Effect of Fuel Injection Distance and Cavity Depth on the Mixing and Combustion Characteristics of a Scramjet Combustor with a Rear-Wall-expansion Cavity

The mixing and combustion characteristics of a model scramjet combustor with a rear-wall-expansion cavity under a Mach 2.0 inflow are experimentally and numerically investigated. Two different cavity depths and two fuel injection distances are compared to show the effect of these parameters. High-speed imaging system is used to capture the oscillation of flamebase and the flame oscillation of cavity shear layer in the transversal direction. Numerical simulation is carried out to reveal the jet mixing process around the cavity. The experimental results show that the combustor with longer fuel injection distance and smaller cavity depth has weaker flame and lower wall pressure due to the significant oscillation. The oscillation of flamebase is found to be stronger in the shallower cavity due to its smaller volume. Cavity with short fuel injection distance and large depth could suppress the oscillation, and the combustor can achieve a robust and strong combustion. The simulation results show that under the shorter injection distance, the center of cavity shear layer is rolled up to the main stream; thus the shear layer can interact with the fuel jets more intensively, which improves the mixing efficiency and promotes strong combustion. Compared to the cavity with larger depth, the shear layer in the smaller cavity has a larger thickness and displays more significant shear layer oscillation in the transverse direction. The oscillation of the cavity shear layer can promote the mixing efficiency but is detrimental to combustion. Therefore, a proper cavity depth should be carefully considered during the design of a cavity flameholder.