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Research Papers: Gas Turbines: Structures and Dynamics

Identification of Structural Stiffness and Energy Dissipation Parameters in a Second Generation Foil Bearing: Effect of Shaft Temperature

[+] Author and Article Information
Luis San Andrés

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

Keun Ryu

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

Tae Ho Kim

Energy Mechanics Research Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Songbuk-gu, Seoul 136-791,Korea

The test FB is proprietary; hence, complete information on its geometry and material properties cannot be listed in this paper. The authors apologize in advance for this limitation.

For the tests at Th=263deg, only three FB motion amplitudes of 7.4μm, 11.1μm, and 14.8μm were selected to prevent permanent damage or distortion of the bump foils.

Prior literature (9,11) shows that some foil bearings have an ad hoc clearance, i.e., a region with very soft or low stiffness, where small forces cause large displacements. In reality, most FBs are designed to work with an interference fit, i.e., a preload.

J. Eng. Gas Turbines Power 133(3), 032501 (Nov 11, 2010) (9 pages) doi:10.1115/1.4002317 History: Received April 06, 2010; Revised July 08, 2010; Published November 11, 2010; Online November 11, 2010

Established high temperature operation of gas foil bearings (GFB) is of great interest for gas turbine applications. The effects of (high) shaft temperature on the structural stiffness and mechanical energy dissipation parameters of a foil bearing (FB) must be assessed experimentally. Presently, a hollow shaft warmed by an electric heater holds a floating second generation FB that is loaded dynamically by an electromagnetic shaker. In tests with the shaft temperature up to 184°C, the measurements of dynamic load and ensuing FB deflection render the bearing structural parameters, stiffness and damping, as a function of excitation frequency and amplitude of motion. The identified FB stiffness and viscous damping coefficients increase with shaft temperature due to an increase in the FB assembly interference or preload. The bearing material structural loss factor best representing mechanical energy dissipation decreases slightly with shaft temperature while increasing with excitation frequency. Separate static load measurements on the bearing also make evident the preload of the test bearing-shaft system at room temperature. The loss factor obtained from the area inside the hysteresis loop of the static load versus the deflection curve agrees remarkably with the loss factor obtained from the dynamic load measurements. The static procedure offers substantial savings in cost and time to determine the energy dissipation characteristics of foil bearings. Post-test inspection of the FB reveals sustained wear at the locations, where the bumps contact the top foil and the bearing sleeve inner surface, thus, evidences the bearing energy dissipation by dry friction.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic view of a radial foil bearing: single bump strip layer

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Figure 2

Setup for dynamic load tests on (nonrotating) foil bearing and hollow shaft warmed by an electric heater

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Figure 3

Measured temperatures at shaft and FB cartridge OD for three cartridge heater temperatures Th=103°C, 183°C, and 263°C. Surrounding ambient temperature is 23°C.

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Figure 4

Real part of test system mechanical impedance versus excitation frequency. Tests with heater at Th=23°C (room temperature) and Th=263°C.

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Figure 5

FB structural stiffness versus excitation frequency for FB motion amplitudes of 7.4 μm, 11.1 μm, 14.8 μm, and 18.5 μm. Tests with heater at Th=23°C (room temperature) and Th=263°C.

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Figure 6

FB structural stiffness K versus excitation frequency. Tests with heater temperature Th=23°C, 103°C, 183°C, and 263°C. FB motion amplitude of 14.8 μm.

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Figure 7

FB viscous damping coefficient versus excitation frequency for FB motion amplitudes of 7.4 μm, 11.1 μm, 14.8 μm, and 18.5 μm. Tests with heater at Th=23°C and Th=263°C.

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Figure 8

FB viscous damping coefficient Ceq versus excitation frequency. Tests with heater temperature Th=23°C, 103°C, 183°C, and 263°C. FB motion amplitude of 14.8 μm.

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Figure 9

FB structural loss factor versus excitation frequency for FB motion amplitudes of 7.4 μm, 11.1 μm, 14.8 μm, and 18.5 μm. Tests with heater at Th=23°C and Th=263°C.

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Figure 10

FB structural loss factor γ versus excitation frequency. Tests with heater temperature Th=23°C, 103°C, 183°C, and 263°C. FB motion amplitude of 14.8 μm.

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Figure 11

Estimated dry friction coefficient versus excitation frequency for heater temperatures Th=23°C, 103°C, 183°C, and 263°C. FB motion amplitude of 14.8 μm.

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Figure 12

Original and post-test condition of foil bearing after dynamic load tests. Top foil and bump strips displaced for phototgraph.

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Figure 13

Schematic view and photograph of experimental setup for FB static load test. Static load 90 deg away from spot weld.

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Figure 14

Recorded FB deflection versus static load and estimated structural stiffness. Data for three cycles of loading-unloading shown. Measurements at room temperature. FB radial assembly preload is 0.005 mm.

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Figure 15

Comparison of static and dynamic load: (a) FB stiffness and (b) FB structural loss factor versus FB deflection. Room temperature.

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