Flame Propagation Induced by Shear Layer Ignition in an Axisymmetric Supersonic Combustor

Characteristics of flame propagation induced by shear layer ignition (SLI) in an axisymmetric supersonic combustor were investigated using laser-induced plasma (LIP). A fully transparent axisymmetric supersonic combustor with a speed of Mach 2.5 and a total temperature of 1486 K was employed to conduct the LIP ignition and optical observation. The characteristics of flame propagation induced by SLI and traditional cavity ignition (CI) with global equivalence ratios (GERs) of 0.18 and 0.24 were comparatively analyzed. The evolution of flow structures was numerically simulated to reveal the acceleration mechanism of flame propagation induced by SLI. Initially, the flame kernel generated by LIP at the shear layer is transported to the cavity ramp and then to the rear cavity, where it resides and forms a flame base that continually ignites the fuel–air mixture from the cavity ramp. Secondly, the resident flame extends from the rear cavity to the leading edge, forming a cavity recirculation region flame. Finally, a circumferential flame propagation is induced to extend the flame throughout the loop-shaped cavity recirculation region and shear layer. SLI can accelerate flame propagation since the temperature within the cavity is increased during establishing the recirculation region flame, which significantly improves the favorable flow conditions for flame propagation. In comparison, CI generates the flame base near the cavity leading edge, forming a shear layer flame. The thin shear layer with a steep velocity gradient and high flow speed is unfavorable for rapid flame propagation. Thus, once the fuel injection is sufficient to sustain the flame kernel near the shear layer, the flame propagation induced by SLI is significantly faster than that induced by traditional CI.