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Research Papers: Hydrodynamic Lubrication

# Transient Thermal Behavior of Preloaded Three-Pad Foil Bearings: Modeling and Experiments

[+] Author and Article Information

Department of Mechanical and Aerospace Engineering, University of Texas at Arlington, 500 West First Street, Arlington, TX 76019

Daejong Kim1

Department of Mechanical and Aerospace Engineering, University of Texas at Arlington, 500 West First Street, Arlington, TX 76019daejongkim@uta.edu

Preliminary investigation on the motion of the bearing reveals that once the external load and bearing reaction force reach the force equilibrium (within a few milliseconds), the transient term in Reynolds equation $∂(ph/Tf)/∂t$ becomes negligible even if the film temperature changes over time.

The break-in operation can be interpreted as initial mild abrasive wear of local minor high spots on the rotor and bearing surfaces when the bearing is first assembled and started. These minor local high spots are due to the manufacturing error, top foil thickness irregularity, uneven oxidation, etc. Most foil bearings with even commercial coatings undergo initial break-in when they are first used. Mild rubbing marks on the bearing and rotor surface are the typical signature marks of the break-in.

1

Corresponding author.

J. Tribol 133(2), 021703 (Mar 22, 2011) (11 pages) doi:10.1115/1.4003561 History: Received September 22, 2010; Revised January 13, 2011; Published March 22, 2011; Online March 22, 2011

## Abstract

Oil-free turbomachinery have emerged as one of the core technologies for the future green power generation systems as stand-alone systems or hybridized with high temperature fuel cells or solar systems. Oil-free technology allows compact, clean, and maintenance-free operation, and foil bearings are at the center of the technology. Since their first commercial applications in the air cycle machines and auxiliary power units in 1970s, significant improvement has been made to the computational models for rotordynamic behavior. However, many technical issues still remain unsolved or poorly understood, and one of them is thermal management. This paper presents transient three-dimensional thermohydrodynamic (3D THD) model of radial foil bearings to predict transient thermal behavior of the bearing-rotor system. The transient model involves transient energy equations applied to all the mechanical structures and gas film. The model was verified through extensive experimental measurements of transient thermal behavior of three-pad foil bearing for various cooling air pressures, external loads, and speeds. The predictions showed very good agreements with the experiments, and also the 3D THD model could predict potential thermal instability observed in the experimental measurements.

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## Figures

Figure 1

Description and photo of a three-pad preloaded air foil bearing (AFB): (a) description of a three-pad configuration with preload and coordinate system and (b) photo of a three-pad foil bearing

Figure 2

Photo of the test rig and description of test section: (a) photo of the entire rig (shown without cooling jacket), (b) photo of the cooling jacket with thermocouple and dummy chamber for pressure pickup, and (c) closed up description of test section with cooling jacket and TCs attached

Figure 3

Cross-section image of the test section

Figure 4

Thermal model of foil structure adopted from Ref. 28: (a) foil structure and (b) heat transfer mechanism around the foil structure

Figure 5

Thermal subsystem of the test rig. Blue arrows indicate cooling air flow and red arrows indicate heat transfer mechanisms.

Figure 6

Measured and predicted temperatures of bearing components (53.4 N, 35,000 rpm): (a) temperature change of the loaded pad and (b) temperature change of the unloaded pad

Figure 7

Measured and predicted temperatures of bearing housing, 53.4 N, 35,000 rpm

Figure 8

Temperature changes of the center of loaded pad for various speeds (53.4 N)

Figure 9

Figure 10

Measured cooling air temperature inside the plenum (75.7 N, 35,000 rpm)

Figure 11

Figure 12

Figure 13

Temperatures of the bearing housing for various plenum pressures (75.5 N, 35,000 rpm)

Figure 14

Predicted air flow rate and channel temperatures for various plenum pressures at steady state, 35,000 rpm and 75.5 N: (a) cooling air flow rate and (b) cooling air channel temperature

Figure 15

Predicted and measured temperatures at the loaded pad center (35,000 rpm): (a) 53.4 N and (b) 75.7 N

Figure 16

Prediction of thermal instability; test at 97.7 N corresponds to initial break-in operation performed at 35,000 rpm: (a) top foil center temperature and (b) predicted temperatures of the rotor center and bearing housing (106.6 N, 35,000 rpm)

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