Acoustic field inconsistency in thermoacoustic oscillations of a swirling non-premixed flame: Forced vs self-sustained regimes

Using a laboratory-scale, swirling non-premixed flame, we combine experiments, large eddy simulation-acoustic simulations, and phase-closure analysis to compare forced and self-sustained regimes. The results show that although the velocity fields upstream and immediately downstream of the swirler remain similar, significant discrepancies emerge within the combustor: in the forced system, velocity perturbations originate from upstream acoustic forcing, whereas in the self-sustained system, the flame itself acts as a volumetric expansion source. In addition, considering the oscillatory fuel jet induced by back-pressure oscillations in the self-sustained regime, acoustic simulations indicate that the jet-velocity pulsation at the porous fuel nozzle reaches about 30% of the mean velocity. Even at this level, simulations show the resulting flame response is relatively modest. The differing combustor acoustic fields, together with the periodic fuel pulsation, lead to pronounced differences between forced and self-sustained oscillations. Consequently, the phase transfer between oscillating velocity, pressure, and heat release rate fluctuations differs markedly between the two regimes. Under weak oscillations and short combustor lengths, the flame responses obtained from forced oscillations reasonably approximate those in self-sustained oscillations. However, at higher oscillation amplitudes or longer combustor lengths, strong velocity fluctuations within the chamber significantly distort flame pulsations, where phase closure error was found to exceed π/2 radian. Even with careful selection of the velocity reference plane in forced oscillation tests, the pronounced differences in the acoustic field and fuel pulsation introduce new challenges for flame transfer function/flame describing function studies.