Initiation pathways in non-premixed methane/oxygen rotating detonation combustors with hollow and annular chambers

To examine the initiation pathways and operating characteristics of non-premixed methane/oxygen rotating detonation combustors (RDCs) under pre-detonator ignition, this study experimentally investigates hollow and annular RDCs with the same injection structure and ignition conditions. Experiments were conducted under near-stoichiometric conditions (ER = 1 ± 0.06) over nominal total mass flow rates of 40–200 g/s. High-speed imaging and high-frequency pressure measurements were used to analyze post-ignition wave evolution, pressure response, and operating modes. The results show that the initiation process does not involve direct evolution from the initial wave system to stable detonation. Instead, it first undergoes initial wave system evolution and an adjustment process, after which the pathway is classified as fast or delayed initiation according to whether a sustained deflagration stage occurs. Within the tested range, the hollow RDC always exhibits fast initiation, whereas the annular RDC mainly exhibits delayed initiation. In fast initiation, the initial disturbances are re-amplified before complete decay and develop into rotating detonation waves (RDWs). In delayed initiation, the initial disturbances decay to near the background fluctuation level, after which RDWs are re-established through stochastic deflagration-to-detonation transition (DDT). At comparable mass flow rates, the higher combustor static pressure in the annular RDC restricts injection recovery during adjustment, resulting in a temporal mismatch between initial wave system decay and reactant replenishment, followed by sustained deflagration. During the matching process and subsequent operation, the RDW number decreases with increasing total mass flow rate. Intermittent operation appears only in the annular RDC at lower flow rates. During a representative extinction event, a serrated front decelerates, reverses direction, and collides with another RDW, causing extinction. These results indicate that initiation pathways are governed by the temporal matching between initial disturbance decay and injection recovery, whereas RDW matching and operating stability are further influenced by reactant replenishment and wave interaction.