Fuel mixing and combustion characteristics in an annular scramjet combustor

An annular scramjet combustor features structural symmetry, uniform flow field, and minimal corner effects. Its relatively small flow path height enhances fuel mixing. This study employs numerical simulations to investigate fuel mixing and combustion characteristics in the annular scramjet combustor at an inflow Mach number of 2.35. At a fixed injection position, increasing the equivalence ratio improves fuel penetration and spanwise spread but decreases mixing efficiency. As the distance from the injector orifice to the cavity leading edge (Lj) increases from 40 to 140 times the injector diameter (d), fuel mixing efficiency improves upstream of the cavity. Three combustion modes are identified: supersonic combustion mode (mode 1), cavity partially choked mode (mode 2), and injection section partially choked mode (mode 3). The combustion mode is determined jointly by injection distance and equivalence ratio. For short injection distances (Lj/d ≤ 80), the combustion zone is mainly in the jet windward shear layer and cavity. At low equivalence ratios, heat release is insufficient to induce thermal choking. The combustion occurs in mode 1. As equivalence ratio increases, thermal choking occurs near the cavity. The combustion is characterized by mode 2. For 80 < Lj/d ≤ 100, increasing the equivalence ratio shifts the thermal choking from the cavity to the injection section. As a result, the combustion mode transitions from mode 1 to mode 2 and eventually to mode 3. For Lj/d > 100, the combustion zone consistently resides upstream of the cavity. At low equivalence ratios, the combustion mode is mode 1, while higher equivalence ratios correspond to mode 3.