Experimental investigation of inclining the upstream wall of a scramjet cavity

Abstract

The cavity flameholder has been widely used in scramjets to improve mixing and combustion rates. While many have investigated modifications to the cavity downstream-wall, few have investigated modifying the upstream-wall - a rear-facing step. The rapid expansion produces a low pressure region and base drag force, while only the larger of the induced twin vortices contributes substantially to mixing. In this paper, the upstream-wall of a scramjet combustor cavity was inclined, aiming to reduce cavity base drag and remove superfluous cavity vortex structures. Upstream-wall angles of 90°, 45° and 22.5° were examined, at cavity length-to-depth (L/D) ratios from 4 to 7. Cavity downstream-wall angles of 90° and 22.5° were examined for each configuration. Reflected shock tunnel experiments were conducted at Mach 7 enthalpy, scramjet combustor conditions. Experimental Schlieren imaging showed that reducing the upstream-wall angle did not significantly affect shear layer separation for L/D ratios of 4 and 5. At L/D ratios 6 and 7, however, reducing the upstream-wall angle saw the shear layer penetrating deeper into the cavity. Reynolds-averaged Navier-Stokes computations examined the internal flow structure and it was shown that reducing the upstream-wall angle to 45° retained the dominant mixing vortex in the cavity, with base drag reduced by 21% compared to the standard 90° upstream-wall case.