Effects of acoustic liners' nonlinear characteristics on attenuating self-sustained combustion oscillations in a bluff-body flame

Acoustic liners are key passive dampers for reducing self-sustained combustion oscillations in gas turbine combustors and afterburners. This study experimentally and numerically explores how acoustic liners mitigate longitudinal combustion oscillations in a combustor featuring a bluff-body diffusion flame. Combustion tests measured oscillation suppression using liners with varying porosities, complemented by acoustic simulations clarifying liner operating mechanisms. Results indicate that optimizing the maximum sound absorption coefficient alone does not yield optimal oscillation attenuation. Instead, lowering the acoustic impedance (dimensionless impedance <0.5) significantly enhances suppression effectiveness. Additionally, attenuating combustion oscillations reduces their frequency by up to 40 Hz, explaining why maximizing absorption and minimizing oscillation intensity cannot be simultaneously optimized. Liners with larger aperture ratios exhibit better suppression performance under identical backing cavity conditions, owing to reduced oscillating velocity within the holes, which lowers the nonlinear acoustic resistance. When the peak sound absorption frequency is approximately 70 Hz lower than the self-sustained oscillation frequency, the liner can reduce the pressure oscillation amplitude by 81% (equivalent to 14.5 dB). This work offers new insights into acoustic liner design for passive control of combustion oscillations in afterburners.