Investigation of the effects of equivalence ratio on combustion oscillation in scramjet combustors

The equivalence ratio (ER) plays a critical role in governing fuel combustion and combustion stability. However, its influence on the evolution of flame-flow-acoustic coupling under high-enthalpy conditions remains inadequately understood. This study investigates the combustion process in a supersonic combustor under varying equivalence ratios using experiments conducted in a ground-connected pulse-combustion wind tunnel. High-speed optical imaging and synchronized multi-point wall pressure measurements were employed to capture the spatiotemporal dynamics of flame oscillations. Modal decomposition and other analytical methods were utilized to identify the primary mechanisms driving combustion oscillations. Results revealed that, across all tested equivalence ratios, combustion oscillations were predominantly influenced by flow-induced mechanisms governing flame upstream propagation and downstream retreat. The dominant oscillation frequency decreased as the equivalence ratio increased, ranging from 228 Hz to 82 Hz. Additionally, as the equivalence ratio rose, the combustion oscillation mechanism became increasingly dominated by acoustic effects, accompanied by a notable enhancement in oscillation energy. Quantitative analysis using the Rayleigh index demonstrated a progressive strengthening of thermoacoustic feedback with increasing equivalence ratio. This study provides quantitative insights into how equivalence ratio affects combustion oscillation modes, frequency shifts, and the intensity of thermoacoustic coupling under high-enthalpy supersonic combustion conditions. These findings are crucial for designing combustion stability margins in scramjet engines, thereby contributing to the advancement of hypersonic vehicle technology.