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Luis San Andrés Mast-Childs Professor Fellow ASME

ASME TURBO EXPO 20 10 , Glasgow, Scotland, UK (June 2010). Identification of Rotordynamic Force Coefficients of a Metal Mesh Foil Bearing using Impact Load Excitations. Thomas Abraham Chirathadam Research Assistant. Luis San Andrés Mast-Childs Professor Fellow ASME. Texas A&M University.

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Luis San Andrés Mast-Childs Professor Fellow ASME

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  1. ASME TURBO EXPO 2010,Glasgow, Scotland, UK (June 2010) Identification of Rotordynamic Force Coefficients of a Metal Mesh Foil Bearing using Impact Load Excitations Thomas Abraham Chirathadam Research Assistant Luis San Andrés Mast-Childs Professor Fellow ASME Texas A&M University ASME GT2010-22440 ASME J. Eng Gas Turbines & Power (in print) Supported by TAMU Turbomachinery Research Consortium

  2. Metal Mesh Foil Bearing (MMFB) Top Foil Metal mesh ring MMFB COMPONENTS: Bearing Cartridge, Metal mesh ring and Top Foil Hydrodynamic air film develops between rotating shaft and top foil Potential applications:ACMs, micro gas turbines, turbo expanders, turbo compressors, turbo blowers, automotive turbochargers, APU • Large damping (material hysteresis) offered by metal mesh • Tolerant to misalignment, and applicable to a wide temperature range • Suitable tribological coatings needed to reduce friction at start-up & shutdown Cartridge

  3. Metal Mesh Foil Bearings (+/-) No lubrication (oil-free). NO High or Low temperature limits. Resilient structure with lots of material damping. Simple construction ( in comparison with other foil bearings) Cheap! • Metal mesh tends to sag or creep over time • Damping NOT viscous. Modeling difficulties • Unknown rotordynamic force coefficients

  4. Past work in Metal Mesh Dampers METAL MESH DAMPERS provide large amounts of damping. Inexpensive. Oil-free Zarzour and Vance (2000)J. Eng. Gas Turb. & Power, Vol. 122 Advantages of Metal Mesh Dampers over SFDs Capable of operating at low and high temperatures No changes in performance if soaked in oil Al-Khateeb and Vance (2001)GT-2001-0247 Test metal mesh donut and squirrel cage( in parallel) Metal Mesh damping not affected by modifying squirrel cage stiffness Choudhry and Vance (2005)Proc. GT2005 Develop design equations, empirically based, to predict structural stiffness and viscous damping coefficient

  5. Past work in MMFBs San Andrés et al. (2010)J. Eng. Gas Turb. & Power, Vol. 132(3) Assembled the first prototype MMFB (L=D=28 mm). Load vs Deflection with hysteresis shows large structural damping (g~ 0.7). Frequency dependentstiffness agree with predictions. San Andrés et al. (2009)ASME GT2009-59920 Demonstrated operation to 45 krpm with early rotor lift off. Educated undergraduate students. San Andrés et al. (2009)Proc. AHS 65th Annual Forum, May, 2009 Start and shut down to measure torque and lift-off speed. Low friction factor ~ 0.01 at high speed 60 krpm.

  6. MMFB assembly Simple construction and assembly procedure METAL MESH RING BEARING CARTRIDGE TOP FOIL

  7. MMFB dimensions and specs PICTURE Mesh thickness= 7 mm

  8. Static load vs. MMFB deflection Start San Andres, L., Chirathadam, T.A., and Kim, T.H., 2010, ASME J. Eng. Gas Turbines Power, 132 (3), p.032503. 3 Cycles: loading & unloading Nonlinear F(X) Large hysteresis loop : Mechanical energy dissipation Displacement: [-0.12,0.12] mm Load: [-120, 150 ]N MMFB Structural Characteristic (wire density ~ 20%)

  9. MMFB structural stiffness K=dF/dx San Andres, L., Chirathadam, T.A., and Kim, T.H., 2010, ASME J. Eng. Gas Turbines Power, 132 (3) During Load reversal : jump in structural stiffness Lower stiffness values for small displacement amplitudes Max. Stiffness ~ 2.5 MN/m MMFB Structure Characterization (wire density ~ 20%)

  10. MMFB structural stiffness vs. freq. 12.7 um San Andres, L., Chirathadam, T.A., and Kim, T.H., 2010, ASME J. Eng. Gas Turbines Power, 132 (3) At low frequencies (25-100 Hz), stiffness decreases At higher frequencies, stiffness gradually increases Motion amplitudeincreases 25.4 um 38.1 um Bearing stiffness is frequency and motion amplitude dependent Al-Khateeb & Vance model: reduction of stiffness with force magnitude (amplitude dependent)

  11. MMFB eq. damping vs. frequency Amplitude increases San Andres, L., Chirathadam, T.A., and Kim, T.H., 2010, ASME J. Eng. Gas Turbines Power, 132 (3) MMFB equiv. viscous damping decreases as the excitation frequency increases and as motion amplitude increases 12.7 μm 25.4 μm 38.1 μm Predicted equivalent viscous damping coefficientsin good agreement with measurements

  12. MMFB rotordynamic test rig MMFB Journal press fitted on Shaft Stub cm 15 10 5 0 TC cross-sectional view Max. operating speed: 75 krpm Turbocharger driven rotor Regulated air supply: 9.30bar Twin ball bearing turbocharger Model T25 Journal: length 55 mm, 28 mm diameter , weight=0.22 kg

  13. Journal speed and torque vs time Constant speed ~ 65 krpm Valve open Valve close 3 N-mm • Applied Load: 17.8 N Rotor starts Rotor stops WD= 3.6 N • Manual speed up to 65 krpm, steady state operation, and deceleration to rest Iift off speed • Startup torque ~ 110 Nmm • Shutdown torque ~ 80 Nmm • Once airborne, drag torque is ~ 3 % of Startup ‘breakaway’ torque San Andres, L., Kim, T.H., Chirathadam, T.A., and Ryu, K., 2009, Proc. AHS 65th Annual forum, Grapevine, TX, May 27-29. Lift off speed at lowest torque : airborne operation Top shaft speed = 65 krpm

  14. Bearing power loss vs rotor speed Hydrodynamiclubrication Mixed lubrication 35.6 N (8 lb) 26.7 N (6 lb) 17.8 N (4 lb) 8.9 N (2 lb) Rotor accelerates Increasing static load (Ws) to 35.6 N (8 lb) Power loss decreases to a minimum during mixed lubrication regime and then increases with increasing rotor speed Dead weight (WD= 3.6 N)

  15. Friction coefficient vs rotor speed 8.9 N (2 lb) 17.8 N (4 lb) 26.7 N (6 lb) 35.6 N (8 lb) Friction coefficient ( f ) decreases with increasing static load f = (Torque/Radius)/(Static load) f ~ 0.01 Rotor accelerates f ~ 0.3-0.4 San Andres, L., Kim, T.H., Chirathadam, T.A., and Ryu, K., 2009, Proc. AHS 65th Annual forum, Grapevine, TX, May 27-29. Dry sliding Airborne (hydrodynamic)

  16. Identification of stiffness and damping coeff. 5 cm IMPACT HAMMER TC MMFB Top foil fixed end Force gauge Journal (28 mm) Flexible string Eddy current sensor Accelerometer Positioning table (FRONT VIEW) (SIDE VIEW) Impact loads

  17. Identification model • Deliver impacts along Y direction only Impact force, fY KYY, CYY Y KXY, CXY Record displacements (relative to rotor) and bearing acceleration Bearing Cartridge Ω Journal X KXX, CXX KYX, CYX Equations of motion: • Assembly mass, M = 0.38 kg

  18. Identification model: freq. domain • Lightly loaded bearing (3.5N). Assume: SHAFT STATIONARY ( 1-DOF) SHAFT ROTATING (2-DOF) • Multiple tests (10) : frequency averages

  19. Impact force and displacements Y • Frequency domain averages of 10 impacts Time domain Time domain Frequency domain Frequency domain Rapid decay of MMFB displacement indicates large material damping • Shaft not rotating

  20. Impact force and displacements Y direction Y direction X direction X direction Synchronous response, 1X Time domain Time domain • Appreciable cross directional motions (X) Frequency domain Frequency domain • Shaft speed = 50 krpm

  21. Bearing acceleration & relative disp. Acceleration from displacement relative to shaft, |ω2Y| Acceleration from displacement relative to shaft Bearing acceleration Y direction Measured X direction SHAFT STATIONARY Acceleration derived from bearing displacement relative to shaft, |-ω2Y|, is not equal to bearing acceleration since the TC shaft stub is rather flexible. Important to measure both: displacements and accelerations (X,Y) SHAFT speed = 50 krpm (833 Hz)

  22. Test MMFB stiffnesses (K, k) Structure K ( No rotation) Direct K Cross k Applied load = 3.5 N. Weight=3.5 N Shaft speed=50 krpm Direct stiffness K at 50 krpm < structure stiffness ( ~ 25 % reduction at 200 hz) Direct and cross coupled stiffnesses (airborne) and bearing structural stiffness increase with frequency

  23. Test MMFB damping (C, c) Structure C Direct C Cross c Applied load = 3.5 N. Weight=3.5 N Shaft speed=50 krpm MMFB direct viscous damping C are similar, with and without shaft rotation. Metal mesh provides ++ damping Equivalent viscous damping decays rapidly with increasing frequency

  24. Test MMFB loss factor (g) 50 krpm No rotation Applied load = 3.5 N. Weight=3.5 N Shaft speed=50 krpm γ=Cω/K Loss factor is large g ~ 0.5, with little variation in frequency Large loss factor magnitude = large energy dissipation mechanism

  25. Not all measurements showed acceptable rotordynamic performance. At certain speeds, rotor-bearing system shows large subharmonic motions. Is this behavior a typical rotordynamic instability or a forced nonlinearity ? Waterfalls of start up

  26. Bearing displacements relative to shaft H load =3.5 N Subharmonic whirl motions of large amplitudes locked at system natural frequency ½ frequeny whirl for lightly loaded bearing

  27. Bearing displacements relative to shaft H load = 18 N Large sub harmonic motions locked at natural frequency ½ frequeny whirl absent with larger applied loads

  28. Conclusions Metal mesh foil bearing assembled using cheap, commercially available materials. • While airborne, bearing power loss increases with rotor speed (little friction). Min power loss found. • MMFB direct stiffness (airborne) slightly < structural stiffness. Cross-stiffness small. • MMFB viscous damping nearly independent of shaft speed though decreasing fast with frequency. LOSS factor is large (~0.50) • Start up shows½ frequency whirl=natural frequency for low static loads & speeds < 50 krpm MMFBs are promising inexpensive bearings for oil-free turbomachinery

  29. Acknowledgments • Thanks support of • Turbomachinery Research Consortium • Honeywell TurboCharging Systems Learn more http://rotorlab.tamu.edu Questions ?

  30. Current work: rotordynamic test rig Rubber belt Force gauge Steel frame/ shield Test bearing Positioning table X-Y 100 N shakers)

  31. Current work: rotordynamic test rig Shakers Test bearing X TC Y Applicable to foil bearings & metal mesh bearings

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