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.

This dissertation addresses the analysis of partially premixed flame configurations and the detection and characterization of their local flame regimes. First, the identification of flame regimes in experimental data is intensively discussed. Current methods for combustion regime characterization, such as the flame index, rely on 3D gradient information that is not accessible with available experimental techniques. Here, a method is proposed for reaction zone detection and characterization, which can be applied to instantaneous 1D Raman/Rayleigh line measurements of major species and temperature as well as to the results of laminar and turbulent flame simulations, without the need for 3D gradient information. Several derived flame markers, namely the mixture fraction, the heat release rate and the chemical explosive mode, are combined to detect and characterize premixed versus non-premixed reaction zones. The methodology is developed and evaluated using fully resolved simulation data from laminar flames. The fully resolved 1D simulation data are spatially filtered to account for the difference in spatial resolution between the experiment and the simulation, and experimental uncertainty is superimposed onto the filtered numerical results to produce Raman/Rayleigh equivalent data. Then, starting from just the temperature and major species, a constrained homogeneous batch reactor calculation gives an approximation of the full thermochemical state at each sample location. Finally, the chemical explosive mode and the heat release rate are calculated from this approximated state and compared to those calculated directly from the simulation data. After successful validation, the approach is applied to Raman/Rayleigh line measurements from laminar counterflow flames, a mildly turbulent lifted flame and turbulent benchmark cases. The results confirm that the reaction zones can be reliably detected and characterized using experimental data. In contrast to other approaches, the presented methodology circumvents uncertainties arising from the use of limited gradient information and offers an alternative to known reaction zone identification methods. Second, this work focuses on the flame structure of partially premixed dimethyl ether (DME) flames. DME flames form significant intermediate hydrocarbons in the reaction zone and are classified as the next more complex fuel candidate in research after methane. To simulate DME combustion processes, accurate predictions by computational combustion models are required. To evaluate such models and to identify appropriate flame regimes, numerical simulations are necessary. Therefore, fully resolved simulations of laminar dimethyl ether flames, defined by different levels of premixing, are performed. Further, the qualitative two-dimensional structures of the partially premixed DME flames are discussed and analyses are carried out at selected slices and compared to each other as well as to experimental data. Further, the flamelet/progress variable (FPV) approach is investigated to predict the partially premixed flame structures of the DME flames. In the context of the FPV approach, a rigorous analysis of the underlying manifold is carried out based on the newly developed regime identification approach and an a priori analysis. The most promising flamelet look-up table is chosen for the fully coupled tabulated chemistry simulations and the results are further compared to the fully resolved simulation data.

Cette thèse est consacrée à l’amélioration des capacités de prédiction des émissions d’oxydes d’azote (NO et NO2) des foyers de combustion aéronautiques. Les travaux, exclusivement numériques, consistent d’abord dans une étude de la cinétique chimique responsable des émissions polluantes. Cetteétude conduit à l’écriture d’un modèle, nommé NOMANI (pour Nitrogen Oxide emission model with one-dimensional MANIfold), basé sur l’approche PCM-FPI (pour Presumed Conditional Moments - Flame Prolongation of ILDM) avec une variable de progrès additionnelle afin calculer l’avancement de la chimie azotée une fois la chimie carbonée à l’équilibre. Différentes validations sur des configurations laminaires simples puis des flammes de laboratoire de Sandia sont présentées. Les résultats en terme de structure de flamme et d'émission de monoxyde d’azote sont confrontés aux mesures expérimentales. Le dernier volet de ces travaux, disponible uniquement dans la version confidentielle du manuscrit, consiste dans le développement d’un modèle de prédiction de polluants associé au modèle TF-LES (pour Thickening Flame for Large Eddy Simulation). Le modèle développé est ensuite appliqué à des calculs d’une chambre de combustion aéronautique. / This thesis is focused on the prediction capabilities of nitrogen oxides (NO and NO2) for numerical tools applied to aeronautical combustion chambers. The modeling work is based on a study of the chemical kinetic that produced the pollutant emissions. This study leads to a model, called NOMANI (Nitrogen Oxide emission model with one-dimensional MANIfold), based on PCM-FPI (Presumed Conditional Moments - Flame Prolongation of ILDM) with an additional progress variable to compute the NO evolution once the carbon chemistry is at the equilibrium. Several benchmarks and test-cases (laminar and turbulent flames) are gathered in this study : Sandia flame have been computed and satisfactory comparisons with measurements are obtained. The last part of this work, only available in the confidential version of the manuscript, is the development of a model to predict pollutant associated with the model TF-LES (for Thickening Flame for Large Eddy Simulation). This model is then applied to computations of a aeronautical combustion chambers.

La combustion stratifiée, qui consiste à brûler un mélange carburant/oxydant inhomogène, et la combustion diluée, consistant à ajouter une quantité limitée de gaz brûlés, sont deux technologies utilisées dans les moteurs à piston pour réduire leur consommation. Cette thèse est dédiée à l’étude de l’allumage dans ces deux types de milieux en régimes laminaire et turbulent. Un nouveau schéma cinétique pour la combustion propane/air a été dérivé et combiné à deux approches de modélisation différentes : la chimie complexe et une approche de chimie tabulée de type FPI. Dans le cas laminaire, les deux approches de modélisation donnent des résultats similaires et un modèle simple a mis en évidence l’importance de la dynamique des gaz frais et des gaz brûlés sur le développement du noyau. Dans le cas turbulent, plusieurs techniques d’analyse ont montré la dépendance de la vitesse absolue de la flamme au champ de vitesse moyen et la décorrélation des fluctuations locales de richesse. / Stratified combustion, which consists in burning an inhomogeneous fuel/air mixture, and diluted combustion, which consists in adding a limited quantity of burnt gases, are two technologies used in internal combustion engines to reduce fuel consumption. This Ph.D is devoted to the study of ignition in these two types of combustion in laminar and turbulent regimes. A new kinetic scheme for propane/air combustion has been derived and combined to two modeling approaches: finite-rate chemistry and an FPI tabulated chemistry approach. In the laminar case, both approaches give similar results and a simplified model has highlighted the importance of fresh and burnt gases dynamics on the kernel development. In the turbulent case, several techniques of analysis have shown the dependency of absolute flame speed on the mean fluid velocity and the lack of correlation to the local equivalence ratio.

This dissertation addresses the analysis of partially premixed flame configurations and the detection and characterization of their local flame regimes. First, the identification of flame regimes in experimental data is intensively discussed. Current methods for combustion regime characterization, such as the flame index, rely on 3D gradient information that is not accessible with available experimental techniques. Here, a method is proposed for reaction zone detection and characterization, which can be applied to instantaneous 1D Raman/Rayleigh line measurements of major species and temperature as well as to the results of laminar and turbulent flame simulations, without the need for 3D gradient information. Several derived flame markers, namely the mixture fraction, the heat release rate and the chemical explosive mode, are combined to detect and characterize premixed versus non-premixed reaction zones. The methodology is developed and evaluated using fully resolved simulation data from laminar flames. The fully resolved 1D simulation data are spatially filtered to account for the difference in spatial resolution between the experiment and the simulation, and experimental uncertainty is superimposed onto the filtered numerical results to produce Raman/Rayleigh equivalent data. Then, starting from just the temperature and major species, a constrained homogeneous batch reactor calculation gives an approximation of the full thermochemical state at each sample location. Finally, the chemical explosive mode and the heat release rate are calculated from this approximated state and compared to those calculated directly from the simulation data. After successful validation, the approach is applied to Raman/Rayleigh line measurements from laminar counterflow flames, a mildly turbulent lifted flame and turbulent benchmark cases. The results confirm that the reaction zones can be reliably detected and characterized using experimental data. In contrast to other approaches, the presented methodology circumvents uncertainties arising from the use of limited gradient information and offers an alternative to known reaction zone identification methods. Second, this work focuses on the flame structure of partially premixed dimethyl ether (DME) flames. DME flames form significant intermediate hydrocarbons in the reaction zone and are classified as the next more complex fuel candidate in research after methane. To simulate DME combustion processes, accurate predictions by computational combustion models are required. To evaluate such models and to identify appropriate flame regimes, numerical simulations are necessary. Therefore, fully resolved simulations of laminar dimethyl ether flames, defined by different levels of premixing, are performed. Further, the qualitative two-dimensional structures of the partially premixed DME flames are discussed and analyses are carried out at selected slices and compared to each other as well as to experimental data. Further, the flamelet/progress variable (FPV) approach is investigated to predict the partially premixed flame structures of the DME flames. In the context of the FPV approach, a rigorous analysis of the underlying manifold is carried out based on the newly developed regime identification approach and an a priori analysis. The most promising flamelet look-up table is chosen for the fully coupled tabulated chemistry simulations and the results are further compared to the fully resolved simulation data.

[pt] Dinâmica dos Fluidos Computacional (CFD) é frequentemente aplicada ao estudo da combustão, permitindo otimizar o processo e controlar a emissão de poluentes. Entretanto, reproduzir o comportamento observado nos sistemas de engenharia tem uma elevada carga computacional. Para superar este custo, técnicas de aprendizagem de máquinas, tais como modelos de ordem reduzida (ROM), têm sido aplicadas a várias aplicações de engenharia com o objetivo de criar modelos para sistemas complexos com custo computacional reduzido. Aqui, o ROM é criado usando dados de simulação de chama laminar não pré-misturada de CFD, decompondo-os, e depois aplicando um algoritmo de aprendizagem de máquinas, criando um ROM estático. Este trabalho analisa o efeito de cinco abordagens diferentes de pré-processamento de dados sobre o ROM, sendo estas: (1) as propriedades tratadas como um sistema desacoplado ou como um sistema acoplado, (2) sem normalização, (3) com temperatura e velocidade normalizadas, (4) todas as propriedades normalizadas, e (5) o logaritmo da espécie química. Para todos os ROM construídos são analisados a energia do processo de redução e a reconstrução dos campos das propriedades da chama. Em relação a análise da energia da redução, o ROM acoplado, exceto o ROM (4), e o ROM do logaritmo convergem rapidamente, semelhante ao ROM da temperatura desacoplado, enquanto o ROM da espécie química minoritária desacoplado exibe uma lenta convergência, tal como o ROM acoplado com todas as propriedades normalizadas. Assim, a aprendizagem é atingida com um número menor de modos para a ROM (2), (3) e (5). Quanto à reconstrução dos campos de propriedades, nota-se que existem regiões de fração mássica negativa, o que sugere que a metodologia do ROM não preserva a monotonicidade ou a delimitação das propriedades. A abordagem do logaritmo mostra que estes problemas são superados e reproduzem os dados originais. / [en] Computational fluid dynamics (CFD) is often applied to the study of combustion, enabling to optimize the process and control the emission of pollutants. However, reproducing the behavior observed in engineering systems has a high computational burden. To overcome this cost, machine learning techniques, such as reduced order models (ROM), have been applied to several engineering applications aiming to create models for complex systems with reduced computational cost. Here, the ROM is created using CFD laminar non premixed flame simulation data, decomposing it, and then applying a machine learning algorithm, creating a static ROM. This work analyzes the effect of five different data pre-processing approaches on the ROM, these being: (1) the properties treated as an uncoupled system or as a coupled system, (2) without normalization, (3) with temperature and velocity normalized, (4) all properties normalized, and (5) the logarithm of the chemical species. For all ROM constructed are analyzed the energy of the reduction process and the reconstruction of the flame properties fields. Regarding the reduction energy analysis, the coupled ROM, except the ROM (4), and the logarithm ROM converges faster, similarly to the uncoupled temperature ROM, whereas the uncoupled minor chemical species ROM exhibits a slower convergence, as does the coupled ROM with all properties normalized. So, the learning is achieved with a smaller number of modes for the ROM (2), (3) and (5). As for the reconstruction of the property fields, it is noted that there are regions of negative mass fraction, which suggest that the ROM methodology does not preserve the monocity or the boundedness of the properties. The logarithm approach shows that these problems are overcome and reproduce the original data.

Le présent travail est une étude expérimentale sur l’interaction entre flamme et turbulence. L’effet de la pression sur le plissement de flammes turbulentes prémélangées est caractérisé à l’aide de diagnostics laser et fil chaud. Dans un premier temps, la caractérisation de la turbulence générée par un système multi-échelles a été réalisée. Il a été démontré que ce dispositif amplifie le taux de turbulence de 40% par rapport à un dispositif mono-grille de maille équivalente. De même, les petites échelles de turbulence sont trouvées expérimentalement plus petites et plus énergétiques pour le système multi-grilles. A partir de ces résultats, l’étude des interactions entre flamme prémélangée et turbulence a été effectuée. En utilisant le diagnostic par tomographie laser, le front de flamme de plusieurs prémélanges a été étudié. En modifiant les conditions de mélange, l’effet des paramètres comme le nombre de Lewis, les conditions de turbulence ou les petites échelles ont pu être observés. Le faible impact des instabilités thermodiffusives sur la courbure du front de flamme et sur la dynamique de la flamme a été démontré. En revanche, l’effet des conditions de turbulence a été démontré comme important sur les caractéristiques du front de flamme. De plus, les résultats obtenus ont montré l’impact majeur de l’échelle de Taylor sur le plissement du front de flamme pour les conditions expérimentales de la présente étude. / The present work is an experimental study on the interactions between flame and turbulence. The pressure effect on the flame front wrinkling is characterised using laser diagnostics and hot wire anemometry. To begin with, the turbulence generated by a multi-grid system is characterised. It is shown that the present system produces a higher turbulence rate by 40% than for an equivalent mesh single-grid system. Moreover, the small turbulence scales sizes are experimentally found smaller with the multi-grid system. From those results, the interactions between premixed flames and turbulence were studied. By using the laser tomography diagnostic, the flame front of several gases premixes was observed. By changing the mixing conditions, the effect of parameters such as the Lewis number, the turbulence conditions and the small scale was observed. The low impact of the thermodiffusives instabilities in our conditions was demonstrated. However, the important effect of the turbulence conditions on the flame front characteristics was observed. Moreover, the present results showed the major impact of the Taylor micro scale on the flame front wrinkling for these study experimental conditions.

Cette thèse est consacrée à l’amélioration des capacités de prédiction des émissions polluantes (CO, NOx . . . ) des foyers de combustion de turboréacteurs. L’étude, exclusivement numérique, repose sur des simulations aux grandes échelles (ou LES pour Large-Eddy Simulation) basées sur des méthodes de tabulation de la chimie détaillée. L’approche PCM-FPI (pour Presumed Conditional Moments - Flame Prolongation of ILDM) a été étendue à la chimie des oxydes d’azote via la modification de la variable d’avancement. Différentes validations sur des configurations laminaires simples puis des flammes de laboratoire (Cabra, Sandia) sont présentées. Les résultats en terme de structure de flamme et de champs d’espèces chimiques sont confrontés aux mesures expérimentales. Le rôle du formaldéhyde comme marqueur de la zone réactionnelle est illustré à l’aide de calculs de flammes laminaires puis confirmé par un calcul 3D LES. Une analyse des spécificités de l’implantation de ce type de modèle sur des machines à architecture massivement parallèle est ensuite menée. Diverses modifications de la structure de la table et des méthodes d’interpolation sont réalisées, servant de base à une étude de sensibilité de maillage appliquée à la flamme Sandia D. Les difficultés relatives à la prédiction du NO dans les flammes turbulentes sont exposées : divers modèles de sous-maille sont alors employés et comparés. / This thesis is focused on the prediction capabilities of pollution (CO, NOx especially) for numerical tools applied to aeronautical combustion chambers. The modeling work is based on Large-Eddy Simulation methods coupled with a tabulated detailed chemistry approach. The PCM-FPI model, which stands for Presumed Conditional Moments - Flame Prolongation of ILDM, has been revised to take into account nitrogen chemistry through a modification of the progress variable. Several benchmarks and test-cases (laminar and turbulent flames) are gathered in this study : Cabra and Sandia flames have been computed and satisfactory comparisons with measurements are obtained. The role of CH2O as a marker of heat release is investigated, first in the frame of laminar premixed flames and then validated through LES runs. The challenges of the implementation of tabulated chemistry methods on massively parallel machines are discussed. Modifications are proposed regarding both the table structure and the interpolation methods leading to a mesh sensitivity review applied to the Sandia D flame. Difficulties arising when dealing with NOx chemistry in turbulent flows are presented : new Sub-Grid Scale models are introduced and investigated.

Cette thèse est une contribution à l’étude des flammes de méthane turbulentes non prémélangées en rotation, dites swirlées, avec ou sans enrichissement en oxygène de l’oxydant. L’étude se focalise sur la stabilité de la flamme, les émissions polluantes et la dynamique du jet en non réactif et réactif. Notre dispositif expérimental se compose d’un brûleur à swirler coaxial avec injection radiale de méthane au voisinage de la sortie du brûleur. Ce dernier est confiné dans une chambre de combustion. La teneur en oxygène dans l’oxydant, le nombre de swirl géométrique et la richesse globale à l’injection sont les principaux paramètres qui peuvent être précisément contrôlés. La stabilité de la flamme est caractérisée par chimiluminescence OH*. Les émissions polluantes sont mesurées par des analyseurs en ligne via un prélèvement dans les gaz brûlés. La dynamique du jet est caractérisée principalement par PIV stéréoscopique dans un plan longitudinal et plusieurs plans transverses. La diffusion du méthane dans le jet swirlé est abordée qualitativement par fluorescence induite par laser de l’acétone dans un plan. À ce jour, peu de travaux portent sur la caractérisation notamment dynamique de ces flammes swirlées avec enrichissement en O2. La mise en rotation du jet est à l’origine d’une zone de recirculation centrale qui favorise la stabilisation de la flamme en régime pauvre et à grand nombre de Reynolds. L’étude des émissions polluantes montre que les régimes de combustion à l’air pour lesquels la flamme est liftée stable sont aussi ceux qui produisent du CO et du CH4 résiduel en des quantités non négligeables. L’enrichissement en oxygène permet alors de convertir les imbrûlés et ce pour de faibles enrichissements tout en améliorant la stabilité de flamme via une diminution de la hauteur d’accrochage et des fluctuations associées comme le confirment de précédentes études. L’augmentation des NOx par la voie thermique a été quantifiée pour des enrichissements en oxygène inférieurs à 30 % vol. L’étude systématique en non réactif et réactif apporte des détails sur la topologie tridimensionnelle du jet swirlé suivant les paramètres de l’étude. L’étude de la décroissance des vitesses et de la décroissance du nombre de swirl dans la direction de l’écoulement permetde mettre en évidence l’effet de la flamme sur le jet swirlé. Un couplage entre l’évolution du taux d’entraînement par la recirculation externe et les émissions polluantes est mis en évidence pour expliquer l’évolution des NOx suivant la richesse globale à l’injection. Nous avons proposé une modélisation des écoulements swirlés qui repose sur les écoulements à vorticité hélicoïdale afin d’identifier les caractéristiques principales des structures hélicoïdales au sein de l’écoulement. / This thesis is a contribution to the study of turbulent non-premixed swirling methane flames with or without oxygen addition in the oxidizer. The study deals with the flame stability, the pollutant emissions and the jet dynamic behaviour in non-reacting and reacting conditions. The burner, operating in a combustion chamber, consists of two concentric tubes with a swirler placed in an annular arrangement, which supplied the oxidant flow (air or oxygen-enriched air). The central pipe delivers fuel (methane) radially just below the burner exit plane. The oxygen content in the oxidizer, the geometric swirl number and the global equivalence ratio are the main parameters, which can be precisely set. OH* chemiluminescence imaging is used to characterize flame stability. Multi-gas analyzers are used to measure pollutant emissions in the exhaust gas. The flow is characterized using stereoscopic PIV measurements in different longitudinal and transverse planes. A qualitative study dealing with the methane diffusion imaging is also conducted by use of acetone planar laser-induced fluorescence. Up to now only few studies have examined the dynamic behavior of this type of swirled flames with oxygen addition. Introducing swirl allows creating a central recirculation zone which favors lean flame stabilization at higher Reynolds numbers. The mapping of the combustion regimes combined with the pollutant emission results show that the stable lifted flames are related to high CO and residual CH4 emission levels in the exhaust gas. Oxygen addition, even by a few percent, allows improving CO and unburned hydrocarbons conversion and increasing flame stability at the same time via a decrease of liftoff heights and the related fluctuations. The NOx emissions increase via the thermal pathway with increasing the oxygen-enrichment rate up to 30 % vol. A comparative study in non-reacting and reacting conditions is conducted to give insight into the tridimensional flow field topology varying the above-mentioned parameters. Mean streamwise velocity and swirl number decay rates show the flame effects on the flow dynamics. A coupling mechanism between the entrainment rate of the surroundings via the external recirculation and the pollutant emissions is proposed to explain the NOx emission trend with the global equivalence ratio. A model is also proposed based on the helical vortices to identify the main features of helix structures in the jet in non-reacting and reacting conditions.