Experimental investigation of combustion-induced starting hysteresis in the scramjet

Abstract

Understanding the hypersonic inlet starting characteristics is the prerequisite for avoiding the abnormal unstart state. To make the work close to the actual situation, an experimental study was performed on a scramjet model at a simulated freestream Mach number of 6.0 with pressure and thrust measurements. The inlet working status is determined by the heat release of the injected ethylene with reciprocating variations. The results show that the critical equivalence ratio of the restart state is lower than that of the unstart state, which means that the combustion weakens the inlet restart capability and raises the unstart/restart hysteresis phenomena. Specifically, two novel unstart/restart hysteresis phenomena are found: one may come from the dual-solution characteristics of the shock–combustion interaction and the other may come from the historical effect of reverse flow. Compared to the former type, the latter type requires greater downstream heat release and generates a larger hysteresis loop. In addition, the engine thrust characteristics of the whole unstart and restart processes are analyzed. The thrust increment in the shock–combustion interaction type exhibits nearly linearly. However, the thrust increment meets abrupt changes and strong oscillations in the reverse flow type, accompanied by the reverse flow's formation and disappearance, making the engine more difficult to restart.