Numerical investigation of fuel mixing with upstream crescent cavities in a scramjet combustor

Cavities are commonly employed in scramjet combustors for flameholding and mixing enhancement, but the mechanism used to enhance mixing is absent at high supersonic Mach numbers, limiting their operational scope. The present study investigates the ability of crescent-shaped cavities placed upstream of a fuel injector to enhance mixing through vorticity generation, a mixing enhancement mechanism that is also effective at high supersonic Mach numbers. The mixing performance of five crescent cavity designs, two of which incorporate hybrid fuelling, is investigated using unsteady Reynolds-Averaged Navier–Stokes (URANS) computations of a chemically frozen flow with hydrogen as the fuel. It is found that the crescent cavities enhance mixing by up to 22.6% without the hybrid fuelling arrangement and by up to 90.1% with the hybrid fuelling arrangement. While vertical jet penetration is lower for all cavity cases, lateral penetration is higher and the cavity cases incur no or negligible total pressure loss compared to the baseline at the domain outflow, within the margin of error. Wall drag is also lower than in the baseline for some cavity cases. The primary mechanism driving mixing is found to be enhanced streamwise vorticity in the vicinity of the cavity, caused by the cavity vortex leaving the cavity and wrapping around the injector. The cavity flowfields are also found to be oscillatory in nature, although the oscillations are lateral and the harmonic frequencies are much lower than those of the longitudinal oscillations characteristic of conventional cavity flow. The mechanisms driving these oscillations are discussed, as are the flowfields for the best performing cavity cases. Several flowfield features of the crescent cavities are also highlighted and discussed, demonstrating how the hybrid injection cavity cases enhance mixing. •Crescent cavities realign cavity vortex with the freestream.•Crescent cavities enhance mixing by up to 90%.•Cavities with hybrid fuelling perform best compared to the flat-plate baseline.•Total pressure loss is similar to the baseline for the cavities, wall drag is lower.•Cavity flowfield is oscillatory in nature with large-scale lateral oscillations.