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Research Papers: Gas Turbines: Structures and Dynamics

Design Space of Foil Bearings for Closed-Loop Supercritical CO2 Power Cycles Based on Three-Dimensional Thermohydrodynamic Analyses

[+] Author and Article Information
Daejong Kim

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

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 13, 2015; final manuscript received August 12, 2015; published online October 6, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(3), 032504 (Oct 06, 2015) (9 pages) Paper No: GTP-15-1280; doi: 10.1115/1.4031433 History: Received July 13, 2015; Revised August 12, 2015

The closed-loop Brayton cycle with supercritical CO2 (S-CO2) as an operating fluid is an attractive alternative to conventional power cycles due to very high power density. Foil gas bearings using CO2 are the most promising for small S-CO2 turbomachinery but there are many problems to address: large power loss due to high flow turbulence, lack of design/analysis tool due to nonideal gas behavior, and lack of load capacity when they are used for large systems. This paper presents high-level design/analysis tool involving three-dimensional (3D) thermohydrodynamic (THD) analyses of radial foil bearings considering real gas effect and flow turbulence inside the film. Simulations are performed for radial foil bearing with 34.9 mm in diameter and lubricated with CO2 and N2 under various ambient conditions up to above 40 bar gauge pressure. The simulation results using the turbulence model still underpredict the measured data in open literature. However, the error between the prediction and measurements decreases as either speed or ambient pressure increases. In addition, general behavior of substantial increase in power loss with ambient pressure agrees with the measured data. The simulation results indicate the importance of detailed THD analysis of the foil bearings for prediction of power loss under severe turbulent condition. A conceptual layout of rotor system for 10 MWe S-CO2 loop is also presented along with realistic rotor weight and bearing load. A hybrid foil bearing with diameter of 102 mm is suggested for gas generator rotor, and its power losses and minimum film thicknesses at various operating conditions are presented.

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References

Figures

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Fig. 1

Exemplary velocity profile: 30 °C/15.5 bar, C = 50 μm, surface speed = 128 m/s, pressure gradient = 1000 Pa/mm, and viscosity = 15.4 μPa·s

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Fig. 2

Shaft-bearing configuration in NASA rig [18], image of bearing and arrow indicating the loading were added by the author

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Fig. 3

Measured friction power taken from Ref. [18] under permission granted by NASA: (a) CO2 and (b) N2

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Fig. 4

Predicted power loss of CO2 foil bearing

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Fig. 5

Predicted power loss of N2 foil bearing

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Fig. 6

Flow Reynolds numbers assuming uniform film temperature of 30 °C

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Fig. 7

Cross-film averaged temperature distribution at 40,000 rpm; drawn for 360 deg along circumferential direction: (a) 14.97 bar (220 psi) and (b) 29.94 bar (440 psi)

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Fig. 8

Specific heat and temperature contours across the film at bearing centerline under 14.97 bar and 40,000 rpm: (a) cp (kJ/kg K) and (b) temperature (°C)

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Fig. 9

Specific heat and temperature contours across the film at bearing centerline under 29.94 bar and 40,000 rpm: (a) cp (kJ/kg K) and (b) temperature (°C)

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Fig. 10

Predicted power loss at high speeds at different ambient conditions

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Fig. 11

Predicted nondimensional power loss at two ambient pressures

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Fig. 12

Concept layout of 10 MWe S-CO2 cycle gas generator rotor

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Fig. 13

Predicted power loss of 102 mm foil bearings at different ambient conditions

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Fig. 14

Centerline pressure profile of 102 mm foil bearing at different speeds when ambient pressure and temperature are 14.97 bar and 100 °C

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Fig. 15

Film thickness at bearing edge of 102 mm foil bearing at different speeds when ambient pressure and temperature are 14.97 bar and 100 °C

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