Mechanisms of the Nonlinear Response of a Swirl Flame to Harmonic Excitation

This paper describes an experimental investigation of the amplitude dependent processes controlling the response of a swirling flame to harmonic excitation. Simultaneous measurements of the acoustic pressure, velocity, and CH* and OH* radical chemiluminescence were obtained over a range of forcing frequencies, amplitudes, and nozzle exit velocities. These were supplemented with OH planar laser induced fluorescence (PLIF) measurements at several representative conditions. The results presented show that there exist substantial nonlinearities in the flame response to forced oscillations. Furthermore, while the flame response monotonically and linearly increases with perturbation amplitude for low forcing levels, its behavior becomes much more complex at higher levels — this includes saturation as well as more complex, non-monotonic behaviors. Analysis of the OH PLIF images show that the observed dynamics result from a superposition of at least five flame/flow processes — (1) the oscillating velocity of the annular jet, oscillations in (2) position and (3) strength of the vortex breakdown bubble and separation bubble, (4) unsteady liftoff of the flame, and (5) an oscillating turbulent flame speed. These processes generally occur simultaneously, with non-monotonic dependencies upon forcing amplitude. This sheds some light on the complex overall flame heat release response measurements and suggests a fruitful area for detailed computational studies that can better elucidate the underlying physics controlling the phenomenon.