Importance of Secondary Flow in Guide Vane Bends

For purposes of analysis, the flow in a guide vane bend is divided into a basic or primary two-dimensional flow with superimposed secondary flow. The two-dimensional flow is reviewed briefly first. It is then shown from experimental data that, for practical purposes, the secondary flow has negligible influence on the two-dimensional deflection, but the two-dimensional head loss is increased materially by the secondary flow. The effect of the secondary flow on head loss can be divided into two parts. The first part causes a loss which can be measured immediately behind the vanes, while the second part causes a loss which occurs between the trailing edges of the vanes and a plane about 4 duct hydraulic diameters behind the miter line of the bend. The second part is considerably larger than the first and may be attributed to increased wall shear downstream of the vanes. The increased wall shear is, in turn, attributable to the redistribution of streamlines by the secondary flow.