Experimental investigations of cavity parameters leading to combustion oscillation in a supersonic crossflow

Abstract

The effects of cavity parameters on combustion oscillation inside an ethylene-fueled scramjet combustor equipped with a cavity flameholder are experimentally investigated for Mach 5.5 flight conditions. Three certain cases, such as i) longer length-to-depth ratio, ii) sharper aft degree of cavity, iii) closer air throttling downstream of the cavity, exhibit quasi-periodic combustion oscillation, which can be separately attributed to i) larger recirculation volume in the cavity and more mass and heat exchange between cavity shear layer and the core flow, ii) the stronger impinging shock wave in the cavity acting on shear layer, iii) improved fuel/air mixing owing to the interaction between separated boundary layer and combustion. High-speed and schlieren images demonstrate that the cavity and downstream of it act as the most sensitive areas, at the same time, the factors mentioned above can form a thermal throat further triggering flame flashback, which is an indispensable key sub-process of combustion oscillation. The quantitative analysis results obtained from iso-luminosity contour have shown the different distribution trends of flame front and distinct differences of quasi-periodic oscillation frequencies, whereas similar flame propagation speed distributions. In addition, a simplified combustion opening system model has been established to analyze combustion oscillation mechanisms, which theoretically demonstrates that above factors can destroy the balance of heat release and dissipation, causing the system cannot self-stabilize once certain temperature fluctuation thresholds in sensitive areas are exceeded.