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Research Papers: Gas Turbines: Structures and Dynamics

Effect of Side Feed Pressurization on the Dynamic Performance of Gas Foil Bearings: A Model Anchored to Test Data

[+] Author and Article Information
Tae Ho Kim

Texas A&M University Turbomachinery Laboratory, Texas A&M University, College Station, TX 77843-3123taeho21c@tamu.edu

Luis San Andrés

Texas A&M University Turbomachinery Laboratory, Texas A&M University, College Station, TX 77843-3123lsanandres@mengr.tamu.edu

The importance of the feed pressure effect on foil bearing stability was unknown prior to testing. Hence, a gas flow meter was not readily available at the time of the measurements. This omission will be corrected in future experimentation.

The effect of temperature on the operation of the test GFBs was minimal, i.e., no significant changes in measured bearing and shaft temperatures were recorded during the experiments.

The flow of gas through the outer region underneath the top foil is only axial, not greatly restricted by the bump strip layers.

J. Eng. Gas Turbines Power 131(1), 012501 (Oct 02, 2008) (8 pages) doi:10.1115/1.2966421 History: Received March 28, 2008; Revised March 29, 2008; Published October 02, 2008

Comprehensive modeling of gas foil bearings (GFBs) anchored to reliable test data will enable the widespread usage of these bearings into novel high speed turbomachinery applications. GFBs often need a forced cooling gas flow, axially fed through one end of the bearing, for adequate thermal management. This paper presents rotordynamic response measurements on a rigid rotor supported on GFBs during rotor speed run-up and coastdown tests with the GFBs supplied with increasing feed gas pressures to 2.8bars. Rotor speed run-up tests to 35krpm show that the bearing end side feed gas pressurization delays the onset speed of rotor subsynchronous whirl motions. The test data validate closely the predictions of the threshold speed of instability and the whirl frequency ratio derived from a GFB model that implements the axial evolution of gas circumferential flow velocity as a function of the imposed side feed pressure. Rotor speed coastdown tests from 25krpm with a low feed pressure of 0.35bar evidence a nearly linear synchronous rotor response for small and moderately large imbalance mass distributions. A structural finite element rotordynamics model integrates linearized synchronous speed GFB force coefficients and predicts synchronous responses, amplitude, and phase angles, agreeing with the test data. The analysis and measurements demonstrate the profound effect of the end side feed gas pressurization on the rotordynamic performance of GFBs.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of “second generation” bump type foil bearing

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Figure 2

Layout of rotor-GFBs test rig and instrumentation

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Figure 3

Flow induced by side pressure in a foil bearing: schematic of the evolution of gas velocities within the inner and outer flow regions, i.e., between the journal and the top foil underneath top foil and bearing housing, respectively

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Figure 4

Waterfalls of rotor free end vertical displacements measured during speed run-up from 10krpm. Base line imbalance condition and air feed gauge pressures (a) 0.34bar(5psigauge) and (b) 2.8bars(40psigauge).

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Figure 5

Measured amplitudes of synchronous and subsynchronous rotor motions versus shaft speed for increasing air feed gauge pressure. Vertical displacements (X-direction) at rotor free end. Nos: onset speed of subsynchronous motion.

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Figure 6

Spectra of measured rotor free end vertical motions for increasing feed (gauge) side pressures and operation at 30krpm(500Hz)

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Figure 7

Measured amplitudes of total shaft motion, and synchronous and subsynchronous components versus magnitude of bearing side (gauge) gas pressure at 30krpm(500Hz)

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Figure 8

Measured normalized amplitude and phase of synchronous rotor response for out-of-phase imbalance masses of 55mg, 110mg, 165mg, and 330mg. Measurements at drive end bearing, vertical plane (DV) with base line subtraction. Bearing end feed pressure at 0.34bar(5psigauge) gauge.

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Figure 9

Predicted GFB journal eccentricity and attitude angle versus side feed (gauge) pressure. Static load 4.9N. Rotor speed: 30krpm(500Hz).

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Figure 10

Predicted GFB drag torque and minimum film thickness versus side feed (gauge) pressure. Static load 4.9N. Rotor speed: 30krpm(500Hz).

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Figure 11

Effect of bearing side pressurization on predicted GFB force coefficients; (a) direct and cross-coupled stiffnesses and (b) direct damping coefficients. The numbers denote magnitude of side feed pressure, Ps [bar (gauge)]. Static load 4.9N (12 rotor weight), rotor speed: 30krpm(500Hz). Bearing structural loss factor γ=0.20.

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Figure 12

Predicted critical mass versus side feed pressure for operation of GFB. Static load 4.9N. Speed: 30krpm(500Hz).

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Figure 13

Predicted synchronous stiffness and damping coefficients versus rotor speed for drive end GFB supporting static load of 6.6N (X-direction). Structural loss factor γ=0.2. Bearing side feed pressure at 0.34bar(5psigauge) gauge.

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Figure 14

Predicted and measured phase angles and normalized rotor amplitude of synchronous response for increasing out-of-phase imbalance mass distributions (drive end bearing, vertical plane). Bearing side feed pressure at 0.34bar(5psigauge) gauge.

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Figure 15

Effective GFB stiffness and damping coefficients estimated from rotor response measurements for small and moderately large imbalance masses. Predictions obtained at a rotor speed of 10krpm.

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