The effect of combustion instability on the structure of recirculation zones in confined swirling flames

A study of the destabilising effect of self-exited bulk-mode oscillations on the shape and size of recirculation zones has been carried out in a swirl burner/furnace. Cycle resolved velocities were measured to examine the effect of the coupling of unsteady pressure and velocity on the orientation of recirculation zones for two different burner exit expansion planes; i) a sudden expansion and ii) a divergent expansion. The sudden expansion exhibited large amplitude axial velocity fluctuations, particularly in the swirling jet. By contrast, the tangential velocity field was found to be comparably insensitive to the unsteady velocity oscillations. Consequently, a disproportionate increase in axial momentum periodically reduced the level of swirl disrupting the recirculation zones. The effect was two fold: i) during decreasing unsteady pressure, a high axial strain field destabilised the central recirculation zone periodically lifting it out of the burner exit. During increasing unsteady pressure, a stable lobed-shaped recirculation zone was re-established and anchored in the burner exit; ii) concurrently, the acceleration of the swirling jet partially entrained flow from the outer recirculation zone. This resulted in an outer recirculation zone that was directionally unstable with regions of counter-rotating flow. By employing a divergent expansion to influence the formation of the recirculation zones, the mean unsteady pressure amplitudes were damped over the whole instability range seeing a 40% reduction between ? = 0.6 to 0.7 and 50% between ? = 0.75 to 0.9. The results illustrate how unsteady pressures in the order of less than 2% of the mean pressure have a significant effect on the amplitude of the velocity field. The results also suggest that flame dynamics are not a direct response to small amplitude pressure oscillations but rather a direct response to changes in the local flow velocity that is determined by the phase velocity at the burner exit