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Research Papers: Gas Turbines: Structures and Dynamics

# Clearance Effects on Rotordynamic Performance of a Long Smooth Seal With Two-Phase, Mainly-Air Mixtures

[+] Author and Article Information
Min Zhang

Development Specialist Praxair, Inc.,
Tonawanda, NY 14228

Dara W. Childs

The Leland T. Jordan Chair
of Mechanical Engineering,
Turbomachinery Laboratory,
Texas A&M University,
College Station, TX 77843

Dung L. Tran, Hari Shrestha

Turbomachinery Laboratory,
Texas A&M University,
College Station, TX 77843

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received November 29, 2017; final manuscript received July 1, 2018; published online October 29, 2018. Assoc. Editor: Philip Bonello.

J. Eng. Gas Turbines Power 141(1), 012502 (Oct 29, 2018) (9 pages) Paper No: GTP-17-1637; doi: 10.1115/1.4040809 History: Received November 29, 2017; Revised July 01, 2018

## Abstract

This paper experimentally studies the effects of changing radial clearance Cr on the performance of a long (length-to-diameter ratio L/D = 0.65) smooth seal under mainly-air (wet-gas) conditions. The test fluid is a mixture of air and silicone oil. Tests are conducted with Cr = 0.188, 0.163, and 0.140 mm, inlet pressure Pi = 62.1 bars, exit pressure Pe = 31 bars, inlet liquid volume fraction LVF = 0%, 2%, 5%, and 8%, and shaft speed ω = 10, 15, and 20 krpm. The seal's complex dynamic stiffness coefficients Hij are measured. The real parts of Hij cannot be fitted by frequency-independent stiffness and virtual-mass coefficients. Therefore, frequency-dependent direct K and cross-coupled k stiffness coefficients are used. The imaginary parts of direct Hij produce frequency-independent direct damping C. Test results show that, for all pure- and mainly-air conditions, decreasing Cr decreases (as expected) the leakage mass flow rate $m˙$. Under mainly-air conditions, decreasing Cr decreases K. This outcome is contrary to the test results at pure-air conditions, where K increases as Cr decreases. Since an unstable centrifugal compressor rotor may precess at approximately 0.5ω, the effective damping Ceff at about 0.5ω is used as an indicator of the impact a seal would have on its associated compressor. For pure-air conditions, when Ω ≈ 0.5ω, decreasing Cr increases Ceff and makes the seal more stabilizing. This trend continues after the oil is added. A bulk-flow model developed by San Andrés (2011, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503) produces predictions to compare with test results. $m˙$ predictions correlate with measurements. Under pure-air conditions, the model correctly predicts the effects of changing Cr on K and the Ceff value near 0.5ω. After the oil is added, as Cr decreases, predicted K increases while measured K decreases. Also, for mainly-air cases and Ω ≈ 0.5ω, decreasing Cr does not discernibly change predicted Ceff but increases the measured value.

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## References

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## Figures

Fig. 1

Section view of the test section [7]

Fig. 2

Test smooth seal (dimensions are in inches) [13]

Fig. 3

Piping and instrumentation diagram of the 2PASS [7]

Fig. 4

Section view and flow illustration of the oil–gas mixer [7]

Fig. 5

Section view of the stator assembly [7]

Fig. 6

Cross section of the zero preswirl ring [7]

Fig. 7

Predictions and measurements of m˙ at ω = 10 krpm

Fig. 8

Measured Re(Hij) (after subtracting baseline data) when Cr = 0.188 mm, inlet LVF = 5%, and ω = 15 krpm

Fig. 9

Measured Im(Hij) (after subtracting baseline data) when Cr = 0.188 mm, inlet LVF = 5%, and ω = 15 krpm

Fig. 10

Variation of KΩ with changes in te when Cr = 0.188 mm, inlet LVF = 5%, and ω = 10 krpm for te = (a) 32.768 s, (b) 65.536 s, and (c) 131.072 s

Fig. 11

Variation of eKΩ/KΩ with changes in te when Cr = 0.188 mm, inlet LVF = 5%, and ω = 10 krpm

Fig. 12

Variation of kΩ with changes in te when Cr = 0.188 mm, inlet LVF = 5%, and ω = 10 krpm for te = (a) 32.768 s, (b) 65.536 s, and (c) 131.072 s

Fig. 13

Variation of ekΩ/kΩ with changes in te when Cr = 0.188 mm, inlet LVF = 5%, and ω = 10 krpm

Fig. 14

Predictions and measurements of K at ω = 10 krpm and inlet LVF = (a) 0% and (b) 2%

Fig. 15

Measured k at Cr = 0.163 mm and ω = 10 krpm

Fig. 16

Predictions and measurements of k¯ at ω = 10 krpm

Fig. 17

Predictions and measurements of C at ω = 10 krpm

Fig. 18

Variation of Ceff with Cr when inlet LVF = 2% and ω = 10 krpm

Fig. 19

Predictions and measurements of Ceff at inlet LVF = 0% and ω = 10 krpm

Fig. 20

Predictions and measurements of Ceff at inlet LVF = 2% and ω = 10 krpm

## Errata

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