Study On Flamelet Model For PAHs And Soot Formation In Gas-solid Two-phase Combustion

Polycyclic aromatic hydrocarbons(PAHs)and soot emissions from coal combustion are harmful to human health and environment.Thus,it is instructive to investigate the formation characteristics of PAHs and soot for the development of clean and efficient combustion technology.In numerical simulation,detailed information of PAHs and soot formation can be provided,which is helpful to analyze PAHs and soot formation characteristics and improve the understanding of PAHs and soot formation mechanism.Therefore,in this paper,detailed chemistry simulation is firstly performed to investigate the effects of strain rate,mass flow rate of pulverized coal and char oxidation on the formation of PAHs in pulverized coal flame.It is found that PAHs are mainly formed in the combustion stage of volatile matter,and with the mass flow increasing and the strain rate decreasing,the peak concentration of PAHs increases.In addition,the char off-gas can inhibit the formation of PAHs.Then,a high-fidelity soot formation simulation method is constructed in this paper,to investigate soot formation characteristics in pulverized coal combustion.In this method,PAHs formation is predicted by flamelet model,to avoid the huge computational cost that is caused by the direct use of detailed chemical reaction mechanism.However,the traditional flamelet model cannot accurately predict the formation of PAHs,and the improved models show some shortcomings.To this end,a dual-scale flamelet/progress variable(DS-FPV)model is developed,which can accurately predict the formation of PAHs in a simple and efficient way.In this model,a novel progress variable CPAHs is introduced,which is used to specifically describe the evolution of PAHs.Through the validation in different flames,it is demonstrated that the developed DSFPV model can significantly improve the performance of the traditional FPV model in the prediction of PAHs formation.Since semi-empirical model cannot accurately describe the process of soot formation,the population balance equation of soot particle number density function is solved to achieve the high-fidelity prediction of soot formation,and it is coupled with the DSFPV model to form a high-fidelity soot formation simulation method.To orderly validate the accuracy of the DS-FPV model coupled with soot formation model,the model validation is firstly conducted in gas-phase flame.Compared with the experimental results of a turbulent ethylene/air diffusion flame,it is found that the distribution trend and the peak value of soot volume fraction can be accurately predicted.Before extending the above coupled model to pulverized coal combustion,the influence of devolatilization on the flamelet structure of pulverized coal combustion is firstly studied,and a two-phase coal flamelet/progress variable(C-FPV)model is developed to consider the influence of devolatilization.Then,the performance of the C-FPV model is evaluated in a mixing layer flame of pulverized coal.The results show that the C-FPV model can better predict the distribution trend and peak value of volatile matter and intermediate species compared with the FPV model.It is also found that the FPV model cannot reasonably predict the temperature of carrier gas,because the traditional mixture fraction Zvol is zero at both the fuel side and the oxidizer side,and it cannot distinguish the oxidizer side and the fuel side.Finally,the DS-FPV model coupled with soot formation model,which has been validated in gas-phase flame,is extended to pulverized coal combustion,to predict the formation of PAHs and soot in a turbulent jet flame of pulverized coal.The results show that in the upstream region,the distribution trend of soot volume fraction can be well predicted,and the peak position of soot volume fraction is also in good agreement with the experimental results.Based on the simulation results,the formation characteristics of PAHs and soot is analyzed.It is found that the distribution of PAHs shows an annular structure in the upstream region and merges together in the downstream region.Compared with laminar condition,the distribution of PAHs is intermittent in turbulent condition.The distribution of soot particle number density is controlled by the soot nucleation source term,and the distribution of soot volume fraction is mainly controlled by surface growth and OH oxidation source terms.