An experimental study was conducted using axisymmetric co-flow laminar diffusion flames of methane-air, methane-oxygen and ethylene-air to examine the effect of pressure on soot formation and the structure of the temperature field. A liquid fuel burner was designed and built to observe the sooting behavior of methanol-air and n-heptane-air laminar diffusion flames at elevated pressures up to 50 atm. A non-intrusive, line-of-sight spectral soot emission (SSE) diagnostic technique was used to determine the temperature and the soot volume fraction of methane-air flames up to 60 atm, methane-oxygen flames up to 90 atm and ethylene-air flames up to 35 atm. The physical flame structure of the methane-air and methane-oxygen diffusion flames were characterized over the pressure range of 10 to 100 atm and up to 35 atm for ethylene-air flames. The flame height, marked by the visible soot radiation emission, remained relatively constant for methane-air and ethylene-air flames over their respected pressure ranges, while the visible flame height for the methane-oxygen flames was reduced by over 50 % between 10 and 100 atm. During methane-air experiments, observations of anomalous occurrence of liquid material formation at 60 atm and above were recorded. The maximum conversion of the carbon in the fuel to soot exhibited a strong power-law dependence on pressure. At pressures 10 to 30 atm, the pressure exponent is approximately 0.73 for methane-air flames. At higher pressures, between 30 and 60 atm, the pressure exponent is approximately 0.33. The maximum fuel carbon conversion to soot is 12.6 % at 60 atm. For methane-oxygen flames, the pressure exponent is approximately 1.2 for pressures between 10 and 40 atm. At pressures between 50 and 70 atm, the pressure exponent is about -3.8 and approximately -12 for 70 to 90 atm. The maximum fuel carbon conversion to soot is 2 % at 40 atm. For ethylene-air flames, the pressure exponent is approximately 1.4 between 10 and 30 atm. The maximum carbon conversion to soot is approximately 6.5 % at 30 atm and remained constant at higher pressures.

An experimental study was conducted using axisymmetric co-flow laminar diffusion flames of methane-air, methane-oxygen and ethylene-air to examine the effect of pressure on soot formation and the structure of the temperature field. A liquid fuel burner was designed and built to observe the sooting behavior of methanol-air and n-heptane-air laminar diffusion flames at elevated pressures up to 50 atm. A non-intrusive, line-of-sight spectral soot emission (SSE) diagnostic technique was used to determine the temperature and the soot volume fraction of methane-air flames up to 60 atm, methane-oxygen flames up to 90 atm and ethylene-air flames up to 35 atm. The physical flame structure of the methane-air and methane-oxygen diffusion flames were characterized over the pressure range of 10 to 100 atm and up to 35 atm for ethylene-air flames. The flame height, marked by the visible soot radiation emission, remained relatively constant for methane-air and ethylene-air flames over their respected pressure ranges, while the visible flame height for the methane-oxygen flames was reduced by over 50 % between 10 and 100 atm. During methane-air experiments, observations of anomalous occurrence of liquid material formation at 60 atm and above were recorded. The maximum conversion of the carbon in the fuel to soot exhibited a strong power-law dependence on pressure. At pressures 10 to 30 atm, the pressure exponent is approximately 0.73 for methane-air flames. At higher pressures, between 30 and 60 atm, the pressure exponent is approximately 0.33. The maximum fuel carbon conversion to soot is 12.6 % at 60 atm. For methane-oxygen flames, the pressure exponent is approximately 1.2 for pressures between 10 and 40 atm. At pressures between 50 and 70 atm, the pressure exponent is about -3.8 and approximately -12 for 70 to 90 atm. The maximum fuel carbon conversion to soot is 2 % at 40 atm. For ethylene-air flames, the pressure exponent is approximately 1.4 between 10 and 30 atm. The maximum carbon conversion to soot is approximately 6.5 % at 30 atm and remained constant at higher pressures.

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Novel sequential and parallel computational fluid dynamic (CFD) codes based on method of lines (MOL) approach were developed for the numerical simulation of multi-component reacting flows using detailed transport and thermodynamic models. Both codes were applied to the prediction of a confined axisymmetric laminar co-flowing methane-air diffusion flame for which experimental data were available in the literature. Flame-sheet model for infinite-rate chemistry and one-, two-, and five- and ten-step reduced finite-rate reaction mechanisms were employed for methane-air combustion sub-model. A second-order high-resolution total variation diminishing (TVD) scheme based on Lagrange interpolation polynomial was proposed in order to alleviate spurious oscillations encountered in time evolution of flame propagation. Steady-state velocity, temperature and species profiles obtained by using infinite- and finite-rate chemistry models were validated against experimental data and other numerical solutions. They were found to be in reasonably good agreement with measurements and numerical results. The proposed difference scheme produced accurate results without spurious oscillations and numerical diffusion encountered in the classical schemes and hence was found to be a successful scheme applicable to strongly convective flow problems with non-uniform grid resolution. The code was also found to be an efficient tool for the prediction and understanding of transient combustion systems. This study constitutes the initial steps in the development of an efficient numerical scheme for direct numerical simulation (DNS) of unsteady, turbulent, multi-dimensional combustion with complex chemistry.

Les suies se présentent sous la forme de fines particules carbonées de diamètres compris entre quelques dizaines de nanomètres à quelques micromètres. Dans l’atmosphère, elles entraînent des enjeux climatiques, de par leurs propriétés radiatives, mais aussi des enjeux sanitaires, du fait de leur faible taille : elles pénètrent facilement dans le système respiratoire et même, pour les plus fines, dans le système sanguin. L’objectif est de parfaire les connaissances sur les propriétés physiques des suies produites par différents systèmes de combustion. C’est dans le but de mieux comprendre l’influence des systèmes de combustion, faisant intervenir des temps de séjours différents, des propriétés de turbulence, d’oxydation et de pression distinctes que nous avons choisi d’étudier trois types de combustion spécifiques : d’une part, des flammes de diffusion laminaires à pression atmosphérique, initiées dans un brûleur développé au cours de ces travaux ; d’autre part, une flamme de diffusion laminaire sous atmosphère pressurisée (3 à 5 bars) ; enfin, une flamme turbulente produite par une chambre tubulaire, elle aussi sous atmosphère pressurisée (1.2 à 3 bar). Un autre enjeu de ce travail était d’approfondir les informations relatives à la combustion de carburants liquides, à savoir le kérosène et le diester. Les travaux effectués visent à déterminer les caractéristiques morphologiques (dimension fractale, diamètre des monomères...) et l’indice complexe m* des suies issues des différents systèmes de combustion. La technique employée pour la mesure de l’indice complexe de réfraction des suies, repose sur l’analyse d’une partie des fumées produites par les flammes. Ces fumées sont acheminées dans un banc d’analyse permettant la mesure de signaux d’extinction et de diffusion, ainsi que de distributions de taille des suies. Par ailleurs, des analyses de clichés obtenus par microscope en transmission d’électrons (TEM) permettent l’obtention d’informations sur la morphologie des agrégats de suies. L’utilisation de la théorie de la diffusion de la lumière pour des agrégats fractals dans la limite de Rayleigh (RDG-FA) permet d’estimer à partir de ces données deux fonctions de l’indice complexe E(m) et F(m), et ainsi de retrouver m*. / Soot are carbonaceous fine particles, which diameters are ranged from a few nanometres to a few micrometers. They have an impact on climate, due to their radiative properties, as well as on health, due to their small size. That’s why particulate matter is an important concern. In order to gain a better understanding of the influence of the combustion devices, which implies specific residence time and also specific turbulence, oxidation and pressure properties, we studied three specific kinds of combustion : first, laminar diffusion flames at atmospheric pressure ; then, a laminar diffusion flame a high pressures (3 to 5 bar) ; finally, a turbulent flame produced in a combustor at high pressures (1,2 to 3 bar). Another objective of this work was to improve the knowledge about soot produced by the combustion of liquid fuels, namely kerosene and biofuel. We studied morphological properties (fractal dimension, primary particle size…) and the refractive index m* of soot produced by these combustion systems. The technique employed to characterize the soot refractive index is based on the analysis of a part of smokes produced by flames. These are transported towards two optical cells, so that extinction and scattering coefficients can be measured, in addition to soot size distributions. Furthermore, a morphological characterization of the aggregates is conducted, using transmission electron microscopy (TEM) photographs. Rayleigh-Debye-Gans theory for fractal aggregates is used to determine two functions of the refractive index E(m) and F(m), so that m* can be deduced.

Le rayonnement joue un rôle fondamental dans les problèmes d'incendie puisque c'est le mode dominant de transfert de chaleur entre la flamme et le milieu environnant. Il contrôle la pyrolyse, et donc la puissance de flamme, et la vitesse de croissance de l'incendie. Étudier les flammes de diffusion contrôlées par les forces de flottabilité est une première étape pour comprendre et de prédire les incendies. Le principal objectif de ce travail est de modéliser le transfert radiatif et les processus de production/destruction de la suie dans ce type de flammes. Premièrement, différents modèles de propriétés radiatives des gaz ont été comparés dans des configurations tests. Il est apparu que le modèle FSCK couplé avec le schéma de mélange de Modest et Riazzi est le meilleur compromis entre précision et temps de calcul, ce modèle étant un bon candidat pour être implémenté dans des codes CFD traitant des problèmes d'incendie. Dans un second temps, un modèle de formation/oxydation des suies semi-détaillé, considérant l'acétylène et le benzène comme précurseurs, a été validé dans des flammes de diffusion laminaires de type coflow sur une large gamme d'hydrocarbures (C1-C3) et pour différentes conditions. Ensuite, le FSCK et le modèle de formation/destruction ont été appliqués pour simuler des feux de nappe de méthane et de propane aux échelles du laboratoire et intermédiaire. Les structures de flamme prédites ainsi que les flux radiatif transférés au milieu environnant ont montré un bon accord avec les résultats expérimentaux disponibles. Finalement, les interactions entre le rayonnement et la turbulence ont été quantifiées. / The radiative heat transfer plays an important role in fire problems since it is the dominant mode of heat transfer between flames and surroundings. It controls the pyrolysis, and therefore the heat release rate, and the growth rate of the fire. In the present work a numerical study of buoyant diffusion flames is carried out, with the main objective of modelling the thermal radiative transfer and the soot formation/destruction processes. In a first step, different radiative property models were tested in benchmark configurations. It was found that the FSCK coupled with the Modest and Riazzi mixing scheme was the best compromise in terms of accuracy and computational requirements, and was a good candidate to be implemented in CFD codes dealing with fire problems. In a second step, a semi-empirical soot model, considering acetylene and benzene as precursor species for soot nucleation, was validated in laminar coflow diffusion flames over a wide range of hydrocarbons (C1-C3) and conditions. In addition, the optically-thin approximation was found to produce large discrepancies in the upper part of these small laminar flames. Reliable predictions of soot volume fractions require the use of an advanced radiation model. Then the FSCK and the semi-empirical soot model were applied to simulate laboratory-scale and intermediate-scale pool fires of methane and propane. Predicted flame structures as well as the radiant heat flux transferred to the surroundings were found to be in good agreement with the available experimental data. Finally, the interaction between radiation and turbulence was quantified.