Limiting Mechanism Of Reactive Shock Bubble Interaction

High performance of combustion and mixing is the critical technique in the sramjet en-gine.Mixing enhancement is usually obtained by the streamwise vortex formed from the strong interaction between shock and fuel through increasing the strength of shock in the supersonic flows.To answer the shock effect on mixing enhancement,the supersonic vortex formed from shock helium bubble interaction with wide range of shock strength（from Ma=1.22 to Ma=4）is numerically studied through the growth of mixedness and scalar dissipation（SD for short）.After shock passage,it is found that the formation of mixing from two kinds of mixing indi-cators has two stages.The first stage is the mixing growing stage,the growth rate of which is found proportional to the main circulation due to the stirring effect,which shows the larger shock strength leading to the higher circulation and quicker mixing process.The second stage is limiting mixing stage with no more mixing growth.It is interesting to find that the second stage of limiting mixing shows that the higher shock strength shows less mixing at final stage.The reason of limiting mixing is found closely related to the production of the secondary baroclinic vorticity（SBV）mainly causing the stagnation of vortex motion and mixing,which is defined as baroclinicity stagnation effect.The maximum baroclinic circulation for the first time which is defined as the characteristic time t_barobased on which all cases show that when t≈2t_baro,the limiting mixing happens.From the limiting mixing mechanism,a new circulation model for SBI is derived and validated.The circulation model gives the direct indication that t_barois closely related to main circulation,which offers the ideal mixing enhancer design.Then,this study presents the numerical investigations on the interaction of a hydrogen bubble with an incident shock wave（Ma=1.22）.The 2D reactive Navier-Stokes equations with detailed chemical mechanisms are used to simulate the reactive shock bubble interaction.The influences of combustion on the unsteady flow structure and vorticity are analyzed by comparing the reacting and inert cases.The combustion process is divided into three stages according to the Damkohler number,which is calculated using a new method for this non-premixed combustion.Numerical results indicate that the combustion efficiency of the pure hydrogen at the final time of 500μs is only 60%,and most of the fuel is wrapped in the core of the main vortex surrounded by a thick flame wall structure.The original part of the unburned hydrogen in the initial bubble and corresponding evolution is confirmed through the Lagrangian particle technique.Based on these analysis,the concentric and eccentric cases with high combustion efficiencies are both presented.Compared with pure hydrogen,the combustion efficiencies of concentric and eccen-tric cases increase by 16%and 21%respectively,at the final time.The detailed reasons for the high combustion performance are also discussed.