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Research Papers: Gas Turbines: Structures and Dynamics

Rotordynamic Performance of a Shaft With Large Overhung Mass Supported by Foil Bearings

[+] Author and Article Information
Nguyen LaTray

Turbomachinery and Energy System Laboratory,
Department of Mechanical
and Aerospace Engineering,
University of Texas at Arlington,
Arlington, TX 76019
e-mail: nttran@mavs.uta.edu

Daejong Kim

Turbomachinery and Energy System Laboratory,
Department of Mechanical
and Aerospace Engineering,
University of Texas at Arlington,
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 7, 2016; final manuscript received August 17, 2016; published online November 16, 2016. Editor: David Wisler.

J. Eng. Gas Turbines Power 139(4), 042506 (Nov 16, 2016) (9 pages) Paper No: GTP-16-1315; doi: 10.1115/1.4034918 History: Received July 07, 2016; Revised August 17, 2016

This work presents the theoretical and experimental rotordynamic evaluations of a rotor–air foil bearing (AFB) system supporting a large overhung mass for high-speed application. The proposed system highlights the compact design of a single shaft rotor configuration with turbomachine components arranged on one side of the bearing span. In this work, low-speed tests up to 45 krpm are performed to measure lift-off speed and to check bearing manufacturing quality. Rotordynamic performance at high speeds is evaluated both analytically and experimentally. In the analytical approach, simulated imbalance responses are studied using both rigid and flexible shaft models with bearing forces calculated from the transient Reynolds equation along with the rotor motion. The simulation predicts that the system experiences small synchronous rigid mode vibration at 20 krpm and bending mode at 200 krpm. A high-speed test rig is designed to experimentally evaluate the rotor–air foil bearing system. The high-speed tests are operated up to 160 krpm. The vibration spectrum indicates that the rotor–air foil bearing system operates under stable conditions. The experimental waterfall plots also show very small subsynchronous vibrations with frequency locked to the system natural frequency. Overall, this work demonstrates potential capability of the air foil bearings in supporting a shaft with a large overhung mass at high speed.

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References

Agrawal, G. , 1997, “ Foil Air/Gas Bearing Technology—An Overview,” ASME Paper No. 97-GT-347.
Mohawk Innovative Technology, Inc., 2015, “ Oil-Free Turbomachinery,” Millenium Business Communication, Albany, NY, accessed Sept. 16, 2015, http://miti.cc/oil-free-turbomachinery
Heshmat, C. , Heshmat, H. , and Valco, M. , 2005, “ Foil Bearings Makes Oil-Free Turbocharger Possible,” World Tribology Congress III, Vol. 2, pp. 63724–63725.
Capstone Turbine Corp., 2015, “ Advanced Engineering Based on Proven Turbine Design,” Equisolve, Chatsworth, CA, accessed Sept. 16, 2015, http://www.capstoneturbine.com/technology
Kim, D. , Lee, A. , and Choi, B. , 2013, “ Evaluation of Foil Bearing Performance and Nonlinear Rotordynamics of 120 kW Oil-Free Gas Turbine Generator,” J. Eng. Gas Turbines Power, 136(3), p. 032504.
Berot, F. , and Dourlens, H. , 1999, “ On Instability of Overhung Centrifugal Compressors,” ASME Paper No. 99-GT-202.
Eldridge, T. , Olsen, A. , and Carney, M. , 2009, “ Morton–Newkirk Effect in Overhung Rotor Supported in Rolling Element Bearings,” ASME Paper No. GT2009-60243.
Kirk, G. , Guo, Z. , and Balbahadur, A. , 2003, “ Synchronous Thermal Instability Prediction for Overhung Rotors,” The Thirty-Second Turbomachinery Symposium-2003, Texas A&M University System Turbomachinery Laboratory, College Station, TX, pp. 121–135.
Yadav, S. , 2013, “ Dynamic Performance of Offset-Preloaded Two Pad Foil Bearings,” Master's thesis, Mechanical Engineering Department, University of Texas at Arlington, Arlington,
Kim, D. , 2006, “ Parametric Studies on Static and Dynamic Performance of Air Foil Bearings With Different Top Foil Geometries and Bump Stiffness Distributions,” ASME J. Tribol., 129(2), pp. 354–364.
Wang, Y. , and Kim, D. , 2013, “ Experimental Identification of Force Coefficients of Large Hybrid Air Foil Bearings,” ASME J. Eng. Gas Turbines Power, 136(3), p. 032503. [CrossRef]
Rudloff, L. , Arghir, M. , and Bonneau, O. , 2011, “ Experimental Analyses of a First Generation Foil Bearing: Startup Torque and Dynamic Coefficients,” ASME J. Eng. Gas Turbines Power, 133(9), p. 092501. [CrossRef]
Chirathadam, T. , 2009, “ Measurement of Drag Torque and Lift Off Speed and Identification of Stiffness and Damping in a Metal Mesh Foil Bearing,” Master's thesis, Mechanical Engineering Department, Texas A&M University, College Station, TX
Lee, D. , and Kim, D. , 2010, “ Five Degrees of Freedom Nonlinear Rotordynamic Model of a Rigid Rotor Supported by Multiple Air Foil Bearings,” 8th IFToMM International Conference on Rotordynamics, Seoul, Korea, pp. 819–826.
Kim, D. , and Varrey, M. , 2012, “ Feasibility Study of Oil-Free F700 Rotorcraft Engine: Hybrid Foil Bearing and Nonlinear Rotordynamics,” American Helicopter Society 68th Annual Forum, AHS International, Fairfax, VA, Vol. 9, pp. 2341–2349.
Pan, C. , and Kim, D. , 2007, “ Stability Characteristics of a Rigid Rotor Supported by a Gas-Lubricated Spiral-Groove Conical Bearing,” ASME J. Tribol., 129(2), pp. 375–383. [CrossRef]

Figures

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Fig. 1

Rotor–AFB diagram with a large overhung mass between the Fwd AFB and the impulse turbine

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Fig. 2

Two-pad offset-preloaded AFB configuration

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Fig. 3

Bump foil geometry

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Fig. 4

Synchronous bearing stiffness coefficients as functions of rotor speed with air properties at 150 °C

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Fig. 5

Synchronous bearing damping coefficients as functions of rotor speed with air properties at 150 °C

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Fig. 6

Bearing stiffness coefficients as functions of excitation frequency at rotor speed of 185 krpm

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Fig. 7

Bearing damping coefficients as functions of excitation frequency at rotor speed of 185 krpm

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Fig. 8

Forward whirling modal impedance as functions of excitation frequency at rotor speed of 185 krpm

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Fig. 11

Torque profile from start to 45 krpm of two candidate AFBs (top). Temperature profile from start to stop (bottom).

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Fig. 12

Measured start-up torque and measured air friction torque of four sets of AFBs are plotted with calculated dry friction torque (μs = 0.6) and calculated air friction torque (μ = 0.01) with static preload approximately 3.3 N

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Fig. 13

Simulated rotordynamic response of rotor–AFB system at the Fwd AFB along the loading direction

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Fig. 14

Simulated rotordynamic response of rotor–AFB system at the Aft AFB along the loading direction

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Fig. 15

Beam element model of the rotor–AFB system, generated using commercial rotordynamic software

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Fig. 16

Free-body diagram of a beam model

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Fig. 17

Eigenvalue map obtained from commercial rotordynamic software

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Fig. 19

Test rig with sensors mounted on the housing

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Fig. 20

Test bed including instrumentations and air supply

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Fig. 21

Waterfall plot of the rotor vibration along the vertical direction (loading direction) near the Fwd AFB

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Fig. 22

Waterfall plot of the rotor vibration along the horizontal direction near the Fwd AFB

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Fig. 23

Waterfall plot of the rotor vibration along the vertical direction (loading direction) near the Aft AFB

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Fig. 24

Waterfall plot of the rotor vibration along the horizontal direction near the Aft AFB

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Fig. 25

Natural frequency map of the rotor–AFB system as functions of excitation frequencies when rotor speed is 122,100 rpm

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Fig. 26

Natural frequency of forward conical mode at different rotor speeds

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