Acoustically perturbed turbulent premixed swirling flames

Abstract

The dynamics of a turbulent premixed confined swirling flame is investigated using large eddy simulation. The flame response is determined by introducing an external acoustic forcing at two modulation frequencies corresponding to characteristic values of the flame transfer function obtained experimentally. These values were found to give different responses in terms of gain in a previous series of experiments. The underlying physical mechanisms identified experimentally are investigated numerically. Simulations confirm that swirl number fluctuations and vortex roll-up govern the flame response. It is also confirmed that the first mechanism is associated with a mode conversion process taking place when acoustic waves impinge on the swirler unit. The axial acoustic velocity disturbance on the upstream side of the swirler generates an axial acoustic wave and an azimuthal convective disturbance in the downstream flow. These combined disturbances are retrieved in the simulation and their effect on the swirl number is extracted. Calculations also indicate that vortex shedding synchronized by the acoustic forcing takes place at the injector lip outlet. Vortices originating from this region are convected in the jet shear layer, impinge on the flame, and roll-up the flame tip. This process interferes with oscillations in the flame angle induced by swirl number fluctuations. The phasing of the flame angle with respect to the instant of vortex shedding from the injector lips determines the lifetime of the vortex before interaction with the flame and controls the strength of this interaction. When this lifetime is reduced, the vortex cannot fully develop and the flame response remains weak. For larger lifetimes, the vortex can fully develop and produce larger heat release rate perturbations. This process depends on the forcing frequency, which determines the phasing between swirl number fluctuations and vortices generation.