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Research Papers

Rotordynamic Stability Effects of Shrouded Centrifugal Impellers With Combined Whirl and Precession

[+] Author and Article Information
Eunseok Kim

Mem. ASME
Department of Mechanical Engineering,
Texas A&M University,
College Station, TX 77840
e-mail: euns670@tamu.edu

Alan Palazzolo

Fellow ASME
Department of Mechanical Engineering,
Texas A&M University,
College Station, TX 77840
e-mail: a-palazzolo@tamu.edu

Contributed by the Technical Committee on Vibration and Sound of ASME for publication in the JOURNAL OF VIBRATION AND ACOUSTICS. Manuscript received April 7, 2017; final manuscript received September 4, 2017; published online October 6, 2017. Assoc. Editor: Patrick S. Keogh.

J. Vib. Acoust 140(2), 021007 (Oct 06, 2017) (12 pages) Paper No: VIB-17-1141; doi: 10.1115/1.4037958 History: Received April 07, 2017; Revised September 04, 2017

Whirling (translational) and precession (tilt) motion of the shrouded centrifugal impeller are possible vibration sources that can cause rotordynamic instability problems. Whirling motion of shrouded impellers and seals has been investigated by test and theory in the literature. However, there has been little study of the effects of coupled motion of whirling and precession of a centrifugal impeller on rotordynamic forces and moments using computational fluid dynamics (CFD). In the present study, the CFD approach for calculating the moment coefficients of the precessing impeller is developed and verified by comparison with the measured data for a precessing centrifugal compressor by Yoshida et al. (1996, “Measurement of the Flow in the Backshroud/Casing Clearance of a Precessing Centrifugal Impeller,” Sixth International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, Honolulu, Hawaii, Vol. 2, pp. 151–160). A full set (4 × 4) of rotordynamic coefficient matrices is calculated, using two separate models of (a) a precessing impeller with a tilt angle and (b) a whirling impeller with dynamic eccentricity to investigate the stability of the impeller. Rotordynamic stability is evaluated by using the whirl frequency ratio of the coupled motion, obtained from the full rotordynamic coefficient matrices, to show that the precession motion has a significant impact on rotordynamic stability. A similar conclusion is reached based on the whirling plus precession response of a finite element (FE) structural rotordynamic model including the 4 × 4 rotordynamic coefficient matrices. A stability analysis using the rotordynamic coefficients indicates that the precession motion with the positive tilt angle increases the tendency toward destabilization of the rotor.

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References

Figures

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Fig. 1

Coordinate systems and moment components [4]

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Fig. 2

Basic geometry of the impeller backshroud [4]

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Fig. 3

Dimensions of the backshroud/casing for 3D CFD modeling

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Fig. 4

Cut plain view of 3D eccentric grid

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Fig. 5

Normalized moment components (a) transverse moment and (b) direct moment

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Fig. 6

Computed CFD velocity vectors along the backshroud leakage path (a) computed velocity vector at backshroud inlet and (b) velocity vector at leakage flow outlet

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Fig. 7

Pure (a) whirling and (b) precession motions of an impeller

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Fig. 8

Nominal configuration of shrouded face-seal impeller

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Fig. 9

Radial (a) and tangential (b) force impedances for whirling motion

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Fig. 10

Direct (a) and transverse (b) moment impedances for whirling motion

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Fig. 11

Radial (a) and tangential (b) force impedances for precession motion

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Fig. 12

Transverse (a) and direct (b) moment impedances for precession motion

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Fig. 13

Impeller located on mode shape where (a) precession and whirl are in phase and (b) precession and whirl are out of phase

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Fig. 14

Finite element model of rotor-bearing system with impeller on right-hand side of shaft (positive αl case)

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Fig. 15

Log-dec versus bearing damping ratio for positive impeller tilt angle, and with cases of only impeller force coefficients, with full set of impeller coefficients, and no impeller coefficients

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Fig. 16

Finite element model of rotor-bearing system with impeller on left-hand side of shaft (negative αl case)

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Fig. 17

Log-dec versus bearing damping ratio for negative impeller tilt angle, and with cases of only impeller force coefficients and with full set of impeller coefficients

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Fig. 18

Combined whirling and precession motion of an impeller

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Fig. 19

(a) Tangential and (b) radial force impedances versus (f = Ω/ω) of the whirling and precession face-seal impeller with three separate precession (tilt) angles

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