Experimental study on combustion instability of a cavity-based flame stabilizer with rounded corners in a scramjet engine

This study investigates the effects of rounded corner configurations at the leading and trailing edges of a cavity flameholder on the unsteady characteristics of a scramjet combustor, using ground-based direct-connect experiments. Tests were conducted at an inflow Mach number of 2.52, total pressure of 1.3 MPa, and total temperature of 1650 K, with ethylene fuel at an equivalence ratio of 0.22. High-speed schlieren imaging and CH* chemiluminescence were used to analyze flow-field structures, wall pressures, flame distributions, and spectral features for rounded corner radii of 0, 5, 7.5, and 10 mm and for rounded corner placements at the leading edge, trailing edge, and both. The results indicate that a leading-edge rounded corner enhances overall combustor pressure and combustion intensity by converting compression waves into expansion waves, deflecting the shear layer into the cavity and significantly promoting fuel-product mixing, and as a result, yielding the highest pressure and most uniform pressure oscillations at a 7.5 mm radius. A trailing-edge rounded corner reduces flow resistance and shifts the flame downstream, improving flame front stability. When rounded corners are applied to both edges, the flame front exhibits a single dominant frequency of 117 Hz, which significantly improves flame front stability compared to the configuration with only a leading-edge rounded corner and also increases the streamwise distribution range of the flame front. Proper Orthogonal Decomposition reveals that rounded corners amplify vertical flame oscillations, and the dual-rounded corner configuration strengthens a dominant mode in the flame oscillation, resulting in the highest energy share in the first ten POD modes among all configurations.