Cavities are widely used as flameholders in supersonic combustors due to their outstanding potential to stabilize combustion without excessive total pressure loss. A review of cavity-stabilized combustion for scramjet applications is provided in this article. The topics cover the fundamental problems and recent advances regarding cavity-organized combustion in high-speed flows, including combustion stabilization modes and mechanisms, flame stability analyses and correlations, combustion oscillations, and other related issues. Remarkable questions such as cavity-coupled fuel injection, flow and combustion coupling, optimal cavity geometry and scale, auto-ignition and flame propagation interactions, and unsteady effects are discussed. Then, an attempt is made to provide some guidelines for the future research of cavity flameholders.

The ignition and flame propagation in an axisymmetric supersonic combustor were investigated. The laser-induced plasma was employed to ignite the supersonic inflow with a speed of Mach 2.5 and a total temperature of 1486 K. A direct-connect axisymmetric model scramjet with a fully transparent glass combustor was built, which enabled the circumferential and axial flame propagation in the cavity-based axisymmetric supersonic combustor to be visualized by the high-speed photography from the endoscopic and external views, respectively. An initial flame kernel is produced by the laser-induced plasma and propagates to the cavity leading edge along the axial direction. The establishment of the cavity shear-layer flame facilitates circumferential flame propagation. The circumferential flame propagation is coupled with the axial propagation, eventually generating a loop-shaped flame with a central-hole. Acceleration of the flame propagation can be observed, especially when the global equivalence ratio is increased. A plausible explanation for the flame propagation in the axisymmetric supersonic combustor was found using URANS numerical simulation. The axisymmetric cavity generates a low-speed loop-shaped recirculation region and thickened cavity shear-layer with an appropriate local equivalence ratio, resulting in the simultaneous axial and circumferential flame propagation. The increased temperature in the cavity and the thickened cavity shear-layer during the flame propagation produce a more intense heat release and mass transfer, leading to faster flame propagation.