Analysis of the secondary vortices in rotating Flow induced by a twisted tape inside a pipe

Abstract

Flow through a twisted-tape (swirler) creates a complicated vortex structure downstream in the pipe. Detailed velocity measurements with Laser Doppler Velocimetry (LDV) along horizontal and vertical axes perpendicular to the axial flow direction have shown a strange flow pattern at the center of the rotating flow - a counter-rotating vortex seems to be present at the center with periodically varying magnitude in the axial direction. In more detailed measurements, it is shown that this behavior is the result of a pair of co-rotating secondary vortices that are superimposed on the primary rotating flow in a helical formation. The source of these secondary vortices has remained unclear. This study presents numerical simulations of the flow through 180o twisted-tape in a pipe, complementing the previous experimental results. The simulations reproduce the characteristics of the helical co-rotating vortices observed in experiments and provide details of the flow field. The results provide insight into the formation of the secondary vortices inside the twisted-tape, explaining the experimental observations. Mechanism of the vortex formation is described, showing that the secondary co-rotating helical vortices are produced by a pair of single co-rotating vortex formed on each side of the twisted-tape.