Fluid flow in a rotating low-specific-speed centrifugal impeller passage

Abstract

Results from experimental investigations of the blade passage flow in a model pump impeller of low specific speed are described. It is found that the flow inside the rotating impeller passages is well described by the flow distribution postulated by potential-flow arguments and boundary-layer considerations. Using a custom-designed two-component laser-Doppler-velocimetry flow measurement system, mounted on a frame co-rotating with an impeller, the radial and circumferential velocity components of the relative flow inside an impeller passage were measured. Measurements were taken from blade to blade, and from this the passagewise fluid velocity was determined. The measured passagewise fluid velocity is compared to the potential-flow solution of the impeller passage-flow velocity that holds asymptotically in a region well away from the blade tips. Additional potential-flow finite-element calculations are used for comparison for those regions where the asymptotic expansion does not hold. The agreement between the experimental and theoretical values is seen to be good for the core flow region, which extends from the blade pressure side to the viscous boundary layer at the blade suction side. Furthermore, the calculation of the boundary-layer development along the suction side of the impeller blades is highlighted. Contrary to the blade pressure side, only there a significant departure from inviscid flow behaviour is observed. Calculations performed in the ordinary fashion, i.e. employment of the common thin-shear-layer approximation, substantiate the conception that for a low-specific-speed impeller the effects of spanwise rotation and modest surface curvature are accommodated in the description of the inviscid core flow.