Secondary Flows in Guide Vane Bends with Some Notes on the Primary Two-Dimensional Flow

Abstract

Incompressible flow in a guide vane bend may be considered as composed of a primary two-dimensional, nearly potential flow upon which is superimposed a secondary flow. The two-dimensional flow lies in the plane of the bend and is determined by the flow boundaries alone. If the flow entering a bend were everywhere irrotational, the two-dimensional flow pattern would be the three-dimensional pattern (nearly). Because the real flow entering a bend has a nonuniform velocity profile in planes normal to the plane of the bend, the streamlines in the nonuniform region are determined not alone by the flow boundaries but also by the entrance profile or vorticity. The main purpose of this research was the study of the influence of secondary flows on the basic two-dimensional flow. It is shown in the report from experimental data that the secondary flows have a negligible influence on the average deflection of the flow by the guide vanes, but they have a major influence on the energy loss. The two-dimensional loss coefficient is increased 100 per cent and more by the secondary flows. The loss attributable to secondary flows may be divided into two parts. The smaller of the two parts occurs at the vanes and the larger part occurs in a region downstream of the vanes extending over three or four duct hydraulic diameters. This larger part of the loss is attributed mainly to increased wall shear downstream of the vanes. The increased wall shear occurs because the secondary flow brings high-energy, high-velocity fluid to the duct wall from the interior.