Recirculation and flowfield regimes of unconfined non-reacting swirling flows

This paper focuses on unconfined swirling flows of air surrounding a bluff-body having a central jet of air. Only one co-flowing primary annular air stream is swirled. The flow conditions investigated here cover a range of swirl numbers and streamwise annular velocities. Emphasis is placed on discerning typical flow structures and on discovering the influence of controlling parameters on the downstream regions of the jet and the recirculation within. The paper is part of a larger program aimed at providing an improved understanding of swirling flows. Laser doppler velocimetry is used to map three components of velocity [u] (v) and [w]. In the flows investigated, an upstream recirculation zone is established above the burner's exit plane. This zone is typical of that found behind bluff-bodies placed in strong axial streams. Additionally, it is shown that the formation of a downstream recirculation zone, hence the onset of vortex breakdown, depends not only on the swirl number but on other flow parameters such as the axial velocity of the primary swirling air, or its Reynolds number. Together. these two parameters appear to control the radial spread of the flow. Spatially. the progression towards downstream recirculation with changes in flow parameters is seen to occur around a fixed location in the streamwise direction. Beyond this axial position, the flowfield seems to be less affected by increases in swirl number. Non-recirculating flow structures are resolved in these flows and are found to cause elevated shear stresses. (C) 2003 Elsevier Science Inc. All rights reserved.