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Research Papers

Clearance Effects on the Evolution of the Flow in the Vaneless Diffuser of a Centrifugal Compressor at Part Load Condition

[+] Author and Article Information
Matthias Schleer1

Turbomachinery Laboratory, Swiss Federal Institute of Technology Zürich, CH 8092 Zürich, Switzerland

Reza S. Abhari

Turbomachinery Laboratory, Swiss Federal Institute of Technology Zürich, CH 8092 Zürich, Switzerland

1

Present address: ABB Turbosystems Ltd., 5401 Baden, Switzerland.

J. Turbomach 130(3), 031009 (May 02, 2008) (9 pages) doi:10.1115/1.2776955 History: Received November 23, 2006; Revised December 10, 2006; Published May 02, 2008

This work reports on flow measurements taken within the vaneless diffuser of a scaled-up model of a small-scale, highly loaded unshrouded compressor with large relative tip clearance. The aims are to describe and to analyze the influence of the clearance flow on the flow structure at the impeller exit in part load operation. The kind of compressor described herein is widely used in distributed power applications and automotive turbocharging. It demands further enhancement of the operation range, as well as a high head rise and an improved efficiency. Therefore, the understanding of flow features and their interaction is crucial. The interaction and mixing of the flow pattern downstream of the impeller are shown using spatially and temporally resolved 3D-velocity data. The measurements have been obtained by using a 3D laser Doppler anemometry system throughout the vaneless parallel wall diffuser. This unique data set provides insight into the development of the flow within the diffuser and allows conclusions on the mixing and migration of the three-dimensional pattern. The flow structure in part load condition is strongly affected by the flow across the large relative tip gap. Due to the large relative tip clearance, a low momentum zone is formed as an additional pattern at the shroud. This clearance flow is highly vortical and interacts with the channel wake structure but remains stable throughout the vaneless diffuser. At the pressure side hub corner, a jet structure is formed, which interacts rapidly with the blade wake. This flow behavior does not comply with the classical jet-wake pattern. It is proposed that in a centrifugal compressor with large relative tip clearance, a modified flow model that includes tip leakage is more appropriate to describe the flow structure at part load condition.

Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 10

Measured contours of the radial and tangential velocities near the shroud (z∕b2=9%) for the base line clearance ratio CR=12.7%

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Figure 11

Measured contours of radial and absolute tangential velocities at R∕R2=104% for CR=12.7%

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Figure 1

Rigi research facility

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Figure 2

Investigated clearance configurations

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Figure 3

Investigated operating conditions

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Figure 4

Optical setup of the LDA system

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Figure 5

Convention of the angles and velocities

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Figure 6

Measured distribution of the radial and tangential velocities in the vaneless diffuser

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Figure 7

Velocity triangles near the hub and shroud wall (z∕b2=15% and 85%, respectively)

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Figure 8

Measured contours of the radial and tangential velocities at midspan (z∕b2=50%) for the base line clearance ratio CR=12.7%

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Figure 9

Measured contours of the radial and tangential velocities near the hub (z∕b2=85%) for the base line clearance ratio CR=12.7%

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Figure 12

Measured contours of radial and absolute tangential velocities at R∕R2=116% for CR=12.7%

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Figure 13

Measured contours of the turbulence level at a radial location (R∕R2=104%) for CR=12.7%

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Figure 14

Velocity triangles of the flow pattern

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Figure 15

Measured contours of radial and absolute tangential velocities at a radial location (R∕R2=105%) for a reduced clearance configuration (CR=4.5%)

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Figure 16

Schematic model of the secondary flow pattern at the impeller exit and downstream in the vaneless diffuser

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