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Research Papers: Gas Turbines: Structures and Dynamics

Effect of Cooling Flow on the Operation of a Hot Rotor-Gas Foil Bearing System

[+] Author and Article Information
Keun Ryu

Senior Development Engineer
Global Commercial Diesel Product Development,
BorgWarner Turbo Systems, Arden, NC 28704,
e-mail: kryu@borgwarner.com

Luis San Andrés

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

The thermal mixing coefficient λ denotes the fraction of upstream gas flow (top foil trailing edge) re-entering the thin film of the GFB at the leading edge of the top foil. λ is an empirical parameter depending on the foil bearing arrangement and the cooling method. Reference [20], a public document, discusses thoroughly the mixing flow and inlet temperature at the leading edge of top foil. References [21,22] derive a similar mixing model and add CFD prediction validations.

1Work conducted as a Graduate Research Assistant at Texas A&M University.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Engineering for Gas Turbines and Power Manuscript received June 21, 2012; final manuscript received June 21, 2012; published online August 22, 2012. Editor: Dilip R. Ballal.

J. Eng. Gas Turbines Power 134(10), 102511 (Aug 22, 2012) (12 pages) doi:10.1115/1.4007067 History: Received June 21, 2012; Revised June 21, 2012

Gas foil bearings (GFBs) operating at high temperature rely on thermal management procedures that supply needed cooling flow streams to keep the bearing and rotor from overheating. Poor thermal management not only makes systems inefficient and costly to operate but could also cause bearing seizure and premature system destruction. This paper presents comprehensive measurements of bearing temperatures and shaft dynamics conducted on a hollow rotor supported on two first generation GFBs. The hollow rotor (1.36 kg, 36.51 mm OD and 17.9 mm ID) is heated from inside to reach an outer surface temperature of 120 °C. Experiments are conducted with rotor speeds to 30 krpm and with forced streams of air cooling the bearings and rotor. Air pressurization in an enclosure at the rotor mid span forces cooling air through the test GFBs. The cooling effect of the forced external flows is most distinct when the rotor is hottest and operating at the highest speed. The temperature drop per unit cooling flow rate significantly decreases as the cooling flow rate increases. Further measurements at thermal steady state conditions and at constant rotor speeds show that the cooling flows do not affect the amplitude and frequency contents of the rotor motions. Other tests while the rotor decelerates from 30 krpm to rest show that the test system (rigid-mode) critical speeds and modal damping ratio remain nearly invariant for operation with increasing rotor temperatures and with increasing cooling flow rates. Computational model predictions reproduce the test data with accuracy. The work adds to the body of knowledge on GFB performance and operation and provides empirically derived guidance for successful rotor-GFB system integration.

Copyright © 2012 by ASME
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References

Figures

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Fig. 1

Photographs of high temperature GFB rotordynamic test rig and bearings. T1T10, Th, Tout, Te, TrDE and TrFE represent locations of temperature measurement. Also shown a bearing sleeve with an axial slot to route a thermocouple installed at the bearing midspan (oblique view).

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Fig. 2

Schematic view with dimensions (mm) of test rotor, cartridge heater, bearing support housing air feed enclosure. Noted locations of thermocouples for feed enclosure air temperature (Te) and reference to control heater set temperature (Ths). Flow paths of cooling air streams also shown.

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Fig. 3

Photographs of test bump-type 1st generation gas foil bearing

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Fig. 4

Test cases #1 and #2. Ths = 100 °C: free end bearing temperature rise (T1∼4 − Tamb) versus air temperature rise in the enclosure (Te − Tamb). Arithmetic mean of (T1T4) shown.

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Fig. 5

Test cases #1 and #2. Heater set temperature 100 °C. No rotor spinning and rotor speed of 10, 20, and 30 krpm: recorded temperature difference of free end bearing sleeve OD above inlet cooling air temperature (T14 − Te) versus cooling flow rate.

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Fig. 6

Test Cases #1 and #2. Heater set temperature 100 °C. No rotor spinning and rotor speed of 10, 20, and 30 krpm: recorded temperature rise on (a) free end bearing sleeve OD (T14 − Tamb) and (b) free end rotor OD per unit cooling flow rate (L/min) versus cooling flow rate.

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Fig. 7

Test case #3: FFTs of rotor responses at rotor drive end, vertical (DV) plane. Rotor speed = 10, 20 and 30 krpm. Cooling flow into bearings from ∼350 L/min to ∼50 L/min for each set rotor speed. Heater set temperature Ths = 65 °C.

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Fig. 8

Test cases #4 (heater off) and #6 (Ths = 100 °C): waterfalls of rotor motion during decelerating from 30 krpm to rest. Heater off, cooling flow rate ∼350 L/min, deceleration = 16.7 Hz/s. Rotor drive end, vertical (DV) plane.

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Fig. 9

Test cases #4∼#6: rotor amplitudes of synchronous response. Slow roll compensation at 2 krpm. Cooling flow rate ∼350 L/min. Speed down ramp rate = 16.7 Hz/s.

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Fig. 10

Prediction versus test data. Test case #2 (Ths = 100 °C): temperature of bearing sleeves (free and drive ends) versus cooling flow rate for operation at three shaft speeds.

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Fig. 11

Prediction: Test case #2. Thin film temperature fields in free end bearing. Ths = 100 °C, rotor speed at 30 krpm. Cooling flow rate per each bearing 175 L/min. Cooling stream inlet temperature = 33 °C, rotor OD temperature = 80 °C uniform, Tamb = 23 °C.

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Fig. 12

Predicted free end FB direct and cross-coupled stiffnesses versus rotor speed. Whirl frequency equals rotor speed. Heater at Ths = 100 °C and increasing air cooling flow rates (per bearing).

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Fig. 13

Predicted free end FB direct and cross-coupled damping coefficients versus rotor speed. Whirl frequency equals rotor speed. Heater at Ths = 100 °C and increasing air cooling flow rates (per bearing).

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Fig. 14

Finite element model of test rotor supported on GFBs. Connecting rod and flexible coupling locate at drive end (left).

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Fig. 15

Comparison of predicted and measured imbalance responses of test rotor supported on foil bearings. Test case #6. Ths = 100 °C. Cooling flow rate 350 L/min (into two bearings).

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Fig. 16

Predicted heat convection coefficient of outer gap flow versus cooling flow rate (per each bearing). Test case #2 (Ths = 100 °C). Rotor speed of 10, 20, and 30 krpm. Free end bearing.

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