La combustion pré mélangée a été depuis longtemps un domaine vaste d’étude au niveau appliqué et fondamental. Bien que la plupart des applications industrielles en combustion aient lieu dans des régimes turbulents, le passage par l’étude laminaire est indispensable pour comprendre les mécanismes fondamentaux des flammes turbulentes. Ce travail de thèse porte essentiellement sur l’étude des différentes instabilités agissant sur un front de flamme laminaire de pré mélange pour des mélanges de propane-air et méthane-air, enrichis ou non en azote. L’étude consiste à mesurer les taux de croissance des perturbations dans un brûleur d’Hele-Shaw formé par deux plaques de verre ($150\times50cm$) très rapprochées (brûleur 2D). Grâce à un système de forçage constitué par des plaques modulées spatialement avec différentes longueurs d’onde, le taux de croissance peut être mesuré même en présence du développement spontané de l’instabilité avec la longueur d’onde la plus instable. A richesse constante et pour des valeurs croissantes de la dilution en oxygène le vecteur d'onde de coupure augmente avec la vitesse de flamme qui devient plus instable. Pour des mélanges de propane ce nombre d’onde augmente également lorsqu'on augmente la richesse à vitesse constante et il diminue dans le cas des mélanges de méthane, cela est en relation avec l'évolution des effets thermodiffusifs dans chacun des cas. Une augmentation de l’épaisseur de la cellule d’Hele-Shaw, aboutit à une augmentation du taux de croissance pour les petits nombres d’onde ainsi que du nombre de Markstein, et à une diminution du taux de croissance pour les grands nombres, du fait des effets des pertes thermiques. / Premixed combustion has been the subject of extensive work, concerning both applications and fundamental aspects. Although in most practical applications combustion occurs in a turbulent environment, the study of laminar flames is important to understand the fundamental mechanisms of turbulent flame propagation. The objective of this work is to study the various instabilities acting on a laminar premixed flame for mixtures of propane-air and methane-air, enriched or not with nitrogen. it consists in measuring the linear growth rates of disturbances in a Hele-Shaw burner formed by two glass plates ($150\times50cm$) separated by a thin gap width (2D burner). Using spatially modulated plates with different wavelengths, the linear growth rate of perturbations can be measured even in the presence of the most unstable wavelength. The experimental values of the linear growth rate as a function of wavenumber are fitted by a linear dispersion relation to estimate the Markstein number and the cutoff wavenumber. For a constant equivalence ratio with increasing values of the dilution in oxygen, the cutoff wavenumber grows with the flame velocity and it is becoming more unstable. The cutoff wave number rises also when the equivalence ratio increases for propane-air mixture and decreases for methane-air mixture, in relation to the evolution of thermal diffusive effects. An enlargement in the thickness of the Hele-Shaw cell results in an increase of the growth rate for small wavenumbers thus in the Markstein number, and in a decline in the growth rate for the large wavenumbers, in relation with the effects of heat losses.

Modelos algébricos para prever o comprimento de uma chama turbulenta têm sido foco de estudo de diversos grupos de pesquisa por suas aplicações na área de engenharia. O método experimental para obter o modelo varia desde visualizações simples, até técnicas fotográficas, este último com parâmetros fotográficos variando entre os autores. Técnicas fotográficas são usadas para estimar a altura de levantamento da base da chama, (Lift-Off) e o comprimento médio visível de chama (Visible Flame Length, VFL). Duas técnicas comuns que podem ser encontradas na literatura: por imagens de chama com baixo tempo de exposição e longo tempo de exposição, são comparados com um terceiro que se baseia na intensidade luminosa e na frequência de imagens de chama que ocupam um pixel. O melhor método foi utilizado para caracterizar o comportamento das chamas turbulentas de gás natural para diferentes regimes de velocidade do escoamento. Modelos algébricos que preveem o comprimento de chama, altura de levantamento e a velocidade crítica de extinção de chama são avaliados com os novos resultados experimentais. Logo após, os coeficientes numéricos dos melhores modelos algébricos são reajustados Finalmente, foram obtidos mapas de estabilidade relacionados à altura de levantamento e à velocidade crítica de extinção de chama para cada diâmetro em função da diluição com CO2 e do número adimensional de Reynolds. A terceira parte deste trabalho está focada no estudo da distribuição de radiação térmica. Em particular, foram consideradas três distâncias radiais medidas em comprimentos de chama (0,5 Lf, 1 Lf, 2 Lf) visando obter a distribuição do fluxo radiante experimental ao longo de um eixo vertical adjacente às chamas. Finalmente, os dados experimentais foram utilizados como dados de entrada em uma análise inversa com o objetivo de calcular os fatores de ponderação do modelo das múltiplas fontes ponderadas (por suas siglas em inglês WMPS). Nesta última parte, são apresentados frações radiantes e distribuições de fluxo de calor radiante de chamas de gás natural diluídas para diversas diluições com dióxido de carbono e diâmetros do queimador. / Predicting models for turbulent diffusion flame lengths have several applications driven the attention of many research groups. Since several studies use photographs to measure the flame length, with photographic parameters varying among authors, in other cases simple visualizations were used. It is important to explore possible discrepancies among measurement technics that could affect the results. Optical visualizations of turbulent diffusion flames are used to estimate the visible average flame length (VFL) and the lift-off. The study presents a study of three different methods to measure the VFL using optical techniques. The effect on the image of the main optic parameters such as focus, exposure time and ISO sensibility are analyzed. The VFL obtained with images in low exposure time and long exposure time are compared with a third optical method that is based on the luminous intensity and the frequency of flame images occupying a pixel. One method was used to characterize the behavior of turbulent diffusion flames of natural gas for a range of flames in function of the flow velocity. Universal non-dimensional models that describe the VFL, lift-off and the blow-out stability limit of gaseous jet diffusion flames in the still air have been compared with new experimental data. The numerical coefficients of the best models are adjusted. Finally, maps of stability related to lift-off and blow-out were obtained for each diameter in function of the dilution with CO2 and flow exit velocity expressed in non-dimensional Reynolds number The third part of this work focuses on the estimation of the thermal distribution of radiative flux from turbulent diffusion flames in laboratory-scale. The experimental measurements were gotten from the previous stability study. In particular, was considered three radial distances measured in flame lengths (0,5 Lf, 1 Lf, 2 Lf) aiming at obtaining the experimental radiant flux along a vertical axis adjacent to the flames. Finally, the experimental data was used as input data in an inverse analysis with the purpose of computing weight coefficients of the weighted multi-point source (WMPS) model. Then, experimental data that include: radiant fractions and radiative heat flux are presents for several flames with different dilutions with carbon dioxide and burner´s diameters.

Increased knowledge into the physics and chemistrycontrolling emissions from flame-surface interactions shouldhelp in the design of combustion engines featuring improvedfuel economy and reduced emissions. The overall aim of this work has been to obtain afundamental understanding of wall-related, premixed combustionusing numerical modeling with detailed chemical kinetics. Thiswork has utilized CHEMKIN®, one of the leading softwarepackages for modeling combustion kinetics. The simple fuels hydrogen and methane as well as the morecomplex fuels propane and gasified biomass have been used inthe model. The main emphasis has been on lean combustion, andthe principal flow field studied is a laminar boundary layerflow in two-dimensional channels. The assumption has been madethat the wall effects may at least in principle be the same forlaminar and turbulent flames. Different flame geometries have been investigated, includingfor example autoignition flames (Papers I and II) and premixedflame fronts propagating toward a wall (Papers III and IV).Analysis of the results has shown that the wall effects arisingdue to the surface chemistry are strongly affected by changesin flame geometry. When a wall material promoting catalyticcombustion (Pt) is used, the homogeneous reactions in theboundary layer are inhibited (Papers I, II and IV). This isexplained by a process whereby water produced by catalyticcombustion increases the rate of the third-body recombinationreaction: H+O2+M ⇔ HO2+M. In addition, the water produced at higherpressures increases the rate of the 2CH3(+M) ⇔ C2H6(+M) reaction, giving rise to increased unburnedhydrocarbon emissions (Paper IV). The thermal coupling between the flame and the wall (theheat transfer and development of the boundary layers) issignificant in lean combustion. This leads to a sloweroxidation rate of the fuel than of the intermediatehydrocarbons (Paper III). Finally in Paper V, a well-known problem in the combustionof gasified biomass has been addressed, being the formation offuel-NOx due to the presence of NH3 in the biogas. A hybridcatalytic gas-turbine combustor has been designed, which cansignificantly reduce fuel-NOx formation. Keywords:wall effects, premixed flames, flamequenching, numerical modeling, CHEMKIN, boundarylayerapproximation, gasified biomass, fuel-NOx, hybrid catalytic combustor. / QC 20100504

In this study, numerical simulations of a low emission lab-scale non-premixed combustor are conducted and analyzed. The objectives are to provide new insight into the physical phenomena in the SPRF (Stagnation Point Reverse Flow) combustor built in the Georgia Tech Combustion Lab, and to compare three Large Eddy Simulation (LES) combustion models (Eddy Break-Up [EBU], Steady Flamelet [SF] and Linear Eddy Model [LEM]) for non-premixed combustion. The nominal operating condition of the SPRF combustor achieves very low NOx and CO emissions by combining turbulent mixing of exhaust gases with preheated reactants and chemical kinetics. The SPRF numerical simulation focuses on capturing the complex interaction between turbulent mixing and heat release. LES simulations have been carried out for a non-reactive case in order to analyze the turbulent mixing inside the combustor. The LES results have been compared to PIV experimental data and the code has been validated. The dominating features of the operational mode of the SPRF combustor (dilution of hot products into reactants, pre-heating and pre-mixing) have been analyzed, and results from the EBU-LES, SF-LES and LEM-LES simulations have been compared. Analysis shows that the LEM-LES simulation achieves the best agreement with the observed flame structure and is the only model that captures the stabilization processes observed in the experiments. EBU-LES and SF-LES do not predict the correct flow pattern because of the inaccurate modeling of sub-grid scale mixing and turbulence-combustion interaction. Limitations of these two models for this type of combustor are discussed.

A full Strouhal number thermo-acoustic model is purposed for the feedback control of self excited noise driven combustors. The inclusion of time delays in the volumetric heat release perturbation models create unique behavioral characteristics which are not properly reproduced within current low Strouhal number thermo acoustic models. New analysis tools using probability density functions are introduced which enable exact expressions for the statistics of a time delayed system. Additionally, preexisting tools from applied mathematics and control theory for spectral analysis of time delay systems are introduced to the combustion community. These new analysis tools can be used to extend sensitivity function analysis used in control theory to explain limits to control effectiveness in self-excited combustors. The control effectiveness of self-excited combustors with actuator constraints are found to be most sensitive to the location of non-minimum phase zeros. Modeling the non-minimum phase zeros correctly require accurate volumetric heat release perturbation models. Designs that removes non-minimum phase zeros are more likely to have poles in the right hand complex plane. As a result, unstable combustors are inherently more responsive to feedback control.

Increasingly stringent pollution and emission controls have caused a rise in the use of combustors operating under lean, premixed conditions. Operating lean (excess air) lowers the level of nitrous oxides (NOx) emitted to the environment. In addition, concerns over climate change due to increased carbon dioxide (CO2) emissions and the need for energy independence in the United States have spurred interest in developing combustors capable of operating with a wide range of fuel compositions. One method to decrease the carbon footprint of modern combustors is the use of high hydrogen content (HHC) fuels. The objective of this research is to develop tools to better understand the physics of turbulent flame propagation in highly stretch sensitive premixed flames in order to predict their behavior at conditions realistic to the environment of gas turbine combustors. This thesis presents the results of an experimental study into the flame propagation characteristics of highly stretch-sensitive, turbulent premixed flames generated in a low swirl burner (LSB). This study uses a scaling law, developed in an earlier thesis from leading point concepts for turbulent premixed flames, to collapse turbulent flame speed data over a wide range of conditions. The flow and flame structure are characterized using high speed particle image velocimetry (PIV) over a wide range of fuel compositions, mean flow velocities, and turbulence levels. The first part of this study looks at turbulent flame speeds for these mixtures and applies the previously developed leading points scaling model in order to test its validity in an alternate geometry. The model was found to collapse the turbulent flame speed data over a wide range of fuel compositions and turbulence levels, giving merit to the leading points model as a method that can produce meaningful results with different geometries and turbulent flame speed definitions. The second part of this thesis examines flame front topologies and stretch statistics of these highly stretch sensitive, turbulent premixed flames. Instantaneous flame front locations and local flow velocities are used to calculate flame curvatures and tangential strain rates. Statistics of these two quantities are calculated both over the entire flame surface and also conditioned at the leading points of the flames. Results presented do not support the arguments made in the development of the leading points model. Only minor effects of fuel composition are noted on curvature statistics, which are mostly dominated by the turbulence. There is a stronger sensitivity for tangential strain rate statistics, however, time-averaged values are still well below the values hypothesized from the leading points model. The results of this study emphasize the importance of local flame topology measurements towards the development of predictive models of the turbulent flame speed.

Η ολοένα αυξανόμενη ανάγκη για γρήγορες, αξιόπιστες και εύκολες στην χρήση αναλυτικές τεχνικές, οι οποίες μπορούν να χρησιμοποιηθούν εντός και εκτός εργαστηρίου, έδωσε ιδιαίτερη ώθηση για ανάπτυξη τεχνικών που να πληρούν τις παραπάνω προϋποθέσεις. Η Φασματοσκοπία Πλάσματος Επαγόμενου από laser (Laser-Induced Breakdown Spectroscopy-LIBS), λόγω των σημαντικών της πλεονεκτημάτων, έχει προταθεί ως μια τεχνική για στοιχειακή ανάλυση υλικών, ανεξαρτήτως της κατάστασής τους (στερεή, υγρή ή αέρια). Κατά την τεχνική LIBS, η ατομοποίηση και η διέγερση των ατόμων του υλικού γίνεται σε ένα στάδιο, ενώ δεν απαιτείται προετοιμασία του δείγματος. Έτσι, τα τελευταία χρόνια η τεχνική LIBS είναι η πιο διαδεδομένη αναλυτική τεχνική βασισμένη στα laser. Στην παρούσα διδακτορική διατριβή η τεχνική LIBS εφαρμόζεται σε δυο διαφορετικά πεδία έρευνας: στις φλόγες υδρογονανθράκων-αέρα και στην μελέτη πολυμερικών/πλαστικών δειγμάτων. Στις φλόγες υδρογονανθράκων-αέρα, η φασματοσκοπία πλάσματος επαγόμενου από laser χρησιμοποιήθηκε για τον καθορισμό του λόγου ισοδυναμίας (φ), ο οποίος εκφράζει την αναλογία καύσιμου προς οξειδωτικό μέσο, σε φλόγες αέριων και υγρών υδρογονανθράκων μέσω των λόγων των εντάσεων των φασματικών γραμμών του υδρογόνου, H, οξυγόνου, O, και άνθρακα, C. Μέσω αυτής της συσχέτισης, πραγματοποιήθηκαν χωρικά αναλυμένες μετρήσεις του φ σε προ-αναμεμιγμένες φλόγες υδρογονανθράκων-αέρα διαφορετικής γεωμετρίας, οι οποίες παρείχαν σημαντικές πληροφορίες για την δομή της φλόγας. Επιπλέον, η τεχνική LIBS χρησιμοποιήθηκε για την ανίχνευση υδρατμών στον αέρα και σε φλόγες υδρογονανθράκων-αέρα και έγινε φανερό πως οι υδρατμοί προκαλούν σημαντικές αλλαγές στην δομή και τα χαρακτηριστικά της φλόγας, ενώ μέσω των φασματικών γραμμών του υδρογόνου, H, οξυγόνου, O, και αζώτου, Ν, οι υδρατμοί μπορούν να καθοριστούν ποιοτικά. Σε ότι αφορά τα πολυμερικά/πλαστικά δείγματα, η τεχνική LIBS μπορεί να αποτελέσει μια σημαντική μέθοδο για την ταυτοποίηση πολυμερικών δειγμάτων, οπότε στην συνέχεια, έγινε μελέτη των χαρακτηριστικών του πλάσματος που δημιουργείται κατά την ακτινοβόληση πολυμερικών/πλαστικών δειγμάτων και πως αυτά επηρεάζονται από τις ιδιότητες της δέσμης laser καθώς και από το περιβάλλον. / The continuously increasing needs for fast, reliable and easy to use analytical techniques operating remotely and in situ, under laboratory and/or field conditions have boosted considerable research efforts towards the development of novel analytical techniques satisfying these requirements. In that view, Laser-Induced Breakdown Spectroscopy (LIBS) has been proposed as an efficient tool for elemental analysis of various types of samples exhibiting several attractive advantages. LIBS is a laser based spectroscopic technique which permits the simultaneous atomization and excitation of the sample in one step, without requiring any sample preparation. In addition, LIBS operates successfully with all kinds of samples while the results are obtained within few seconds. Because of these advantages and its attractive simplicity, LIBS has become rapidly the most popular laser based analytical technique. In this work, LIBS is used in two different fields: the study of combustible hydrocarbon-air mixtures and the study of the created plasma in polymeric samples. In hydrocarbon –air flames, LIBS was applied for the determination of the local equivalence ratio in different hydrocarbon (gaseous and liquid) -air mixtures. In particular, it is shown that the ratio of the intensities of atomic spectral lines of H, C and O, emitted from a laser induced spark in the gaseous mixture, can be used for the rapid and accurate determination of the local equivalence ratio. There are also obtained spatially resolved equivalence ratio profiles in laminar premixed flames of different geometries (Bunsen type and impinging flames), which are used in order to reveal and quantify important flame structure features. Additionally, it is shown that LIBS can be used for the detection of humidity in air and in hydrocarbon-air flames. The results obtained showed that humidity causes significant changes in flame characteristics, and through the atomic spectral lines of H, N and O, humidity can be qualitatively be determined. In polymeric samples, LIBS technique is used for the identification of polymers and plastics for recycling purposes. Due to the importance of this application, the properties of the plasma created are extensively studied. It is shown that plasma generation and expansion is effected by the laser properties (laser energy, laser pulse) and by the environmental conditions (pressure).

Le monoxyde d’azote (NO) est un polluant atmosphérique responsable d’effets nuisibles sur l’environnement et la santé. Afin de mieux contrôler ces émissions, il est indispensable de comprendre et de maîtriser leur formation,en particulier lors de la combustion à haute pression, domaine d’application industrielle (cas des turbines à gaz,des moteurs…). On distingue quatre voies principales de formation de NO : la voie thermique, la voie du NO précoce, la voie NNH et la voie N2O. L’objectif de cette thèse à caractère expérimentale est de compléter la base de données expérimentale déjà existante nécessaire à la compréhension et à l’identification de la contribution de chaque voie à la formation du NO à haute pression.Dans cette thèse, un dispositif de brûleurs à contre-courants a été utilisé pour étudier la structure de flammes laminaires, prémélangées à haute pression. Les profils de concentration de NO dans les flammes CH4/O2/N2 à différentes richesses (Фc =0,7-1,2) et différentes pressions (P=0,1-0,7 MPa) ont été mesurés par Fluorescence Induite par Laser. L’effet de l’ajout d’hydrogène (80%CH4/20%H2 : Application Hythane®) sur la formation de NO a également été étudié dans les flammes pauvres CH4/O2/N2. Le mécanisme cinétique GDF-Kin®3.0_NCN a été comparé aux mesures de NO disponibles dans la littérature ainsi qu’aux simulations des mécanismes cinétiques du Gaz Research Institute (version 2.11 et 3.0). Ces trois mécanismes ont été ensuite comparés aux mesures expérimentales réalisées dans ces travaux de thèse. / The nitric oxide (NO) is a pollutant responsible of detrimental effects on the environment and health. To better control these emissions, it’s crucial to understand and to control their formation, in particular during the combustion process at high pressure, area of industrial applications (gas turbines, engines…).There are four major routes of the NO formation: the thermal route, the prompt-NO route, the NNH route and theN2O route. The aim of this experimental thesis is to complete the existing experimental database which isnecessary to the understanding and the identification of the contribution from each route to the NO formation at high pressure.In this thesis, a facility of two twin counter-flow burners was used to study the structure of the laminar, premixed flames at high pressure. Experimental NO concentration profiles have been measured in CH4/O2/N2 flames for arange of equivalence ratio (from 0.7 to 1.2) and pressures (from 0.1 to 0.7 MPa) by Laser Induced Fluorescence.The effect of adding hydrogen (80%CH4/20%H2: Hythane® application) on the NO formation has been also studied in lean CH4/O2/N2 flames. The GDF-Kin®3.0_NCN kinetic mechanism has been compared to experimental data from the literature and also compared to the simulations from the Gas Research Institute mechanisms (version 2.11 and 3.0). These three mechanisms have been finally compared to the experimental data from this thesis.

Le contexte de cette étude est l’optimisation de la combustion turbulente prémélangée de syngaz pour la production propre d’énergie. Un brûleur CH4/air de type bec Bunsen avec forçage turbulent multi-échelles produit par un système de trois grilles, est simulé numériquement par différentes techniques de simulation des grandes échelles (SGE), et les résultats sont comparés à l’expérience. On a développé et appliqué une formulation bas-Mach du solveur Navier-Stokes basé sur différents schémas numériques, allant des différences finies centrées d’ordre 4 à des versions avancées des schémas WENO d’ordre 5. La méthodologie est évaluée sur une série de cas-tests classiques (flamme laminaire 1D prémélangée, turbulence homogène et isotrope en auto-amortissement), et sur des simulations 2D de la flamme turbulente prémélangée expérimentale. Les SGE implicites (ILES), i.e. sans aucune modélisation sous-maille, et explicites avec le modèle de flamme épaissie et un modèle de plissement sous-maille nouvellement élaboré (TFLES), sont appliquées à la simulation 3D du brûleur expérimental. Les résultats montrent que l’approche TFLES avec un schéma d’ordre élevé à faible dissipation numérique prédit correctement la longueur de la flamme et la densité de surface de flamme. La SGE implicite avec un schéma WENO avancé produit une flamme trop courte mais réaliste à condition que la taille de la maille soit de l’ordre de l’épaisseur de flamme laminaire. La représentation des interactions flamme/turbulence est néanmoins très différente entre TFLES et ILES. / The context of this study is the optimization of premixed turbulent combustion of syngas for clean energy production. A Bunsen-type CH4/air turbulent premixed burner with a multi-scale grid generator is simulated with different Large Eddy Simulation (LES) strategies and compared to experimental results. A low-Mach formulation of a compressible Navier-Stokes solver based on different numerical methods, ranging from 4th order central finite difference to 5th order advanced WENO schemes, is developed and applied. Classical test cases (1D laminar premixed flame, decaying HIT), and 2D simulations of the turbulent premixed flame are performed to assess the numerical methodology. Implicit LES (ILES), i.e. LES without any explicit subgrid modeling, and explicit LES with the Thickened Flame model and subgrid scale flame wrinkling modelling (TFLES) are applied to simulate numerically the 3D experimental burner. Results show that TFLES with a high-order low dissipation scheme predicts quite well the experimental flame length and flame surface density. ILES with advanced WENO schemes produces a slightly shorter although realistic flame provided the grid spacing is of order of the laminar flame thickness. The representation of flame/turbulence interactions in TFLES and ILES are however quite different.

Les énergies fossiles sont largement utilisées depuis les années 1900 pour satisfaire l’augmentation mondiale de la demande d’énergie. Cependant, la combustion est un procédé qui libère des polluants comme le CO2 et les NOx. Un des principaux challenges du 21ème siècle est de réduire ces émissions et les constructeurs automobiles sont impliqués dans cette course. Pour augmenter le rendement des moteurs à pistons, des solutions techniques, tels que le "downsizing", sont développées. Cette technique consiste à réduire la cylindrée des moteurs tout en maintenant leurs performances grâce à un turbocompresseur qui permet d’augmenter la masse enfermée dans la chambre de combustion. Malheureusement, l’augmentation de la pression dans les cylindres induite par le turbocompresseur est à l’origine de combustions anormales : des variations cycles à cycles importantes apparaissent, les gaz frais peuvent s’auto-allumer (allumage avant le claquage de la bougie) entrainant des phénomènes de cliquetis ou de rumble. La Simulation aux Grandes Echelles (SGE) a déjà prouvé qu’elle était un outil fiable pour prédire ces combustions anormales. Cependant ces calculs reposent sur des modèles pour prédire la propagation de la flamme dans la chambre de combustion. Ces modèles sont généralement issus de corrélations réalisées dans des cas où la turbulence est supposée homogène et isotrope. Définir théoriquement ou numériquement une telle turbulence est relativement simple mais expérimentalement la tâche est plus délicate. Cette thèse s’intéresse à un dispositif classiquement utilisé: une enceinte fermée dans laquelle la turbulence est générée par des ventilateurs. L’objectif de ce travail est donc double: 1. caractériser la turbulence générée dans ce type d’enceinte pour vérifier si elle est homogène et isotrope. 2. caractériser finement la combustion, laminaire et turbulente, afin d’enrichir les connaissances dans ce domaine et ainsi améliorer les modèles utilisés. Une première étude sur la propagation des flammes laminaires a été menée. Elle présente les effets de l’étirement et du confinement sur la vitesse de flamme laminaire. La principale difficulté pour la simulation de l’enceinte complète consiste à trouver une méthode numérique permettant de reproduire précisément l’écoulement généré par un ventilateur mais aussi d’en gérer plusieurs simultanément. Deux méthodes ont alors été testées. Premièrement, une méthode type Frontières Immergées a été implémentée dans le code de calcul AVBP. Malgré les bons résultats obtenus sur des cas tests simples, cette méthode ne s’est pas montrée adaptée pour reproduire précisément l’écoulement généré par un seul ventilateur. Une autre approche, provenant du monde du calcul des turbomachines, et basée sur le couplage de codes (appelée MISCOG), a quant à elle démontré ses capacités à le faire et est donc utilisée pour calculer l’écoulement généré par les six ventilateurs à l’intérieur de l’enceinte. L’écoulement non réactif est d’abord analysé: les résultats montrent qu’il existe une zone d’environ 6 cm de diamètre au centre de l’enceinte dans laquelle la vitesse moyenne de l’écoulement est proche de zéro et dans laquelle la turbulence est quasiment homogène et isotrope. Enfin, le pré-mélange de gaz frais est allumé en déposant un noyau de gaz chauds au centre de l’enceinte et la phase de propagation turbulente est analysée. En particulier, il est montré que la température des gaz brulés déposés au moment de l’allumage est un paramètre critique. / Fossil energy is widely used since the 1900s to satisfy the global increasing energy demand. However, combustion is a process releasing pollutants such as CO2 and NOx. One of the major challenges of the 21th century is to reduce these emissions and car manufacturers are involved in this race. To increase fuel efficiency of piston engines, some technical solutions are developed such as ‘downsizing’. It consists in reducing the engine size while maintaining its performances using a turbocharger to increase the trapped mass in the combustion chamber. Unfortunately, downsizing can lead to abnormal combustions: intense cycle to cycle variations can appear, the fresh mixture can auto-ignite (ignition before spark-plug ignition) leading to knock or rumble. Large Eddy Simulation has proven to be a reliable tool to predict these abnormal combustions in real engines. However, such computations are performed using models to predict the flame propagation in the combustion chamber. Theses models are generally based on correlations derived in cases where turbulence is assumed to be homogeneous and isotropic. Defining theoretically or numerically such a turbulence is a simple task but experimentally it is more challenging. This thesis focuses on a apparatus used in most experimental systems: fans stirred vessel. The objective of this work is twofold: 1. characterize the turbulence generated inside the vessel to check wether it is homogeneous and isotropic or not, 2. finely characterize laminar and turbulent combustion in this setup in order to increase the knowledge in this field, and thereby improve models used. First, a laminar flame propagation study has been conducted to address both confinement and curvature effects on the laminar flame speed in a spherical configuration. The main difficulty to perform the simulation of the whole configuration consists in finding a numerical method able to compute accurately the flow generated by one fan and able to handle six fans simultaneously too. Two numerical methodologies have been tested. First an Immersed Boundaries method was implemented. Despite good results obtained on academic test cases, this method was shown to be unadapted to compute accurately the flow generated by one fan. On the other hand, a numerical approach, coming from turbomachinery calculations and based on code coupling (called MISCOG), demonstrates its ability to do it and it is used to compute the flow generated by the six fans inside the closed vessel. Non-reacting flow is first analyzed and reveals a zone at the vessel center of around 6 cm of diameter where mean velocity is near zero and turbulence is almost homogeneous and isotropic. After that, the premixed fresh mixture is ignited depositing a hot gases kernel at the vessel center and the turbulent propagation phase is analyzed. In particular, it is shown that the amount of energy deposited at ignition is a critical parameter.