Experimental investigation of relationship between deflagration and detonation in a rotating detonation engine

Deflagration is prevalent in Rotating Detonation Engines (RDEs), yet systematic investigations remain inadequate. This study investigates the relationship between deflagration and detonation in an RDE fueled by ethylene/oxygen-enriched air. By varying the oxygen content and equivalence ratio in the experiments, two distinct Rotating Detonation Wave (RDW) propagation modes are identified. The classical single-wave mode exhibits compact reaction zones and steep pressure peaks, whereas the sawtooth-wave mode demonstrates dispersed reaction zones, lower pressure amplitudes, and irregular waveforms. The results reveal that RDWs in sawtooth-wave mode are accompanied by extensive deflagrations, consuming more propellants and destabilizing leading shock waves. Meanwhile, RDWs in sawtooth-wave mode exhibit poor coupling between leading shock waves and reaction zones despite consistent propagation frequencies. Deflagration in the combustor significantly influences the propagation of RDW. Specifically, the escalation of deflagration contributes to the formation of new detonation waves. Then, substantial pre-ignition of propellants emerges ahead of the RDW (termed “parasitic combustion”), raising the combustor temperature, ultimately establishing the sawtooth-wave mode. Although the single-wave and sawtooth-wave modes exhibit comparable propagation frequencies, their self-sustaining mechanisms diverge fundamentally. The latter closely resembles the high-frequency combustion instabilities in rocket engines.