Analysis of air inlet and fuel plenum behavior in a rotating detonation combustor

•Pressure fluctuations in the reactants plenum of an RDC are analyzed experimentally.•A low frequency sinusoidal base oscillation is discovered in the air inlet.•This low frequency is shown to be a probable cause of Helmholtz resonance.•A novel way to determine the operating frequency of the RDC is established. The behavior of the oxidizer inlet and the fuel injection plenums during the operation of a Rotating Detonation Combustor (RDC) is studied using pressure sensors in the air injection gap, the fuel plenum, and in the combustor. Significant pressure feedback from the rotating detonation wave is observed in the air injection gap. Pressure feedback into the fuel plenum is relatively weaker. The average normalized cross-correlation between the pressure–time series in the air injection gap and within the combustor is greater than 0.3. The air injection gap has a considerable base sinusoidal oscillation in the same frequency range as a previously discovered waxing-and-waning instability in the combustor. The fundamental frequency in the air injection gap is the same as the RDC operation frequency for almost all test cases, indicating the high efficacy of the sensors in the air inlet to attain the operating frequency. Frequency analysis reveals notable spatial variation in the fuel plenum dynamics. The low frequency oscillation in the air injection gap is found to be constant at 235 (±2.5) Hz for all the air flow rates and equivalence ratios tested.