Flow choking induced by combustion and mass injection in a circular model scramjet at mach 4.5

This paper provides a comparison between choking and shock propagation induced by combustion heat release and/or mass injection. The experiments were performed in a supersonic (Mach 4.5) high-enthalpy flow at representative conditions of a scramjet engine. A constant area model scramjet, having circular cross section and a conical inlet, was tested in the ACT-II hypersonic facility. An ethylene-fueled reacting case was compared with an air-injected non-reacting case to investigate similarities and differences between the two blockage mechanism. In the reacting case, flow choking was achieved by combustion heat release; whereas in the non-reacting case, by the aerodynamic blockage induced by the air jets. The two cases were analyzed using simultaneous time-resolved pressure measurements and high-speed (20 kHz) flow visualization in both isolator and combustor. It was observed that, matching the isolator pressure gradient induced by the two mechanisms, the resulting flow dynamics was qualitatively and quantitatively very similar. In particular, the isolator shock oscillations exhibited same amplitude and frequency in both cases. The overall pseudo-shock propagation time throughout the combustor was also found to be independent on the nature of choking. Nevertheless, substantial differences were observed in the propagation mode of the pseudo-shock, that was smooth and uniform in non-reacting case, and characterized by large fluctuations in the reacting case, possibly due to combustion instabilities.