Simulações de chamas empregando mecanismos cinéticos detalhados são problemas computacionalmente demandantes. Por esse motivo, mecanismos reduzidos e técnicas de redução de cinética química vêm sendo desenvolvidos buscando uma melhor eficiência computacional. Mecanismos globais de poucos passos são particularmente populares pela simplicidade de programação nos códigos disponíveis. Assim, o objetivo da presente dissertação é avaliar modelagens simplificadas de cinética química na simulação numérica de chamas laminares 1D e 2D de metano diluído com dióxido de carbono. Mecanismos globais de 1, 2 e 4-passos são avaliados em comparação com o mecanismo detalhado GRI-Mech 3.0 na simulação unidimensional de chamas difusivas contra-corrente. O mecanismo global de melhor desempenho é então usado nas simulações bidimensionais de chamas difusivas tipo jato em comparação com a técnica de redução Flamelet-Generated Manifold. Observou-se que o mecanismo de 4-passos estudado apresenta bons resultados para o campo de temperaturas e para as principais espécies químicas, tanto nas simulações unidimensionais quanto nas bidimensionais. No entanto, espécies minoritárias como o CO e H2 não são bem reproduzidas. Fenômenos como posição de estabilização e penetração de oxidante na base de chamas tipo jato também não são capturadas quando o mecanismo global é usado. Por outro lado, a técnica FGM se mostrou capaz de prever tais fenômenos e resultou, adicionalmente, em um ganho computacional expressivo. / Numerical simulations of flames employing detailed kinetic mechanisms are computationally demanding problems. For this reason, reduced mechanisms and techniques of chemical kinetic reduction have been developed aiming better computational efficiency. Global mechanisms formed by few steps are particularly popular due to the simplicity of programing them in available codes. Thus, the objective of the present dissertation is to evaluate simplified chemical kinetics models in 1D and 2D numerical simulations of methane diluted with carbon dioxide laminar flames. Global mechanisms formed by 1, 2 and 4-steps are evaluated in comparison with the detailed mechanism GRI-Mech 3.0 in one-dimensional simulations of counterflow diffusive flames. The global mechanism with best performance is then used in two-dimensional simulations of diffusive jet flames for a comparison with the chemical reduction technique FGM. It was observed that a 4-step mechanism presented good results for temperature and major chemical species for both one and two-dimensional simulations. However, minor species like CO and H2 are not well reproduced. Phenomena such as stabilization position and oxygen penetration in the jet flame base are also not captured when the global mechanism is used. On the other hand, the technique Flamelet- Generated Manifold demonstrated to predict those phenomena and resulted, additionally, in an expressive computational gain.

Simulações de chamas empregando mecanismos cinéticos detalhados são problemas computacionalmente demandantes. Por esse motivo, mecanismos reduzidos e técnicas de redução de cinética química vêm sendo desenvolvidos buscando uma melhor eficiência computacional. Mecanismos globais de poucos passos são particularmente populares pela simplicidade de programação nos códigos disponíveis. Assim, o objetivo da presente dissertação é avaliar modelagens simplificadas de cinética química na simulação numérica de chamas laminares 1D e 2D de metano diluído com dióxido de carbono. Mecanismos globais de 1, 2 e 4-passos são avaliados em comparação com o mecanismo detalhado GRI-Mech 3.0 na simulação unidimensional de chamas difusivas contra-corrente. O mecanismo global de melhor desempenho é então usado nas simulações bidimensionais de chamas difusivas tipo jato em comparação com a técnica de redução Flamelet-Generated Manifold. Observou-se que o mecanismo de 4-passos estudado apresenta bons resultados para o campo de temperaturas e para as principais espécies químicas, tanto nas simulações unidimensionais quanto nas bidimensionais. No entanto, espécies minoritárias como o CO e H2 não são bem reproduzidas. Fenômenos como posição de estabilização e penetração de oxidante na base de chamas tipo jato também não são capturadas quando o mecanismo global é usado. Por outro lado, a técnica FGM se mostrou capaz de prever tais fenômenos e resultou, adicionalmente, em um ganho computacional expressivo. / Numerical simulations of flames employing detailed kinetic mechanisms are computationally demanding problems. For this reason, reduced mechanisms and techniques of chemical kinetic reduction have been developed aiming better computational efficiency. Global mechanisms formed by few steps are particularly popular due to the simplicity of programing them in available codes. Thus, the objective of the present dissertation is to evaluate simplified chemical kinetics models in 1D and 2D numerical simulations of methane diluted with carbon dioxide laminar flames. Global mechanisms formed by 1, 2 and 4-steps are evaluated in comparison with the detailed mechanism GRI-Mech 3.0 in one-dimensional simulations of counterflow diffusive flames. The global mechanism with best performance is then used in two-dimensional simulations of diffusive jet flames for a comparison with the chemical reduction technique FGM. It was observed that a 4-step mechanism presented good results for temperature and major chemical species for both one and two-dimensional simulations. However, minor species like CO and H2 are not well reproduced. Phenomena such as stabilization position and oxygen penetration in the jet flame base are also not captured when the global mechanism is used. On the other hand, the technique Flamelet- Generated Manifold demonstrated to predict those phenomena and resulted, additionally, in an expressive computational gain.

Simulações de chamas empregando mecanismos cinéticos detalhados são problemas computacionalmente demandantes. Por esse motivo, mecanismos reduzidos e técnicas de redução de cinética química vêm sendo desenvolvidos buscando uma melhor eficiência computacional. Mecanismos globais de poucos passos são particularmente populares pela simplicidade de programação nos códigos disponíveis. Assim, o objetivo da presente dissertação é avaliar modelagens simplificadas de cinética química na simulação numérica de chamas laminares 1D e 2D de metano diluído com dióxido de carbono. Mecanismos globais de 1, 2 e 4-passos são avaliados em comparação com o mecanismo detalhado GRI-Mech 3.0 na simulação unidimensional de chamas difusivas contra-corrente. O mecanismo global de melhor desempenho é então usado nas simulações bidimensionais de chamas difusivas tipo jato em comparação com a técnica de redução Flamelet-Generated Manifold. Observou-se que o mecanismo de 4-passos estudado apresenta bons resultados para o campo de temperaturas e para as principais espécies químicas, tanto nas simulações unidimensionais quanto nas bidimensionais. No entanto, espécies minoritárias como o CO e H2 não são bem reproduzidas. Fenômenos como posição de estabilização e penetração de oxidante na base de chamas tipo jato também não são capturadas quando o mecanismo global é usado. Por outro lado, a técnica FGM se mostrou capaz de prever tais fenômenos e resultou, adicionalmente, em um ganho computacional expressivo. / Numerical simulations of flames employing detailed kinetic mechanisms are computationally demanding problems. For this reason, reduced mechanisms and techniques of chemical kinetic reduction have been developed aiming better computational efficiency. Global mechanisms formed by few steps are particularly popular due to the simplicity of programing them in available codes. Thus, the objective of the present dissertation is to evaluate simplified chemical kinetics models in 1D and 2D numerical simulations of methane diluted with carbon dioxide laminar flames. Global mechanisms formed by 1, 2 and 4-steps are evaluated in comparison with the detailed mechanism GRI-Mech 3.0 in one-dimensional simulations of counterflow diffusive flames. The global mechanism with best performance is then used in two-dimensional simulations of diffusive jet flames for a comparison with the chemical reduction technique FGM. It was observed that a 4-step mechanism presented good results for temperature and major chemical species for both one and two-dimensional simulations. However, minor species like CO and H2 are not well reproduced. Phenomena such as stabilization position and oxygen penetration in the jet flame base are also not captured when the global mechanism is used. On the other hand, the technique Flamelet- Generated Manifold demonstrated to predict those phenomena and resulted, additionally, in an expressive computational gain.

The present thesis deals with the fundamental aspects of turbulent mixingand non-premixed combustion in wall-jet flows. Direct numerical simulations(DNS) of compressible turbulent flows are performed in a wall-jet configura-tion, which has a close resemblance to many industrial combustion applica-tions. The triple ”turbulence-chemistry-wall” interactions are also present inthis flow set-up. These interactions have been addressed by first focusing onturbulent flow effects on the isothermal reaction, including the near-wall issues.Then, by adding heat-release to the simulations, it has been concentrated onheat-release effects on various phenomena that occur in the reacting turbulentwall-jet flow. In the computational domain, fuel and oxidizer enter separatelyin a non-premixed manner and the flow is fully turbulent and subsonic in allsimulations. In the first phase of this study, the case of a turbulent wall-jetincluding an isothermal reaction without heat release is addressed in order toisolate the near-wall effects and the mixing characteristics of the flow and thekey statistics for combustion are studied in the absence of thermal effects. Adeeper insight into three-dimensional mixing and reaction characteristics in aturbulent wall-jet has been gained through investigation of the probability den-sity functions, higher order moments of velocities and reacting scalars and thescalar dissipation rates of different species. In the second phase, DNS of turbu-lent reacting wall-jets including heat release is performed, where a single-stepglobal exothermic reaction with an Arrhenius-type reaction rate is considered.The main target was to identify the heat-release effects on different mixingscales of turbulent wall-jet flow. The scalar dissipation rates, time scale ratios,two-point correlations, one and two-dimensional premultiplied spectra are usedto illustrate the heat release induced modifications. It is observed that heatrelease effects delay the transition process in the chemically reacting cases andenlarge the fluctuation intensities of density and pressure, but have a dampingeffect on all velocity fluctuation intensities. Finer small mixing scales were ob-served in the isothermal simulations and larger vortical structures formed afteradding significant amounts of heat-release. Simulations with different Damk ̈h-  oler numbers, but comparable temperature-rise are performed and the expectedbehavior, a thinner flame with increasing Damk ̈hler number, is observed. Finally, some heat transfer related quantities are examined. The wall heat fluxand the corresponding Nusselt numbers are addressed. The near-wall reactioneffects on the skin friction coefficient are studied and further the reaction char-acteristics are investigated throughout the domain. / QC 20110908

Commercial fuels are mixtures with large numbers of components. Continuous thermodynamics is a technique for modelling fuel mixtures using a probability density function rather than dealing with each discreet component. The mean and standard deviation of the distribution are then used to model the chemical reactions of the mixture. This thesis develops the necessary theory to apply the technique of continuous thermodynamics to the oxidation reactions of hydrocarbon fuels. The theory is applied to three simplified models of hydrocarbon oxidation: a global one-step reaction, a two-step reaction with CO as the intermediate product, and the four-step reaction of Müller et al. (1992), which contains a high- and a low-temperature branch. These are all greatly simplified models of the complex reaction kinetics of hydrocarbons, and in this thesis they are applied specifically to n-paraffin hydrocarbons in the range from n-heptane to n-hexadecane. The model is tested numerically using a simple constant pressure homogeneous ignition problem using Cantera and compared to simplified and detailed mechanisms for n-heptane. The continuous thermodynamics models are able not only to predict ignition delay times and the development of temperature and species concentrations with time, but also changes in the mixture composition as reaction proceeds as represented by the mean and standard deviation of the distribution function. Continuous thermodynamics is therefore shown to be a useful tool for reactions of multicomponent mixtures, and an alternative to the "surrogate fuel" approach often used at present.