Ethylene Flame Dynamics and Inlet Unstart in a Model Scramjet

Ethylene flame dynamics in a model scramjet installed in an open test section of an arc-heated hypersonic wind tunnel are experimentally investigated with freestreams of Mach 4.5–9. An ethylene fuel jet is injected into the model scramjet that is autoignited by high-enthalpy flows compressed and decelerated by a train of shock waves and boundary layers developing in the scramjet isolator/combustor. The ethylene flame behavior is captured by a high-speed movie camera through optical access windows on the model and characterized by surface pressure/temperature traces recorded at five different locations along the flowpath through the model scramjet. Downstream pressure buildup due to combustion heat release, shock-induced compression, and mass loading via the fuel jet triggers the flame propagation toward upstream, stabilizes the flame at a location in the combustor, or pushes the flame further upstream to cause inlet unstart. In particular, flame dynamics while the scramjet undergoes the inlet unstart with Mach 4.5 freestreams are investigated intensively. In addition, stagnation temperature, overall fuel concentration, and oxygen concentration in freestream flows are varied to investigate their influences on the flame dynamics. The flame propagates faster and anchors further upstream with increased total temperature, oxygen enrichment, and overall equivalence ratio slightly above unity.