Effect of Rotating Detonation on a Ramjet Inlet in Mach 4 Free-Jet Experiments

A hydrogen-fueled rotating detonation ramjet was tested in the TUSQ wind tunnel at conditions corresponding to Mach 4 flight at 15 km. The purpose of the experiments was to demonstrate the engine operation, assess the effect of rotating detonation on a ramjet inlet and define the conditions at which the inlet unstarts. Fluctuations of Pitot pressure, measured with a probe outside the engine cowl, coupled with schlieren imaging, were used to detect inlet unstart. Four modes of combustion were identified, which primarily depended on nozzle contraction ratio and equivalence ratio, namely: deflagration, started rotating detonation, unstarted rotating detonation, and pulsed detonation. Started and unstarted rotating detonation modes were achieved at lower nozzle contraction and equivalence ratios. In both rotating detonation modes, a single detonation wave was observed, propagating at circumferential velocity of 45% to 63% of Chapman-Jouguet model (normal velocity of 64% to 89% assuming detonation wave propagating at 45°). It was found that a periodic inlet unstart and pulsed detonation phenomena were coupled. Potential cause could be either periodic auto-ignition or flame-holding attributed to the backward facing step. For both deflagration and detonation, an unstart occurs at similar isolator exit pressure of about 530 kPa.