Experimental study by PIV of swirling flow induced by trapezoid-winglets

The characteristics of the longitudinal vortex induced by trapezoid-winglets in a circular tube are investigated by the Particle Image Velocimetry (PIV) Technique with flow Reynolds number in the range of 500–13 000. In the experimental test section, four trapezoid-winglets are fixed symmetrically on the tube wall in two different ways: up-flow and down-flow. The results show that a counter-rotating vortex pair is formed behind each winglet and they distribute as a symmetrical vortex array in the transverse section. Between the two vortexes in a vortex pair the fluid flows towards the wall in the up-flow winglet case and away from the wall in the down-flow winglet case, corresponding also to the regions of peak values of the velocity components normal to the mainstream. Both of the flow patterns enhance the velocity in the near wall region, leading to the intensification of the transverse mixing and the mass transfer in the tube. With Reynolds number increasing, the flow maintains the vortex pattern in the case of the up-flow winglets, indicating better persistence of the longitudinal vortex, while the vortexes in the case of the down-flow winglets are more scattered and tend to breaking into small eddies. The trapezoid winglet shows the preferable turbulent disturbance characteristics in the tube and the experimental results provide benchmark data for further CFD studies.