Turbulence-chemistry interactions in non-premixed swirling flames

Abstract

Swirling flames with varying departures from blow-off have recently been studied using a simple, yet representative swirl burner. Extensive measurements of flow, stability and composition fields have already been made in turbulent non-premixed swirling flames covering a range of fuel mixtures and swirl numbers. The data, which are now made available on the World Wide Web, have revealed complex flame structures involving vortex breakdown leading to swirl-induced recirculation zones, flow instability, and the occurrence of localized extinction. These effects, which are also typical of what actually occurs in many practical combustors, are most likely inter-related rather than isolated phenomena. The nature of these interactions remains poorly understood. This paper brings together flow-field, stability and composition measurements with the aim of shedding more light on the interaction between complex fluid dynamics and finite-rate chemistry. A threshold for the occurrence of localized extinction is defined and examined with regard to rates of localized flow rotation, the occurrence of flow reversals and the velocity shear stresses. It is found that local extinction occurs in regions of high shear stress that do not necessarily overlap with the mean stoichiometric contours. The formation of an elongated recirculation (bluff-body stabilized) zone or a second, downstream region of flow recirculation is largely controlled by the swirl number and the ratio of momentum in the swirling annulus and central fuel jet.