Analysis of supersonic combustion characteristics of ethylene/methane fuel mixture on high-speed measurements of CH* chemiluminescence

Supersonic combustion behaviors in a Mach 2.0 scramjet model combustor were experimentally investigated at stagnation temperatures of 1700–2300 K. An ethylene/methane fuel mixture was used, and the mole fraction of ethylene and the equivalence ratio were varied. Six typical combustion modes were classified based on high-speed imaging of CH* chemiluminescence at 50,000 fps, shadowgraph imaging at 4000 fps and pressure distributions. Ram mode combustion was observed, without thermal choke at the nozzle throat. Mode maps indicated that an increase in the ethylene concentration improved the supersonic combustion performance. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) of sequential images of CH* chemiluminescence were also conducted. POD and DMD mode spectra showed large peaks in the frequency range of 100–500 Hz for cavity shear-layer stabilized combustion. Although the power of the spectra decreased, peaks were also observed in the same frequency range for jet-wake stabilized and ram combustion. In the case of the ram combustion mode, the peak heights decreased. The FFT of several primary POD modes and the power spectra of DMD modes showed peaks with similar frequencies. Combustion modes could be classified from these spectra. The fundamental combustion frequencies were captured by the modal decomposition of the high-speed images of CH* chemiluminescence.