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Research Papers

Analysis and Validation of a Unified Slip Factor Model for Impellers at Design and Off-Design Conditions

[+] Author and Article Information
Xuwen Qiu

 Concepts NREC, 217 Billings Farm Road, White River Junction, VT 05001xq@conceptsnrec.com

David Japikse, Jinhui Zhao, Mark R. Anderson

 Concepts NREC, 217 Billings Farm Road, White River Junction, VT 05001

J. Turbomach. 133(4), 041018 (Apr 25, 2011) (9 pages) doi:10.1115/1.4003022 History: Received June 28, 2010; Revised July 09, 2010; Published April 25, 2011; Online April 25, 2011

This paper presents a unified slip model for axial, radial, and mixed-flow impellers. The core assumption of the model is that the flow deviation or the slip velocity at the impeller exit is mainly originated from the blade loading near the discharge of an impeller and its subsequent relative eddy in the impeller passage. The blade loading is estimated and then used to derive the slip velocity using Stodola’s assumption. The final form of the slip factor model can be successfully related to Carter’s rule for axial impellers and Stodola’s slip model for radial impellers, making the case for this model applicable to axial, radial, and mixed-flow impellers. Unlike conventional slip factor models for radial impellers, the new slip model suggests that the flow coefficient at the impeller exit is an important variable for the slip factor when there is significant blade turning at the impeller discharge. This explains the interesting off-design trends for slip factor observed from experiments, such as the rise of the slip factor with flow coefficient in the Eckardt A impeller. Extensive validation results for this new model are presented in this paper. Several cases are studied in detail to demonstrate how this new model can capture the slip factor variation at the off-design conditions. Furthermore, a large number of test data from more than 90 different compressors, pumps, and blowers were collected. Most cases are radial impellers, but a few axial impellers are also included. The test data and model predictions of the slip factor are compared at both design and off-design flow conditions. In total, over 1650 different flow conditions are evaluated. The unified model shows a clear advantage over the traditional slip factor correlations, such as the Busemann–Wiesner model, when off-design conditions are considered.

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Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Loading calculation at impeller discharge

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Figure 2

Comparison of the model prediction of Carter’s m factor for circular arc airfoils

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Figure 3

(a) Eckardt rotor O geometry and (b) Eckardt rotor O slip factor

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Figure 4

(a) Eckardt rotor A geometry and (b) Eckardt rotor A slip factor

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Figure 5

(a) Rotor 37 geometry, (b) rotor 37 deviation angle, and (c) rotor 37 slip factor

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Figure 6

(a) Pump case geometry and (b) pump case slip factor

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Figure 7

(a) Blower case geometry and (b) blower case slip factor

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Figure 8

(a) Compressor slip factor predicted by the unified slip model and (b) compressor slip factor predicted by the Wiesner–Busemann slip model

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Figure 9

Pump slip factor predicted by the unified slip model

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