Influence of fuel plenum on thermoacoustic oscillations inside a swirl combustor

Large eddy simulation (LES) of a lean methane/air flame exhibiting self-excited thermoacoustic oscillations inside a swirl combustor is performed. The influence of fuel plenum on the combustor dynamics is studied. The velocity, temperature and mixture fraction statistics computed from the two LES cases are compared with the measurements. The results show good overall agreement for both cases with slight improvements by including the fuel plenum. In particular, the near-field mixing and flame lift-off height are captured more accurately. The dominant unstable Helmholtz frequency (≈270 Hz) computed by including the plenum is closer to the measured value (≈290 Hz) compared to that (≈250 Hz) obtained without the plenum. Mode shape analysis shows that including the fuel plenum is necessary for obtaining the phase difference between the pressure oscillations in the combustion chamber and air plenum. It is also found that self-excited oscillations are amplified with an increase by 6% in the mean Rayleigh index and 50% in the gain of the flame transfer function due to the equivalence ratio fluctuations when the fuel plenum is included.