Past experience with ramjet engines suggests that combustion instabilities and associated flow oscillations may also occur in scramjet engines, although the problem has not been recognized and explored yet. The present work investigated thermoacoustic instabilities inside an ethylene-fueled scramjet combustor with a recessed-cavity flameholder. Detailed characterization was made of flame oscillation and acoustic motion using high-speed photography and pressure measurements. The effects of fuel/air equivalence ratio, fueling scheme, and simulated flight conditions on the stability characteristics in the engine flowpath were examined systematically. In addition, an analytical analysis based on the acoustic and convective feedback loops among the precombustion shock, fuel injection, and distributed flame was established to help understand and correlate the observed oscillation phenomena. The oscillation frequencies fall in a range of 100-400 Hz, with the upper bound dictated by the shock- flame coupling mechanism and the lower bound by the injector-flame interaction.
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