LDA STUDY OF THE FLOW DEVELOPMENT THROUGH AN ORTHOGONALLY ROTATING U-BEND OF STRONG CURVATURE AND RIB-ROUGHENED WALLS

This paper reports laser-Doppler anemometry (LDA) and wall pressure me asurements of turbulent flow in a square-sectioned, rotating U-bend, t ypical of coolant passages employed in modern gas turbine blades. In t he upstream and downstream tangents, the pressure and suction (inner a nd outer) surfaces are roughened with discrete square-sectioned ribs i n a staggered arrangement for a rib-height to duct-diameter ratio of 0 .1. Three cases have been examined at a passage Reynolds number of 10( 5): a stationary case; a case of positive rotation (the pressure side coinciding with the outer side of the U-bend) at a rotation number (Ro = Omega D/U-m) of 0.2; and a case of negative rotation at Ro = -0.2. Measurements have been obtained along the symmetry plane of the duct. In the upstream section, the separation bubble behind each rib is abou t 2.5 rib heights long. Rotation displaces the high-momentum fluid tow ard the pressure side, enhances turbulence along the pressure side, an d suppresses turbulence along the suction side. The introduction of ri bs in the straight sections reduces the size of the separation bubble along the inner wall of the U-bend, by raising turbulence levels at th e bend entry; it also causes the formation of an additional separation bubble over the first ribs interval along the outer wall, downstream of the bend exit. Rotation also modifies the mean flow development wit hin the U-bend, with negative rotation speeding up the flow along the inner wall and causing a wider inner-wall separation bubble at exit. T urbulence levels within the bend are generally increased by rotation a nd, over the first two diameters downstream of the bend, negative rota tion increases turbulence while positive rotation on the whole has the opposite effect.