Combustion Stabilization Mode In The Scramjet Under High Stagnation Temperature

There have been considerable interests in the characteristics of the flow,mixing and combustion in the scramjet engine in recent decades.Based on the experiments,numerical simulation and theoretical analysis,this study is focused on the combustion stabilization mode and transition mechanism under high stagnation temperature.The NPLS(Nano-particle Planar Laser Scattering)technique and the large eddy simulation were used to investigate the mixing process.The effects of the injection distance,the momentum ratio,and the injection schemes were analyzed.With long injection distance,the interactions of the jet and the cavity are greatly reduced.The K-H(Kelvin-Helmholtz)instability dominates the development of the vortex structures in the cavity shear layer.With a shorter injection distance,the vortex structures in the cavity shear layer is affected by the counter-rotating vortex pair in the jet.Increasing the injection distance and the jet-to-air momentum ratio will improve the distribution of local equivalence ratio in the cavity.Under the same global equivalence ratio,compared to the single injection,three parallel injections will enhance the spanwise spreading of the fuel,meanwhile increase the fuel entrainment into the cavity.The fuel mass exchange rate between the cavity and the jet will also be increased.The combustion stabilization modes under high stagnation temperature were experimentally studied.Increasing the equivalence ratio,the flame transfers from the cavity stabilized mode to the jet-wake stabilized mode in the scramjet operation.When fuel is parallel injected,the reduction in the injection distance could enhance the flame intensity and reduces the critical equivalence ratio for the mode transition.With a single injector,the transition of the combustion stabilization mode highly depends on the jetcavity interactions and the distribution of equivalence ratio in the cavity,especially the interactions of the reflected shock wave of bow shock and the cavity shear layer.With the injection distance of 100 mm,the critical equivalence ratio for the mode transition is the lowest.Comparing the hydrogen flame with the ethylene flame,it is found that the ethylene flame changes directly from the cavity stabilized mode in the scramjet operation to the jet-wake stabilized mode in the ramjet operation.During the transition process,the sidewall effects cause the large-scale boundary layer separation.In the cavity stabilized mode,the shear layer supports the combustion stabilization.The flame front oscillation,which performs in the three main frequencies,are dominated by the shear-layer oscillation mode.When the flame is in the jet-wake stabilized mode,the oscillation of the flame is mainly affected by the oscillation of the jet.When the flame front is upstream of the injector,it is the large-scale boundary layer separation upstream of the jet which affects the flame fluctuation rather than the jet itself.Based on the experimental observation and simulation results,“Triple reaction zones” mechanism was proposed to illustrate the mode transition of the cavity flame from cavity stabilized mode to jet-wake stabilized mode.The entire combustion regions are identified as a combination of the cavity reaction zone,the jet reaction zone around the cavity,and the jet wake reaction zone upstream of the cavity.The equivalence ratio in the cavity reaction zone is the dominant factor of flame stabilization inside the cavity.While the interactions between the jet and the cavity contributed to flame formation in the jet around the cavity.The flow parameters in the jet wake upstream of the cavity will greatly affect the flame stabilization in the jet wake.The large-scale boundary layer separation helps the flame to propagate upstream and stabilized the flame in the jet wake.The auto-ignition effect under high total temperature was investigated.It is found that the auto-ignition could lead to the initial flame core,and further generated some independent new flame in the jet-wake region during the flameout/re-ignition process,which in turns induced the re-ignition process of the jet-wake.Theoretical analysis indicated that the inflow temperature is the major factor for auto-ignition effect and flame propagation.The auto-ignition effect increases exponentially with the increase of the temperature,and the auto-ignition effect will have a significant impact if the static temperature goes up to more than 1000 K.As the temperature further rises,the autoignition effect will eventually be dominant in the whole combusting flowfield.For the partially premixed hydrogen flame in the jet-wake stabilized mode,the diffusion flame is mainly located in the regions along the boundary of the jet flame,while the rich premixed flame is distributed inside the jet and the lean premixed flame is located in the toroidal vortex of the jet leeward region and in the cavity.For the jet-wake flame under the inflow total temperature of 1600 K,the auto-ignition is mainly concentrated in the shear-layer of jet upwind region and the near-wall jet-wake region.This auto-ignition region is believed to play an important role in stabilizing the flame base.While for combustion regions in the main stream,the flame propagation process dominates the flame stabilization mechanism.A laminar chemical reaction database,which combines the mixture fraction,the reaction progress variable and the temperature,was established and verified.Nonequilibrium chemistry tabulation was used to calculate the auto-ignition delay time of the two-dimensional and three-dimensional partial premixed flames.The results prove the feasibility of the present tabulation model.Although it is difficulty to precisely capture the strong nonlinear characteristics of those reactive regions with high temperature,the auto-ignition delay time is found to be quite accurate.The numerical verification shows that the non-equilibrium chemistry tabulation method effectively improves the computing efficiency of chemical reaction.