Investigation of thermo-acoustic instabilities and flame structures in a partially premixed combustor

Self-excited instability experiments were performed to investigate the thermo-acoustic instabilities and their relationship with heat release rate fluctuations in swirled partially premixed combustors. The reconstructed phase space was used to figure out the pressure fluctuations, and their relationship was analyzed by using Rayleigh index. The proper orthogonal decomposition method was employed to extract flame coherent structures. Results indicated that pressure fluctuations presented evolutions of low-amplitude, intermittent, limit cycle and low-amplitude oscillations, with the increase of equivalence ratio. The fluctuation frequency was controlled by the first order pure acoustic mode of the cavity (eigenfrequency was around 80.8 Hz). The Rayleigh index was around zero value when oscillations occurred, while this parameter maintained positive at the thermo-acoustic instability state. The proper orthogonal decomposition results showed that the flame distributions reflected by the first two order modes (the total energy of these two mode was above 55%) became bright and dark alternately along the longitudinal direction at the limit cycle, resulting from continuous vortex sheddings. The frequency of time coefficients (83.2 Hz) was consistent with that of pressure pulsations (83.3 Hz). At the low equivalence ratio, the mode spatial distribution had no regular pattern. When oscillations were intermittent, the dominant mode was of an axisymmetric pattern. Only the outer edge of the flame had large-scale longitudinal motions at a high equivalence ratio. © 2022 BUAA Press. All rights reserved.