Viscous Effects on Slip Factor of Centrifugal Blowers

Abstract

For the design of centrifugal blowers it is essential to estimate the slip factor correctly. There are several empirical equations as well as theoretical ones for estimating slip factors. These equations predict the slip factor correctly for conventional blowers and pumps, and they are widely used for design. According to our experience for high specific speed blowers, however, the experimental slip factor was considerably smaller than the prediction based on these equations, and as a result the blowers which were designed using the predicted slip factor did not accomplish the design goal. In this paper the flow in shrouded centrifugal impellers with backward leaning blades is analyzed assuming that the flow is quasi-two-dimensional, steady, subsonic and inviscid, and then the slip factor is corrected to include viscous effects such as blockage of flow passage and variation of blade shape due to boundary layer, as well as the change of moment of momentum due to the wall shear force inside the impeller. By incorporating these corrections the slip factor based on inviscid theory agrees well with experimental slip factor for various impellers in a wide range of specific speed.