Standing oblique detonation for hypersonic propulsion: A review

Standing oblique detonation is a unique pressure-gain combustion phenomenon for hypersonic ramjet propulsion, and its research has been related with supersonic combustion in scramjet engines since its births, for example, absent treatment in its early stage and re-consideration in recent decades. Standing oblique detonations and supersonic combustion share the same features of supersonic chemically-reacting flows, and can be considered as different flow development stages. Combustion instability in a chemically-reacting flow is reviewed first to identify its fundamental mechanisms, and the upstream-propagating shock wave is identified as one of intrinsic characteristics and taken as the key problem for developing hypersonic ramjet propulsion. Critical conditions for the standing oblique detonation are summarized as a theoretical base for standing oblique detonation ramjet engines. Three key parameters are included, that is, the maximum heat that can drive local flow states from supersonic to sonic after combustion, the critical inflow Mach number of combustors, at which supersonic combustion becomes stable, and the critical wedge angle at which a standing oblique detonation can be initiated. The evolution of the standing oblique detonation is reviewed by placing emphasis on its complex wave structure that was found to develop via three stages, that is, shock-induced initiation, the decaying stage and the fully-developed stage. Finally, progress in experimental research is reviewed with detailed discussions on stabilization of the standing oblique detonation, experimental methods and development of adequate test facilities. In conclusion, the stable operation of hypersonic ramjet propulsion is a critical issue to approach its engineering application, and the standing oblique detonation ramjet engine is recommended as a promising candidate, deserving more attention in the future.