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Research Papers: Hydrodynamic Lubrication

Experimental Feasibility Study of Radial Injection Cooling of Three-Pad Air Foil Bearings

[+] Author and Article Information
Daejong Kim

e-mail: Daejongkim@uta.edu
Mechanical and Aerospace Engineering Department,
University of Texas at Arlington,
500 W. 1st Street,
Arlington, TX 76019

Young Cheol Kim

Korea Institute of Machinery and Materials,
171 Jang-dong, Yuseong-Gu,
Daejeon, 305-343, South Korea

1Corresponding author.

Contributed by the Tribology Division of ASME for publication in the JOURNAL OF TRIBOLOGY. Manuscript received October 12, 2012; final manuscript received May 11, 2013; published online June 24, 2013. Assoc. Editor: J. Jeffrey Moore.

J. Tribol 135(4), 041703 (Jun 24, 2013) (9 pages) Paper No: TRIB-12-1175; doi: 10.1115/1.4024547 History: Received October 12, 2012; Revised May 11, 2013

The foil bearing (FB) is one type of hydrodynamic bearing using air or another gas as a lubricant. When FBs are designed, installed, and operated properly, they are a very cost-effective and reliable solution for oil-free turbomachinery. Because there is no mechanical contact between the rotor and its bearings, quiet operation with very low friction is possible once the rotor lifts off the bearings. However, because of the high speed of operation, thermal management is a very important design factor to consider. The most widely accepted cooling method for FBs is axial flow cooling, which uses cooling air or gas passing through heat-exchange channels formed underneath the top foil. The advantage of axial cooling is that no hardware modification is necessary to implement it, because the elastic foundation structures of the FB serve as the heat-exchange channels. Its disadvantage is that an axial temperature gradient exists on the journal shaft and bearing. In this paper, the cooling characteristics of axial cooling are compared with those of multipoint radial injection, which uses high-speed injection of cooling air onto the shaft at multiple locations. Experiments were performed on a three-pad FB 49 mm in diameter and 37.5 mm in length, at speeds of 30,000 rpm and 40,000 rpm. Injection speeds were chosen to be higher than the journal surface speed, but the total cooling air flow rate was matched to that of the axial cooling cases. Experimental results show that radial injection cooling is comparable to axial cooling at 30,000 rpm, in terms of cooling performance. Tests at 40,000 rpm reveal that the axial cooling performance reaches saturation when the pressure drop across the bearing is larger than 1000 Pa, while the cooling performance of radial injection is proportional to the cooling air flow rate and does not become saturated. Overall, multipoint radial injection is better than axial cooling at high rotor speeds.

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References

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Figures

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Fig. 1

Schematic view of three-pad foil bearings with preload; CS is the minimum radial clearance at the converging end of the film when rotor is at the center of the bearing

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Fig. 2

Leading edge groove region where exit flow from trailing edge is mixed with cooling air (provided through axial cooling or radial injection)

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Fig. 3

Three-pad FB with radial injection holes at the leading edge groove region

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Fig. 4

Cooling air flow passages for different cooling methods. (a) Axial flow through bump foils. (b) Radial injection through bearing sleeve.

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Fig. 5

Test rig configuration shown with radial injection cooling configuration

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Fig. 6

Instrumentation for plenum temperature and pressure measurement

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Fig. 7

Photo of FB (clockwise shaft rotation) inside the bearing housing and TCs; TC locations: ± 30 degrees from top, ± 30 degrees from bottom, all 7.5 mm in depth

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Fig. 9

Pressure drop versus air volumetric flow rate across the bearings, measured at stationary conditions

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Fig. 10

Temperature evolution of bearing sleeve at bottom (see Fig. 7) with different flow rates; 30,000 rpm, 45 -N external loading to the foil bearing. (a) Radial injection cooling. (b) Axial cooling.

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Fig. 11

Temperature evolution of bearing sleeve at top (see Fig. 7) with different flow rates; 30,000 rpm, 45 -N external loading to the foil bearing. (a) Radial injection. (b) Axial cooling.

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Fig. 12

Ball bearing housing temperature at the test section; 30,000 rpm, 45 -N external loading to the foil bearing. (a) Radial injection. (b) Axial cooling.

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Fig. 13

Temperature distribution of bearing sleeve; 30,000 rpm, 45 -N external loading to the foil bearing; flow rates for axial cooling cases were inferred from pressure-flow calibration shown in Fig. 9. (a) Radial injection. (b) Axial cooling.

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Fig. 14

Temperature of plenums; 30,000 rpm, 45 -N external loading to the foil bearing; flow rates for axial cooling cases were inferred from pressure-flow calibration shown in Fig. 9. (a) Radial injection. (b) Axial cooling.

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Fig. 15

Temperature distribution of bearing sleeve; 40,000 rpm, 45 -N external loading to the foil bearing; flow rates for axial cooling cases were inferred from pressure-flow calibration shown in Fig. 9. (a) Radial injection. (b) Axial cooling.

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Fig. 16

Temperature of plenums; 40,000 rpm, 45 -N external loading to the foil bearing; flow rates for axial cooling cases were inferred from pressure-flow calibration shown in Fig. 9. (a) Radial injection. (b) Axial cooling.

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Fig. 17

TC locations on the shaft and ball bearing housings; TCs on the ball bearing housings are fixed on top of the housing, and TCs for the shaft do not touch shaft but remain close to it until the shaft stops

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