Zugl.: Aachen, Techn. Hochsch., Diss., 2008.

Zugl.: Karlsruhe, Univ., Diss., 2008 / Hergestellt on demand

Zugl.: Aachen, Techn. Hochsch., Diss., 2006.

A simulação numérica de chamas turbulentas é ainda hoje um desafio para as práticas de mecânica dos fluidos computacional. Compreendendo que as abordagens numéricas mais completas e realísticas atualmente disponíveis podem ser computacionalmente proibitivas, diversos modelos vêm sendo desenvolvidos com o objetivo de reproduzir os fenômenos envolvidos na combustão de uma forma simplificada, mas ainda fisicamente consistente. Este trabalho é, portanto, dedicado à comparação de diferentes modelos de fechamento para a turbulência baseados nas equações de Navier-Stokes em médias de Reynolds e de modelos para simplificação da cinética química baseados no conceito de flamelets, com e sem a modelagem da radiação térmica, esta última através do modelo de soma-ponderada-de-gasescinzas. Para tanto, na primeira parte do presente trabalho são comparados seis modelos de turbulência na solução de um jato turbulento de propano, não reativo e isotérmico, circundado por uma corrente paralela de ar, quanto a sua eficiência na predição dos valores médios da velocidade longitudinal e transversal, fração mássica de propano e massa específica da mistura. Os modelos são o k- Padrão (empregado na sua versão original e com mais duas modificações nas suas constantes conforme propostas encontradas na literatura), o k- Realizable, o k- Padrão e o k- Shear-Stress Transport. Um dos modelos de melhor desempenho é então usado na simulação de uma chama turbulenta não pré-misturada de metano/hidrogênio/nitrogênio circundada por um escoamento coaxial de ar de baixa velocidade, no qual são então comparados os modelos para redução da cinética química baseados no conceito de flamelets, o Steady Laminar Diffusion Flamelet (SLDF) e o Flamelet-Generated Manifold (FGM), tendo os seus resultados comparados aos dados experimentais para os valores médios da velocidade longitudinal, fração de mistura, temperatura e frações mássicas das espécies químicas. Dentre os modelos de turbulência avaliados, é observado que as duas versões ajustadas do k- Padrão e o k- Padrão se mostraram com melhor concordância em relação às medições experimentais do que os demais. No presente estudo é também avaliada a consistência dos dados experimentais reportados e uma discrepância é identificada neste jato, mas que, conforme verificado, não compromete a comparação dos modelos aqui proposta. Na solução do escoamento reativo, o modelo SLDF se mostrou com resultados bastante próximos aos resultados experimentais (exceto para o NO), sendo aprimorados ainda mais com a inclusão da modelagem da radiação térmica, sobretudo para regiões mais distantes do bico injetor do combustível, após o pico de temperatura da chama. O modelo FGM, contudo, apresentou resultados muito aquém dos esperados, sobretudo para as frações mássicas das espécies químicas, mesmo utilizando malhas com nível de refinamento muito maior e com o teste de diversas combinações de espécies para a variável de progresso da reação, e no qual a inclusão da radiação na modelagem também não trouxe benefícios perceptíveis. Todas as simulações numéricas foram realizadas empregando o código comercial ANSYS Fluent, versão 15.0.0. / The numerical simulation of turbulent flames is still a challenge for today's computational fluid dynamics practices. Understanding that the most complete and realistic numerical approaches available today may be computationally prohibitive, several models have been developed in order to reproduce the phenomena involved in combustion in a simplified, but still physically consistent, way. Therefore, this work is dedicated to compare different models for turbulence closure based on the Reynolds-averaged Navier-Stokes equations and models for simplification of the chemical kinetics based on the flamelet concept, with and without thermal radiation modeling through the weighted-sum-of-gray-gases model. Thus, in the first part of the current work six turbulence models are employed to solve a turbulent nonreactive isothermal flow, a propane jet surrounded by a parallel stream of air. The models are compared through their effectiveness in predicting the mean values of longitudinal and transversal velocities, propane mass fraction and mixture density. The models are the Standard k- (employed in its original version and with two modifications according to proposals found in the literature), the Realizable k- , the Standard k- and the Shear-Stress Transport k- . One of the best performing models is then used to simulate a turbulent nonpremixed flame of methane/hydrogen/nitrogen surrounded by a low-velocity air coflow, in which are compared the models to reduce the chemical kinetics based on the flamelets concept, the Steady Laminar Diffusion Flamelet (SLDF) and the Flamelet-Generated Manifold (FGM), being the numerical results compared to the experimental data for the mean values of longitudinal velocity, mixture fraction, temperature and species mass fractions. Among the six turbulence models evaluated, it is observed that the two adjusted versions of the Standard k- and the Standard k- showed better agreement with the experimental measurements than the other models. In the current study it is also evaluated the consistency of the reported experimental data and a discrepancy is identified, which, as verified, does not compromise the models comparison here proposed. In the solution of the reactive flow, the SLDF model showed results very close to the experimental results (except for NO), being further enhanced with the inclusion of the thermal radiation modeling, especially for regions far from fuel nozzle, after the peak of temperature of the flame. The FGM model, however, showed results far below the expected, especially for the mass fractions of chemical species, even using meshes with much higher refinement level and testing of various species combinations for the reaction progress variable. The inclusion of the radiation modeling did not brought noticeable benefits. All the numerical simulations were performed employing the ANSYS Fluent version 15.0.0 commercial code.

A simulação numérica de chamas turbulentas é ainda hoje um desafio para as práticas de mecânica dos fluidos computacional. Compreendendo que as abordagens numéricas mais completas e realísticas atualmente disponíveis podem ser computacionalmente proibitivas, diversos modelos vêm sendo desenvolvidos com o objetivo de reproduzir os fenômenos envolvidos na combustão de uma forma simplificada, mas ainda fisicamente consistente. Este trabalho é, portanto, dedicado à comparação de diferentes modelos de fechamento para a turbulência baseados nas equações de Navier-Stokes em médias de Reynolds e de modelos para simplificação da cinética química baseados no conceito de flamelets, com e sem a modelagem da radiação térmica, esta última através do modelo de soma-ponderada-de-gasescinzas. Para tanto, na primeira parte do presente trabalho são comparados seis modelos de turbulência na solução de um jato turbulento de propano, não reativo e isotérmico, circundado por uma corrente paralela de ar, quanto a sua eficiência na predição dos valores médios da velocidade longitudinal e transversal, fração mássica de propano e massa específica da mistura. Os modelos são o k- Padrão (empregado na sua versão original e com mais duas modificações nas suas constantes conforme propostas encontradas na literatura), o k- Realizable, o k- Padrão e o k- Shear-Stress Transport. Um dos modelos de melhor desempenho é então usado na simulação de uma chama turbulenta não pré-misturada de metano/hidrogênio/nitrogênio circundada por um escoamento coaxial de ar de baixa velocidade, no qual são então comparados os modelos para redução da cinética química baseados no conceito de flamelets, o Steady Laminar Diffusion Flamelet (SLDF) e o Flamelet-Generated Manifold (FGM), tendo os seus resultados comparados aos dados experimentais para os valores médios da velocidade longitudinal, fração de mistura, temperatura e frações mássicas das espécies químicas. Dentre os modelos de turbulência avaliados, é observado que as duas versões ajustadas do k- Padrão e o k- Padrão se mostraram com melhor concordância em relação às medições experimentais do que os demais. No presente estudo é também avaliada a consistência dos dados experimentais reportados e uma discrepância é identificada neste jato, mas que, conforme verificado, não compromete a comparação dos modelos aqui proposta. Na solução do escoamento reativo, o modelo SLDF se mostrou com resultados bastante próximos aos resultados experimentais (exceto para o NO), sendo aprimorados ainda mais com a inclusão da modelagem da radiação térmica, sobretudo para regiões mais distantes do bico injetor do combustível, após o pico de temperatura da chama. O modelo FGM, contudo, apresentou resultados muito aquém dos esperados, sobretudo para as frações mássicas das espécies químicas, mesmo utilizando malhas com nível de refinamento muito maior e com o teste de diversas combinações de espécies para a variável de progresso da reação, e no qual a inclusão da radiação na modelagem também não trouxe benefícios perceptíveis. Todas as simulações numéricas foram realizadas empregando o código comercial ANSYS Fluent, versão 15.0.0. / The numerical simulation of turbulent flames is still a challenge for today's computational fluid dynamics practices. Understanding that the most complete and realistic numerical approaches available today may be computationally prohibitive, several models have been developed in order to reproduce the phenomena involved in combustion in a simplified, but still physically consistent, way. Therefore, this work is dedicated to compare different models for turbulence closure based on the Reynolds-averaged Navier-Stokes equations and models for simplification of the chemical kinetics based on the flamelet concept, with and without thermal radiation modeling through the weighted-sum-of-gray-gases model. Thus, in the first part of the current work six turbulence models are employed to solve a turbulent nonreactive isothermal flow, a propane jet surrounded by a parallel stream of air. The models are compared through their effectiveness in predicting the mean values of longitudinal and transversal velocities, propane mass fraction and mixture density. The models are the Standard k- (employed in its original version and with two modifications according to proposals found in the literature), the Realizable k- , the Standard k- and the Shear-Stress Transport k- . One of the best performing models is then used to simulate a turbulent nonpremixed flame of methane/hydrogen/nitrogen surrounded by a low-velocity air coflow, in which are compared the models to reduce the chemical kinetics based on the flamelets concept, the Steady Laminar Diffusion Flamelet (SLDF) and the Flamelet-Generated Manifold (FGM), being the numerical results compared to the experimental data for the mean values of longitudinal velocity, mixture fraction, temperature and species mass fractions. Among the six turbulence models evaluated, it is observed that the two adjusted versions of the Standard k- and the Standard k- showed better agreement with the experimental measurements than the other models. In the current study it is also evaluated the consistency of the reported experimental data and a discrepancy is identified, which, as verified, does not compromise the models comparison here proposed. In the solution of the reactive flow, the SLDF model showed results very close to the experimental results (except for NO), being further enhanced with the inclusion of the thermal radiation modeling, especially for regions far from fuel nozzle, after the peak of temperature of the flame. The FGM model, however, showed results far below the expected, especially for the mass fractions of chemical species, even using meshes with much higher refinement level and testing of various species combinations for the reaction progress variable. The inclusion of the radiation modeling did not brought noticeable benefits. All the numerical simulations were performed employing the ANSYS Fluent version 15.0.0 commercial code.

A simulação numérica de chamas turbulentas é ainda hoje um desafio para as práticas de mecânica dos fluidos computacional. Compreendendo que as abordagens numéricas mais completas e realísticas atualmente disponíveis podem ser computacionalmente proibitivas, diversos modelos vêm sendo desenvolvidos com o objetivo de reproduzir os fenômenos envolvidos na combustão de uma forma simplificada, mas ainda fisicamente consistente. Este trabalho é, portanto, dedicado à comparação de diferentes modelos de fechamento para a turbulência baseados nas equações de Navier-Stokes em médias de Reynolds e de modelos para simplificação da cinética química baseados no conceito de flamelets, com e sem a modelagem da radiação térmica, esta última através do modelo de soma-ponderada-de-gasescinzas. Para tanto, na primeira parte do presente trabalho são comparados seis modelos de turbulência na solução de um jato turbulento de propano, não reativo e isotérmico, circundado por uma corrente paralela de ar, quanto a sua eficiência na predição dos valores médios da velocidade longitudinal e transversal, fração mássica de propano e massa específica da mistura. Os modelos são o k- Padrão (empregado na sua versão original e com mais duas modificações nas suas constantes conforme propostas encontradas na literatura), o k- Realizable, o k- Padrão e o k- Shear-Stress Transport. Um dos modelos de melhor desempenho é então usado na simulação de uma chama turbulenta não pré-misturada de metano/hidrogênio/nitrogênio circundada por um escoamento coaxial de ar de baixa velocidade, no qual são então comparados os modelos para redução da cinética química baseados no conceito de flamelets, o Steady Laminar Diffusion Flamelet (SLDF) e o Flamelet-Generated Manifold (FGM), tendo os seus resultados comparados aos dados experimentais para os valores médios da velocidade longitudinal, fração de mistura, temperatura e frações mássicas das espécies químicas. Dentre os modelos de turbulência avaliados, é observado que as duas versões ajustadas do k- Padrão e o k- Padrão se mostraram com melhor concordância em relação às medições experimentais do que os demais. No presente estudo é também avaliada a consistência dos dados experimentais reportados e uma discrepância é identificada neste jato, mas que, conforme verificado, não compromete a comparação dos modelos aqui proposta. Na solução do escoamento reativo, o modelo SLDF se mostrou com resultados bastante próximos aos resultados experimentais (exceto para o NO), sendo aprimorados ainda mais com a inclusão da modelagem da radiação térmica, sobretudo para regiões mais distantes do bico injetor do combustível, após o pico de temperatura da chama. O modelo FGM, contudo, apresentou resultados muito aquém dos esperados, sobretudo para as frações mássicas das espécies químicas, mesmo utilizando malhas com nível de refinamento muito maior e com o teste de diversas combinações de espécies para a variável de progresso da reação, e no qual a inclusão da radiação na modelagem também não trouxe benefícios perceptíveis. Todas as simulações numéricas foram realizadas empregando o código comercial ANSYS Fluent, versão 15.0.0. / The numerical simulation of turbulent flames is still a challenge for today's computational fluid dynamics practices. Understanding that the most complete and realistic numerical approaches available today may be computationally prohibitive, several models have been developed in order to reproduce the phenomena involved in combustion in a simplified, but still physically consistent, way. Therefore, this work is dedicated to compare different models for turbulence closure based on the Reynolds-averaged Navier-Stokes equations and models for simplification of the chemical kinetics based on the flamelet concept, with and without thermal radiation modeling through the weighted-sum-of-gray-gases model. Thus, in the first part of the current work six turbulence models are employed to solve a turbulent nonreactive isothermal flow, a propane jet surrounded by a parallel stream of air. The models are compared through their effectiveness in predicting the mean values of longitudinal and transversal velocities, propane mass fraction and mixture density. The models are the Standard k- (employed in its original version and with two modifications according to proposals found in the literature), the Realizable k- , the Standard k- and the Shear-Stress Transport k- . One of the best performing models is then used to simulate a turbulent nonpremixed flame of methane/hydrogen/nitrogen surrounded by a low-velocity air coflow, in which are compared the models to reduce the chemical kinetics based on the flamelets concept, the Steady Laminar Diffusion Flamelet (SLDF) and the Flamelet-Generated Manifold (FGM), being the numerical results compared to the experimental data for the mean values of longitudinal velocity, mixture fraction, temperature and species mass fractions. Among the six turbulence models evaluated, it is observed that the two adjusted versions of the Standard k- and the Standard k- showed better agreement with the experimental measurements than the other models. In the current study it is also evaluated the consistency of the reported experimental data and a discrepancy is identified, which, as verified, does not compromise the models comparison here proposed. In the solution of the reactive flow, the SLDF model showed results very close to the experimental results (except for NO), being further enhanced with the inclusion of the thermal radiation modeling, especially for regions far from fuel nozzle, after the peak of temperature of the flame. The FGM model, however, showed results far below the expected, especially for the mass fractions of chemical species, even using meshes with much higher refinement level and testing of various species combinations for the reaction progress variable. The inclusion of the radiation modeling did not brought noticeable benefits. All the numerical simulations were performed employing the ANSYS Fluent version 15.0.0 commercial code.

Die Arbeit befasst sich mit der Flamelet Modellierung für die Verbrennung von Kohlenstaub. Dabei liegt der Fokus sowohl auf der detaillierten Betrachtung der Gasphasenchemie als auch auf der Interaktion der Kohle mit der Gasphase. Ziel der Arbeit ist die Entwicklung einer Methode für die Simulation großtechnischer Kohlestaubfeuerungen. Die energetische Umsetzung von Kohle läuft in drei wesentlichen Schritten ab: Verdampfung der Feuchtigkeit, Ausgasung der Kohle (Pyrolyse) und schließlich der Koksabbrand. Da die Struktur der Kohle als fossiler Brennstoff hoch komplex ist, existieren viele prädiktive, rechenaufwändige Modelle zur Beschreibung dieser Prozesse [1–4]. Diese Modelle können nicht direkt in numerischen Strömungssimulationen genutzt werden, dienen aber zur Kalibrierung einfacherer kinetischer Modelle. Diese in der Arbeit angewendete Prozedur wird in [5] beschrieben. Zur detaillierten Beschreibung des Abbaus der entstehenden höheren Kohlenwasserstoffe werden in der Simulation große Reaktionsmechanismen benötigt. Die Benutzung solcher Mechanismen ist mit großen Rechenzeiten verbunden und daher bleibt deren Anwendbarkeit auf einfache Anwendungsfälle beschränkt. Der Vorteil der Flamelet Modellierung besteht darin, dass unter bestimmten Voraussetzungen der komplette thermo-chemische Zustand, bestehend aus Temperatur, Druck und Zusammensetzung, mit nur wenigen charakterisierenden Kontrollvariablen abgebildet werden kann. Durch Vorgabe und Variation der Kontrollvariablen können diese Zustände mittels kanonischer Flammenkonfigurationen vorberechnet und in sogenannten Flamelettabellen abgespeichert werden. Für das klassische Flamelet / Fortschrittsvariablen Modell [6] wird der thermo-chemische Zustand über Mischungsbruch und Fortschrittsvariablen parametriert, dabei bestimmt der Mischungsbruch den Anteil an Brenn- stoff im Gemisch und die Fortschrittsvariable den Fortschritt der chemischen Reaktion. Die Kontrollvariablen werden in der numerischen Simulation transportiert, an Stelle der Energie- und Speziesgleichungen. Dies stellt für große Mechanismen eine dramatische Reduktion der zu lösenden Gleichungen dar. Der thermo-chemische Zustand ergibt sich per Look-up aus den Flamelettabellen. Im Zuge der Verbrennung trockener Kohle werden zwei Brennstoffe durch Pyrolyse und Koksabbrand freigesetzt. Für die Flamelet Modellierung bedeutet dies entsprechend je einen Mischungsbruch für Pyrolysegas und Produkte aus dem Koksabbrand. Neben der Fortschrittsvariablen wird ebenfalls die Enthalpie der Gasphase als Kontrollvariable benötigt aufgrund des intensiven Wärmeaustauschs zwischen Kohle und Gasphase. In der Arbeit erfolgt die Vorstellung der benötigten Transportgleichungen sowie die Beschreibung verschiedener Methoden zur Integration nicht-adiabater Zustände in Flamelettabellen. Dabei unterscheiden sich die vorgestellten Tabellierungstrategien hauptsächlich in der betrachteten Verbrennungsart. IV Erfolgt die Mischung von Brennstoff und Oxidationsmittel erst in der untersuchten Flammenkonfiguration, spricht man von Diffusionsflammenstrukturen; sind beide schon gemischt, so entstehen Vormischflammenstrukturen. Die Detektion solcher Strukturen erfolgt in der Arbeit anhand einer Flammenstrukturanalyse mittels Flammenmarker. Die prinzipielle Übertragbarkeit des Flamelet / Fortschrittsvariablen Modells auf turbulente Kohlestaubfeuerung wurde von Watanabe [7] gezeigt, jedoch ist die Bewertung der eingesetzten Flamelet Modellierung in Grobstruktursimulationen nicht ohne weiteres möglich. Deshalb werden zur Verifizierung der entwickelten Tabellierungstrategie in der Arbeit einfache Flammenkonfigurationen betrachtet, die es erlauben, direkte Chemielösungen mit den Lösungen der tabellierten Chemie zu vergleichen. Für den entsprechenden Vergleich erfolgt die Vorstellung zweier Analysen. Bei der a priori Analyse wird der thermo-chemische Zustand der detaillierten Lösung mit dem tabellierten Zustand verglichen. Für den Look-up werden dabei die Kontrollvariablen der direkten Chemiesimulation benutzt. Die a posteriori Analyse ist der Vergleich einer voll gekoppelten Rechnung unter Benutzung der Tabellierungstrategie mit der zugehörigen detaillierten Rechnung. Die erste untersuchte Konfiguration stellt eine Gegenstromanordnung mit vorgewärmter Luft und Kohlebeladung dar. Die Hauptergebnisse dieser rein numerischen Studie wurden bereits veröffentlicht [8] und es konnte die erfolgreiche Applikation der vorgestellten Tabellierungstrategie in dieser Anordnung für Tabellen basierend auf Diffusionflammenstrukturen gezeigt werden. Für die Validierung der detaillierten Rechnungen erfolgt die Nutzung experimenteller Daten [9, 10] für magere Methan-Sauerstoff-Stickstoff Mischungen in Staupunktströmungen. Es zeigt sich, dass diese Konfigurationen stark von den vorgemischten Gasflammen dominiert werden und somit Tabellen basierend auf Vormischflammenstrukturen einzusetzen sind. Die entwickelte Tabellierungsmethode ist in der Lage, auch diese Flammenstrukturen abzubilden. Abschließend wird numerisch eine Parametervariation hinsichtlich Einlassgeschwindigkeit und Kohlebeladung vorgestellt, um die Robustheit und breite Anwendbarkeit der entwickelten Tabellierungstrategie aufzuzeigen. Zusammenfassend konnte mittels Flammenstrukturanalyse für jede vorgestellte Konfiguration der zu verwendende Typ der Tabelle bestimmt werden. In den untersuchten Konfigurationen führte deren Anwendung zu einer guten Übereinstimmung mit den detaillierten Rechnungen. Damit legt diese Arbeit den Grundstein für weiterführende Betrachtung zur Simulation großtechnischer Kohlestaubfeuerungen.

<div>Combustion instability due to thermo-acoustic interactions in high-speed propulsion devices such as gas turbines and rocket engines result from pressure waves with very large amplitudes propagating back and forth in the combustion chamber. Exposure to the pressure fluctuations over a long period of time can lead to a cataclysmic failure of engines. The underlying physics governing the generation of the thermo-acoustic instability is a complex interaction among heat release, turbulence, and acoustic waves. Currently, it is very difficult to accurately predict the expected level of oscillations in a combustor. Hence development of strategies and engineering solutions to mitigate thermo-acoustic instability is an active area of research in both academia and industry. In this work, we carry out numerical modeling of thermo-acoustic instability in a self-excited, laboratory scale, model rocket combustor developed at Purdue University. Two different turbulent combustion models to account for turbulence-chemistry interactions are considered in this study, the flamelet model and the transported probability density function (PDF) method. </div><div><br></div><div>In the flamelet modeling approach, detailed chemical kinetics can be easily incorporated at a relatively low cost in comparison to other turbulent combustion models and it also accounts for turbulence-chemistry interactions. The flamelet model study is divided into two parts. In first part, we examine the effect of different numerical approaches for implementing the flamelet model. In advanced modeling and simulations of turbulent combustion, the accuracy of model predictions is affected by physical model errors as well as errors that arise from the numerical implementation of models in simulation codes. Here we are mainly concerned with the effect of numerical implementation on model predictions of turbulent combustion. Particularly, we employ the flamelet/progress variable (FPV) model and examine the effect of various numerical approaches for the flamelet table integration, with presumed shapes of PDF, on the FPV modeling results. Three different presumed-PDF table integration approaches are examined in detail by employing different numerical integration strategies. The effect of the different presumed-PDF table integration approaches is examined on predictions of two real flames, a laboratory-scale turbulent free jet flame, Sandia Flame D and the self-excited resonance model rocket combustor. Significant difference is observed in the predictions both of the flames. The results in this study further support the claims made in previous studies that it is imperative to preserve the laminar flamelet structure during integration while using the flamelet model to achieve better predictions in simulations. In the second part of the flamelet modeling study, computational investigations of the coupling between the transient flame dynamics such as the ignition delay and local extinction and the thermo-acoustic instability developed in a self-excited resonance combustor to gain deep insights into the mechanisms of thermo-acoustic instability. A modeling framework that employs different flamelet models (the steady flamelet model and the flamelet/progress variable approach) is developed to enable the examination of the effect of the transient flame dynamics caused by the strong coupling of the turbulent mixing and finite-rate chemical kinetics on the occurrence of thermo-acoustic instability. The models are validated by using the available experimental data for the pressure signal. Parametric studies are performed to examine the effect of the occurrence of the transient flame dynamics, the effect of artificial amplification of the Damkohler number, and the effect of neglecting mixture fraction fluctuations on the predictions of the thermo-acoustic instability. The parametric studies reveal that the occurrence of transient flame dynamics has a strong influence on the onset of the thermo-acoustic instability. Further analysis is then conducted to localize the effect of a particular flame dynamic event, the ignition delay, on the thermo-acoustic instability. The reverse effect of the occurrence of the thermo-acoustic instability on the transient flame dynamics in the combustor is also investigated by examining the temporal evolution of the local flame events in conjunction with the pressure wave propagation. The above observed two-way coupling between the transient flame dynamics (the ignition delay) and the thermo-acoustic instability provides a plausible mechanism of the self-excited and sustained thermo-acoustic instability observed in the combustor.</div><div><br></div><div>The second turbulent combustion model considered in this study is the transported PDF method. The transported PDF method is one of the most attractive models because it treats the highly-nonlinear chemical reaction source term without a closure requirement and it is a generalized model for a wide range of turbulent combustion problems.</div><div>Traditionally, the transported PDF method has been used to model low-Mach number, incompressible flows where the pressure is assumed to be thermodynamically constant. Since there is significant pressure fluctuations in the model rocket combustor, the flow is highly compressible and it is necessary to account for this compressibility in the transported PDF method. In the past there has been very little work to model compressible reactive flows using the transported PDF and no effort has been made to model thermo-acoustic instability using the transported PDF method. There is a pressing need to further examine and develop the transported PDF method for compressible reactive flows to broaden our understanding of physical phenomenon like thermo-acoustic instability, interaction between combustion and strong shock and expansion waves, coupling between acoustic and heat release which are observed in high-speed turbulent combustion problems. To address this, a modeling framework for compressible turbulent reactive flows by the using the transported PDF method is developed. This framework is validated in a series of test cases ranging from pure mixing to a supersonic turbulent jet flame. The framework is then used to study the thermo-acoustic interactions in the self-excited model rocket combustor.</div>

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Zugl.: Darmstadt, Techn. Univ., Diss.