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Research Papers: Flows in Complex Systems

Impact of Fan Gap Flow on the Centrifugal Impeller Aerodynamics

[+] Author and Article Information
Yu-Tai Lee

 Naval Surface Warfare Center, Carderock Division, 9500 MacArthur Blvd., West Bethesda, MD 20817yu.lee@navy.mil

J. Fluids Eng 132(9), 091103 (Sep 23, 2010) (9 pages) doi:10.1115/1.4002450 History: Received February 22, 2010; Revised August 10, 2010; Published September 23, 2010; Online September 23, 2010

The effect of a gap between an inlet duct and a rotating impeller in a centrifugal fan is often neglected in the impeller design calculations or design-related computational fluid dynamics (CFD) analyses. This leads to an arbitrary determination of the gap size for the final fan configuration. Since the gap guides the volute flow back to the impeller flow field near the shroud high-curvature turning area, the low-momentum jet formed by the gap flow could prevent local flow from separation, reducing the local flow turning losses. However, this jet flow has enlarged flow separation in the blade passage, producing shedding vorticity in the downstream passage-flow. The passage-flow separation and the downstream volute flow, which is also affected by the passage-flow separation, have a higher impact on flow losses than the blade leading edge separation. If the gap size is not selected carefully, the combined effect of the passage-flow separation and downstream volute flow losses reduces the fan’s overall performance between 2% points and 5% points as demonstrated in the current study. In this paper, local impeller velocity distributions obtained from both design-CFD and analysis-CFD calculations are compared along the shroud from the gap to the blade trailing edge. The overall impeller flow fields with and without the gap and volute effects are also compared and discussed based on the CFD solutions. Finally, an example of controlling the gap effect is shown.

Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Centrifugal fan geometry used for the current study (only half of the fan is shown)

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Figure 2

The bellmouth and the DWDI impeller

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Figure 3

Impeller blade arrangement

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Figure 4

Gap between the bellmouth and the impeller shroud

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Figure 5

Component representation for impeller/volute coupling calculation for half of the fan

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Figure 6

Gridding for the impeller

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Figure 7

Grid density calculations

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Figure 8

Centrifugal fan test rig

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Figure 9

Predicted lift pressure compared with measurement

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Figure 10

Predicted power compared with measurement

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Figure 11

Impeller flow schematic

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Figure 12

Meridional velocity on axial planes at SC (HC), SD, SE, and HD (HE) obtained from calculation without gap

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Figure 13

Meridional velocity on axial planes at SC (HC), SD, SE, and HD (HE) obtained from calculation with gap

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Figure 14

Meridional velocities along the hub and shroud profiles with and without gap

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Figure 15

Blade flow traces obtained from calculation without gap

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Figure 16

Hub and shroud flow traces obtained from calculation without gap

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Figure 17

Blade flow traces obtained from calculation with gap

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Figure 18

Hub and shroud flow traces obtained from calculation with gap

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Figure 19

Gap flow traces

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Figure 20

Vorticity contours at the interface shown in Fig. 5

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Figure 21

Fangap with the original gap size of 1.27 mm

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Figure 22

Fangap with a larger gap size of 3 mm

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Figure 23

Fangap with the larger gap size of 3 mm and the modified shroud profile

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