Numerical study of periodic flame flashback in a cavity-based scramjet combustor

The periodic flame flashback phenomenon in an ethylene-fueled cavity-based scramjet combustor was numerically investigated by a three-dimensional unsteady Reynolds-averaged Navier-Stokes solver with two-step kinetics. The air inflow stagnation temperature is 1225 K, and its Mach number is 2.6. Spectral analyses revealed the combustion oscillations with flame flashbacks maintained in the separated scramjet mode with the establishment/vanishment of flow separation near the fuel injector, differing from previous studies of flame flashbacks connected to the ramjet/scramjet mode transitions. A mechanism with four evolution stages was proposed to elucidate the flow-flame interaction. In stage I, a rapid flame flashback upstream and shock-train extension were caused by the high-temperature induced auto-ignition tendency of well-mixed unburned gas in the near-sidewall low-speed region. In stage II, the combustion-induced back pressure and shock train gradually achieved an aerodynamic balance. The combustion flow barely changed in stage III. Meanwhile, a simplified model suggested that the gradual temperature rises occurring upstream of the cavity and away from the sidewall were caused by spanwise heat conduction. The higher temperatures would cause upstream flame propagation with enhanced heat release due to auto-ignition. However, the enhanced heat release occurred mostly in the subsonic flow, resulting in pressure decreases according to one-dimensional flow equations. A smaller near-sidewall separation was produced by the lower back-pressures, which prompted the rapid flame recession downstream in stage IV. Moreover, a simplified flame-spreading model was proposed to illuminate the flame propagation nature. The comparison of flame speeds 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.