Research On The Unsteady Combustion Process In Scramjet Combustor

In this paper,the unsteady combustion process under the conditions of the flight Mach number of 4 in the single-side expansion scramjet combustor is the research object.The flameholding characteristics in the combustor,the transient process,induction and formation mechanism,characteristics and suppression of combustion oscillation were studied through a large number of direct-connected experiments combined with numerical calculation.The flameholding characteristics in the combustor under different equivalence ratios were studied experimentally.It was found that in the range of equivalent ratio of 0.13-0.53,the following multiple steady and unsteady combustion processes will occur in the combustor: the cavity-stabilized combustion,the cavity assisted jet-wake stabilized combustion,the intermittent oscillating combustion dominated by the cavity assisted jet-wake stabilized combustion,the intermittent oscillating combustion dominated by the cavity-stabilized combustion and the periodic oscillating combustion.The higher the equivalence ratio is,the easier the unsteady combustion occurs and the more obvious the periodic characteristic of combustion oscillation is.Moreover,in the single-side expansion scramjet combustor,the wall expansion effect of the flameholding characteristics is very significant.When the injector and flameholder is installed in the expansion wall,the oscillating combustion is more likely to occur.The combustion oscillation process in the combustor was studied in detail through experiments and numerical calculation.The results show that the large-scale unsteady combustion process in the combustor has strong low-frequency oscillation characteristics,and most of the oscillation energy is distributed in the low frequency domain below 200 Hz.A typical combustion oscillation period is a complex process including flame cavity-stablized stage,flame flashback stage,the stage of flame propagation and stabilization in the jet wake region and flame blowoff stage,accompanied by the combustion mode switching between the supersonic combustion mode and the subsonic combustion mode,the flameholding mode transition between the cavity-stabilized flame and the cavity assisted jet-wake stabilized flame.Flame flashback is triggered by the upstream propagation to the cavity recirculation zone of the downstream flame along the sidewall boundary layer led by the interaction between the unsteady combustion process in the upstream cavity recirculation zone and in the downstream outlet separation zone.Flame flashback is an unsteady combustion process with very significant three-dimensional features,which proceeding in the corner recirculation and the sidewall boundary layer,the flame in the corner recirculation lead the flame flashback in the sidewall boundary layer.The flame flashback acceleration is caused by the positive feedback effect of the strong coupling between the shock and the flame,which is an incomplete DDT process.The periodic low-frequency combustion oscillation in the combustor is a specific frequency oscillation formed by the mutual coupling of the incomplete DDT process and the flame blowout process of the jet wake flame.The influence of various factors on the combustion oscillation in the combustor was studied experimentally.The results show that the fuel type,fuel injecting and mixing,flow passage area ratio,air throttling and incoming flow condition have important influence on the combustion oscillation characteristics in the combustor,while the parameters of the flameholding cavity itself are less affected.The suppression methods of combustion oscillation were studied by experiments.It was found that piloted hydrogen combustion method and distributed injection method can effectively reduce the frequency of the large amplitude combustion oscillation in the combustor,decrease the amplitude of combustion oscillation,and weaken the energy of combustion oscillation.