Dual-mode scramjets have the potential to provide efficient, air-breathing propulsion at high flight Mach numbers. Flame stabilization and spreading are a challenge in such engines due to the very high flow velocities. Combustion occurs in a complex regime where both flame properties and auto-ignition reactions are expected to be important. The focus of the current study is to improve the physical understanding of the combustion mechanism and its practical implication in such combustors. Topics of interest are the combustion stabilization locations, the detailed structure of the reaction zone, and the physical mechanisms controlling the heat release distribution. A facility was developed for the experimental investigation of a laboratory dual-mode scramjet combustor at conditions equivalent to flight Mach numbers of 4.3 to 5.5. The combustor contained flush wall fuel injection and a cavity flameholder, which are the basic flow elements in many proposed practical designs. The diagnostics used include high speed movies of the chemiluminescence, wall pressure measurements, and planar laser induced fluorescence of CH, and simultaneous OH/formaldehyde. The study revealed two distinct reaction zone structures that are caused by two flame anchoring locations. Cavity stabilized combustion occurs at low stagnation temperatures. For these conditions the reaction zone is anchored at the cavity leading edge and the flame spreading is controlled by premixed flame propagation. Jet-wake stabilized combustion occurs at high stagnation temperatures. The reaction zone is a lifted jet flame, which has a premixed base and a downstream diffusion flame. In all cases, initial auto-ignition reactions occur well upstream of the primary reaction zone, resulting in an auto-ignition assisted flame base. The results are useful for developing physics based models of the combustion
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