LDA INVESTIGATION OF THE FLOW DEVELOPMENT THROUGH ROTATING U-DUCTS

This paper reports results from the use of laser-Doppler anemometry (L DA) to measure the mean and fluctuating flow field in a U-bend of stro ng curvature, Rc/D = 0.65, that is either stationary or rotating in or thogonal mode (the axis of rotation being parallel to the axis of curv ature). The data acquisition system enables a stationary optical fiber probe to collect flow data from a rotating U-bend sweeping past it. T hree cases have been examined all concerning a flow Reynolds number of 100,000; a stationary case, a case of positive rotation (the pressure side of the the duct coincides with the outer side of the U-bend) at a rotational number (Omega D/U-m) of 0.2, and a case of negative rotat ion at a rotational number of -0.2. Measurements have been obtained al ong the symmetry plane of the duct and also along a plane near the top wall. The most important influence on the development of the mean and turbulence flow fields is exerted by the streamwise pressure gradient s that occur over the entry and exit regions of the U-bend. In the sta tionary case a three-dimensional separation bubble is formed along the inner wall at the 90 deg location and it extends to about two diamete rs downstream of the bend, causing the generation of high-turbulence l evels. Along the outer side, opposite the separation bubble, turbulenc e levels are suppressed due to streamwise flow acceleration. For the r otation numbers examined, the Coriolis force also has a significant ef fect on the flow development. Positive rotation doubles the length of the separation bubble and generally suppresses turbulence levels. Nega tive rotation causes an extra separation bubble at the bend entry, rai ses turbulence levels within and downstream of the bend, increases vel ocity fluctuations in the cross-duct direction within the bend, and ge nerates strong secondary motion after the bend exit. It is hoped that the detailed information produced in this study will assist in the dev elopment of turbulence models suitable for the numerical computation o f flow and heat transfer inside blade-cooling passages.