Research In Mechanism Of The Shock-Induced Detonation Waves

There are two kinds of design for hypersonic air-breathing propulsion system at present:1)the supersonic combustion ramjet engine(scramjet)based on diffusing-effects-controlled combustion;2)the shock-induced combustion ramjet engine(shcramjet)based on shock-induced combustion.Apart from the supersonic combustion ramjet(scramjet)engine,the shock-induced combustion ramjet(shcramjet)engine is a promising concept with simpler ignition mechanism and more compact construction.Meanwhile,it has an operational capability at higher Mach number than scramjet.Although with these advantages,shcramjet is confined by its complicate combustion mechanism which is hard to control.Among so much shock-induced propulsion design,the oblique detonation wave engine(ODWE)is the most remarkable one.A lot of researches were conducted in the past decades.Pioneers found when hypersonic inflow strikes a ramp,oblique shock waves form on the leading edge of the ramp.Mixed gas is ignited in the following area which results in reaction waves these reaction waves couple with shock waves and give birth to oblique detonation.Numbers of experimental and simulating results prove that the coupling structure can exist stably.This study was performed to research the mechanism of shock induced oblique detonation.The transition of oblique detonation wave under changeable boundary conditions was emphatically researched with CFD methods.Ususally,oblique detonation waves(ODWs)were induced by inclined ramp.Due to the fast combustion and the rigorous inflow conditions,real detonation is difficult to capture.So most researches were implemented in numerical methods.Stable boundary conditions were introduced in most of them.But in real cases,boundary conditions are transient.In light of this,researches with changeable boundary conditions are necessary and meaningful.Considering the inflow temperature is sensitive to the nonuniform mixture and other factors,the temperature disturbance was introduced into a combustion chamber.Numerical results show that the ODWs can adjust with the temperature disturbance and the transition progress is smooth enough.But the inherent instability of ODWs was found to be further released by disturbance.The existing form of disturbance was researched quantitatively and qualitatively,which is complex of shock waves,expansion waves and weak compression wave.Comparison between results from two kinds of disturbance was conducted,which demonstrates that the distribution of waves is basically the same in detonation zone,while reversed in deflagration zone.It is caused by the difference in the strength of weak compression wave in two cases.It also testifies the importance of this kind of wave,usually neglected by other researches.Furthermore,at decreasing temperature,the complex of three kinds of wave propagates downstream along the ramp and shock wave appears in four types of form which are bow-like shock,mach reflection,regular reflection and normal shock nearly vertical to the ramp.There is an enormous bifurcation at increasing temperature,where the waves propagate along the surface of ODWs and the form of waves remains stable relatively.In form researches about oblique detonation waves,less attention was put in the effect of variation of inflow species on the oblique detonation structure.In light of this,simulations on the effects of equivalence ratio on oblique detonation structure were performed.Numerical results show that the dependence of induction length on equivalence ratio is a U-shape curve and temperature is the key factor influencing initiation length.The strength of compression waves near the end of initiation zone was found to dominate the transition type of initiation zone.The critical range for equivalence ratio is 0.6 to 2.5,in which compression waves are strong enough to lead to abrupt transition.When equivalence ratio is out of this range,compression waves get weak and transition type turns to be smooth.Furthermore,theoretical analysis based on the constant volume combustion(CVC)theory was also performed.Comparison between numerical and theoretical results demonstrates that initiation length is controlled by chemical kinetic effects in initiation zone which leads the initiation length to move in a U-shape curve like the CVC theory predicts.Nevertheless gasdynamic effects dominate the range and varying rate of initiation length,preventing the initiation length to change extensively like the theoretical results.Furthermore,this study also includes researches on the effects of nitrogen on oblique detonation structure.Numerical results showed that mixed gas reburns in reaction zone when the content of nitrogen is low,which induces strong compression wave.As the content of nitrogen increases,the reburning of mixed gas and compression waves are weakened.Furthermore,the influences of temperature on oblique detonation wave were discussed.Quantitative analysis reveals that the reburning of mixed gas is enhanced by decreasing temperature.