Current Developments in Gas Bearings for Microturbomachinery

Transcription

1 Turbomachinery Laboratory, Texas A&M University Mechanical Engineering Department Current Developments in Gas Bearings for Microturbomachinery Luis San Andrés Mast-Childs Professor October 2007 CIBIM 8 8 th Congreso Iberoamericano de Ingenieria Mecancia Cusco-Peru

2 MICROTURBOMACHINERY Justification 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. Meso-scale or MEMS turbomachinery (< 100W) for Next Generation Land Warriors, Micro vehicles & robots, Portable electronic devices and systems, Smart munitions

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

4 MTM POWER RANGE < 250 kwatt < 100 Watt Distribute power (Gas turbine &Fuel Cell Hybrid) ASME Paper No. GT Auto engine and part / Industrial compressor Oilfree/turbocharger.htm mart_economy/2006/09/microgas_tu rbin.html Portable Electronic Devices Honeywell, Hydrogen and Fuel Cells Merit Review hpcompressozr.pdf Kang, S., Ph D dissertation (Stanford Univ.)

5 Industrial applications (30 kw 1 MW) Compressor source: Air Cycle Machine Turbojet Engines Aircraft Gas Turbine Engines

6 Industrial applications source: Turbocharger Turbopumps Turboexpander Auxiliary Power Units

7 Capstone s C30 Engine Diffuser Oil-Free Radial Bearing Oil-Free Foil Compressor Thrust Runner Oil-Free Thrust Bearing Turbine Nozzle Turbine Bearings: >500 C Proprietary bearing design and coating Thin Dense Chrome journals 1.4 MDN (idle) 3.1 MDN (full speed) ~1.5 L/D 1.6 psi static load Demonstrated Life: >40k hours; >6k cycles and over 11 Mhrs field life source: Dan Lubell, 2006 IJTC, Capstone Turbine Corportation

8 MICRO GAS TURBINES Cogeneration systems with high efficiency Multiple fuels (best if free) 99.99X% Reliability Low emissions Reduced maintenance Lower lifecycle cost 60kW MGT source: Dan Lubell, 2006 IJTC, Capstone Turbine Corportation Microturbine Power Conversion Technology Review, ORNL/TM-2003/74. MANUFACTURER Bowman Capstone Elliott Energy Systems General Electric Ingersoll Rand Turbec, ABB & Volvo OUTPUT POWER (kw) 25, 80 30, 60, , 60, 80, , Hybrid System : MGT with Fuel Cell can reach efficiency > 60% Ideal to replace reciprocating engines. Low footprint desirable

9 Capstone MicroTurbine Cooling fins Air intake Generator Compressor Air bearings Exhaust output Recuperator Turbine Fuel injector Combustion chamber No gearbox or other mechanicals Low scheduled maintenance Only one moving part No coolants or lubricants Contaminant-free exhaust Compact and lightweight Super-low CO & NO X source: Dan Lubell, 2006 IJTC, Capstone Turbine Corportation

10 MTM Expectations & Requirements Low cost driven by materials Low maintenance driven by design Long life defined by the bearings and materials Efficient driven by design Fully integrated solutions system design source: Dan Lubell, 2006 IJTC, Capstone Turbine Corportation

11 Differences between MTM & Aero engine Power density is less important than cost Low pressure ratios work well with recuperators Efficiency is important to compete against alternate technologies Maintenance infrastructure is not as prolific within customer infrastructures, making low maintenance very important source: Dan Lubell, 2006 IJTC, Capstone Turbine Corportation

12 ULTRA MICROTURBOMACHINERY MEMS MTM Meso-scale MTM Palm-size power source Brayton cycle Gas foil bearings GT Silicon wafer 1.2 Million rpm Thrust 0.1 N Spiral groove and hydrostatic gas bearings 2007, Journal of Micromechanics and Microengineering, Vol Watt & less Small unmanned vehicles and to replace batteries in portable electronic devices

13 Applications of Meso/MEMS MTM Micro Gas Turbine RescueRobot Portable Generator 5000 POWER DENSITY (MW/m3) Micro Reactor Micro Solar Cell Micro-Lithium Battery LiSO 2 Battery (BA5590) oducts.php4 Large Scale Combustor Solar Cell Large Scale Combustor UAV om/product/448_448.htm Mobile electronic equipment view.com/ efly.com/ SIZE (cm3)

14 MEMS MTM at MIT Thrust: 11g (17 watts) Turbine inlet temp : 1600 K Fuel burn: 16 gram/hr Rotor Speed: 1.2 M rpm Weight: 2 grams Exhaust gas temp : 1243 K Source: GT

15 Mesoscale MTM at Stanford ~1997: DARPA M-Dot project Palm size gas turbine engine (thrust type) φ25 mm turbine, 400k rpm All metal components Ran a few minutes. Turbine blades melted! 1998: DARPA M-Dot Stanford Carnegie Mellon project Replace the inlet nozzle to improve specific thrust density. Inlet nozzle: major ceramic part. Tested in 1,250 C gas 7% performance (thrust/weight) improvement expected Ceramic turbine built but not tested. M-DOT micro-turbine engine Silicon nitride inlet nozzle and turbine Figures and text: Kang, S.,2001, Ph.D dissertation, Stanford Univ. & Personal communication with Kang, S.

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

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

18 Foil Gas Bearings for Microturbomachinery Dr. Luis San Andres Mast-Childs Professor October 2007 CIBIM 8 8 th Congreso Iberoamericano de Ingenieria Mecancia Cusco-Peru

19 Gas Bearings for Oil-Free Turbomachinery Advantages of gas-lubricated bearings over oillubricated bearings for high-speed, small-scale turbomachinery Process gas offers more cleanliness and eliminates contamination by buffer lubricants Gases are more stable at extreme temperature 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! Gas Bearings Must Be Simple!

20 Ideal gas bearings for micro turbomachinery (< 0.25 MW ) must be: 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) readily available

21 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. 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

22 Gas Foil Bearings +/- Increased reliability: large load capacity (< 100 psi) No lubricant supply system, i.e. reduce weight High and low temperature capability (up to 2,500 K) No scheduled maintenance Ability to sustain high vibration and shock load. Quiet operation Less load capacity than rolling or oil bearings Wear during start up & shut down No test data for rotordynamic force coefficients Thermal management issues Predictive models lack validation. Difficulties in modeling complex interaction between film and foilbumps support (dry-friction damping)

23 Foil Bearing Test Rig Electromagnet loader Driving motor (1HP, 50 krpm) Optical Tachometer Start motor (2HP, 25 krpm) Test rotor Flexible coupling Centrifugal clutch (Engaged at ~50 krpm) Cluth shoes Foil bearing housing Spring Ω Wear ring

24 ROTOR and TEST FOIL BEARINGS D = L = C nominal = 1.4 mil Shaft bearing locations = Thin dense chrome (TDC) coating Rotor mass = 2.2 lb without coupling Free free mode natural frequencies = 4096 Hz and 9850 Hz BUMP FOILS TOP FOIL

25 Foil Bearing: structural load and stiffness 140 Foil bearing structural load and stiffness are non linear, i.e. hardening as shaft displacement increases. Rotor displacement (μm) Rotor displacement (μm) c DE 2 c FE Magnetic Load FE Load DE Load Free end bearing Drive end bearing Bearing loads (N) Stiffness varies with deflection F K X Static Static Load (N) (N) Predictions Experimental Results D 1 = mm θ = 45º -225º FB Deflection (mm) FB Displacement (mm) Structural Stiffness (kn/m) Structural Stiffness (N/mm) FB Deflection (mm) FB Displacement (mm)

26 PERFORMANCE OF SMALL ROTOR ON GAS FOIL BEARINGS X FE Y FE X DE Y DE Free end 2 krpm g Drive end Incipient sub sync motions 25 krpm WATERFALL PLOTS OF BASELINE CONDITION no imbalance

27 PERFORMANCE OF SMALL ROTOR ON GAS FOIL BEARINGS Amplitude [microns] Displacement Amplitude Amplitude [microns] (μm) Displacement Amplitude (μm) X 0.5X u = 7.4 μm 1X Frequency [Hz](Hz) u = 10.5 μm 1X 2X 2X Frequency [Hz] (Hz) 2.6 krpm 8.5 krpm 25.7 krpm 2.6 krpm 12.5 krpm 20.5 krpm 25.7 krpm Imbalance + Speed + Amplitudes of subsynchronous motions INCREASE as Imbalance increases (forced nonlinearity) Waterfall plots

28 Displacement Amp. (μm, 0-pk) Amplitude (μm) Displacement Amp. (μm, 0-pk) Amplitude (μm) Displacement Amp. (μm, 0-pk) t Amplitude (μm) (, ) Direct Synchronous Synchronous component Direct component (, ) Direct Synchronous Synchronous component Direct component Imbalance mass location X DE FE Synchronous component Direct Synchronous Direct component X DE DE X DE Small U Large U Effect of imbalance on system response Medium U Small Subsynchronous motions at 15 krpm. Critical speed at ~ 8.2 krpm Subsynchronous motions are most severe from 15 krpm to 12 krpm. Supersynchronous through critical speed

29 PERFORMANCE OF SMALL ROTOR ON GAS FOIL BEARINGS -FEED AIR PRESSURE increases 40 kpa [6 psig] to 340 kpa [50 psig] AIR SUPPLY Small change in stiffness or rotor centering shaft cool ROTORDYNAMIC TESTS Air Pressurization

30 PERFORMANCE OF SMALL ROTOR ON GAS FOIL BEARINGS Displacement Amplitude (μm) (μm) splacement Amplitude Dis Subsynchronous components components X DRIVE END subsync (a) Running speed Running speed 40.8 kpa = kpa (b) Frequency (Hz) Frequency (Hz) (c) 16 krpm 204 kpa = kpa kpa = kpa 340 Feed pressure increases Notable reduction in subsynchron ous amplitudes when increasing air feed pressure

31 PERFORMANCE OF SMALL ROTOR ON GAS FOIL BEARINGS, No feed pressure krpm Amplitude (μm, [microns] 0-pk) krpm Speed (-) krpm Frequency [Hz] 1X Severe subsynchronous motions at natural frequency

32 Closure: PERFORMANCE OF SMALL ROTOR ON GAS FOIL BEARINGS Subsynchronous motions (onset and amplitude) are sensitive to level of rotor imbalance FORCED NONLINEARITY Subsynchronous frequencies track shaft speed at approximately 50% and lock at natural frequency. Bifurcations to other WFRs found Air pressurization reduces amplitude of synchronous motions at critical speed. Increasing air pressures reduce amplitudes of subsynchronous motions Foil Bearings survived severe instabilities. Synchronous motions may be predictable!

33 Closure: Gas Bearings for Oil-Free MTM Dominant challenge for gas bearing technology: intermittent contact and damaging wear at startup & shut down, and temporary rubs during normal operating conditions Low gas viscosity requires minute clearances to generate load capacity. Damping & rotor stability are crucial Inexpensive coatings required to reduce drag and wear (Low speeds and transient HS rubs) Bearing design & manufacturing process well known Thermal management to extend operating envelope to high temperature applications Current research focuses on coatings (materials), rotordynamics (stability) & high temperature (thermal management) Need Low Cost & Long Life Solution!