On The Excitation Of Flame Flashback And Flame Propagation Mechanism In Supersonic Flow

In the present study,the flame propagation in scramjet is taken as the research object.Combined with experimental observation,numerical simulation and theoretical analysis,the excitation of flame flashback and flame propagation mechanism in supersonic flow are systematically studied.First,the numerical schemes used in numerical simulation have been investigated.A standardized procedure for quantitative evaluation of shock-capturing schemes(including empirical dispersion relation,computational efficiency,shock-capturing error)has been developed,which is intended to go beyond traditional case-by-case analysis,by setting objective metrics for cross-comparison of flow solvers.Appling the above the systematic framework for the evaluation of shock sensors in hybrid compact/WENO algorithms.A new shock sensor which can be easily implemented in existing WENO-based codes has been also developed.Finally,EWENO and EWCNS schemes are developed based on the new shock sensor to achieve higher accuracy,computational efficiency and numerical robustness in consideration of the shock capture capability.Secondly,the turbulent combustion flamelet model is improved.The inherent flaws of the traditional flamelet model will lead to non-physical solution.The Flamelet/Progress Variable(FPV)Model was established for considering premixed combustion.The advantages of new model were verified from the combustion flow pattern and quantitative temperature distribution curve.An improved flamelet model has been developed considering the effect of nonhomogeneous of pressure in a supersonic combustor.The new model can not only ensure the accuracy of the calculation,but also effectively reduce the capacity of flamelet database and greatly reduce the computational costs.Finally,through the experimental study,numerical simulation and theoretical analysis,the excitation of flame flashback and flame propagation mechanism inside an ethylene-fueled scramjet were systematically investigated.Concentrate on the effects of injection and cavity parameters,the internal mechanism of flame flashback is summarized as follows :(1)the expansion of low-speed separation region downstream of the cavity provides favorable conditions for violent combustion;(2)the high temperature,high pressure and low speed regions formed downstream of the jet bow shock wave are conducive to combustion enhancement;(3)the longer fuel residence time improved the mixing of fuel and air,providing assurance for high-intensity combustion;(4)the large length-to-depth ratio promoted the mass and heat exchange between the cavity shear layer and the main flow;(5)the impact shock wave formed by the trailing edge of the cavity acts on the shear layer of the cavity and lifted it up,which is similar to the separation of the boundary layer;(6)nitrogen injection near the cavity formed a strong adverse pressure gradient,which provided a driving force for the flame flashback.The strong interaction between boundary layer and flame,which were observed in high-speed photography and schlieren images,leaded to the gradual separation of boundary layer and the formation of thermodynamic throat,causing mainstream congestion and inducing flame flashback.On this basis,the large-scale numerical simulation was carried out,revealing that the boundary layer downstream of the cavity is a sensitive area of flame flashback,and temperature fluctuation is a sensitive parameter of flame flashback.Many factors can induce flame flashback,such as increasing the turbulent boundary layer,increasing the thermodynamic disturbance,and improving the local mixing efficiency downstream of the cavity.Under the condition of these factors,the strong interaction between the boundary layer and the flame caused the boundary layer to gradually separate,thereby forming a thermodynamic throat and inducing a flashback phenomenon.The theoretical model of flame flashback was established.By analyzing the effects of three dimensionless parameters,the internal mechanism of induced flame flashback in experimental and numerical simulations is effectively explained: initial temperature fluctuations,disturbance dwell time and total heat release in the system can significantly affect the stability of the combustion system.When the range of thermodynamic disturbance changes exceeds a certain threshold,the small disturbance is exponentially amplified to finally induce the flame flashback phenomenon.