Combustion oscillation characteristics of a supersonic ethylene jet flame using high-speed planar laser-induced fluorescence and dynamic mode decomposition

Flame dynamics and combustion oscillation are complex problems in propulsion systems. In this study, the combustion oscillation characteristics of a supersonic ethylene jet flame in a hot coflow were investigated utilizing a 5 kHz high-speed hydroxyl planar laser-induced fluorescence (OH-PLIF) technique and an advanced postprocessing method, namely, dynamic mode decomposition (DMD). A PLIF system equipped with a large laser sheet was used to collect the dynamic development process of the jet flame. An ethylene jet flow was burned at a hot coflow temperature of 1900 K at different Mach numbers (Mach 0.55–1.6). The dynamic evolution of flame microstructures was clearly obtained. The local extinction events and flame area distribution in the flame were statistically analyzed to characterize the instability of the jet flame. The results indicated that flame instability was enhanced with increasing Mach number, but the jet velocity did not affect the global flame oscillation frequency. Based on DMD analysis, the spatiotemporal three-dimensional oscillation characteristics of the jet flame were quantified. The DMD results indicated that during different time periods, the jet flame is dominated by coherent structures with different frequencies. For supersonic flow, increased jet velocity might lead the dominant mode frequency shift to a higher level.