusions, imperative findings and further works available in these studies are given in Chapter 7.

LITERATURE REVIEW

Biofuels and Alcohols in general

Recent growing alarm on declination of conventional petroleum based fuels particularly in global automotive industry has caused significant interest in biofuels research. Biofuels such as pure vegetable oils, biodiesel based on cross transesterification of vegetable oils or animal fats and alcohols based on biochemical processes are currently readily available. These biofuels issued from the biomasses which are known to be renewable, biodegradable, oxygenated and produce fewer pollutants [START_REF] Agarwal | Biofuels (Alcohols and Biodiesel) Applications as Fuels for Internal Combustion Engines[END_REF] than conventional fuels require further and more extensive investigations especially in terms of fundamental characteristics such as vaporization and oxidation behaviours. Biofuels are usually referred to liquid, gas and solid fuels predominantly for transport produced from biomass. A variety of fuels can be produced from biomass such as ethanol, methanol, biodiesel, Fischer-Tropsch diesel, hydrogen and methane. They could be pure biofuels or blend fuels in such a proportion that they can substitute conventional motor fuels without or minimized altering of the car performance. Figure 2.1 (adopted from Tran et al.

(2012)) shows variety of oxygenated fuels can be obtained through wide range of processes involving fermentation and catalytic reactions. From this figure, alcohols as the main interest biofuels in this study are generally derived from the fermentation process of starch and sugar.

The lower molecular weight alcohols such as methanol and ethanol could be also produced through gasification process from lignocelluloses sources such as crop and forest waste. Nigam and Singh (2011) have generally classified the biofuels into two major categories (Figure 2.2). Primary biofuels referred to as natural and unprocessed biomass such as firewood, wood chips and pellets, and are mainly those where the organic material is utilised essentially in its natural and non-modified chemical form. Primary fuels are directly combusted, usually to supply cooking fuel, heating or electricity production. Meanwhile the secondary biofuels are adapted primary fuels, which have been processed and produced in the form of solids (e.g. charcoal), or liquids (e.g. ethanol, biodiesel and bio-oil), or gases (e.g. biogas, synthesis gas and hydrogen). Secondary fuels can be used for multiple ranges of applications, including transport and high temperature industrial processes. The secondary biofuels are further classified to first, second and third generation biofuels based on their raw material and technology used for their production.

The physical and chemical properties of alcohols-Ethanol and 1-propanol

Alcohols is an interesting alternative fuel for transportation purposes as they have properties that allow its use in existing internal combustion engines without or with minor hardware modifications. Comparisons of typical fuel physical properties have been made between alcohols such as ethanol (C 2 H 5 OH) and 1-propanol (C 3 H 7 OH) as the main subjects in this current study and gasoline as summarized in the engine, thus allows for use at higher compression ratios and significantly improves the engine performance. A fuel with a higher octane number can endure higher compression ratios before engine starts knocking, thus giving engine an ability to deliver more power efficiently and economically. They also have a density close to the gasoline which is suitable to the existing internal engines without any further adjustment. In the meantime the presence of oxygen in the formula of alcohols, can provide a more homogeneous fuel/air mixing and consequently a decrease in unburned or partially burned molecule emissions (HC and CO) resulting in cleaner environment due to less pollutants.. A high latent heat of vaporization enabling a "cooling effect" of air and consequently can enhance the volumetric efficiency.

However, the existence of oxygen included in the molecule (~30% wt) of alcohols can induce an increase in the fuel volumetric consumption. Meanwhile the high latent heat of vaporization can tempt running difficulties in cold conditions, especially cold start. To lower this phenomenon, some technical solutions may be applied. The most frequently used solution is to use E85 (85% ethanol) instead of pure ethanol. The addition of 15% light hydrocarbon fraction in ethanol induces a strong increase in volatility and consequently an easier engine start. Ethanol is more and more largely used in spark ignition (SI) engines as pure fuel or "flex-fuel" blends up to 85 % in gasoline in Europe. With this high percentage, ethanol vaporization as fundamental process in combustion phenomenon must be correctly characterized. When compared to gasoline, with a higher latent heat of vaporization, ethanol can cause difficulties at start-up in cold conditions. With a lower heat value, it is necessary to ignite richer mixtures that are unfavourable for pollutant emissions. The introduction of engines that can be fuelled by gasoline, ethanol or a blend of both at any appropriate proportion are frequently called ''flex-fuel", and became attractive because car owners do no longer depend on ethanol price and market availability. Ethanol also leads to azeotropes with light hydrocarbon fractions and can lead to volatility issues. Ethanol and 1-propanol are also miscible with wa-ter, which can cause demixing issues when blended with hydrocarbons. The high oxygen content in alcohols and its ability to oxidize into acetic acid provoke compatibility issues with some materials used in the engine, such as metals or polymers. Alcohol, especially 1propanol combustion in engines encourages formation of aldehydes emissions, which can have a negative impact on health (Koshland, (1994)). Aldehydes also play an important role in formation of photochemical smog.

The historical background of alcohols in internal combustion engines

Alcohols particularly ethanol has been known as a fuel for many years. In fact, when Henry Ford designed the Model T in early 19th century, his expectation was that ethanol; made from renewable biological materials would be the most important fuel for automobile. Nevertheless, gasoline emerged as the dominant transportation fuel in the early twentieth century because of two main factors; ease of operation of gasoline engines with the materials then available for engine construction, and a rising supply of cheaper petroleum from oil field discoveries all around the world. Nevertheless gasoline had many drawbacks and less attractive properties as an automotive fuel. It had a lower octane rating than ethanol, was much more toxic (particularly when blended with tetra-ethyl lead and other compounds to enhance octane rating), and emitted harmful air pollutants. Gasoline was more likely to explode and burn accidentally; gum would form on storage surfaces, and carbon deposits would form in combustion chamber. Petroleum was much more physically and chemically varied than ethanol, requiring intricate refining procedures to ensure the manufacture of a consistent ''gasoline'' product. Because of its lower octane rating relative to ethanol, the use of gasoline meant the use of lower compression engines and larger cooling systems. Diesel engine technology, which developed soon after the emergence of gasoline as the dominant transportation fuel, also resulted in the generation of large quantities of pollutants. However, despite these envi-ronmental flaws, fuels made from petroleum have dominated automobile transportation for the past three-quarters of a century. According to [START_REF] Agarwal | Biofuels (Alcohols and Biodiesel) Applications as Fuels for Internal Combustion Engines[END_REF] For decades, regulations have been made and implemented to provide a 'greener' and 'cleaner' environment towards the usage of conventional fossil fuels. Engine requires certain minimum levels of octane to run smoothly and to resist knocking. Aromatics and alcohols have been the most popular choices. Aromatic compounds, such as benzene and toluene are known to have higher octane levels, but the presence of these compounds produces more smoke, smog, as well as benzene [START_REF] Surisetty | Alcohols as alternative fuels: An overview[END_REF]. The US Environmental Protection Agency, EPA has approved using several alcohols and ethers in unleaded gasoline. For years, ethanol and methyl tertiary butyl ethers (MTBE) are the two most popular additives. According to [START_REF] Rasskazchikova | Ethanol as highoctane additive to automotive gasolines. Production and use in Russia and abroad[END_REF], the use of ethanol as high octane additive has been justified despite its higher cost due to its low toxicity, reduced environmental pressure when burning ethanol-containing fuel and a fact that it was produced from renewable sources.

Due to the phase out of lead in all gasoline grades and the adverse health and environmental effects of MTBE, the synthesis of higher alcohols, from synthesis gas has attracted considerable interest. Low molecular weight alcohols such as ethanol have replaced other additives as octane boosters in automotive fuels. Adding alcohols to petroleum products permits the fuel to combust more completely due to the presence of oxygen, which increases the combustion efficiency and reduces air pollution. However, the presence of alcohols in fuel can cause corrosion to metallic fuel system components. In order to make the best use of alcohols as alternative fuels; the engine or the vehicle can be redesigned; one or more additives to ethanol or methanol can be blended to improve its characteristics.

Using ethanol as a fuel additive to unleaded gasoline causes an improvement in engine per- mentally investigated the engine performance and pollutant emissions of commercial spark ignition (SI) engine using ethanol-diesel blends at various blended rates. Results showed that with increasing the ethanol content, the heating value of the blended fuels is decreased, while the octane number of the blended fuels increases. They also found that with increasing the ethanol content, the Reid vapour pressure of the blended fuels initially increases to a maximum at 10% ethanol addition, and then decreases. Results of the engine test indicated that using ethanol-gasoline blended fuels, torque output and fuel consumption of the engine slightly increase; CO and HC emissions decrease dramatically as a result of the leaning effect caused by the ethanol addition; and CO 2 emission increases because of the improved combustion whereas NO x emission depends on the engine operating condition rather than the ethanol content. Exhaust gas temperatures and lubricating oil temperatures were lower for ethanol diesel blends than mineral diesel. The engine could be started normally both hot and cold.

Costa and Sodre (2004) have studied the comparison of performance and emissions from a four-stroke engine fuelled with hydrous ethanol or 78% gasoline-22% ethanol blend. The results showed that torque and brake mean effective pressure were higher when gasolineethanol blend was used at low engine speeds. However, higher torque and brake mean effective pressure were achieved when hydrous ethanol was used at high engine speeds.

Alcohols such as methanol and ethanol have been studied extensively and they are used currently either as gasoline additives or pure fuel. Both alcohols however, have low energy densities, relatively high vapour pressures, and they are notably hygroscopic. On the other hand, saturated C 3 alcohols, namely n-propanol and iso-propanol have a better energy density and lower affinity for water compared to methanol and ethanol. Though both propanol isomers can be produced commercially via fermentation, currently they are produced largely from petrochemical feedstocks. Recently, [START_REF] Shen | E.coli for 1-butanol and 21-propanol production via keto-acid pathway[END_REF] and [START_REF] Atsumi | Directed evolution of Methanococcus jannaschii citramalate synthase for biosynthesis of 1-propanol and 1-butanol by Escherichia coli[END_REF] demonstrated techniques to produce n-propanol from glucose using bacteria Escherichia coli.

As such, there is growing interest in C 3 (saturated) alcohols as potential alternatives to lower molecular weight alcohols.

2.2 Isolated Liquid Fuel Droplet Vaporization

Liquid fuel droplet vaporization

Liquid fuel droplet vaporization is fundamental mechanism in spray combustion for various applications such as internal combustion (IC) engines, aerospace-propulsion engines and industrial burners. In these systems, fuel is injected into combustion chamber as a spray or jet. This spray or jet breaks down into droplets that evaporate due to the surrounding atmosphere forming a combustible mixture that ignites once appropriate conditions are achieved.

Numerous works on investigating the major spray characteristics (spray macroscopic and microscopic characteristics) have been carried on conventional fuels such as gasoline and diesel (2011)) as they influence the combustion efficiency and exhaust emissions. The evaporation process controls the combustion process that is eventually controls the design of combustion chamber and performance of engine. However it is highly necessary also to study the vaporization of a single droplet before completely characterizing spray vaporization and combustion. For many years, studies have been conducted experimentally and numerically in various environments to highlight the influence of the isolated parameters.

Quasi-steady Theory

The so-called 'quasi-steady' model was established by the work of Godsave (1λ53) and [START_REF] Spalding | The combustion of liquid fuels, 4 th Symposium[END_REF] c. The gas-phase quasi-steadiness: This assumption indicates that the gas-phase immediately adjusts itself to the local boundary conditions and droplet size at each instant of time.

d. The processes are considered as spatially isobaric; the pressure is equal to that of ambient.

e. The properties of transport in gas phase are constant. Such properties are thermal conductivity and calorific capacity. The number of Lewis must be equal to 1. That means the thermal diffusivity will always equal to the mass diffusivity.

(2-1)

where λ is thermal conductivity, ρ density, C p the specific heat per unit mass and D binary diffusion coefficient.

f. The change of the liquid phase: the mechanism of transport is quicker in the gas phase. At the instant time, at the surface of the droplet, there will be equilibrium between the liquid and the vapour where the saturated vapour pressure corresponds to the temperature at the surface of the droplet.

g. The effect of Soret and Dufour are negligible.

h. Absence of droplet-droplet interaction. Only an isolated droplet in an infinite oxidant ambient is considered. If the fuel vaporization rate at the droplet surface is equal to the consumption rate at the droplet surface, then [START_REF] Agarwal | Biofuels (Alcohols and Biodiesel) Applications as Fuels for Internal Combustion Engines[END_REF][START_REF] Al-Farayedhi | Experimental investigation of SI engine performance using oxygenated fuel[END_REF](4)(5)[START_REF]Close valve 1 and open valve 2, observe discharge[END_REF](7)(8)(9)[START_REF] Balakrishnan | Combustion of a fuel droplet in a mixed convective environment[END_REF][START_REF] Barata | Modelling of biofuel droplets dispersion and evaporation[END_REF][START_REF] Beerer | An experimental and kinetic study of alkane autoignition at high pressures and intermediate temperatures[END_REF][START_REF] Botero | Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures[END_REF] Substituting Eq. 2-13 into Eq. 2.-10 and integrating gives, [START_REF] Agarwal | Biofuels (Alcohols and Biodiesel) Applications as Fuels for Internal Combustion Engines[END_REF][START_REF] Al-Farayedhi | Experimental investigation of SI engine performance using oxygenated fuel[END_REF](4)(5)[START_REF]Close valve 1 and open valve 2, observe discharge[END_REF](7)(8)(9)[START_REF] Balakrishnan | Combustion of a fuel droplet in a mixed convective environment[END_REF][START_REF] Barata | Modelling of biofuel droplets dispersion and evaporation[END_REF][START_REF] Beerer | An experimental and kinetic study of alkane autoignition at high pressures and intermediate temperatures[END_REF][START_REF] Botero | Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures[END_REF][START_REF] Brett | Simulation of methane autoignition in a rapid compression machine with creviced pistons[END_REF] where d is the diameter of the droplet, d 0 is its initial diameter and K is the vaporization rate and t is the time. K is a constant and could be written in terms of binary diffusion coefficient as [START_REF] Agarwal | Biofuels (Alcohols and Biodiesel) Applications as Fuels for Internal Combustion Engines[END_REF][START_REF] Al-Farayedhi | Experimental investigation of SI engine performance using oxygenated fuel[END_REF](4)(5)[START_REF]Close valve 1 and open valve 2, observe discharge[END_REF](7)(8)(9)[START_REF] Balakrishnan | Combustion of a fuel droplet in a mixed convective environment[END_REF][START_REF] Barata | Modelling of biofuel droplets dispersion and evaporation[END_REF][START_REF] Beerer | An experimental and kinetic study of alkane autoignition at high pressures and intermediate temperatures[END_REF][START_REF] Botero | Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures[END_REF][START_REF] Brett | Simulation of methane autoignition in a rapid compression machine with creviced pistons[END_REF][START_REF] Canada | Fuel droplet burning rates at high pressures[END_REF] By utilise the hypothesis of Lewis Number equal to unity, the vaporization rate K could also be written in another form that consists mixture of thermodynamics properties, [START_REF] Agarwal | Biofuels (Alcohols and Biodiesel) Applications as Fuels for Internal Combustion Engines[END_REF][START_REF] Al-Farayedhi | Experimental investigation of SI engine performance using oxygenated fuel[END_REF](4)(5)[START_REF]Close valve 1 and open valve 2, observe discharge[END_REF](7)(8)(9)[START_REF] Balakrishnan | Combustion of a fuel droplet in a mixed convective environment[END_REF][START_REF] Barata | Modelling of biofuel droplets dispersion and evaporation[END_REF][START_REF] Beerer | An experimental and kinetic study of alkane autoignition at high pressures and intermediate temperatures[END_REF][START_REF] Botero | Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures[END_REF][START_REF] Brett | Simulation of methane autoignition in a rapid compression machine with creviced pistons[END_REF][START_REF] Canada | Fuel droplet burning rates at high pressures[END_REF][START_REF] Castanet | Heat transfer within combusting droplets[END_REF] From above equations, it is shown that the evolution of the squared-diameter of the droplet against time is linear under the 'quasi-steady' assumption.

The limitations of the theory

Eventhough the 'quasi-steady' theory or also called the d 2 -law is very successful in describing the gasification process of a fuel droplet, the hypotheses upon which the theory has been developed are subjected to several experimental and numerical analyses and also critisms due to its simplicity. The most controversial critisms which are already identified and constantly discussed are as follows:

b. Unity of Lewis number

The Lewis number is not always a unity and it keeps changing during the process of vaporization (Raghunandan and Mukunda, (1977); Udeotok, (2012)).

c. Spherical symmetry

This assumption is used in derivation of the d 2 -law, means that the convection either natural or forced is absent and therefore the droplet and the flow, temperature and species field surrounding it remain spherical during the entire droplet lifetime.

This assumption facilitates the analytical development. However, the assumption is no longer valid as under practical applications, the droplet deforms during gasifica-tion due to the pressure of natural convection or strong forced flow. Eventhough numerous experimental and numerical studies have been undertaken to verify the implications of this assumption, the results shows that the d 2 -law still holds even under convective flow conditions but under the condition that the droplet reaches its thermal equilibrium first. Nonetheless, a new improved method of suspended drop- The findings showed that the droplet is retaining its spherical symmetry shape by improving the method of the support fibre. Instead of single suspended fibre with a droplet hanging at the end of the fibre, the new method consists of two perpendiculars of fine quartz fibres. The droplet is positioning at the intersection of these two fibres. The details of this method will further discussed and elaborated at chapter 3 under experimental set up. By comparing to the existing literature, the effects of heat transfer are significantly minimized.

There are other affects known as 'thermocapillary effects' where the spherical symmetry is not only consist of the spherical shape but also due to the temperature distribution inside the droplet. Thermocapillary effects can modify the shape of inside of the spherical symmetry. Studies showed that the theoretical results are dissimilar with the surface which is free or not [START_REF] Wilson | The onset of steady Marangoni convection[END_REF]; Ha and Lai (2001)).

d. Soret and Dufour effects

Soret effect describes the flow of matter caused by a temperature gradient (thermal diffusion), while Dufour effect describes the flow of heat caused by concentration gradients. The two effects occur simultaneously. Both effects are believed to be small in most cases although sometimes their contribution may be significant (Coelho and Silva, (2002); Postelnicu, (2004); Gopalakrishnan and Abraham,

e. Quasi-steadiness

Several studies have shown that the deficiency of the d 2 -law is a result of the nonsteadiness of the gas-phase surrounding the droplet or of the vapour accumulation near the droplet interface (Law et al., 1980;Waldman, 1975;Law and Faeth, 1994).

However, the effects of this assumption on the d 2 -law are not yet completely recognized (Faeth, 1983).

The Experimental Techniques and Methodology

There are a few known techniques or methods available in determining the liquid fuel droplet vaporization experiments. The most known techniques are as following:

a. A porous sphere with liquid fuel being fed to its interior at such rate that the surface is just wetted to support combustion; b. A free-fall single droplet or droplet stream eventually levitating; c. Acoustic levitation d. A single droplet suspended at the end of a thin quartz fibre.

Each technique has been utilised for so many years and impose its own merits and limitations.

Porous sphere

The porous sphere experiment is an accurately steady-state and thus is the one that most closely conforms to the steady-state assumption of the d 2 -law [START_REF] Godsave | Studies of the Combustion of Drops in a Fuel Spray, the Burning of Single Drops of Fuel, 4 th[END_REF]; Williams, (1973)). In combustion experiment, it allows detailed probing of the flame structure [START_REF] Canada | Fuel droplet burning rates at high pressures[END_REF].The main downsides of this method are the excessively large size of drop-let and the deterrence of observing certain transient phenomena which is inherently present in droplet combustion. In this method the diameter of the sphere made up of an inert porous material is maintained constant during combustion. Fuel is supplied to the surface of the sphere at a rate equal to the rate of its combustion which depends on the diameter of the sphere and ambient conditions. In recent years, a number of studies employing this method have been carried out on measuring burning rates of liquid fuel. out experimental study using porous sphere technique to determine the burning rates of ethanol and ethanol-blended fossil fuels. They found that the mass burning rate of fuel increases with sphere size and air velocity, and when water is added to ethanol, the mass burning rate decreases. For ethanol blended with diesel, the mass burning rate does not vary significantly.

For ethanol blended with gasoline, the mass burning rate increases with increasing gasoline content due to higher volatility of gasoline.

Free-fall droplets

Meanwhile, free droplets experiments offer the advantages of small sizes, non-interference from suspension fibre and the capability of using volatile fuels. Nevertheless, this method provides more complex and elusive experimental methodology. Moreover, as the droplets are not stationary, more additional equipment is required to obtain detailed photography. The free fall motions also entail that the intensity of forced convection is continuously changed as the droplets size is incessantly diminished. A number of studies have employed this technique in investigating the vaporization and combustion behaviour of hydrocarbon and alcohols droplets. One of the earliest experimental studies have been carried out by [START_REF] Kumagai | Combustion of free fuel droplets in a freely falling chamber[END_REF] in developing a successful technique for achieving spherical combustion of free fuel droplets under a zero-gravity condition in a freely-falling chamber. It was followed by an improved experimental apparatus set up by [START_REF] Okajima | Further investigations of combustion of free droplets in a freely falling chamber including moving drplets[END_REF] showed that the evaporation distance and the velocity of the droplets decreases with elevated pressures. A comparison with theoretical calculation showed an excellent agreement of the measured results.

Acoustic levitation

The acoustic levitation of droplets is a valuable tool for studying heat and mass transfer at the droplet surface because it allows steady droplet positioning. However, acoustic levitation results in an acoustic streaming near the droplet surface (which may affect the heat and mass transfer rate). A key element of the heat and mass transfer processes at the surface of levitated droplets is the acoustic streaming. This technique has been actively pursued by researchers especially from University of Erlangen-Nurnberg (Yarin et al. (1999) and Yarin et al. (2002)). The acoustic levitation of single droplets is a recent development that attempts to avoid some of the errors intrinsic to the previously-used experimental methods of droplet free flight, free fall or pendant suspension on a capillary or filament.

Suspended droplet on support fibre

The fourth method, which is the main technique applied in this study is the suspended droplet experiment. First advantage of this method is the easiness in set up and performance.

As the droplet is motionless, the detailed cine-microphotography could be taken of its evapo- the ambient pressure increased, the evaporation is also increased. However, at high temperature and higher ambient pressure, evaporation rate is increased to a maximum value around 2.0 MPa and then decreases.

Influence of gravity and convection

Most non-convective droplet evaporation experiments have been conducted at normal gravity. The presence of natural convection enhances the evaporation slightly for low pressure but significantly strong at high temperatures [START_REF] Ristau | Theoretical and experimental investigations on droplet evaporation and droplet ignition at high pressures[END_REF]). However, droplet evaporation experiments at microgravity have been carried out from atmospheric pressure to supercritical pressure. These experiments are significant not only for microgravity applications but serve as comparison bases for accuracy test of evaporation models. The purpose of creating a microgravity environment condition for droplet vaporization and combustion is to remove the influence of buoyancy. The aim is to create a situation in which the evaporation induced or Stefan velocity is much larger than the relative velocity between the droplet and ambience that created either by buoyancy or a forced convection. Burning behaviour of a suspended noctane droplet under both normal and microgravity fields has been studied experimentally by Sato (1990). The studies concerned the effects of natural convection at high ambient pressure levels up to four times the fuel critical pressure. Experimental results showed that the burning rate constant increases with the increase of the ambient pressure at subcritical pressures and decreases at supercritical pressures for both microgravity and normal gravity fields. That means the natural convection increases the burning rate constant and its effect become stronger as the ambient pressure increases. [START_REF] Nomura | Experimental study on high-pressure droplet evaporation using microgravity conditions[END_REF] studied the evaporation of suspended n-heptane droplet under microgravity in a closed chamber. Microgravity conditions were used to repress the effect of natural convection in the ambient gas. They studied the effect of temperature and pressure on the evaporation rate. They also studied the effect of these parameters on the heating and evaporation time. The effects of forced and natural convection were studied in isolation by [START_REF] Okajima | Experimental studies on combustion of fuel droplets in flowing air under zero-and high-gravity conditions[END_REF]. They used a free falling chamber provided with a wind tunnel. This was used to study the effect of forced convection The natural convection could be due to thermal or solutal buoyancy. However, according to Mandal and Bakshi (2012), these effects were insignificant as they found out that the internal circulation can be induced by a small temperature variation caused by the droplet evaporation. Therefore, the internal circulation is responsible for the enhanced evaporation rate in otherwise stationary environment in a closed chamber.

Their work showed that there is evaporation induced internal circulation within certain droplets while evaporating even under atmospheric conditions. This circulation enhances the evaporation rate significantly as compared to diffusion-driven evaporation. These findings seemed to agree with [START_REF] Hegseth | Natural convection in droplet evaporation[END_REF] on suspended methanol droplet experiment where the results showed that when a droplet evaporates sufficiently fast, it exhibits a vigorous inte-rior flow. This flow was driven by surface tension gradients. [START_REF] Chauveau | An Analysis of the d 2 -Law Departure during Droplet Evaporation in Microgravity[END_REF] have carried out experiments of n-decane droplet vaporization under both normal and microgravity in stagnant hot atmospheric environment. By using the improved 'cross-fibre' suspended droplet technique, the results showed that for ambient temperature below 950 K, deviation from the d 2 -law is observed during droplet vaporization in microgravity condition. However, for temperatures beyond 950 K, the experimental results demonstrate that the d 2 -law holds throughout the entire lifetime. Based on their results, they concluded that microgravity condition is not necessarily guarantee that the d 2 -law holds during droplet vaporization. These observations are based on their argument that once the flow radial velocity is attained beyond the critical velocity, the effect of natural convection becomes unimportant as the corresponding radial evaporation characteristic velocity becomes significantly influential.

Influence of external heat transfer

As our study is concerned with the vaporization of a single droplet suspended by a support fibre, therefore it is essential to discuss the effect of heat transfer conduction from the support fibre. In the early work for high pressure droplet evaporation under natural convection, [START_REF] Kadota | Evaporation of a single droplet at elevated pressures and temperatures[END_REF] have considered the effects of fibre conduction and liquid-phase radiative absorption in simplified manner. They evaluated the fibre conduction with a simple one-dimensional steady-state analysis where the radiative analysis was assumed to occur on the droplet surface. Eventhough it was only a qualitative significance, their calculations indicated enhancement on the evaporation rate. Shih and Megaridis (1995) have numerically analyzed the effect of fibre conduction on droplet evaporation under forced convection. They have observed that for a fibre parallel to the flow direction, only small enhancement of evaporation was found. Less heat input through the fibre is resulted. [START_REF] Avedisian | Soot patterns around suspended nheptane droplets in a convection free environment[END_REF] have observed the effect of support fibre on the soot patterns for droplets burning in a stagnant ambience in reduced gravity. They have observed the nonlinearity in the variation of d 2 /d 0 2 due to the influence of fibre. The soot that aggregated forming inside the flame was also found to be evolved into nonsymmetrical. The effect became more significant with thicker fibre. A study to investigate the effect of heat conduction through the support fibre on evaporation of a droplet in weakly convective flow was initiated by [START_REF] Yang | On the discrepancies between theoretical and experimental results for microgravity droplet evaporation[END_REF].

A droplet of n-heptane or n-hexadecane was suspended at the tip of a horizontal or vertical fibre in an upward hot gas flow. In general, they found that the heat conduction through the fibre enhances the evaporation, with a stronger effect for a lower gas temperature and a thicker fibre. Also, the evaporation rate is enhanced in an oxygen-containing gas flow due to the additional heating from the oxidation around the droplet.

Conclusions

Temperature and pressure play a significant role in effecting the behaviour of vaporization of isolated droplet fuel. However, previous studies on most alkanes droplet showed that the later has less impact on the vaporization rate. Studies also showed that the natural convection is significantly noticeable at higher temperatures conditions.

In our study, all experimental works are carried out at normal gravity and ambient atmospheric pressure. Eventhough the gravitational force and ambient pressure play a significant role in influencing the vaporization rate of the droplet; it will not be covered in this study.

Concerning the effect of external heat transfer from the supporting fibre, Chauveau et al.

(2008), in their studies of the effects of heat conduction through a support fiber showed that their results obtained are important in the sense that they make it possible to clearly show that the effects of the suspension fiber can be very important and can even hide the benefit of the experiments conducted in reduced gravity if the fiber has a too important size. Taking into consideration these results, it would be thoughtful to revisit the studies carried out previously on the effect of the pressure on the droplet vaporization rates, because all these experiments were conducted with fibers having large sizes, introducing therefore the possibility of a systematic over-estimation of the measured vaporization rates. The effect of the droplet suspending technique such as buoyancy and natural convection is eliminated by using a novel cross micro-fiber system. This technique enables to preserve the spherical shape of the droplet throughout the vaporization process in normal gravity atmosphere. As this study involved a single value of ambient pressure i.e. at normal atmospheric pressure, thus the influence of natural convection which is dependent on elevated pressures is negligible.

The heat conduction from the suspended fibre seems to play a big role in influencing the vaporization of the droplet. The 'extra' heat is used to enhance the vaporization rateν therefore the actual vaporization rate is not achievable and overestimated. Studies showed that the 'extra' heat transfer from the fibre increases with the thickness of the fibre. Fortunately, an

5 Vaporization and combustion issues and challenges of droplet alcohols

Recent concern regarding environmental issues due to hydrocarbon fuel has intensified the interest in alternative fuels such as alcohols. However, due to the latent heat of vaporization properties of alcohols that is relatively higher than conventional hydrocarbon fuels, there exist some significant worries over their vaporization efficiency and therefore the heterogeneity and uniformity of the fuel/air mixture for combustion process. Alcohols are also known to possess higher miscibility property with water. Therefore, it is our main concern in this study to properly characterize the extent of this property to the vaporization behaviour of alcohols. ethanol-water droplet under three different ambient conditions. The results showed that the decrease of surface concentration of ethanol is fastest in the case of the hot nitrogen blowing over the surface of the droplet. Also, the slow convection at atmospheric temperature was found to enhance the rate of evaporation of ethanol eventhough the total rate of evaporation is not significantly changed.

Conclusions

Most of the experimental studies of the vaporization behaviour on the hydrogenated fuels such as alcohols are always correlated with the interference of water existence. The higher miscibility with water and the hygroscopic nature of alcohols seem to change the overall performance of alcohols fuels in terms of vaporization and combustion characteristics. Most of the observations and findings from the literature suggested that alcohols droplets have freely absorbed water from the environment and subsequently dissolved therefore changing the vaporization behaviour of ethanol vaporization. However the extent to which these water impact on actual vaporization behaviour of alcohols is still lack in literature.

Therefore, in our present study, experimental results concerning vaporization behaviour of both lower and higher molecular weight alcohols; ethanol and 1-propanol droplets respectively are presented. A new and wide range of temperatures is covered in this work. The d 2 -law will be further examined and the histories of the instantaneous vaporization rates will be presented to have more comprehensible understanding on the actual vaporization behavior of alcohols. The so-called impact of water vapour on the overall alcohols droplets vaporization behavior is also investigated. To further emphasis on this issue, a quantitative measurement of water content during vaporization of alcohols is carried out. Due to the lack of experimental data in the literature, the "quasi-steady" model calculation has been used in order to compare with our experimental results.

Alcohol blends vaporization

Apart from the study on pure alcohols, blends of alcohols with diesel oil or fuel oil have been anticipated for a number of applications in engines and combustion appliances. The addition of some alcohols to a hydrocarbon fuel allows some use of energy from renewable sources without seriously changing the characteristics of the fuel and also it reduces pollutant emissions. However there are some limitations on alcohol-hydrocarbon mixtures. The limited miscibility of the components is the major constraint. Alcohols also readily absorb water, out experimental study to determine the burning rates of ethanol and ethanol-blended fossil fuels. They found out that the fuel mass burning rate increases with sphere size and air velocity, and when water is added to ethanol, the mass burning rate decreases. For ethanol blended with diesel, the mass burning rate does not vary significantly. For ethanol blended with gasoline, the mass burning rate increases with increasing gasoline content due to higher volatility of gasoline.

which

Conclusions

Most commonly, alcohols are used as a pure fuel or blended with either gasoline or diesel.

As our studies concern the vaporization behaviour of alcohols and the effect of water vapour during the process, the results and the data are imperative for the future works of alcohol/hydrocarbon blends especially in internal combustion engines applications.

Autoignition Studies

Autoignition process is defined as a spontantaneous process where a mixture of fuel and air undergoes a chemical reaction leading to ignition and combustion without the aid of external sources such as a flame or spark. Generally, autoignition is always associated with ignition delay time (IDT), and usually measured in milliseconds. Ignition delay time is a key characteristic of a fuel which indicates its relative reactivity at given conditions. Common definition of ignition delay time is the time period between the creation of the combustible mixture (at the end of compression) and heat release (Figure 2.3). If a fuel/oxidiser mixture is compressed and heated to temperatures and pressures high enough to allow combustion to occur, ignition is not instantaneous but instead takes place after some time later. During this period, the fuel molecules decompose (initiation) and react chemically with the oxidiser to produce reactive radical species. These radicals then undergo a chain reaction with more fuel molecules producing more and more reactive radical species (propagation), resulting in the exponential growth of what is termed the "radical pool" (chain-branching). When the radical pool becomes of critical mass, the remaining fuel fraction is consumed instantaneously (chain-terminating) leading to an explosive release of energy (ignition). Ignition delay measurement is significantly important especially for the design of superior performing engines, as well as for gas turbine design and chemical kinetics research. The main key to improve the efficiency of internal combustion engines is the understanding of the chemistry that takes place when a fuel burns. To predict the chemistry of fuel oxidation at wider range of temperatures and pressures requires a complete qualitative and quantitative characterization of these chemical reactions. Nonetheless, to study the details of fuels chemistry in an internal combustion engine is impractical, not uncomplicated and not effortless since they involve turbulent reactive flows that are complex to analyse or repeat under controllable conditions. Its environment is also plagued by varying conditions of temperature and pressure, combined with intricate fluid motions (laminar, transitional or turbulent). To overcome some of the challenges in predicting the chemistry of the fuel oxidation, some simplified experimental laboratory devices offer an alternative to complex engine environments.

They eliminate some of the complexities that exist in real engines but at the same time preserve the ability to work efficiently under engine-relevant conditions.

The option of simplified experimental devices is restricted by the range of temperatures and pressures at which they can operate; and only the RCM and shock tube can reach enginerelevant temperatures and pressures rapidly enough and yet endure the high pressures that transpire after the ignition event.

Shock tube

Shock tubes are usually used at higher temperatures (T > 1000 K) due to their capability to rapidly bring the mixture to test conditions. In the shock tube, the premixed gas is heated by a shock wave in approximately 1 ns to pre-selected temperatures and pressures. The shock wave is usually generated by the rupturing of a diaphragm that separates two sections containing high-pressure and low-pressure gas respectively. Typically reflected shock tempera-ture and pressure range in the tube shock are 1000-3000 K and 1-17 atm. However, the timescale behind the shock wave is very short; normally fall between 10-1000 µs, which limits the test period, and therefore unfit for testing at lower temperatures (Crossley et

Rapid Compression Machine (RCM)

Meanwhile RCMs have been widely used in the low to intermediate temperature region

Conclusions

Studies show that the choice of simplified experimental devices is limited by the range of temperatures and pressures at which they can operate. It is also observed that only the shock tube and rapid compression machine (RCM) can reach engine-relevant temperatures and pressures rapidly enough and at the same time withstand the high pressures that occur after the ignition event. The shock tube is known to be accommodating for the study of hightemperature and high-pressure reactions, while intermediate and low-temperature reactions can be studied at various ranges of pressures in rapid compression machine. Both devices provide useful data as they could present significant data on ignition delay time of reactive fuel gases.

Development of Autoignition Studies on Alcohols

To better assess the effect of the presence of these new molecules in fuels on the engine ef- bar, with equivalence ratio, φ varied from 0.25 to 2. In general, the results showed that an increase in total pressure was accompanied by a uniform decrease in the observed ignition delay for any particular reaction mixture. The results showed that the observed delay time decreases as the initial reactant concentration increases.

As a continuation from their first experimental studies on ethanol oxidation, Dunphy et al. water. Stable HCCI operation was obtained for fuels containing up to 40% water. Incomplete combustion and excessive intake temperatures limited the operating range at higher water concentrations. The maximum value of the cumulative heat release profiles decreases with an increase in water concentration. Exhaust emissions data is also presented and discussed. Hydrocarbon and carbon monoxide emissions tend to increase with increasing fuel water content while NO x levels are low, which is typical in HCCI engines.

Conclusions

The autoignition of alcohols has been studied by numerous authors either in shock tubes or rapid compression machines (RCMs). The experimental works either in shock tube or RCMs have been structured to adapt the realistic conditions in internal combustion engines. The developments of autoignition study on oxygenated fuels have attracted interests especially on alcohols. Starting with the lower molecular weight alcohols such as ethanol and methanol, the development was already extended to heavier alcohols, namely n-propanol and n-butanol.

The experimental results of various mixtures have provided the data for future improvement of kinetics mechanisms. In our current work, the effect of the impact of water addition to alcohols autoignition behaviour is wholly initiated by the lack of the study concerned in the literature.

Conclusions

In the first part, the literature review underlines the important results and findings concerning the vaporization of an isolated single droplet. The key assumptions that govern the 'quasisteady' theory have been conferred, with the lack or supports have been identified. The influ-ences of various parameters on vaporization of alcohols have been discussed to give a clearer view on their impact to vaporization behaviour. However, our main concern in this study is the miscibility property of alcohols with water; therefore the effect of water addition to the vaporization of alcohols especially on ethanol is really significant and required further investigation. The effects of water on overall vaporization of droplet ethanol will be explored in details in terms of d 2 -law compliance and the effects on instantaneous vaporization rate at various ambient temperatures.

In the second part of the review, the results and findings concerning the autoignition characteristics study have been emphasized. The common typical devices for the autoignition study; i.e. shock tube and RCMs was briefly explained. The developments of autoignition study on oxygenated fuels have been considered. Nonetheless, there is still a lack of autoignition study on the effect of the impact of water addition to alcohols. Kinetics mechanism from literature will be applied to capture the impact of water addition. Sensitivity analysis will be employed so that the actual impact of water addition in ethanol autoignition behaviour is known and discovered.

In the next chapter, a detailed interpretation of the vaporization of an isolated droplet has been carried out in this experimental study aimed at investigating ethanol and 1-propanol droplets. An alcohol droplet is located at the intersection of the cross quartz fiber with a controlled initial at various ambient temperatures and at atmospheric pressure. The real impact of the water concentration on the vaporization rate of an ethanol droplet in a wide range of temperature will be thoroughly examined.

The autoignition experiments of ethanol, 1-propanol and blends of ethanol and water have been carried out in a rapid compression machine (RCM).

EXPERIMENTAL STUDIES

3.1 Experimental studies of an isolated droplet of alcohol vaporization

Experimental Apparatus and Equipment

The main apparatus that has been utilized in this study is known as Multi User Combustion Chamber, courtesy of CNRS, Orleans, France. The MUCC has been developed in year 1988

for the purpose of combustion test. It has undergone a number of modifications since then and has been utilized for many applications. Under its actual configuration, the MUCC allows the experimental study of evaporation for various combustible fuels even in poor oxygen environment. It is possible to carry out evaporation experimental works either a single droplet or a series of several droplets (up to 9 droplets). It also permits us to carry out experiments with different criteria or parameters such as droplet size, gaseous nitrogen pressure and temperature in the chamber. Every experimental result could also be recorded in a video form. Therefore we are able to determine the time and speed of droplet evaporation at various configurations.

The experimental apparatus MUCC (see Appendix A-1 for more details) utilised in characterising the mechanism of isolated droplet vaporization are composed of these three main elements:

a. Under-pressure chamber that consists of furnace, mobile piezo-electric injector (three motors that allow the displacement of frame in three dimensions), a moving frame, and other support equipments.

b. Programmable Logic Controller (PLC) and temperature operation panel that control the temperature, pressure, lighting and the cooling system of the furnace and its inverter.

c. A computer to permit the supervision of the experiments, post-processing data such as data collection and data analysis.

Cross Fibre Technique

The cross-fibre technique is a novel method in investigating the vaporization of an isolated droplet in a closed chamber. It is an 'improved' version of a conventional droplet suspended method (Figure 3.3). Instead of a single support fibre, it consists of two fine intersecting quartz fibre of 14 µm thickness each (Figure 3.4). The droplet under investigation will be formed at the intersecting point of the two perpendicular quartz fibres.

The droplet injection on the frame is carried out in a region of the vaporization chamber which is located at the lower part of the chamber at ambient temperature, called the 'cold zone' in order to avoid any pre-vaporization before the start of the experiment. A piezoelectric injector is utilized to generate the droplet, by supplying a monodisperse liquid jet impacting the support. Once the droplet of controlled initial diameter (300 -600 µm) is formed on the intersection of the quartz fibres, it is then introduced into the furnace by the aid of mo-torized displacement system. The average total transfer time measured from the lower region of the chamber into the furnace is about 700 ms. As soon as the droplet is exposed to the hot environment in the furnace; the temporal evolution is recorded using a high-speed video camera with various frame rates from 20 to 400 fps dependent on the ambient temperature. Nitrogen (99.95% purity) fills the medium of the furnace to allow pure vaporization and to avoid any oxidation or ignition to occur particularly at elevated temperatures.

Purging of pressure chamber

To ensure pure vaporization and to prevent any droplet combustion to occur during experiment, a high level of pure nitrogen is essential. Therefore, it is necessary to conduct a purge to the pressure chamber. The purge is performed during heating of the furnace. In a worst case, the pressure chamber will be filled with atmospheric air. The air is commonly composed of approximately 21% of oxygen (O 2 ) and 78% nitrogen (N 2 ). However, to run the experiments only gaseous Nitrogen is needed in the system. The principle and procedure of purge is simple. It consists of depressurization and pressurization of air in the chamber.

The percentage of oxygen is reduced by half in each purge. Therefore it is possible to say that after four successive purges, the oxygen percentage will reduce to approximately 1.3%.

(And indeed, in some cases, the pressure chamber is filled with nitrogen with a small proportion of tiny fuel residual injected during experiment).

Materials and Fuels

Alcohols such as anhydrous ethanol with high purity GC grade (Sigma -Aldrich contain 99.6% of ethanol), and standard ethanol (in this study we refer this ethanol as ethanol 95%) that contain approximately 5 % water (Ethyl Alcohol 96.2 Re Puro by Carlo Erba) and 1propanol (Sigma-Aldrich) are used for these experiments. Ethanol (CH 3 CH 2 OH) is a lower molecular weight alcohol; its molecular structure shows a polar fraction due to the hydroxyl radical and a non polar fraction in its carbon chain. Due to its short carbon chain, the properties of ethanol polar fraction overcome the non polar properties. That explains the hygroscopic nature of ethanol. Conversely, 1-propanol (CH 3 CH 2 CH 2 OH) is an alcohol which is having almost equally both polar and non polar fractions in its molecules. However, the polarity frac-tion in 1-propanol molecules is fewer comparatively to ethanol due to its longer carbon chain. 1-propanol also exhibits higher boiling temperature than ethanol.

The physical and chemical properties of ethanol and 1-propanol are presented in Table 3.1. In all experiments, the pressure in the furnace is kept at atmospheric at P ∞ = 0.1 MPa, while the ambient temperature is varied from 298 to 973 K. The homogeneity of the temperature is controlled by three thermocouples K-type placed inside the furnace. The ambient relative humidity is measured by VAISALA HMT333 Humidity and Temperature Transmitter. For each experiment set, a minimum of 700 images are captured and recorded to permit sufficient temporal resolution with at least six experiments performed for each test condition.

Experimental Instability

The instabilities occur during the motion of droplet from lower chamber to the furnace (Figure 3.5). This movement is accompanied by vibrations and subsequently induces oscilla-tions. It was decided then to show the evolution of surface area of the vaporized droplet only when the droplet was stabilized.

Computations and Post-Treatment of the Data

The images captured by the high speed video camera are transferred to a computer and analysed by post-processing to deduce the droplet instantaneous surface area and hence its diameter temporal variations.

Visualization of the vaporization phenomena is carried out using a fast video camera (Phantom v5). The vaporization sequence is first recorded in the camera memory and then transferred on the hard disk of the acquisition computer. The maximum frame rate of the camera at full resolution of 1024 x 1024 pixels is 1200 fps.The objectives coupled to the camera allow getting a resolution of approximately 9.8 m /pixel.

The first step of the treatment is to convert the film obtained in series of consecutive images. To perform this task, the software Cine Viewer 640™ is used.

Then, we define a grey level threshold (usually at nominal value, S = 70/256) in order to "binarize" the image. These binarized pictures represent the droplet projected surface. From these pictures, we can extract an equivalent diameter for each drop. These dimensions in pixels are converted into metric sizes by knowing the resolution of the optical system, obtained by in-situ calibration.

Estimation of the Droplet Size Measurement

It is possible to determine the droplet size directly from the image captured. As the coordinates of the image are in pixels, one can deduce the droplet size as the conversion of pixel to the width is known (1 pixel corresponds to 9.8 micrometer).

Measurement of Uncertainties and Experimental Reproducibility

The method for determining the droplet diameter mainly includes two sources of errors; the first is the conversion factor from pixels to actual measured size in mm. To determine this factor we use a calibration target types USAF that has been positioned in place of the droplet.

The manual operation causes a pointing accuracy of ± 1 pixel on each of the lines of sight to give a conversion factor of 9.5 µm/pixel of ±0.06. The rod (0.5 mm square) had eight squares in X and Y direction. A standard measurement (Photoshop) gave us H = 421 pixels and L = 420 pixels with a magnification factor of Gr = 9.5 µm /pixel. If now we consider a pointing error of 1 pixel outward in each direction we obtain 423 pixels in H and L where the magnification factor, Gr = 9.46 µm/pixel. If we now consider a pointing error of 1 pixel inward in each direction, we obtain 418 pixels in H and L where Gr = 9.57 µm/pixel. Therefore, the magnification factor is defined as, Gr = 9.5 ± 0.06% µm/pixel.

The second source of error is from the automatic treatment of images. To determine the surface droplet in squared-pixel, it is necessary for the binarization of the droplet and the bottom. However, this procedure uses a threshold value which is a constant for any given sequence of vaporization. A variation of the voluntary value of this threshold (± 20 grey level) around its nominal value (S = 70/256) causes a variation of ± 3.5% in the droplet diameter of a 450 µm initial diameter of a droplet. However it should be noted that this error is not constant throughout the life of the drop, and significantly increases at the end of droplet lifetime (> 10%). If a real calculation of uncertainty is taking into account the possible error in the beginning and at the end of life of the droplet, with addition on the magnification and the With the images treatment utilized in this study, the vaporization rates are obtained between 3 and 5% of error depending on the size of the initial diameter of the droplet. With one manual analysis of the images, we could not obtain a good precision result. However, with our treatment technique, we could achieve a considerable time of treatment with sufficient images to analyze. In our present experimental work, minimum of 700 images are captured and recorded for each experiment with at least 6 repetitive experiments performed for each test condition to permit sufficient temporal resolution.

Concerning the variability and reproducibility between the different tests, mean and standard deviation of the average vaporization rate for all test runs at each condition are calculated. Figure 3.7 shows the measurements of the droplet vaporization rate of 1-propanol at T ∞ = 673K were repeatable within ± 2%. In this section a detailed description of experimental studies on autoignition behaviour of ethanol, propanol and ethanol/water mixture will be given. The experimental works have been carried out using a rapid compression machine (RCM) courtesy of Chemistry of Combustion Centre, National University of Ireland, Galway (NUIG), Ireland (Director: Dr. Henry Curran).

Experimental Apparatus and Equipment

A rapid compression machine (RCM) is a device designed to perform the compression stroke of a reciprocating Diesel engine, such that the autoignition of fuels may be studied un- The procedure for performing an experiment with the RCM is briefly described below, with details in Appendix A-2.

Mixture preparation

In this study test mixtures are prepared in one of the three stainless steel mixing tanks using standard manometric methods. Oxygen and diluents gases are obtained from BOC Ireland Ltd. and are presented in Table 3-4 and are used without further purification. All fuels are obtained from Aldrich Chemical Co. Ltd. and are presented in Table 3- Each compressed temperature condition is repeated at least three times to ensure reproducibility.

The reaction chamber is fitted with pressure and temperature sensing devices to measure the initial conditions in the reaction chamber. A pressure transducer (Kistler type 603B, SN 51740) is mounted flush with the reaction chamber wall and is used to monitor the change in pressure inside the reaction chamber during compression and any post-compression includeng ignition. The pressure experienced by the transducer is recorded as a charge signal.

This signal is sent to a charge proportional amplifier (Kistler type 5001) where it is amplified to a known setting determined by the user, before being recorded as a voltage-time profile by an oscilloscope (Nicolet TDS). For all experiments with the rapid sampling chamber the charge amplifier is set to 20 Mechanical Units/Volt and the transducer sensitivity is set to 4.684 pC/V, as calibrated by the manufacturer for this specific pressure transducer (SN 51740). The temperature is measured by thermocouple type-J. There is first a characterisation of the inside gas temperature as a function of the wall temperature. This is then used in order to determine the initial gas temperature before the shot.

Determination of gas compressed temperature, T c

The adiabatic compression/expansion facility in the application GasEq (C. Morley, http://www.gaseq.co.uk/) is used to calculate the initial test gas pressure required to reach a specific compressed gas temperature, T c . To do so GasEq requires the initial mixture composition, the values of initial pressure, P i and initial temperature, T i as well as an accurate value of P c. The compressed temperature T c can be defined from adiabatic process as;

(3-3)
where is the ratio of specific heat at constant pressure (C p ) to specific heat at constant volume (C v ),

(3-4)

Experimental Operating Conditions

The details of operating conditions for autoignition experiments for alcohols and alcohol/water mixture have been performed using a NUIG rapid compression machine and are presented as below (Table 3-6): neous surface area and hence its diameter temporal variation. Note that the error analysis in determining the droplet diameter is calculated to be around 3%.

The study of autoignition of ethanol, 1-propanol and blends of ethanol and water have been performed in a rapid compression machine (RCM) at a compressed pressure of 30 bar over a temperature range of 750-860 K for stoichiometric mixture of fuel and air. The thermodynamic conditions are pertinent to those encountered in internal combustion engines. The experiments have been carried out in the twin piston at NUIG RCM. The compressed gas temperature was changed by adjusted the initial temperature. Fuel-oxidiser mixtures were prepared manometrically in stainless steel tanks. In present studies, each compressed pressure and temperature condition is repeated at least three times to ensure good reproducibility. The mean and standard deviation of the ignition delay for all test runs at each condition are calculated at less than 10%, as an indication of reproducibility.

THEORETICAL MODEL CALCULATIONS

A requirement of comparison between experimental and theoretical vaporization rate, K and K th respectively is realized in this chapter. Theoretical vaporization rate, K th is calculated by two methods; the first method based on the calculation of the binary diffusion coefficient, D, and the second method based on the estimation of the thermodynamic properties.

For these two methods we determine the vaporization rate based on the equations [START_REF] Agarwal | Biofuels (Alcohols and Biodiesel) Applications as Fuels for Internal Combustion Engines[END_REF][START_REF] Al-Farayedhi | Experimental investigation of SI engine performance using oxygenated fuel[END_REF](4)(5)[START_REF]Close valve 1 and open valve 2, observe discharge[END_REF](7)(8)(9)[START_REF] Balakrishnan | Combustion of a fuel droplet in a mixed convective environment[END_REF][START_REF] Barata | Modelling of biofuel droplets dispersion and evaporation[END_REF][START_REF] Beerer | An experimental and kinetic study of alkane autoignition at high pressures and intermediate temperatures[END_REF][START_REF] Botero | Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures[END_REF][START_REF] Brett | Simulation of methane autoignition in a rapid compression machine with creviced pistons[END_REF][START_REF] Canada | Fuel droplet burning rates at high pressures[END_REF] and (2-16) that have been discussed and derived from the Chapter 2 based on assumption of unity value of Lewis number, where the vaporization rate, K is defined as;

(2-15)

(2-16)

First method: calculation and estimation of binary diffusion coefficient

In this calculation of the first method, the vaporization rate is calculated from equation [START_REF] Agarwal | Biofuels (Alcohols and Biodiesel) Applications as Fuels for Internal Combustion Engines[END_REF][START_REF] Al-Farayedhi | Experimental investigation of SI engine performance using oxygenated fuel[END_REF](4)(5)[START_REF]Close valve 1 and open valve 2, observe discharge[END_REF](7)(8)(9)[START_REF] Balakrishnan | Combustion of a fuel droplet in a mixed convective environment[END_REF][START_REF] Barata | Modelling of biofuel droplets dispersion and evaporation[END_REF][START_REF] Beerer | An experimental and kinetic study of alkane autoignition at high pressures and intermediate temperatures[END_REF][START_REF] Botero | Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures[END_REF][START_REF] Brett | Simulation of methane autoignition in a rapid compression machine with creviced pistons[END_REF][START_REF] Canada | Fuel droplet burning rates at high pressures[END_REF]. We should determine all the unknown and known variables such as; ρ F the density of the fuel where the subscript of l and g corresponds to the state of fuel liquid and gaseous respectively D the binary diffusion coefficient B the Spalding transfer number This type of calculations was inspired mostly by [START_REF] Chesneau | Vaporisation et combustion de gouttes isolée de combustibles liquides-Influence de la pression[END_REF] and Morin (1999).

In the case of droplet vaporization, the derivation of conservation equations shows the existence of thermal transfer number B T and mass transfer number B M . These numbers are equal under the assumption of quasi-steady condition. The transfer number represents the ratio of the driving force for vaporization to the resistance to vaporization.

Expression of mass transfer number, B M

The mass transfer number is given by;

Expression of thermal transfer number

The thermal transfer number, B T is given as;

(4-8)

The calorific capacity is calculated according to the reference temperature, T r as defined by [START_REF] Sparrow | The variable fluid property problem in free convection[END_REF] where and the latent heat of vaporization at normal boiling temperature, L vTb is calculated based on the method suggested by Vetere [START_REF] Reid | The Properties of Gases and Liquids[END_REF]) and defined as;

(4- 13
)
where R is the perfect gas constant, T b is the normal boiling temperature; T c and P c are the critical temperature and the pressure respectively. For alcohols, the average absolute percentage of error between calculated (Vetere method) and experimental values of latent heat of vaporization at normal boiling point is estimated around 3.8% [START_REF] Reid | The Properties of Gases and Liquids[END_REF]).

Calculation of binary diffusion coefficient, D

In the case where experimental values are not available, such as ethanol-water gas diffusivity, ethanol-nitrogen gas diffusivity, and water-nitrogen gas diffusivity, the Chapman-Enskog correlation [START_REF] Reid | The Properties of Gases and Liquids[END_REF] was used to approximate the binary diffusivity coefficient of combustible fuel F in ambient O.

(4-14)
where T r is the reference temperature, P is the pressure, M is the molecular weight, is the characteristic length and  D , the diffusion collision integral.

This equation is derived directly from the resolution of the equation of Boltzmann, indicated for diffusion in a binary system.

M FO is given by the molar masses of components F and O;

(4-15)

The diffusion collision integral,  D , depends strongly on temperature and the intermolecu- lar forces between the collided molecules. The function of Lennard-Jones potential gives a good description of the transport properties where it relies on the intermolecular energy between these two molecules, ψ and their separation distance, r;

(4-16)
with ε and are the characteristic energy and length of Lennard-Jones respectively.

The integral collision depends on term defined as kT/ε AB , with k is Boltzmann constant and is given by the relationship of Neufield [START_REF] Reid | The Properties of Gases and Liquids[END_REF]); where k is Boltzmann constant.

The characteristic energy and length are defined by: The calculation of this method is using the similar methodology than the previous first method. By the assumption of equality of mass transfer number B M and thermal transfer number B T , the droplet surface temperature, T s is determined iteratively and used for the calculation of thermodynamics and transport properties of the combustible droplet. The main difference between the previous method and this method is the utilisation of the thermodynamics and transport properties of the mixtures.

The expression of transfer numbers, B M and B T (4-24)

where Y Fs is defined in terms of molecular weight and pressure as; where M O and M F are the molecular weight of the gaseous oxidant and liquid combustible fuel respectively. P sat is defined as the saturated vapor pressure calculated at the droplet surface temperature (equation 4-8). with

Estimation and calculation of thermodynamic and transport properties

The physical parameters in the droplet film region such as thermodynamics (C pg ) and transport properties (k g ) are evaluated at temperature defined as reference temperature, T r [START_REF] Hubbard | Droplet evaporation: effects of transient and variable properties[END_REF][START_REF] Lefebvre | Atomization and Sprays[END_REF];

(4-26)
where A r is the averaging parameter. For the one-third rule, A r = 1/3.

To calculate the specific heat of gaseous mixture C pg , one needs to calculate first the individual molar specific heat of the oxidant and the combustible fuel, represented by C pO and C pF respectively at reference temperature, T r and by utilizing the method of Joback.

However, in our study we define the molar fraction of combustible fuel as; where P amb is the ambient gas pressure.

Therefore by considering the concentration of combustible fuel droplet at infinity is zero, the molar concentration of oxidant will be;

(4-28)

The specific heat of the gaseous mixture of combustible fuel and oxidant C pg , therefore could be written off as; As mentioned before, the increase of average vaporization rate, K is influenced by the droplet surface temperature and binary diffusion. From the theoretical calculation, it demonstrates that as the temperature increases, both droplet surface temperature T s and the binary diffusion D, for both 1-propanol and anhydrous ethanol droplets also increase accordingly (Figure 4.1 and Figure 4.3). The value of the binary diffusion D is higher for anhydrous ethanol at all temperatures. On contrary, 1-propanol droplet surface temperature T s is higher than anhydrous ethanol at all temperatures. age vaporization rate of anhydrous ethanol and 1-propanol droplets respectively. At a temperature T ∞ lower to 473 K, the value of the average vaporization K of anhydrous ethanol droplet is higher than that of 1-propanol. Nevertheless, as the temperature increases beyond this limit, the average vaporization rate of 1-propanol droplet is superior to anhydrous ethanol. By the presentation of both K th and K values for both alcohols, one can conclude that for temperature T ∞ lower than 473 K, the average vaporization rate of both alcohols is mainly influenced by the diffusion factor and at T ∞ higher to 473 K, the droplet surface temperature, T s is more dominant in increasing the average vaporization rate, K compared to the binary diffusion coefficient.

Estimation and calculation of Grashof , Prandlt, Schmidt and Lewis Numbers

From the theoretical calculation, a number of dimensionless properties could also be calculated. The Grashof number is defined in terms of surface droplet temperature and other thermodynamics properties by [START_REF] Ebrahimian | Towards A Predictive Evaporation Model For Multi-Component Hydrocarbon Droplets at All Pressure Condition[END_REF] :

(4-35)
with g the gravitational acceleration, ρ g is the volumetric mass of ambient gas, T amb the gas ambient temperature, T s the droplet surface temperature, µ O the dynamic viscosity of ambient gas, d o is the droplet initial diameter, The volumetric mass and the dynamic viscosity are determined by the reference temperature.

Grashof number is calculated and estimated to determine the effect of buoyancy on the droplet vaporization. Figure 4.6 shows the variation of Grashof number with ambient temperature, T ∞ . At lower temperature range (T ∞ < 373 K), the Gr number is increased with T ∞ , however it is then negatively dependent at higher T ∞ range. Meanwhile, the Prandtl number could be defined as: where the specific heat and thermal conductivity of gaseous mixture are estimated by the reference composition and the temperature according to equation .

In order to calculate the Lewis number, a dimensionless Schmidt number could be defined as:

(4-37)

where µ g the viscosity and ρ g the density of gaseous mixture and D the binary diffusion coefficient of fuel-oxidant. τne of the major assumptions applied in 'Quasi-Steady' model utilised in this study is that the properties of transport in gas phase are always constant. Such properties are thermal conductivity and molar specific heat. Therefore, the Lewis number must be equal to unity. In other words, the thermal diffusivity will always equal the mass diffusivity. The Lewis number is defined as:

(4-38)
where the thermal conductivity λ g , the density ρ g and the specific heat C pg of the gaseous mixture are estimated and calculated based on the reference temperature and composition and D the binary diffusion coefficient.

Conclusions

Theoretical calculation of average vaporization rate for both anhydrous ethanol and 1propanol droplets have been realized in this chapter. The theoretical calculations are based on the concept of 'quasi-steady' where the assumptions as explained in chapter 2 are applied.

Two different methods; one is based on the functionality and influence of binary diffusion coefficient D and the other is determined by evaluating the thermodynamics properties at a reference temperature and also at reference composition. The comparison of experimental average vaporization rate, K for both alcohols with these theoretical calculated vaporization rates, K th1 and K th2 is in a very good agreement. Various dimensionless numbers are calculated to determine the influence of buoyancy (Gr σumber) and the assumption of 'quasisteadiness' (Le number). versus t/d 0 2 curves for all temperatures are deviated from the d 2 -law. The initial average vaporization rate, K i ceases to be constant at a certain point through the droplet life. For ethanol (95%) the deviation from the linear part occurs at mid stage of the vaporization of the droplet (and d 2 /d 0 2 < 0.4) whereas for anhydrous ethanol, the deviation only starts towards the end of the droplet life (and d 2 /d 0 2 < 0.2). When the droplets are formed in a closed chamber filled with nitrogen gas, one prevents the combustion of the droplets and excludes the effect of ambient moisture on the evaporation process. Even so, there is still some humidity in the chamber (leak, wall adsorption…). This could explain why even anhydrous ethanol produces a non-linear d²-law. As shown in Figure 5.6, the average vaporization rates, K i calculated from the first linear part of the d 2 curves are similar for both ethanol forms. Therefore it shall be noted that the first linear part of d 2 curves for both ethanol (95%) and anhydrous ethanol may entirely and totally consist only of ethanol vaporization. Ethanol is the major component evaporating at this first stage as it has lower boiling temperature than water.

RESULTS AND DISCUSSIONS

Meanwhile, a second 'linear' part of the d 2 -curves could be observed towards the end of the curve for all temperatures of anhydrous ethanol vaporization. We define previously that the gradient at this period as a final vaporization rate, K f . For the calculation of the final average vaporization rate, K f , a comparison is made with the theoretical calculation of water vaporization rate. The theoretical water vaporization rate, K th1 H 2 0 and K th2 H 2 0 are calculated based on the 'quasi-steady' model of equations 4-1 and 4-2. As observed in Figure 5.6, the evolution of K f for anhydrous ethanol is in a very good agreement with the theoretical water vaporization rate, K th1 H 2 0. This comparison is important as to correlate the 'deviation' of the anhydrous droplet from the quasi-steady of the

Instantaneous Vaporization Rate

The instantaneous vaporization rate, K inst , is calculated from the d 2 -curves by determining the derivative of these curves. According to [START_REF] Law | Alcohol Droplet Vaporization in Humid Air[END_REF], the derivative of the d 2 (t) will give the instantaneous vaporization rate,

(

In order to avoid errors in the calculation of the derivative, a smoothing is carried out on the d 2 (t) curves by using a FFT (Fast Fourier Transfer) low-pass filter. This method removes only the high frequency components with a parabolic window (Origin  function). Then the derivative is calculated on this smoothed curve. unsteady for the initial part corresponding to the ethanol vaporisation period, K i . The second period, K f , attributed to the water vaporization is almost quasi steady. In order to illustrate that, the equivalent average value of K inst is plotted in Figures 5.9 and 5.10, corresponding to the vaporizing rate K i and K f respectively. That shows that although it is easy to determine a linear trend on the d 2 (t) curve, the unsteadiness of the phenomenon is clearly revealed by the evolution of K inst according to time. This evolution is certainly due to the interference of water concentration on the ethanol droplet vaporization and also to the water condensation from the ambient moisture, due to the temperature decrease at droplet surface. In order to compare these evolutions for different temperatures, a normalization of these curves has been conducted. The time has been normalized by the droplet total vaporization time t vap . Figures 5.11 and 5.12 show the variation of normalized instantaneous vaporization rate, K inst for both ethanol (95%) and anhydrous ethanol against the time normalized by t vap . It is observed that by considering normalized time by t vap , the instantaneous vaporization rate, K inst presents the two domains, previously described, ethanol vaporization first and then the water vaporization. In this figure one can observe that the first part of the vaporization process occurs mainly at 1/3 of the total vaporization time for the ethanol (95%), even though this occurs around at 70% of the total vaporization time for the anhydrous ethanol.

Nevertheless, one can observe in Figure 5.13, that the instantaneous vaporization rate for the anhydrous ethanol is always higher than the ethanol (95%) for the initial part of the total vaporization time, corresponding to the K i . This can be explained by the fact that for the calculation of a mean vaporization rate the variation of instant vaporization rates is divided by a time period. For the anhydrous form the time period considered is longer thus compensating the systematic higher values observed for instant vaporization rates and therefore making mean values equivalent to (95%) form.

Another interesting observation that could be made from Figure 5.13 is the behaviour of anhydrous ethanol at temperature 673 K where the K inst is always almost a constant, 'quasisteady' and equal to K i throughout droplet lifetime. The disappearance of ethanol component only occurs at the end of the lifetime, as underlined also by [START_REF] Marchese | The Effect of Liquid Mass Transport on the Combustion and Extinction of Bicomponent Droplets of Methanol and Water[END_REF]. It seems that at this higher temperature anhydrous ethanol behaves as a single component without or with slight water concentration interference. [START_REF] Morin | Droplet vaporisation characteristics of vegetable oil derived biofuels at high temperatures[END_REF] had studied the vaporization of n-alkanes droplet. The results showed that the instantaneous vaporization rate increases with time. However, for alcohol fuels such as ethanol and 1-propanol as in our current studies show the opposite results. The K inst decreases with time. The nature of alcohols which is miscible with water has changed the overall process of vaporization. A plot of average vaporization rates against ambient gas temperature is shown in Figure 5.18 where a polynomial fit of degree two is plotted (with the value of R 2 = 0.999873). It is clearly shown that at all ambient temperatures, the average vaporization K follow the fit predominantly, with slightly lower values observed at higher temperatures.

Instantaneous Vaporization Rate

The calculation of instantaneous vaporization rate of 1-propanol is similar to that of ethanol droplet. In order to avoid errors in the calculation of the derivative, a smoothing is carried out on the d 2 (t) curves by using a FFT filter. This method removes only the high frequency components with 40 to 50 points of window (Origin  function). Then the derivative is simply calculated on this smoothed curve. In order to compare lucidly these evolutions for different temperatures, a normalization of these curves has been carried out. The time has been normalized by the droplet total vaporization time, t vap . We can observe from Figure 5.20 that the quasi-steady behaviour of 1propanol droplet is achievable for all lower temperatures up to T ∞ = 673 K. However, an interesting observation could be seen beyond this temperature, where the instantaneous vaporization rate, K inst reveals a bit of unsteady behaviour with the values of K inst gradually decreasing over time. The experimental instantaneous vaporization rates are calculated and show that in both cases, they are significantly unsteady especially at the initial part corresponding to ethanol vaporization. This unsteadiness is certainly due to the interference of water concentration on the ethanol droplet vaporization and also to the water condensation from the ambient moisture, due to the temperature decrease at droplet surface. The miscibility nature of ethanol to water has changed the overall process of vaporization.

The effect of various ambient temperatures on the vaporization of 1-propanol droplet shows that at various temperatures, the d 2 -law holds quite steadily and the 'quasi-steady' behaviour is preserved. The time histories of instantaneous vaporization rate, K inst confirm this stationary aspect of 1-propanol droplet at various ambient temperatures. The results also conclusively demonstrate that the 1-propanol vaporization is not affected by the water vapour from the environment even though it posses the miscibility property with water.

An attempt to correlate the effect of ambient relative humidity on the behaviour of K i and K f has been carried out. It is clearly demonstrated that the initial vaporization rate K i is always constant despite the change of ambient relative humidity. On the other hand, the value of final vaporization rate K f is apparently affected when the ambient relative humidity is changed. The value of K f is observed to decrease as the ambient relative humidity increases.

This observation might conclude that the values of the first linear part of d 2 curves (K i ) for ethanol consist entirely and totally only of ethanol vaporization. Ethanol is the major component evaporating at this first stage since it has lower boiling temperature than water. In the meantime, K f undoubtedly correlates to the water vaporization phenomenon as these values are negative dependent on the ambient.

Eventhough the droplet initial diameter d 0 has been varied during the experiments, the actual effect on the ethanol remains ambiguous due to the fact that ethanol are extremely affected by the ambient humidity. However, an obvious observation of d 0 impact can be made on 1-propanol vaporization. Results showed that the average vaporization rate K, increases as the droplet initial diameter d 0 increased.

In experiments of different initial water content in ethanol, as expected, a so-called 'quasisteady' period for ethanol in all cases occurs two times throughout the droplet lifetime. The results also show that the deviation of the curve from the linearity of the d 2 -law becomes more prominent with the increase of initial water content and instead the values of the first linear part of all d 2 -curves remain unchanged. The droplet lifetime is prolonged with increase of ambient relative humidity.

AUTOIGNITION CHARACTERISTIC AND KINETICS MECHANISM OF ETHANOL AND 1-PROPANOL

The chemical structure of biofuels such as alcohols significantly differs from fossil fuels due to the incorporation of oxygen atoms into the alkyl chain, as such the chemical details of their combustion is currently poorly understood relative to more conventional fuels. The chemical processes of a combustion reaction are extremely complicated. This has become obvious with the continued development of kinetic models and their capacity to predict with accuracy the observations of sophisticated and well defined experiments. It has been shown

that large numbers of chemical species and an even larger number of chemical reactions are required to predict experimental observations such as the evolution and consumption of intermediate species. In addition, kinetic modelling has guide to an understanding of how the chemistry of these species affects the reactivity of the global system. Since so many intermediates species can be produced during the combustion process, the number of reactions required to describe this process can be up to hundreds or even thousands of chemical reactions depending on the size of the fuel molecule undergoing oxidation.

Both experimental and kinetic modelling techniques have shed light upon the chemical mechanism of combustion as being a radical chain reaction. Although the well known chain initiating, branching, propagation and termination reactions are dependent on the chemical composition and structure of the fuel, it is also reliant in a complicated non-linear way on the temperature and pressure at which the combustion is occurring.

The Arrhenius Power Law Expression

The measured and computed ignition delay times for most experimental conditions are correlated to an Arrhenius, power law expression. According to [START_REF] Johnson | A shock tube study of n-and iso-propanol ignition[END_REF] this corre-lation form has been applied successfully in many previous studies for straight, branched and cyclic alkanes. The rate expressions which are the building blocks of the kinetic model consist of three main parameters, in terms of the modified Arrhenius-type plot of logarithm of the delay time versus reciprocal temperature equation;

(6-1)
Where is the rate constant, A the frequency factor which has units of cm 3 mol -1 s -1 , T is the temperature in Kelvin, n the temperature exponent which is a constant, R the universal gas constant (cal K -1 mol -1 ) and E A the activation energy (cal mol -1 ).

Nonetheless, there were also several findings from literature showed the occurrences of "two-stage" autoignition that led to the existence of "negative temperature coefficient" where P i and T i are the initial pressure and temperature respectively.

In summary, the modelling simulation is based on a few main assumptions:

Sensitivity Analysis

A sensitivity analysis was performed on the kinetic model to explicate the important channels of reaction for alcohols under these rapid compression machine conditions. The sensitivity analysis was employed by multiplying the forward and reverse rate constants of a reaction or reaction class by a factor of two thereby leaving the thermo chemistry or thermodynamic equilibrium constant within the chemical reaction unaffected and noting the effect on the computed ignition delay time (Marinov (1997)). The sensitivity coefficient, , is defined as:

Ignition Delay Time, Measurements

To further explain the definition and correlation of ignition delay time, an Arrhenius plot has been plot for all alcohols autoignition. Figure 6.9 shows the comparison of experimental autoignition delay times between ethanol, 1-propanol and mixture ethanol/water. Note that the ignition delay time for all fuels varies linearly with reciprocal temperature and hence exhibits Arrhenius behaviour. The ignition delay time of ethanol is observed to be longer in magnitude than 1-propanol. Therefore it shows that the higher molecular weight alcohol, 1propanol is more reactive. As mentioned by [START_REF] Cooke | A Shock-Tube Study of Tthe Ignition of Methanol and Ethanol with Oxygen[END_REF] who investigated the ignition delay measurements of shock-heated ethanol-oxygen in argon mixes, on comparison of reac-tivity between different molecular weight alcohols, have shown that ethanol is more reactive than methanol. In the meantime, the addition of water in ethanol appears to decrease the overall ignition delay time. The existence of water in ethanol/water mixture also shows an extended reactivity. Aramco mechanism shows the increase of reactivity as water is added in ethanol, with an over prediction of the model to the experimental data. The Aramco modelling simulations predict that at higher compressed temperature Tc > 830 K approximately, the water addition to ethanol oxidation produces an increased in reactivity, which is in an agreement with the experimental results. However, at lower compressed temperature Tc < 830 K the opposite occurs, i.e. reduced reactivity is observed and ignition delay prolongs. Unfortunately, in order to avoid condensation of water before the test run, the initial temperature is higher than 373 K, therefore our experiments on ethanol/water mixture could not be carried out at these lower compressed temperatures and therefore no definite conclusions could be made at these lower temperatures. In order to explain the effect of water addition to ethanol oxidation process, the sensitivity analysis results by Aramco mechanism have been separately plotted for both ethanol and ethanol/water mixtures. As seen previously in Figures 6.11 where the comparison between experimental and model predicted ignition delay times has been carried out, Frassoldati et al.

mechanism do not exactly capture the effect of water addition to ethanol oxidation process.

Figure 6.14 shows the most sensitive reactions at compressed temperature of 860 K for ethanol and ethanol/water respectively. As shown in previous Figure 6. [START_REF] Botero | Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures[END_REF], both ethanol and ethanol/water oxidation are dominated by the similar reactions. However, the results do not definitively explain the increase of reactivity due to water addition to ethanol oxidation. As we compare between these Figures 6.14a showed that H-abstraction was also the most sensitive reaction occurred with formation of αhydroxybutyl radical. By increasing the temperature, the sensitivity of the system to this reaction (R12) is increased. However, for reaction (R13), the sensitivity of the system to this reac- However, the increase of reactivity due to water addition to ethanol is only captured by Aramco mechanism. Sensitivity analyses by both mechanisms demonstrate a similarity in terms of reactions that administer the experimental ignition delay time of ethanol and ethanol/water blend. The sensitivity analysis on ethanol/water by Aramco mechanism also shows that one reaction might be liable for the reactivity increase observed in the system. For 1-propanol using Johnson et al. mechanism, the system is clearly most sensitive to reactions of hydrogen abstraction which involve the radicals such as metylperoxy and hydroperoxyl.

Perspective and future works

As mentioned, there is still much to be investigated before biofuels especially isolated alcohols droplet vaporization are fully understood. Future research should focus on a more de-tailed determination of the data which could only be estimated at this time. Improvements could be made in the experimental design and the experimental scope by including broadening the test temperature, pressure and fluid mixture range. Improvements and modification could also be carried out on the design of the droplet injector nozzle to adapt to that higher density fuels such as biodiesel, and heavier molecular weight alcohol such as n-butanol. Furthering the study by including other alcohols and/or alcohols mixtures would lead to an increased generality in the vaporization characterization. Improvement of theoretical modeling and calculation can be adapted by incorporating the effect of water fraction in alcohols vaporization rate calculation. Some measurements by Infrared (IR) camera will enhance and verify the important factors such as temperature gradient and internal flow of droplets during vaporization process. For better understanding and comparison purposes of the possible behaviour of alcohols vaporization, modeling aspect need to be established.

In this present study, the kinetic impact of water on ethanol ignition process has been carried out only at single percentage of water addition, i.e. 30 %. For better assessment on the actual behaviour and to raise the confidence level of existing findings, a wider experimental scope of water percentage is required as more data are needed as to improve the existing kinetic mechanisms.

viii 7 . 1 Figure 2 . 1 :

 7121 Figure 2.1: Summary of fuel components derived from biomass (Adopted from Tran et

Figure 2 . 2 :

 22 Figure 2.2: Classification of biofuels (Nigam and Singh (2010)). ............................................

Figure 2 . 3 :

 23 Figure 2.3: Definition of ignition delay time used in this study ..............................................

Figure 3 . 1 :

 31 Figure 3.1: The main apparatus of Multi User Combustion Chamber (MUCC) at CNRS,

Figure 3 . 2 :

 32 Figure 3.2: Schematic diagram of the whole experimental apparatus. ....................................

Figure 3 . 3 :

 33 Figure 3.3: Single fibre suspended droplet technique (Chauveau et al. 2011) ........................

Figure 3 . 4 :

 34 Figure 3.4: Cross-fibre supported droplet technique used in MUCC. .....................................

Figure 3 . 5 :

 35 Figure 3.5: Representative set of runs showing the instabilities during the motion of the

Figure 3 . 6 :

 36 Figure 3.6: The evolution of the droplet life time for anhydrous ethanol at 473K (a)

Figure 3 . 7 :

 37 Figure 3.7: Representative set of runs showing the reproducibility of droplet

Figure 3 . 8 :

 38 Figure 3.8: The NUIG RCM ....................................................................................................

Figure 3 . 9 :

 39 Figure 3.9: Schematic diagram of the NUIG RCM .................................................................

Figure 3 . 10 :

 310 Figure 3.10: Representative set of runs showing the reproducibility of experiments. ............

Figure 4 . 1 :

 41 Figure 4.1: Calculated droplet surface temperature, T s for 1-propanol and anhydrous

Figure 4 . 2 :Figure 4 . 3 :

 4243 Figure 4.2: Calculated latent heat of vaporization, L v for 1-propanol and anhydrous

Figure 4 . 4 :

 44 Figure 4.4: Theoretical and experimental average vaporization rates, K th and K for

Figure 4 . 5 :

 45 Figure 4.5: Theoretical and experimental average vaporization rates, K th and K for 1-

Figure 4 . 6 :

 46 Figure 4.6: Evolution of Grashof number in function of temperature for ethanol and 1-

Figure 4 . 7 :

 47 Figure 4.7: Evolution of Prandtl number in function of temperature for ethanol and 1-

Figure 4 . 8 :

 48 Figure 4.8: Evolution of Schmidt number in function of temperature for ethanol and 1-

Figure 4 . 9 :

 49 Figure 4.9: Evolution of Lewis number in function of ambient temperature for ethanol

Figure 5 . 1 :

 51 Figure 5.1: Definition and calculation of initial average initial, K i and final vaporization rates, K f from the d 2 (t) curve for anhydrous ethanol droplet; T ∞ = 473 K and

Figure 5 . 2 :Figure 5 . 3 :

 5253 Figure 5.2: Normalized squared-diameter curves for ethanol (95%) at different

Figure 5 . 4 :

 54 Figure 5.4: Normalized squared-diameter curves for ethanol (95%) and anhydrous

Figure 5 . 5 :

 55 Figure 5.5: Normalized squared-diameter curves for ethanol (95%) and anhydrous

Figure 5 . 6 :

 56 Figure 5.6: Average vaporization rates K i for anhydrous ethanol and ethanol (95%) at

Figure 5 . 7 :

 57 Figure 5.7: Average vaporization rates calculated during second linear part of the d 2

Figure 5 . 8 :

 58 Figure 5.8: Theoretical and experimental average vaporization rates of the second part

Figure 5 . 9 :

 59 Figure 5.9: Evolution of the instantaneous vaporization rate, K inst (t) and the squared droplet diameter for ethanol (λ5%) droplet at (a) T∞= 473 K (d0 = 407 m) and (b) T∞= 673 K (d0 = 523 m)ν P∞= 0.1 MPa. .............................................

Figure 5 . 10 :

 510 Figure 5.10: Evolution of the instantaneous vaporization rate, K inst (t) and the squared

Figure 5 . 11 :Figure 5 . 12 :

 511512 Figure 5.11: Comparison of the instantaneous vaporization rate, K inst (t) of ethanol

Figure 5 . 13 :

 513 Figure 5.13: Comparison of the instantaneous vaporization rate, K inst (t) of anhydrous

Figure 5 . 14 :

 514 Figure 5.14: Example of determination of water diameter in the droplet from d 2 (t)

Figure 5 . 15 :

 515 Figure 5.15: Volume percentage of condensed water at different temperatures for

Figure 5 . 16 :

 516 Figure 5.16: Evolution of squared diameter, d 2 against time, t for 1-propanol droplet at

Figure 5 .

 5 Figure 5.17: d 2 (t) curves for 1-propanol at various temperaturesν P∞ = 0.1 MPa ................

Figure 5 . 18 :

 518 Figure 5.18: Average vaporization rates, K for 1-propanol droplet at various temperaturesν P∞ = 0.1 MPa. The dotted line is the polynomial fit of the

Figure 5 . 19 :

 519 Figure 5.19: Evolution of the instantaneous vaporization rate, K inst (t) and the squared

Figure 5 . 20 :

 520 Figure 5.20: Comparison of the instantaneous vaporization rate, K inst (t) of 1-propanol

Figure 5 . 21 :

 521 Figure 5.21: Normalized squared-diameter curves for anhydrous ethanol and 1-propanol

Figure 5 . 22 :Figure 5 . 23 :

 522523 Figure 5.22: Evolution of the instantaneous vaporization rate, K inst (t) and the

Figure 5 . 24 :

 524 Figure 5.24: The disparity of average vaporization rate of 1-propanol droplets at various

Figure 5 . 25 :

 525 Figure 5.25: Normalized d 2 -curves for ethanol droplets (absolute, no additional water

Figure 5 . 26 :

 526 Figure 5.26: Normalized d 2 -curves for ethanol droplets (initial water content = 5%

Figure 5 . 27 :

 527 Figure 5.27: Normalized d 2 -diameter curves for ethanol droplets (initial water content =

Figure 5 . 28 :

 528 Figure 5.28: Normalized d 2 -diameter curves for ethanol droplets (initial water content =

Figure 5 . 29 :

 529 Figure 5.29: Normalized d 2 -diameter curves for ethanol droplets (initial water content =

Figure 6 . 1 :

 61 Figure 6.1: Presentation of ignition delay time calculation steps by modelling

Figure 6 . 2 :

 62 Figure 6.2: Alcohol oxidation pathway (Adopted from Norton et al., (1991)). ....................

Figure 6 . 3 :Figure 6 . 4 :

 6364 Figure 6.3: Experimental pressure profile measured in the RCM of Ethanol at all temperatures; P c =30.0 bar, φ=1.0. .......................................................................

Figure 6 . 5 :

 65 Figure 6.5: Experimental pressure profile measured in the RCM of Ethanol (70% vol)/water (30% vol) mixture at all temperatures; P c =28.0 bar, φ=1.0. ..............

Figure 6 . 6 :

 66 Figure 6.6: Comparison of experimental pressure profile measured in RCM of Ethanol

Figure 6 . 7 :

 67 Figure 6.7: Comparison of calculated pressure profile of Ethanol and 1-propanol

Figure 6 . 8 :

 68 Figure 6.8: Comparison of calculated pressure profile of Ethanol and Ethanol (70%

Figure 6 . 9 :

 69 Figure 6.9: Experimental ignition delay time of Ethanol, 1-propanol and Ethanol (70% vol)/water (30% vol) fuels at all temperatures; P c =34-35 bar, φ=1.0. Lines

Figure 6 .

 6 Figure 6.10: Experimental (symbols) and model predictions (lines) ignition delay time of Ethanol and 1-propanol fuels at all temperatures; P c =34-35 bar, φ=1.0. .......

Figure 6 . 11 :

 611 Figure 6.11: Experimental (symbols) and model predictions (lines) ignition delay time

Figure 6 . 12 :Figure 6 . 13 :Figure 6 . 14 :

 612613614 Figure 6.12: Evaluation of the most sensitive reactions with the Frassoldati mechanism

Figure 6 . 15 :

 615 Figure 6.15: Evaluation of the most sensitive reactions with the Johnson mechanism of

Figure A. 1 :

 1 Figure A.1: A 3D image of main elements of under-pressure chamber ................................

Figure A. 2 :

 2 Figure A.2: Droplet injector data acquisition Microdrop™ ..................................................

Figure A. 3 :

 3 Figure A.3: The RCM operation manifold ............................................................................

Figure A. 4 :

 4 Figure A.4: Schematic diagram of NUIG RCM. ...................................................................

Figure C. 1 :

 1 Figure C.1: The disparity of initial vaporization rate K i and final vaporization rate K f of

Figure C. 2 :

 2 Figure C.2: The disparity of average vaporization rate of 1-propanol droplets at various

Figure D. 1 :

 1 Figure D.1: Comparison of experimental pressure profile measured in RCM of Ethanol

Figure D. 2 :Figure D. 3 :

 23 Figure D.2: Pressure profile of Ethanol at all temperatures using the mechanism of (a) Frassoldati et al. (b) Aramco; P c =30.0 bar, φ=1.0. .............................................

Figure D. 4 :

 4 Figure D.4: Pressure profile of Ethanol (70% vol)/water (30% vol) mixture at all

Figure F. 1 :

 1 Figure F.1: Evaluation of the most sensitive reactions with the Frassoldati et al.

Figure F. 2 :

 2 Figure F.2: Evaluation of the most sensitive reactions with the Aramco mechanism of

Figure 2 . 1 :

 21 Figure 2.1: Summary of fuel components derived from biomass (Adopted from Tran et al.

Figure 2 . 2 :

 22 Figure 2.2: Classification of biofuels (Nigam and Singh (2010)).

  formance and exhaust emissions(Al Hassan, (2003);[START_REF] Al-Farayedhi | Experimental investigation of SI engine performance using oxygenated fuel[END_REF]). Ethanol addition resulted in an improvement in brake power, brake thermal efficiency, volumetric efficiency and fuel consumption; however the brake specific fuel consumption and equivalence air-fuel ratio decreased because of lower calorific value of the gasoline-alcohol fuel blends.Using an ethanol-unleaded gasoline blend leads to a significant reduction in exhaust emissions of CO and HC for all engine speeds. Ethanol diesel blends up to 20% can very well be used in present day constant speed compressed ignition engines without any hardware modification (Meiring et al. (1983); Mouloungui et al. (2001)). Hsieh et al. (2002) have experi-

(

  Hiroyasu and Arai (1990); Zhao et al. (1997); Zhao et al. (1999); Kong et al. (1999); Desantes et al. (1999); Payri et al. (2005); Desantes et al. (2005)ν Taşkiran and Ergeneman

i.At

  No internal liquid motion inside the droplet. j. Single component fuel. By applying those hypotheses above, the problem is solved by the main three conservation equations; continuity equation, conservation of energy equation and conservation of species equation. conditions of: At r = r s: T = T s and Y F = Y Fs (2-6) As r → ∞μ T = T ∞ and Y F = 0 the mass fraction of the fuel in gaseous form and the Spalding transfer number B represents the ratio of the driving force for vaporization to the resistance to vaporization. B T and B M are thermal transfer number and mass transfer number respectively. These numbers are equal under the condition of quasi-steadiness; i.e. Le = 1.

a.

  Constant and uniform droplet temperature Based on previous studies (Chin and Lefebvre, (1983); Nomura et al., (1996), Sazhin et al., (2005)) there are a lot of evidences that show the existence of the transient heat-up period of the droplet preceeding to the occurrence of the d 2 -law.

  let has been studied by Renaud et al. (2004), Mikami et al. (2005) and Chauveau et al. (2007).

  Balakrishnan et al. (2001) have investigated quasi-steady burning of spherical fuel particles in a mixed convective environment using the porous sphere method and suggested a correlation for the variation of mass burning rate with the free stream Reynolds number. Raghavan et al. (2009) studied methanol combustion using the porous technique to measure the mass burning rates and suggested correlations for the same envelope and wake flame regimes. Recently, Parag and Raghavan (2009) carried

  in their further investigation of the combustion of free droplet, where the combustion of fuel droplets in weak forced convection has been studied for the first time. Wang et al. (1984) studied the combustion characteristics of isolated, low Reynolds number, multicomponent droplets freely falling in a hot, oxidizing gas flow. Lee and Law (1992) have experimentally studied vaporization and combustion of methanol and ethanol droplets in both dry and humid environments. Their results demonstrated that the alcohol droplets had freely absorbed water from wet environment whether the water is present in the ambient gas or is generated at the droplet flame. Stengele et al. (1999) conducted an experimental set-up where the evaporation of free-falling, noninteracting binary mixtures of n-pentane and n-nonane droplets was investigated. The results

  rating or burning sequences. Most of the isolated droplet evaporation experiments have been conducted with the droplet suspended on a support fiber to avoid the experimental difficulties for free-falling droplets[START_REF] Hiroyasu | Fuel droplet size distribution in diesel combustion chamber[END_REF];[START_REF] Nomura | Experimental study on high-pressure droplet evaporation using microgravity conditions[END_REF];[START_REF] Morin | Droplet vaporisation characteristics of vegetable oil derived biofuels at high temperatures[END_REF];[START_REF] Ghasemmi | Experimental study of evaporation of kerosene droplet at elevated pressures and temperatures[END_REF]). However, due to the thickness of the suspension fibre, it is quite difficult to suspend a droplet much smaller than 1000 micrometers. This large size of droplet is much larger than the standard size of typical droplet in sprays. Recent study by[START_REF] Daho | Study of droplet vaporization of various vegetable oils and blends of domestic fuel oil-cottonseed oil under different ambient temperature conditions[END_REF] on droplet vaporization of various vegetable oils and blends domestic fuel oil-cottonseed oil at different temperatures have utilized the fibre-suspended technique with diameter of 400 µm. The droplet diameters are in range between 1000 to 1420 µm. The support fibre also usually has relatively larger thickness (around 150µm) and therefore is able to increase the vaporization rate of the droplet due to the induced heat transfer from the fibre to the droplet during vaporization process. The suspension fibre also significantly distorts the droplet's spherical shape. Therefore, the assumption of spherical symmetry of the droplet is no longer valid. However, recent developments[START_REF] Renaud | On the effects of supporting fibers and natural convection on fuel droplet vaporization[END_REF], Mikami et al. (2005) and Chauveau et al. (2007)) have improved the suspended droplet method to provide more accurate data results by diminishing or minimizing the effect of heat transfer from the support fibre to the droplet. This new improved technique of suspended droplet will be further elaborated in details in chapter 3. Conclusions Most droplet evaporation and combustion experiments have been conducted with the droplet suspended on a support fiber to avoid the experimental difficulties for free-falling droplets, such as for obtaining high-resolution droplets images. The main concern about the suspended droplet technique is the existence of the heat conduction effects from the support fibre to the evaporating droplet. The literature reports many studies appreciably improving the technique by reducing as much as possible the fiber diameter, others by taking into account in numerical models this phenomenon, then correlating their results with experimental data. It is only very recently that experimental studies could implement extremely fine suspension fibres, allowing the production of new results with improved technique (Renaud et al. (2004), Mikami et al. (2005) and Chauveau et al. (2007)).2.4 Influence of different parameters2.4.1 Influence of temperatures and pressureIn the early work for high-pressure evaporation under natural convection, Kadota and Hiroyasu (1976) have considered a mathematical model of a single droplet evaporating in high pressure and high temperature gaseous environments. The calculation covered the unsteady and steady state of droplet evaporation considering the effect of natural convection. The calculated results showed the reverse effect of ambient gas pressure on droplet lifetime; i.e. the droplet lifetime decreases with an increase in pressure at high temperatures and with a decrease in pressure at low temperatures.[START_REF] Hartfield | Droplet vaporization in a high pressure gas[END_REF], have studied the vaporization of single refrigerant R-113 (trichlorotrifluoroethane) and n-heptane droplets experi-mentally. They have observed that the gas temperature affected strongly droplet vaporization whereas gas pressure had a weaker effect.[START_REF] Stengele | Experimental and theoretical study of one-and two-component droplet vaporization in a high pressure environment[END_REF] conducted an experimental set up to study the evaporation of free falling, non interacting droplets in a higher pressure environments. The experiments were carried out with binary mixtures of n-pentane and nnonane.[START_REF] Ghasemmi | Experimental study of evaporation of kerosene droplet at elevated pressures and temperatures[END_REF] on the vaporization of kerosene droplet experimentally investigated at high temperatures and high pressures under normal gravity. The evaporation rate increased monotonically with an increase in gas temperature. At low temperature, when

  without natural convection. Runge et al. (1998) and Gökalp et al. (1994) investigated evaporation of droplets of binary mixtures to bring in the effect of multiple components in droplet evaporation process. These experiments were done in a convective environment and at ambient pressures. Daif et al. (1999) reported an enhancement of the evaporation rate of a droplet in an environment without forced convection due to flow induced by natural convection in the gas phase surrounding the droplet.

  advanced and novel technique, the 'cross-fibre' technique (Renaud et al. (2004), Mikami et al. (2005) and Chauveau et al. (2007)), employed in this current studies apparently reduced the said effect of the heat conduction from the suspended fibre.

  Alcohols droplet vaporization mechanism has been first investigated more than three decades ago by[START_REF] Law | Fuel spray vaporization in humid environment[END_REF] through a theoretical study of spray vaporization of a monodisperse fuel spray in a cold and humid environment. The results showed that the associated condensation heat release is considerable and significantly enhance the fuel vaporization rate heterogeneously on the droplet surface. The subsequent study was carried out experimentally by[START_REF] Law | Alcohol Droplet Vaporization in Humid Air[END_REF] with the vaporization of suspended alcohols droplets such as methanol and ethanol in cold and humid environment. They observed that the condensation and subsequent dissolution of water into the alcohols droplet is significant and resulting in the deviation of diameter-squared evaluation of the droplet from the classical d 2 -law.Meanwhile Choi et al. (1988[START_REF] Lefebvre | Atomization and Sprays[END_REF] on the combustion of methanol droplet suggested the potential importance of surface condensation of the matter vapour produced at droplet flame. Those claims were confirmed byChoi et al. (1990),Lee et al. (1990) andLee (1990) by experimental sampling.[START_REF] Lee | An Experimental Investigation on the Vaporization and Combustion of Methanol and Ethanol Droplets[END_REF] continued to study the effect of water condensation on alcohols droplet through experiments of free-falling methanol and ethanol droplets combustion in both dry and wet environments. Their results demonstrated that the alcohol droplets had freely absorbed water from wet environment whether the water is present in the ambient gas or is generated at the droplet flame.[START_REF] Marchese | The Effect of Liquid Mass Transport on the Combustion and Extinction of Bicomponent Droplets of Methanol and Water[END_REF] simulated a time-dependent combustion of isolated, bicomponent liquid droplets of methanol and water using a spherosymmetric, finite element, chemically reacting flow model. The results are then compared with previously reported data from microgravity drop tower, freely falling isolated droplet and suspended droplet combustion experiments. Results suggest that droplet experiments using methanol-water mixtures should strongly characterize the magnitude of the liquid mass transport rate in a given experimental configuration. Numerical results are consistent with experiments when it is speculated that sufficient internal liquid phase motion is present to reduce the effective liquid mass Peclet number (dimensionless number used in calculations involving convective heat transfer. It is the ratio of the thermal energy transferred to the fluid by con-vection to the thermal energy conducted within the fluid) to the order of one. Such internal motion has been noted in droplet combustion experiments and most likely arises from droplet generation/deployment techniques and/or surface tension gradients. From Mukhopadhyay and Sanyal (2001), a theoretical model for combustion of alcohol droplets has been developed by considering the quasi-steady sphericosymmetric gas phase equations. The results showed that for alcohols with boiling temperatures lower than that of water, an amount of moisture that is generated during combustion is absorbed by the droplet. It prolongs droplet lifetime and consequently reduces flame temperature. A study to clarify the effect of watery vapour concentration in hot ambient on droplet evaporation of a single suspended droplet of ethanol, which posses the hydrophilic property and n -hexadecane, n-heptane droplets with dehydrophile property has been carried out by Lee et al. (2001). The results showed that the watery vapour increases the evaporation of the ethanol droplet after entering into the droplet and it promotes evaporation velocity with occasional micro-explosion. In recent studies, Hopkins and Reid (2005) and Hopkins et al. (2006) studied multicomponent droplets of methanol/water, ethanol/water and 1-propanol/water. The results showed that the evaporation and growth of a multicomponent droplet depend on kinetic and thermodynamic parameters, including the gas and liquid-phase diffusion coefficients and the activity coefficients and vapour pressures of the constituents. Raj et al. (2010) have studied the evaporation of ethanolwater and methanol-water droplets by a technique measuring the surface tension concentration variation during evaporation process. The results showed evidence of rapid evaporation of more volatile component at initial phase followed by a diffusion-controlled slow evaporation towards the end of droplet lifetime. Mandal and Bakshi (2011) recently proposed the same measurement of surface concentration of an evaporating multicomponent droplet on an

  further reduces the range of miscibility. The behaviour of evaporating or burning droplets of alcohols-hydrocarbon mixtures has been little studied. Hallet et al.,(2010) have studied the measurements of single suspended droplet evaporation behaviour for mixtures of pure and denatured ethanol with No. 2 fuel oil (a complex combination of hydrocarbons with carbon numbers in the range C 9 and higher produced from the distillation of petroleum crude).No. 2 fuel oil is usually used for heating and is very similar to diesel fuels. The results showed that the mixtures behave identically to pure ethanol up to the point where the ethanol disappears, after which the evaporation rate becomes that of pure fuel oil. The departure from the d 2 -law is due largely to the inclusion of natural convection. Burning characteristics of free-falling droplets of diesel/ethanol and biodiesel/ethanol mixtures have been experimentally studied by[START_REF] Botero | Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures[END_REF]. They observed that the diffusion-limited mechanism for multicomponent droplet burning with highly disparate boiling points was demonstrated for diesel/ethanol and biodiesel/ethanol mixtures. Three phases have been identified; steady burning with more volatile components with lower boiling points, an intermediate transient heating period as the dominant surface components transition from more volatile to less volatile and the last phase consists of steady burning by the co-gasification of both components. Ethanol micro explodes earlier during burning with stronger intensity. The addition of ethanol also reduces the yellow luminosity of the flame at early stage of droplet lifetime, indicat-ing an overall reduction in sooting propensity. Experimental studies on Brazilian gasoline type C and hydrated ethanol mixture at various concentrations have been carried out by Delgado et al. (2006). The physio-chemical properties for a better comprehension of the effects caused by the flex-fuel technology have been evaluated. The results show that the mixtures of hydrated alcohol-gasoline increased the octane properties such as Motor Octane Number (MON), (RON) and Anti-Detonant-Index. The specific mass and electric conductivity also increased in values with the addition of ethanol. Parag and Raghavan (2009) carried

Figure 2 . 3 :

 23 Figure 2.3: Definition of ignition delay time used in this study

(

  700 K < T < 1100K). The RCM is a device that can rapidly compress a premixed fuel/oxygen/diluents gas mixture to a preselected temperature and pressure. It can simulate only a single stroke of the combustion engine and thus allows the study of autoignition under more favourable conditions than those in real engine. Post-compression conditions of temperature and pressure in RCM are typically in the range of 700-1200 K and 1-6 MPa. The typical test times are in the region of 1-200 ms. One shortcoming with RCMs is the inevitable loss to the walls which is due from relatively longer test times in the RCM. For years, RCMs have been utilized in autoignition and oxidation studies of alkane fuels and recently on oxygenated fuels at low to intermediate temperature range.Minetti et al. (1995) have studied n-heptane oxidation and autoignition in a rapid compression machine at low to intermediate temperature regimes and high pressures. N-heptane exhibits a high reactivity characterized by a relatively short ignition delay and by a relatively low ignition limit in accordance with the octane number of this fuel. The delay times show a remarkable negative dependence upon gas temperature in the range of compressed temperature 700-860 K.A detailed experimental study of the nine isomers of heptane has been performed in a rapid compression machine by[START_REF] Silke | The influence of fuel structure on combustion as demonstrated by the isomers of heptane: a rapid compression machine study[END_REF]. The interest in the study lies in determining the role of molecular structure of the C 7 H 16 hydrocarbons on the rate of combustion of the various isomers. Ignition delay times were measured, and their dependence on the reaction conditions of temperature and pressure was studied, and the comparative reactivity profiles of the different isomers were obtained. The study has resulted in an RCM data for the nine isomers of heptane. In general, results showed shorter ignition delay times for isomers with low research octane number (RON) with the longest ignition delays and/or failure to ignite for isomers with high RON. Healy et al. (2008) have presented an extensive range of experimental data for methane/propane mixtures in the temperature range of 740 -1550 K at various compressed gas pressures and equivalence ratio in both shock tube and rapid compression machine. Lee et al. (1993) have reported the autoignition characteristics of methanol, ethanol and metyl tert-butyl ether (MTBE) in a RCM at pressure range 20-40 atm and temperature within 750-1000 K.An RCM also has been used to study the effects of fuel structure and additives on the Homogeneous Charge Compression Ignition (HCCI) of pure hydrocarbon fuels and mixtures under well determined conditions byTanaka et al., (2002). The results indicated that for HCCI combustion, the ignition delay and the burn rate can be independently controlled using various fuel mixtures and additives.Flow reactorsAnother common experimental device to study the autoignition behaviour of fuels is flow reactors. Flow reactors imitate the flow conditions inside the gas turbine premixers therefore their data generally used for both gas turbine design[START_REF] Spadaccini | Autoignition characteristics of aircrafttype fuels[END_REF]) as well as chemical kinetics research[START_REF] Gokulakrishnan | Experimental and kinetic modelling of kerosene-type fuels at gas turbine operating conditions[END_REF]). Recent experimental works on flow reactors by[START_REF] Beerer | An experimental and kinetic study of alkane autoignition at high pressures and intermediate temperatures[END_REF] on alkane autoignition at high pressures and intermediate temperatures showed a number of differences in ignition delays trend identified between high and intermediate temperatures, including overall activation energies, relative reactivity of ethane and propane and impact of small quantities of ethane or propane mixed with methane on the ignition delay time. They concluded that these contrasting trends are attributed to the different elementary reactions that control the ignition process.

  ficiency and the pollutants formation, it is imperative to carry out experimental investigations and to develop well validated detailed kinetics models for these oxygenated components of biofuels. Major parts of the experimental studies of oxidation and ignition of alcohols have only been performed recently. However, most of the experimental investigations of alcohols have been carried out at temperature above 770 K. This is mostly due to a lack of reactivity of these compounds at lower temperature[START_REF] Tran | Progress in Detailed Kinetic Modeling of the Combustion of Oxygenated Components of Biofuels[END_REF]).[START_REF] Natarajan | An Experimental and Analytical Study of Methanol Ignition Behind Shock Waves[END_REF] have performed an experimental and analytical investigation of the ignition of ethanol-oxygen-argon mixtures behind reflected shock waves over the temperature range of 1300-1700 K at pressures of 1.0 and 2.0 atm. The equivalence ratio, φ is varied at 0.5, 1.0 and 2.0. The experimental ignition delay data were found to correlate with initial ethanol and oxygen concentrations and with initial temperature. A 56 steps kinetic model for ethanol oxidation in the temperature range mentioned was assembled using published rate coefficient data wherever available.Dunphy and Simmie (1991) have studied the ignition characteristics of ethanol-oxygen mixtures behind reflected shock waves from 1080 to 1660 K in the pressure range of 1.8-4.6

( 1991 )

 1991 have modelled a high-temperature oxidation of ethanol in a 97-steps, 30-species reaction mechanism and the results of the calculations were then compared to recent measurements of the ignition delays of mixtures of ethanol, oxygen and argon behind the reflected shock waves in a range of 1080-1660 K at pressure of 1.8-4.6 bar with equivalence ratio of 0.25 to 2.0.[START_REF] Lee | Autoignition of alcohols and ethers in a rapid compression machine[END_REF] have investigated the autoignition characteristics of alcohols such as methanol and ethanol and ether (methyl-tert-butyl ether) in a rapid compression machine in the range of 20-40 atm and at low temperatures of 750-1000 K. The results showed higher autoignition temperatures than paraffins which is consistent with the high octane number of these fuels. It also confirmed the intrinsic resistance to autoignition of oxygenated fuels relative to reference fuels. Marinov (1998) has studied a detailed chemical kinetic model for high temperature ethanol oxidation. The model has been developed and validated against a variety of experimental data. The laminar speed data obtained from a constant volume bomb and counterflow thinflame, ignition delay data behind the reflected shock wave, ethanol oxidation product profiles from a jet-stirred and turbulent flow reactor were used for computational study. The results showed that high temperature ethanol oxidation exhibit strong sensitivity to the fall-off kinetics of ethanol decomposition. Li et al. (2007) have reported the experimental profile of stable species mole fraction for ethanol oxidation in a Variable Pressure Flow Reactor (VPFR) at initial temperature range of 800 K to 950 K, constant pressure of 3 to 12 atm and various equivalence ratios from 0.3 to 1.4. A new updated ethanol mechanism has been proposed and validated against wide range set of data and showed a significant improvement of predictions. The detailed kinetics mechanism for ethanol combustion was developed, taking into consideration of a hierarchical manner of reacting system.. Yates et al. (2010) have carried out a detailed chemical kinetic modelling study to characterize the autoignition behaviour of full range of blends of both methanol and ethanol with a Primary Reference Fuel (PRF) 80 base fuel. The study provided a few main observations.The cool-flame temperature rise was progressively reduced in proportion to the blend fraction and it primarily determined the characteristics of the blend autoignition chemistry.In comparison with methanol and ethanol, studies of combustion and oxidation on higher molecular weight alcohols such as propanol and butanol were limited and only recently performed.[START_REF] Norton | The flow reactor oxidation of C1-C4 alcohols and MTBE[END_REF] have presented experimental results for the flow oxidation of methanol, ethanol, n-propanol, iso-propanol, tert-butyl alcohol (TBA) and methyl tertbutyl ether (MTBE) at initial temperatures of 1020-1120 K and at atmospheric pressure. The results demonstrated that in comparison with alkanes, alcohols have a more complex oxidation mechanism, which involves the production of both oxygenated and non-oxygenated intermediates directly from the fuel. According to their observations also, the primary alcohols are more inclined to dehydrogenation than to dehydration because of the weakness of the C-H bond. The direct production of aldehydes from primary alcohols causes these fuels to have much shorter reaction times than do the corresponding alkanes. Meanwhile the secondary alcohols react both by dehydration to alkanes and by dehydrogenation to ketones. Tertiary alcohols are susceptible to unimolecular dehydration. Sinha and Thompson (2004) studied diffusion flames of C 3 -oxygenated hydrocarbons and their mixtures including iso-propanol, dimethoxy methane and dimethyl carbonate. They concluded that the intermediate pools in their flames were strongly related to the fuel structural features. Johnson et al. (2009) have studied the ignition characteristics of the two isomers of propanol (n-propanol and iso-propanol) in a shock tube device. Ignition delay times for propanoloxygen-argon mixtures have been measured behind reflected shock waves at high temperatures, range of 1350-2000 K at atmospheric pressure with equivalence ratio of 0.5, 1.0 and 2.0. The experimental results illustrated that ignition times for the n-propanol mixtures are faster than for iso-propanol in all cases. A kinetic model has been developed to describe the decomposition and ignition pathway of both n-propanol and iso-propanol isomers in this temperature range. It is based on previously validated C 3 -chemistry with sub mechanisms added for the propanol isomers. The results showed that the overall trends in the data are captured fairly well by the mechanisms which include a greater level of reactivity for the npropanol mixtures relative to iso-propanol. Frassoldati et al. (2010) have developed a kinetic model to describe the combustion of npropanol and iso-propanol. It was validated by comparing predictions made using this kinetic model with new experimental data on structures of counterflow non-premixed flames. The kinetics mechanism was made up of more than 7000 reactions among 300 species. The agreement between this kinetic model and experimental data showed satisfactory results. In general, they observed that the structures and overall combustion characteristics of npropanol and iso-propanol flames are similar.Veloo and Egolfopoulos (2011) have measured laminar flame speeds and extinction strain rates of n-propanol/air, iso-propanol/air and propane/air mixtures. A model was also presented which predicted experiments accurately, with deviations at rich conditions of npropanol/air and propane/air flames.[START_REF] Moss | An experimental and kinetic modeling study of the oxidation of the four isomers of butanol[END_REF] have carried out the autoignition measurements of four isomers of butanol using shock tube at 1 and 4 bar pressure and higher temperature of 1200-1800 K at various equivalence ratio and fuel mole percentage. Kinetic modelling indicates that the consumption of 1-butanol and iso-butanol which are the most reactive isomers takes place primarily by H-atom abstraction resulting in the formation of radicals, the decomposition of which yields highly reactive branching agents. Heufer et al. (2011) reported high pressure ignition delay results of stoichiometric nbutanol/air mixtures under the conditions behind the reflected shock of approximately 10-42 bar and temperature 770-1250 K. The results showed non-Arrhenius behaviour at temperatures lower than about 1000 K for the pressure range studied. They found that the rate of increase of ignition delay with decreasing temperature appeared to change around 1000 K. Autoignition experiments for n-butanol have been performed by Weber et al. (2011) using a heated rapid compression machine at compressed pressures of 15 and 30 bar, in the lowintermediate compressed temperature range of 675-925 K. Over the conditions studied, the ignition delay decreased monotonically as temperature increased and the experimental data was performed and the reactivity in terms of the inverse of ignition delay showed nearly second order dependence on the initial oxygen mole fraction and slightly greater than first order dependence on initial fuel mole fraction and compressed pressure.Effect of water on combustion and autoignion behaviour of alcoholsPrevious studies by Christensen and Johansson (1999) have shown that water injection in an HCCI engine significantly delays combustion timing, thus increasing the required intake temperature for a specific operating point when compared to pure fuel. This method was found successful in terms of controlling the ignition timing. A later investigation by Steinhilber and Sattelmayer (2006) found that a fuel-water mixture, or an emulsion, is more effective at retarding combustion timing and reducing pressure rise rates in comparison with separate injections. A study by Megaritis et al. (2007) used forced induction and residual gas through negative valve overlap (NVO) to run an HCCI engine on wet ethanol containing up to 20% water, the findings suggest increased air heating can extend the operating range of ethanol-inwater mixtures beyond the limitations of their experimental set-up. Mack et al. (2009) discussed the experimental results from a HCCI engine running on wet ethanol. Fuel mixtures studied range from pure ethanol to mixtures containing as high as 60%

Figure 3 . 1 :

 31 Figure 3.1: The main apparatus of Multi User Combustion Chamber (MUCC) at CNRS, Or-

Figure 3 . 2 :

 32 Figure 3.2: Schematic diagram of the whole experimental apparatus.

Figure 3 . 3 :

 33 Figure 3.3: Single fibre suspended droplet technique (Chauveau et al. 2011)

Figure 3 . 5 :

 35 Figure 3.5: Representative set of runs showing the instabilities during the motion of the drop-

Figure 3 .Figure 3 . 6 :

 336 Figure 3.6 displays the representative sequences of anhydrous ethanol droplet vaporization

Figure 3 . 7 : 3 . 2

 3732 Figure 3.7: Representative set of runs showing the reproducibility of droplet vaporization ex-

Figure 3 . 9 :

 39 Figure 3.9: Schematic diagram of the NUIG RCM

5 .

 5 Gaseous fuels of anhydrous ethanol and propanol also are used without further purification. The mixing tank is always flushed with the diluent as it is to contain for the next test mixture before being evacuated to 10 -2 Torr (~10 -5 bar). Liquid fuel is then allowed to vaporize into the evacuated mixing tank. Partial pressures of fuel and all gases are measured using a 2000 mbar digital manometer (Chell cd101) to an accuracy of ±0.2 mbar. Test gas mixtures are typically made up to a final pressure of 2000 mbar and are allowed standing for at least a couple of hours to ensure homogeneity. The preheat temperature is set above the saturation temperature of each alcohol to ensure complete vaporization of the fuel. A magnetic stirrer mixes the reactants which are heated to avoid condensation of the mixtures. The temperature inside the mixing tank is allowed approximately 1.5 to 2 hours to reach a steady-state condition. Both reactive and non reactive (absent of O 2 ) mixtures are prepared for all experiments. The non reactive (NR) mixtures are prepared as a reference to be used for species reaction calculation and kinetics modelling.

3. 2 . 4

 24 Experimental Reproducibility The typical uncertainty in ignition delay time measured by NUIG RCM based on the observation and measurement of Wurmel et al. (2007) is around ±10%. This uncertainty is mostly due to the properties change with temperature and pressure. In present studies, each compressed pressure and temperature condition is repeated at least three times to ensure good reproducibility. The mean and standard deviation of the ignition delay for all test runs at each condition are calculated, as an indication of reproducibility (cf. Figure 3.10). The results show that the standard deviation calculation is less than 10% of the mean in every case. This estimation of uncertainties is in agreement with Weber et al. (2011) and Lee et al. (1993) in terms of RCM calculated error of ignition delay that represents the indication of reproducibility.

Figure 3 . 10 :

 310 Figure 3.10: Representative set of runs showing the reproducibility of experiments.

(4- 1 )

 1 and when Y Famb = 0 and Y Fl ≈ 1, equation 4.1 becomes, with Y Fs the mass fraction of gaseous combustible fuel at droplet surface, C pg the molar calorific capacity of gaseous mixture, L v the molar latent heat of vaporization, T amb the ambient gas temperature and T s the droplet surface temperature.The equality of these two numbers permits us to determine the droplet surface temperature, T s by iterations. For all calculation, we have utilized the software of mathematical calculation, MATHCAD™.

Figure 4 .

 4 Figure 4.1 shows the calculated droplet surface temperature T s of ethanol and 1-propanol

Figure 4 . 1 :

 41 Figure 4.1: Calculated droplet surface temperature, T s for 1-propanol and anhydrous ethanol

6 )

 6 The saturated vapor pressure, P sat is calculated from equation of Wagner for alcohols studies[START_REF] Reid | The Properties of Gases and Liquids[END_REF]); (4-7) with , A, B, C and D are constants for alcohols properties.

(4- 9 )

 9 and it is estimated by the method of Joback[START_REF] Reid | The Properties of Gases and Liquids[END_REF] where(4)(5)[START_REF]Close valve 1 and open valve 2, observe discharge[END_REF](7)(8)(9)[START_REF] Balakrishnan | Combustion of a fuel droplet in a mixed convective environment[END_REF] with A, B, C and D are constants.To calculate the latent heat of vaporization at droplet surface temperature, the Watson relationship is used[START_REF] Reid | The Properties of Gases and Liquids[END_REF];(4)(5)[START_REF]Close valve 1 and open valve 2, observe discharge[END_REF](7)(8)(9)[START_REF] Balakrishnan | Combustion of a fuel droplet in a mixed convective environment[END_REF][START_REF] Barata | Modelling of biofuel droplets dispersion and evaporation[END_REF] where n is defined as per relation of Viswanath and Kuloor (Rei et al. 1987);(4)(5)[START_REF]Close valve 1 and open valve 2, observe discharge[END_REF](7)(8)(9)[START_REF] Balakrishnan | Combustion of a fuel droplet in a mixed convective environment[END_REF][START_REF] Barata | Modelling of biofuel droplets dispersion and evaporation[END_REF][START_REF] Beerer | An experimental and kinetic study of alkane autoignition at high pressures and intermediate temperatures[END_REF]

Figure 4 . 2 shows

 42 the comparison of L v between ethanol and 1-propanol in terms of various surface temperature of the droplet T s .

Figure 4 . 2 :

 42 Figure 4.2: Calculated latent heat of vaporization, L v for 1-propanol and anhydrous ethanol

(4- 17 )

 17 with A, B, C, D, E, F, G, H are constants, and(4)(5)[START_REF]Close valve 1 and open valve 2, observe discharge[END_REF](7)(8)(9)[START_REF] Balakrishnan | Combustion of a fuel droplet in a mixed convective environment[END_REF][START_REF] Barata | Modelling of biofuel droplets dispersion and evaporation[END_REF][START_REF] Beerer | An experimental and kinetic study of alkane autoignition at high pressures and intermediate temperatures[END_REF][START_REF] Botero | Synergistic combustion of droplets of ethanol, diesel and biodiesel mixtures[END_REF][START_REF] Brett | Simulation of methane autoignition in a rapid compression machine with creviced pistons[END_REF][START_REF] Canada | Fuel droplet burning rates at high pressures[END_REF][START_REF] Castanet | Heat transfer within combusting droplets[END_REF][START_REF] Chauveau | Why d²-law does not hold during droplet vaporization in microgravity conditions?[END_REF][START_REF] Chauveau | An Experimental Study on the Droplet Vaporization: Effects of Heat Conduction through the Support Fiber[END_REF]

20 )

 20 The relations between  F ,  O ,  F ,  O , and the critical parameters are used to determine the values (Hirschfelder et al. N is Avogadro number.

Figure 4 .

 4 Figure 4.3 shows the calculated value of binary diffusion coefficient, D of ethanol and 1-

Figure 4 . 3 :

 43 Figure 4.3: Calculated binary diffusion coefficient, D for 1-propanol and anhydrous ethanol

(4- 29 ) 3

 293 And the thermal conductivity of gaseous mixture is estimated by utilizing the relation proposed by Euckan[START_REF] Reid | The Properties of Gases and Liquids[END_REF] for polyatomic gases; µ F and µ O are the viscosities of combustible fuel and oxidant respectively and they are defined by equation proposed by Chapman-Enskog[START_REF] Reid | The Properties of Gases and Liquids[END_REF]) as; Comparison between the two methods K th1 and K th2 are the theoretical vaporization rates that are calculated using two different calculations. The difference between these two methods comes from the calculation of thermodynamic and transport properties. In the second method, K th2 is determined by evaluating the thermodynamics properties at a reference temperature) as the first method, but also by evaluating a reference composition. Therefore, it is closer to the real case of droplet vaporization. In the first method of calculation, we use semi-empirical relationships to determine the binary diffusion coefficient of the liquid into the mixture using the characteristic length and the diffusion collision integral of the molecules.Droplet surface temperature, T s and binary diffusion coefficient, D are the main factors in influencing the increase of vaporization rate of both alcohols droplets. These two factors are calculated theoretically for both 1-propanol and anhydrous ethanol and plotted against various ambient temperatures in Figure 4.1 and Figure 4.3.

Figure 4 . 4 and

 44 Figure 4.4 and Figure 4.5 show the measured experimental and calculated theoretical aver-

Figure 4 . 4 :

 44 Figure 4.4: Theoretical and experimental average vaporization rates, K th and K for ethanol

Figure 4 . 5 :

 45 Figure 4.5: Theoretical and experimental average vaporization rates, K th and K for 1-propanol

Figure 4 . 6 :

 46 Figure 4.6: Evolution of Grashof number in function of temperature for ethanol and 1-

Figure 4 . 7 :

 47 Figure 4.7: Evolution of Prandtl number in function of temperature for ethanol and 1-

Figure 4 . 8 :

 48 Figure 4.8: Evolution of Schmidt number in function of temperature for ethanol and 1-

5. 1

 1 Ethanol and Anhydrous Ethanol 5.1.1 Average Vaporization Rate Average vaporization rate from the experiments is calculated by a linear least-square fit in the quasi steady zone of the d 2 curves. However in the case of both ethanol (95%) and anhydrous ethanol, the apparent 'quasi-steady' period occurs two times throughout the droplet lifetime (Figure 5.1). The normalized temporal evolutions of squared-diameter, d 2 against time of anhydrous ethanol and ethanol 95% droplets, at various ambient temperatures are shown in Figure 5.2 and 5.3 respectively. In these experiments, the so-called 'quasi-steady' period for both ethanol (95%) and anhydrous ethanol occurs two times throughout the droplet lifetime. Average vaporization rates are deduced from the d²-curves presenting two parts: the first linear part allows in determining a first average vaporization rate called hereafter "initial vaporization rate K i ", and the second linear part a second average vaporization rate called "final vaporization rate K f ". Both Figures 5.2 and 5.3 show few common features, the d 2 /d 0 2

Figure 5 . 2 :

 52 Figure 5.2: Normalized squared-diameter curves for ethanol (95%) at different temperatures;

Figure 5 . 3 :

 53 Figure 5.3: Normalized squared-diameter curves for anhydrous ethanol at different tempera-

Figure 5 . 4 :

 54 Figure 5.4: Normalized squared-diameter curves for ethanol (95%) and anhydrous ethanol at

Figure 5 . 5 :

 55 Figure 5.5: Normalized squared-diameter curves for ethanol (95%) and anhydrous ethanol at

d 2 -Figure 5 . 6 :

 256 Figure 5.6: Average vaporization rates K i for anhydrous ethanol and ethanol (95%) at differ-

Figure 5 . 7 :Figure 5 . 8 :

 5758 Figure 5.7: Average vaporization rates calculated during second linear part of the d 2 curves

Figure 5 . 9 and

 59 Figure 5.9 and Figure 5.10 show the original d 2 (t) curve and their corresponding instanta-

Figure 5 . 9 :Figure 5 . 10 :

 59510 Figure 5.9: Evolution of the instantaneous vaporization rate, K inst (t) and the squared droplet diameter for ethanol (λ5%) droplet at (a) T∞= 473 K (d0 = 407 m) and (b) T∞= 673 K (d0 = 523 m)ν P∞= 0.1 MPa.

Figure 5 . 11 :Figure 5 . 12 :Figure 5 . 13 :

 511512513 Figure 5.11: Comparison of the instantaneous vaporization rate, K inst (t) of ethanol (95%)

Figure 5 . 14 :

 514 Figure 5.14: Example of determination of water diameter in the droplet from d 2 (t) curves for

Figure 5 . 15 :Figure 5 . 8 ,

 51558 Figure 5.15: Volume percentage of condensed water at different temperatures for anhydrous

Figure 5 .

 5 Figure 5.17: d 2 (t) curves for 1-propanol at various temperatures; P∞ = 0.1 MPa

Figure 5 . 18 :

 518 Figure 5.18: Average vaporization rates, K for 1-propanol droplet at various temperatures; P∞ = 0.1 MPa. The dotted line is the polynomial fit of the data.

Figure 5 .

 5 19a shows the original d 2 (t) curve and their corresponding instantaneous vaporization rate, K inst against time for 1-propanol droplet at ambient gas temperature of 473 K. It is clearly illustrated that a 'quasi-steady' vaporization period occurs throughout the droplet lifetime. The same behaviour but with a slight 'unsteadiness' is observed at ambient gas temperature of 673 K (Figure5.19b).

Figure 5 . 19 :

 519 Figure 5.19: Evolution of the instantaneous vaporization rate, K inst (t) and the squared droplet

Figure 5 . 20 :Figure 5 . 21 :Figure 5 . 22 :Figure 5 . 23 :Figure 5 . 24 :

 520521522523524 Figure 5.20: Comparison of the instantaneous vaporization rate, K inst (t) of 1-propanol droplet

Figure 5 . 25 :

 525 Figure 5.25: Normalized d 2 -curves for ethanol droplets (absolute, no additional water con-

Figure 5 . 26 :

 526 Figure 5.26: Normalized d 2 -curves for ethanol droplets (initial water content = 5% volume)

Figure 5 . 27 :

 527 Figure 5.27: Normalized d 2 -diameter curves for ethanol droplets (initial water content = 10%

Figure 5 . 28 :

 528 Figure 5.28: Normalized d 2 -diameter curves for ethanol droplets (initial water content = 20%

Figure 5 . 29 :

 529 Figure 5.29: Normalized d 2 -diameter curves for ethanol droplets (initial water content = 30%

Figure 6 . 1 :

 61 Figure 6.1: Presentation of ignition delay time calculation steps by modelling simulation.

  where the activation energy equal to the bond energy by assumption of a reference frequency factor, to the metathesis reactions to define the reactivity of the H atoms in hydroxyl position and the H atoms in α position. Decomposition reactions of the corresponding alkoxy and parent radicals from alcohols fuels and finally the class of the four-centre molecular dehydration reactions are required to complete the kinetics mechanism of fuel ethanol and 1-propanol. For 1-propanol fuel, a mechanisms developed by Johnson et al. (2009) has been utilised in this study. The model consists of 1415 reactions involving 237 species. The 1-propanol mechanism was developed based on the hierarchical structure of chemical kinetic mechanisms and uses the updated C 3 -chemistry (Bourque et al. (2008) and Johnson et al. (2009))for the baseline chemistry, with the propanol isomer sub mechanisms added. These sub mechanisms were systematically generated considering (i) unimolecular fuel decomposition reactions, (ii) hydrogen atom abstraction reactions, and (iii) -scission reactions associated with the alkyl/alkoxy radicals generated from the parent fuel.

Figure 6 . 2 :

 62 Figure 6.2: Alcohol oxidation pathway (Adopted from Norton et al., (1991)).

(6- 2 )where 1

 21 Figure 6.3 shows the major feature of the RCM, namely the ability to vary compressed

Figure 6 . 3 :

 63 Figure 6.3: Experimental pressure profile measured in the RCM of Ethanol at all temperatures; P c =30.0 bar, φ=1.0.

Figure 6 . 4 :

 64 Figure 6.4: Experimental pressure profile measured in the RCM of 1-propanol at all temperatures; P c =30.0 bar, φ=1.0.

Figure 6 . 5 :

 65 Figure 6.5: Experimental pressure profile measured in the RCM of Ethanol (70% vol)/water (30% vol) mixture at all temperatures; P c =28.0 bar, φ=1.0.

Figure 6 .

 6 Figure 6.6 illustrates the comparison of measured pressure profile between ethanol, 1-

Figure 6 . 6 :

 66 Figure 6.6: Comparison of experimental pressure profile measured in RCM of Ethanol and 1-

Figure 6 .

 6 Figure 6.7 shows in more detailed the comparison between calculated pressure profiles of

Figure 6 . 7 :Figure 6 . 8 :

 6768 Figure 6.7: Comparison of calculated pressure profile of Ethanol and 1-propanol mixture us-

Figure 6 . 9 :

 69 Figure 6.9: Experimental ignition delay time of Ethanol, 1-propanol and Ethanol (70% vol)/water (30% vol) fuels at all temperatures; P c =34-35 bar, φ=1.0. Lines are linear square

Figure 6 .

 6 Figure 6.10 shows the comparison of experimental and model predicted ignition delay time

Figure 6 . 10 :

 610 Figure 6.10: Experimental (symbols) and model predictions (lines) ignition delay time of Ethanol and 1-propanol fuels at all temperatures; P c =34-35 bar, φ=1.0.

Figure 6 . 11 :Figure 6 . 12 :Figure 6 . 13 :

 611612613 Figure 6.11: Experimental (symbols) and model predictions (lines) ignition delay time of

Figure 6 . 14 :

 614 Figure 6.14: Evaluation of the most sensitive reactions with the Aramco mechanism of Etha-

Figure 6 . 15 :

 615 Figure 6.15: Evaluation of the most sensitive reactions with the Johnson mechanism of 1-

Figure

  Figure A.2μ Droplet injector data acquisition Microdrop™

=Figure C. 1 :Figure C. 2 :Figure D. 2 :

 122 Figure C.1: The disparity of initial vaporization rate K i and final vaporization rate K f of etha-

Figure D. 3 :

 3 Figure D.3: Pressure profile of 1-Propanol at all temperatures using the mechanism of Johnson et al.; P c =30.0 bar, φ=1.0.

Figure D. 4 :

 4 Figure D.4: Pressure profile of Ethanol (70% vol)/water (30% vol) mixture at all temperatures using the mechanism of (a) Frassoldati et al. and (b) Aramco; P c =28.0 bar, φ=1.0.

Figure F. 1 :Figure F. 2 :

 12 Figure F.1: Evaluation of the most sensitive reactions with the Frassoldati et al. mechanism

Table 2

 2

-1.

Table 2 .

 2 1: Common physical properties a of Ethanol, 1-propanol and Gasoline a All data from Jeuland et al. (2004) unless stated otherwise. b data from Reid et al. (1987), c data from Tran et al. (2012) and d data from Anderson et al. (2010).A number of observations can be made from Table2.1 regarding the properties of alcohols as an alternative fuel or addictive to gasoline. Ethanol and 1-propanol have a very high octane number, which induces a strong resistance to knock and consequently the ability to optimize

		Ethanol 1-Propanol Gasoline
	Molar Mass (g/mol)	46.07 b	60.1 b	102.5
	C (wt. %)	52.2	59.96	86.5
	H (wt. %)	13.1	13.42	13.5
	O (wt. %)	34.7	26.62	0
	Density (kg/mm 3 )	790 c	803 c	735-760
	Boiling Temperature (K)	351.6 b	370.3 b	303-473 c
	Autoignition Temperature (K)	763 c	753 c	723 c
	Latent Heat of Vaporization (kJ/kg) 854	689	289
	Lower Heating Value (MJ/kg)	26.8 c	30.9 c	42.7 c
	Stoichiometric Ratio	8.95	10.28	14.4
	Research Octane Number (RON)	109 d	112 c	91-99 d

  in early 1950 and has led to the development of a theoretical model capable of describing the gasification process of a fuel droplet. The model, renowned as 'Quasisteady model' or else called the d 2 -law, shows that during the gasification process, the droplet surface area, represented by the droplet-squared diameter, changes linearly during its lifetime.

	The d 2 -law consists much of the crucial physics and rough approximations of droplet gasifi-
	cation. However, this model is based on a few vital hypotheses. The most common hypothe-
	ses are as listed below:
	a. Constant and uniform droplet temperature: The mechanisms of the heating and mass
	transport inside the liquid phase are negligible.
	b. The droplet is always symmetrically spherical: This means that both natural and
	forced convections are always absent and thus the droplet remains spherical during its
	lifetime. The consequence of this assumption is that non-radial motion in the gas-
	phase is absent. Hence the analysis reduces to one dimension i.e. in radial direction
	only.

  al. (1972), Lifshitz, (2001)). According to Würmell et al. (2009), shock tube experiments are generallycarried out with a premixed mixture of fuel, oxygen, and argon, since the shock wave behaviour is optimized in monatomic carrier gases, such as argon. Dilute fuel mixtures that contain only small proportions of fuel and oxygen in more than 90% of argon are therefore studied under the most optimal shock wave conditions, since the fraction of polyatomic gas is kept small.

Table 3 -

 3 1: Physical and chemical properties of ethanol and 1-propanol

	Properties

Table 3 -

 3 4: Diluent gases used in RCM experiments.

	Gas	Purity (%) Major Contaminations (volume per million)
		Ar <250 vpm
	N 2 (CP Grade)	99.95
		O 2 <1 vpm
		Hydrocarbon < 0.0014%
		H 2 O < 0.025%
	O 2	99.50
		CO 2 < 0.0014%
		CO < 0.0014%

Table 3 -

 3 5: Fuels used in RCM experiments.

	Fuel	Purity (%)
	Anhydrous Ethanol	99.5
	1-Propanol	99.9

Table 3 -

 3 6: Autoignition operating conditions.

	Fuel	P i (bar)	T i (K)	P c (bar)	T c (K)
	C 2 H 5 OH	1	330 -380	30	780 -860
	C 3 H 7 OH	1	340 -380	30	750 -850
	C 2 H 5 OH (70% vol)/H 2 O(30% vol)	1	350 -370	30	790 -835
	Anhydrous ethanol (Sigma-Aldrich, 99.5%), 1-propanol (Sigma-Aldrich, 99.9%) and pure
	For molar calculations,
					(R1)
	Where m =
	For Ethanol,
	m= 3,
	C 2 H 6 O + 3 O 2 + 11.28 N 2 → 2Cτ 2 + 3H 2 O + 11.28N 2

water (Mili-Q-Milipore, 0.22 µm) are used as the reactants. To determine the mixture composition, the mass of fuel, equivalence ratio and oxidizer ratio are specified. The diluents gas used is Nitrogen (N 2 ). Equivalence ratio is fixed to φ = 1.0 (stoichiometric condition) at all experiments. Proportions of O 2 and N 2 in the mixture are determined manometrically and added at room temperature (Table

3

-7, 3-8 and 3-9).

Table 3 -

 3 7: Mixture Preparation of Anhydrous Ethanol for both Reactive and Non Reactive.

	Reactive	Non Reactive

Table 3 -

 3 8: Mixture Preparation of 1-Propanol for both Reactive and Non Reactive.

	Reactive	Non Reactive

Table 3 -

 3 9: Mixture Preparation of Anhydrous Ethanol/H 2 O for both Reactive and Non Reactive.

	Reactive	Non Reactive

  Adiabatic expansion of the core due to the cooling of the boundary layer (test gas).2. Non Reactive mixtures, i.e. O 2 are replaced by N 2 (due to its similar thermody-yet to be published) is also applied for comparison purposes and it consists of 1542 reactions with 253 species. A kinetic mechanism of methanol and ethanol by[START_REF] Frassoldati | An Experimental and Kinetic Modeling Study of n-Propanol and iso-Propanol Combustion[END_REF] is reasonably well-known and has been revised recently by numerous authors. According to them, the mechanism is evolving from the initiation reactions

	1. namics properties). still underdevelopment and
	INPUT 6. 3 Detailed Kinetics Mechanism of Ethanol and 1-propanol
	Non Reactive Folder
	Kinetic Mechanisms Alcohol have been defined as organic compounds characterized by a hydroxyl functional Non Reactive Profile
	group OH, attached to a main carbon root, R. Figure 6.2 shows the oxidation pathways of al-
	adi_P2V cohols, depends on which bond hydrogen abstraction occurs (Norton et al. (1991)). The reac-
	Non Reactive Volume Profile
	tions governing ignition delay time or chemical induction period combustion can be broken
	down into four categories:
	Reactive Volume Profile
	1. Chain-initiating reactions: fuel is decomposed, usually by uni-molecular decompo-
	SEGMENT FIT sition (pyrolysis at high temperatures and by RH+O 2 → +HO 2 reaction at lower
	Approximated Volume Profile
	temperatures).
	CHEMKIN 3.7
	Pressure Profile 2. Chain-propagation reactions: keep the radicals concentration constant.
	Post -Processing 3. Chain-branching reactions: increase the radical pool where intermediate species are
	formed and radicals (reactive species with an unpaired electron) are released.
	P i , T i , P c , T c , 10 4 /T c , IDT
	4. Chain-terminating reactions: decrease the radical pool where the final stable prod-
	ucts such as H 2 O and CO 2 are formed.
	The pyrolysis and oxidation mechanism of ethanol and 1-propanol are very similar to those
	for hydrocarbon fuels. The development of a complete set of primary propagation reactions
	of fuel ethanol has been under studied and defined by Frassoldati et al. (2010) with a few new
	kinetics parameters for reactions involving bonds and H-atoms near to the OH group. This
	kinetic model consists of 1416 reactions involving 80 species. Another kinetics mechanism
	developed by C3 NUIG researchers called Aramco mechanism (courtesy of C3, NUIG and

Table B -

 B 1: Physical properties of all compounds used in this study.NameFormula Molecular Weight (g/mol) T b (K) T c (K) P c (bar) V c (cm 3 /mol)

	Nitrogen	N 2	28.013	77.4	126.2	33.9	89.8
	Ethanol	C 2 H 5 OH	46.069	351.4 513.9	61.4	167.1
	1-propanol C 3 H 7 OH	60.096	370.3 536.8	51.7	219.0
	Water	H 2 O	18.015	373.2 647.3	221.2	57.1
	T b = normal boiling point
	T c = critical temperature
	P c = critical pressure
	V c = critical volume

Table B -

 B 2: Constants to calculate the saturated vapour pressure by using equation of Wagner (Equation 4-8).

	Name	Formula	A	B	C	D
	Ethanol	C 2 H 5 OH -8.51838	0.34163	-5.73683 8.32581
	1-propanol C 3 H 7 OH -8.05594 0.04.25183 -7.51296 6.89004
	Water	H 2 O	-7.76451	1.45838	-2.77580 -1.23303

Table B -

 B 3: Constants to calculate the isobaric heat capacity of the ideal gas by method of Joback (Equation 4-11) and Lennard-Jones potentials of all compounds used in this study. , C and D = constants to calculate isobaric heat capacity of the ideal gas

	Name	Formula	A	B	C	D	(Å) ε/k (K)
	Nitrogen	N 2	3.115E+1 -1.357E-2 2.680E-5 -1.168E-8 3.798 71.4
	Ethanol	C 2 H 5 OH 9.014E+0 2.141E-1 -8.390E-5 1.373E-9 4.530 362.6
	1-propanol C 3 H 7 OH 2.470E+0 3.325E-1 -1.855E-4 4.296E-8 4.549 576.7
	Water	H 2 O	3.224E+1 1.924E-3 1.055E-5 -3.596E-9 2.641 809.1
	A, B

Table E -

 E 2: Experimental ignition delay times of 1-Propanol at various compressed temperature T c ; P c = 34-35 barν φ= 1.0.P i (mbar) T i (K) P c (bar) T c (K) 10000/Tc (K -1) , (dP/dt) max (ms)

	339.41 34.79	798	12.53	61
	339.44 33.65	794	12.59	64.5
	339.42 34.90	796	12.56	59
	339.24 34.12			Non Reactive
	329.80 34.44	776	12.89	153
	329.08 33.71	771	12.97	174
	328.01 35.28	775	12.90	150.4
	332.01 34.74			Non Reactive
	349.61 34.44	815	12.27	30
	349.06 34.29	813	12.30	30.4
	348.95 34.30	813	12.30	30
	348.73 34.05			Non Reactive
	358.54 34.69	834	11.99	15.2
	358.58 33.84	831	12.03	15.6
	358.54 34.92	834	11.99	15.4
	358.60 34.38			Non Reactive
	368.29 33.68	849	11.78	8.6
	368.15 34.07	851	11.75	8.2
	368.07 33.84	849	11.78	8.2
	368.17 35.07			Non Reactive
	378.02 34.53	871	11.48	4.2
	377.89 34.04	870	11.49	4.2
	377.91 34.04	870	11.49	3.9
	377.85 34.36			Non Reactive

Table E -

 E 5: Simulated ignition delay times of ethanol by Aramco mechanism at various compressed temperature Tcν Pc= 30 barν φ= 1.0.P i (mbar) T i (K) P c (bar) T c (K) 10000/T c (K -1 ) , (dP/dt) max (ms)

	959	340	29.60 776.7	12.87	329.6
	980	349	30.10 793.8	12.6	129.8
	967	358	29.90	812	12.32	65.8
	958	368	30.10 834.2	11.99	31.0
	958	378	30.10 853.1	11.72	16.4
	958	388	30.10	872	11.47	9.0
	958	393	30.10 881.5	11.34	6.6

Table E -

 E 6: Simulated ignition delay times of ethanol/water by Frassoldati et al. mechanism at various compressed temperature T c ; P c = 28 barν φ= 1.0.Table E-7: Simulated ignition delay times of ethanol/water by Aramco mechanism at various compressed temperature T c ; P c = 28 barν φ= 1.0. P i (mbar) T i (K) P c (bar) T c (K) 10000/T c (K -1 ) , (dP/dt) max (ms)

	968	349	27.9	781.2	12.8	346.4
	980	360	28.6	804.5	12.43	110.2
	984	368	28.2	817	12.24	55.0
	984	378	28.2	835.7	11.97	26.8
	984	388	28.2	854.3	11.71	14.0
	984	393	28.2	863.5	11.58	10.2
	984	400	28.2	876.5	11.41	6.6
	968	349	27.9	781.4	12.8	410.6
	980	360	28.6	804.8	12.43	106.2
	984	368	28.2	817.4	12.23	50.2
	984	378	28.2	836.3	11.96	23.4
	984	388	28.2	855.3	11.69	11.6
	984	393	28.3	864.9	11.56	8.4
	984	400	28.3	878.8	11.38	5.2

P i (mbar) T i (K) P c (bar) T c (K) 10000/T c (K -1 ) , (dP/dt )max (ms)

Table E -

 E 8: Simulated ignition delay times of 1-propanol by Johnson et al. mechanism at various compressed temperature T c ; P c = 30 barν φ= 1.0.

	P i (mbar) T i (K) P c (bar) T c (K) 10000/T c (K -1 ) , (dP/dt) max (ms)
	959	332	29.8	760.3	13.15	111.6
	968	340	29.4	771.3	12.97	55.8
	968	349	29.3	789.1	12.67	29.2
	960	358	29.6	810	12.35	15.6
	967	368	30.2	830.6	12.04	9.0
	964	378	29.6	848.1	11.79	5.2

C. Supporting figures for the effects of initial droplet diameter and ambient relative humidity to average vaporization rate.

(a)

laughter, and the good tears.

Financial support over the course of my Ph.D. studies was provided by Ministry of Higher Education Malaysia and is gratefully appreciated.

der more defined conditions than those found in an engine by excluding the complicating influences of blow-by, spatial inhomogeneities etc. A RCM must be capable of the nearadiabatic compression of a low-pressure test gas into a confined volume of elevated pressure and temperature and of maintaining these conditions. Achievable post-compression conditions in RCM studies are ≈ 10-60 atm and ≈ 600-1100 K, and as such the RCM is a valuable tool for the study of the principles of homogenous charge compression ignition (HCCI).

HCCI involves the compression of a homogenous (fuel) lean mixture of fuel/air. The use of a dilute and premixed fuel/air mixture allows ignition to occur at many points simultaneously when the piston is close to top-dead-centre, preventing thermal runaway of the combusting mixture and eliminating the high temperature combustion zones responsible for NO x and particulate matter production. The RCM in NUIG has its origins in the Shell-Thornton (Affleck and Thomas, (1969)) research laboratory, where it operated since its creation in 1λ6λ until the mid 1λ80's. The machine arrived in NUIG, Galway in 1995 where it was re-commissioned and modified to operate in its new environment. A description of these initial minor modifications and a detailed description of the workings of the machine are given by [START_REF] Brett | Simulation of methane autoignition in a rapid compression machine with creviced pistons[END_REF]. A schematic of the NUIG RCM is given in Figure 3.9 (Affleck and Thomas, (1968)). Briefly, to compress the test gas the RCM uses two horizontally opposed pistons which are tightly sealed inside two compression sleeves which adjoin the reaction chamber. The RCM is symmetrical; two large drive chambers are positioned behind each piston and serve as a reservoir for high pressure compressed air to drive the pistons forward at high speed. The two halves of the RCM are distinguished by the terms "fixed" and "free" referring to the non identical compression sleeves which allow the reaction chamber to be attached in the centre of the RCM. Following compression the pistons may be withdrawn to their pre-fired position by applying a vacuum to these same drive chambers. Adjacent to each drive chamber is an oil-containing hydraulic chamber through which the piston must pass as part of a complicated oil-hydraulic system which controls the starting, stopping and velocity of the pistons. By pressurising the hydraulic system the pistons are held in the pre-fired position whilst the drive pressure is applied.

Once the desired pressure is contained in the drive chamber and the test gas has been admitted to the reaction chamber assembly, the pistons may be driven forward instantaneously by venting a portion of the pressure applied to the hydraulic system. This motion confines the test gas in the reaction chamber at an elevated temperature and pressure. All experiments were performed with creviced piston heads, an idea that was first engineered by [START_REF] Park | Rapid compression machine measurements of ignition delay times for PRD Mixtures[END_REF] and further developed by Lee and Hochgreb (1998). According to Silke et al.

(2007), provided their optimal size and shape, piston head crevices effectively swallow the cooler boundary that is scraped from the chamber wall during the piston movement, thus preventing it from mixing with the hot compressed gas. The net effect is a more homogeneous distribution of temperature during the post-compression period. Since the rates of chemical reactions are extremely sensitive to temperature, non-homogeneous temperature fields render realistic kinetic modelling very difficult or even impossible. [START_REF] Würmel | CFD studies of a twin-piston rapid compression machine[END_REF] highlighted the importance of an optimal piston head design. It was shown, by means of a computational fluid dynamic (CFD) study, that the crevice volume, its distance from the chamber and the ease with which the gas can flow into the crevice are crucial design considerations.

In our case of ethanol and 1-propanol fuels vaporization, the calculations of the Lewis number (Figure 4.9) show that the values are not unity (the Lewis number varies between 1 and 2). At lower temperature 1-propanol fuel seems to have higher value of Lewis number compared to ethanol fuel. Nevertheless, the Lewis number seems to converge to unity for both alcohols as the ambient temperature increases, therefore they are in almost agreement with the principal assumption of quasi-steady theory. Note that these figures do not exhibit the droplet heat-up periods for the reason that during this period of the droplet lifetime, the droplet is moved from the so-called 'cold chamber' to the furnace. This movement is accompanied by vibrations and subsequently induces oscillations. It was decided then to show the evolution of surface area of the vaporized droplet only when the droplet was stabilized.

Sensitivity Analysis Results

A sensitivity analysis study was conducted for the purposes of determining the most important reactions which strongly influence the overall rate of ethanol oxidation. Hydrogen peroxide radical and fuel dehydrogenation reactions control ignition delay time of ethanol and ethanol/water mixtures. Figures 6.12a The main chamber is consists of high-temperature furnace, three dimensional motorized displacement system of piezo-electric injector, movable frame and other various related equipment. The cylindrical chamber is made from aluminium alloy with the furnace made from stainless steel. The furnace is a cylinder with an inner diameter of 68 mm and 100 mm height. The heating system is by Joule effect. The furnace is capable of generating temperatures up to 1200 K by the Joule effect and it is placed in a steel chamber that could be pressurized if required.

Programmable Logic Controller (PLC) and temperature operation panel

The panel consists of adjustment and display of the resistance intensity controller, visualisation of actual temperature in the furnace, visualization of pressure, furnace ignition controller, selection of thermocouple and lights controller. RCM pre-firing procedure 1. Turn air extractor fan on.

2. Turn pump on, close tap 1 (to atmosphere).

3. Turn oscilloscope, reaction chamber assembly thermometer and charge amplifier on.

4. Confirm vacuum and pressure gauges are on and that the reaction chamber assembly is at good vacuum (≈ 10 -2 Torr).

5.

Prepare oscilloscope for acquisition by, "File" _ "Recall" _ "Recall Setup τnly".

RCM firing procedure 1. Following a compression valve 4 should be closed, valves 1, 2, 3 and 5, as well as the air admittance tap should be open: confirm this is the case.

2. Open the solenoid valve to allow the depressurised hydraulic fluid to equilibrate around the entire hydraulic system by depressing the fire button for five seconds.