Longitudinal and azimuthal thermoacoustic modes in a pressurized annular combustor with bluff-body-stabilized methane-hydrogen flames

In this experimental study on a methane-hydrogen fired laboratory-scale pressurized annular combustor, various steady and transient operating conditions were explored. Under steady operation, the thermoacoustic stability depended strongly on the equivalence ratio, but only weakly on the mass flow rate and hydrogen power fraction. During transient equivalence-ratio ramps, the predominant instability of the system switched between standing and spinning azimuthal modes. The ramping rate influenced the thermoacoustic amplitude during the switching, as well as the onset and decay of the different modes. The strong acoustic reflection at the choked outlet produced significant harmonic content during both steady and transient operation. Nonlinear time-series analysis and clustering algorithms showed that the Jensen-Shannon complexity and the permutation entropy could accurately classify the modal dynamics in an unsupervised manner. The classification revealed that standing azimuthal modes, with several preferred orientations, dominated under steady operation. The amplitude of the heat-release-rate (HRR) oscillations around the annulus depended on the mode orientation. The energy contribution from the harmonic components was isolated via bandpass filtering, revealing that the orientation of the first harmonic (n = 2) followed the fundamental mode, eliminating negligible HRR oscillation regions at the pressure node of the fundamental mode. By filtering out the higher-order harmonic content, intermittent switching between high-amplitude periodicity and lowamplitude chaos in the fundamental azimuthal mode was uncovered. Tools from dynamical systems theory confirmed the existence of chaos and demonstrated that the intermittency belongs to the type-II Pomeau-Manneville class. This study provides evidence of the typeII intermittency route to chaos in an annular combustor with longitudinal and azimuthal modes.