Experimental study on the coupling process of flow and combustion phenomena during pilot ignition transients

Abstract

The pilot ignition of kerosene under different pre-established hydrogen flames is performed in a model scramjet combustor with the isolator entrance Mach number of 2.52, the total pressure of 1.6 MPa, and stagnation temperature of 1486 K, which corresponds to Ma 5.5 flight condition. The flame dynamics and evolvement of the precombustion shock train during pilot ignition transients are acquired through high-speed photography and schlieren photography. The flame stabilization mode of pilot hydrogen belongs to cavity-recirculation-region stabilized combustion with smaller equivalence ratio. While the flame stabilization mode turns into jet-wake stabilized combustion as the equivalence ratio of pilot hydrogen increases. Liquid kerosene can not be ignited by cavity-recirculation-region stabilized flame, while it can be ignited successfully by pilot hydrogen flame stabilized in the jet-wake. Thermal choking resulted from combustion of the pilot fuel should be large enough to produce and sustain the precombustion shock train in the combustor for igniting liquid kerosene. There is a positive feedback mechanism between combustion and flow dynamics. The backpressure caused by combustion produces and sustains the precombustion shock train, which in turn increases combustion intensity. The positive feedback mechanism lasts until a new balance between combustion and flow phenomena is achieved. Boundary-layer separation is the main reason for the upstream propagation of flame, which is a result of the large adverse pressure gradient caused by heat release.