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Large Eddy Simulation of Turbulent Reacting Flows With Radiative Heat Transfer

Cavalcanti Miranda, Flavia (2019):
Large Eddy Simulation of Turbulent Reacting Flows With Radiative Heat Transfer.
Darmstadt, Technische Universität, [Online-Edition: https://tuprints.ulb.tu-darmstadt.de/8390],
[Ph.D. Thesis]

Abstract

Combustion is the most common method of energy conversion constituting about 85 % of the primary energy consumption. However, combustion is responsible for emissions of CO, NOx , CO2 , soot and others pollutants. Hence, having the expected increase of the global energy demand in mind as well as the challenge of a global reduction in the greenhouse gases emissions and the current difficulties in developing renewable energy sources for a foreseeable future, combustion science will continue being a very important topic and a very active field of technology. Moreover, since the parameters involved in combustion systems are affected by heat transfer, the understanding and development of mathematical models for analyses of heat transfer is crucial. Studying such flows is not a straightforward task because of the high nonlinear interaction among the involving processes, which include chemical kinetics, turbulence and thermal radiation. In this context, Large Eddy Simulation (LES) is an outstanding approach and has become a common model to deal with such complex flows. In this approach, the instantaneous form of the governing equations are filtered. As a consequence of the filter procedure, unclosed terms appear that correspond to effects of Turbulence-Chemistry Interactions (TCI) and Turbulence Radiation Interactions (TRI). The importance of the TCI has long been recognized and it is a very active research topic in the combustion community. Furthermore, the relevance of TRI has been gaining recognition but just a few LES studies considered their effects of TRI. This thesis deals with the simulation of turbulent flames by taking into account radiative heat transfer. The focus of this work is on the development and application of a radiation solver for computing turbulent reacting flows. In this study, the role of the TRI in the context of LES is analyzed for two important and widely investigated configurations: Sandia flame D and bluff-body stabilized non-premixed flame. The radiation solver was implemented by considering the complete radiative transfer equation, including the emission, absorption and scattering terms. The finite volume method, which is a variation of the Discrete Ordinates Method (DOM), was applied to discretize this equation. To account for the spectral behavior of the combustion gases involved, the Weighted Sum of Gray Gases (WSGG) method is used. To include thermal radiation in the LES framework, the filtered radiative source term should be computed. The contribution of the resolved scales can be explicitly calculated, whereas the terms involving the subgrid-scale contributions are unclosed and require approximations. The Optically Thin Fluctuation Assumption (OTFA) is applied for approximating the filtered absorption term and the Eulerian Stochastic Field (ESF) method is employed for representing the emission TRI. Following, the importance of considering the subgrid-scale contributions is analyzed. For this aim, simulations are performed by considering and neglecting these contributions. The Sandia flame D, bluff-body flame and their corresponding four times scaled flames are the configurations studied here for analyzing the TRI. The scaled flames are additionally investigated in order to have a more pronounced radiation effect. For all cases, the difference between the radiative source term computed by accounting and neglecting the subgrid-scale contribution is not significant, which indicates that considering these terms is not important in the context of LES. Additionally, the radiative source term is computed with the mean fields of temperature and species concentrations to show the difference to Reynolds Averaged Navier-Stokes equations (RANS), since this procedure corresponds to compute the radiative source term in a RANS framework. As expected, the results for this case presented a significant difference to the remaining procedures, which demonstrates that considering the subgrid-scale contributions are relevant for RANS simulations.

Item Type: Ph.D. Thesis
Erschienen: 2019
Creators: Cavalcanti Miranda, Flavia
Title: Large Eddy Simulation of Turbulent Reacting Flows With Radiative Heat Transfer
Language: English
Abstract:

Combustion is the most common method of energy conversion constituting about 85 % of the primary energy consumption. However, combustion is responsible for emissions of CO, NOx , CO2 , soot and others pollutants. Hence, having the expected increase of the global energy demand in mind as well as the challenge of a global reduction in the greenhouse gases emissions and the current difficulties in developing renewable energy sources for a foreseeable future, combustion science will continue being a very important topic and a very active field of technology. Moreover, since the parameters involved in combustion systems are affected by heat transfer, the understanding and development of mathematical models for analyses of heat transfer is crucial. Studying such flows is not a straightforward task because of the high nonlinear interaction among the involving processes, which include chemical kinetics, turbulence and thermal radiation. In this context, Large Eddy Simulation (LES) is an outstanding approach and has become a common model to deal with such complex flows. In this approach, the instantaneous form of the governing equations are filtered. As a consequence of the filter procedure, unclosed terms appear that correspond to effects of Turbulence-Chemistry Interactions (TCI) and Turbulence Radiation Interactions (TRI). The importance of the TCI has long been recognized and it is a very active research topic in the combustion community. Furthermore, the relevance of TRI has been gaining recognition but just a few LES studies considered their effects of TRI. This thesis deals with the simulation of turbulent flames by taking into account radiative heat transfer. The focus of this work is on the development and application of a radiation solver for computing turbulent reacting flows. In this study, the role of the TRI in the context of LES is analyzed for two important and widely investigated configurations: Sandia flame D and bluff-body stabilized non-premixed flame. The radiation solver was implemented by considering the complete radiative transfer equation, including the emission, absorption and scattering terms. The finite volume method, which is a variation of the Discrete Ordinates Method (DOM), was applied to discretize this equation. To account for the spectral behavior of the combustion gases involved, the Weighted Sum of Gray Gases (WSGG) method is used. To include thermal radiation in the LES framework, the filtered radiative source term should be computed. The contribution of the resolved scales can be explicitly calculated, whereas the terms involving the subgrid-scale contributions are unclosed and require approximations. The Optically Thin Fluctuation Assumption (OTFA) is applied for approximating the filtered absorption term and the Eulerian Stochastic Field (ESF) method is employed for representing the emission TRI. Following, the importance of considering the subgrid-scale contributions is analyzed. For this aim, simulations are performed by considering and neglecting these contributions. The Sandia flame D, bluff-body flame and their corresponding four times scaled flames are the configurations studied here for analyzing the TRI. The scaled flames are additionally investigated in order to have a more pronounced radiation effect. For all cases, the difference between the radiative source term computed by accounting and neglecting the subgrid-scale contribution is not significant, which indicates that considering these terms is not important in the context of LES. Additionally, the radiative source term is computed with the mean fields of temperature and species concentrations to show the difference to Reynolds Averaged Navier-Stokes equations (RANS), since this procedure corresponds to compute the radiative source term in a RANS framework. As expected, the results for this case presented a significant difference to the remaining procedures, which demonstrates that considering the subgrid-scale contributions are relevant for RANS simulations.

Place of Publication: Darmstadt
Divisions: 16 Department of Mechanical Engineering
16 Department of Mechanical Engineering > Institute for Energy and Power Plant Technology (EKT)
Date Deposited: 10 Feb 2019 20:55
Official URL: https://tuprints.ulb.tu-darmstadt.de/8390
URN: urn:nbn:de:tuda-tuprints-83907
Referees: Janicka, Prof. Dr. Johannes and Coelho, Prof. Pedro
Refereed / Verteidigung / mdl. Prüfung: 26 June 2018
Alternative Abstract:
Alternative abstract Language
Verbrennung ist mit einem Anteil von circa 85 % des Primärenergieverbrauchs die am häufigsten verwendete Methode der Energieumwandlung. Gleichzeit ist sie aber auch für die Emission von CO, NOx , CO2 , Ruß und anderen Schadstoffen Verantwortlich. Aufgrund der erwarteten steigenden, globalen Nachfrage nach Energie, der Herausforderung die globalen Treibhausgasemissionen zu reduzieren und ungelöster Schwierigkeiten bei der Substitution durch erneuerbarer Energien in absehbarer Zeit wird die Verbrennungsforschung ein sehr wichtiges Thema bleiben. Da die Verbrennungseffizienz unter anderem vom Wärmetransfer abhängt, ist die Entwicklung von mathematischen Modellen für die Analyse und das Verständnis der Wärmeübertragung von entscheidender Wichtigkeit. Die Untersuchung solcher Verbrennungsströme ist aufgrund der stark nichtlinearen Wechselwirkungen zwischen den beteiligten Prozessen, welche Reaktionskinetiken, Turbulenzen und thermische Strahlung beinhalten, keine einfache Aufgabe. Das Simulationsmodel Large Eddy Simulation (LES) ist hierzu ein herausragender und gebräuchlicher Lösungsansatz für solche komplexen Strömungen. Hierbei wird die momentane Form der aktuell dominierenden Gleichungen herausgefiltert. Als Konsequenz aus dem Filtervorgang erscheinen nicht geschlossene Terme, welche den Effekten der Turbulence-Chemistry Interaction (TCI) und Turbulence Radiation Interaction (TRI) entsprechen. Die Wichtigkeit der TCI wurde schon seit langem erkannt und ist ein sehr populäres Gebiet in der Verbrennungsforschung. Ferner gewinnt auch die Bedeutung der TRI an Anerkennung. Allerdings berücksichtigen nur ein paar LES Studien die Effekte der TRI. Diese Dissertation behandelt die Simulation von turbulenten Flammen unter Berücksichtigung des Einflusses der thermischen Strahlung. Der Fokus dieser Arbeit liegt in der Entwicklung und Anwendung von mathematischen Lösern für den Strahlungsanteil bei der Berechnung turbulenter und chemisch reagierender Ströme. In dieser Arbeit wurde der Einfluss der TRI im LES Zusammenhang für zwei wichtige und breit untersuchte Konstellationen analysiert: Die Sandia Flamme D und eine durch Staukörper stabilisiert Diffusionsflamme. Der implementierte Strahlungslöser berücksichtigt die komplette Strahlungstransportgleichung einschließlich Emission, Absorption und Streuungstermen. Um diese Gleichung zu diskretisieren wurde die Finite Volumen Methode verwendet, welche eine Variante der Discrete Ordinates Method (DOM) ist. Um das spektrale Verhalten der Verbrennungsgase zu berücksichtigen, wurde die Weighted Sum of Gray Gases (WSGG) Methode angewandt. Um den Beitrag der thermischen Strahlung in der LES zu berücksichtigen, sollte die Strahlungsgleichung gefiltert berechnet werden. Der Beitrag der aufgelösten Skalen kann explizit berechnet werden, wohingegen der Term der Untergitterbeiträge nicht geschlossen ist und daher eine Modellierung erfordern. Die Optically Thin Fluctuation Assumption (OTFA) wurde zur Annäherung des gefilterten Absorptionsteils verwendet und die Eulerian Stochastic Field (ESF) Methode wurde angewandt um die Emissions TRI abzubilden. Daraufhin wurde die Wichtigkeit der Untergitterbeiträge analysiert, indem Simulationen mit und ohne diese Beiträge verglichen wurden. Es wurden die Sandia Flamme D, die Staukörper Flamme und deren viermal vergrößerte Flammen als Varianten für die Analyse der TRI untersucht. Die vergrößerten Flammen wurden zusätzlich analysiert, da erwartungsgemäß in diesem Fall die Strahlungseffekte ersichtlicher sind. In allen Fällen war der Unterschied im Strahlungsquellterm, welcher durch das Berücksichtigen und Vernachlässigen der Untergitterbeiträge entstand, unbedeutend. Dies deutet darauf hin, dass deren Berücksichtigung für die LES Simulation unbedeutend ist. Zudem wurde der Strahlungsquellenterm mit den mittleren Feldern der Temperatur- und Konzentrationswerte berechnet. Dieses Vorgehen entspricht der Berechnung des Strahlungsquellenterms im Rahmen der Averaged Navier-Stokes Gleichungen (RANS). Wie erwartet zeigten die Ergebnisse dieses Falles einen deutlichen Unterschied zu den restlichen Vorgehensweisen was verdeutlicht, dass die Untergitterbeiträge für die RANS relevant sind.German
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