Research Papers

Natural Frequency Shift in a Centrifugal Compressor Impeller for High-Density Gas Applications

[+] Author and Article Information
Yohei Magara

e-mail: yohei.magara.bc@hitachi.com

Kazuyuki Yamaguchi

e-mail: kazuyuki.yamaguchi.jg@hitachi.com
Hitachi, Ltd.
Hitachi Research Laboratory
832-2 Horiguchi, Hitachinaka
Ibaraki 312-0034, Japan

Haruo Miura

e-mail: haruo.miura.wm@hitachi-pt.com

Naohiko Takahashi

e-mail: naohiko.takahashi.qb@hitachi-pt.com

Mitsuhiro Narita

e-mail: mitsuhiro.narita.nq@hitachi-pt.com
Hitachi Plant Technologies, Ltd.
603 Kandatsu-machi, Tsuchiura
Ibaraki 300-0013, Japan

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received August 1, 2011; final manuscript received August 20, 2011; published online November 6, 2012. Editor: David Wisler.

J. Turbomach 135(1), 011014 (Nov 06, 2012) (8 pages) Paper No: TURBO-11-1170; doi: 10.1115/1.4006423 History: Received August 01, 2011; Revised August 20, 2011

In designing an impeller for centrifugal compressors, it is important to predict the natural frequencies accurately in order to avoid resonance caused by pressure fluctuations due to rotor-stator interaction. However, the natural frequencies of an impeller change under high-density fluid conditions. The natural frequencies of pump impellers are lower in water than in air because of the added mass effect of water, and in high-pressure compressors the mass density of the discharge gas can be about one-third that of water. So to predict the natural frequencies of centrifugal compressor impellers, the influence of the gas must be considered. We previously found in the nonrotating case that some natural frequencies of an impeller decreased under high-density gas conditions but others increased and that the increase of natural frequencies is caused by fluid-structure interaction, not only the added mass effect but also effect of the stiffness of the gas. In order to develop a method for predicting natural frequencies of centrifugal compressor impellers for high-density gas applications, this paper presents experimental results obtained using a variable-speed centrifugal compressor with vaned diffusers. The maximum mass density of its discharge gas is approximately 300 kg/m3. The vibration stress on an impeller when the compressor was speeding up or slowing down was measured by strain gauges, and the natural frequencies were determined by resonance frequencies. The results indicate that for high-density centrifugal compressors, some natural frequencies of an impeller increased in high-density gas. To predict this behavior, we developed a calculation method based on the theoretical analysis of a rotating disk. Its predictions are in good agreement with experimental results.

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Fig. 2

Three-dimensional model of test compressor

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Fig. 3

Flow path in the third stage

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Fig. 4

Vibrational mode shapes of impeller with 3 (left) and 5 (right) nodal diameters

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Fig. 5

Relation between the rotation speed and the 19th- and 38th-order vibrational stresses (suction pressure: 0.2 MPa)

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Fig. 6

Rotor of the test compressor

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Fig. 7

Strain gauge measuring points

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Fig. 8

19th-order (i.e., diffuser vane passing frequency) amplitude of impeller vibration during acceleration

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Fig. 9

Phase difference of 19th-order impeller vibration relative to that at SG-02

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Fig. 10

Schematic diagram of a disk in a gas-filled cylindrical enclosure

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Fig. 11

Acoustic mode shapes with three nodal diameters and zero nodal circles in the impeller side gap (left: shroud side, right: hub-disk side)

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Fig. 12

Instantaneous distribution of fluctuating pressure

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Fig. 13

Coupling factor L between the gas and the impeller vibration for the mode with three nodal diameters

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Fig. 14

Relation between the number of the modes used for calculation and the calculated results for the mode with three nodal diameters (ρf = 179 kg/m3, c = 277 m/s)

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Fig. 15

Comparison of experimental and calculation results of the natural frequencies of the impeller with three nodal diameters




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