Vortex dynamics in different combustion regions of a cavity-based scramjet

A hybrid RANS/LES study of a cavity-based scramjet was performed and reasonable agreements were found between simulation results and experimental measurements. In the current case, the flame was stabilized by the subsonic cavity shear layer and propagated downstream into the supersonic flow. The vortex dynamic in the flow, mixing, and combustion regions was comparatively investigated. The averaged vorticity in the combustion regions was lower by 55% compared to the mixing region, primarily due to dilatation as a result of the heat release. Furthermore, the combustion zone was decomposed into four regions based on premixed/diffusion flame and subsonic/supersonic combustion. Then the vorticity and its transport in the four regions were compared. The averaged vorticity in the premixed combustion regions was only slightly larger than that in the diffusion combustion regions. However, the averaged heat release rate was nearly 3 times larger in the premixed regions, leading to higher contributions of dilatation and baroclinic torque in the premixed regions, with an overall weak positive impact on the vorticity generation. In the subsonic combustion regions, the vorticity was three times larger than that in the supersonic combustion regions, despite similar heat release rates on average. It could be explained by the relatively large magnitude of dilatation and baroclinic torque in the supersonic flow. Vortex stretching and dilatation were comparable in the supersonic flame but the former became two times larger than the latter in the subsonic flame. Moreover, the baroclinic torque had larger contributions than diffusion in the supersonic flame whereas the opposite trend was found in the subsonic flame. The results highlight that the subsonic combustion regions in the cavity shear layer and near the walls significantly contribute to the vortex dynamics and mixing process, in addition to flame stabilization.