Casing boundary layer decelopment in axial impellers with TIP clearence

Abstract

Casing wall boundary layers are heavily skewed under the action of secondary and tip clearance flows, relative wall motion and the transverse forces associated with the blade rows. The growth of the casing boundary layers contributes greatly to the total losses and the blockage effects. Inviscid secondary flow theories have had some success in predicting the angle variations across and outside the boundary layers but are inherently incapable of taking into account the boundary layer growth as it passes through the blade rows. The latter can be achieved by treating the flow in the wall region emerging from a blade row as, in some sense, a full three-dimensional boundary layer. This thesis describes a study of the boundary layer flow in the tip region of a high-stagger single compressor rotor. Extensive measurements of the flow quantities upstream and downstream of the rotor were made covering a wide range of operating conditions with varying inlet boundary layer thickness, tip clearance and flow coefficient. These measurements were used to estimate the magnitude of some of the terms that appear in the momentum integral equations as a consequence of the blade-to-blade averaging process. The boundary layer growth across the blade row has also been theoretically predicted and the effect of blade force defect, axial velocity gradient, blade force direction, etc., studied. An approximate method has been suggested for estimating the force defect for blades moving through an approaching boundary layer.