Recirculation effects on detonation in a flow-through ramjet combustor

The application of detonation combustion to enhance the combustion performance of ramjets requires careful consideration of combustion stability. Recirculation zones are critical for flame stabilization and detonation enhancement, yet their governing role in detonation-dominated combustion regimes remain poorly understood. This work numerically examines the cavity-induced recirculation effects on rotating detonation wave (RDW) behavior in a flow-through ramjet rotating detonation combustor by varying the cavity front-step expansion angle θ over five configurations: 90°, 58°, 38°, 21°, and 11°. A weak abrupt expansion (11°) fails to establish a recirculation zone and results in deflagration regime. Moderate expansion generates stable recirculation and sustains detonation regime, whereas a further increase toward 90° causes RDW intensity to first increase and then decrease. The optimal detonation performance occurs at θ = 58°, corresponding to the highest radial pressure peaks and the maximum combustion efficiency. Recirculation zones deliver thermal preconditioning and reactive radicals at the contact region of the unburned mixture and product to facilitate rapid detonation heat release, yet excessive recirculation promotes parasitic combustion that competes with the detonation front. Appropriately reducing θ within the detonation-sustaining region weakens parasitic combustion at the interface and improves the regularity and stability of RDW propagation. The RDW-induced counterflow provides analogous recirculation benefits by prolonging reactant residence time and enhancing mixing, underscoring the importance of a properly scaled recirculation zone. This study elucidates the mechanism by which the cavity front-step angle regulates recirculation to control RDW propagation and stabilization, and establishes a novel design paradigm for ramjet rotating detonation engines that prioritizes optimal RDW propagation over large flame-holding recirculation, supporting the design and active control of high-performance rotating detonation engines.