Research Papers: Gas Turbines: Structures and Dynamics

On the Failure of a Gas Foil Bearing: High Temperature Operation Without Cooling Flow

[+] Author and Article Information
Keun Ryu

Assistant Professor
Department of Mechanical Engineering,
Hanyang University,
Ansan, Gyeonggi-do 426-791, South Korea
e-mail: kryu@hanyang.ac.kr

Luis San Andrés

Mast-Childs Professor
Fellow ASME
Department of Mechanical Engineering,
Texas A&M University,
College Station, TX 77843
e-mail: LSanAndres@tamu.edu

This operating procedure is not advised under any circumstance when the rotor and bearings are exposed to high temperatures. The practice detailed here offers a cautionary tale for other foil bearing end users.

Thermal runaway is a consequence of the thermal growth mismatch between the shaft and bearing, manifesting itself as a continual increase in preload [18].

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 10, 2013; final manuscript received July 12, 2013; published online September 17, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 135(11), 112506 (Sep 17, 2013) (10 pages) Paper No: GTP-13-1254; doi: 10.1115/1.4025079 History: Received July 10, 2013; Revised July 12, 2013

Implementing gas foil bearings (GFBs) in micro gas turbine engines is a proven approach to improve system efficiency and reliability. Adequate thermal management for operation at high temperatures, such as in a gas turbine or a turbocharger, is important to control thermal growth of components and to remove efficiently mechanical energy from the rotor mainly. The paper presents a test rotor supported on GFBs operating with a heated shaft and reports components temperatures and shaft motions at an operating speed of 37 krpm. An electric cartridge heater loosely inserted in the hollow rotor warms the test system. Thermocouples and noncontact infrared thermometers record temperatures on the bearing sleeve and rotor outer diameter (OD), respectively. No forced cooling air flow streams were supplied to the bearings and rotor, in spite of the high temperature induced by the heater on the shaft outer surface. With the rotor spinning, the tests consisted of heating the rotor to a set temperature, recording the system component temperatures until reaching thermal equilibrium in  ∼ 60 min, and stepping the heater set temperature by 200 °C. The experiments proceeded without incident until the heater set temperature equaled 600 °C. Ten minutes into the test, noise became apparent and the rotor stopped abruptly. The unusual operating condition, without cooling flow and a too large increment in rotor temperature, reaching 250 °C, led to the incident which destroyed one of the foil bearings. Post-test inspection evidenced seizure of the hottest bearing (closest to the heater) with melting of the top foil at the locations where it rests on the underspring crests (bumps). Analysis reveals a notable reduction in bearing clearance as the rotor temperature increases until seizure occurs. Upon contact between the rotor and top foil, dry-friction quickly generated vast amounts of energy that melted the protective coating and metal top foil. Rather than a reliability issue with the foil bearings, the experimental results show poor operating procedure and ignorance on the system behavior (predictions). A cautionary tale and a lesson in humility follow.

Copyright © 2013 by ASME
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Fig. 1

Photographs of one test foil bearing. Pristine condition before experiments.

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Fig. 2

Photographs of high temperature GFB rotordynamic test rig and locations of temperature measurement. T1 ∼ T10, TrDE, and TrFE are locations of temperature measurement.

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Fig. 3

Schematic view for dimensions (mm) of test rotor, cartridge heater, and enclosure with locations of thermocouples for feed enclosure air temperature (Te) and reference to control heater set temperature (Th)

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Fig. 4

Recorded test rig temperatures versus elapsed time with rotor spinning at 37 krpm (617 Hz): (a) heater (Th), free end (TrFE), and drive end (TrDE) of rotor OD; (b) test rig housing (T5 and T10) and ambient (Tamb); (c) FE bearing sleeve OD (T1 ∼ T4); and (d) DE bearing sleeve OD (T6 ∼ T9). No cooling flow into bearings. Note different vertical scales.

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Fig. 5

Measured steady state temperatures of free end and drive end rotor OD (TrFE and TrDE), air inside closed enclosure of housing (Te), and free end and drive end bearing sleeve OD (T4 and T9) for operation with heater off and with heater on at Th = 200 °C, 400 °C, and 600 °C

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Fig. 6

Waterfall of rotor response while the rotor is heated up. Free end vertical plane. Fixed rotor speed at 37 krpm (617 Hz). No cooling flow into bearings.

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Fig. 7

Synchronous speed rotor motion orbits for operation with a hot rotor at 37 krpm. Measurements at rotor free end. Heater temperature (Th) increased to 200 °C, 400 °C, and 600 °C at ∼60 min intervals.

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Fig. 8

Photographs of rotor showing its OD surface conditions: original and post failure

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Fig. 9

Photograph of drive end foil bearing after failure. Post-tests inspection (bearing still functional).

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Fig. 10

Photograph of free end foil bearing after failure. Post-tests inspection.

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Fig. 11

Close up view of (inboard) side of free end bearing. Post-test inspection.

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Fig. 12

Schematic representation of deformation in top foil and bump strips

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Fig. 13

Predicted thermal expansion of rotor at free end bearing location and bearing sleeve contraction versus temperature. Note different vertical and horizontal scales.

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Fig. 14

Estimated radial clearances for test foil bearings versus temperatures. Rotor speed = 37 krpm. Top: free end bearing, bottom: drive end bearing.

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Fig. 15

Measured and predicted mean temperatures at FE and DE FB sleeves versus rotor temperature. Operation without forced cooling flow. Rotor speed 37 krpm.

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Fig. 16

Predicted top foil temperature when rotor contacts foil at a speed of 36 krpm. Initial rotor and foil temperatures = 300 °C.




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