Numerical investigation of combustion characteristics of upstream crescent cavities in a scramjet combustor

This study investigates the combustion performance of a number of novel, crescent shaped cavities placed directly in front of the fuel injection point in a scramjet combustor. Cavities are commonly used for flameholding in scramjet combustors, however they are almost exclusively placed downstream of the fuel injection point and their effect on mixing performance is limited. The crescent cavities in this study were previously found to enhance mixing in chemically frozen flow and the purpose of this study is to investigate whether this performance enhancement translates to chemically reacting flow. The performance of the cavities was numerically studied using unsteady Reynolds-Averaged Navier-Stokes (URANS) modelling with hydrogen as the fuel and an isothermal 1800 K combustor wall, and performance was compared to a no-cavity baseline. The cavities were found to enhance combustion by up to 114% and heat release by up to 143% compared to the baseline when hybrid fuelling is employed and by 47.4% and 54.7%, respectively, when no hybrid fuelling is used. Total pressure loss in the cavity cases was similar to the baseline case and viscous drag was lower in the cavity cases due to the presence of combustion and fuel near the wall. Flame penetration was lower in the cavity cases than in the baseline however as a result of the increased fuel concentration on the wall, while mean heat flux is lower in the cavity cases than in the baseline. Ignition is also found to occur more rapidly in the cavity cases and the flow inside the cavity is oscillatory in nature, as was the case in chemically frozen flow. Given that the crescent cavities in this study significantly enhance combustion performance with limited drawbacks, especially when hybrid fuelling is used, they are an attractive option for operational scramjet combustor design.