Flame Stabilization Mode Transition Induced by Pilot Injection in a Cavity-Based Scramjet

The effects of pilot hot gas injection on flame stabilization in a cavity-based scramjet were investigated using three-dimensional unsteady Reynolds-averaged Navier–Stokes simulations. Ethylene fuel was injected within a dual-solution regime exhibiting two possible flame stabilization modes. Pilot injection assisted both ignition and flame stabilization. Two representative pilot heating levels, low and high enough, were examined to assess their impact on flame behavior and mode transition. Results show that flame stabilization was highly sensitive to pilot heating power: low heating sustained cavity shear-layer stabilization, whereas high enough heating induced a transition to jet-wake stabilization. This transition was primarily driven by reduced ignition delay in the unburned jet-wake region due to elevated temperatures from pilot heating. Additionally, pilot injection enhanced fuel–air mixing through vortex generation. Both effects intensified with increasing pilot heating power. Consequently, high enough heating power facilitated upstream flame propagation and flow separation, ultimately triggering the transition. A theoretical analysis based on the Semenov thermal ignition theory further showed that high enough pilot heating promoted chemical heat release within a control volume in the unburned jet-wake region, which exceeded the enthalpy outflow in the initial cavity shear-layer stabilization mode. This energy imbalance led to a sustained temperature rise, initiating the transition.