Numerical Investigation Of Combustion In Three Dimensional Spatially Developing Supersonic Mixing Layers

Combustion in supersonic mixing layer is a typical flow and combustion pattern in combustion chamber of scramjet engines.As a result,direct numerical simulation of combustion in three dimensional spatially developing supersonic mixing layers,as well as understanding the evolution of vortex,shock-wave and flame structure and the variation of mixing and combustion efficiency in supersonic mixing layer are of great scientific significance and broad practical applications.The main research work and corresponding conclusions include:A compatible method for setting inlet condition of supersonic reacting mixing layer is proposed.Based on multi-component NS equations which couple with detailed reaction mechanisms and fifth-order compact-WENO hybrid scheme which could resolve both vortex and shock-wave,direct numerical simulations of combustion in three dimensional spatially developing air/hydrogen supersonic mixing layers are realized.The verification computation demonstrates that the predicted growth rate of shear layer and turbulent stresses are consistent with the results in literatures.The instantaneous characteristics of flow,thermodynamic and component parameters of supersonic mixing layer under various incoming air freestream temperatures are investigated,vortex,shock-wave and flame structures are obtained,and overall analysis are conducted.The results demonstrate that as the incoming air freestream temperature rises,the supersonic mixing layer experiences transition between four combustion patterns,they are incombustible,premixed combustion near outlet,premixed combustion with large region plus diffusion combustion along the interface between shear layer and air freestream,and diffusion combustion plus premixed combustion with small region.The region and intense of heat release are different for four combustion patterns,so different shock-wave structures are induced,as a result,the spatial distributions of flow,thermodynamic and component parameters are also different.The evolution of vortex,flame and shock-wave structures of supersonic mixing layer under 1319 K incoming air freestream temperature are investigated.The results demonstrate that the evolution of vortex experiences spanwise vortex,(43)vortex,hairpin vortex and slender vortex.The evolution of flame experiences local ignition,convection and deformation and propagation of initial ignition region,and local extinction.The evolution of shock wave undergoes a scratch,from weak to strong the development process;oblique shock-wave appears first and then normal shock-wave appears;after the whole shock-wave system forms,the streamwise position of shock-wave surface oscillates with time.The first-order moments of velocity,temperature,pressure,density and component mass fraction,the correlation functions of fluctuating velocity-velocity,component mass fraction-component mass fraction,temperature-velocities and temperature-component mass fractions,as well as the streamwise variation of vorticity thickness,mixing efficiency and combustion efficiency are obtained.The results demonstrate that the distribution of the first-order and second-order moments relates to combustion patterns,heat release and the shock-wave system.The vorticity thickness and combustion efficiency are affected mainly by combustion pattern,while the mixing efficiency is affected mainly by incoming freestream condition and heat release.Understanding the instantaneous,time-averaged and macroscopic characteristics of combustion in supersonic mixing layer is of great importance for realizing efficient mixing,reliable ignition and stable combustion in combustion chamber of scramjet engines.