gas bearings for microturbomachinery - texas a&m …rotorlab.tamu.edu/me626/notes_pdf/gas...
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Gas Bearings for Microturbomachinery
Rotordynamic Performance & Stability
Turbomachinery Laboratory, Texas A&M University Mechanical Engineering Department
The 8th IFToMM International Conference on Rotordynamics
Luis San AndrésMast-Childs Professor
ASME, STLE Fellow
September 2010
2
Oil-Free Bearings for Turbomachinery
JustificationCurrent 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 (< 60 kW) in microengines to midsize gas turbines (< 250 kW) for distributed power and hybrid vehicles.
Gas Bearings allow• weight reduction, energy and complexity savings• higher temperatures, without needs for cooling air • improved overall engine efficiency
3
Microturbomachinery as per IGTIMicroturbomachinery as per IGTI
ASME Paper No. GT2002-30404
Honeywell, Hydrogen and Fuel Cells Merit Review
Automotive turbochargers, turbo expanders, compressors,
Distributed power(Hybrid Gas turbine & Fuel Cell), Hybrid vehicles
Drivers:deregulation in distributed power, environmental needs, increased reliability & efficiency
International Gas Turbine Institute
Max. Power ~ 250 kWatt
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Micro Gas TurbinesMicro Gas Turbines
100Turbec, ABB & Volvo
70, 250IngersollRand
175General Electric
35, 60, 80, 150Elliott EnergySystems
30, 60, 200Capstone
25, 80Bowman
OUTPUT POWER (kW)MANUFACTURER
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
60kW MGT
www.microturbine.com
Hybrid System : MGT with Fuel Cell can reach efficiency > 60%
Ideal to replace reciprocating engines. Low footprint desirable
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MEMS MTM
Ultra MicroturbomachineryUltra Microturbomachinery
GT-2003-38866
Meso-scale MTM
PowerMEMS 2004
• Palm-size power source• Brayton cycle• Gas foil bearings
www.m-dot.com
Small unmanned vehicles and to replace batteries in portable electronic devices 100 Watt & less
• Silicon wafer• 1.2 Million rpm• Thrust 0.1 N• Spiral groove and hydrostatic gas bearings
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MTM materials & fabricationMTM materials & fabrication
Materials & Reliability
• High temperature durability• Light weight
GT2004-53493
Fabrication
•Mold SDM•Precision 3D Milling•MEMS
GT2003-38866
DRIE process
Mold SDM process
GT2003-38933
GT2003- 38151
3D Milling
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• Oil-Free • NO DN limit• Low friction and power loss• Thermal management
GAS BEARINGS
AIAA-2004-5720-984
Gas Foil Bearing
GT 2004-53621
Flexure Pivot Bearing
AIAA 2004-4189
Rolling element bearings
PowerMEMS 2003
• Low temperatures• Low DN limit (< 2 M)• Need lubrication system
NICH Center, Tohoku University
Herringbone grooved bearing
• Precision fabrication process• Low load capacity and stiffness and little damping
Available Bearing TechnologiesAvailable Bearing Technologies
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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 wearCeramic rotors and components
Automated agile processesCost & number
Low-NOx combustors for liquid & gas fuelsTH scaling (low Reynolds #)
Best if free (bio-fuels)
MTM – Needs, Hurdles & Issues
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Gas Bearings for Oil-Free MTM
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!Gas Bearings Must Be Simple!
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Ideal gas bearings for MTM
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) available
11
Example: Turbochargers
Increased engine efficiency and performance relies on a robust rotor-bearing system
RBSWith Fully Floating
Bearing RBSWith Semi Floating
BearingRBS
With Ball BearingOil Less
Bearing System (2007)
DETC2007-34136
RBS: Rotor Bearing System
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Bearing technologies• Preformed bushings• Ceramic rolling element bearings• Rigid geometry gas bearings • Flexure tilting pad gas bearings• Foil gas bearings • Solid lubricant films (coatings)• Active and passive magnetic bearings
Technical requirements for turbochargers
8.77 M6 MP V (psi ft/s)752800Tip speed (m/s) 5.75 M3.7 MDN (rpm-mm)61 to 157169 to 247Max speed (krpm)459 to 669068 to 182Rotor weight (gram)71 to 18341 to 60Compressor wheel diameter (mm)CV turboPV turbo
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Preformed BushingsSelf-lubricating material, typically a thermoplastic or a graphite derivative. The bushings may have a rigid support backing (steel or bronze).
Advantages: Disadvantages: No lubrication required Simple design and construction Moderate friction and wear Inexpensive Off the shelf items Low conductivity – electrical and
thermal insulation.
Dynamic behavior unknown Restricted to low P-V numbers. Relatively Low temperature limit
(Graphalloy bushings to 1000 F). Simple cylindrical configurations
molded Not suitable for high speeds Unknown hydrodynamic performance
Vespel, Torlon, PeekGraphalloy
Limits:PV 300,000 psi ft/minLow Temperature
14
Ceramic Rolling Element Bearingssilicon nitride (Si3N4) balls and hardened steel outer and inner raceways. Cage retainer made of thermoplastics (PTFE with bronze impregnated, Vespel™, Thorlon™, Peek™, Nylon)
Advantages: Disadvantages: High speed and acceleration - up to 3
million DN Increased stiffness Less friction, less heat Low thermal expansion, higher
accuracy Non-conductive Extended operating life Ability to support thrust (axial) loads.
Requires some form of lubricant for extended usage.
Higher cost for parts and lubricant Operating temperature limited by
lubricant Dissimilar heat expansion of materials
can cause seizure. Too stiff, little damping.
Dry friction μ= 0.17(compare to 0.42
steel/steel)Need lubrication to
last, Expensive
15
Rigid Geometry Gas Bearings
Gas film bearings (hydrostatic/hybrid) give low friction and support load (< 1 bar)
journal rotation
Plain journalbearing
pressure
Major issues: Little damping, Wear at start & stop,Instability (whirl & hammer)
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Gas Foil BearingsFoil bearings integrate a gas film in series with an elastic substructure. Bump foil and multi-leaf foil bearings are used. Foils must be coated to avoid wear and seizure, and to reduce drag friction during frequent start and stops.
Issues:Excessive cost / Protected technology
Extensive testing for coatingsRotordynamic instability or forced
nonlinearityThermal management Issues
Complicated analysis tools
Currently used in micro power (<100 kW) systems and secondary cryogenic turbopumps
Source: NASA Oil Free MTM program
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Solid lubricant films ensure low friction and reduced surface wear rates while promoting early rotor lift off.Applied with plasma spray, ion beam deposition, sputtering, and chemical vapor deposition.PS304, Near Frictionless Carbon Coating (NFC™) , UltraC Diamond™, Casidiam™ , Fluoropolymer (EMRALON™), Molybdenum and Graphite coatings
Low friction coefficients in air are as low as 0.10
Still costly for mass production !
Coatings (Solid Lubricant Films)
Procedure for depositing PS304 coating on shaft
Undercut shaft First plasma spray Second plasma spray Polished
Source: NASA Oil Free MTM program
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Active Magnetic BearingsAttraction type – controlled 5 axes
Permanent magnet – reluctance bearings
Passive Magnetic Bearings
Limitations- Cost,
- Low temperature,- Complexity
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Rankings for gas bearings Technology Details Cost/unit RRD RRA A Preformed
bushings Thermoplastics & Graphite derivatives
$30.00 unit 1 4
B Ceramic rolling element bearings
Ceramic – steel Contained lubricant
$33.00 unit 1 2
C Rigid geometry gas bearings
Hydrodynamic/ hydrostatic
$2-$4 unit
2 3
D Tilting pad gas bearings
Hydrodynamic/ hydrostatic
$50 Hypad© 2 3
E Foil bearings Hydrodynamic $ 100 MITI 3 2 G Active magnetic
bearings $50-100
system (bulk) 3 4
H Passive magnetic bearings
NA 5 5
F Solid film
lubricants. Applicable to products C-E
Coatings on shaft.
Diamond like coating (DLC). PS 304
$0.75 part $50 part + grinding
2
2
2
3
Foil coatings Teflon (Emralon™) $ 0.50 part (bulk)
1 2
RRD: relative ranking of current state of development. (1): off the shelf item with proven results over a wide range of applications(2): readily available technology, engineering analysis or experimentation required (3): known technology, both engineering andtesting required (4): some applications known, largely empirical development (5): unknown product, research phase.
RRA: ranking for readiness of application to oil-free TCs(1): readily available engineered product proven for TC application (2): technology known, engineering analysis in progress, manufacturing process and performance issues for TC application. (3): technology available, proven engineering analysis, product development needed to extend limits of application.(4): some applications known, largely empirical development at this time(5): unknown application to oil-free TC.
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Gas Bearings for MTM
What are the needs?
Make READY technology for industrial application by PUSHING development to
• make 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
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Gas Bearings for Oil-Free MTM
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,Instability (whirl & hammer)
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Gas Bearing Research at TAMU
2001/2 - Three Lobe Bearings
2003/4 - Rayleigh Step Bearings
2002-09 - Flexure Pivot Tilting Pad Bearings
2004-10: Bump-type Foil Bearings
2008-10: 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
23
Overview of experience with gas bearings
• Three Lobe Bearings
• Flexure Pivot Tilting Pad Bearings
• Bump-type Foil Bearings
• Metal Mesh Foil Bearings
24
Positioning Bolt
X
Y
Load
LOP
Rotor/motor
Bearing
Sensors
Load cell
Air supply
Thrust pin
Rotor: 826 gramsBearings: L= 30 mm, D=29 mm, C < 25 um
190 mm, 29 mm diam
Hybrid Gas Bearing Test Rig (1)
25
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0 10000 20000 30000 40000 50000 60000
Operating Speed (rpm)
Dis
plac
emen
t Am
plitu
de (m
mp-
p)
Pratio = 5.08
Pratio = 3.72
Pratio = 3.02
As supply Pressure
increases, critical
speed rises & damping ratio drops
displacement
20 psig
40 psig
60 psig
Imbalance response
C=66 um, r:0.32 , do: 1mm
Three lobe gas bearings
ASME GT2003-38859
26
Flexure Pivot Bearings
27
Flexure-Pivot Tilting Pad BearingsO-ring
Grove
1.5”
AlignmentPins
1.5”
O-ring Grove
1.5”
AlignmentPins
1.5”
X
Y
Load
Wire-EDM, low cost, common material, engineered design, no stack-up tolerances,
PROVEN record in HP compressors (oil lubricated)
External pressurization
28
Ps=3.8 bar (40 psig)
Three-lobe Bearing (WFR=0.37)
unstable
Stable to 99 krpm
Clean 1X rotor response(no instability –
hydrodynamic type)
60 KRPM
GT 2004-53621
Waterfalls of rotor motion
GT 2003-38859
29
Rotordynamic response
Predictions vs. test data
L R
V: verticalH: horizontal
Good correlation: tests and rotor-gas bearing model predictions
IJTC 2006-12026ASME GT2008-50393
0
5
10
15
20
0 5000 10000 15000 20000 25000 30000 35000 40000Speed [rpm]
Am
plitu
de [μ
m, p
k-pk
]
5.08 bar
3.72 bar
2.36bar TEST
PREDICT
μm
As supply Pressure
increases, critical speed
raises &damping
ratio drops
30
0
5
10
15
20
0 10000 20000 30000 40000 50000Speed [rpm]
Am
plitu
de [μ
m, 0
-pk]
5.08 bar3.72 bar2.36 barNo feed pressure
5.08 bar
No feed pressure
2.36 bar
3.72 bar
A way to control rotordynamics?
Question:If shaft speed
regulates feed
pressure, could large
rotor motions be
suppressed ?
ASME 2007-27127
31
Towards suppression of critical speeds
As pressure supply increases, critical speed raises anddamping ratio decreases
L R
V: verticalH: horizontal
0
5
10
15
20
0 5000 10000 15000 20000 25000 30000 35000 40000Speed [rpm]
Am
plitu
de [μ
m, p
k-pk
]
5.08 bar
3.72 bar
2.36 bar
5.08 bar
3.72 bar
2.36bar
20 psig
40 psig
60 psig
Speed range for Ps control
ASME GT2008-50393
32
Rotor speed during coast down
0
10
20
30
40
50
0 50 100 150 200Coast down time [sec]
Rot
or s
peed
[krp
m]
15 krpm ~ 14 krpm
58.0 sec ~ 62.7 sec
~ 2 minute
Speed decays exponentially with time. Little viscous drag.
2.36 bar
210 rpm/s
Speed region for control of feed
pressure
Towards suppression of critical speeds
ASME GT2008-50393
33
Control of feed pressure
Simple on/off valve ASME GT2008-50393
34
0
5
10
15
20
0 5000 10000 15000 20000 25000 30000 35000 40000
Speed [rpm]
Am
plitu
de [μ
m, p
k-pk
]
Controller activated system
Displacements at RB(H)
L R
V: verticalH: horizontal
5.08 bar
2.36 barBlue line: Coast downRed line: Set speed
2.36 bar5.08 bar
Rotor peak amplitude eliminated by sudden increase in supply pressure
Suppression of critical speeds ASME GT2008-50393
35
0
5
10
15
20
0 5000 10000 15000 20000 25000 30000 35000 40000Speed [rpm]
Am
plitu
de [μ
m, p
k-pk
]
Test-O.C. #5, active Ps control, coast down
Prediction
5.08 bar 2.36 bar Supply pressure
2 bar
5 bar
speed
Ps
TestPrediction
TEST
PREDICT
Tests vs predictions
Good agreement! (predictable RBS performance)
IJTC 2006-12026
ASME GT2008-50393
36
Closure: Stable to 99 krpm!• External pressurization stiffens bearings and reduces
their modal damping, thus raising critical speeds.• Speed coast down tests with controlled supply pressure
show displacement of critical speed and elimination of peak displacements.
• Predictive codes reproduce well test rotor response; even for large rotor motions and controlled supply pressure!
• Hybrid bearings demonstrated reliable rotordynamic performance free of sub sync. whirl.
• OTHER measurements show reliability of gas bearings to external shocks and base periodic motions.
Flexure Pivot Gas Bearings
37
Gas Foil Bearings
38
Gas Foil Bearings
Advertised advantages: high load capacity (>20 psig), rotordynamically stable, tolerance of misalignment and shocks
: Bump spot weld X
Rotor spinning
Housing
Bump strip layer
Top foil
Gas film
Y
Top foil spot weld
Ω
Ө
39
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
40
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 data for rotordynamic force coefficients & operation with limit cycles (sub harmonic motions)
• Thermal management for high temperature applications (gas turbines, turbochargers)
• Predictive models lack validation for GFB operation at HIGH TEMPERATURE
41
No. of finiteelementsbetween bumps
Axially distributed linear spring
Fixed end
Thin Shell
Shell Length Shell width
Finite element
Shell thickness
ASME JGT 2008, v130, 2010, v132; ASME GT 2007-27249ASME J. Tribol., 2006, v128
0
1
2
3
4
5
6
0 90 180 270 360Angular location [deg]
Dim
ensi
onle
ss p
ress
ure,
p/pa[-]
Predicted gas film pressure field
P/Pa
Ω
X
Y
Θ
W
Foil Bearing Models
Fast PC codes integrate foil
structure with gas film
hydrodynamics –GUI driven
42
Accuracy of Foil Bearing Model Predictions
100
1000
10000
100000
0 10 20 30
Time [sec]
Rot
or s
peed
[rpm
]
1
10
100
1000
0 10 20 30
Time [sec]
Torq
ue [N
-mm
] 10,000 cycle test data (Kim et al., 2003)
Prediction
1
10
100
1000
0 10 20 30
Time [sec]
Torq
ue [N
-mm
] 10,000 cycle test data (Kim et al., 2003)
Prediction
Prediction
Shutdown
Test data
StartupStartup
0
100
200300
400
500
600700
0 5000 10000 15000 20000
Rotor speed [rpm]
Load
cap
acity
[N]
Test data
Prediction
KIST test data (2003)
0
4
8
12
0 2000 4000 6000 8000 10000
Rotor speed [rpm]
Torq
ue [N
-mm
]
Static load: 52 N
Rotor speed decreases
Driving motor Shutdowns
Prediction10,000 cyclesTest
data:5,000 cycles
Model validation: Static load performance AIAA-2007-5094
43
Foil Bearing Test Rig
Driving motor (1HP, 50 krpm)
Flexible coupling
Optical Tachometer
Start motor(2HP, 25 krpm)
Foil bearing housing
Electromagnet loader
Test rotor
Shaft Diameter = 1.500”Rotor mass = 2.2 lb
ASME GT2006-91238
44Amplitudes of subsynchronous motions INCREASE as imbalance increases (forced nonlinearity)
Effect of imbalance on RBS response
Speed (-)
Imbalance +
0 100 200 300 400 500 600 700 800
20
40
60
80
100
Frequency [Hz]
Am
plitu
de [m
icro
ns]
Dis
plac
emen
t Am
plitu
de (μ
m)
Frequency (Hz)
u = 7.4 μm
1X0.5X
2X
u = 7.4 μm
0 100 200 300 400 500 600 700 8000
20
40
60
80
100
Frequency [Hz]
y ( )
1X0.5X
2X
25.7 krpm
2.6 krpm
12.5 krpm
20.5 krpm
u = 10.5 μm
Frequency (Hz)
u = 10.5 μm
26 krpm
Limit cycle: large subsync motions
aggravated by imbalance
ASME GT2006-91238
45
Dis
plac
emen
t Am
plitu
de (μ
m)
0 5 10 15 20 250
20
40
60
SUB SYNCSYNCHRONOUS
Rotor speed (krpm)D
ispl
acem
ent A
mpl
itude
(μm
) 0 5 10 15 20 25
0
20
40
60
SUB SYNCSYNCHRONOUS
Rotor speed (krpm)
u = 7.4 μm u = 10.5 μm
Synchronous and subsynchronous motions for two imbalances
Severe subsync motions (limit cycle) aggravated by imbalance. Non linear forced response
AIAA-2007-5094
SynchronousSubsynchronous
Effect of imbalance on RBS whirl motions
46
50 krpm0 200 400 600 800 1000
0
20
40
60
80
100
Frequency [Hz]
Am
plitu
de [m
icro
ns]
2 krpm
1X 26 krpmA
mpl
itude
(μm
, 0-p
k)
Subsynchronous motions
Large amplitudes locked at natural frequency (50 to 27 krpm) …… but stable limit cycle!
Coast down from 50 krpm (833 Hz)
Rotor speed
decreases
Rotordynamic instability?Rotordynamic instability?ASME JGT, 209, v31
47
Gas Foil Bearings
All GFB models predict (linearized) rotordynamic force coefficients.
No model readily available to predict nonlinear rotordynamic forced response
What causes the subsynchronous motions?
What causes the excitation of natural frequency?
48
FB: stiffness & energy dissipation
FB structure is non linear (stiffness hardening), a typical source of sub harmonic motions for large (dynamic) loads. Hysteresis loop gives energy dissipation
-300
-200
-100
0
100
200
300
-50 -40 -30 -20 -10 0 10 20 30 40 50
Static load [N]
Bea
ring
disp
lace
men
t [μm
]
Test 1Test 2Test 3Test 4Test 5Test 6Test 7Test 8
Eight cyclic load -unload structural
tests
Loading
Loading
Unloading
Unloading
F ≠ K X2 3
1 2 3sFBF K r K r K r= + +
AIAA-2007-5094
49
0 100 200 300 400 500 600 700 800
50
0
50
100
150Waterfall
Excitation frequency [Hz]
Am
plitu
de [u
m]
X1.2 krpm
36 krpm
Am
plitu
de [μ
m]
1X Subsynchronous
rotor motions
Rotor speed: 30 →1.2 krpm (600 →20 Hz)Imbalance displacement, u = 12 μm (Vertical motion)
Subsynchronous (sub harmonic) whirl motions of large amplitude
AIAA-2007-5094
Waterfall of predictions for rotorWaterfall of predictions for rotor--GFB systemGFB system
50
0 100 200 300 400 500 6000
20
40
60
80
PredictionExperiment
Frequency [Hz]
Am
plitu
de [u
m]
Am
plitu
de [μ
m]
Comparison to test data
Whirl frequencies within a narrow band enclosing natural frequency (132 Hz) of RBS
0 5 10 15 20 25 300
40
80
120
160
200
PredictionExperiment
Rotor speed [krpm]Fr
eque
ncy
[Hz]
Amplitude vs. frequency Frequency vs. rotor speed
Test dataPredictions
Test data
Predictions
Rotor speed (krpm)Frequency (Hz)
AIAA-2007-5094
Amplitude and frequency of whirl motionsAmplitude and frequency of whirl motions
51
6 8 10 12 14 16 18 20 22 24 26 28 300
0.25
0.5
0.75
1
Rotor speed [krpm]
Whi
rl fr
eque
ncy
ratio
(WFR
)
WFR
P& T show bifurcation of motion into 1/2 & 1/3 WFRs
Test data(San Andres et al, 2006)
Predictions Test data(Kim and San Andres et al, 2007)
Rotor speed (krpm)
AIAA-2007-5094
Whirl frequency ratio of rotor bearing systemWhirl frequency ratio of rotor bearing system
52
• FB structure is highly non linear, i.e. stiffness hardening: a common source of sub harmonic motions for large (dynamic) loads.
• Subsynchronous frequencies track shaft speed at ~ ½ to 1/3 whirl ratios, locking at system natural frequency.
• Model predictions agree well with rotor response measurements (Duffing oscillator with multiple frequency response).
Gas Foil Bearings
Closure 1 Instability or forced nonlinearity?
AIAA-2007-5094
53
AIR SUPPLY
Effect of cooling flow on rotordynamics
Ps
Rotating journal
Pa
Bump spring
Top foilBearing housing
Circumferential velocity
ΩRJAxial
velocity ΩRJ
Outer gap
Inner gas film
XY
zx
Typically foil bearings 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
Side effect: Axial flow retards evolution of circumferential flow velocity
ASME JGT, 209, v31
54
Effect of side flow on rotordynamics
0
20
40
60
0
20
40
60
Am
plitu
de [μ
m, 0
-pk]
0
20
40
60
0 100 200 300 400 500 600
Frequency [Hz]
(a) 0.35 bar
(b) 1.4 bar
(c) 2.8 bar
Whirl frequency locks at RBS
natural frequency ( not
affected by level of feed pressure
For Ps ≥ 2.8 bar rotor subsync. whirl motions
disappear;(stable rotor
response)ωsub= 132 Hz
ωsub= 147 Hz
ωsub= 127 HzSubsynchronous ωsyn= 508 Hz
Synchronous
FFT of shaft motions at 30 krpm
ASME JGT, 209, v31
55Onset of subsynchronous whirl motions
0
20
40
60
0
20
40
60
Am
plitu
de [μ
m, 0
-pk]
0
20
40
60
10 15 20 25 30
Rotor speed [krpm]
(a) 0.35 bar
(b) 1.4 bar
(c) 2.8 bar
SynchronousSubsynchronous
NOS: 25 krpm
NOS: 30.5 krpm
NOS: 27 krpm
Delay of large
amplitude subsynchro
nous rotor motions
with increase in
axial cooling flow
Effect of side flow on rotordynamics
ASME JGT, 209, v31
56
(a) Gas foil bearing (b) Gas foil bearing with three shims
JournalY
Θl
Ω Ω
JournalY
X
Θp
ts
Θl
Θs Housing
Structural bump
Thin foil
Shim g
XOriginal GFB Shimmed GFBShimmed GFB
Effect of preload on rotordynamics
Original GFB
Inserting metal shims underneath bump strips introduces a preload (centering stiffness) at low cost – typical industrial practice
Trib Trans 2009, v52
57
400 200 0 200 400 600 800 10000
20
40
60
80
100Free end, vertical direction
Frequency [Hz]
Am
plitu
de [u
m]
50 krpm
2 krpm
25 krpm
1X
Am
plitu
de [μ
m]
(a) Waterfall
0 10 20 30 40 500
20
40
60
SUB SYNCSYNCHRONOUS
Rotor speed [krpm]
Ampl
itude
[um
, 0-p
k]
27 krpm
Am
plitu
de [μ
m, 0
-pk]
Original GFBs0.35 bar (5 psig)
Am
plitu
de (μ
m)
Rotor speed (krpm)
Am
plitu
de (μ
m, 0
-pk) Frequency (Hz)
400 200 0 200 400 600 800 10000
20
40
60
80
100
Frequency [Hz]A
mpl
itude
[mic
rons
]A
mpl
itude
[μm
]
50 krpm
2 krpm
25 krpm
1X
0 10 20 30 40 500
20
40
60
SUB SYNCSYNCHRONOUS
Rotor speed [krpm]
Am
plitu
de [
um, 0
-pk]
40 krpm
Am
plitu
de [μ
m, 0
-pk]
Shimmed GFBs0.35 bar (5 psig)
Rotor speed (krpm)
Am
plitu
de (μ
m, 0
-pk)
Am
plitu
de (μ
m)
Frequency (Hz)
Gas Foil Bearing with Metal Shims Trib Trans 2009, v52
58
• Predictive foil bearing FE model (structure + gas film) benchmarked by test data.• (Cooling) side pressure reduces amplitude of sub sync whirl motions• Preloads (shims) increase bearing stiffness and raise onset speed of subsync. whirl.•Predicted rotor 1X response and GFB force coefficients agree well with measurements.30
Gas Foil Bearings
Closure 2
Foil Bearings survive severe subsynchronous motions and abusive operation!
Improved stability with pressure and shims
59
Foil Bearings Thermal Management
What is effect of rotor temperature on dynamic performance of rotor-GFB system?
• Measure bearing & rotor temperatures & rotordynamic performance for increasing shaft temperatures
• Realize effectiveness of side cooling flow on thermal management
Gases have little thermal capacity. Cooling flow needed for thermal management: to remove heat from shear drag or to reduce thermal gradients from hot to cold engine sections
60
Temperatures in test rig
Foil bearings
Cartridge heaterHeater stand
T14
T10
T12
T13
Coupling cooling air
Drive motor
T15T16Th
Hollow shaftT5
T11
TambΩ
T1
T4
T3
T2
45º
Free end (FE) GFB
g
45º
Ω
T6
T7
T8
T9
Drive end (DE) GFB
g
Insulated safety cover
Cooling air
Thermocouples: 1 x heater, 2 x 4 FB outboard, 2 x Bearing housing outer surface, 1x Drive motor, 1 x ambient + infrared thermometers 2 x rotor, rotor surface temperature (Total = 17)
61
Rotordynamic response for hot rotor
As Tc increases, critical speed increases by ~ 2 krpm and the peak amplitude decreases.
0
20
40
60
80
100
0 5000 10000 15000 20000 25000 30000
Rotor speed [rpm]
Am
plitu
de [μ
m, 0
-pk]
Tc= 22 °C
Tc= 93 °C
Tc= 132 °C
Cartridge temperature (Tc) increases
Coastdown
ASME J. Tribol., 2010, v132
DEV
62
FH
FV
DH
DVg
Baseline. No forced cooling
Similar responses – free of sub synchronous whirl motions
Waterfalls of rotor motionDEH
(a) Heater off
(a) Heater on, Ths=360 C
0 200 400 600 800 10000
0.02
0.04
0.06
0.08
trace 1
X_leftWF
0.707
X_leftXaxis0 200 400 600 800 1000
Frequency [Hz]
80
60
40
20
0
Am
plitu
de [µ
m, 0
-pk]
1X2X
30 krpm
2 krpm
14 krpm
0 200 400 600 800 10000
0.02
0.04
0.06
0.08
trace 1
WF
X_leftXaxis
0 200 400 600 800 10000
0.02
0.04
0.06
0.08
trace 1
WF
X_leftXaxis
1X
0 200 400 600 800 1000Frequency [Hz]
80
60
40
20
0
2X
ASME GT 2010-22981
63
0
10
20
30
40
50
0 5 10 15 20 25 30
Speed [krpm]
Am
plitu
de [μ
m, 0
-pk]
DVDHFVFH
0
10
20
30
40
50
0 5 10 15 20 25 30
Speed [krpm]
Am
plitu
de [μ
m, 0
-pk]
DVDHFVFH
No forced cooling
Rotor 1X response for cold & hot conds
No heating Ths=360C
Critical speed (Rigid body) ~ 13 krpmElastic mode at 29 krpm
No major differences in responses between cold and hot
Free End Drive EndNo heating
T11=37º C T12=26º C
Free End Drive End
T11=157º C T12=107º C
Ths=360º C
ASME GT 2010-22981
64
Free End Drive EndNo heating
T11=37º C T12=26º C
Free End Drive End
T11=157º C T12=107º C
Ths=360º C
Free End Drive End
T11=92º C T12=70º C
Ths=200º C
FH
FV
DH
DVg
0
10
20
30
40
50
0 5 10 15 20 25 30
Speed [krpm]
Am
plitu
de [μ
m, 0
-pk]
No heatingThs=100ºCThs=200ºCThs=300ºCThs=360ºC
Heater up to 360C. No forced cooling
Cartridge temperature (Ths) increases
As Ths increases to 360ºC, peak amplitudes between 7~15 krpm decrease
natural frequency
Rotor 1X response for cold & hot condsD
HASME GT 2010-22981
65
1
10
100
0 10 20 30 40 50 60 70 80Coast down time [sec]
Rot
or s
peed
[krp
m]
No heatingThs=100ºCThs=200ºCThs=300ºCThs=360ºC
Time for rotor coast down
Heater up to 360C.No forced cooling
Coastdown time reduces as rotor heats (lesser clearance)
Cartridge temperature (Ths) increases
Exponential decay
Long time to coastdown :
very low viscous drag
Effect of temperature
Coastdown time (s)
Speed (krpm)
ASME GT 2010-22981
66
Foil Bearings survived high temperature operation– Still working !
Thermal management with cooling streams works best at high temperatures and flow rates ensuring turbulent flow.
As rotor and bearing temperatures increase, air becomes more viscous and bearing clearances decrease; hence coastdown time decreases.
For operation with hot shaft, rotor motion reduces while crossing critical speed.
Gas Foil Bearings
Closure 3 Thermal management ASME GT2010-22981
67
Novel developments
Metal Mesh Foil Bearings
68
Metal Mesh Foil Bearing (MMFB)
Bearing Cartridge, metal mesh ring and top foilHydrodynamic air film between rotating shaft and top foil
Applications: ACMs, micro gas turbines, turbo expanders, turbo compressors, turbo blowers, automotive turbochargers, APUs
• Large damping (material hysteresis) offered by metal mesh
• Tolerant to misalignment,• Wide temperature range• Coatings to reduce friction at start-up &
shutdownCartridgeMetal mesh ring
TopFoil
69
MMFB Assembly
BEARING CARTRIDGE
METAL MESH RING TOP FOIL
Simple construction and assembly procedure
ASME GT2009-59315
70
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 timeDamping NOT viscous. Modeling difficultiesUnknown rotordynamic force coefficients
71
MMFB rotordynamic test rig
Max. operating speed: 75 krpmTurbocharger driven rotorRegulated air supply: 9.30bar
Journal: length 55 mm, 28 mm diameter , weight=0.22 kg
Journal press fitted on Shaft Stub
TC cross-sectional view
Twin ball bearing turbocharger Model T25
MMFB
ASME GT2010-22440
72
020406080
0 5 10 15 20 25 30Time [sec]
Spee
d [k
rpm
]
0
50
100
0 5 10 15 20 25 30Time [sec]B
earin
g to
rque
[N-m
Lift off speed and torqueRotor starts
Constant speed ~ 65 krpm
Valve open
Valve close
3 N-mm
Rotor stops
START UP to 65 krpm and SHUT DOWN
Once airborne, drag torque is ~ 3 % of
Startup ‘breakaway’torque
Iift off speed
Lift off speed at lowest torque : airborne operation
Load: 18 N
WD= 3.6 N
2009 AHS 65th Annual forum
73
0
5
10
15
20
25
30
35
40
45
50
0 10000 20000 30000 40000 50000 60000Journal speed [rpm]
Pow
er lo
ss [W
]Load 8.9 NLoad 17.8 NLoad 26.7 NLoad 35.6 N
MMFB power loss vs rotor speed
8.9 N (2 lb)
17.8 N (4 lb)
26.7 N (6 lb)35.6 N (8 lb)
Rotor accelerates
Power loss decreases to min during mixed lubrication, then increases with increasing rotor speed
Dead weight
(WD= 3.6 N)
Increasing static load (Ws) to 35.6 N (8 lb)
Mixed lubrication
Hydrodynamiclubrication
ASME GT2010-22440
74
0
0.01
0.02
0.03
0.04
0.05
0 10 20 30 40 50 60Rotor speed [krpm]
Fric
tion
coef
ficie
nt [-
]
Rotor accelerates
8.9 N (2 lb)
17.8 N (4 lb)26.7 N (6 lb)35.6 N (8 lb)
Friction coefficient vs rotor speed
f ~ 0.01
Friction coefficient ( f ) decreases with increasing static load
Dry sliding Airborne
f = (Torque/Radius)/(Static load)
f ~ 0.3-0.4
2009 AHS 65th Annual forum
75
Waterfall of rotor-MMFB motions
H load = 18 N
Rot
or s
peed
[krp
m]
Frequency [Hz]
Rot
or d
ecel
erat
ing
to re
st
Reverse whirl Forward whirl
Large sub harmonic
motions locked at natural
frequency
½ frequeny whirl absent with
larger applied loads
Metal mesh foil bearings have similar “forced”nonlinearity issues as bump-type foil bearings
ASME GT2010-22440
76
Metal Mesh Foil Bearings
ClosureWhile airborne, MMFB power loss increases with rotor
speed (little friction). Min power loss found.
MMFB structural stiffness and damping coefficients are frequency and amplitude dependent. Predictive model benchmarked against test data.
Measurements of MMFB rotordynamic force coefficients underway.
MMFBs are inexpensive gas bearings for oil-free MTM. Use cheap commercially available materials.
What we’ve learned so far
77
Turbocharger Systems
Increased engine efficiency and performance relies on robust rotor-bearing system
conventionalOil-Bearing
Oil LessBearing System (2007)
DETC2007-34136
Foil Bearingsor Metal Mesh Bearingsor Flex Pivot Bearings
Bearing Cartridge Metal mesh
donut
Formed top foil
A challenge!
78
Gas Bearings for MTM
The road ahead
79
Closure: Gas Bearings for MTM
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
80
Closure: Gas Bearings for MTM
Other pressing challenges for gas bearing technology:intermittent contact and damaging wear at
startup & shut down, andtemporary rubs during normal operating
conditions
Current research focuses oncoatings (materials), rotordynamics (stability) & high temperature (thermal management)
Need Low Cost & Long Life Solution!
81
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
Materials
Manufacturing
Processes & Cycles
Fuels
Rotordynamics & (Oil-free) Bearings & Sealing
Coatings: surface conditioning for low friction and wearCeramic rotors and components
Automated agile processesCost & number
Low-NOx combustors for liquid & gas fuelsTH scaling (low Reynolds #)
Best if free (bio-fuels)
MTM – Needs & issues reassessed
82
AcknowledgmentsThanks toNSF (Grant # 0322925)NASA GRC (Program NNH06ZEA001N-SSRW2),Capstone Turbines, Inc.,Honeywell Turbocharging Systems,Korea Institute of Science and TechnologyFoster-Miller, MiTI, TAMU Turbomachinery Research Consortium (TRC)
Learn more:http://rotorlab.tamu.edu
83
References
Gas Bearings for MTM
84
References
Osborne, D.A., and San Andrés, L., 2006, “Comparison of Rotordynamic Analysis Predictions with the Test Response of Simple Gas Hybrid Bearings for Oil Free Turbomachinery,” ASME J. Eng. Gas Turbines and Power, v128.
ASME GT2003-38859
Osborne, D.A., and San Andrés, L., 2006, “Experimental Response of Simple Gas Hybrid Bearings for Oil-Free Turbomachinery,” ASME J. Eng. Gas Turbines and Power, v128. 2003 BEST Rotordynamics Paper Award –ASME (IGTI)
ASME GT 2003-38833
San Andrés, L., 2006, “Hybrid Flexure Pivot-Tilting Pad Gas Bearings: Analysis and Experimental Validation,” ASME J. Tribology, 128.
IJTC 2006-12026
Zhu, S. and L., San Andrés, 2007, “Rotordynamic Performance of Flexure Pivot Hydrostatic Gas Bearings for Oil-Free Turbomachinery,” ASME J. Eng. Gas Turbines and Power, v129
ASME GT 2004-53621
San Andrés, L., and K. Ryu, 2008, “Flexure Pivot Tilting Pad Gas Bearings: Operation with Worn Clearances and Two Load-Pad Configurations,” ASME J. Eng. Gas Turbines and Power, v130
ASME GT2007-27127
San Andrés, L., and Ryu, K., 2008, “Hybrid Gas Bearings with Controlled Supply Pressure to Eliminate Rotor Vibrations while Crossing System Critical Speeds,” ASME J. Eng. Gas Turbines and Power, v130
ASME GT2008-50393
San Andrés, L., and Ryu, K., 2009, “Dynamic Forced Response of a Rotor-Hybrid Gas Bearing System Due to Intermittent Shocks.”
ASME GT2009-59199
San Andrés, L., Niu, Y., and Ryu, K, 2010, “Dynamic Response of a Rotor-Hybrid Gas Bearing System Due To Base Induced Periodic Motions.”
ASME GT2010-22277
Flexure pivot gas bearings
85
References Foil Bearings
San Andrés, L., D. Rubio, and T.H. Kim, 2007, “Rotordynamic Performance of a Rotor Supported on Bump Type Foil Gas Bearings: Experiments and Predictions,” ASME J. Eng. Gas Turbines and Power, v129
ASME GT2006-91238
Kim, T.H., and L. San Andrés, 2008, “Heavily Loaded Gas Foil Bearings: a Model Anchored to Test Data,” ASME J. Eng. Gas Turbines and Power, v130
ASME GT2005-68486
San Andrés, L., and T.H. Kim, 2007, “Issues on Instability and Force Nonlinearity in Gas Foil Bearing Supported Rotors,” 43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cincinnati, OH, July 9-11
AIAA-2007-5094
San Andrés, L., and Kim, T.H., 2009, “Analysis of Gas Foil Bearings Integrating FE Top Foil Models,” Tribology International, v42
ASME GT2007-27249
Kim, T. H., and San Andrés, L., 2009, “Effect of Side End Pressurization on the Dynamic Performance of Gas Foil Bearings – A Model Anchored to Test Data,” ASME J. Eng. Gas Turbines and Power, v131. 2008 Best PAPER Rotordynamics IGTI
ASME GT2008-50571IJTC2007-44047
Kim, T.H., and San Andrés, L., 2009, "Effects of a Mechanical Preload on the Dynamic Force Response of Gas Foil Bearings - Measurements and Model Predictions," Tribology Transactions, v52
IJTC2008-71195
San Andrés, L., and Kim, T.H., 2009, “Thermohydrodynamic Analysis of Bump Type gas Foil Bearings: A Model Anchored to Test Data,” ASME J. Eng. Gas Turbines and Power, v132
ASME GT2009-59919
Kim, T. H., and San Andrés, L., 2010, “Thermohydrodynamic Model Predictions and Performance Measurements of Bump-Type Foil Bearing for Oil-Free Turboshaft Engines in Rotorcraft Propulsion Systems,” ASME J. of Tribology, v132
AHS 2009 paper
San Andrés, L., Ryu, K., and Kim, T-H, 2010, “Thermal Management and Rotordynamic Performance of a Hot Rotor-Gas Foil Bearings System. Part 1: Measurements.
ASME GT2010-22981
86
References
Gjika, K., C. Groves, L. San Andrés, and G. LaRue, 2007, “Nonlinear Dynamic Behavior of Turbocharger Rotor-Bearing Systems with Hydrodynamic Oil Film and Squeeze Film Damper in Series: Prediction and Experiment.”
ASME DETC2007-34136
Other
San Andrés, L., Kim, T.H., Chirathadam, T.A., and Ryu, K., 2009, “Measurements of Drag Torque, Lift-Off Journal Speed and Temperature in a Metal Mesh Foil Bearing,” American Helicopter Society 65th Annual Forum, Grapevine, Texas, May 27-29
AHS Paper
San Andrés, L., Chirathadam, T. A., and Kim, T.H., 2009, “Measurements of Structural Stiffness and Damping Coefficients in a Metal Mesh Foil Bearing.”
ASME GT2009-59315
San Andrés, L., and Chirathadam T.A., 2010, “Identification of Rotordynamic Force Coefficients of a Metal Mesh Foil Bearing Using Impact Load Excitations.”
ASME GT2010-22440
Metal mesh foil bearings
Learn more:http://rotorlab.tamu.edu
87
Gas Bearings for MTM
Copyright©Luis San Andres 2010