Metal Mesh Foil Gas Bearings for Oil-Free Turbomachinery

Transcription

1 12 th National Conference on Gas Lubrication, Dry Gas Seal and Micro Gravity in China May 218 Metal Mesh Foil Gas Bearings for Oil-Free Turbomachinery An introduction to Oil-Free MTM requirements and metal mesh foil gas bearings for stable rotor support Dr. Luis San Andres Mast-Childs Chair Professor 1

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. DOE, DARPA, NASA interests range from applications as portable fuel cells (< 6 kw) in microengines to midsize gas turbines (< 25 kw) for distributed power and hybrid vehicles. 225 mandate on + efficiency for IC engines: materials and oil-free bearing systems (turbochargers) will enable 55 miles/gallon (2km/L) 2

3 Microturbomachinery Oil-free gas turbines and generators: (mid size to.5 MW) foil gas bearings, damper seals Meso-micro turbomachinery: portable packs (5 kw), + 1 Mrpm Coatings gas lubrication & rotordynamics Rotordynamics, 3D printing, materials Hybrid vehicles 225: +5 miles/gal & Nox, turbochargers fuel cells Issues: high temperature, materials and NL rotordynamics 214 KIST (Lee, Kim & Kim) 37

4 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 Rotordynamics & (Oil-free) Bearings & Sealing Materials Coatings: surface conditioning for low friction and wear Ceramic rotors and components Manufacturing Automated agile processes 3D printing - Cost & number Processes & Cycles Low-NOx combustors for liquid & gas fuels TH scaling (low Reynolds #) Fuels Best if free (bio-fuels) 4

5 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 5

6 Example: Turbochargers RBS: Rotor Bearing System RBS With Fully Floating Bearing RBS With Semi Floating Bearing RBS With Ball Bearing Increased engine efficiency and performance relies on a robust rotorbearing system Oil Less Bearing System (27) 6

7 Gas Foil Bearings Advertised advantages: high load capacity (>2 psig)?, rotor dynamically stable?, tolerance of misalignment and shocks. 7

8 Gas Foil Bearings Bump type (BFB) Series of corrugated foil structures (bumps) assembled within a bearing sleeve. Hydrodynamic gas film in series with one or more structural sublayers. Applications: micro gas turbines, cryo turbo expanders, blowers Endurance: performance at start up & shut down Thermal management for high temperature applications (gas turbines, turbochargers) Prone to sub synchronous shaft whirl limit cycle operation. 8

9 Bump foil bearing components Gen I Top Foil.12 mm top foil Chrome-Nickel alloy Rockwell 4/45 Heat treated at ~ 45 ºC for 4 hours and allowed to cool. Foil retains arc shape after heat treatment Sprayed with MoS 2 sacrificial coating Bump foil Made by compressing a flat steel strip in specially made die Simple to manufacture but its engineered design demands time and $$ Bearing cartridge (+top foil+ bump foil) Bump foil and top foil inserted in steel bearing cartridge. 9

10 Experience at TAMU Gen II foil bearings (Teflon coated) Shaft Diameter = 1.5 Rotor mass = 2.2 lb 1

11 Measured rotor motions Rotor speed: 24 krpm 1.2 krpm (4 2 Hz) Imbalance displacement, u = medium 11

12 Subsynchronous motions: increase in imbalance Amplitude [microns] Amplitude [μm], [μm Amplitude [microns, -pk] Air feed pressure = 34.4 kpa (5 psig) p k Imbalance mass: 55 mg 1X 4 krpm i Frequency 6 [Hz] 27 krpm Rotor speed [krpm] SUB SYNC SYNCHRONOUS 6 23 krpm 48 Subsync. 48 whirl for 24 speeds 24 > krpm Sub Sub sync sync Frequency [Hz] [Hz] Sub sync Frequency [Hz] krpm 35 krpm Amplitude [microns] Amplitude [μm] Amplitude [microns, [μm, -pk] -pk] Rotor speed 18 [krpm] Rotor speed [krpm] Rotor speed [krpm] WFR Excitation frequency [Hz] 1X 3/4X 1/2X 1/3X 1/4X Imbalance mass: 33 mg 18 krpm Rotor speed [krpm] SUB SYNC SYNCHRONOUS (b) Synchronous and subsynchronous 1X 5 4 krpm 18 krpm Worse 4 whirl for 3 speeds > Sub sync Frequency [Hz] 2 18 krpm 1 28 krpm 1 Rotor speed [krp 12

13 What causes the subsynchronous motions? What causes the excitation of natural frequency? Hardening of bump-foil structure with amplitude of deformation! (forced nonlinearity) 13

14 Effect of feed pressure (cooling flow) on a rotor-gfb performance Outer gap x z Bearing housing Bump spring Top foil P s P a Typically FBs DO not require pressurization. Cooling flow is for thermal management: to remove heat from drag or to reduce thermal gradients in hot/cold engine sections Inner gas film Axial velocity Circumferential velocity ΩR J Rotating journal Ω R J X Y Axial flow retards evolution of circumferential flow velocity 14

15 Waterfalls recorded at rotor free end Amplitude [μm] Rotor responses during acceleration for two feed pressures Amplitude [μm] Amplitude [um] [μm] Speed up (a).34 bar 1X 2X [Hz] Frequency [Hz] 28 krpm 28 krpm krpm 1 1 krpm Amplitude [μm] [microns] Speed up (b) 2.8 bar 1X 2X [Hz] Frequency [Hz] krpm krpm 1 1 krpm Side feed pressure: 5 psi Side feed pressure: 4 psi Increase in side feed pressure delays onset rotor speed with large amplitude subsynchronous rotor motions 15

16 Rotor speed-up test for increasing feed pressure DE & FE GFB temperatures (F) Drive motor temperatures (F) pressure gauge Rotor speed [rpm] 5 psig At 33 krpm (55 Hz), increase pressure to 6 psig Free end rotor motion Side feed pressure suppresses sub synchronous whirl motions 16

17 Effect of mechanical preload (shims) on the dynamics of a rotor-gfb system

18 Add a mechanical preload to a foil bearing Thin foil Θ l Structural bump Θ l Ω Ω Journal Y Journal Y g Housing Θ p Θ s t s Shim X (a) Gas Original foil bearing GFB Original GFB X (b) Gas foil Shimmed bearing with GFBthree shims Shimmed GFB Inserting metal shims underneath bump strips introduces a preload (centering stiffness) at low cost typical industrial practice 18

19 Amplitude (μm, -pk) Amplitude (μm) Gas Foil Bearing with Metal Shims Amplitude (μm) Amplitude (μm, -pk) Amplitude [um] [μm] Original GFBs Free end, vertical direction.35 bar (5 psig) 2 krpm 25 krpm Amplitude [ microns] Amplitude [μm] Shimmed GFBs.35 bar (5 psig) 2 krpm 25 krpm 2 2 Amplitude [μm, [um, -pk] krpm Frequency [Hz](Hz) (a) Waterfall 1X 27 krpm Rotor speed (krpm) Rotor speed [krpm] SUB SYNC SYNCHRONOUS Amplitude [μm, [um, -pk] Frequency [Hz] Frequency (Hz) 1X 4 krpm 5 krpm Rotor [krpm] SUB SYNC Rotor speed (krpm) SYNCHRONOUS 19

20 Gas Foil Bearings Closure Improved stability with side pressure and shims (Cooling) side pressure reduces amplitude of sub sync whirl motions Preloads (shims) increase bearing stiffness and rise onset speed of subsynchronous whirl. Foil Bearings survive severe subsynchronous motions and abusive operation! 2

21 Metal Mesh Foil Bearings 21

22 MMFB components Simple to manufacture and assemble Top Foil.12 mm top foil Chrome-Nickel alloy Rockwell 4/45 Heat treated at ~ 45 ºC for 4 hours and allowed to cool. Foil retains arc shape after heat treatment Sprayed with MoS 2 sacrificial coating Metal mesh pads Compressed weave of copper wires Compactness (density)=2% Stiffness and damping of MMFB depend on metal mesh compactness Bearing cartridge (+top foil+ metal mesh) Metal mesh pads and top foil inserted inside bearing cartridge. Top foil firmly affixed in a thin slot made with wire-edm machining 22

23 MMFB Assembly Simple construction and assembly procedure BEARING CARTRIDGE METAL MESH RING TOP FOIL 23

24 Metal Mesh Foil Bearings (+/-) NO temperature limits Resilient structure with lots of material damping. Simple construction ( in comparison to other foil bearings) Cheap! Metal mesh tends to sag or creep over time Damping NOT viscous. Modeling difficulties 24

25 Major question (?) Metal mesh pad Top foil fixed end Bump foil Spinning shaft (a) Metal mesh foil bearing Gas film Bearing cartridge (b) Bump type foil bearing (generation I) Is a metal mesh foil bearing as good as a bump type foil bearing (generation I)? 25

26 MMFB and BFB specifications MMFB BFB Bearing inner diameter (mm) Bearing diametral clearance (mm)..11 Bearing axial length, L (mm) No of bumps 26 Bump pitch (mm) 4.3 Bump length (mm) 2.1 Bump height (mm).54 5 cm a) Metal mesh foil bearing Copper mesh outer diameter (mm) 42.7 mesh inner diameter (mm) Copper mesh density 2 % Wire diameter (mm).3 Steel top foil thickness (mm) Shaft OD static load-deflection tests (mm) Journal OD rotordynamic tests (mm) cm b) Bump type foil bearing 26

27 MMFB rotordynamic test rig MMFB TC cross-sectional view Max. operating speed: 75 krpm Turbocharger driven rotor Regulated air supply: 9.3bar Journal: length 55 mm, 28 mm diameter, weight=.22 kg Journal press fitted on Shaft Stub Twin ball bearing turbocharger Model T25 27

28 Torque & bearing lift-off vs. shaft speed Rotor speed [krpm] Bearing torque [Nmm] s s s T T Time Time [s] [s] Time Time [s] [s] (a) (a) BFB (c) (c) Rotor speed [krpm] MMFB Bearing torque [Nmm] Rotor Rotor starts starts spinning spinning MMFB MMFB 3 2 MMFB MMFB 1 s Rotor Rotor stops stops Bearing torque [Nmm] Rotor speed [krpm] 8 6 Bearing torque [Nmm] Rotor speed [krpm] Rotor Rotor starts starts spinning spinning BFB BFB 3 BFB BFB 2 1 T Rotor Rotor stops stops T 36 N load Static load W Y Time Time [s] [s] Time Time [s] [s] (b) MMFB (b) MMFB torque torque (d) BFB (d) BFB torque torque 28

29 Measurements: friction factor f = (Torque/Radius)/(Static load) 36 N/LD=3.8 psi Static load Friction factor [-] 1.1 MMFB f DRY ~ N 26.7N 17.8N Friction factor [-] BFB f ~.3 f ~ N 26.7N 17.8N Rotor accelerates Rotor speed [krpm] Dry sliding Airborne.1 Rotor accelerates Rotor speed [krpm] Friction decreases with load and rotor speed (due to lift-off). MMFB lifts earlier (lower speed) than BFB 29

30 Rotordynamics test rig (X-Y 1 N shakers) Dynamic load : 25-4 N Rotor speed 7 krpm Freq. identification range: 2 to 4 Hz Motion amplitudes : 2mm, 25 mm & 3 mm Static load: 22 N 3

31 Identification model EOM: M S a X X CS v X X KS X C X XX CXY x K XX K XY x FX M a C v K Y C C y K K y F SY Y SY Y SY YX YY YX YY Y K S,C S : soft SQ stiffness and damping M S : effective mass K ij,c ij : test bearing stiffness and damping 31

32 Stiffness vs. frequency Dynamic load load along along X X Shaft speed=5 krpm (833 Hz) X X Y Y Dynamic load load along along Y Y Experimental 15.5 N 22 N/LD=2.3 psi 15.5 N Fixed end Stiffness [MN/m] Stiffness [MN/m] Stiffness [MN/m] K XX K XX K YX K YX K YY K YY Kxx kxx Kxx kxx Kyx kyx Kyx kyx Stiffness [MN/m] Stiffness [MN/m] Stiffness [MN/m] Stiffness [MN/m] 1 K XY K XY Kxy kxy Kxy kxy Kyy kyy Kyy kyy Frequency [Hz] [Hz] [Hz] Frequency Frequency [Hz] [Hz] [Hz] (a) (a) MMFB (b) (b) BFB K YY K YY K XX K XX K XY K XY K YX K YX kxxkxx kyxkyx kxykxy kyykyy 22 N MMFB has lower dynamic stiffness. BFB hardens with frequency. Both bearings show little cross-coupled stiffnesses 32

33 Damping vs. frequency Experimental Shaft speed=5 krpm (833 Hz) 22 N/LD=2.3 psi 15.5 N Fixed end 15.5 N Eq. viscous damping [Ns/m] Eq. viscous damping [Ns/m] C XX C XX C YY C YY C YX C YX C XY C XY Cxx Cyx Eq. viscous damping [Ns/m] CxxCxy Cxy CyxCyy Cyy Eq. viscous damping [Ns/m] C XY C XY C YY C YY C XX C XX C YX C YX Frequency Frequency [Hz] [Hz] Frequency Frequency [Hz] [Hz] MMFB (a) MMFB (a) MMFB (b) BFB (b) BFB Cxx Cyx Cxx Cxy Cxy Cyx Cyy Cyy 22 N MMFB has lower viscous damping. Both bearings show little cross-coupled damping. 33

34 Loss factor vs. frequency Shaft speed=5 krpm (833 Hz) 22 N/LD=2.3 psi Estimation 15.5 N 15.5 N Fixed end Loss factor ~ C K Loss factor [-] MMFB rpm MMFB 5 krpm MMFB 5 krpm BFB krpm BFB MMFB BFB 22 N BFB Frequency [Hz] MMFB has much more structural damping (ability to dissipate mechanical energy). 34

35 Conclusions MMFB vs BFB Static load-deflection: BFB has larger mechanical hysteresis loop, loss factor is ~ 2-3 times that of BFB. Drag torque increases with increase in static load When airborne, the friction factor (f ) ~.3 for both test bearings. MMFB has larger dry-friction torque prior to lift-off. Dynamic load tests: MMFB has lesser dynamic stiffness and viscous damping than BFB. MMFB structural damping > than BFB s. MMFBs are inexpensive gas bearings for oil-free MTM. Use cheap commercially available materials. 35

36 Acknowledgments Honeywell Turbocharging Systems, Foster-Miller, TAMU Turbomachinery Research Consortium (TRC) Learn more: 36