Large eddy simulation of turbulent swirling flames

Large eddy simulation (LES) is attractive as it provides a reasonable compromise between accuracy and cost, and is rapidly evolving as a practical approach for many engineering applications. This thesis is concerned with the application of large eddy simulation to unconfined swirl in turbulent non-premixed flames and isothermal flows. The LES methodology has been applied for the prediction of turbulent swirling reacting and non-reacting flows based on laboratory scale swirl burner known as the Sydney swirl burner, which has been a target flame of the workshop series of turbulent non-premixed flames (TNF). For that purpose a LES code was developed that can run wide range of applications. An algorithm was developed for LES of variable density reacting flow calculations. Particular attention was given to primitive conservation (mass, momentum and scalar) and kinetic energy of the flow and mixing field. The algorithm uses the primitive variables, which are staggered in both space and time. A steady laminar flamelet model which includes the detailed chemical kinetics and multi component mass diffusion, has been implemented in the LES code. An artificial inlet boundary condition method was implemented to generate instantaneous turbulent velocity fields that are imposed on the inflow boundary of the Cartesian grid. To improve the applicability of the code, various approaches were developed to improve stability and efficiency. LES calculations for isothermal turbulent swirling jets were successful in predicting experimentally measured mean velocities, their rms fluctuations and Reynolds shear stresses. The phenomenon of vortex breakdown (VB) and recirculation flow structures at different swirl and Reynolds numbers were successfully reproduced by the present large eddy simulations indicating that LES is capable of predicting VB phenomena which occurs only at certain conditions. For swirling flames, the LES predictions were able to capture the unsteady flow field, flame dynamics and showed good agreement with experimental measurements. The LES predictions for the mean temperature and major species were also successful

Advanced flamelet modelling of turbulent nonpremixed and partialy premixed combustion

Current work focuses on the development and performance evaluation of advanced flamelet models for turbulent non-premixed and partially premixed combustion in RANS and large eddy simulation (LES) based modelling. A RANS based combustion modelling strategy which has the ability to capture the detailed structure of turbulent non-premixed flames, including the pollutant NO, and account for the effects of radiation heat loss and transient evolution of NO, has been developed and incorporated into the in-house RANS code. The strategy employs an 'enthalpy-defect' based non-adiabatic flamelet model in conjunction with steady or unsteady nonadiabatic flamelets based NO submodels. The performanceo f the non-adiabaticm odel and its NO submodelsh asb eena ssessed against experimental measurements and steady flamelet model predictions for turbulent CH4/H2 bluff-body stabilized and CH4-air piloted jet flames. Appreciable improvements in the mean thermal structure predictions have been observed in the piloted jet flames by consideration of radiation heat loss through the non-adiabatic flamelet model. Since transient effects were weaker in the piloted jet flame, both unsteady and steady non-adiabatic NO submodels provided similar level of improvement in the pollutant NO predictions in comparison to their adiabatic counterpartsT. ransiente ffectsw ere, however,d ominanti n the bluff-body flame. The unsteady non-adiabatic NO submodel provided excellent agreement with measured NO distribution in comparison to the appreciably overpredicted distribution by its steadyc ounterpart.T he strategyo f non-adiabaticf lamelet model in conjunctionw ith unsteady non-adiabatic NO submodel seems to provide an accurate and robust alternative to the conventional strategy of steady flamelet model with steady NO submodel. While addressing the limitations of steady flamelet model in regard to radiation and slow chemistry of NO is one objective of this research, extending the applicability of the model to partially premixed combustion has been pursued as the second objective. Flamelet/progress variable (FPV) approach based combustion models, which have the potential to describe both non-premixed and partially premixed combustion, have been incorporated in the in-house RANS and LES codes. Based on the form of the PDF for reaction progress variable, two different formulations, FPV-8 function model and FPV-P function model, have been derived. (Continues...)

Numerical simulations of stationary and transient spray combustion for aircraft gas turbine applications

Le développement des turbines à gaz d’aviation actuelles et futures est principalement axé sur la sécurité, la performance, la minimisation de la consommation de l’énergie, et de plus en plus sur la réduction des émissions d’espèces polluantes. Ainsi, les phases de design de moteurs sont soumises auxaméliorations continues par des études expérimentales et numériques. La présente thèse se consacre à l’étude numérique des phases transitoires et stationnaires de la combustion au sein d’une turbine à gaz d’aviation opérant à divers modes de combustion. Une attention particulière est accordée à la précision des résultats, aux coûts de calcul, et à la facilité de manipulation de l’outil numérique d’un point de vue industriel. Un code de calcul commercial largement utilisé en industrie est donc choisi comme outil numérique. Une méthodologie de Mécanique des Fluides Numériques (MFN) constituée de modèles avancés de turbulence et de combustion jumelés avec un modèle d’allumage sous-maille, est formulé pour prédire les différentes phases de la séquence d’allumage sous différentes conditions d’allumage par temps froid et de rallumage en altitude, ainsi que les propriétés de la flamme en régime stationnaire. Dans un premier temps, l’attention est focalisée sur le régime de combustion stationnaire. Trois méthodologies MFN sont formulées en exploitant trois modèles de turbulence, notamment, le modèle basé sur les équations moyennées de Navier-Stokes instationnaires (URANS), l’adaptation aux échelles de l’écoulement (SAS), et sur la simulation aux grandes échelles (LES). Pour évaluer la pertinence de l’incorporation d’un modèle de chimie détaillée ainsi que celle des effets de chimie hors-équilibre, deux différentes hypothèses sont considérées : l’hypothèse de chimie-infiniment-rapide à travers le modèle d’équilibre-partiel, et l’hypothèse de chimie-finie via le modèle de flammelettes de diffusion. Pour chacune des deux hypothèses, un carburant à une composante, et un autre à deux composantes sont utilisés comme substituts du kérosène (Jet A-1). Les méthodologies MFN résultantes sont appliquées à une chambre de combustion dont l’écoulement est stabilisé par l’effet swirl afin d’évaluer l’aptitude de chacune d’elle à prédire les propriétés de combustion en régime stationnaire. Par la suite, les rapports entre le coût de calcul et la précision des résultats pour les trois méthodologies MFN formulées sont explicitement comparés. La deuxième étude intermédiaire est dédiée au régime de combustion transitoire, notamment à la séquence d’allumage précédant le régime de combustion stationnaire. Un brûleur de combustibles gazeux, muni d’une bougie d’allumage, et dont la flamme est stabilisée par un accroche-flamme, est utilisé pour calibrer le modèle MFN formulé. Ce brûleur, de géométrie relativement simple, peut aider à la compréhension des caractéristiques d’écoulements réactifs complexes, en l’occurrence l’allumabilité et la stabilité. La méthodologie MFN la plus robuste issue de la précédente étude est reconsidérée. Puisque le brûleur fonctionne en mode partiellement pré-mélangé, le modèle de combustion paramétré par la fraction de mélange et la variable de progrès est adopté avec les hypothèses de chimie-infiniment-rapide et de chimie-finie, respectivement à travers le modèle de Bray-Moss-Libby (BML) et un modèle de flammelettes multidimensionnel (FGM). Le modèle d’allumage sous-maille est préalablement ajusté via l’implémentation des propriétés de la flamme considérée. Par la suite, le modèle d’allumage est couplé au solveur LES, puis successivement aux modèles BML et FGM. Pour évaluer les capacités prédictives des méthodologies résultantes, ces dernières sont utilisées pour prédire les évènements d’allumage résultant d’un dépôt d’énergie par étincelles à diverses positions du brûleur, et les résultats sont qualitativement et quantitativement validés en comparant ceux-ci à leurs homologues expérimentaux. Finalement, la méthodologie MFN validée en configuration gazeuse est étendue à la combustion diphasique en la couplant au module de la phase liquide, et en incorporant les propriétés de la flamme de kérosène dans le modèle d’allumage. La méthodologie MFN résultant de cette adaptation, est préalablement appliquée à la chambre de combustion étudiée antérieurement, pour prédire la séquence d’allumage et améliorer les prédictions antérieures des propriétés de la flamme en régime stationnaire. Par la suite, elle est appliquée à une chambre de combustion plus réaliste pour prédire des évènements d’allumage sous différentes conditions d’allumage par temps froid, et de rallumage en altitude. L’aptitude de la nouvelle méthodologie MFN à prédire les deux types d’allumage considérés est mesurée quantitativement et qualitativement en confrontant les résultats des simulations numériques avec les enveloppes d’allumage expérimentales et les images d’une séquence d’allumage enregistrée avec une caméra infrarouge.The development of current and future aero gas turbine engines is mainly focused on the safety, the performance, the energy consumption, and increasingly on the reduction of pollutants and noise level. To this end, the engine’s design phases are subjected to improving processes continuously through experimental and numerical investigations. The present thesis is concerned with the simulation of transient and steady combustion regimes in an aircraft gas turbine operating under various combustion modes. Particular attention is paid to the accuracy of the results, the computational cost, and the ease of handling the numerical tool from an industrial standpoint. Thus, a commercial Computational Fluid Dynamics (CFD) code widely used in industry is selected as the numerical tool. A CFD methodology consisting of its advanced turbulence and combustion models, coupled with a subgrid spark-based ignition model, is formulated with the final goal of predicting the whole ignition sequence under cold start and altitude relight conditions, and the main flame trends in the steady combustion regime. At first, attention is focused on the steady combustion regime. Various CFD methodologies are formulated using three turbulence models, namely, the Unsteady Reynolds-Averaged Navier-Stokes (URANS), the Scale-Adaptive Simulation (SAS), and the Large Eddy Simulation (LES) models. To appraise the relevance of incorporating a realistic chemistry model and chemical non-equilibrium effects, two different assumptions are considered, namely, the infinitely-fast chemistry through the partial equilibrium model, and the finite-rate chemistry through the diffusion flamelet model. For each of the two assumptions, both one-component and two-component fuels are considered as surrogates for kerosene (Jet A-1). The resulting CFD models are applied to a swirl-stabilized combustion chamber to assess their ability to retrieve the spray flow and combustion properties in the steady combustion regime. Subsequently, the ratios between the accuracy of the results and the computational cost of the three CFD methodologies are explicitly compared. The second intermediate study is devoted to the ignition sequence preceding the steady combustion regime. A bluff-body stabilized burner based on gaseous fuel, and employing a spark-based igniter, is considered to calibrate the CFD model formulated. This burner of relatively simple geometry can provide greater understanding of complex reactive flow features, especially with regard to ignitability and stability. The most robust of the CFD methodologies formulated in the previous configuration is reconsidered. As this burner involves a partially-premixed combustion mode, a combustion model based on the mixture fraction-progress variable formulation is adopted with the assumptions of infinitely-fast chemistry and finite-rate chemistry through the Bray-Moss-Libby (BML) and Flamelet Generated Manifold (FGM) models, respectively. The ignition model is first customized by implementing the properties of the flame considered. Thereafter, the customized ignition model is coupled to the LES solver and combustion models based on the two above-listed assumptions. To assess the predictive capabilities of the resulting CFD methodologies, the latter are used to predict ignition events resulting from the spark deposition at various locations of the burner, and the results are quantitatively and qualitatively validated by comparing the latter to their experimental counterparts. Finally, the CFD methodology validated in the gaseous configuration is extended to spray combustion by first coupling the latter to the spray module, and by implementing the flame properties of kerosene in the ignition model. The resulting CFD model is first applied to the swirl-stabilized combustor investigated previously, with the aim of predicting the whole ignition sequence and improving the previous predictions of the combustion properties in the resulting steady regime. Subsequently, the CFD methodology is applied to a scaled can combustor with the aim of predicting ignition events under cold start and altitude relight operating conditions. The ability of the CFD methodology to predict ignition events under the two operating conditions is assessed by contrasting the numerical predictions to the corresponding experimental ignition envelopes. A qualitative validation of the ignition sequence is also done by comparing the numerical ignition sequence to the high-speed camera images of the corresponding ignition event

Advanced flamelet modelling of turbulent non-premixed and partially premixed combustion

Current work focuses on the development and performance evaluation of advanced flamelet models for turbulent non-premixed and partially premixed combustion in RANS and large eddy simulation (LES) based modelling. A RANS-based combustion modelling strategy which has the ability to capture the detailed structure of turbulent non-premixed flames, including the pollutant NO, and account for the effects of radiation heat loss and transient evolution of NO, has been developed and incorporated into the in-house RANS code. The strategy employs an 'enthalpy defect'-based non-adiabatic flamelet model in conjunction with steady or unsteady nonadiabatic flamelets based NO submodels. [Continues.

CFD modelling of gas turbine combustion processes

Stationary gas turbines manufacturers and operators are under constant scrutiny to both reduce environmentally harmful emissions and obtain efficient combustion. Numerical simulations have become an integral part of the development and optimisation of gas turbine combustors. In this thesis work, the gas turbine combustion process is analysed in two parts, a study on air-fuel mixing and turbulent combustion. For computational fluid dynamic analysis work the open-source CFD code OpenFOAM and STAR-CCM+ are used. A fuel jet injected to cross-flowing air flow is simplified air-fuel mixing arrangement, and this problem is analysed numerically in the first part of the thesis using both Reynolds Averaged Navier Stokes (RANS) method and Large Eddy Simulation (LES) methods. Several turbulence models are compared against experimental data in this work, and the complex turbulent vortex structures their effect on mixing field prediction is observed. Furthermore, the numerical methods are extended to study twin jets in cross-flow interaction which is relevant in predicting air-fuel mixing with arrays of fuel injection nozzles. LES methods showed good results by resolving the complex turbulent structures, and the interaction of two jets is also visualised. In this work, all three turbulent combustion regimes non-premixed, premixed, partially premixed are modelled using different combustion models. Hydrogen blended fuels have drawn particular interest recently due to enhanced flame stabilisation, reduced CO2 emissions, and is an alternative method to store energy from renewable energy sources. Therefore, the well known Sydney swirl flame which uses CH4: H2 blended fuel mixture is modelled using the steady laminar flamelet model. This flame has been found challenging to model numerically by previous researchers, and in this work, this problem has been addressed with improved combustion modelling approach with tabulated chemistry. Recognizing that the current and future gas turbine combustors operate on a mixed combustion regime during its full operational cycle, combustion simulations of premixed/partially premixed flames are also performed in this thesis work. Dynamical artificially thickened flame model is implemented in OpenFOAM and validated using propagating and stationary premixed flames. Flamelet Generated Manifold (FGM) methods are used in the modelling of turbulent stratified flames which is a relatively new field of under investigation, and both experimental and numerical analysis is required to understand the physics. The recent experiments of the Cambridge stratified burner are studied using the FGM method in this thesis work, and good agreement is obtained for mixing field and temperature field predictions

Advanced flamelet modelling of turbulent nonpremixed and partialy premixed combustion

Current work focuses on the development and performance evaluation of advanced flamelet models for turbulent non-premixed and partially premixed combustion in RANS and large eddy simulation (LES) based modelling. A RANS based combustion modelling strategy which has the ability to capture the detailed structure of turbulent non-premixed flames, including the pollutant NO, and account for the effects of radiation heat loss and transient evolution of NO, has been developed and incorporated into the in-house RANS code. The strategy employs an 'enthalpy-defect' based non-adiabatic flamelet model in conjunction with steady or unsteady nonadiabatic flamelets based NO submodels. The performanceo f the non-adiabaticm odel and its NO submodelsh asb eena ssessed against experimental measurements and steady flamelet model predictions for turbulent CH4/H2 bluff-body stabilized and CH4-air piloted jet flames. Appreciable improvements in the mean thermal structure predictions have been observed in the piloted jet flames by consideration of radiation heat loss through the non-adiabatic flamelet model. Since transient effects were weaker in the piloted jet flame, both unsteady and steady non-adiabatic NO submodels provided similar level of improvement in the pollutant NO predictions in comparison to their adiabatic counterpartsT. ransiente ffectsw ere, however,d ominanti n the bluff-body flame. The unsteady non-adiabatic NO submodel provided excellent agreement with measured NO distribution in comparison to the appreciably overpredicted distribution by its steadyc ounterpart.T he strategyo f non-adiabaticf lamelet model in conjunctionw ith unsteady non-adiabatic NO submodel seems to provide an accurate and robust alternative to the conventional strategy of steady flamelet model with steady NO submodel. While addressing the limitations of steady flamelet model in regard to radiation and slow chemistry of NO is one objective of this research, extending the applicability of the model to partially premixed combustion has been pursued as the second objective. Flamelet/progress variable (FPV) approach based combustion models, which have the potential to describe both non-premixed and partially premixed combustion, have been incorporated in the in-house RANS and LES codes. Based on the form of the PDF for reaction progress variable, two different formulations, FPV-8 function model and FPV-P function model, have been derived. (Continues...).EThOS - Electronic Theses Online ServiceGBUnited Kingdo

LES modelling of non-premixed and partially premixed turbulent flames

A large eddy simulation (LES) model has been developed and validated for turbulent non-premixed and partially premixed combustion systems. LES based combustion modelling strategy has the ability to capture the detailed structure of turbulent flames and account for the effects of radiation heat loss. Effects of radiation heat loss is modelled by employing an enthalpy-defect based non-adiabatic flamelet model (NAFM) in conjunction with a steady non-adiabatic flamelet approach. The steady laminar flamelet model (SLFM) is used with multiple flamelet solutions through the development of pre-integrated look up tables. The performance of the non-adiabatic model is assessed against experimental measurements of turbulent CH4/H2 bluff-body stabilized and swirl stabilized jet flames carried out by the University of Sydney combustion group. Significant enhancements in the predictions of mean thermal structure have been observed with both bluff body and swirl stabilized flames by the consideration of radiation heat loss through the non-adiabatic flamelet model. In particular, mass fractions of product species like CO2 and H2O have been improved with the consideration of radiation heat loss. From the Sydney University data the HM3e flame was also investigated with SLFM using multiple flamelet strategy and reasonably fair amount of success has been achieved. In this work, unsteady flamelet/progress variable (UFPV) approach based combustion model which has the potential to describe both non-premixed and partially premixed combustion, has been developed and incorporated in an in-house LES code. The probability density function (PDF) for reaction progress variable and scalar dissipation rate is assumed to follow a delta distribution while mixture fraction takes the shape of a beta PDF. The performance of the developed model in predicting the thermal structure of a partially premixed lifted turbulent jet flame in vitiated co-flow has been evaluated. The UFPV model has been found to successfully predict the flame lift-off, in contrast SLFM results in a false attached flame. The mean lift-off height is however over-predicted by UFPV-δ function model by ~20% for methane based flame and under-predicted by ~50% for hydrogen based flame. The form of the PDF for the reaction progress variable and inclusion of a scalar dissipation rate thus seems to have a strong influence on the predictions of gross characteristics of the flame. Inclusion of scalar dissipation rate in the calculations appears to be successful in predicting the flame extinction and re-ignition phenomena. The beta PDF distribution for the reaction progress variable would be a true prospect for extending the current simulation to predict the flame characteristics to a higher degree.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Modeling lifted jet flames in a heated coflow using an optimized Eddy dissipation concept model

Moderate or intense low oxygen dilution (MILD) combustion has been established as a combustion regime with improved thermal efficiency and decreased pollutant emissions, including NOx and soot. MILD combustion has been the subject of numerous experimental studies, and presents a challenge for computational modeling due to the strong turbulence–chemistry coupling within the homogeneous reaction zone. Models of flames in the jet in hot coflow (JHC) burner have typically had limited success using the eddy dissipation concept (EDC) combustion model, which incorporates finite-rate kinetics at low computational expense. A modified EDC model is presented, which successfully simulates an ethylene-nitrogen flame in a 9% O2 coflow. It is found by means of a systematic study in which adjusting the parameters and from the default 0.4082 and 2.1377 to 3.0 and 1.0 gives significantly improved performance of the EDC model under these conditions. This modified EDC model has subsequently been applied to other ethylene- and methane-based fuel jets in a range of coflow oxidant stream conditions. The modified EDC offers results comparable to the more sophisticated, and computationally expensive, transport probability density function (PDF) approach. The optimized EDC models give better agreement with experimental measurements of temperature, hydroxyl (OH), and formaldehyde (CH2O) profiles. The visual boundary of a chosen flame is subsequently defined using a kinetic mechanism for OH* and CH*, showing good agreement with experimental observations. This model also appears more robust to variations in the fuel jet inlet temperature and turbulence intensity than the standard EDC model trialed in previous studies. The sensitivity of the newly modified model to the chemical composition of the heated coflow boundary also demonstrates robustness and qualitative agreement with previous works. The presented modified EDC model offers improved agreement with experimental data profiles than has been achieved previously, and offers a viable alternative to significantly more computationally expensive modeling methods for lifted flames in a heated and vitiated coflow. Finally, the visually lifted flame behavior observed experimentally in this configuration is replicated, a phenomenon that has not been successfully reproduced using the EDC model in the past.M. J. Evans, P. R. Medwell & Z. F. Tian

Modelling of turbulent flames with transported probability density function and rate-controlled constrained equilibrium methods

In this study, turbulent diffusion flames have been modelled using the Transported Probability Density Function (PDF) method and chemistry reduction with the Rate-Controlled Constrained Equilibrium (RCCE). RCCE is a systematic method of chemistry reduction which is employed to simulate the evolution of the chemical composition with a reduced number of species. It is based on the principle of chemical time-scale separation and is formulated in a generalised and systematic manner that allows a reduced mechanism to be derived given a set of constraint species. The transported scalar PDF method was coupled with RANS turbulence modelling and this PDF-RANS methodology was exploited to simulate several turbulent diffusion flames with detailed and RCCE-reduced chemistry. The phenomena of extinction and reignition, soot formation and thermal radiation in these flames are explored. Sandia Flames D, E and F have been simulated with both the detailed GRI-3.0 mechanism and RCCE reduced mechanisms. Scatter plots show that PDF methods with simple mixing models are able to reproduce different degrees of local extinction in Sandia piloted flames. The PDF-RCCE results are compared with PDF simulations with the detailed mechanism and with measurements of Sandia flames. The RCCE method predicted the three flames with the same level of accuracy of the detailed mechanism. The methodology has also been applied to sooting flames with radiative heat transfer. Semi-empirical soot model and Optically-thin radiation model have been combined with the PDF-RCCE method to compute these flames. Methane flames measured by Brooks and Moss [26] have been predicted using several RCCE mechanisms with good agreement with measurements. The propane flame with preheated air [162] has also been simulated with the PDF-RCCE methodology. Gaseous species profiles of the propane flame compare reasonably with measurements but soot and temperature predictions in this flame were weak and improvements are still needed.Open Acces