Effect of Rotating Detonation on a Ramjet Inlet in Freejet Experiments

A hydrogen-fueled rotating detonation ramjet was tested in the TUSQ wind tunnel under conditions corresponding to Mach 4 flight at 15 km. The purpose of the experiments was to demonstrate 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: 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. Although in most cases rotating detonation was sustained throughout an experiment, the detonation wave was not stabilized in any of the experiments. In both rotating detonation modes, a transition between single-wave detonation mode and irregular wave propagation was observed. The propagation velocities normal to the detonation wave were about 60 and 52% of the Chapman–Jouguet model for the unstarted and started cases, respectively. It was found that a periodic inlet unstart and pulsed detonation phenomena were coupled. The potential cause could be either periodic auto-ignition or flameholding attributed to the backward-facing step. For both deflagration and detonation, an unstart occurred at a similar isolator exit pressure of about 530 kPa.