Sub-grid Scale Model And Flamelet Model For Pulverized Coal Combustion

As large-scale computational power has become more widely available,computational fluid dynamics(CFD)has become an important tool to describe the pulverized coal combustion process.However,the capability of the pulverized coal combustion submodels used nowadays is low and its prediction accuracy is dependent on the researchers' experiences.The purpose of the present work is to develop high-fidelity sub-grid scale(SGS)models and gas combustion models to improve the prediction accuracy of pulverized coal combustion.The gas combustion models developed in the framework of both laminar/direct numerical simulation(DNS)and large-eddy simulation(LES)include a two-mixture fraction flamelet model,a three-mixture fraction/three-stream flamelet model,a multi-regime flamelet model,dynamic artificially thickened flame model,and the finite rate detailed chemistry model for model validation.The computational setup for the pulverized coal flame include two-dimensional(2D)laminar counterflow flame,3D turbulent counterflow flame,turbulent flame jet in cross-flow(JICF),piloted jet flame,swirled-stabilized flame,jet flame near a planar wall,etc.To consider the SGS effects of the turbulence velocity and scalar fluctuation of the gas flow on the coal particles,a velocity-scalar joint filtered density function(VSJFDF)model is developed.The performance of the VSJFDF model is evaluated by simulating an isopropyl alcohol spray and a piloted pulverized coal jet flame.The simulation results are compared to the experimental data,and the results from the non-VSJFDF model(no fluctuation is considered)and VFDF model(only velocity fluctuation is considered).The results show that compared to the non-VSJFDF model and the VFDF model,VSJFDF model can improve the prediction accuracy of thermo-chemical variables.In addition,it is found the VSJFDF model can not only describe the SGS effects on the particle behaviors,but also can represent the coupling interaction between the large scale flow fields,SGS flow field and the particle motion.To validate the two-mixture fraction flamelet model for laminar pulverized coal combustion,2D counterflow flames operated under various conditions are simulated with detailed chemistry(DC).The suitability of the flamelet chemtable and trajectory variables specifically for pulver-ized coal combustion is together evaluated.The two-mixture fraction flamelet model is evaluated through both a priori and a posterori tests.Compared to the DC solutions,it is found that the stable species and gas temperature can be correctly predicted by the flamelet model,while the intermedi-ate species such as CO and H2 in the premixed flame reaction zone cannot be correctly predicted by the diffusion-flame based flamelet model.It is also found that the operating conditions influ-ence the prediction capability of the flamelet model.The prediction accuracy deteriorates as the strain rate increases.To validate the three-mixture fraction flamelet model for laminar pulverized coal combustion,a 2D counterflow pulverized coal/methane flame and a piloted pulverized coal jet flame are simulated.The flamelet model is evaluated in an a priori sense.Compared to the DC solutions,it is found that for the counterflow flame,the stable species and gas temperature can be correctly predicted while the intermediate species in the premixed flame reaction zone can-not,while for the piloted pulverized coal jet flame,although the overall agreement is achieved,both the stable and intermediate species in the downstream cannot be predicted due to the mul-tidimensionality of the flamelet.To validate the two-mixture fraction flamelet model for LES of pulverized coal combustion,a laboratory-scale pulverized coal flame is simulated.The simulation results are compared to the experimental data and the results from the infinitely fast chemistry EBU model.The comparisons show that the flamelet predictions agree with the experimental data,and the flamelet model performs much better than the EBU model on prediction of the species mass fractions.To validate the three-stream flamelet model,a semi-industrial scale swirl-stabilized pul-verized coal flame is simulated.The simulation results are compared to the experimental data and the results from the EBU model in the literature.The comparisons show that the flamelet model performs better then the EBU model on prediction of gas temperature and species mass fractions.Although the flamelet predictions agree with the experimental data in the upstream,discrepancies can be observed in the downstream.To validate the multi-regime flamelet model,a laboratory-scale pulverized coal flame is simulated.The simulation results are compared to the results from the diffusion-flame based flamelet model and the EBU model.It is found that the multi-regime flamelet model performs better than the diffusion-flame based flamelet model in the region where premixed combustion is dominant.The EBU model cannot give correct predictions for the species mass fractions.To further extend the flamelet model and understand the pulverized coal combustion charac-teristics,we investigate the 3D pulverized coal flame in detail under various operating conditions.The results show that the radiative heat transfer,coal mass flow rate and strain rate influence the pulverized coal flame structure significantly.We also find that the oxidizer temperature in the com-bustion system can be varied due to interphase heat transfer,which indicates the flamelet model pro-posed above with fixed value of oxidizer temperature can be improved.According to the findings from the 3D counterflow flame results,a new flamelet model is developed based on the assump-tion of thermo-equilibrium assumption in which the oxidizer temperature is varied in the flamelet chemtable.The validity of the new flamelet model is evaluated on different configurations,and its performance is compared to the two-mixture fraction flamelet model and three-stream flamelet model described above.It is found that compared to the two-mixture fraction flamelet model,the prediction of the gas temperature and species mass fractions can be significantly improved by the new flamelet model;compared to the three-stream flamelet model,the new flamelet model can im-prove the prediction accuracy of peak gas temperature without introducing new trajectory variable.The particle diameter and the wall effect on the performance of the flamelet model are also inves-tigated.The results show that the particle size influence the pulverized coal structure significantly and the performance of the flamelet model.Although the major thermo-chemical variables can be correctly predicted by the flamelet model,the fuel species in the volatile matter and char off-gases near the wall show discrepancies with the reference results.Finally,LES of a pulverized coal flame jet in cross-flow is conducted with the new flamelet model.The influences of the flame-flame interactions and flame-wall interactions on the pulverized coal combustion characteristics and thermo-chemical quantities distributions are investigated in detail.The results show that the overall flame temperature in the twin jets in cross-flow(TJICF)is higher than that in the single jet in cross-flow(SJICF).The particle trajectory does not only depend on the momentum ratio of the main jet and the cross-flow,but also on the flame-flame interactions.The findings reported in the present work can provide valuable guidance for the design and safe operation of the pulverized coal boilers.