Multi-scale flame oscillation characteristics coupled with combustion modes in a dual-cavity scramjet engine

This study experimentally investigates the combustion modes and multi-scale flame oscillation characteristics in a tandem dual-cavity scramjet engine operating under Mach 2.52 inflow conditions. Utilizing synchronized high-speed CH* chemiluminescence imaging, schlieren visualization, and wall pressure measurements, four distinct combustion modes are identified: downstream-cavity mode, dual-cavity mode, upstream-cavity mode, and ramjet mode. Statistical analysis reveals the progressive upstream propagation and vertical elevation of flame centroids with increasing equivalence ratios, while the luminosity standard deviation peaks in the dual-cavity mode. Frequency analysis, using power spectral density and dynamic mode decomposition methods, reveals distinct dominant frequencies and mode behaviors in each combustion mode. The sparsity-promoting dynamic mode decomposition analysis further illustrates that the multi-scale flame oscillations evolve from being dominated by a few modes to a multi-mode regime. The study provides insights into the combustion mechanisms and oscillation characteristics of dual-cavity supersonic combustors, which are crucial for the design of reliable scramjet engines.