Structure of a swirling, recirculating coaxial free jet and its effect on particle motion

A developing coaxial swirling jet with sufficient annular swirl for flow re circulation to occur and the effect of the now field on particle motion wer e investigated. Experiments were conducted for a single flow condition with a velocity ratio (annular to inner) of 1.15, inner flow Reynolds number of 13,000, swirl number of 0.92 and five particle Stokes numbers ranging from 1 to 5. Instantaneous axial flow visualization illustrated a complex shear layer with a wide range of scales of turbulent motions without easily iden tifiable quasi-periodic toroidal structures found in non-swirling shear lay ers. However, toroidal structures amenable to single frequency plane wave e xcitation were identified, similar to those found in non-swirling shear lay ers. Using phase-averaged single hot-wire velocity measurements, the toroid al structures in the swirling shear layer were shown to have similar signat ure to those found in non-swirling shear layers. The instantaneous particle concentration field was measured using digital processing of pulsed laser sheet images. The coherent structures more effectively dispersed particles when compared to the natural jet for the five particle Stokes numbers inves tigated with the most significant effect occurring for Stokes number near u nity. Similar to other single jet results, the particles were preferentiall y concentrated in the regions between the vortex rings. (C) 2001 Elsevier S cience Ltd. All rights reserved.