Detonation instability mechanisms under transverse dilution jets in rotating detonation turbine engines

Effective air dilution for turbine inlet cooling without compromising detonation stability is essential yet challenging for rotating detonation turbine engines (RDTEs). This study investigates detonation instability mechanisms under downstream transverse dilution jets in a methane-fueled open-loop RDTE. Air jets introduced during operation raise combustor pressure and suppress reactant accumulation, leading to transient wave attenuation or detonation decoupling under stronger dilution. Under kilohertz oblique-shock sweeping, transverse jets undergo periodic blockage and penetration, producing non-uniform cold-air packets that break the wave-strength balance in four-wave collision modes. RDTE experiments show that jet-wave coupling is governed by the jet recovery timescale relative to successive detonation sweeping, rather than by air flow rate alone. When the two timescales become comparable, the jets recover and penetrate more deeply, promoting detonation mode adjustment. The resulting feedback between energy loss caused by dilution disturbance and energy gain associated with reactant accumulation periodically adjusts wave strength, driving wave-spacing oscillation between co-propagating detonation waves. This intermediate regime between stable propagation and complete instability provides a new perspective on mode stability boundaries in RDTEs under air dilution disturbances. Novelty and significance statement Effective air dilution is essential for turbine inlet cooling in rotating detonation turbine engines (RDTEs). Existing studies mainly address cooling feasibility, leaving its influence on detonation stability unresolved. This study develops a methane-fueled RDTE with secondary-air injection, extends RDTE research beyond the hydrogen fuel, and achieves sustained engine operation over 2 min. The detonation instability mechanisms induced by transverse air jets under sustained turbine-coupled operation are revealed for the first time. Unlike quasi-steady mixing in conventional combustors, dilution jets are found to act as unsteady aerodynamic disturbances coupled to kilohertz detonation-wave propagation. A key novelty is identifying the relative timescale of jet recovery and detonation sweeping as the governing parameter for instability onset, and elucidating the self-sustained mechanism of wave-spacing oscillations. These findings provide a physical basis for balancing turbine cooling requirements against detonation stability constraints in RDTEs.