Giovanni Bègue
email: giovanni.begue@ineris.fr

Lionel Aufauvre
email: lionel.aufauvre@ineris.fr

Christian Michot

Certification of pyrotechnic articles for vehicles according to the EU directive 2007/23/EC on the placing on the market of pyrotechnic articles

 on the placing on the market of pyrotechnic articles. Following the "new approach" this Directive aims at ensuring the free movement of such products within the European Union (EU), improving safety of both consumers and professionals and implementing harmonized safety requirements for these articles in all Member States. Therefore a European frame for the approval of pyrotechnic articles for vehicles is now set up by introducing harmonized Essential Safety Requirements (ESR), CE marking and certification procedures. For manufacturers and importers that had to comply with a different regulation in each country to get technical approvals of their products the principle "tested once, accepted everywhere" is a great achievement.

Although this EU directive will reduce the administrative burden for everybody, its substitution and application instead of the current national regulations may create difficulties at the beginning.

As Notified Body and Official Laboratory in France INERIS has gained lot of experience in testing, certifying and approving explosives and pyrotechnic articles. In this paper we give an insight of the different parts of the Directive 2007/23/EC and present the frame of the CE marking and certification procedures for pyrotechnic articles for vehicles.

INTRODUCTION

On May 23, 2007 the European Parliament and the Council adopted the Directive 2007/23/EC [1] on the placing on the market of pyrotechnic articles. This newly published text is another piece of the European regulation concerning explosives and pyrotechnic articles. It supplements the Directive 93/15/EEC of 5 April 1993 [2] on the harmonization of the provision relating to the placing on the market and supervision of explosives for civil uses. These two Directives following the "new approach" aim at ensuring the free movement of products within the European Union (EU), improving safety of both consumers and professionals and implementing harmonized safety requirements for such products in all Members States.

If pyrotechnic articles and ammunitions, identified in the Directive 2004/57/EC of 23 April 2004 [3], were explicitly excluded from the scope of the Directive 93/15/EEC, the Directive 2007/23/EC applies now specifically to pyrotechnic articles including fireworks and pyrotechnic articles for vehicles.

INERIS, who is Notified Body for the certification of the explosives according to the former Directive 93/15/EEC, has acquired experience as Official Laboratory in France (by order of February 11, 1994) in the approving of explosives and pyrotechnical automotive safety equipment, since 1990 and the publication of the French decree n°90-153 of February 16, 1990 [4]. This paper presents the different points of the Directive 2007/23/EC related to pyrotechnic articles for vehicles, although the Directive also deals with fireworks and other pyrotechnic articles. The certification procedures are reviewed, taking into account the regulation and the actual practices developed to assess the conformity to the Essential Safety Requirements (ESR) of this new Directive for the EC certification of pyrotechnic articles for vehicles.

DEFINITIONS AND GENERALITIES

Pyrotechnic articles for vehicles: components of safety devices in vehicles which contain pyrotechnic substances used to activate these or other devices.

Type: sample representative of the production envisaged, placed at the disposal of the Notified Body by the applicant, in order to its evaluation according to the ESR in the annex I of the Directive.

For pyrotechnic articles for vehicles seven main kinds of products have been identified: -Igniter (squib, initiator…), -micro gas generator, -seatbelt pretensioner (buckle, retractor…), -airbag gas generator or inflator, -airbag module (driver, passenger, side, knee…), -actuator (retractor, pusher, circuit breaker, switch…), -semi-finished assembly. Category P1 pyrotechnic articles: pyrotechnic articles other than fireworks and theatrical pyrotechnic articles which present a low hazard.

Category P2 pyrotechnic articles: pyrotechnic articles other than fireworks and theatrical pyrotechnic articles which are intended for handling or use only by persons with specialist knowledge.

Person with specialist knowledge: person authorized by a Member State to handle and/or use on its territory, category P2 pyrotechnic articles.

Manufacturers, importers and distributors shall sell or otherwise make available pyrotechnic articles of category P2 only to persons with specialist knowledge. Pyrotechnic articles of category P1 shall be sold or made available only to consumers above 18 years. However the Directive 2007/23/EC admits that pyrotechnic articles for vehicles are supplied exclusively to professional users.

As far as it concerns pyrotechnic articles for vehicles the relevant date to be considered are the following:

-July 4, 2007: date of entry into force of the Directive 2007/23/EC -January 4, 2010: Member States shall have adopted and published the laws, regulations and administrative provisions necessary to comply with the Directive 2007/23/EC; -July 4, 2013: Member States shall apply the provisions of the Directive 2007/23/EC; -Approvals issued before July 4, 2013 remain valid until their expiry date or up to July 4, 2017 whichever is shorter (by way of derogation from that, national approvals for pyrotechnic articles for vehicles issued before July 4, 2013 shall continue to be valid until their expiry).

CONFORMITY ASSESSMENT PROCEDURES FOR PYROTECHNIC ARTICLES

The manufacturer or the importer of the pyrotechnic articles, established in the Community, has to lodge its application for the assessment of conformity by the means of one of the following procedures allowed by the Directive 2007/23/EC:

(a) the EC type-examination (Module B) procedure, and, at the choice of the manufacturer, either: (i) the conformity to type (Module C) procedure, (ii) the production quality assurance (Module D) procedure, (iii) the product quality assurance (Module E) procedure;

(b) the unit verification (Module G) procedure. This module is applied alone for each single article and therefore seems less appropriate for pyrotechnic articles for vehicles.

INITIAL CERTIFICATION

Application for initial certification of pyrotechnic articles for vehicles can be made directly to INERIS [START_REF]Application and documentation to be sent: by mail to INERIS Certification Division -Parc Technologique Alata BP2 -60550[END_REF] or by use of an official application form available on request to INERIS. The initial application shall contain the following elements:  name and address of the applicant or its proxy if not in the EU,  name and address of the manufacturer(s),  commercial name(s) of the product to certify,  conformity assessment procedure selected by the applicant,  declaration of the applicant saying that "the application for the EC certification under the Directive 2007/23/EC of May 27, 2007, for the products designated, has not been lodged with any other Notified Body".

-a technical documentation covering the design (diagrams of components, subassemblies, composition and mass of each pyrotechnic element) of the article and its variants, -a technical documentation describing the manufacturing process and the controls (description of test procedures and sampling) applied during production and on finished products, -the instructions for use, -a safety data sheet, For modules C, D or E, this application is to be supplemented with a documentation demonstrating the measures implemented the applicant or the manufacturer to ensure the conformity to the type, to declare the conformity to the type and to affix the CE marking.

On reception of an application INERIS makes a preliminary analysis of the documentation received to determine the validity of the application, the tests to be carried out, the number of samples needed, eventually the additional information necessary from the applicant. On that basis INERIS sends to the applicant a quote covering all the jobs for the assessment of the request (testing, audit visit of manufacturing plant, paperwork...), the cost is essentially depending of the number of products and of manufacturing plants to be assessed. A time schedule is also established after discussion with the applicant. The overall process from the reception of the application to the emission of the certification documents can take one month if everything goes smoothly. The first step consists of either the EC type-examination (module B) or the unit verification (module G). When it is passed successfully a certificate is issued for the product assessed. In case of the module B the applicant has to choose between the conformity to type (module C), the production quality assurance (module D) or the product quality assurance (module E) for the evaluation of the manufacturing phase. INERIS carries out the assessment according to these modules by the means of periodical audit visits. If the result of the audit is favourable, INERIS notifies to the manufacturer(s) its authorization for the EC marking according to the module chosen. This notification remains valid until the date of the next audit.

As far as it will concern pyrotechnic articles for vehicles most of the assessments will likely be carried out according to the combination of module B (EC type-examination) and module D (production quality assurance) due to the fact that in the automotive industry the ISO 9001 and ISO/TS 16949 standards are widely implemented.

On the one hand, module B covers the design phase and is to be applied for each type to be defined (general design of a product), the examination is carried out to assess the conformity of a design with the ESR.

And on the other hand module D covers the production phase and is to be applied for several products, the evaluation is carried out to guarantee the conformity of manufactured objects with the corresponding type approved.

AMENDMENT OF A CERTIFICATION

When an initial certification has been granted for a particular design of product and manufacturer, some situations may require amendment of this certification for example:

-transfer of certification from a Company to another one, -addition and/or withdrawal of manufacturer(s), -addition and/or withdrawal of commercial name(s), -addition of a variant of the design certified. The variant will be evaluated following the same conformity assessment procedures as the initial model. But depending to what extend the variant differs from the initial design, parts of the initial test results will be reused for the assessment of the variant. These kinds of amendments can be managed either by addition to the existing certificates (modifying and/or extending the type) or by issue of new certificates.

In some cases an audit of the manufacturer(s) is performed if:

-the new manufacturer(s) has never been audited, -the manufacturing process of the variant has never been audited, or if the addition of the variant changes significantly the system of production and control of the products certified, -the validity of the certification of the quality system matures.

CONVERSION OF FRENCH APPROVAL TO EC TYPE-EXAMINATION CERTIFICATION

Companies that hold valid French approvals for pyrotechnic articles for vehicles will beneficiate of an adapted procedure for the conversion of them into EC type-examination certificates, but it will not be automatic and it will be done on request. A preliminary study of the application will be performed to verify if all the documents and test results available are sufficient to answer to the ESR of the Directive 2007/23/EC and allow INERIS to carry out the EC type-examination. In the case of no additional documents or tests is required the procedure will continue with the necessary paperwork. In the other cases (need of additional documents and/or test results) the petitioner will have to complete its application.

In addition, the quality system of the manufacturer will be assessed according to the module of its choice.

REQUIRED DOCUMENTS AND INFORMATION FOR THE APPLICATION OF CONFORMITY ASSESMENT

EC TYPE-EXAMINATION (MODULE B)

The application is constituted of the following elements:  the name and address of the manufacturer,  a written declaration that the same application has not been lodged with any other notified body,  a technical documentation covering the design, manufacture and operation of the article and containing:

-a general type-description, -conceptual design and manufacturing drawings and diagrams of components, subassemblies, circuits, etc., -descriptions and explanations necessary for the understanding of the drawings and diagrams and the operation of the article, -a list of the harmonized standards referred to in Article 8 of this Directive, applied in full or in part, and descriptions of the solutions adopted to meet the essential safety requirements of this Directive where the harmonized standards referred to in Article 8 of this Directive have not been applied, -results of design calculations made, examinations carried out, etc., -test reports.

CONFORMITY TO TYPE (MODULE C)

This module is always applied with the module B. This module is applied when a manufacturer does not operate a quality system or has a quality system not externally approved. In this case, INERIS must perform or ask to perform a control of a sample of finished articles taken on the spot at random intervals.

PRODUCTION QUALITY ASSURANCE (MODULE D)

This module is always applied with the module B. This module is applied when the manufacturer operates quality system externally approved (for example ISO 9001, ISO/TS 16949…) for production, final product inspection and testing. However, the manufacturer must lodge an application for assessment of his quality system with INERIS.

The application must include  all relevant information for the pyrotechnic article category envisaged,  the documents concerning the quality system,  the technical documents pertaining to the approved type and a copy of the EC typeexamination certificate.

INERIS will ensure a monitoring to make sure that the manufacturer duly fulfils the obligations arising out of the approved quality system. For that INERIS carries out periodically audits to make sure that the manufacturer maintains and applies the quality system. During these audits the manufacturer must allow access to the manufacturing, inspection, testing and storage premises and provide all necessary information, in particular:  the quality system documents,  the quality records, such as inspection reports and test data, calibration data, and qualification reports of the personnel concerned.

PRODUCT QUALITY ASSURANCE (MODULE E)

This module is always applied with the module B. This module is applied when the manufacturer operates quality system externally approved (for example ISO 9001, ISO/TS 16949…) for final product inspection and testing.

However, the manufacturer must lodge an application for assessment of his quality system with INERIS. The application must include  all relevant information for the pyrotechnic article category envisaged,  the documents concerning the quality system,  the technical documents pertaining to the approved type and a copy of the EC typeexamination certificate. INERIS will ensure a monitoring to make sure that the manufacturer duly fulfils the obligations arising out of the approved quality system. For that INERIS carries out periodically audits to make sure that the manufacturer maintains and applies the quality system. During these audits the manufacturer must allow access to the manufacturing, inspection, testing and storage premises and provide all necessary information, in particular:  the quality system documents,  the technical documents,  the quality records, such as inspection reports and test data, calibration data, and qualification reports of the personnel concerned.

UNIT VERIFICATION (MODULE G)

This module is always applied alone. This module is applied when the product to certify is unique. It is mainly carried out on the base of technical documents enabling the assessment of the conformity with the ESR, the understanding of the design, manufacture and operation of the pyrotechnic article. The application consists of documents containing:  a general description of the type,  conceptual design and manufacturing drawings and schemes of components, subassemblies and circuits,  the descriptions and explanations necessary for the understanding of the conceptual design and manufacturing drawings, the schemes of components, sub-assemblies and circuits and the operation of the pyrotechnic article,  a list of the harmonised standards referred to in Article 8 of this Directive, applied in full or in part, and descriptions of the solutions adopted to meet the essential safety requirements of this Directive where the harmonised standards referred to in Article 8 of this Directive have not been applied,  results of design calculations made and examinations carried out,  test reports.

The Essential Safety Requirements (ESR) are minimum requirements to be satisfied by the pyrotechnic articles in order to ensure a high level of protection of human health and safety and the protection of consumers and professional end users. Although the ESR do not really intend to guarantee any performance reliabilities in the final use of the articles, an adequate and complete operation of the articles according to what it is expected is in some way a basic condition for safety.

In order to facilitate the process of demonstrating compliance with the essential safety requirements, harmonized standards for the design, manufacture and testing of pyrotechnic articles are being developed by the CEN Technical Committee 212. With regard to pyrotechnic articles for vehicles, these standards are developed under the Vienna agreement to take into account the international orientation of the vehicle supply industry.

Pyrotechnic articles manufactured in compliance with harmonized standards will benefit from a presumption of conformity with the essential safety requirements provided for in the Directive 2007/23/EC. However, the compliance with harmonized standards is never compulsory. In addition, the preparation and the complete adoption of these harmonized standards will required few years during which it will be necessary to carry out assessment procedure. Therefore, there is a necessity to use existing test procedures or standards during the transitory period.

The following table gives a list of existing methods used in the frame of the French approval of pyrotechnical automotive safety equipments [START_REF] Aufauvre | French Approval Procedures for Pyrotechnical Automotive Safety Equipment[END_REF], [START_REF] Lionel | How to deal with the Essential Safety Requirements of the EU directive on the placing on the market of pyrotechnic articles[END_REF].

1.

Each pyrotechnic article must attain the performance characteristics specified by the manufacturer to the notified body in order to ensure maximum safety and reliability.

Operation of the article is compared to what it is suppose to be. Examination of the results of operation in conditions defined by the manufacturer (may require manufacturer's testing facilities).

A1, N7, R8

2.

Each pyrotechnic article must be designed and manufactured in such a way that it can be disposed of safely by a suitable process with minimum effect on the environment.

The way of disposal must be given in the documentation. The chemical composition of the different pyrotechnic mixtures must be given as well as information on their toxicity profile.

Compliance with Directive 2000/53/EC [START_REF][END_REF] is taken into account.

3.

Each pyrotechnic article must function correctly when used for its intended purpose.

Each pyrotechnic article must be tested under realistic conditions. If this is not possible in a laboratory, the tests must be carried out in the conditions in which the pyrotechnic article is to be used.

Articles are tested with or without conditioning. Function correctly can also signify no ignition or deployment for some articles in realistic conditions such as handling.

See also ESR 1.

a)

Design, construction and characteristic properties, including detailed chemical composition (mass and percentage of substances used) and dimensions.

These information have to be provided in the technical documentation and drawings. Careful examination of the samples submitted allows checking of these data. A1

b)

The physical and chemical stability of the pyrotechnic article in all normal, foreseeable environmental conditions.

Indications on what are the foreseeable environmental conditions have to be given in the technical documentation (minimum and maximum temperatures and hygrometry recommended for storage and use, shelf life time…). Thermal stability tests and ageing have to be carried out as well as mechanical resistance tests (vibrations, impact...) E1, E10, J8

d)

Compatibility of all components as regards their chemical stability.

Needs no further examination if the contained pyrotechnic compound is certified. If not the case, the chemical composition is examined.

A1, E1, E10

3. e)

Resistance of the pyrotechnic article to moisture where it is intended to be used in humid or wet conditions and where its safety or reliability may be adversely affected by moisture.

The technical documentation should specify the environmental conditions where the article is intended to be use (minimum and maximum temperatures and hygrometry recommended for storage and use…).

Testing in harsh environmental conditions or after thermal cycles or ageing.

E1, E10, F2 § Essential Safety Requirements (ESR) Comments Test methods

f)

Resistance to low and high temperatures, where the pyrotechnic article is intended to be kept or used at such temperatures and its safety or reliability may be adversely affected by cooling or heating of a component or of the pyrotechnic article as a whole. The technical documentation and/or specific instructions for the user have to described or give these information. This is important for articles handled as spare parts. Pyrotechnic articles must not contain military explosives.

5.B. 1)

Pyrotechnic articles must be designed in such a way as to minimize risk to health, property and the environment during normal use.

The description of the design, assembling and of all necessary accessories for a correct operation must be given in the technical documentation.

Examination of the documentation and the results obtained in the different test procedures carried out. The correct and complete functioning of the pyrotechnic article is checked to assess the conformity to the expected effect Description (dimensions, mass, labelling) and comparison of the sample with the technical data or drawing submitted.

A1, N7

5.B. 2)

The method of ignition must be clearly visible or must be indicated by labelling or instructions.

5.B. 3)

The pyrotechnic article must be designed in such a way as to minimize risk to health, property and the environment from debris when initiated inadvertently

5.B. 4)

Where appropriate, the pyrotechnic article must function properly until the 'use by' date specified by the manufacturer.

In addition, when ignition devices are concerned the following ESR have to be fulfilled: § Essential Safety Requirements (ESR) Comments Test methods

5.C. 1)

Ignition devices must be capable of being reliably initiated and be of sufficient initiation capability under all normal, foreseeable conditions of use. Determination of the recommended firing current, percussion, friction or optical power level from statistical tests (Bruceton type or similar). Electric igniters must be protected against electromagnetic fields under normal, foreseeable conditions of storage and use.

5.C. 4)

The covering of fuses must be of adequate mechanical strength and adequately protect the explosive filling when exposed to normal, foreseeable mechanical stress.

See ESR 3. b) and 3. c) E1, E10, I5, I10, I11, J8

5.C. 5)

The parameters for the burning times of fuses must be provided with the article.

Determination of the burning time when applicable

5.C. 6)

The electrical characteristics (e.g. no-fire current, resistance, etc.) of electric igniters must be provided with the article.

Verification of information provided by the manufacturer. A1

5.C. 7)

The wires of electric igniters must be sufficiently insulated and must be of sufficient mechanical strength, including the solidity of the link to the igniter, taking account of their intended use.

Verification of the resistance of insulation and the mechanical strength of the wires when applicable

A1

The technical documentation and/or the Material Safety Data Sheet (MSDS) have to describe or give enough information to allow the assessment of the ESR. In the case of pyrotechnic articles for vehicles Article 13 of the Directive specifies the minimum information for the labelling and refers to the Directive 2001/58/EC amending the Directive 91/155/EEC for the drafting of the MSDS [9].

SAMPLING FOR EACH TYPE

An analysis of the table listing the essential safety requirements in the previous paragraph gives a minimum sample number in order to cover all the testing for each article type, see table hereafter. These minimum sample number are given in the hypothesis that some tests perform at a sub-level can be reused at an upper level of integration. For example, if electrical test results carried out at the igniter level are available then they will be reused at the upper level. If the conclusions of the evaluation are satisfying, INERIS issues an EC type-examination certificate. This certificate includes an identification number linked to the certified article, the date of the certification, the commercial name(s) of the article, the name and the address of the applicant and of the manufacturer(s), and the data necessary for the identification of certified type.

Article type Minimum sample number

The identification number is as follow:

Registration number of the Notified Body issuing the certificate (INERIS), given by the EU.

Category of the pyrotechnic articles abbreviated (P1 or P2).

Chrono number used by the Notified Body. Year of the certification.

0080.XX.AA.YYYY

INERIS communicates to the other Notified Bodies the relevant information concerning EC type-examination certificates and additions issued or withdrawn, by means of a database published on its Website (www.ineris.fr).

In the case of an addition to certification, INERIS issues an addition to EC type-examination certificate, including the update data. This addition has to be joined to the initial certificate.

CONCLUSION

In this document we have presented the important points of the Directive 2007/23/EC [1] related to pyrotechnic articles for vehicles and have given some of our interpretations.

For more than 15 years INERIS as Notified Body has demonstrated its competence and its know-how worldwide by taking a leading position in the field of certification of explosives for civil uses and is now ready to act for the certification of all pyrotechnic articles. INERIS makes its services for certification accessible to all applicants whose activities fall within the scope of Directive 2007/23/EC [START_REF]INERIS is also Notified Body for the following EU Directives: 93/15/EEC Explosives for civil uses[END_REF]. The access is not conditional upon the applicant's nationality, size or membership of any association or group, nor the certification is conditional upon the number of certificates already issued.

  ormity to type (Annex II.2) MODULE D Production quality assurance (Annex II.3) MODULE E Product quality assurance (Annex II.4) Schematic view of the procedures for conformity assessment (Modules described in the annex II of the directive 2007/23/EC)

  The logical scheme applied by INERIS for an initial certification of a pyrotechnic article for vehicles can be represented by the following flow chart: Application Examination by INERIS of the evaluation conclusions Module B -EC type-examination Evaluation of the conformity according to the ESR (annex I of the Directive) by a /2 years, on the quality system for the production and the control of the final product) Module E Product quality assurance (1 audit/year, on the quality system for the control of the final productof the module) EC type-examination certificate Module G -Unit verification Evaluation of the conformity according to the ESR (annex I of the Directive) by a

  results obtained in mechanical tests (shock, vibration, drop…) on bared or packaged article. I5,I10, I11, J8

  not contain commercial blasting agents, except for black powder or flash composition. Examination of the technical documentation on the base of an agreed resolution of CEN Technical Committee 212. A1, P8 4. b)

K5 5 .

 5 C. 2) Ignition devices must be protected against electrostatic discharge under normal, foreseeable conditions of storage and use. The article is submitted to electrical spark of different energy, a level of minimum energy of no-reaction is determined. Verification in the design of the presence of effective protection features (shunt, ESD/EMI protection…) See also ESR 3. g) K1 5.C. 3)

ESR WITH THE CORRESPONDING TEST METHODS ABLE TO EVALUATE THE CONFORMITY The

  ESR of the Directive 2007/23/EC about the pyrotechnic articles for vehicles are listed with comments in the following table. The last column gives the references of examples of test methods that can be carried out by INERIS for the assessment of conformity of the product.

	5.1 §	Essential Safety Requirements (ESR)	Comments	Test methods
	Ref. Title	Description
	A1	Description	Description and comparison of the samples with the technical data or drawing submitted. Examination of technical documentation.
	E1	Stability	The article is submitted to a temperature of 75°C during 48 hours
	E10	Stability	The article is submitted to a temperature of 50°C and a relative humidity of 60% during 4 weeks
	F2	Progressive	The article is progressively heated (heating rate 5°C/min), the temperature of
		heating	runaway is determined
	I5*	Fall	The article is dropped on a hard floor from a height of 1.20 m
	I10	Fall	The article is dropped on a hard floor from a height of 1.20 m
	I11	Impact	The article is submitted to the free fall of a 30-kg fallhammer dropped from 0.25 m and 0.50 m high on 2 different directions
	J8	Vibration	The article is submitted to vibrations (frequency 50 Hz, amplitude 0.5 mm) during 60 minutes
	K1	Sensitiveness to	The article is submitted to electrical spark of different energy, a level of minimum
		electrical spark	energy of no-reaction is determined.
	K5	Ignition	Determination of firing impulse (electrical, mechanical, optical)
	M7	Heating	The article is exposed to a gas burner until reaction
	N7	Operation	Operation of the article is compared to what it is suppose to be
	P8*	Detonability	The ability of the article to initiate a cartridge of pentrite is tested
	R8** Combustion products	Analysis of the nature and quantity of the products emitted when the article is initiated in a 2.5 m 3 chamber
	* Applied to igniter
	** Applied to gas generator


Salma Halelfadl

Patrice Estellé
email: patrice.estelle@univ-rennes1.fr

Thierry Maré

Heat transfer properties of aqueous carbon nanotubes nanofluids in coaxial heat exchanger under laminar regime

Keywords: heat transfer enhancement, coaxial heat exchanger, nanofluids, carbon nanotubes, thermal conductivity, viscosity

The thermal performance of water-based multi-wall carbon nanotubes nanofluids are measured in a coaxial heat exchanger under laminar regime within the range of Reynolds numbers 500-2500. The convective heat transfer properties with constant wall temperature are evaluated for four different multi-wall carbon nanotubes based nanofluids at low concentration of 0.05% in weight (0.026% in volume). The measurements of thermal and rheological properties of the nanofluids with operating temperature were investigated experimentally. The effects of the aspect ratio of carbon nanotubes, the type of base fluid and surfactant on viscosity, thermal conductivity and laminar convective heat transfer were studied. Based on the experimental results, we reported the shear-thinning behaviour of nanofluids, the nanofluid viscosity being dependant on the base fluid type in the Newtonian region. We also showed that the enhancement of the thermal conductivity and the average convective heat transfer of nanofluids increased with the aspect ratio of nanotubes and decreased when the thermal conductivity of the base fluid increases. This enhancement attains at least 10% in comparison to base fluid even with the low content of nanotubes used.

Nomenclature

Introduction

One developing issue in heat and mass transfer enhancement relies on the use of nanofluids [START_REF] Choi | Enhancing thermal conductivity of fluids with nanoparticules[END_REF][START_REF] Li | A review on development of nanofluid preparation and characterization[END_REF][START_REF] John | Thermal properties of nanofluids[END_REF].

Nanofluids consist of nanometer-sized particles with high thermal conductivity dispersed in a common base fluid such as water or engine oil. Due to their high thermal conductivity compared to base fluids and their performance in energy devices [START_REF] John | Thermal properties of nanofluids[END_REF][START_REF] Daungthongsuk | A critical review of convective heat transfer of nanofluids[END_REF][START_REF] Huminic | Application of nanofluids in heat exchangers: A review[END_REF], nanofluids are interesting candidates for heat transfer enhancement in many fields and applications [START_REF] Huminic | Application of nanofluids in heat exchangers: A review[END_REF][START_REF] Mahian | Wongwises A review of the applications of nanofluids in solar energy[END_REF][START_REF] Colangelo | A new solution for reduced sedimentation flat panel solar thermal collector using nanofluids[END_REF]. Several studies have been previously performed on the heat transfer enhancement of nanofluids flowing through a tube under laminar regime. It is well admitted that experimental convective heat transfer coefficients of nanofluids varied with increase of the flow velocity and volume fraction. These coefficients are far higher than the ones of base fluids under the same conditions, as reported in [START_REF] Hussein | A review of forced convection heat transfer enhancement and hydrodynamic characteristics of a nanofluid[END_REF] for a wide variety of nanofluids containing Cu or Al 2 O 3 spherical nanoparticles. Similar results were also summarized in [START_REF] Kleinstreuer | Experimental and theoretical studies of nanofluid thermal conductivity enhancement: a review[END_REF] considering other nanoparticle natures. The authors mentioned in particular that the measured Nusselt numbers of nanofluids are higher than the ones of base fluids, and increase with higher Reynolds numbers. However, few works reported the heat transfer coefficient measurement of nanofluids containing nanotubes and carbon nanotubes in particular, while they have a high thermal conductivity [START_REF] Xie | Nanofluids containing multiwalled carbon nanotubes and their enhanced thermal conductivities[END_REF][START_REF] Liu | Enhancement of thermal conductivity with carbon nanotube for nanofluids[END_REF][START_REF] Maré | Comparison of the thermal performances of two nanofluids at low temperature in a plate heat exchanger[END_REF]. Hence, the potential of aqueous multi-wall carbon nanotubes (MWCNT) nanofluids for heat transfer enhancement in coaxial tube exchanger under laminar flow regime is here investigated from experiments.

Ding et al. [START_REF] Ding | Heat transfer of aqueous suspensions of carbon nanoubes (CNT nanofluids)[END_REF] first reported the large convective heat transfer enhancement of aqueous CNT under laminar flow in comparison with water for both low Reynolds number and weight fraction in nanotubes. Ko et al. [START_REF] Ko | An experimental study of the pressure drop of nanofluids containing carbon nanotubes in a horizontal tube[END_REF] measured the flow properties of water-based CNT nanofluids through a horizontal tube. They investigated the effect of SDS as surfactant and the introduction of oxygen containing functional groups. They also reported the effect of shear-thinning of nanofluids with concentration, which induces the increase of pressure drop under laminar regime in comparison with base fluid. A similar result was also reported by Halelfadl et al. [START_REF] Halelfadl | Experimental investigation of rheological behavior and pressure drop of aqueous suspensions of carbon nanotubes in a horizontal tube[END_REF] with water based nanofluids containing CNT and SDBS as surfactant for a low weight fraction of 0.01% and flowing in a tube. The heat transfer properties of MWCNT nanofluids with volume fraction lower than 0.24% in intertube falling-film flow were studied by Ruan and Jacobi [START_REF] Ruan | Heat transfer characteristics of multiwall carbon nanotube suspensions (MWCNT nanofluids) in intertube falling-film flow[END_REF]. They showed that the heat transfer coefficient of nanofluids vary with base fluids and nanofluid volume fraction. Under laminar regime and same Reynolds number, higher enhancement was achieved with ethylene glycol (EG) than water. Garg et al. [START_REF] Garg | An experimental study on the effect of ultrasonication on viscosity and heat transfer performance of multi-wall carbon nanotube-based aqueous nanofluids[END_REF] studied the heat transfer performance of MWCNT aqueous nanofluids in horizontal tube under laminar flow regime, showing that the heat transfer coefficient for a fixed axial distance decreases with the Reynolds number and increases with the axial distance.

Chen et al. [START_REF] Chen | Heat transfer and flow behaviour of aqueous suspensions of titanate nanotubes (nanofluids)[END_REF] measured the convective heat transfer of titanate aqueous nanotubes in a vertical tube for Reynolds number lower than 2500. The enhancement in convective heat transfer appeared to be strongly dependant on nanoparticle concentration and aspect ratio, Reynolds number and axial position. Also, heat transfer coefficient increased with Reynolds number and the highest enhancement was observed at the entrance region of the tube.

It is well known that the thermal conductivity enhancement of nanofluids depends on the addition of nanoparticles and the nanoparticle aspect ratio, as shown in [START_REF] John | Thermal properties of nanofluids[END_REF][START_REF] Colangelo | Results of experimental investigations on the heat conductivity of nanofluids based on diathermic oil for high temperature applications[END_REF]. However, this increases the viscosity of nanofluids [START_REF] Aladag | Experimental investigations of the viscosity of nanofluids at low temperatures[END_REF][START_REF] Yiamsawas | Experimental studies on the viscosity of TiO2 and Al2O3 nanoparticles suspended in a mixture of ethylene glycol and water for high temperature applications[END_REF] and can penalize the thermal benefits of nanofluids in energy systems. For the above reasons, we have here investigated nanofluids containing nanoparticle with both high aspect ratio and conductivity such as MWCNT and low weight fraction of 0.05%. As the heat transfer efficiency of nanofluids in flowing systems is also closely related to their thermo physical properties [START_REF] Halelfadl | Efficiency of carbon nanotubes water based nanofluids as coolants[END_REF], the thermal conductivity and viscosity of nanofluids are first experimentally evaluated. Then, we report the measurement of convective heat transfer enhancement of nanofluids in a coaxial heat exchanger under laminar flow regime within the range of Reynolds number 500-2500. The experimental results are reported for an operating temperature of 45°C and considering the effect of surfactants and base fluids used to produce the nanofluids.

The influence of the nanotube aspect ratio is also discussed.

Nanofluids

In this work, we studied four types of nanofluids containing multi-walled carbon nanotubes with a low weight fraction of 0.05%. Due to the true density of nanotubes, this leads to a volume fraction around 0.026%. Table 1 summarizes the composition and the properties of the different nanotubes and nanofluids. For each studied nanofluid, a starting suspension containing 1% in weigth fraction of nanotubes and 2% in weight fraction of surfactant initially prepared by Nanocyl, was diluted with the base fluid. The dilution was performed to maintain a constant surfactant/carbon nanotubes weight ratio of 2 and followed by mechanical stirring to disperse the nanoparticles in the base fluid.

The type and quantity of surfactants, which was used to stabilize the nanoparticles within the base fluid, were selected by Nanocyl. They consisted of ionic agents, such as lignin and sodium polycarboxylate. Lignin is a by-product of paper industry which is suitable as a dispersing agent for multiwalled carbon nanotubes [START_REF] Rochez | Dispersion of multiwalled carbon nanotubes in water by lignin[END_REF]. So, we investigated experimentally the effect of base fluid type, the effect of carbon nanotubes aspect ratio and the type of surfactant. As shown in Table 1, nanofluids N 1 and N 3 differ by their base fluid. The aspect ratio of the nanotubes dispersed in the nanofluid N 2 is approximately two times higher than the one of the nanofluid N 4 . Lignin and sodium polycarboxylate were used as surfactants for N 2 and N 4 respectively.

Thermo physical properties of nanofluids

The thermo physical properties of the nanofluids were evaluated in this study at the average temperature of T av =45°C. This temperature corresponds to the average of the inlet and outlet temperatures of the working fluid in the inner tube of the coaxial heat exchanger test section (see section 4). The thermal conductivity and the viscosity of the nanofluids were experimentally measured at the working temperature following the procedures previously used in [START_REF] Halelfadl | Efficiency of carbon nanotubes water based nanofluids as coolants[END_REF][START_REF] Halelfadl | Optimization of thermal performance and pressure drop of a rectangular microchannel heat sink using aqueous carbon nanotubes based nanofluid[END_REF][START_REF] Estellé | Shear flow history effect on the viscosity of carbon nanotubes water based nanofluid[END_REF][START_REF] Halelfadl | Viscosity of carbon nanotubes waterbased nanofluids: Influence of concentration and temperature[END_REF] and described thereafter. The density and the heat capacity of the nanofluids were evaluated from wellknown theoretical correlations stated later.

Thermal conductivity and viscosity measurements

Thermal conductivity measurement of nanofluids was carried out from the transient hot wire method. We used a KD2 Pro thermal property analyzer (Decagon Devices Inc.) equipped with a KS-1 probe. Once both the probe and the nanofluid sample equilibrated at 45°C for 30 min, an average of over ten measurements of thermal conductivity was performed with a time interval of 5 min for each nanofluid. The accuracy and reliability of the thermal conductivity measurement system were previously reported, leading to maximum relative deviation of 3.5% [START_REF] Halelfadl | Optimization of thermal performance and pressure drop of a rectangular microchannel heat sink using aqueous carbon nanotubes based nanofluid[END_REF].

Rheological measurements of the nanofluids were conducted with a Kinexus Pro rheometer (Malvern) with well controlled temperature under ±0.01°C. Given the low viscosity of nanofluids, all the experiments were performed with a cone of 60mm in diameter and 1° in angle. The measurement procedure and the validation of the experimental protocol are detailed and reported in previous works [START_REF] Estellé | Shear flow history effect on the viscosity of carbon nanotubes water based nanofluid[END_REF][START_REF] Halelfadl | Viscosity of carbon nanotubes waterbased nanofluids: Influence of concentration and temperature[END_REF], leading to maximum relative deviation in viscosity value of 4%. Once equilibrated at 45°C for 5 min between the cone and plate, each nanofluid sample was subjected to a logarithmic stress ramp under steady-state conditions. The tests were done in two replicates indicating the measurement repeatability and the stability of the nanofluids. Rheological measurement of the base fluids, e.g. distilled water and distilled water and ethylene glycol (50/50), was also performed.

Density and heat capacity evaluation

The following standard formulae were used to evaluate the density of the studied nanofluids [START_REF] Pak | Hydrodynamic and heat transfer study of dispersed fluids with sub-micron metallic particles[END_REF] and their specific heat capacity [START_REF] Hanley | Measurement and model validation of nanofluid specific heat capacity with Differential Scanning Calorimetry[END_REF], respectively described by equations ( 1) and ( 2).

( )

np v bf v nf ρ + ρ = ρ ϕ ϕ - 1 (1) ( ) ( )( ) ( ) bf v np v bf p v np p v nf p, ρ + ρ ρC + ρC = C ϕ ϕ - - 1 1
(2)

Coaxial heat exchanger 4.1 Experimental set-up

The experimental set up used to measure the thermal performances and the convective heat transfer coefficient with fixed wall temperature boundary condition is shown in figure 1. It involved a test section finely designed and instrumented. This test section consists of a stainless steel coaxial heat exchanger with inner tube length of 0.66m, inner diameter of 18.7mm and a thickness of 1.3mm and an annular tube diameter of 47.6mm and thickness of 1.7mm. An adiabatic section of 0.2m in entrance length preceded the test section, allowing the measurement to be made after the hydrodynamic and thermal entry. The nanofluids are flowed through the inner tube with a fixed inlet temperature of 50°C. Distilled water was used as a cooling fluid with a fixed inlet temperature of 10°C and was circulated in the annular tube of the coaxial heat exchanger. For both the inner and the annular tubes, the temperature was controlled using two thermostatic baths (Fisher). Four platinum probes with a maximum accuracy of 0.1°C after calibration, are inserted into the flow at the inlet and outlet of each tube. As shown in figure 1, eight K-type thermocouples with an accuracy of 0.1°C after calibration, were mounted at axial positions in mm of 5(T1), 20(T2), 30 (T3), 60(T4), 140(T5), 185(T6), 245(T7) and 385(T8) from the inlet of the test section to evaluate the wall temperature distribution along the tube. The positions of the thermocouples have been chosen in order to evaluate the wall temperature distribution especially in the thermal entrance region thereafter the adiabatic entry section. As detailed by figure 1, it should be noted that the thermocouples are inserted inside the wall of the inner tube. The pressure drops of the inner tube were measured using piezo-resistive pressure transmitters (Rosemount) over a range of 0-5bar with an accuracy of 0.075%. All the data was recorded by a data acquisition system (Labview). The entire test section was insulated in order to minimize the heat losses.

The measurements are made for a co-current flow. This allows the heat transfer of nanofluids to be investigated at the entry of the heat exchanger. The water flow rate in the annular tube is maintained constant for all tests at 336l/h. This volumetric flow rate is very high compared to that of the inner tube, which varies between 5 and 80l/h. These flow rates were chosen to achieve the fixed wall temperature boundary condition and were adjusted for each nanofluid to compare their properties at same Reynolds number. As reported later, this also leads to laminar flow condition within the inner tube containing nanofluids.

The nanofluid flow rate was measured directly from the time required to accumulate a fixed volume of the nanofluid using a 3-way valve. This 3-way valve was also used for flow system cleaning between runs even with the same nanofluid. For each volumetric flow rate, the data acquisition is performed when a steady state is reached. This was achieved when the difference between inlet and outlet temperature of both inner and annular tubes becomes constant. For all flow rates, the average temperature of the operating temperature in the inner tube is about 45°C.

Data analysis

The heat transfer between the nanofluids and distillated water within the coaxial heat exchanger was obtained according to equation 3:

∆T C m = nf p nf nf & φ (3)
Where nf m & and nf p C are nanofluid's mass flow rate and heat capacity respectively. ∆T is the inlet and outlet temperature difference of the nanofluid in the inner tube of the coaxial heat exchanger.

The overall and the average convective heat transfer coefficients, respectively denoted U and h nf , were obtained according to the following equations:

ln T S = U nf ∆ φ (4) with ( ) ( )         - - - - - i w i nf o w o nf i w i nf o w o nf T T T T T T T T = ∆T ln ln (5) and 1 1 2 1 1 2λ ln - -             - s nf D D D U = h (6)
In the previous equation, D 2 and D 1 are the external and the internal diameter of the inner tube of the heat exchanger as shown in figure 1.b. S is the heat transfer surface area.

The Reynolds and the Nusselt numbers were calculated using the following equations:

h nf nf D πµ m = & 4 Re h D (7) nf h nf D D h = u h λ N ( 8
)
nf µ and nf λ are the dynamic viscosity and the thermal conductivity of the nanofluid respectively.

These thermophysical properties were evaluated experimentally as presented previously.

Uncertainty and validation of experimental set-up

The uncertainty of the heat transfer characteristics with the coaxial heat exchanger was computed from the work of [START_REF] Kline | Describing uncertainties in single simple experiments[END_REF] and the values summarized in table 3. Hence, the uncertainty in Reynolds number Re, convective heat transfer coefficient h and Nusselt number Nu was evaluated to ±6%, ±7.45% and ±10.95% respectively. Before studying the nanofluids, the experimental coaxial heat exchanger was tested and calibrated with distilled water. Thus, the experimental results for Nusselt number of distilled water where reported in Figure 3, and compared to the theoretical prediction under laminar regime depicted by equation 9 [START_REF] Seider | Heat transfer and pressure drop of liquid in tubes[END_REF].

0.14 3 1 / RePr 1.86                 p h µ µ D L = Nu (9)
where p µ is the dynamic viscosity of distillated water at wall temperature. p µ was evaluated based on wall temperature measured by the thermocouples. Figure 3 presents the experimental and theoretical Nusselt numbers, which increase linearly with Reynolds number as expected. Figure 3 shows that the Nusselt numbers calculated through the experimental results and equation ( 8) are in good agreement. The maximum deviation between equation ( 8) and equation ( 9) is 6%. This is of the same order of magnitude as the experimental uncertainty in Nusselt number.

Results and Discussion

The relative thermal conductivity of the nanofluids investigated in this work at the operating temperature of 45°C is presented in Table 2. As expected, Table 2 first shows that relative thermal conductivity of nanofluids is enhanced with the presence of nanotubes. From Table 2, we also observe that, at tested nanotubes weight fraction, the thermal conductivity of the base fluid has got a significant effect on the enhancement of the thermal conductivity of the nanofluids. In fact, the thermal conductivity increases by 15.4% for N 3 with a mixture of water and ethylene glycol (50/50 in weight) as a base fluid. The thermal conductivity enhancement reduces to 11.3% for N 1 when water is only used as a base fluid. This result agrees with that of Chen et al. [START_REF] Chen | Nanofluids containing carbon nanotubes treated by mechanochemical reaction[END_REF] and John and Shima [START_REF] John | Thermal properties of nanofluids[END_REF] who revealed that the thermal conductivity showed a lower enhancement for aqueous CNT based nanofluid as compared to ethylene glycol CNT based nanofluid. Table 2 also shows an enhancement of thermal conductivity with increase of CNT aspect ratio. The thermal conductivity increases by 9.6% for the nanofluid N 4 and by 11% for the nanofluid N 2 . It is also shown that the role of surfactant in thermal conductivity enhancement is negligible at that weight fraction in comparison to the effect of the base fluid type and the aspect ratio.

Figure 3 shows the evolution of the apparent dynamic viscosity as a function of shear rate. The results showed that the apparent viscosity first decreases with shear rate. For higher shear rates, the apparent viscosity tends to a Newtonian plateau where the apparent viscosity becomes independent of the shear rate. This means that the aqueous CNT based nanofluids (N 1 , N 2 , N 3 and N 4 ) behave like shear thinning fluids. Thus, at the initiation of shear, the nanotubes network tends to align in the direction of flow. In the Newtonian region, the viscosity of nanofluids N 1 , N 2 and N 4 is quite close of the viscosity of distilled water. This indicates that the aspect ratio and the surfactant do not have a significant role on viscosity at high shear rate for the weight fraction of CNT investigated in the current work, and with water as base fluid. Figure 3 also shows that the viscosity of the nanofluid N 3 , with a mixture of ethylene glycol and water as base fluid, is three times higher than that of the aqueous CNT based nanofluids (N 1 , N 2 and N 4 ). Besides, the addition of a small amount of CNT (0.05% in weight) slightly increases the viscosity of the base fluid (mixture of water and ethylene glycol) of about 10%.

Prior to discuss the effect of base fluid, surfactant and CNT aspect ratio on the relative convective heat transfer of the nanofluids, figure 4 first shows the wall temperature distribution along the axial distance of the coaxial heat exchanger for the different nanofluids at Re=950. As it is seen in figure 4, for all nanofluids the wall temperature decreases with the increase of axial distance for very low axial distance then stabilizes for axial distance higher than 0.25m. This result shows that the heat exchange is particularly significant in the entrance region of the heat exchanger, and decreases with axial distance. The decreasing in wall temperature distribution depends also on nanofluids composition, in particular on base fluid. Indeed, the wall temperature distribution is lower with the mixture of water and EG as base fluid. It is important to note that similar trends are obtained when distilled water flows through the inner tube instead of nanofluids and also when the Reynolds number varies within the laminar regime.

Figure 5 shows the evolution of the average heat transfer coefficient of all nanofluids and associated base fluids in function of Reynolds numbers. It is observed that these coefficients increase with Reynolds number enhancement, as reported in [START_REF] Chen | Heat transfer and flow behaviour of aqueous suspensions of titanate nanotubes (nanofluids)[END_REF], and also evolve similarly according to the Reynolds number. Consequently, the relative heat transfer coefficients h nf /h bf , which are defined as the ratio of average heat transfer coefficient of nanofluids to average heat transfer coefficient of base fluids, appear to be quite constant with Re in Figure 6. Comparing N 1 and N 3 , Figure 6 shows a significant enhancement of heat transfer coefficient by about 16% with N 3 (CNT dispersed on a mixture of ethylene glycol with water), whereas the enhancement for aqueous CNT based nanofluid N 1 is only about 12%. The convective heat transfer enhancement is reduced with an increase in the thermal conductivity of the base fluid.

Figure 6, in turn, shows that the enhancement of the heat transfer increased with the aspect ratio.

The enhancement of the convective heat transfer coefficient is relatively higher for nanofluid N 2 with a high aspect ratio. The convective heat coefficient increased by about 12.5% for N 2 , while it increased by about 10% for N 4 compared to the base fluid.

Finally, the enhancement of the convective heat transfer coefficient differs for both nanofluids N 1 and N 2 , especially for Reynolds number lower than 700. As shown in figure 6, the enhancement of the convective heat transfer coefficient of N 1 is quite constant within the range of Reynolds numbers while the convective heat transfer coefficient of N 2 increases up to Re=700, then tends to a plateau.

The maximum enhancement of the convective heat transfer coefficient for both nanofluids is about 12.5%.

Conclusion

The heat transfer performance of carbon nanotubes based nanofluids flowing in a coaxial heat exchanger under laminar flow regime is investigated experimentally. The convective heat transfer properties with a constant wall temperature of 45°C are evaluated for four different multi-wall carbon nanotubes based nanofluids at low concentration of 0.05% in weight (0.026% in volume).

The effect of surfactant, aspect ratio and base fluid type was also considered. First, the viscosity and the thermal conductivity of nanofluids were measured under the operating temperature of 45°C. It is reported that all CNT based nanofluids behave like shear thinning fluids. In the Newtonian region, situated 100s -1 , viscosity of aqueous carbon nanotubes based nanofluids, N 1 , N 2 and N 4 , is close to that of base fluid. With distilled water and ethylene glycol (50/50) as base fluid, the presence of CNT also increases the viscosity. It is also shown that the effect of the aspect ratio of

CNT and the surfactant nature on the nanofluids viscosity is low at the CNT concentration investigated here. It is revealed that both the thermal conductivity of the base fluid and nanotube aspect ratio plays a significant role on the enhancement of the thermal conductivity of nanofluids as compared to surfactant. Higher improvement in thermal conductivity is obtained for nanofluids with higher nanotube aspect ratio and lower thermal conductivity of base fluid. Finally, the results indicate that a low volume fraction of 0.026% in CNT leads to an average convective heat transfer enhancement higher than 12% in comparison with base fluids. Nanotubes aspect ratio increase and base fluid with lower thermal conductivity contribute to better enhance the convective heat transfer of nanofluids.

Figure 1 .

 1 Figure 1. Experimental set up of coaxial heat exchanger (a) -View of the thermocouples location along the inner tube (b).

Figure 2 .

 2 Figure 2. Comparison of the experimental Nusselt number of distilled water with the result of Eq. (9) in function of Reynolds number.

Figure 3 .

 3 Figure 3. Apparent viscosity of nanofluids and base fluids as a function of shear rate at operating temperature of 45°C.

Figure 4 .

 4 Figure 4. Wall temperature distribution along the axial distance of heat exchanger for the different nanofluids at Re=950.

Figure 5 .

 5 Figure 5. Average convective heat transfer coefficient of nanofluids and base fluids versus Reynolds number.

Figure 6 .

 6 Figure 6. Evolution of the relative convective heat transfer coefficient versus Reynolds number for the studied nanofluids.

Figure 1 .

 1 Figure 1. Experimental set up of coaxial heat exchanger (a) -View of the thermocouples location along the inner tube (b).

Figure 2 .Figure 3 .Figure 4 .Figure 5 .

 2345 Figure 2. Comparison of the experimental Nusselt number of distilled water with the result of Eq. (9) in function of Reynolds number.

Figure 6 .

 6 Figure 6. Evolution of the relative convective heat transfer coefficient versus Reynolds number for the studied nanofluids.

Table Captions Table 1 .

 Captions1 Nanotubes and nanofluids properties

Table 2 .

 2 Relative thermal conductivity of nanofluids

Table 3 .

 3 Accuracy or relative accuracy of the measuring instruments

Table 1 .

 1 Nanotubes and nanofluids properties

		N 1	N 2	N 3	N 4
	Nanotube average	9.2	9.2	9.2	11.4
	diameter (nm)
	Nanotube average	1.5	1.5	1.5	≈1
	length (µm)
	Average aspect	r≈160	r≈160	r≈160	r≈90
	ratio
	Purity (wt.%)	90	90	90	90
	Base fluid	Distilled	Distilled water	Ethylene Glycol (50%)	Distilled water
		water (W)	(W)	+ Distilled water (50%)	(W)
				(W+EG)
	Surfactant	Lignin	Sodium	Lignin	Sodium
			polycarboxylate		polycarboxylate

Table 2 .

 2 Relative thermal conductivity of nanofluids.

	Nanofluid	bf λ ∆λ	=	nf λ	bf λ -	bf λ	( ) %
	N 1			11.3
	N 2			11
	N 3			15.4
	N 4			9.6

Table 3 .

 3 Accuracy or relative accuracy of the measuring instruments

	Description	Model	Accuracy/relative
			accuracy
	Inlet/outlet	RTD PT100	±0.1°C
	flow rate	platinum
		probe
	Wall	Type K,	±0.1°C
	temperature	thermocouple
	Thermal	KD2 Pro	±3.5%
	conductivity
	Fluid flow rate	--	±2%
	Dynamic	Rheometer,	±4%
	viscosity	Malvern
		Kinexus Pro

Acknowledgments

Nanocycl Belgium is gratefully acknowledged for the preparation and supply of CNT water based nanofluids.

Highlights


Paul Lauga
email: paul.lauga@telecom-paristech.fr

Alper Koz
email: alper.koz@telecom-paristech.fr

Giuseppe Valenzise
email: giuseppe.valenzise@telecom-paristech.fr

Frédéric Dufaux
email: frederic.dufaux@telecom-paristech.fr

REGION-BASED TONE MAPPING FOR EFFICIENT HIGH DYNAMIC RANGE VIDEO CODING

Keywords: High Dynamic Range, video compression, tone mapping, Perceptually Uniform encoding, H.264/AVC

In this paper, we address the problem of efficient High Dynamic Range (HDR) video coding. For this purpose, we introduce a new representation which segments the HDR image into low-range and highrange regions which are independently tone mapped. This representation allows preserving more information, leading to a higher visual quality of the reconstructed HDR image.

INTRODUCTION

High Dynamic Range (HDR) [START_REF] Banterle | Advanced High Dynamic Range Imaging[END_REF] is gaining a lot of interests nowadays, thanks to its ability to provide more appealing and immersive images and video sequences. An HDR image (or video) is scene referred, i.e. pixel values describe the real physical luminance values (in cd/m²) present in the scene. Therefore, HDR images can represent accurately information about the luminance in a very high contrast scene from the level of cd/m² for extreme darkness to cd/m² for a bright sunshine. To represent this entire range, pixels have to be encoded with a higher numerical precision than the conventional imagery, e.g. up to 32 bits, in HDR formats such as OpenEXR or RGBE [F. Kainz As a consequence, HDR images and video sequences require a very large amount of data to be stored and transmitted which is significantly larger than common Low Dynamic Range (LDR) content. Hence, efficient compression is required. However, standard coding technologies, such as H.264/AVC (Advanced Video Coding) codec [START_REF] Wiegand | Overview of the H.264/AVC video coding standard[END_REF], were not design to handle bit depths higher than 8 bits. Even if more recent extensions such as the High Profile of H.264/AVC can handle up to 14 bits per data sample, this is still not enough for HDR content.

Scalable (or layered) HDR video coding techniques have been proposed in 0[4] [5]. In these techniques, a tone mapping operator is first applied on the HDR sequence in order to generate an LDR video. This LDR layer can be encoded using the usual H.264/AVC with 8 bits depth, providing backward compatibility with standard 8 bits displays. Next, the HDR layer can be predicted from the LDR layer in an inter-layer prediction stage. In turn, the prediction error (or residue) can also be encoded using H.264/AVC. Another approach consists in using the High Profile of H.264/AVC. This setup enables to use a higher number of bits (up to 14) to represent luminance values; however, this video content is not directly displayable on current LDR screens. The authors of [6] introduce the Adaptive LogLuv Transform, which is an extension of the LogLuv representation proposed in [START_REF] Larson | Overcoming gamut and dynamic range limitations in digital images[END_REF], to represent the luminance on 12 bits and then, to encode it using the High Profile of H.264/AVC. The Adaptive LogLuv Transform [6] compresses the range of HDR values using a logarithm function. This is motivated by the fact that the response for the luminance sensitivity of the Human Visual System (HVS) to the luminance can be approximated by a logarithmic function, as stated by Weber's law [x]. More recently, the Perceptually Uniform (PU) encoding [START_REF] Aydin | Extending Quality Metrics to Full Luminance Range Images[END_REF] has also been proposed as a more precise model of the HVS.

A limitation of most of the tone mapping algorithms is that one has to choose which part of the luminance range would be reproduced with the best fidelity. Typically, tone mapping algorithms tend to reproduce the luminance range where most of the signal is concentrated as close as possible. The luminance values outside of this range would be mostly saturated. The resulting loss of information can produce annoying distortions when the reconstructed HDR image is viewed on an HDR display.

In this paper, we propose a new tone mapping operator for efficient HDR video compression using H.264/AVC. More specifically, the objective of the proposed tone mapping is to preserve as much information as possible from the HDR data, in order to enable the reconstruction of a high visual quality HDR image. However, this is in contrast with backward-compatible tone mapping operators, which aim at producing the most visually pleasing LDR image to display on LDR devices.

To this end, we propose a tone mapping algorithm which consists of two parts. In the first part, we apply a tone mapping on low luminance values and obtain an image where low values are well preserved and high values are saturated. Similarly, in the second part, we apply in parallel another tone mapping on the high luminance range. Both representations are then combined and encoded using H.264/AVC High Profile. In this way, we have recorded the whole range of the HDR image into the available bit depth. At the decoder side, the HDR image is reconstructed with the help of a binary mask to identify both the low and the high luminance regions.

The rest of the paper is structured as follows. Section 2 explains in more details Adaptive LogLuv Transform [6], which is used as a base to our proposed method. In Section 3 we present our proposed approach for HDR representation and compression. We assess its performance in Section 4. Finally, we give the conclusions and future work in Section 5.

ADAPTIVE LOG LUV TRANFORM

The reference method used for the compression and decompression is the Adaptive LogLuv Transform by Motra and Thoma [6]. This method takes into account the minimum and the maximum luminance of the HDR image to be encoded to map the luminance input to the range [ ] where n is the bit-depth of the compressed luminance video (selected as 12 for the implementation).. Chrominance channels (u and v) are compressed in 8 bits.

More specifically, the Adaptive LogLuv Transformation is defined by the following steps. First, the RGB channels of the HDR input is transformed into the XYZ color space: After defining the transition variables, x and y, as :

                                B G
Z Y X X x    (2) Z Y X Y y    (3)
the Adaptive LogLuv representation is given by

          3 12 2 4 410 y x x u e (4)           3 12 2 9 410 y x y v e (5)           otherwise 0 if ) ( log 0 2 Y b Y a L (6) with the parameters ) min( ) max( log 1 2 2 Y Y a n   (7) )) (min( log 1 2 Y a b   (8).
In the definition of a and b, min(Y) and max(Y) denote the minimum and maximum of the luminance component, Y, respectively. Note that both min(Y) and max(Y) are strictly positive values. The parameters a and b are sent as metadata to the decoder, as they are required to reconstruct the HDR content.

The backward transformation at the decoder is applied as follows: [START_REF] Aydin | Dynamic Range Independent Image Quality Assessment[END_REF].

               otherwise 0 if 2 0 5 . 0 L L b a L (9) 410 5 . 0   e u u ( 10
) 410 5 . 0   e v v
The X and Z components are then obtained by 12 16 6

9    v u u x (12) 12 16 6 4    v u v y (13) y Yx X  (14) y y x Y Z ) 1 (    (15).
Finally, the reconstructed RGB values are given by 

                                  Z Y X B G R

PROPOSED SCHEME

In this paper, we propose a new representation for HDR video content with the objective to better preserve the HDR information in the luminance signal. A block diagram of the proposed scheme is shown in Figure 1.

The Adaptive LogLuv Transform applies a logarithmic transformation to luminance values prior to the coding. Instead, in our algorithm, we use the luminance values [START_REF] Aydin | Extending Quality Metrics to Full Luminance Range Images[END_REF] which are proposed as a closer approximation to the luminance sensitivity of the HVS. The PU encoding is obtained from the Contrast Sensitivity Function (CSF), which models the contrast masking of the HVS. For low luminance values, PU encoding is closer to a logarithmic transformation. However, they diverge for higher luminance values.

With both PU encoding or logarithm transformation, a significant range of the luminance is well represented and preserved. However, the dim and the bright regions are typically saturated, causing a loss of information. Moreover, we observe that in most HDR image histograms, the energy of the signal is concentrated in a small range (most of the time in the low values). Nevertheless, a significant amount of information is contained in a much wider range of luminance values.

Taking into account these observations, the input HDR image is segmented into two regions, which will then be processed independently. To perform this binary segmentation, a threshold on the luminance is defined, in such a way to identify the small range which contains most of the signal energy. In this paper, we set this threshold so that 70 % of the luminance values are below the threshold. This threshold is decided empirically in our tests by observing the rate-distortion (RD) curves with respect to the different threholds values.

Applying a tone mapping algorithm on the low-range region, rather than the whole HDR image, will preserve more information. In parallel, another tone mapping algorithm is applied to the high-range regions. Hence, saturation is avoided in bright regions, which are conserving details. More specifically, both the low-range and the high-range regions are PU encoded. In order to stretch the histogram and fully exploit the whole range available, we next apply a logarithm on the PU values.

An example is illustrated in Figure 2. More precisely, the figure shows an original sample frame from the HDR video sequence Sun [START_REF]HDR image Gallery[END_REF], which has been tone mapped by Adaptive LogLuv in (a), the binary segmentation mask defining the low-range and high-range regions for our proposal in (c), and the tone mapped image obtained by the proposed method in (b) and (d). Note that the objective of the proposed method is to preserve most information from the HDR data. Therefore, the tone mapped image resulting from the proposed method may appear unnatural. However, it carries more information and enables to reconstruct a higher quality HDR image.

Figure 3 (a) shows the histogram of the luminance, in the logarithmic scale for the first frame of Sun. Similarly, the histograms for the case of the Adaptive LogLuv Transform and the proposed method are shown in Figure 3 In other words, the mapping , is applied. In the following, we detail more formally the proposed segmentation-based tone mapping algorithm. First, the PU encoding is applied to the luminance values (Y) to obtain the luma values: Compression is then applied on the Luv channels, using H.264/AVC.

) (Y PUencode Luma  (17)         )) (max( )) (min( ) ( max min Y PUencode L Y PUencode L Y PUencode Luma Threshold Threshold (18)                                              otherwise log log log log 1 ) 1 2 ( Luma Luma if log log log log ) 1 2 ( 2 max 2
As they are required at the decoder to reconstruct the HDR content, the three parameters , and are transmitted as metadata information to the decoder site.

(a) (b) (c) (d)

Figure 2 (a) The first frame of the HDR Sun sequence, which is tone mapped by using Adaptive LogLuv Transform,(b) the same image tone mapped by using our proposal without inversing the second part of the tone mapping curve, (c) the binary mask obtained with our proposal, (d) same image tone mapped using our proposal after inversing the second part of the tone mapping curve. A binary segmentation map is also produced, defined by: (20) This mask, which is required at the decoder side, is compressed using JIBG2 bi-level image compression standard [START_REF] Ono | JBIG2-the ultimate bi-level image coding standard[END_REF] and transmitted to the decoder as illustrated in Figure 1.

At the decoder side, the backward transformation is applied as follows:

                              1 M ask if 2 Luma 0 M ask if 2 2 max 2 min 2 min 2 log 1 1 2 log log 1 2 log Thres n Thres n Thresh Luma L Luma L L L L Luma Luma (21) ) ( 1 Luma PUencode Y   (22).
Then, we use equation ( 16) to reconstruct RGB values.

EXPERIMENTAL RESULTS

To assess the performance of the proposed scheme, we have carried out experiments using the Sun and Tunnel sequences [START_REF]HDR image Gallery[END_REF]. These are two HDR videos taken from inside a car which is driving on a highway during a sunny day for the first one and in a tunnel for the second one. The frame size of the video sequences is 640 × 480 pixels and the frame rate is 30 fps. In our experiments, we process the first 30 frames of the sequences. The overall dynamic range represented in these sequences is for Sun and for Tunnel. We assess the compression performance of the proposed algorithm in comparison to, Adaptive LogLuv method. In both cases, Y is coded with 12 bits and u and v are coded with 8 bits. We use the H.264/AVC High Profile in Intra coding mode and with a fixed set of Quantization Parameters (QP). After decoding, the HDR images are reconstructed using the backward Adaptive LogLuv transform and our proposed scheme. In both cases, metadata are needed for reconstruction. Therefore, these two values have to be transmitted as metadata along with the compressed H.264/AVC stream. Moreover, we compress the binary mask in our proposed scheme by using JBIG2 [START_REF] Ono | JBIG2-the ultimate bi-level image coding standard[END_REF].

As our proposed scheme preserves more information in the tone mapped image compared to Adaptive LogLuv, it requires a higher bitrate for the same QP. However, it also produced reconstructed HDR images with better quality.

In order to compare the visual quality of the reconstructed HDR images, we use the perceptually-based HDR-VDP metric [START_REF] Aydin | Dynamic Range Independent Image Quality Assessment[END_REF][12] [START_REF] Mantiuk | Visible Difference Predicator for High Dynamic Range Images[END_REF]. We choose this metric for its relevance to the HVS. The HDR-VDP metric takes as inputs both a processed and a reference HDR image. The output is an image of the same size, which estimates the probability for each pixel in the two HDR images to be seen different by a human observer. From this output image, we extract two numbers: the percentage of the pixels in the image which has a probability higher than 75% and 95 % to be detected. Finally, these percentages are averaged over all frames of the video sequence.

Figure 4 and 5 show the rate-distortion performance of the proposed method, compared to Adaptive LogLuv for Sun and Tunnel sequences. At both 75 % and 95 % probability thresholds, it can be observed that our proposed method consistently outperforms Adaptive LogLuv method.

CONCLUSIONS

In this paper we proposed a new scheme for HDR video content compression. It is based on a new representation which conserves more information in the HDR image. More precisely, the input HDR image is segmented into low-range and high-range regions. Each region is then tone mapped using PU encoding and a logarithmic scale. The resulting image is compressed using H.264/AVC. Rate-distortion results, using the perceptuallybased HDR-VDP metric, show the consistently superior performance of the proposed scheme when compared to the Adaptive LogLuv representation. Future work will aim at an improved temporal coherence of the proposed representation in order to improve interframe coding performance in H.264/AVC. Moreover, we will focus on finding the optimal value for each frame using the HDR histogram.

  , R. Bogart, "Technical Introduction to OpenEXR", Industrial Light & Magic, 18 February 2009.].

Figure 1 :

 1 Figure 1: Block diagram of the proposed scheme.

Figure 3

 3 Figure 3 (a) Histogram of the HDR image in logarithm domain. The image used is the first frame of Sun and we can see tone mapped versions on figure 2). (b) histogram of the tone mapped image using Adaptive LogLuv. (c) histogram of the tone mapped image using our propose without inversing the second part of the tone mapping curve. (d) histogram of the tone mapped image using our propose after inversing the second part of the tone mapping curve.

Figure 4 :

 4 Figure4: HDR-VDP rate-distortion results for Sun sequence. First row: 75 % probability to be detected; second row: 95 % probability to be detected.

Figure

  Figure5: HDR-VDP rate-distortion results for Tunnel sequence. First row: 75% probability to be detected; second row: 95% probability to be detected.

  5: HDR-VDP rate-distortion results for Tunnel sequence. First row: 75% probability to be detected; second row: 95% probability to be detected. Video Compression," IEEE Transactions on Image Processing, 20 (6), pp. 1558-1571, June 2011. [4] A. Segall, "Scalable Coding of High Dynamic Range Video," in IEEE Proc. International Conference on Image Processing (ICIP), San Antonio, TX, Sept. 2007. [5] S. Liu, W.-S. Kim, and A. Vetro, "Bit-depth scalable coding for high dynamic range video," in SPIE Proc. Visual Communications Image Processing, San Jose, CA, January 2008. [6] A. Motra, and H. Thoma, "An adaptive LogLuv Transform for High Dynamic Range Video Compression," in IEEE Proc. International Conference on. Image Processing (ICIP), Hong Kong, September 2010.

ACKNOWLEDGEMENTS

This work was partly funded by Région Ile de France in the framework of the NEVEx project.