Chang, MinwookRotating Detonation Engines (RDE) utilize detonative combustion processes for heat release instead of deflagration, which is more commonly used in conventional combustors. Potential benefits of RDE include pressure gain combustion, efficient energy conversion, and simpler designs that avoid combustion instability problems due to their cyclic nature of operation. It has been observed, however, that RDE operation can become unsteady due to the onset of counter-rotating detonation waves. In addition, the random presence of residual liquid fuel droplets and their unexpected breakup could also affect the periodic operation of RDE. This study aims to better understand the physical mechanisms that destabilize the RDE’s periodic processes and lead to unsteady operation. Specifically, the investigation focuses on understanding physical mechanisms associated with two key off-design scenarios: (i) the onset of counter-rotating detonation waves and their impact on next cycle fuel injection, and (ii) the breakup of liquid fuel droplets by detonation wave and decoupled detonation wave which consists of shock and flame fronts. Experiments using either hydrogen-oxygen or ethylene-oxygen detonation in linear channel simulate an unwrapped RDE combustor process. For the counter-rotating wave study, detonation waves are initiated from both ends of the channel, and complex recovery behavior associated with colliding detonation waves is examined providing insights into RDE slapping mode operation. For the fuel droplet breakup study, 2-mm diameter ethanol and JP-8 droplets are placed on the downstream path of detonation waves and decoupled shock-flame fronts, which propagate at average wave speeds of Mach 7.3, 3.6, and 2.6, respectively. Liquid droplets break up faster when exposed to slower decoupled shock-flame waves compared to faster detonation waves. This unexpected difference is attributed to the initial slip flow Mach number around the droplet, which is subsonic for detonation waves but supersonic for decoupled waves. Research findings suggest that the slip flow Mach number, along with the Weber number, plays a crucial role in RDE fuel droplet breakup.enDissertationAerospace engineeringCounter Propagating Detonation WaveDetonation-droplet interactionInjector Refresh JetLiquid Fuel Droplet BreakupRotating Detonation EngineShock-droplet interaction