Combustion Instability due to Combustion Mode Transition in a Cavity-Stabilized Scramjet

This study experimentally investigates flame dynamics during a combustion-driven mode transition in a cavity-stabilized scramjet model with an inlet Mach number of 2.2 using a split fuel injection scheme. Fueling into the cavity was gradually increased while keeping a constant global equivalence ratio of 0.3. A cavity-fueling percentage below 15% resulted in low-intensity combustion that corresponded to operation in scram mode. Redirecting 29% of the fuel from the upstream injection port to the cavity yielded flame spreading into the main flowpath, inducing intense combustion. A sharp pressure rise and an intense combustion region in the diverging section suggest dual-mode combustion, with thermal choking and a pseudoshock formation downstream of the cavity. An intermediate cavity-fueling fraction of 21% led to a low-frequency instability with intermittent transitions between the combustion modes, indicating operation at the thermal choking limit. Spectral and modal analyses of chemiluminescence images and static-pressure measurement indicate peak response at 10–20 Hz and , originating from oscillations in the air heater and a fuel jet instability, respectively. Thermally chocked conditions significantly amplify oscillations at these frequencies. At the thermal choking limit, there was high-amplitude response at , preventing stable dual-mode operation.