Combustion Hysteresis Phenomenon in a Dual-Mode Scramjet

The combustion hysteresis of a hydrogen-fueled ramp-based dual-mode scramjet under a flight Mach number of 5 was examined by using IDDES based on up to 115.75 million cells and a detailed 13s/33r [Formula: see text] mechanism. As is evident by the wall pressure measurements, the scramjet and ramjet modes were successfully reproduced for four global fuel equivalence ratios ([Formula: see text]). However, a hysteresis in the mode transition occurs under the [Formula: see text]-increasing and [Formula: see text]-decreasing paths. A smooth ram-to-scramjet mode transition with gradual pressure drop occurs when [Formula: see text] decreases to 0.08 under the [Formula: see text]-decreasing path, whereas a jumped scram-to-ram mode transition with abrupt pressure rise occurs at [Formula: see text] under the [Formula: see text]-increasing path. The combustion efficiency under the [Formula: see text]-decreasing path is always higher than those under the [Formula: see text]-increasing path within the hysteresis loop. This higher efficiency can be attributed to mixing enhancement and kinetic strengthening. For the former, longer residence time and richer vortexes promote the mixing. Meanwhile, the latter kinetic strengthening is contributed by the higher pressure, higher temperature, and richer radicals inherited from the former combustion state. A dynamic regulation mechanism realized through a feedback loop with a characteristic response time of 0.89 flush through time resides the pseudo-shock wave in the isolator and sustains the ramjet mode. Hysteresis causes more total pressure loss due to stronger pseudo-shock structure and more heat addition. Within the hysteresis loop, higher combustion efficiency dominates the higher thrust under [Formula: see text]-decreasing path.