An experimental study of a rib-roughened rotating U-bend flow

In this paper we report an experimental investigation of turbulent flow thr ough a square-sectioned U-bend of strong curvature, in which the inner and outer walls of the upstream and downstream sections are artificially roughe ned with square ribs, in a staggered arrangement. The U-bend is either stat ionary or rotates about an axis parallel to that of curvature, with positiv e rates, i.e. so that the secondary flows provoked by curvature and rotatio n are in the same sense. The main objective is to provide CFD validation da ta for flows which contain most of the flow features encountered in blade-c ooling passages, but which are numerically easier to compute, while retaini ng the modelling challenges provided by a real gas-turbine blade. In earlie r investigations we showed that the introduction of ribs in the upstream an d downstream sections: (a) raises overall turbulence levels; (b) reduces th e size of the separation bubble formed along the inner wall of the U-bend a nd (c) causes the formation of a large separation bubble along the outer wa ll, as the flow encounters the first outer-wall rib, after the bend exit. H ere we: (a) explore how the location of the first outer-wall rib, after the bend exit, affects the development of the downstream flow and (b) focus on the three-dimensional character of these flows, by providing data along a plane close to the top (flat) wall, in addition to data along the duct symm etry plane. We show that, for both stationary and rotating conditions, as t he first outer-wall rib is moved further away from the bend exit, the size of the separation bubble along the outer wall is reduced. The separation bu bble along the inner wall, however, increases in size but, overall, turbule nce levels are reduced. The flow within and immediately downstream of the U -bend is highly three-dimensional, showing strong variations from the symme try plane to the top wall. Rotation generates the expected secondary motion in the straight sections that, at the duct centre, convects the faster flu id towards the pressure side, and along the top and bottom (flat) walls, ha s the opposite effect. Within and downstream of the bend, positive rotation reduces the three-dimensionality of the flow. (C) 1999 Elsevier Science In c. All rights reserved.