Investigation On The Counter Gradient Transport Phenomenon In Trubulent Combustion Under The Action Of Multiple Gradient Coupling

Counter gradient transport(CGT)phenomenon widely exists in turbulent flames and actual combustion,but the kinetic mechanism of CGT is not clear so for.However,the CGT phenomenon is directly related to the establishment,validation and evaluation of turbulent model and combustion model.The CGT phenomenon is also very important for the research and control of various types of turbulent flames.The present paper focuses on the CGT phenomenon in turbulent combustion.Through numerical simulation and theoretical analysis,the formation mechanism and influencing factors of CGT phenomenon were explored.The main work and conclusion are as follows:Through numerical simulation,the effectiveness of various types of turbulent model and combustion model were evaluated.Based on gradient transport assumption,traditional models do not distinguish the anisotropic large eddy vortex and isotropic small eddy vortex,thus can not capture the CGT phenomenon.By introducing the transport equation of Reynolds stress,the RSM model avoids the simple linear modeling of Reynolds stress,and can simulate the CGT phenomenon well.The TFC combustion model considers the effect of turbulent mixing and thermal expansion,and can reproduce the CGT phenomenon in premixed combustion.The LES-EDC model distinguishes large and small vortex,and can be used to simulate the CGT phenomenon in non-premixed combustion.By using RSM model,the present paper simulated the channel flow with an airfoil to explore the formation mechanism and influencing factorsof CGT phenomenon in turbulent flows.The results show that the effect of multi-scale turbulence and the asymmetric distribution of mean velocity are the important cause of the CGT phenomenon.The CGT phenomenon is often associated with the negative rate of turbulence kinetic energy.However,in the CGT region,the turbulence kinetic energy can be either positive or negative,they are not the same.Reynolds number has little influence on the CGT region.Pressure gradient affects the CGT region,and the favorable pressure gradient increases the CGT region.The TFC combustion model was used to simulate the Moreau burner through RANS and LES approaches,and the formation mechanism and influencing factors of CGT phenomenon in premixed flames were investigated.The results show that the CGT in premixed flames is related to the different acceleration of heavy reactants and light products caused by thermal expansion.In the initial stage of the Moreau burner,gradient-transport(GT)phenomenon occurs.In the early stage of the basic segment,the GT and CGT phenomenon coexist.The CGT phenomenon dominates the later part of the basic segment.The CGT phenomenon is more obvious when the velocity gradient of the incoming flow is larger.However,the pressure gradient and temperature gradient of the incoming flow have little effect on the CGT phenomenon.Numerical simulation of non-premixed combustion requires chemical kinetic mechanisms.The large scale detailed mechanism will introduce the problems of large computational cost and stiffness.Traditional reduction method can not balance between accuracy and computational amount,the combined reduced and optimized mechanism(CROM)was proposed in the present paper to solve this.Two mechanisms,including high temperature mechanism(T>1100K)and low temperature mechanism(T>650K),have been successfully constructed for RP-3 aviation kerosene using CROM.The obtained optimized mechanism owns high accuracy,wide operating conditions and moderate scale,which is of great application value.According to the mechanism of methane Gri-mech3.0,CROM was used to construct the 14 component,18 steps optimized mechanism.The obtained optimized mechanism was used in non-premixed combustion simulation,and achieved good results.The piloted methane-air jet flames(Flames-D)was simulated using LES-EDC approach to explore the formation mechanism and influencing factors of CGT phenomenon in non-premixed combustion.The results show that chemical reactions occur drastically in the transition zone of the central axis,and the different acceleration effect due to thermal expansion causes the CGT phenomenon.In the shear layer,turbulent flow,species diffusion and heat release are strongly coupled.The GT caused by turbulent mixing interacts with the CGT caused by thermal expansion,which causes the CGT phenomenon to exist intermittently in the the shear layer.The CGT phenomenon has become a research hot spot and critical issue because of its important application background and complicated transport mechanism.The research achievements of present paper give some advice to the improving or even proposing new turbulent model and combustion model.