SUMMARY

The present study investigates the flow characteristics of turbulent swirling flows induced by twisted tape inserts in circular pipes. Experimental tests show the existence of two co-rotating helical vortices superimposed over the main swirling flow. The close proximity of the two co-rotating vortices creates a local reversing flow at the pipe centerline. The flow is investigated experimentally using LDV measurements and high speed camera visualization with fine air bubbles seeding. Images and movies recorded with regular and high speed cameras clearly show that the helical vortices are 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 the same, approximately half of the bulk velocity. The pitch of the helical vortices is found to be independent of Reynolds number. The experimental findings are confirmed by numerical simulations. The numerical results show that the helical vortices originate inside the swirler and evolve from single co-rotating vortices on each side of the tape. Swirlers with multiple twists are shown to have less stable secondary motion and could be employed in applications were the secondary motion is detrimental.