Pressure Rise Due to Flame Compressibility in a Cavity-Fueled Scramjet Combustor

In this work, the effects of distributed fueling and pressure rise in a cavity-based scramjet combustor are investigated, while simulating relevant flight enthalpies, in a vitiated, Mach 2.8 direct connect wind tunnel. Schlieren imaging, broadband chemiluminescence imaging, and pressure transducers were utilized to demonstrate flow behavior in the cavity test section. It was seen that for very lean fuel-air mixtures, defined with an equivalence ratio ($\phi$) of less than 0.1, the behavior of the system agreed with one-dimensional compressible flow computation predictions. However, for higher fuel-air mixtures that were still lean, above a ($\phi$) of 0.2, a unique behavior was noticed. Using test section pressure readings, combustion efficiency was calculated for all cases, where a significant decrease in efficiency was seen as the global choking limit is realized. This phenomenon is conjectured to arise from adverse pressure gradients at the cavity lip coupled with greater local heat release. Consequently, it is theorized that this can cause a combustion-supported oblique shock to stabilize near the cavity lip region, tapering the combustive efficiency.