LAUSR

Abstract The flameholding characteristics of a model scramjet were investigated experimentally. Experiments were performed in a shock tunnel at a nominal freestream Mach number of 4. The considered total enthalpy was approximately 1.7 MJ/kg. The test model consisted of a double- ramp inlet, combustor with cavity, and cowl. The investigated fuel was ethylene, with a fuel–air equivalence ratio ranging from 0.05 to 0.21. Shadowgraph and flame luminescence images were obtained. The effects of injection location, angle, and equivalence ratio on flameholding were investigated. As the equivalence ratio was increased, the flame signals in the cavity shear layer strengthened with a near-constant spreading angle for inlet injection. The flame was maintained at the shear layer above the cavity. Unlike in the inlet case, the cavity upstream injection indicated that the flame was quite unstable during steady flow, and in general, the flame height downstream of the cavity increased as the equivalence ratio increased. In line with the literature, the flame stabilization mode for inlet injection was categorized as a “cavity shear layer-stabilized flame,” while the cavity upstream injection was a “combined cavity shear layer/recirculation zone-stabilized flame.” The effect of the injection angle on flameholding indicated that transverse injection exhibited superior fuel–air mixing compared to angled injection for the inlet case. With the cavity upstream injection, the flame was effectively maintained for both the transverse and angled injections, with approximately equivalent flame spreading angles downstream of the cavity.