The structure of the central recirculation zone in enclosed swirling flow.

Abstract

Swirling flows are widely used in fuel burners and combustors to provide a central recirculation zone which intensifies mixing and stabilizes the flame. In general, in a flow with sufficiently high swirl entering through a sudden expansion, the change of the tangential velocity produces an axial pressure gradient and forms a back flow near the axis which results in a central recirculation zone. In this study, the recirculation zone was measured in an experimental apparatus consisting of a sudden expansion fed by a swirl generator of the axial-tangential type, whose swirl intensity could be varied over a wide range. Three expansion ratios of 2.813, 2.0 and 1.625 were chosen in this experimental work. Velocity components at the inlet and at a number of stations downstream of the sudden expansion were measured. From the experiments, the recirculation zone was found to increase with swirl intensities and expansion ratios. Although the recirculating mass flow increases with swirl intensities, the diameter of the recirculation does not change greatly. In order to predict the structure of the central recirculation zone in enclosed swirling flow, a simple mathematical model of the momentum integral type is developed based on assumed velocity profile shapes both at the inlet and at the location of maximum diameter of the recirculation zone. The momentum integral method is used to solve for the recirculating mass flow. To test the model parameters against the measured values, a fitting routine for model velocity profiles is also presented. The results of comparisons between the model and the experiment show that the model is satisfactory.