Flow visualization and measurements of a square jet with mixing tabs

Abstract A square jet with mixing tabs was investigated using laser induced fluorescence and laser Doppler anemometer (LDA). The triangular tabs were tilted either downstream (i.e., at an inclination, θ=45°) or upstream (i.e., at θ=135°). With four tabs, the jet flow was bifurcated into a four-finger structure and there was a “mushroom” structure behind each tab for θ=135°. Secondary velocity vector measurements using LDA in the y–z plane for the θ=45° and 135° with tabs at x=0.25DH and 0.075DH respectively showed a vortex pair formed behind each tab. These streamwise vortices spread the jet fluids “outward” along the diagonal direction of the jet cross-section, resulting in the four-finger structure. At the two corners of each finger, a pair of streamwise vortices, which were the legs of weaker horseshoe vortices, were also found. For θ=135°, a third pair of streamwise vortices proposed by Reeder and Samimy [J. Fluid Mech. 311 (1996) 73] was observed at the tip of each tab, which explained the existence of the unique mushroom structure. LDA measurements of mean velocity contours on the two tabbed jets confirmed the flow visualization results that the jet core had bifurcated into a four-finger structure. Furthermore, with tabs at x=0.75DH and 1.25DH, the downstream tilted tab jet produced a larger cross-sectional area of the jet core than the jet with θ=135° and the jet without tab. Downstream tilted tabs were thus more efficient in mixing enhancement than upstream tilted tabs.