Subgrid Scale Modelling And Its Application For Large Eddy Simulation Of Turbulent Premixed/Stratified Combustion

Considering performance requirements for low pollutant emissions in combustion chamber,fuel-lean premixed combustion is of increasing importance in many industrial applications(such as land based gas turbines and aero-engines).Compared with turbulent non-premixed combustion,turbulent fuel-lean premixed combustion plays a predominant role in reducing NOx emission due to the low peak temperature.Space and time requirements on such combustion devices may pose constraints that influence the premixing of the fuel and oxidiser,resulting in an inhomogeneous mixture prior to combustion;this may lead to combustion taking place under stratified conditions.To predict the complex flow structure like swirling flow,the occurrence of unsteady phenomena,and the complex interaction between turbulence and flame occurring within many combustion devices,large eddy simulation(LES)has been proved to be a suitable and widely-used tool.Due to the mutual coupling of non-linear multi-scale turbulence and multi-scale chemical reactions makes accurately modelling filtered reaction source terms and solving the species transport equations a significant challenge for LES of turbulent combustion.In the present study several sub-grid scale(SGS)models are developed with the scope of turbulent premixed and stratified combustion,and the proposed SGS models in conjunction with LES are then employed for the simulations of typical turbulent flames consisting of different flow conditions,different flame stabilization regimes and different combustion regimes.The main work and innovation are as follows.Firstly,LES is performed to study the non-reacting flow fields of a Cambridge swirl burner.The LES statistical results are in reasonable agreement with experimental data,and the accuracy of the numerical method is verified.The vortex structures near the shear layers,visualized by the iso-surface of Q-criterion,display ring structures in non-swirling flow and helical structures in swirling flow near the burner exit.Spectral analysis is employed to predict the occurrence of flow oscillations induced by vortex shedding and precessing vortex core(PVC).A three-dimensional proper orthogonal decomposition(POD)method is developed to extract the unsteady large-scale structures in swirling flow accurately.The POD analysis reveals that flow fields contain several flow instabilities associated with the PVC,vortex shedding and bluff-body recirculation zone.Secondly,a method of detailed chemistry tabulation combined with presumed probability density function(PDF)is applied to simulate piloted premixed jet burner flames with high Karlovitz number using LES.Thermo-chemistry states are tabulated by the combination of auto-ignition and premixed flame propagation model.To evaluate the predictive capability of the proposed tabulation method to represent the thermo-chemistry states under the condition of different fresh gases temperatures,a priori study is conducted by performing idealized transient one-dimensional laminar premixed flame simulations.Presumed PDF is used to involve the interaction of turbulence and flame with Dirichlet distribution to represent the interaction between two mixture fractions that are associated with three inlet streams.The overall LES results are evaluated with respect to the experiments in terms of the predictive capability of different chemistry tabulation approaches,and the influence of different presumed PDF models.Therewith,the flow structures and flame structures of the piloted premixed jet burner flames with high Karlovitz number are analyzed qualitatively.Thirdly,a SGS model by combining dynamic thickened flame(DTF)with flamelet generated manifolds(FGM)tabulation approach(i.e.DTF-FGM)is developed for investigating turbulent premixed and stratified combustion under thin reaction zone regime.The dynamic thickening factor of the DTF model is determined from the flame sensor,which is obtained from the normalized gradient of the reaction progress variable.Therewith the DTF model can ensure that the thickening of the flame is limited to the regions where it is numerically necessary.To describe the thermo-chemistry states,LES transport equations for two characteristic scalars(the mixture fraction and the reaction progress variable)and relevant sub-grid variances in the DTF-FGM model are presented.For turbulent stratified flames,the statistical correlation between the mixture fraction and the reaction progress variable is considered using a copula method,and the LES transport equation for covariance in the DTF-FGM model is deduced.As to the evaluation of different SGS models,another model by utilizing the combination of presumed probability density function(PPDF)and FGM(i.e.PPDF-FGM)is also described.LES of a variety of cases with or without swirl in premixed and stratified regimes of the Cambridge swirl burner flames are performed to evaluate the developed SGS combustion models.Based on the simulation results of turbulent premixed flames,sensitivity analyses of important parameters in the DTF-FGM model,such as wrinkling factor,thickening factor,and sub-grid variance model,are carried out.The ability of DTF-FGM and PPDF-FGM models to describe turbulent premixed and stratified flame structures under thin reaction zone is evaluated.Therewith,the stratified flame structures are studied quantitatively by using the PDF distribution of the equivalence ratio,and the PDF distribution of the alignment angle between the gradients of equivalence ratio and reaction progress variable.Finally,the SGS model by utilizing the combination of detailed chemistry tabulation and presumed PDF is also applied to study a turbulent lifted jet flame in vitiated co-flow.Thermo-chemistry states are tabulated by combining auto-ignition model with premixed flame propagation model.Meanwhile,the joint PDF distributions of the mixture fraction and the reaction progress variable,including their statistical correlation,are obtained using a copula method.Afterwards,the LES transport equations for characteristic scalars,sub-grid variances and covariance are presented.Using available instantaneous species and temperature measurements,a prior tests are performed to estimate the predictive capability of different chemistry tabulation approaches,and the influence of different presumed PDF distributions.The flame stabilization regimes and combustion regimes of the turbulent lifted jet flame in vitiated co-flow are discussed by using a simplified formula for turbulent flame propagation speed,and performing a hybrid flamelet combustion regimes analysis.