Non-linear response of a swirl-stabilized flame to high-frequency transverse acoustic excitation

This study provides a detailed experimental investigation into the non-linear response of a swirl-stabilized, partially premixed methane–air flame under large-amplitude in-phase transverse acoustic excitation. Motivated by azimuthal thermo-acoustic instabilities in gas turbine combustors, experiments are conducted in a model gas turbine combustor using simultaneous high-speed particle image velocimetry, OH* chemiluminescence, and unsteady pressure measurements. The flame is excited at one of the dominant resonant modes of the combustor (1500 Hz) with varying forcing amplitudes, corresponding to 1.6%–5.0% of chamber pressure. At low forcing amplitudes, the flame maintains a periodic V-shaped structure with stable, linear acoustic-flame coupling. Increasing the forcing amplitude induces amplitude modulations and spectral broadening, signaling the onset of non-linear interactions. Recurrence analyses reveal a transition to type-II intermittency, characterized by bursts of high-amplitude pressure oscillations interspersed with low-amplitude states. At higher amplitudes, strong intermittency and chaotic behavior are observed, leading to flame blow-off. The flame dynamics are closely linked to transitions in the underlying flow structure, with the swirling jet evolving from a columnar vortex breakdown (V-flame) to a wall-jet (wall-flame) configuration under strong transverse excitation. This transition promotes heat loss to the combustor walls, leading to intermittent local extinction and re-ignition events that destabilize the flame. Cross wavelet transform analysis highlights a progressive loss of phase-locking between pressure and heat release rate fluctuations with increasing forcing amplitude. The results emphasize the critical role of non-linear flame dynamics and flow-flame-acoustic coupling in driving combustion instabilities at high-frequency and high-amplitude excitation. This study provides new insights into the mechanisms governing intermittent behavior and blow-off in swirl-stabilized flames, with implications for the design of more robust, low-emission combustion systems.