Experimental And Numerical Investigation On Flame Propagation Process In Supersonic Flow

The present study is based on the gas flow,mixing and combustion in the scramjet.Combining with numerical simulation,theoretical analysis and experimental research,the flame flashback and acceleration phenomenon in the scramjet engine is studied systematically and deeplyFirst of all,the flame acceleration phenomena in the tube with obstacles are studied by theoretical modeling and numerical simulation in this paper.The effectiveness of the theoretical model is verified by comparison with the experiment.Then flame acceleration under different equivalent ratios are calculated numerically by using the detailed chemical reaction mechanism.Simulation results show that the flame acceleration rate is in accordance with the theory under the condition of stoichiometric equivalent ratio.When the equivalence ratio is less than the stoichiometric equivalent ratio,there is a big divergence between the acceleration rate and the theoretical value.In this paper,we consider the influence of the obstacle configuration on the flame acceleration.When the thickness of the obstacle is less than the width between the obstacles,the enhanced turbulence is more dominant than the effective volume of the jet flow which contributes to flame acceleration,the flame acceleration rate is bigger than that of the infinite thin configuration.As the thickness of the obstacle continues to increase,the role of reducing the volume between the obstacles dominates,the flame acceleration rate of declines rapidly.Secondly,in order to analyze the fuel mixing state before the flame process,the fuel mixing characteristics of injection upstream the cavity in the combustor under different injection positions and different flow conditions are revealed by numerical simulations.It can be seen from the results that under the same turbulent boundary layer,the mixing efficiency of the two injections far from the cavity is the highest compared with the two injections near the cavity and the two injections near and far from the cavity.In the upstream of the cavity rear wall,the effect of turbulence is more obvious and the mixing efficiency is increased when the inlet turbulence boundary layer thickness is increased.Thirdly,the combustion phenomena at different injection positions are studied by means of high speed photography,high speed schlieren and high frequency pressure sensors in the equal cross section combustor of the scramjet engine.The results show that the dominant flame front oscillation frequency is about 95 Hz,and the main frequency of the flame front oscillation is very close to that of the pressure oscillation.This shows that the periodic oscillations of the flame which is characterized by the process of flame flashback and flame blowing,dominate the pressure oscillations in the combustor.Simulaiton results show that the condition of the flame oscillation is related to the chemical reaction mechanism.A higher oscillation frequency is obtained than the experimental oscillation frequency when a stronger the chemical reaction mechanism is applied.The numerical simulation results show that when the inflow boundary layer is laminar,the combustion in the combustor is cavity-stabilized.When the turbulent inlet boundary layer is adopted,the flames in the downstream of the cavity increase and propagate to the upstream.Increasing the thickness of the turbulent inflow boundary layer can make the flame flashback earlier when turbulent inlet boundary condition is applied.The numerical simulation results show that the flame can not flashback under low total temperature of the inflow.The influence of the inflow total temperature is more dominant in the initial process of the flame accumulation in the downstream of the cavity under high total temperature of the inflow.The increase of the total temperature of the flow makes the flame downstream of the cavity flashback earlier.The quasi-periodic flame flashback phenomenon is also observed in the experiments of injections far from the cavity in the equal cross section combustor with two parallel cavities.The results show that the main oscillation frequency of the pressure oscillation is not obvious,but the periodic oscillation of the flame can be also considered to dominate the pressure oscillation in the combustion chamber.The flame flashback phenomenon in the combustior with two parallel cavities is also studied by numerical simulations.The numerical results show that the flame could propagate asymmetrically in the upstream of the cavity.Finally,the combustion mode of the flame flashback process is analyzed.The results show that the flames in the cavity and the downstream remain in the rich premixed combustion mode in the initial process of the flame accumulation.As the flame flashback,the flame front remains in the combustion premixed combustion mode while the combustion in the shear layer near the center of the flow channel rapidly changes into the diffusion combustion mode.In this paper,the G / Z equation model of supersonic premixed combustion is improved and verified when accounting for the partially premixed combustion phenomenon.Simulation results show that the partial ly premixed combustion G / Z equation model is more efficient than the finite rate reaction model when compared with the experimental data.It is reasonable that the G / Z equation model can capture the characteristics of partially premixed combustion in the supersonic flow field.