SUMMARY

The present study tries to elucidate the flow characteristics of turbulent swirling flows induced by twisted tape inserts in circular pipes. During previous investigations an unexpected counter-rotating flow was encountered in the LDV measurements near the pipe centerline, downstream of a twisted tape swirl promoter in a 1 inch diameter pipe. Initial tests showed the existence of two co-rotating helical vortices superimposed over the main swirling flow (see figure). The close proximity of the two co-rotating 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 bubbles seeding. Three swirlers are investigated with pitch 45, 60 and 90 mm. Through a reconstruction technique, the tangential flow field induced by a twisted tape swirler with 60 mm pitch at 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 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. This study will also try to explain the helical vortices formation and confirm the proposed mechanism with numerical simulations.