A New Slip Factor Model for Axial and Radial Impellers

Abstract

This paper proposes a unified slip model for axial, radial, and mixed flow impellers. For many years, engineers designing axial and radial turbomachines have applied completely different deviation or slip factor models. For axial applications, the most commonly used deviation model has been Carter’s rule or its derivatives. For centrifugal impellers, Wiesner’s correlation has been the most popular choice. Is there a common thread linking these seemingly unrelated models? This question becomes particularly important when designing a mixed flow impeller where one has to choose between axial or radial slip models. The proposed model in this paper is based on blade loading, i.e., the velocity difference between the pressure and suction surfaces, near the discharge of the impeller. The loading function includes the effect of blade rotation, blade turning, and the passage area variation. This velocity difference is then used to calculate the slip velocity using Stodola’s assumption. The final slip model can then be related to Carter’s rule for axial impellers and Stodola’s slip model for radial impellers. This new slip model suggests that the flow coefficient at the impeller exit is an important variable for the slip factor when there is blade turning at the impeller discharge. This may explain the interesting slip factor trend observed from experiments, such as the rise of the slip factor with flow coefficient in Eckardt A impeller. Some validation results of this new model are presented for a variety of applications, such as radial compressors, axial compressors, pumps, and blowers.