Parametric Study of Bump Foil Gas Bearings for Industrial Applications

1. 2011 ASME Turbo Expo Congress & Exhibition Parametric Study of Bump Foil Gas Bearings for Industrial Applications GT2011-46767 http://rotorlab.tamu.edu http://www.ciateq.mx/

2. Oil-Free Bearings for Turbomachinery Justification Current advancements in vehicle turbochargers and midsize gas turbines need of proven gas bearing technology to procure compact units with improved efficiency in an oil-free environment. Also, Oil-freeturbomachinery and subsea compression are among major focuses in modern energy industry. DOE, DARPA, NASA interests range from applications as portable fuel cells (< 60 kW) in microengines to midsize gas turbines (< 250 kW) for distributed power and hybrid vehicles. • Gas Bearingsallow • weight reduction, energy and complexity savings • higher temperatures, without needs for cooling air • improved overall engine efficiency

3. Available Bearing Technologies Magnetic bearings • Low to medium temperatures • Moderate loads • Need control systems • Need back-up bearings • Long history of operation in some specific industrial applications www.skf.com, 2011 www.synchrony.com, 2011 Schweitzer/Maslen 2009 • Current Magnetic Bearing solution is expensive and even more expensive (and difficult) to make it reliable

4. Available Bearing Technologies Flexure Pivot Bearing Herringbone grooved bearing NICH Center, Tohoku University • Precision fabrication process • Low load capacity and stiffness and little damping GT 2004-53621 GAS BEARINGS Gas Foil Bearing • Oil-Free • NO DN limit • Low friction and power loss • Thermal management AIAA-2004-5720-984 Rolling element bearings • Low temperatures • Low DN limit (< 2 M) • Need lubrication system AIAA 2004-4189 PowerMEMS 2003

5. Microturbomachinery as per IGTI ASME Paper No. GT2002-30404 Drivers: deregulation in distributed power, environmental needs, increased reliability & efficiency Distributed power (Hybrid Gas turbine & Fuel Cell), Hybrid vehicles Automotive turbochargers, turbo expanders, compressors, Honeywell, Hydrogen and Fuel Cells Merit Review Max. Power ~ 250 kWatt International Gas Turbine Institute

6. Micro Gas Turbines 60kW MGT www.microturbine.com Microturbine Power Conversion Technology Review, ORNL/TM-2003/74. Cogeneration systems with high efficiency • Multiple fuels (best if free) • 99.99X% Reliability • Low emissions • Reduced maintenance • Lower lifecycle cost Hybrid System : MGT with Fuel Cell can reach efficiency > 60% Ideal to replace reciprocating engines. Low footprint desirable

7. Examples of commercial applications • Micro Turbines • Capstone of California • Turbo chargers • Honeywell “on the race”

8. Examples of commercial applications • Industrial Air Compressors • Samsung’s successful Micro Turbo Master line of compressors feature gas foil bearings • Pressures up to 130 psig • Powers up to 0.13 MW • Samsung has another line (Turbo Master) of air compressors with pressures up to 300 psig and power up to 2.4 MW (~20x larger). Run on TPJB.What’s next ?? www.samsungtechwin.com, 2011

9. MTM – Needs, Hurdles & Issues Largest power to weight ratio, Compact & low # of parts High energy density Reliability and efficiency, Low maintenance Extreme temperature and pressure Environmentally safe (low emissions) Lower lifecycle cost ($ kW) High speed Materials Manufacturing Processes & Cycles Fuels Rotordynamics & (Oil-free) Bearings & Sealing Coatings: surface conditioning for low friction and wear Ceramic rotors and components Automated agile processes Cost & number Low-NOx combustors for liquid & gas fuels TH scaling (low Reynolds #) Best if free (bio-fuels)

10. Gas Bearings for Oil-FreeMTM • Advantages of gas bearings over oil-lubricated bearings • Process gas is cleaner and eliminates contamination by buffer lubricants • Gases are more stable at extreme temperatures and speeds (no lubricant vaporization, cavitation, solidification, or decomposition) • Gas bearing systems are lower in cost: less power usage and small friction, enabling savings in weight and piping Gas Bearings Must Be Simple!

11. Ideal gas bearings for MTM Load Tolerant – capable of handling both normal and extreme bearing loads without compromising the integrity of the rotor system. Simple – low cost, small geometry, low part count, constructed from common materials, manufactured with elementary methods. 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) available

12. Gas Bearings for MTM What are the needs? • Make READY technology for industrial application by PUSHING development to • make out of the shelf item with proven results for a wide range of applications; • engineered product with well known manufacturing process; • known (verifiable) performance with solid laboratory and field experiences

13. Gas Bearings Research at TAMU See References at end Thrust: 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,Nonlinear behavior (subsync. whirl)

14. Gas Bearing Research at TAMU 2001/2 - Three Lobe Bearings 2003/4 - Rayleigh Step Bearings 2002-09 - Flexure Pivot Tilting Pad Bearings 2004-11:Bump-type Foil Bearings 2008-12:Metal Mesh Foil Bearings Stability depends on feed pressure. Stable to 80 krpm with 5 bar pressure Worst performance to date with grooved bearings Stable to 93 krpm w/o feed pressure. Operation to 100 krpm w/o problems. Easy to install and align. Industry standard. Reliable but costly. Models anchored to test data. Cheap technology. Still infant. Users needed See References at end

15. Gas Foil Bearings

16. Gas Foil Bearings Advertised advantages:high load capacity (>20 psig), rotordynamically stable, tolerance of misalignment and shocks

17. Gas Foil Bearings – Bump type • Series of corrugated foil structures (bumps) assembled within a bearing sleeve. • Integrate a hydrodynamic gas film in series with one or more structural layers. Applications:APUs, ACMs, micro gas turbines, turbo expanders • Reliable • Tolerant to misalignment and debris, also high temperature • Need coatings to reduce friction at start-up & shutdown • Damping from dry-friction and operation with limit cycles

18. Foil Bearings (+/-) • Increased reliability: load capacity (< 20 psi) • No lubricant supply system, i.e. reduce weight • High and low temperature capability (> 1,000 C) • No scheduled maintenance • Tolerate high vibration and shock load. Quiet operation • Endurance:performance at start up & shut down (lift off speed) • Little test datafor rotordynamic force coefficients & operation with limit cycles (sub harmonic motions) • Thermal managementfor high temperature applications (gas turbines, turbochargers) • Predictive models lack validation for GFB operation at HIGH TEMPERATURE

19. Computational analysis

20. Theoretical basis • Solve Reynolds equation for compressible flow (isothermal case). • Coupled to bump metal sheet deformation (non-linear stiffness and damping). • Iterative solution to find bearing equilibrium position. • Perturbation analysis to find dynamic performance (frequency-dependent stiffness and damping coefficients).Refs: San Andrés (2009), Arghir (2004), Iordanoff (1999), Heshmat (1992)

21. The computational program • Windows OS and MS Excel 2003 (minimum requirements) • Fortran 99 Executables for FE underspring structure and gas film analyses. Prediction of forced – static & dynamic- performance. • Excel® Graphical User Interface (US and SI physical units). Input & output (graphical) • Compatible with XLTRC2 and XLROTOR codes Code: XL_GFBTHD

22. Graphical User Interface Worksheet: Shaft & Bearing models (I)

23. Graphical User Interface Worksheet: Shaft & Bearing models (II)

24. Graphical User Interface Worksheet: Top Foil and Bump Models

25. Graphical User Interface Worksheet: Foil Bearing (Operation and Results)

26. Parametric Study

27. Results • Example bearing (Ref [3]): • Bump unit area stiffness lowers as bump pitch increases • Bump unit area stiffness increases with foil thickness

28. Results Measured load capacity (Ref [3]) Current predictions, constant load of 31 psi 31 psi Calibration point 1 Calibration point 2 • Benchmarking with independent experiments – Generation 1 bearing (Ref [3]). • Used to find practical limit of film thickness

29. Results • Base bearing (Ref [3]):

30. Rule of thumb for design From observations of bump stiffness and bearing performance predictions: • Bearing scaling: use Della Corte´s rule:W ~ N L D^2 • Bump scaling:Knew = Korig / f ; f is de diameter scale factor

31. Application example • Industrial compressor for injection service • 8 impellers, 640 lb rotor • Re-configured rotor – move bearings INBOARD of gas seals • Use larger diameter at bearing location Bearing characteristics Expected speed range:3 to 20 krpm 13-15 krpm MCOS most typical

32. Application - rotordynamics Linear stability analysis Predicted stiffness range Compressor can´t cross these speeds (requires more damping) 5th

33. Observations • Conceptually, scaled gas foil bearings can support an industrial, flexible rotor. • Re-location of bearings is necessary to decrease unit load, but it is feasible in the compressor working environment. • Rotor-bearing system requires additional damping to control shaft vibration at critical speeds.

34. Closure Dominant challenges for gas bearing technology: • Low gas viscosity requires minute clearances to generate load capacity. • Damping & rotor stability are crucial • Inexpensive coatings to reduce drag and wear • Bearing design & manufacturing process well known • Adequate thermal management to extend operating envelope into high temperatures

35. Closure Other pressing challenges for gas bearing technology: intermittent contact and damaging wear at startup & shut down, and temporary rubs during normal operating conditions Current research focuses oncoatings (materials), rotordynamics (stability) & high temperature (thermal management) Need Low Cost & Long Life Solution!

36. Oil-Free Bearings for Turbomachinery References

37. References Foil Bearings

38. References Foil Bearings

39. References Metal mesh foil bearings

40. CIATEQ´s full-size rotordynamic rig

41. Acknowledgments Learn more: http://rotorlab.tamu.edu Thanks to NSF (Grant # 0322925) NASA GRC (Program NNH06ZEA001N-SSRW2), Capstone Turbines, Inc., Honeywell Turbocharging Systems, Korea Institute of Science and Technology (Dr. Tae-Ho Kim) Foster-Miller, MiTI, TAMU Turbomachinery Research Consortium (TRC) CIATEQ A.C.