Thermoacoustic Instabilities in an Annular Rotating Detonation Combustor Under Off-Design Condition

Experiments are performed to investigate the combustion features in a laboratory-scale hydrogen (H-2)/air annular rotating detonation combustor under the off-design condition of low air mass flow rates. The air mass flow rate is ranged from 10 to 115 g/s, and the overall equivalence ratio is ranged from 0.6 to 1.0. The unstable longitudinal pulsed deflagration (ULPD), rotating fast deflagration (RFD), and stable longitudinal pulsed deflagration (SLPD) are found to exist in the combustor according to the analysis of pressure traces obtained at different circumferential and axial positions. The RFD is such a combustion instability coupled with the first tangential acoustic mode, but it occurs very rarely. The propagation speed of the RFD waves is approximately 500 m/s, and the pressure wave amplitudes are as low as 0.2 MPa. The SLPD appears with stronger and periodic pressure oscillations, and it is coupled with the first longitudinal acoustic mode of the combustor. The ULPD found in the present experiment can be regarded as the longitudinal mode even though the main frequency band is wider. The pressure oscillations are very irregular in the ULPD situation, but they are traveling forward and backward along the axial direction of the combustor. The variation of the air mass flow rate has more influence on the oscillation intensity of the deflagration wave than the variation of the equivalence ratio. It is also found that the pressure oscillations in the air and hydrogen plenums are forced by the unstable combustion process in the annular combustor. The unchoked condition of the injectors contributes the system coupling of the combustor and plenum.