Experimental study of flame stabilization in a single-side expansion scramjet combustor with different cavity length-to-depth ratios

Flame stabilization with cavity-strut injection of ethylene in a single-side expansion scramjet combustor is experimentally investigated in a Mach 2.1 supersonic flow. Two cavity geometries with different length-to-depth ratios of 3.2 and 4.5 and two injection schemes with different injection distances upstream the cavity are used to compare their combustion characteristics in this study. Flame dynamics with different cavity geometries and injection schemes are visualized comprehensively using high-speed imaging and static pressure measurements. Two different combustion stabilization modes are recognized, including the jet wake mode and the cavity shear layer mode. The transition process between the two modes is also observed in the experiments. Flame oscillations are often accompanied by the transition of the combustion modes. It is indicated that although increasing the cavity length-to-depth ratio can strengthen the exchange of momentum and heat to promote combustion, it also leads to intensified flame oscillations. When the injection distance upstream of the cavity increases, the flame oscillations become more intense, and the total heat release from the combustor decreases accordingly. In order to obtain a robust combustion process, it is suggested that the flame should be stabilized in the cavity shear layer mode and the mode transition should be suppressed to some extent.