Dual-Mode Scramjet Flameholding Operability Measurements

Measurements were made in a direct-connect combustor facility designed to simulate cavity flameholding in a hydrocarbon-fueled dual-mode scramjet combustor where the presence of a shock train upstream of the flameholder has a significant impact on the inlet flow to the combustor and on flameholding limits. A mechanical throttle was installed in the downstream end of the test rig to provide the backpressurization needed to form the shock train and to decouple the operation of the flameholder from the backpressure formed by heat release and thermal choking, as it would be in a flight engine. The flameholding limits were measured by ramping inlet air temperature down until blowout was observed. The test facility used a vitiated air heater, Mach 2.2 and 3.3 inlet nozzles, a 0.65 in.-deep cavity, and ethylene and heated JP-7 fuel. A Mean blowout temperature of 1502 R was measured at the baseline condition which used a Mach 2.2 inlet, a cavity pressure of 21 psia, and ethylene fuel. The blowout temperature was found to be most sensitive to fuel injection location and fuel flow rates, and relatively insensitive to inlet Mach number and operating pressure. Video imaging showed unsteady flame structures with significant movements laterally and upstream of the flameholder.