Experimental investigation of the swirling flow and the helical vortices induced by a twisted tape inside a circular pipe

The present paper describes the experimental investigation of the turbulent swirling flow induced by a twisted tape (or fin) into a 1 in. diameter pipe. The investigation shows the existence of two corotating helical vortices superimposed over the main swirling flow. The close proximity of the two corotating vortices creates a local reversing flow at the pipe centerline. The flow is investigated using LDV measurements and high speed camera visualization with fine air bubble seeding. Three swirlers with 45, 60, and 90 mm pitch are investigated. Through a reconstruction technique, the tangential flow field induced by a twisted tape swirler with 60 mm pitch at Re=7.7x10(4) is described by superposition of 385 measurements. Images and movies recorded with regular and high speed cameras clearly show that the helical vortices are very stable and that they rotate around their own axis, confirming the measurements. After extracting the characteristic tangential velocity profiles of the main vortex and of the two secondary vortices, it was observed that the maximum tangential velocity of all three vortices is almost the same, approximately half of the bulk velocity. The pitch of the helical vortices was found to be 4/3 of the pitch of the twisted tape for all three swirlers investigated, independent of Reynolds number. We hypothesize that the helical vortices are generated by vortices originating inside the twisted tape swirler. The main rotational flow accelerates the corotating vortices and decelerates the counter-rotating vortices. As a result, the counter-rotating vortices disappear while the corotating vortices reach the same maximum tangential velocity as the main flow. Thus the tangential velocity near the wall is approximately doubled by the presence of the secondary vortices.