Destabilizing effects of oxidation-driven mass injection on the second mode of high-speed flows under conjugate heat transfer

Flow stability in high-speed boundary layers is critically influenced by high-temperature gas effects and surface reactions occurring on thermal protection materials. In this study, linear stability theory has been employed to investigate the effects of wall catalysis and ablation on flow stability during unsteady flow-solid conjugate heat transfer over a blunt cone with a Mach number of 15.99. The key physical insight is that oxidation-driven mass injection is the primary mechanism destabilizing the second-mode disturbance. Specifically, the oxidation reaction injects mass at the wall, thickening the boundary layer and shifting the second mode to a lower frequency with an enhanced growth rate, which expands the unstable region upstream. In contrast, a temporal increase in wall temperature, regardless of whether catalysis or ablation occurs, gradually stabilizes the second mode by lowering its frequency and reducing its growth rate. Boundary conditions for mass fraction and velocity perturbations are found to have negligible influence on the linear stability theory results.