Influences of cavity on combustion stabilization in an axisymmetric scramjet

A study of cavity-enhanced combustion for a scramjet in Mach 4.5 high-enthalpy flow was conducted. In order to investigate the combustion and flameholding physics without corner boundary-layer effects present in rectangular geometries, an axisymmetric model was developed. The modular scramjet design allowed for parallel testing of both a cavity and constant-area combustor configuration. Fuel (ethylene gas) was injected into the supersonic core flow via a single row of equally spaced, azimuthally distributed choked holes on an injector annulus mounted flush with the combustor entrance. The fuel undergoes autoignition at the combustor entrance as a result of the high-enthalpy flow. Characterization of combustion dynamics were achieved by means of high-speed flame chemiluminescence imaging, wall pressure measurements, and combustor exit heat flux measurements. Flame structures were resolved via instantaneous hydroxyl (OH) planar laser-induced fluorescence (PLIF) imaging. It was observed that the cavity flameholder enhances combustion through improved fuel-air mixing and promotes the transition of quasi-laminar flame shapes to large-scale turbulent flame structures in the diverging combustor. The direct heat addition into the core flow through mixing dominates the combustor performance and can lead to thermal choking. It was observed that the major effect of the cavity on combustion performance is fuel-air mixing enhancement rather than the reduction of ignition induction time by cavity induced radical recirculation.