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 recirculation to occur and the effect of the flow field on particle motion were 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 identifiable quasi-periodic toroidal structures found in non-swirling shear layers. However, toroidal structures amenable to single frequency plane wave excitation were identified, similar to those found in non-swirling shear layers. Using phase-averaged single hot-wire velocity measurements, the toroidal structures in the swirling shear layer were shown to have similar signature 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 investigated with the most significant effect occurring for Stokes number near unity. Similar to other single jet results, the particles were preferentially concentrated in the regions between the vortex rings.