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Research Papers: Gas Turbines: Structures and Dynamics

Numerical Comparison of Rotordynamic Characteristics for a Fully Partitioned Pocket Damper Seal and a Labyrinth Seal With High Positive and Negative Inlet Preswirl

[+] Author and Article Information
Zhigang Li, Zhenping Feng

Institute of Turbomachinery,
School of Energy and Power Engineering,
Xi’an Jiaotong University,
Xi’an 710049, China

Jun Li

Institute of Turbomachinery,
School of Energy and Power Engineering,
Xi’an Jiaotong University,
Xi’an 710049, China;
Collaborative Innovation Center
of Advanced Aero-Engine,
Beijing 100191, China
e-mail: junli@mail.xjtu.edu.cn

1Corresponding author.

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received August 7, 2015; final manuscript received August 29, 2015; published online October 21, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(4), 042505 (Oct 21, 2015) (13 pages) Paper No: GTP-15-1398; doi: 10.1115/1.4031545 History: Received August 07, 2015; Revised August 29, 2015

Pocket damper seals (PDSs) are used as replacements for labyrinth seals in high-pressure centrifugal compressors at the balance-piston location or center seal location to enhance rotordynamic stability. A concern exists that this enhanced stability will be lost at high positive inlet preswirl. Numerical results of frequency-dependent rotordynamic force coefficients and leakage flow rates were presented and compared for a fully partitioned PDS (FPDS) and a labyrinth seal at high positive and negative inlet preswirl, using a proposed transient computational fluid dynamics (CFD) method based on the multifrequency elliptical orbit whirling model. The negative preswirl indicates a fluid swirl in a direction opposite to rotor rotation at seal inlet. Both seals have identical diameter and sealing clearance. The full 3D concentric CFD model and mesh were built for the labyrinth seal and FPDS, respectively. The accuracy and availability of the present transient CFD numerical method were demonstrated with the experiment data of frequency-dependent rotordynamic coefficients of the labyrinth seal and FPDS at zero and high positive preswirl conditions. The numerical boundary conditions include two high positive preswirl, two high negative preswirl, and a zero preswirl. Numerical results show that the effect of inlet preswirl on the direct force coefficients is weak, but the effect on the cross-coupling stiffness and effective damping is dramatic. Both seals possess negative effective damping at lower excitation frequencies due to positive preswirl, and the crossover frequency of effective damping term increases with increasing positive preswirl. Negative preswirl produces negative cross-coupling stiffness and positive effective damping over the whole excitation frequency range. Increasing negative preswirl is a stabilizing factor for annular gas seals, which results in a significant increase in the effective damping and a decrease in the crossover frequency. It is desirable to reduce the inlet preswirl to zero or even negative through applications of negative-swirl brakes and negative injection devices.

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Figures

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Fig. 1

Twelve-bladed PDS: (a) conventional PDS and (b) fully partitioned PDS

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Fig. 2

Geometries of experimental annular gas seals [17]: (a) labyrinth seal and (b) FPDS

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Fig. 3

Computational models and meshes of two types of annular gas seals: (a) labyrinth seal and (b) FPDS

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Fig. 4

Elliptical orbit whirling model for the rotor vibration with a single frequency: (a) x-direction excitation and (b) y-direction excitation

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Fig. 5

Rotordynamic coefficients versus vibration frequency without and with inlet preswirl: (a) labyrinth seal and (b) FPDS

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Fig. 6

Seal leakage flow rate versus positive preswirl ratio

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Fig. 7

Rotordynamic coefficients versus vibration frequency at different positive inlet preswirls: (a) labyrinth seal and (b) FPDS

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Fig. 8

Static pressure contours on the cross section through the middle of annular cavity 7 or pocket cavity 3 and phasor diagram of the response force at different positive preswirls (x excitation, T=0.1  s): (a) labyrinth seal and (b) FPDS

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Fig. 9

Seal leakage flow rate versus negative preswirl ratio

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Fig. 10

Rotordynamic coefficients versus vibration frequency at different negative inlet preswrils: (a) labyrinth seal and (b) FPDS

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Fig. 11

Static pressure contours on the cross section through the middle of annular cavity 7 or pocket cavity 3 and phasor diagram of the response force at high negative preswirl (λ=−1.0, x excitation, T=0.1  s): (a) labyrinth seal and (b) FPDS

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Fig. 12

Swirl velocity distribution along relative cavity depth at different inlet preswirl velocity: (a) labyrinth seal and (b) FPDS

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Fig. 13

Cross-coupling stiffness representation of follower force on a forward rotor mode

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