Mechanisms and control of combustion-mode transitions in a hydrogen-fueled air-breathing rotating detonation engine

Precise regulation of combustion modes is essential for achieving stable and efficient operation of air-breathing rotating detonation engines (RDEs). However, the physical mechanisms governing mode transitions remain insufficiently understood, which significantly hinders the development of effective control strategies. In this study, a systematic experimental investigation is conducted on a hydrogen-fueled air-breathing RDE using a direct-connect test platform to elucidate the dominant mechanisms responsible for combustion-mode transitions. The experimental results demonstrate that the observed transitions among multiple combustion modes are governed by two primary physical factors. First, fuel–air mixing quality determines the transition between the collision-wave mode and the stable single-wave mode. Enhancing mixing—either by advancing the fuel injection location upstream to the combustor inlet or by introducing a hydrogen premixing fraction of at least 40%—promotes the formation of a stable single-wave mode. Conversely, degraded mixing at elevated inlet total temperatures leads to a reversion from the stable single-wave mode to the collision-wave mode. Second, the transition from the stable single-wave mode to unstable combustion regimes is controlled by the coupling strength between forward-propagating pressure disturbances and the intake airflow. Weak coupling results in a hybrid pulsed-rotating detonation mode, whereas strong coupling disrupts the intake process and induces chaotic combustion. These mechanisms are further validated by demonstrating that controlled variations in inlet total temperature, fuel injection location, and combustor configuration can reliably induce predictable transitions among four distinct combustion modes: stable single-wave, collision-wave, oscillating single-wave, and chaotic combustion modes. The results establish a unified physical framework linking fuel–air mixing, pressure–intake coupling, and combustion stability, providing practical guidance for combustion-mode regulation and optimization in air-breathing rotating detonation engines.