Study on the Operability of Cavity Flameholders inside a Scramjet Combustor

Abstract

Performance and operating limits of an ethylene-fueled recessed cavity flameholder with various cavity lengths were investigated both experimentally and numerically, using an AFRL research scramjet flowpath at Wright-Patterson Air Force Base. Flush-wall low-angled injectors were used as main fuel injectors. Discrete flight conditions from Mach 3.5 to 5 at flight dynamic pressures up to 2000 psf were simulated with Mach 1.8 and 2.2 facility nozzles. Cavities with length-to-depth ratio (L/D) of 4, 5, and 6 were tested. Each recessed cavity features an array of fueling ports on the aft ramp for direct cavity fueling. The cavity operating conditions include 1) direct cavity fueling, 2) direct cavity fueling with back pressurization, and 3) fueling from main injectors with direct cavity fueling. It was found that the L/D=6 cavity exhibits the poorest performance among the three cavities, in terms of the lean blowout limit at cavity-only operation. With back pressurization, both the lean ignition limit and the lean blowout limit increase as compared to the case without back pressurization. The L/D=6 cavity also has a poorer lean operability when back pressurized with air throttle. Despite the poorer LBL observed above, the flowpath equipped with the L/D=6 cavity actually performs better in terms of overall combustor operability and thrust generation. The flowpath equipped with the L/D=4 cavity performs worst among these three cavity configurations both experimentally and numerically. The measured dominant frequency for acoustic pressure oscillation lies between 100 and 300 Hz, which agrees with the previous measurements inside the same flowpath equipped with the L/D=5 cavity. The acoustic oscillation does not depend on the cavity length or the injector location for the present flowpath. The L/D=6 cavity exhibits better RBL while the L/D=4 cavity exhibits worse RBL, probably due to the difference in cavity volume.