Flame Characteristics and Fuel Entrainment Inside a Cavity Flame Holder in a Scramjet Combustor (Postprint)

Flame structures and operating limits of an ethylene-fueled recessed cavity flameholder were investigated both experimentally and numerically, using a newly developed AFRL research scramjet flowpath at Wright-Patterson Air Force Base. Flush-wall low-angled injectors were used as main fuel injectors. The 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 and without direct cavity fueling. With direct cavity fueling, significant variation in the shape and spatial distribution of the cavity flame was observed at various fuel flow rates with and without back pressurization. It was found that both lean ignition and blowout limits increase with the characteristic air flow rate. The lean blowout limit is decreased toward a lower value as the shock train is pushed toward upstream. With fueling from main injectors, the flame is mainly distributed within the body wall corners for the present flowpath. The rich blowout limit for a cavity fueled with both main and cavity fuel is lower than for the case with cavity fuel alone, due to main fuel entrainment from the low-angle injectors. Qualitative composition analysis indicates that the gas mixture inside the cavity mainly contains combustion products and is relatively rich with main fuel only. Consequently, additional fuel injection into the cavity increases the probability of blowing out the entire flame by disabling the flame holding capability of the recessed cavity for the present flowpath and injector designs. The rich blowout limit with main fuel injection was found to increase with the body-side fuel flow rate. Merging of fuel plumes injected from upstream injectors creates an aerodynamic blockage for air entrainment into the cavity and, consequently, reduces the rich blowout limit. NOMENCLATURE

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