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Research Papers: Gas Turbines: Structures and Dynamics

Heavily Loaded Gas Foil Bearings: A Model Anchored to Test Data

[+] Author and Article Information
Tae Ho Kim

Mechanical Engineering Department,  Texas A&M University, College Station, TX 77843-3123

Luis San Andrés

Mechanical Engineering Department,  Texas A&M University, College Station, TX 77843-3123lsanandres@mengr.tamu.edu

J. Eng. Gas Turbines Power 130(1), 012504 (Jan 09, 2008) (8 pages) doi:10.1115/1.2770494 History: Received April 14, 2006; Revised December 13, 2006; Published January 09, 2008

Widespread usage of gas foil bearings (FBs) into microturbomachinery to midsize gas turbine engines requires accurate performance predictions anchored to reliable test data. This paper presents a simple yet accurate model predicting the static and dynamic force characteristics of gas FBs. The analysis couples the Reynolds equation for a thin gas film to a simple elastic foundation model for the top foil and bump strip layer. An exact flow advection model is adopted to solve the partial differential equations for the zeroth- and first-order pressure fields that render the FB load capacity and frequency-dependent force coefficients. As the static load imposed on the foil bearing increases, predictions show that the journal center displaces to eccentricities exceeding the bearing nominal clearance. A nearly constant FB static stiffness, independent of journal speed, is estimated for operation with large loads, and approaching closely the bearing structural stiffness derived from contact operation without rotor spinning. Predicted minimum film thickness and journal attitude angle demonstrate good agreement with archival test data for a first-generation gas FB. The bump-foil-strip structural loss factor, exemplifying a dry-friction dissipation mechanism, aids to largely enhance the bearing direct damping force coefficients. At high loads, the bump-foil structure influences most the stiffness and damping coefficients. The predictions demonstrate that FBs have greatly different static and dynamic force characteristics when operating at journal eccentricities in excess of the bearing clearance from those obtained for operation at low loads, i.e., small journal eccentricity.

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Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic view of first-generation FB

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Figure 2

Geometry of a journal and arcuate foil with arc preload

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Figure 3

Equivalent foil structural model for elastic foundation (bump strip)

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Figure 4

Schematic view of nonrotating journal and FB in contact

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Figure 5

Configuration of top foil and bump-foil strip

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Figure 6

Predicted journal eccentricity versus static load. Compliance coefficient S=0.67. Bearing configuration given in Ref. 16.

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Figure 7

Minimum film thickness versus static load. Compliance coefficient S=0.67. Comparison to test data in Ref. 16.

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Figure 8

Journal attitude angle versus static load. Compliance coefficient S=0.67. Comparison to test data in Ref. 16.

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Figure 9

Predicted FB stiffness coefficients versus excitation frequency for various static loads at 30krpm. Compliance coefficient S=0.67, loss factor γ=0.0.

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Figure 10

Predicted damping coefficients versus excitation frequency for various static loads at 30krpm. Compliance coefficient S=0.67, loss factor γ=0.0 and 0.4.

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