Secondary vortices in swirling flow

TL;DR

This study investigates the complex flow structures induced by twisted tapes in a pipe, revealing secondary vortices and flow reversals through experimental measurements and simulations.

Contribution

It uncovers the existence of secondary co-rotating vortices and flow reversals in swirling flow induced by twisted tapes, using combined experimental and numerical methods.

Findings

Identification of secondary vortices causing flow reversal

Observation of flow transition along the pipe axis

Validation through high-speed imaging and simulations

Abstract

Twisted tapes are used to induce swirling flow and improve mixing. The flow induced by a 180 degree twisted tape with length (pitch) 60 mm and diameter 25.4 mm in a circular pipe was investigated using Laser Doppler Velocimetry (LDV) measurements. Tangential velocity profiles downstream of the twisted tape swirler were measured at multiple locations along the pipe axis, across the horizontal diameter of the pipe. The profiles showed an unexpected transition along the pipe axis from regular swirling flow to an apparent counter-rotation near the pipe axis, and then reverting back to regular swirling flow. Injecting fine air bubbles into the flow showed the existence of two co-rotating helical vortices superimposed over the main swirling flow. The close proximity of the two co-rotating vortices creates the local reversing flow at the pipe centerline. The secondary vortices are analyzed with high speed camera videos and numerical simulations.