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Research Papers

Clearance Effects on the Onset of Instability in a Centrifugal Compressor

[+] Author and Article Information
Matthias Schleer1

Turbomachinery Laboratory, Swiss Federal Institute of Technology Zurich, 8092 Zurich, Switzerland

Seung Jin Song

School of Mechanical and Aerospace Engineering, Seoul National University, 151-742 Seoul, Korea

Reza S. Abhari

Turbomachinery Laboratory, Swiss Federal Institute of Technology Zurich, 8092 Zurich, Switzerland

1

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

J. Turbomach 130(3), 031002 (Apr 01, 2008) (11 pages) doi:10.1115/1.2776956 History: Received November 23, 2006; Revised December 10, 2006; Published April 01, 2008

This report intends to shed an insight into the effect of large relative tip clearances on the onset of instability in a highly loaded centrifugal compressor. Time-resolved pressure measurements have been performed along the casing of a scaled-up model of a small compressor for two clearances at a wide range of operating conditions. Based on these time-resolved measurements, the pressure distribution along the meridional length and the blade loading distribution are calculated for each operating condition. In addition, the phase locked pressure fluctuation and its deviation are computed. The results show the behavior of each subcomponent of the compressor at different flow conditions and explain the role of the relative tip clearance on the onset of instability. For high mass-flow rates, the steady pressure distribution along the casing reveals that the inducer acts as an accelerating nozzle. Pressure is only built up in the radial part due to the centrifugal forces and in the subsequent diffuser due to area change. For off-design conditions, incidence effects are seen in the blade loading distribution at the leading edge while the inducer is unloaded. A region of high pressure deviation originates at the leading edge of the main blade and convects downstream. This feature is interpreted as the trajectory of the leakage vortex. The trajectory of these vortices is strongly affected by the mass-flow coefficient. If the mass-flow rate is sufficiently small, the trajectory of the leakage vortex becomes perpendicular to the axis of rotation, the leakage vortex interacts with the adjacent blade, and inlet tip recirculation is triggered. If the flow rate is further reduced, the leakage vortex vanishes and rotating stall is initiated in the diffuser. For larger clearances, stronger vortices are formed, stall is triggered at higher flow rates, and the overall compressor performance deteriorates.

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Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 14

Deviation of the measured periodic pressure fluctuation for the reduced clearance case (CR=4.5%) at different mass-flow rates

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Figure 1

“Rigi” research facility

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Figure 2

Investigated clearance configurations

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Figure 3

Comparison of the operating map at base line and reduced clearance

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Figure 4

Investigated operating conditions

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Figure 5

Location of the pressure transducer (mm)

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Figure 6

Measured steady wall pressure distribution for the base line clearance ratio (CR=12.7%), normalized by the inlet static pressure

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Figure 7

Representation used for plotting the periodic pressure fluctuation and deviation

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Figure 8

Measured periodic pressure coefficient for the base line clearance case (CR=12.7%) at different mass-flow rates

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Figure 9

Designation of the local blade loading

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Figure 10

Measured blade loading distribution for the base line clearance case (CR=12.7%)

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Figure 11

Deviation of the measured periodic pressure fluctuation for the base line clearance case (CR=12.7%) at different mass-flow rates

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Figure 12

Measured steady wall pressure distribution for base line clearance ratio (CR=12.7%) and the reduced clearance ratio (CR=4.5%); normalized by the Impeller inlet static pressure

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Figure 13

Measured blade loading distribution for the reduced clearance case (CR=4.5%) at different mass-flow rates

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