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Doctor of Philosophy(PhD) / This body of research uses numerical and experimental investigative techniques to further the understanding of autoignition. Hydrogen/nitrogen and methane/air fuel configurations of turbulent lifted flames in a vitiated coflow burner are used as model flames for this investigation. Characterisation was undertaken to understand the impact of controlling parameters and the overall behaviour of the flames, and to provide a body of data for modelling comparisons. Modelling of the flames was conducted using the PDF-RANS technique with detailed chemistry incorporated using In-situ Adaptive Tabulation (ISAT) within the commercial CFD package, FLUENT 6.2. From these investigations, two numerical indicators for autoignition were developed: convection-reaction balance in the species transport budget at the mean flame base; and the build-up of ignition precursors prior to key ignition species. These indicators were tested in well defined autoignition and premixed flame cases, and subsequently used with the calculated turbulent lifted flames to identify if these are stabilised through autoignition. Based on learnings from the modelling, a quantitative, high-resolution simultaneous imaging experiment was designed to investigate the correlations of an ignition precursor (formaldehyde: CH2O) with a key flame radical (OH) and temperature. Rayleigh scattering was used to measure temperature, while Laser Induced Fluorescence (LIF) was used to measure OH and CH2O concentrations. The high resolution in the Rayleigh imaging permitted the extraction of temperature gradient data, and the product of the OH and CH2O images was shown to be a valid and useful proxy for peak heat release rate in autoigniting and transient flames. The experimental data confirmed the presence of formaldehyde as a precursor for autoignition in methane flames and led to the identification of other indicators. Sequenced images of CH2O, OH and temperature show clearly that formaldehyde exists before OH and peaks when autoignition occurs, as confirmed by images of heat release. The CH2O peaks decrease later while those of OH remain almost unchanged in the reaction zone.

Doctor of Philosophy(PhD) / This body of research uses numerical and experimental investigative techniques to further the understanding of autoignition. Hydrogen/nitrogen and methane/air fuel configurations of turbulent lifted flames in a vitiated coflow burner are used as model flames for this investigation. Characterisation was undertaken to understand the impact of controlling parameters and the overall behaviour of the flames, and to provide a body of data for modelling comparisons. Modelling of the flames was conducted using the PDF-RANS technique with detailed chemistry incorporated using In-situ Adaptive Tabulation (ISAT) within the commercial CFD package, FLUENT 6.2. From these investigations, two numerical indicators for autoignition were developed: convection-reaction balance in the species transport budget at the mean flame base; and the build-up of ignition precursors prior to key ignition species. These indicators were tested in well defined autoignition and premixed flame cases, and subsequently used with the calculated turbulent lifted flames to identify if these are stabilised through autoignition. Based on learnings from the modelling, a quantitative, high-resolution simultaneous imaging experiment was designed to investigate the correlations of an ignition precursor (formaldehyde: CH2O) with a key flame radical (OH) and temperature. Rayleigh scattering was used to measure temperature, while Laser Induced Fluorescence (LIF) was used to measure OH and CH2O concentrations. The high resolution in the Rayleigh imaging permitted the extraction of temperature gradient data, and the product of the OH and CH2O images was shown to be a valid and useful proxy for peak heat release rate in autoigniting and transient flames. The experimental data confirmed the presence of formaldehyde as a precursor for autoignition in methane flames and led to the identification of other indicators. Sequenced images of CH2O, OH and temperature show clearly that formaldehyde exists before OH and peaks when autoignition occurs, as confirmed by images of heat release. The CH2O peaks decrease later while those of OH remain almost unchanged in the reaction zone.

No description available.

Fossil fuel reserves are projected to be decreasing, and emission regulations are becoming more stringent due to increasing atmospheric pollution. Alternative fuels for power generation in industrial gas turbines are thus required able to meet the above demands. Examples of such fuels are synthetic gas, blast furnace gas and coke oven gas. A common characteristic of these fuels is that they are multi-component fuels, whose composition varies greatly depending on their production process. This implies that their combustion characteristics will also vary significantly. Thus, accurate and yet flexible enough combustion sub-models are required for such fuels, which are used during the design stage, to ensure optimum performance during practical operating conditions. Most combustion sub-model development and validation is based on Direct Numerical Simulation (DNS) studies. DNS however is computationally expensive. This, has so far limited DNS to single-component fuels such as methane and hydrogen. Furthermore, the majority of DNS conducted to date used one-step chemistry in 3D, and skeletal chemistry in 2D only. The need for 3D DNS using skeletal chemistry is thus apparent. In this study, an accurate reduced chemical mechanism suitable for multi-component fuel-air combustion is developed from a skeletal mechanism. Three-dimensional DNS of a freely propagating turbulent premixed flame is then conducted using both mechanisms to shed some light into the flame structure and turbulence-scalar interaction of such multi-component fuel flames. It is found that for the multi-component fuel flame heat is released over a wider temperature range contrary to a methane flame. This, results from the presence of individual species reactions zones which do not all overlap. The performance of the reduced mechanism is also validated using the DNS data. Results suggest it to be a good substitute of the skeletal mechanism, resulting in significant time and memory savings. The flame markers commonly used to visualize heat release rate in laser diagnostics are found to be inadequate for the multi-component fuel flame, and alternative markers are proposed. Finally, some popular mean reaction rate closures are tested for the multi-component fuel flame. Significant differences are observed between the models’ performance at the highest turbulence level considered in this study. These arise from the chemical complexity of the fuel, and further parametric studies using skeletal chemistry DNS would be useful for the refinement of the models.

Malgré l'augmentation constante des ressources informatiques dédiées au calcul scientifique, simuler des écoulements réactifs mettant en jeu une chimie complexe reste aujourd'hui encore un véritable challenge. L'objectif de cette thèse est le développement de la méthode Hybrid Transported-Tabulated Chemistry (HTTC), destinée aux simulations DNS/LES de flammes avec des mécanismes cinétiques détaillés, en offrant un temps de calcul acceptable. Cette nouvelle approche consiste à transporter les espèces majoritaires de l'écoulement, tandis que les espèces minoritaires sont extraites d'une table chimique. La méthode HTTC a été implémentée dans un code DNS/LES et validée sur des flammes 1D de méthane et de kérosène, mettant en évidence une réduction extrêmement importante du temps de calcul, comparé aux solveurs classiques de chimie détaillée. HTTC a ensuite été mis en œuvre avec succès sur des flammes triples de méthane en présence de forts gradients de fraction de mélange. L'impact des méthodes choisies pour prolonger la table chimique et pour calculer les variables de contrôle, utilisées pour paramétrer la table, a été étudiée avec une attention particulière. Un très bon accord a été trouvé avec les résultats de référence, obtenus avec un solveur de chimie détaillée. / Even if significant progress is being made to improve the power of high-performance computers, the numerical simulation of reactive flows involving complex chemistry is still a challenging task. The objective of this work is the development of the Hybrid Transported-Tabulated Chemistry method (HTTC), designed for the DNS/LES simulations of flames with detailed kinetic mechanisms, with an acceptable cost. This novel approach combines the transport of the main species in the flow with the tabulation of the radical species. It has been implemented in a DNS/LES code and validated on 1D methane and kerosene flames. The cost of the simulations has been considerably decreased, compared to classic detailed chemistry solvers. Then, simulations of methane edge flames, featuring large gradients of mixture fraction, have been performed with HTTC. In particular, the impact of the methods used to extend the chemical tables and to compute the control variables have been analyzed in details. A very good agreement has been found by comparison with detailed chemistry.

La propagation de flammes turbulentes dans des milieux réactifs inhomogènes concerne un grand nombre d’applications pratiques, y compris celles qui reposent sur des cycles de combustion à volume constant. Les hétérogénéités de composition (richesse, température,dilution par des gaz brûlés, etc.) sont issues de plusieurs facteurs distincts tels que la dispersion du spray de gouttelettes de combustible et son évaporation, la topologie de l’écoulement ainsi que la présence éventuelle de gaz brûlés résiduels issus du cycle précédent. La structure des flammes partiellement prémélangées qui en résultent est significativement plus complexe que celles des flammes plus classiques de diffusion ou de prémélange. L’objectif de ce travail de thèse est donc de contribuer à l’amélioration de leur connaissance, en s’appuyant sur la génération et l’analyse de base de données de simulations numériques directes ou DNS (Direct Numerical Simulation). Celles-ci sont conduites avec le code de calcul Asphodele qui est basé sur l’approximation de faible nombre de Mach. Le combustible de référence retenu est l’iso-octane.La base de données est structurée suivant cinq paramètres qui permettent de caractériser l’écoulement turbulent ainsi que l’hétérogénéité de composition du milieu réactif. Dans un premier temps, des configurations bidimensionnelles ont été considérées en raison du coût élevé induit par la description détaillée de la cinétique chimique. L’étude des ces différents cas de calcul a permis de mettre en lumière plusieurs mécanismes fondamentaux de propagation dans les milieux hétérogènes en composition. Une réduction significative des coûts de calcula pu ensuite être obtenue grâce au développement d’un modèle chimique simplifié optimisé.Son utilisation a permis d’étendre les analyses à de / The propagation of turbulent flames in non-homogeneous reactive mixtures of reactants concerns a large number of practical applications, including those based on constant volume combustion cycles. The composition heterogeneities (equivalence ratio, temperature, dilution by burnt gases, etc.) result from several distinct factors such as the dispersion of the spray of fuel droplets and its evaporation, the flow field topology as well as the possible presence of residual burnt gases issued from the previous cycle. The resulting partially premixed flames structure is significantly more complex than the one of more conventional diffusion or premixed flames.The aim of this thesis work is therefore to contribute to the improvement of their understanding, by proceeding to the generation and analysis of a new set of direct numerical simulations (DNS) databases. The present computations are performed with the low-Mach number DNS solver Asphodele. The database is structured according to five parameters that characterize the turbulent flow as well as the composition heterogeneity of the reactive mixture. First, because of the high numerical costs induced by the detailed description of chemical kinetics, two-dimensional configurations were considered. The study of these various simulations highlights several fundamental mechanisms of flame propagation in heterogeneous mixtures. Then, a significant computational cost saving has been achieved through the development of an optimized simplified chemistry model. The use of the latter allowed to overcome the major bottleneck of high CPU costs related to chemical kinetics description and thus to extend the analysis to three-dimensional configurations. Some of the conclusions obtained previously were reinforced.