Flow and mixing characteristics of swirling double-concentric jets influenced by a control disc at low central jet Reynolds numbers

The flow characteristics and mixing properties of the swirling double-concentric jets subject to the influences of a control disk installed at a short distance above the primary disk were studied. The instantaneous flow patterns were illustrated using the laser-light sheet-assisted smoke wire flow visualization method. The velocity characteristics were measured by a particle image velocimeter (PIV). The axial and radial mean velocities and turbulence intensities in the symmetry plane of the flow field were obtained from the PIV measurements. The dispersion properties of the central jet fluid were measured by a gas analyzer using carbon dioxide as the tracer gas. The control disk installed at a short distance above the primary disk caused the central jet to impinge upon the bottom face of the control disk, deflect radially outward, turn downstream, and interact with the swirling annular jet to form four characteristic near-field flow structures (close-top toroidal vortical wake, open-top toroidal vortical wake, turbulent forward-flow wake, and turbulent reverse-flow wake) in different regimes of annular and central jets' Reynolds numbers. The reverse flow on the central axis of the turbulent reverse-flow wake caused dramatically large turbulence intensities to occur in the near wake-region when compared to the open-top toroidal vortical wake, which contained no stagnation point or reverse flow on the central axis. Therefore, this induced a drastic large radial dispersion of the central jet fluid. The mixing index of the turbulent reverse-flow wake might be enhanced by approximately 90% when compared to that of the swirling double-concentric jets without the influence of a control disk.