Numerical Study of Periodic Flame Flashback In A Cavity-Based Scramjet Combustor

The phenomenon of periodic flame flashback in an ethylene-fueled scramjet combustor with cavity flameholders was numerically investigated. The air inflow has a Mach number of 2.6 and a stagnation temperature of 1225 K. The overall equivalence ratio was 0.29. A 3-D URANS method with a two-step kinetic model was adopted. Results demonstrated a low-frequency periodic oscillation process with sizable flame-front movements and large-amplitude variations of wall-pressures and thrust. A mechanism with four evolution stages was proposed to elucidate the flow-flame interaction. In Stage I, the auto-ignition tendency of well-mixed unburned gas in the near-sidewall high-temperature region contributed to the rapid upstream flame flashback and shock-train extension. In Stage II, the combustion-induced back pressure and shock train gradually achieved an aerodynamic balance. In Stage III, the combustion flow changed little, while temperature gradually increased upstream of the cavity in the region away from the sidewall. A simplified heat conduction model was proposed to speculate the spanwise heat conduction as the reason for this increase. The increased temperature would trigger upstream flame propagation with enhanced heat release due to auto-ignition in Stage IV. However, the enhanced heat release was mostly in subsonic flow, resulting in pressure decreases according to 1-D flow equations. Thus, the separation near-sidewall became smaller, triggering the rapid downstream flame recession. Besides, the comparison of flame propagation speed with theoretical estimations indicated that the current flame was in the regime of turbulent flame propagation, rather than the C-J detonation or deflagration speculated in previous studies.