Experimental Study on Combustion Modes and Oscillations in a Cavity-Based Scramjet Combustor

This study conducted experiments to investigate flame behaviors in a cavity-based scramjet combustor operating under Mach 2.92 supersonic inflow. Pressure measurements and flame chemiluminescence observations were combined to study the initial flame formation and combustion mode transitions. In addition, flame luminosity standard deviation and fast Fourier transform power spectral density (PSD) were post-processed to assess the impact of equivalence ratio and aft wall height on combustion modes as well as oscillations. It is indicated that increasing the equivalence ratio leads to a transition from lifted shear-layer mode to ramjet mode. Elevating the cavity aft wall height improves the combustor's self-ignition capability and promotes the transition from lifted shear-layer mode to ramjet mode as long as flame stabilization is achieved. It is found that increasing the equivalence ratio and aft wall height also results in more intense combustion oscillations; besides, raising the cavity aft wall height could accentuate the predominant frequency of low-frequency oscillations in the combustor. Finally, it is demonstrated that the application of spark plasma effectively suppresses these low-frequency combustion oscillations, which is a promising active control method in the scramjet engine.