Acoustic Characterization of an Ethylene-Fueled Scramjet Combustor with a Cavity Flameholder

*† ‡ § ** †† ‡‡ The occurrence of combustion oscillations has recently raised serious concerns about the development of scramjet engines. Previous studies on supersonic combustion for high-speed airbreathing propulsion applications indicated that combustion may take place in subsonic regions, such as boundary layers and recirculation zones in flame-holding cavities. During this process, a longitudinal mode of thermoacoustic instability may develop in a spatial domain reaching from the shock train to the flame zone. The present work experimentally and analytically investigates such thermoacoustic instabilities inside an ethylene-fueled scramjet combustor with a recessed cavity flameholder. High-speed pressure transducers are utilized to record acoustic signals. The effects of fuel/air equivalence ratio, fueling scheme, and simulated flight conditions on the stability characteristics of the combustor are examined systematically. A companion analytical analysis is also established to help explore the underlying mechanisms responsible for driving and sustaining thermoacoustic flow instabilities. In particular, the interactions between the unsteady heat release, fuel injection and mixing, and shock response are examined. The measured oscillation frequencies agree well with the characteristic frequencies related to the acoustic feedback loop between the shock and flame and the acoustic-convective feedback loop between the fuel injection and flame.