Unstart Phenomena Induced by Forced Excitation in a Dual-Mode Scramjet

Abstract

Scramjet engine with no moving parts and extensive speed range is suitable for developing cheaper vehicles to outer space. The isolator, which isolates the inlet from the pressure buildup inside the combustor, is a critical component of the scramjet engine. In an actual scramjet, the combustor undergoes highly transient combustion giving rise to high amplitude fluctuating backpressure, which may cause the inlet to unstart. In general, combustion in a scramjet is a complex process that involves flows subjected to considerable pressure gradients, coexisting subsonic and supersonic flow regions in the presence of shock waves. Moreover, shock-induced boundary-layer separation patterns are different due to asymmetrical boundary layer development at the walls. Until now, no experimental study on the forced dynamic excitation in hypersonic flows is reported, mainly due to the limited available ground facilities that can reproduce such conditions. Such dynamic excitation can give to flow fields that are significantly different from that reported in other studies. The present study numerically addresses forced excitation inside the combustor generated by an elliptical-shaped rotor. During the unstart process, several stages were identified, such as separation and its growth in both the upper and lower walls of the combustor, isolator, inlet, and finally, shock excursion leading to unstart. The results provide ample insight into the upstream propagation of flow disturbances originating from the combustor, its interaction with the asymmetric boundary layers on top and bottom walls, and the dynamics of inlet unstart.