Swirling flows in three circular-to-rectangular transition ducts, each of them with an aspect ratio of 2.0 at the rectangular exit, have been studied experimentally. The swirling flows were produced by swirlers whose vane angles were at 5 ~ 10 ~ and 20 ~ inclinded to the incoming flow, respectively. Flow visualization experiments were made at selected cross-sectional planes in the transition ducts. Pressure and velocity measurements were obtained on the contoured duct walls and at the rectangular exit planes, respectively. The results show that the swirling flow pattern evolves into a skewed structure at the exit plane which, in connection with the wall pressure distributions, is asymmetric with respect to the centerline of either the top and bottom walls or the side walls. Moreover, an analysis based on the mean streamwise vorticity equation with the velocity data obtained immediately downstream of the rectangular exit plane indicates that the cross-stream Reynolds stress plays an important role in transporting the streamwise vorticity of the swirling flow into the surrounding fluid.

This work reports on flow behaviors near the wall and secondary flow structures in a circular-to-rectangular transition duct at Reynolds numbers of 10*. Flow visualization experiments were conducted under the conditions of uniform and swirling inlet flows.As found, flow structures observed at the rectangular exit under the swirling inlet condition appear very differently from those observed under the uniform inlet condition. In the former case the secondary flows are developed due to the detachments of the swirling flow from the contoured wall, while in the latter case the secondary flows are known due to the mechanism of meanflow straining in the corner regions.