Effects of single- and dual-blockage disks on swirling coaxial jets at high annulus Reynolds numbers

Using the laser-assisted smoke wire flow visualization technique, particle image velocimetry, and concentration detection method, this study examines the effects of two circular disks, arranged in tandem, on the flow structures, turbulence properties, and mixing characteristics of swirling double-concentric jets at high annulus Reynolds numbers. The smoke flow patterns, velocity vector fields, streamlines, normalized velocity contours, fluctuation intensity contours, vorticity contours, and distributions of carbon dioxide are presented and discussed. When a single-blockage disk is installed at the exit of swirling double-concentric jets, two pairs of counter-rotating vortices are formed in the near wake. The central jet fluids penetrate the recirculation zone so that no stagnation point, and therefore no reverse flow induced by annular jet flow, is found along the central axis. This causes high values of carbon dioxide concentration along the central axis. When a second circular disk is installed a short distance downstream of the disk at the jet exit, the axial momentum of the central jet transfers radially, turns downstream at the edge of the downstream disk, and then merges with the swirling annular flow. For this dual-disk case, a large reverse-flow zone is observed in the near wake in the condition of high annulus Reynolds and swirl numbers. Large turbulence intensity region is therefore induced downstream disks. Consequently, the mixing capability of dual-disk-controlled swirling double-concentric jets is significant improved compared to that of the single-blockage disk.