Investigation Of The Blowout Characteristics Of The Partially Premixed Cavity Flame In Supersoninc Flows

The stability of the global flame in the combustor is crucial to the persistent output of the scramjet engine.However,supersoninc flows and global equivalence ratio may change drasticly when the engine perform condition transition and other tasks.Accompanied by the strong coupling of the turbulence and chemical reactions,the flame in the combustor is particularly prone to be blowout.Investigating the blowout characteristics of the cavity flame during the condition transition is important to ensuring the safe and reliable flight.This thesis presents investigations of the combustion modes and the unsteady characteristics of the flame with varieties of combustion diagnosis techniques under background of the condition transition process in the scramjet engine.The dynamic response characteristics of the flame blowout under different initial reactive flowfields and injection pressure differentials are discussed in detail,and the critical blowout limits during condition transition are confirmed.The combustion modes and the unsteady characteristics of the flame when the injection pressure decreases during consition transition are studied at first.The results reveal that stable combustion mode,oscillation combustion mode,and flame blowout mode appear in turn after the condition transition as the injetcion pressure differential increases.The flame can be shifted from initial stable state to the other stable state under stable combustion mode.Alternate local extinction and reignition phenomena can be observed intermittently at cavity ramp and the downstream rear wall under oscillation combustion mode.When the injection pressure differential exceed a critcial value,the combustion intensity gradually decreases during consition transition and the flame will be blowout absolutely in the combustor.The dynamic response characteristics of the flame blowout under different initial reactive flowfields and injection pressure differential s are confirmed sudsequently.The flame can be maintaned in the combustor for more time when the initial combustion intensity is stronger,and more products with high temperature can be resided in the combustor before the flame blowout.As the injection pressure differential increases,initial flame evolution process shows no clear difference during the consition transition while the unsteady chracteristics of the flame before blowout are explicit.It can be observed that the jet flame can be able to lats longer in the local extinction and reignition state.The flame can be maintaned in the combustor at a lower global equivalence ratio when the chemical reactions are more violent in the initial flowfields.The unsteasy characteristics of the flame when the injection pressure increases during the consition transition are investigated at last.When the initial combustion intensity is relatively weak,the combustion mode are stable combustion mode and blowout mode as the injection pressure differential increases afer the consition transition.The flame can be blowout rapidly during the transition process once the the injection pressure differentical reaches a critical value.When the initial combustion intensity is relatively strong,two different stable combutsion mode and flame flashback mode appear in turn afer the consition transition.The flame pulsation process has an obvious periodicity in the flashback mode.In addition,the contrast of the combustion intensity is even more pronounced and the frequency of the combustion oscillation is lower as the injection pressure differentical increases.The critical limits for the flame blowout in the combustor increases with the initial global equivalence ratio.