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Research Papers: Gas Turbines: Structures and Dynamics

Evaluation of Foil Bearing Performance and Nonlinear Rotordynamics of 120 kW Oil-Free Gas Turbine Generator

[+] Author and Article Information
Daejong Kim

University of Texas at Arlington,
Arlington, TX 76019
e-mail: daejongkim@uta.edu

An Sung Lee

e-mail: aslee@kimm.re.kr

Bum Seog Choi

e-mail: bschoi@kimm.re.kr
Korea Institute of Machinery and Materials,
104 Shinsungro, Yuseonggu,
Daejon City 305-600, Korea

At excitation frequency ratio of 0.05.

1Corresponding author.

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received August 29, 2013; final manuscript received August 30, 2013; published online November 27, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(3), 032504 (Nov 27, 2013) (8 pages) Paper No: GTP-13-1329; doi: 10.1115/1.4025898 History: Received August 29, 2013; Revised August 30, 2013

This paper presents a design approach of air foil bearings (AFBs) for a 120 kWe gas turbine generator, which is a single spool configuration with gas generator turbine and alternator rotor connected by a diaphragm coupling. A total of four radial AFBs support the two rotors, and one set of double acting thrust foil bearing is located inside the gas generator turbine. The rotor configuration results in eight degree of freedom (DOF) rotordynamic motions, which are two cylindrical modes and two conical modes from the two rotors. Stiffness of bump foils of candidate AFB was estimated from measured structural stiffness of the bearing, and implemented to the computational model for linear stiffness and damping coefficients of the bearing and frequency-domain modal impedances for cylindrical and conical modes. Stiffness of the diaphragm coupling was evaluated using finite element analysis and implemented to nonlinear rotordynamic analyses of the entire engine. Analyses show the conical mode of the turbine rotor is the main source of instability of the entire engine when AFB clearance is not selected properly. Optimum AFB clearance is suggested from frequency domain modal analyses and nonlinear transient analyses.

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References

Figures

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Fig. 1

120 kW MGT configuration, rated speed = 45,800 rpm, maximum over speed = 50,400 rpm

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Fig. 2

Photo of diaphragm coupling: (a) under lateral load; (b) under moment

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Fig. 3

FEA results of diaphragm coupling: (a) with top foil removed; (b) complete assembly

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Fig. 4

Photo of radial foil bearing

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Fig. 5

Photo of load-deflection test rig for radial foil bearing

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Fig. 6

Load-deflection curve of radial foil bearing: (a) 4 DOF for generator shaft; (b) 4 DOF for turbine shaft

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Fig. 7

Coordinate system and variables for 3D rotordynamics simulation

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Fig. 8

Log-decrements of four damped natural frequencies

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Fig. 10

Transient response from nonlinear rotordynamics analysis: (a) cylindrical mode; (b) conical mode

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Fig. 9

Transient response using linear synchronous coefficients

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Fig. 11

Modal impedance curves as a function of excitation frequency at 50,400 rpm

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Fig. 12

Transient response from nonlinear rotordynamics analysis with clearance increased by 50 μm

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Fig. 13

Modal impedance curves for conical mode with radial clearances increased by 50 μm

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Fig. 14

Ratio of synchronous modal damping to maximum negative subsynchronous damping for conical mode of turbine rotor with different AFB clearances

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Fig. 15

Trend of modal impedance curves for conical mode with different radial clearances

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Fig. 16

Stable transient response, C = 215 μm

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Fig. 17

Unstable transient response, C = 205 μm

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