Frequency transition in thermoacoustic oscillations of a lean premixed swirl burner

Thermoacoustic instabilities remain a significant challenge in lean premixed gas turbine applications, as they can lead to severe vibrations, performance degradation, and even structural damage. The oscillation frequency of thermoacoustic modes is often influenced by the flame dynamics, particularly the heat release structure and response characteristics. However, the mechanisms responsible for frequency shifts during self-excited thermoacoustic instabilities in swirl combustors are not yet fully understood. This study investigates the phenomenon of frequency transition in combustion oscillations under a constant operating condition in a lean premixed swirl-bluff body stabilized combustor. To systematically explore these dynamics, we analyze the instability patterns of thermoacoustic modes using our in-house low order acoustic network modeling code. The experimental results reveal a clear correlation between changes in flame shape and a frequency reduction. Modeling and externally flame exciting results indicate that large amplitude oscillations can trigger a transition in the flame response, affecting both the gain and time delay of the flame transfer function. The modulation of the interaction between the flame and the acoustic mode leads to a transfer of energy from the dynamic combustion process to the pressure field under a specific acoustic mode. Optical diagnostic measurements provide additional insights, showing that as the frequency decreases, the flame detaches from its initial position and reattaches to the rim of the bluff body. The findings suggest that the frequency reduction is primarily driven by a flame mode transition induced by large velocity perturbations, altering the FTF gain and time delay.