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 (ER) 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.

Machine Learning-Based Backpressure Unstart Prediction and Warning Method for Combined Cycle Engine Hypersonic Inlet-Oriented Wide Speed Range

“…It is well known that the unstart phenomenon is one of the most important issues of the hypersonic inlet, which should be prevented [10] or taken effective measures to realize the restart when falling into the unstart state [11][12][13]. However, once the inlet unstart occurs during flight, it will increase the working burden of the hypersonic engine, and the hysteresis phenomena in the restarting process of the hypersonic inlet are widespread [14]. Especially in some certain cases, it is even impossible to restart during the engine operation [15].…”

Self-Start Characteristics of Hypersonic Inlet When Multiple Unstart Modes Exist

“…Most of the time, unstart results from the combined action of both factors [7,8], and hysteresis occurs in the conversion between start and unstart. The specific performance issue is that the inlet may not restart immediately after returning to the original path when the status converts from start into unstart, and restart does not take place until the situation is resolved [9][10][11]. Usually, the process of transitioning from unstart to start is called "self-start".…”