Regulating scramjet combustor mode transition using fuel distribution control

Dual-mode scramjets can operate efficiently over a range of flight speeds from moderate supersonic to hypersonic conditions. Depending on the fueling and flight conditions, the combustion mode operates in either a thermally-choked mode or a supersonic combustion mode. Direct-connect experiments were conducted using a laboratory-scale scramjet combustor with hydrogen as fuel, and its combustion mode transition behavior was characterized over various equivalence ratios. It was observed that the combustor became susceptible to combustion instability when mode transition was occurring naturally. To explore the possibility of actively triggering combustion mode transition while alleviating the combustion instability concerns, a new strategy of changing fuel injection distribution was formulated and a series of spatially distributed fuel injection experiments were conducted. The results showed that the critical amount of fueling for mode transition depends on the degree of fuel distribution. Subsequent experiments demonstrated that the combustion mode transition timing could be effectively controlled by scheduling spatial distribution of fuel injection. When fuel was injected at one location, most of heat release was concentrated near the cavity flame-holder, leading to thermal choking at a relatively low equivalence ratio. With distributed fuel injection, heat release became more evenly distributed across the expanding portion of the combustor, effectively delaying the mode transition to a higher equivalence ratio. Through the use of fast-acting solenoid valves, it was shown that changing fuel injection distribution could be used to trigger a timely combustor mode transition while holding the total fuel flow rate unchanged. When mode transition was actively triggered, the entire transition process occurred over a significantly shorter time scale compared to the natural mode transition process. The results indicate that combustion mode transition process could be controlled at the desired timing by actively scheduling fuel injection distribution, with reduced risks of encountering combustion instabilities while transitioning.