Flame transfer function saturation mechanisms in a swirl-stabilized combustor

An understanding of the amplitude dependence of the flame response to harmonic acoustic excitation is required in order to predict and/or correlate combustion instability amplitudes. This paper describes an experimental investigation of the mechanisms for nonlinearity of the heat release response to imposed acoustic oscillations using phase-locked, two-dimensional OH PLIF imaging. It focuses upon two representative conditions from a larger test set, corresponding to fundamentally different mechanisms of nonlinearity in flame response. The first mechanism is vortex roll-up at large disturbance amplitudes, similar to the observations of Balachandran et al. [R. Balachandran, B.O. Ayoola, C.F. Kaminski, A.P. Dowling, E. Mastorakos, Combust. Flame 143 (2005) 37–55.]. This roll-up causes the destruction rate of flame surface area by flame propagation to grow with disturbance amplitude, resulting in a flame surface area which does not grow proportionately with the disturbance amplitude. The second mechanism is unsteady flame liftoff, which occurs during the phase of the cycle of peak instantaneous axial velocity. This causes the flame attachment point to move off of the center body to a downstream location for part of the cycle. During this part of the cycle, the flame surface area decreases due to a merging of flame branches. Interestingly, while the flow field is highly three-dimensional and non-axisymmetric, the two key mechanisms identified here are essentially axisymmetric in nature. Furthermore, both mechanisms of nonlinearity are ultimately due to reduction in flame area by flame propagation normal to itself.