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=7.7×104 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 se...