Investigation of back-pressure driven flame propagation induced by fuel switching in a supersonic flow

This study investigates the back-pressure driven flame propagation induced by kerosene–ethylene fuel switching in a dual-cavity supersonic combustor under Ma 1.6 inflow conditions through direct-connect experiments. By comprehensively utilizing high-speed schlieren imaging, excited state methylidyne radical chemiluminescence imaging, and reduced-order variational mode decomposition (RVMD), the dynamic responses under two injection strategies, “kerosene in upper cavity switch to ethylene in lower cavity” (Case I) and “kerosene in upper cavity switch to ethylene in upper cavity” (Case II), are compared and analyzed. The results indicate that in Case I, downstream thermal choking induces the progressive upstream propagation of a pre-combustion shock train, which forms a strongly coupled positive feedback loop with heat release, leading to flame flashback. In contrast, in Case II, due to the merging of upstream jet shocks weakening the combustor's resistance to back-pressure, downstream thermal choking triggers a detached oblique shock that rapidly propagates upstream. The flame response lags behind the abrupt flow-field change, exhibiting a sequential mechanism of “shock propagation—flow-field abrupt change—flame reconstruction.” RVMD analysis further reveals that the low-frequency mode in Case I corresponds to the coupled process of “heat release—recirculation zone expansion,” whereas Case II exhibits a shock-triggered flame flashback mode. This study clarifies the different back-pressure driven flame propagation in the fuel switching process, providing insights for combustion control in scramjet engines.