Research On Turbulent Combustion Model Of Large Gradient Shear-Mixing Layer

In order to improve the numerical simulation accuracy of the large gradient shear-mixing layer combustion flow in the rocket-based combined cycle(RBCC)engine,a flamelet time scale model was established based on the steady laminar flamelet model,which provides a reliable basis for engine performance prediction.The outstanding advantage of the steady laminar flamelet model is the decoupling of the turbulent flow and the chemical reaction,so that they could be solved separately.However,for the relatively slow chemical reaction zones in large gradient shear-mixing layer of hydrocarbon liquid fuels combustion,the limitations of SLF Model are more prominent.In order to study the influence of unsteady chemical kinetics on laminar diffusion flame,the paper used the FlameMaster to solve the kerosene/air unsteady laminar counter-flow flame and compare the result with the steady laminar counter-flame.The comparison results shown that,in the initial stage of the reaction,the former mainly carried out the decomposition reaction of the large molecular weight component in aviation kerosene,and then the rapid chemical reaction of the small molecular weight component was started.After reaching the chemical equilibrium,the mass fraction of the reaction product of the unsteady laminar diffusion flame is lower than the steady solution,and the mass fraction distribution of the intermediate component is wider.These results shown that,even considering the effect of finite chemical reaction rate on combustion,the governing equation of steady laminar diffusion flame still overestimates the chemical reaction rate and the mass fraction of the reaction product.In order to improve the accuracy of the prediction of the combustion in large gradient shear-mixing layer,the flamelet time scale model has been established.In the flamelet time scale model,the influence of unsteady chemical kinetic factors on turbulent combustion has been considered,and the advantages of steady laminar flamelet model have been preserved.The basic idea of the flamelet time scale model is that,turbulent diffusion combustion consists of turbulent flow and unsteady laminar diffusion flames which are embedded in the flow field.The interaction of turbulence and flame can be described as,the turbulent vortexes,which are larger than the characteristic size of the chemical reaction zone,is a random transport to the unsteady laminar flame surface.And the turbulent vortex can be deepened into the reaction zone to slow down the chemical reaction rate,and the turbulent combustion is controlled by chemical kinetic factors.In this paper,the solver of the flamelet time scale model was written in the open source CFD solution package,OpenFOAM.The large gradient shear-mixing layer combustion of RBCC engine combustion chamber was taken as the research object,and the RANS method was used to verify the calculation accuracy of the flamelet time scale model.The results show that the flamelet time scale model can simulate the complex chemical reaction process inside the RBCC engine.The calculation accuracy of the flamelet time scale model established in this paper is better than the steady laminar flamelet model,which can control the calculation error within 15%.