Keun Ryu Research Assistant

1. ASME TURBO EXPO 2009,Orlando, Fla Dynamic Forced Response of a Rotor-Hybrid Gas Bearing System due to Intermittent Shocks Dr. Luis San Andrés Mast-Childs Professor Fellow ASME Keun Ryu Research Assistant TURBOMACHINERY LABORATORY TEXAS A&M UNIVERSITY ASME paper GT2009-59199 Supported by TAMU Turbomachinery Research Consortium

2. Flexure pivot Bearing GT 2004-53621 ASME Paper No. GT2002-30404 Gas bearings Gas Foil Bearing • Oil-Free bearing • High rotating speed (DN value>4M) • Simple configuration • Lower friction and power losses • Compact size AIAA-2004-5720-984 Micro Turbomachinery (< 0.5 MW) ADVANTAGES • High energy density • Compact and fewer parts • Portable and easily sized • Lower pollutant emissions • Low operation cost http://www.grc.nasa.gov/WWW/Oilfree/turbocharger.htm

3. Ideal gas bearings for micro turbomachinery (< 0.5 MW ) must be: Simple – low cost, small geometry, low part count, constructed from common materials, manufactured with elementary methods. Load Tolerant – capable of handling both normal and extreme bearing loads without compromising the integrity of the rotor system. High Rotor Speeds – no specific speed limit (such as DN) restricting shaft sizes. Small Power losses. Good Dynamic Properties – predictable and repeatable stiffness and damping over a wide temperature range. Reliable – capable of operation without significant wear or required maintenance, able to tolerate extended storage and handling without performance degradation. +++Modeling/Analysis (anchored to test data) readily available

4. Thrust of research program: Investigate conventional bearings of low cost, easy to manufacture (common materials) and easy to install & align. Combine hybrid (hydrostatic/hydrodynamic) bearings with low cost coating to allow for rub-free operation at start up and shut down Major issues: Little damping, Wear at start & stop, Instability (whirl & hammer), & reliability under shock operation

5. Bearing shell and Load cells Bearing cover Rig housing Gas bearing Shaft and DC motor Test rig Max. operating speed: 100 kpm 3.5 kW (5 Hp) AC integral motor Rotor: length 190 mm, 28.6 mm diameter, weight=0.826 kg Components of high-speed gas bearing test rig

6. Rotor/motor Load cell Bearing Sensors Thrust pin Air supply Test rig Positioning Bolt LOP

7. Flexure pivot tilting pad hybrid bearing Promote stability: no cross-coupledstiffnesses Eliminatepivot wear, contact stresses, pad flutter Minimizemanufacturing and assemblytolerances’ stack-up worn pads surfaces Clearances Cp =38 & 45 mm, Preload =7 & 5 mm (~20%) Web rotational stiffness=20 Nm/rad

8. TAMU work on flexure pivot tilting bearings Zhu & San Andres (2004)GT 2004-53621 Gas bearing for oil-free applications. Good comparisons with: GT 2004-53621 60 KRPM Stable to 99 krpm Delgado & San Andres (2004) Computational model for hydrodynamic operation, with application to hybrid brush seals GT 2004-53614 San Andres (2006) Computational model for hybrid operation validated by Zhu (2004) measurements. Code used by 20+ companies Journal of Tribology, 129 San Andres & Ryu (2007) Operation with worn clearances and LOP/LBP configuration J. Eng. Gas Turbines and Power, 2008, 130

9. J. Eng. Gas Turbines and Power, 2008, v. 130 2008: Control of bearing stiffness / critical speed Displacements at RB(H) 5.08 bar 2.36 bar 5.08 bar Blue line: Coast down 2.36 bar Red line: Set speed Controller activated system Peak motion at “critical speed” eliminated by controlling supply pressure into bearings

10. Objectives: Demonstrate the rotordynamic performance, reliability, and durability of hybrid gas bearings • Rotor motion measurements for increasing gas feed pressures and speed range to 60 krpm. • Install electromagnetic pusher to deliver impact loads into test rig. • Perform shock loads (e-pusher & lift-drop) tests to assess reliability of gas bearings to withstand intermittent shocks without damage.

11. 2008 Gas bearing test rig layout E-pusher : Push type solenoid 240 N at 1 inch stroke

12. Electromagnetic pusher tests Multiple impact Impact duration ~20 ms E-force ~400 N (pk-pk)

13. Manual lift & drop tests Multiple impact Lift off to 5~15 cm (10~30° rotation)

14. Coast down: E-pusher tests Ps=5.08 bar (ab) Displacements at LB(H) Intermittent shocks Impact force 100~400 N 46 krpm Shock ~15 g Transient rotor response ~ 40 µm

15. Coast down: manual lift & drop tests Shock induced acceleration At base 5~20 g At housing 5~10 g Ps=3.72 bar (ab) Beyond critical speed: Synchronous frequency is isolated from shocks Below 20 krpm: Large fluctuation of synchronous response Displacements at LB(H)

16. Waterfall: manual lift & drop tests Rotor speed decreases Displacements at LB(H) Ps=2.36 bar (ab) Excitation of rotor natural frequency. NOT a rotordynamic instability!

17. Rotor response: manual lift & drop tests Ps=2.36 bar (ab) Shock loads applied Shock loads applied Overall rotor amplitude increases largely. Subsynchronous amplitudes larger than synchronous

18. Rotor response: manual lift & drop tests Ps=2.36 bar (ab) Natural frequency of rotor-bearing system(150~190 Hz) Natural frequency of test rig (~40 Hz) Rotor-bearing natural frequency increases with rotor speed. Natural frequency of test rig also excited.

19. Rotor response: manual lift & drop tests Ps=2.36 bar (ab) 15 krpm Drop induced shocks ~30 g Transient response Full recovery within ~ 0.1 sec.

20. Rotor speed vs time (No shocks) Dry friction (contact) With feed pressure: long time to coast down demonstrates very low viscous drag!

21. Rotor speed vs time (manual lift-drop tests) Overall coast down time reduces with shock loads (~ 20 sec) No shocks Exponential decay (No rubs) even under severe external shocks No shocks

22. Conclusions: • Under shock loads ( up to ~30 g), natural frequency of rotor-bearing system (150-200 Hz) and test rig base (~ 40 Hz) excited. However, rotor transient motions quickly die! • For all feed pressures (2-5 bar), rotor transient responses from shocks restore to their before impact amplitude within 0.1 second. Peak instant amplitudes (do not exceed ~50 µm) • Even under shock impacts, viscous drag effects are dominant, i.e., no contact between the rotor and bearing. • Hybrid bearings demonstrate reliable dynamic performance even withWORN PAD SURFACES

23. Dominant challenges in gas bearing technology • Bearing design & manufacturing process better known. Load capacity needs minute clearances since gas viscosity is low. • Damping & rotor stability are crucial • Inexpensive coatings to reduce drag and wear at low speeds and transient rubs at high speeds • Engineeredthermal managementto extend operating envelope to high temperatures Current research focuses on coatings (materials), rotordynamics (stability) & high temperature (thermal management) Need Low Cost & Long Life Solution!

24. 2009 Gas bearing test rig layout connecting rod pushes base plate!

25. Rotor speed coast down tests Ps = 2.36 bar (ab) Shaker input frequency: 12Hz • Subsynchronous response: • 24 Hz (Harmonic of 12 Hz) • Natural frequency 193 Hz Synchronous Dominant! excitation of system natural frequency is NOT an instability!