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Research Papers: Gas Turbines: Structures and Dynamics

Measurement Versus Predictions of Rotordynamic Coefficients of a Hole-Pattern Gas Seal With Negative Preswirl

[+] Author and Article Information
Philip D. Brown

Drive Systems Engineer
Bell Helicopter,
Hurst, TX 76053
e-mail: pbrown@bellhelicopter.textron.com

Dara W. Childs

The Leland T. Jordan Chair of Mechanical Engineering,
Turbomachinery Laboratory,
Texas A&M University,
College Station, TX 77843
e-mail: dchilds@tamu.edu

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received June 20, 2012; final manuscript received June 27, 2012; published online October 11, 2012. Editor: Dilip R. Ballal.

J. Eng. Gas Turbines Power 134(12), 122503 (Oct 11, 2012) (11 pages) doi:10.1115/1.4007331 History: Received June 20, 2012; Revised June 27, 2012

Test results are presented for the rotordynamic coefficients of a hole-pattern annular gas seal at supply pressures to 84 bar and running speeds to 20200 rpm. The principal test variable of interest was negative preswirl. Preswirl signifies the circumferential fluid flow entering a seal and negative preswirl indicates a fluid swirl in a direction opposite to rotor rotation. The influences of the pressure ratio and rotor speed were also investigated. The measured results produce direct and cross-coupled stiffness and damping coefficients that are a function of the excitation frequency Ω. Changes in the pressure ratio had only small effects on most rotordynamic coefficients. Cross-coupled stiffness showed slightly different profiles through the midrange of Ω values. Increasing rotor speed significantly increased the cross-coupled stiffness and cross-coupled damping. At 10,200 RPM, high negative inlet preswirl produced negative cross-coupled stiffness over an excitation frequency range of 200–250 Hz. Negative preswirl did not affect the direct stiffness and damping coefficients. Effective damping combines the stabilizing effect of direct damping and the destabilizing effect of cross-coupled stiffness. The crossover frequency is the precession frequency where effective damping transitions from a negative value to a positive value with increasing frequency. At 20,200 rpm with a pressure ratio of 50%, the peak effective damping was increased by 50%, and the crossover frequency was reduced by 50% for high-negative preswirl versus zero preswirl. Hence, reverse swirl can greatly enhance the stabilizing capacity of a hole-pattern balance-piston or division-wall seals for compressors. A two-control-volume model that uses the ideal gas law at constant temperature was used to predict rotordynamic coefficients. The model predicted direct rotordynamic coefficients well, however, substantially under-predicted cross-coupled rotordynamic coefficients, especially at high negative preswirls.

Copyright © 2012 by ASME
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References

Kleynhans, G., and Childs, D., 1997, “The Acoustic Influence of Cell Depth on the Rotordynamic Characteristics of Smooth-Rotor/Honeycomb-Stator Annular Gas Seals,” ASME J. Eng. Gas Turbines Power, 119, pp. 949–957. [CrossRef]
Childs, D., and Wade, J., 2004, “Rotordynamic-Coefficient and Leakage Characteristics for Hole-Pattern-Stator Annular Gas Seals-Measurements Versus Predictions,” ASME J. Tribol., 126, pp. 326–333. [CrossRef]
Benckert, H., and Wachter, J., 1980, “Flow Induced Spring Coefficients of Labyrinth Seal for Applications in Rotordynamics,” Proceedings of the Rotordynamic Instability Problems in High-Performance Turbomachinery Workshop, Texas A&M University, College Station, TX, pp. 189–212, NASA Paper No. CP-2133.
Kanki, H., Katayama, K., Morii, S., Mouri, Y., Umemura, S., Ozawa, U., and Oda, T., 1988, “High Stability Design for New Centrifugal Compressor,” Proceedings of the Rotordynamic Instability Problems in High Performance Turbomachinery Workshop, Texas A&M University, College Station, TX,pp. 445–459, NASA Paper No. CP-3026.
Moore, J., Walker, S., and Kuzdal, M., 2002, “Rotordynamic Stability Measurement During Full-Load Full-Pressure Testing of a 6000 psi Reinjection Centrifugal Compressor,” Proceedings of the 31st Turbomachinery Symposium, Texas A&M University, College Station, pp. 29–38.
Gans, B., 2007, “Reverse-Swirl Swirl Brakes Retrofitting With Brush Seals,” Turbomachinery International, September/October, pp. 48–49.
Childs, D., and Hale, K., 1994, “A Test Apparatus and Facility to Identify the Rotordynamic Coefficients of High-Speed Hydrostatic Bearings,” ASME J. Tribol., 116, pp. 337–334. [CrossRef]
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Weatherwax, M. and Childs, D.2003, “The Influence of Eccentricity on the Leakage and Rotordynamic Coefficients of a High Pressure, Honeycomb, Annular Gas Seal: Measurements Versus Predictions,” ASME J. Tribol., 125, pp. 422–429. [CrossRef]
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Figures

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Fig. 1

Two-control-volume model in honeycomb seal [1]

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Fig. 2

Hole-pattern seal with positive fluid swirl [2]

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Fig. 3

Shunt injection in a labyrinth balance-piston seal [4]

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Fig. 4

Swirl brakes in a centrifugal compressor [5]

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Fig. 5

Steam turbine seal with antiswirl vanes [6]

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Fig. 6

Cross section of the air seal test rig [2]

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Fig. 7

Test stator assembly [2]

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Fig. 8

Cross-sectional view of the preswirl rings and Pitot tube location [2]

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Fig. 9

Preswirl ring and preswirl measurement [2]

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Fig. 14

The k(Ω) measurements versus the predictions. Left-hand views: three speeds, high-negative preswirl, and PR = 50%. Right-hand views: three preswirls, PR = 50%, and 20,200 rpm.

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Fig. 13

The Ceff(Ω) for (a) three pressure ratios, ω = 20,200 RPM, and high-negative preswirl, (b) three rotor speeds, PR = 50%, and high-negative preswirl, and (c) three preswirls, PR = 50%, and ω = 20,200 rpm

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Fig. 12

Direct damping and cross-coupled damping versus excitation frequency for (a) three pressure ratios, ω = 20,200 rpm, and high-negative preswirl, (b) three rotor speeds, PR = 50%, and high-negative preswirl, and (c) three preswirls, PR = 50%, and ω = 20,200 rpm

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Fig. 11

Direct stiffness and cross-coupled stiffness versus excitation frequency for (a) three pressure ratios, ω = 20,200 rpm, and high-negative preswirl, (b) three rotor speeds, PR = 50%, and high-negative preswirl, and (c) three preswirls, PR = 50%, and ω = 20,200 rpm

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Fig. 10

Drawing of the test seal hole pattern

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Fig. 15

The C(Ω) measurements versus the predictions. Left-hand views: three speeds, high-negative preswirl, and PR = 50%. Right-hand views: three preswirls, PR = 50%, and ω = 20,200 rpm.

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Fig. 16

The Ceff(Ω) measurements versus the predictions. Left-hand views: three speeds, high-negative preswirl, and PR = 50%. Right-hand views: three preswirls, PR = 50%, and ω = 20,200 rpm.

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