Evolution of swirling flow in a circular pipe after a vane swirler at moderate Reynolds numbers

The experimental results on the swirling flow structure behind a vane swirler in a smooth pipe at an axial flow Reynolds number varying within Re = 240 – 1640 are presented. A change in the degree of flow swirl along the pipe length is analyzed as a function of the Reynolds number. The main patterns in the evolution of the profiles of longitudinal and circumferential velocity components and the distribution of the root-mean-square fluctuations of the longitudinal velocity component with increasing distance from the swirler are determined. It is shown that at Re = 1640, flow instability develops in the pipe, which is a consequence of the formation of a reverse flow region on the pipe axis in the immediate vicinity of the swirler. The influence of flow swirl on the appearance of signs of a local laminar-turbulent transition in vicinity of the pipe axis and near its wall is analyzed (a sharp increase in the root-mean-square velocity fluctuations with increasing Reynolds number and the appearance of intermittency in the flow velocity oscillograms). It has been established that in the near-wall region, local turbulence of the flow is caused by the interaction between the swirler vane wakes and the wall.