Study On The Effect Of Recess Ratio On Spray Combustion Characteristics For LOX/CH4 Swirl Coaxial Injectors

Liquid oxygen/methane bipropellant liquid rocket engine has become a research hotspot all over the world because of its advantages such as high specific impulse,stable combustion,no pollution and so on.The gas-liquid swirl coaxial injector is widely used in this kind of rocket engine because of its good atomization performance.However,the atomization and combustion mechanism of this kind of injector is complex and prone to self-pulsation,which affects the combustion stability.In this thesis,the self-pulsation process,spray combustion process and spray/flame dynamic characteristics of recessed gas-liquid swirl coaxial injectors are studied by means of experimental observation,numerical simulation and theoretical analysis.Firstly,the injector self-pulsation process is studied.The effects of gas-liquid ratio,backpressure and recess ratio on self-pulsation are analyzed,the flow field structure in the recess chamber and the motion process of liquid sheet are clarified,and the theoretical model of self-pulsation is established.The results show that under the condition of backpressure,the increase of recess ratio will restrain the occurrence of self-pulsation.The self-pulsation intensity increases at first and then decreases with increasing the backpressure,and finally the spray changes to a stable state.The increase of the annular gap gas pressure drop leads to the expansion wave and oblique shock wave at the annular exit of the recess chamber,and the air flow separates after the shock wave,resulting in a vortex.The gas-liquid swirl coaxial injector with large recess ratio will appear the phenomenon of “break” of self-pulsation with the increase of gas-liquid ratio,and the self-sustaining mechanism of self-pulsation before and after “break” is different.Secondly,the combustion process of recessed liquid oxygen/methane swirl coaxial injector is studied.The ignition shock characteristics of liquid oxygen/methane engine,flame spatial distribution,combustion efficiency variation and flame oscillation characteristics are analyzed.The results show that the ignition delay is the direct cause of detonation in liquid oxygen/methane engine,and the detonation phenomenon can be effectively suppressed by increasing ignition energy.The low frequency oscillation of liquid oxygen phase in the process of liquid oxygen/methane combustion will lead to the low frequency oscillation of flame and finally lead to low frequency unstable combustion.After the phase state of liquid oxygen is stable,the low frequency oscillation disappears and the combustion state changes to quasi-steady combustion.The combustion efficiency of the low frequency unstable combustion process is the highest,and the flame length and normalized flame area of the quasi-steady combustion process are the largest.Finally,the effect of recess ratio on the spray and flame dynamic characteristics of liquid oxygen/methane swirl coaxial injector is studied.The effects of recess ratio on the spatial distribution of liquid oxygen/methane flame and combustion efficiency are analyzed.In addition,the spray/flame modal analysis of injectors with different recess ratios was carried out.It is found that the combustion efficiency is positively correlated with the flame length,and increasing the flame length can improve the combustion efficiency.With the increase of recess ratio,the flame shape changes from stable state to oscillatory state.When the flame is stable,increasing the recess ratio will improve the combustion efficiency.When the flame oscillates,the combustion efficiency of the increased recess ratio first decreases and then increases,and the combustion efficiency of injector with the critical recess ratio is the lowest.