Comparison of Three Slip Velocity Models for a Centrifugal Impeller

Abstract

The slip velocity model has been created to account for the effects produced by the difference in velocities between the suction side and the pressure side of the blade at the centrifugal impeller exit. The model is applied to reduce the ideal tangential velocity and provides an estimation of the impeller exit velocity and blade loading for use in turbomachinery meanline modeling. Wiesner reviewed all the proposed models in 1967 and came up with a set of formulations which are only dependent on the impeller geometry, such as number of blades, exit blade angle and blade inlet-to-exit radius ratio. In order to cover flow induced effects and off-design conditions, Qiu recently proposed a formula which contains the effects from the radial rotation, blade turning and passage width variation. For a centrifugal impeller, often the impeller exit condition is affected by downstream components. Pfleiderer’s model of 1961 includes effects of downstream vaned diffuser, vaneless diffuser, or a volute. This paper focuses on the comparisons of the three models in predicting blade loading for a centrifugal-fan impeller with a double-discharge volute at both design and off-design conditions. The comparisons reveal not only each model’s prediction accuracy, but also the flexibility of each model.