Experimental investigation of flame dynamics in scramjet combustor under various combustion modes using laser-induced breakdown spectroscopy

Supersonic combustion mechanisms were investigated experimentally in a transversely injected ethylene-fueled scramjet model combustor with a cavity flame holder. Laser-Induced Breakdown Spectroscopy (LIBS) was employed to quantify local equivalence ratios, with simultaneous CH* chemiluminescence measurements to identify heat-release regions. The LIBS measurement locations were scanned precisely using a 3-axis stage controller actuated by stepping motors. Experiments were conducted in a Mach 2.0 airflow at a stagnation temperature of 1900 K and a stagnation pressure of 0.37 MPa. Four global equivalence ratios, 0.071, 0.154, 0.194, and 0.239, were examined, corresponding to cavity shear-layer stabilized, jet-wake stabilized, oscillation between jet-wake and ram, and ram combustion modes, respectively. The results revealed a common flame-stabilization structure across all combustion modes. The flame tip was located near a fuel-rich region at an equivalence ratio of 2–3, whereas the primary reaction zone, indicated by strong CH* chemiluminescence, appeared farther downstream in the near-stoichiometric region. Despite substantial changes in the overall flame structure, this feature persisted for all combustion modes. Together with our previous results obtained using a different fuel and injection configuration, the same flame stabilization structure was observed, indicating that this characteristic behavior is consistent in cavity-stabilized supersonic combustion. Moreover, the time-averaged LIBS results provided insight into the instability mechanism between the jet-wake and ram combustion modes. Relative to the jet-wake mode, the oscillation mode exhibited a broader near-stoichiometric mean mixture field, whereas the ram mode showed a lower-penetration and richer distribution closer to the wall and cavity shear layer. Combined with previous image-based instability analyses, these support an interpretation in which vortex-induced fuel transport promotes a mean mixture distribution favorable for heat release and drives the transition between jet-wake and ram combustion states. Novelty and significance statement This study provides a quantitative characterization of local equivalence-ratio distributions associated with combustion-mode transition and instability in a cavity-stabilized dual-mode scramjet combustor under realistic high-enthalpy supersonic flow conditions. By applying laser-induced breakdown spectroscopy together with CH* chemiluminescence, the work directly links localized mixture structure to flame stabilization across cavity shear-layer, jet-wake, oscillatory, and ram combustion modes. A common flame-stabilization structure is identified in all modes: a fuel-rich flame-tip region and a downstream near-stoichiometric main reaction zone. The measurements further reveal that the oscillatory mode corresponds to an intermediate mixture state between the high-penetration jet-wake mode and the lower-penetration ram mode, providing new experimental evidence for the role of localized fuel transport in mode transition. These findings advance the physical understanding of supersonic combustion instability and provide a useful diagnostic and design basis for low-instability, high-performance scramjet combustors.