500-kHz OH PLIF and OH chemiluminescence imaging of deflagration and rotating detonation in CH4-O2 and H2-air mixtures

Deflagrative preburning and incomplete mixing of reactants are considered to be major loss mechanisms impairing the effective implementation of rotating detonation in air breathing and rocket engine systems. In this work, simultaneous orthogonal 500-kHz OH planar laser-induced fluorescence (PLIF) and OH* chemiluminescence imaging are employed to investigate key processes including turbulent mixing, deflagrative preburning, and detonation wave propagation in an optically accessible rotating detonation engine (RDE) operating on H2-air and CH4O2, which have widely varying levels of reactivity. A custom-built optical parametric oscillator (OPO) is coupled with a high-repetition-rate burst-mode laser for 284 nm excitation of the Q1(9) transition of the OH radical along a radial-axial slice of the annular combustor, while OH* chemiluminescence is used to track the azimuthal-axial progress of the detonation wave. Similar leading and trailing wave systems are observed for both the H2-air and CH4O2 reactants, indicating that partial consumption is occurring at the leading detonation wave due to incomplete mixing and local static properties in the injection near field. The OH* chemiluminescence data effectively mark the detonation wave propagation characteristics but are not sensitive enough to capture deflagrative preburning during the refill process. In contrast, the OH PLIF data reveal substantial OH production during the interaction between the cold incoming reactants and the burned products, which confirms the presence of deflagrative preburning caused by recirculation and turbulent mixing before the detonation wave arrives. Contact burning at the leading edge of the refill zone is less prevalent due to limited mixing between reactants and products, and is mostly absent in H2-air mixtures, likely due to lower reactivity. These data provide qualitative insights on the inlet flow and combustion phenomena associated with deflagrative loss mechanisms for propellants with widely varying reactivity, as revealed through spatio-temporally resolved planar fluorescence and OH* chemiluminescence.