Velocity fields and mixing properties of swirling double-concentric jets using two separated circular disks as center bodies

This study investigated the flow features, turbulence characteristics, and mixing properties of swirling double-concentric jets by using two separated circular disks as the center bodies. The upstream circular disk was placed at the exit of the central jet, while the downstream circular disk was located at a short distance downstream the upstream circular disk. The laser-assisted smoke-wire technique was used for flow visualization, particle image velocimetry (PIV) was utilized to investigate the velocity field, and the tracer gas concentration detection method was used to examine mixing property. When one circular disk was used as the center body, the flow patterns obtained through flow visualization and PIV measurement revealed two pairs of counter-rotating vortices existing in the near wake. The central jet penetrated the recirculation zone without any reversal, so low mixing between the central jet fluids and the annular swirling jet was observed. When two circular disks were used as the center body, the central jet momentum was transferred from the axial direction to the radial direction, and then it was dispersed and diffused into the annular swirling jet in the wake. Two flow modes were identified in the near wake: close-top toroidal wake and turbulent reverse-flow wake. In both characteristic flow modes, flows on the central axis were reversed in the near wake region. The axial and radial turbulence intensities were increased, and large values were observed inside the recirculation zone. Consequently, the carbon dioxide released from the central jet exit detected in the wake of the two-disk case was drastically lower than that of the one-disk case. The maximum improvement of mixing capabilities of the swirling double-concentric jets could attain about 96% when one circular disk was replaced by two disks.