Strain Characteristics Near the Flame Attachment Point in a Swirling Flow

Abstract

Swirling flows are widely used in industrial burners and gas turbine combustors for flame stabilization. In many cases, the flame is stabilized in the separating shear layer near the centerbody or abrupt expansion, where the high-speed nozzle flow transitions into the larger combustor. Several prior studies have shown that the flame position becomes increasingly unsteady as it approaches blowoff, due to local extinction and reattachment of the flame at one or both of these locations. This is apparently due to the local strain rate irregularly oscillating about the extinction strain rate values near the attachment point. In order to characterize these flame strain characteristics, particle image velocimetry measurements were obtained of several hydrogen/methane mixtures in this attachment region. The fluid mechanic straining of the flame in this region is dominated by two gradients in velocity—that due to the strong shear near the centerbody and to the bulk flow deceleration as it expands from the smaller diameter nozzle into the combustor. These two velocity gradients cause positive and negative stretching of the flame sheet, respectively. The shearing velocity gradient is an order of magnitude larger than the flow deceleration term but, due to the fact that the flame is essentially parallel to the shear layer, actually has a secondary influence relative to the flow deceleration. As a result, the dominant flame straining term near the attachment point is compressive – a somewhat counter-intuitive result, given that the flame is stabilized in the positively straining shear layer.