The laminar burning velocity is an important fundamental property of a fuel-air mixture at given conditions of temperature and pressure. Knowledge of burning velocities is required as an input for combustion models, including engine simulations, and the validation of chemical kinetic mechanisms. It is also important to understand the effect of stretch upon laminar flames, to correct for stretch and determine true (unstretched) laminar burning velocities, but also for modelling combustion where stretch rates are high, such as turbulent combustion models. A constant volume combustion vessel has been used in this work to determine burning velocities using two methods: a) flame speed measurements during the constant pressure period, and b) analysis of the pressure rise data. Consistency between these two techniques has been demonstrated for the first time. Flame front imaging and linear extrapolation of flame speed has been used to determine unstretched flame speeds at constant pressure and burned gas Markstein lengths. Measurement of the pressure rise during constant volume combustion has been used along with a numerical multi-zone combustion model to determine burning velocities for elevated temperatures and pressures as the unburned gas ahead of the spherically expanding flame front is compressed isentropically. This burning velocity data is correlated using a 14 term correlation to account for the effects of equivalence ratio, temperature, pressure and fraction of diluents. This correlation has been modified from an existing 12 term correlation to more accurately represent the dependence of burning velocity upon temperature and pressure. A number of fuels have been tested in the combustion vessel. Biogas (mixtures of CH₄ and CO₂) has been tested for a range of equivalence ratios (0.7–1.4), with initial temperatures of 298, 380 and 450 K, initial pressures of 1, 2 and 4 bar and CO₂ fractions of up to 40% by mole. Hydrous ethanol has been tested at the same conditions (apart from 298 K due to the need to vaporise the ethanol), and for fractions of water up to 40% by volume. Binary, ternary and quaternary blends of toluene, n-heptane, ethanol and iso-octane (THEO) have been tested for stoichiometric mixtures only, at 380 and 450 K, and 1, 2 and 4 bar, to represent surrogate gasoline blended with ethanol. For all fuels, correlation coefficients have been obtained to represent the burning velocities over wide ranging conditions. Common trends are seen, such as the reduction in burning velocity with pressure and increase with temperature. In the case of biogas, increasing CO₂ results in a decrease in burning velocity, a shift in peak burning velocity towards stoichiometric, a decrease in burned gas Markstein length and a delayed onset of cellularity. For hydrous ethanol the reduction in burning velocity as H₂O content is increased is more noticeably non-linear, and whilst the onset of cellularity is delayed, the effect on Markstein length is minor. Chemical kinetic simulations are performed to replicate the conditions for biogas mixtures using the GRI 3.0 mechanism and the FlameMaster package. For hydrous ethanol, simulations were performed by Carsten Olm at Eötvös Loránd University, using the OpenSMOKE 1D premixed flame solver. In both cases, good agreement with experimental results is seen. Tests have also been performed using a single cylinder optical engine to compare the results of the hydrous ethanol tests with early burn combustion, and a good comparison is seen. Results from tests on THEO fuels are compared with mixing rules developed in the literature to enable burning velocities of blends to be determined from knowledge of that of the pure components alone. A variety of rules are compared, and it is found that in most cases, the best approximation is found by using the rule in which the burning velocity of the blend is represented by weighting by the energy fraction of the individual components.

No presente trabalho é apresentado um estudo do efeito da diluição com inertes sobre as características da transferência de calor por radiação em chamas laminares não pré-misturadas de metano. O trabalho também apresenta um estudo sobre a modelagem do fluxo radiativo proveniente de chamas turbulentas visando à obtenção de fatores de ponderação para o modelo das múltiplas fontes pontuais. Em ambos os estudos, a distribuição do fluxo radiante é obtida através de medições ao longo do eixo da chama e os valores de fração radiante são calculados a partir da integração dessa distribuição. É mostrado qualitativamente que a adição de gás inerte ao combustível propicia a inibição da formação de fuligem. É mostrado quantitativamente que a adição de gás inerte pode reduzir ou ampliar a fração radiante da chama, dependendo do tipo de gás, dos níveis de diluição e do tempo de residência característico da chama. São reportados valores para os fatores de ponderação utilizados no modelo de múltiplas fontes pontuais obtidos experimentalmente a partir de medições nas chamas. O formato da curva formada pelos fatores de ponderação é semelhante para os diferentes níveis de diluição com gás carbônico. A utilização desses fatores de ponderação no modelo de múltiplas fontes pontuais apresenta resultados satisfatórios em comparação com a distribuição dos fluxos radiativos medidos ao longo do eixo da chama. / In this work it is presented a study of the effect of fuel dilution with inerts on the radiative heat transfer characteristics of laminar non premixed methane flames. A study on the radiative flux distribution from turbulent flames is conducted in order to obtain weighting factors for the model of multiple point sources. The distribution of radiative heat flux is obtained by measuring the fluxes along the axis of the flame and the radiant fraction is calculated by the integration of such distribution. It is qualitatively shown that the addition of inert gas in the fuel leads to the inhibition of soot formation. It is quantitatively shown that the addition of inert may decrease or increase the radiant fraction depending on gas type, dilution levels and characteristic residence times of the flame. It is also reported values for the weighting factors used in the model of multiple point sources experimentally obtained from measurements. The shapes of the curves formed by the weighting factors are similar for the different dilution levels of carbon dioxide. The use of these weighting factors in the multi-point source model shows satisfactory results in comparison to the distribution of radiative fluxes measured along the flame axis.

No presente trabalho é apresentado um estudo do efeito da diluição com inertes sobre as características da transferência de calor por radiação em chamas laminares não pré-misturadas de metano. O trabalho também apresenta um estudo sobre a modelagem do fluxo radiativo proveniente de chamas turbulentas visando à obtenção de fatores de ponderação para o modelo das múltiplas fontes pontuais. Em ambos os estudos, a distribuição do fluxo radiante é obtida através de medições ao longo do eixo da chama e os valores de fração radiante são calculados a partir da integração dessa distribuição. É mostrado qualitativamente que a adição de gás inerte ao combustível propicia a inibição da formação de fuligem. É mostrado quantitativamente que a adição de gás inerte pode reduzir ou ampliar a fração radiante da chama, dependendo do tipo de gás, dos níveis de diluição e do tempo de residência característico da chama. São reportados valores para os fatores de ponderação utilizados no modelo de múltiplas fontes pontuais obtidos experimentalmente a partir de medições nas chamas. O formato da curva formada pelos fatores de ponderação é semelhante para os diferentes níveis de diluição com gás carbônico. A utilização desses fatores de ponderação no modelo de múltiplas fontes pontuais apresenta resultados satisfatórios em comparação com a distribuição dos fluxos radiativos medidos ao longo do eixo da chama. / In this work it is presented a study of the effect of fuel dilution with inerts on the radiative heat transfer characteristics of laminar non premixed methane flames. A study on the radiative flux distribution from turbulent flames is conducted in order to obtain weighting factors for the model of multiple point sources. The distribution of radiative heat flux is obtained by measuring the fluxes along the axis of the flame and the radiant fraction is calculated by the integration of such distribution. It is qualitatively shown that the addition of inert gas in the fuel leads to the inhibition of soot formation. It is quantitatively shown that the addition of inert may decrease or increase the radiant fraction depending on gas type, dilution levels and characteristic residence times of the flame. It is also reported values for the weighting factors used in the model of multiple point sources experimentally obtained from measurements. The shapes of the curves formed by the weighting factors are similar for the different dilution levels of carbon dioxide. The use of these weighting factors in the multi-point source model shows satisfactory results in comparison to the distribution of radiative fluxes measured along the flame axis.

No presente trabalho é apresentado um estudo do efeito da diluição com inertes sobre as características da transferência de calor por radiação em chamas laminares não pré-misturadas de metano. O trabalho também apresenta um estudo sobre a modelagem do fluxo radiativo proveniente de chamas turbulentas visando à obtenção de fatores de ponderação para o modelo das múltiplas fontes pontuais. Em ambos os estudos, a distribuição do fluxo radiante é obtida através de medições ao longo do eixo da chama e os valores de fração radiante são calculados a partir da integração dessa distribuição. É mostrado qualitativamente que a adição de gás inerte ao combustível propicia a inibição da formação de fuligem. É mostrado quantitativamente que a adição de gás inerte pode reduzir ou ampliar a fração radiante da chama, dependendo do tipo de gás, dos níveis de diluição e do tempo de residência característico da chama. São reportados valores para os fatores de ponderação utilizados no modelo de múltiplas fontes pontuais obtidos experimentalmente a partir de medições nas chamas. O formato da curva formada pelos fatores de ponderação é semelhante para os diferentes níveis de diluição com gás carbônico. A utilização desses fatores de ponderação no modelo de múltiplas fontes pontuais apresenta resultados satisfatórios em comparação com a distribuição dos fluxos radiativos medidos ao longo do eixo da chama. / In this work it is presented a study of the effect of fuel dilution with inerts on the radiative heat transfer characteristics of laminar non premixed methane flames. A study on the radiative flux distribution from turbulent flames is conducted in order to obtain weighting factors for the model of multiple point sources. The distribution of radiative heat flux is obtained by measuring the fluxes along the axis of the flame and the radiant fraction is calculated by the integration of such distribution. It is qualitatively shown that the addition of inert gas in the fuel leads to the inhibition of soot formation. It is quantitatively shown that the addition of inert may decrease or increase the radiant fraction depending on gas type, dilution levels and characteristic residence times of the flame. It is also reported values for the weighting factors used in the model of multiple point sources experimentally obtained from measurements. The shapes of the curves formed by the weighting factors are similar for the different dilution levels of carbon dioxide. The use of these weighting factors in the multi-point source model shows satisfactory results in comparison to the distribution of radiative fluxes measured along the flame axis.

L'allumage d'ergols injectés dans une chambre de combustion, la propagation du noyau de flamme puis sa stabilisation sont autant de paramètres déterminants pour la conception d'un moteur fusée. Pour ce type d'application, il est nécessaire - du point de vue de la modélisation - de tenir compte du couplage existant entre les effets de compressibilité, les processus de mélange turbulent ainsi que de cinétique chimique, dans un environnement diphasique puisque les ergols sont injectés à l'état liquide. Un modèle Lagrangien a été implanté dans le code de calcul compressible N3S-Natur afin de disposer d'un outil numérique capable de simuler le transitoire d'allumage d'un moteur fusée. La physique représentative de chacun des processus physiques impliqués pendant la phase d'allumage a été incorporée puis validée sur des configurations académiques. Ce travail a permis de mettre en évidence l'importance de la description du mélange à petites échelles pour capturer correctement le développement de la flamme. Il a aussi mis en exergue la nécessité de prendre en compte le transitoire thermique des gouttes d'oxygène liquide afin de reproduire fidèlement sa stabilisation. Enfin, il a nécessité l'extension de la notion de fraction de mélange à des cas pratiques présentant plus de deux entrées afin d'être en mesure de simuler la propagation de l'allumage sur la plaque d'injection. Cette approche basée sur l'introduction d'un injecteur fictif est non seulement utile pour la simulation de l'allumage des moteurs-fusées mais peut aussi être employée dans tout autre système impliquant le mélange entre des courants de réactifs issus de deux entrées ou plus. / In the field of liquid rocket propulsion, ignition, propagation and stabilization of the flame are of first importance for the design of the engine. Computational fluid dynamics (CFD) solvers may provide a great deal of help to proceed with the primary design choice but need to be fed with suited physical models. Important modelling efforts are therefore required to provide reliable computational representations able to take into account compressibility effects, turbulent mixing and chemical kinetics in two-phase flows since ergols are injected at the liquid state. A Lagrangian model has been implemented in the compressible solver N3S-Natur so as to obtain a computational tool able to compute the transient ignition of rocket engines. The physical processes involved at each step of this ignition sequence have been integrated and validated on academically configurations. Three significant contributions rose from this work. First of all, it is highlighted that the description of the micro-mixing is of first importance to correctly capture the flame development. This study also emphasized the need to consider the transient heating of liquid oxygen droplets in order to accurately compute the flame stabilization. Finally, the notion of mixture fraction must be extended to practical devices implying more than two inlets. The proposed approach which is based on the introduction of a fictive injector is not only well suited to rocket engine ignition application but also to deal with other practical devices implying two inlets and more.

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

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.

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.

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.