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Peter A. Chapman, Jr., P.E.Principal Engineer

Mechanical Solutions, Inc.pac@mechsol.com

Albany, NYWhippany, NJ

Denver, COwww.mechsol.com

973-326-9920

2016 University Turbine Systems Research WorkshopNovember 1-3, 2016

Blacksburg, VA

Advanced Gas Foil Bearing Design for Supercritical CO2 Power Cycles

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Today’s Presentation

Project Background Overview of foil bearings:

A primer on various types, typical applications, etc.

Application in sCO2 Power Cycle MachinesDesign Considerations:Fluid propertiesMaterial selectionLoad Capacity, DampingPower Loss

Progress to Date Ongoing Work

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Project Background

Funding provided by the Department of Energy (DOE) Office of Fossil Energy Phase 1: June 2015 – March 2016 Phase 2: August 2016 – July 2018

Goal: develop a reliable, high performance foil bearing system using sCO2 as the working fluid Temperatures up to 800°C Pressures up to 300 bar

Key elements of the design: An advanced hydrostatically-assisted hydrodynamic foil bearing with higher

load capacity An integral gas delivery system to distribute flow throughout the bearing Addition of overload protection to handle large shaft excursions during

severe system transients Use of high temperature materials and coatings to prolong life and enabling

sufficient start/stop cycles

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High speed Extreme-temperature and/or oil-less environment Permits a hermetically-sealed system (eliminate

end seals) Insensitive to system pressure Applicable to high energy density turbomachineryMotors and generators are being designed to

run faster and with more torque, with reduced size & weight

Direct drive is a trend Long, maintenance-free life

Why Foil Bearings?

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Bump Style Radial Foil Bearing with Preload

Smooth top/inner foil – one or several segments

Support foil contains cylindrical bumps One or several segments One or several layers Sometimes slotted for improved edge loading, misalignment tolerance, etc.

Unidirectional, with shaft rotating from free end to fixed end

Is a hydrodynamic bearing – gas or liquid film OK

Compliant – reduces the need for high dimensional accuracy & roundness

General Features of a Bump-Style Radial Foil Bearing

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Application Spectrum for Foil Bearings

Journal Bearing

Diameter (inches)

4

3

2

1

020 40 60 80 100 120 140 160 180

Rotor Speed (thousand of rpm)

ACM

ACM

Solar Chiller RVP Gas

Turbine

Turboexpander

Turbocharger

Heat Pump Compressor

Motor Blower

Cryocompressor

Test Spindle

Air Compressor

Auto Gas Turbines

High-Speed Motor

Turbocharger

Textile Spindles

- Size and speed being studied under this project

Proprietary Gas Turbine

Turbocompressors

Heat Pump Compressor

Blower

Blower

ORC Turbine Gen.

ACM = Air Cycle Machine

Anticipated envelope for sCO2 power cycles in the multi-MW class

4 - 6 inches

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Foil Bearings for Miniature Gas Turbine Engine

160,000 rpm 66 mm journal

diameter

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Journal Bearings

Combination Bearings

Thrust Bearings

Foil Bearings for Turbocompressor

180,000 rpm 16 mm journal

diameter

Radial Foil Bearing

Foil Bearing Sizes

Thrust Bearing

Journal BearingTop foil removed

Foil Bearings for Centrifugal Air Compressor

93 mm journal diameter 45,000 rpm

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Applying Foil Bearings to Supercritical CO2 Machinery

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CO2 Fluid Properties

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Source: National Institute of Standards and Technology (NIST)http://webbook.nist.gov/chemistry/fluid/

Above 200°C, viscosity increasesas temperature increases Characteristic of gases Insensitive to pressure

variations Similar to air

Below 200°C, viscosity decreases as temperature increases Characteristic of liquids Very sensitive to pressure

variations A potentially unstable thermal

condition Start-up sequence may be

critical for proper performance

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800

Visc

osity

(µPa

-s)

Temperature (°C)

Viscosity of CO2

5 MPa (vapor)10 MPa20 MPa30 MPaAir (1 atm)

Viscosity Hydrodynamic lubrication simplified by using the Reynolds equation Viscosity is the only property taken into account

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CO2 Fluid Properties

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Source: National Institute of Standards and Technology (NIST)http://webbook.nist.gov/chemistry/fluid/

As flow exits the laminar regime, density becomes an important parameter

Studies have shown a significant increase in power loss as pressure (density) increases (Bruckner and Dellacorte1, Milone2)

The additional power loss must be accounted for in overall system efficiency

The heat generated must be managed to avoid overheating and potential thermal instability

Density The more compressible the fluid, the more other fluid properties influence

bearing design

0

100

200

300

400

500

600

700

800

900

0 200 400 600 800

Den

sity

(kg/

m3)

Temperature (°C)

Density of CO2

5 MPa (vapor)10 MPa20 MPa30 MPa

1. Bruckner, R.J. and DellaCorte, C., “Windage Power Loss in Gas Foil Bearings and the Rotor-Stator Clearance of High Speed GeneratorsOperating in High Pressure Carbon Dioxide Environments”, Supercritical CO2 Power Cycle Symposium, Rensselaer Polytechnic Institute, April29-30, 2009, Troy, NY.

2. Milone, D., "Windage and Gas Foil Bearing Losses in a Supercritical Carbon Dioxide Turbine Generator;" Supercritical CO2 Power CycleSymposium, May 24-25, 2011, Boulder, CO.

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Material Considerations - Strength

Strength of Several Nickel Alloys (Courtesy Haynes International, Inc.)

Inconel X-750 commonly used foil material High strength at elevated temperatures Good fatigue and corrosion resistance Available in a variety of foil thicknesses

René 41 a potential alternative

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Advanced Coatings

Under normal, steady-state operating conditions, there is no contact between the shaft and bearing

During start-up and shut-down, contact is inevitable This is most often the life-limiting aspect of foil bearings By energizing the hydrostatic feature, rubbing during

start-up/shut-down may be avoided

Characteristics of a good coating include Low friction Resistance to wear Good adhesion to the substrate

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Advanced Coatings

A number of high temperature coatings were previously evaluated up to 650°C, with promising results

The best candidates were then evaluated up to 800°C

A new family of coatings is starting to become available, known as Adaptive coatings, or “chameleon” coatings Named due to their ability to adapt to changing

temperature by preserving good tribological properties from 25°C to 1000°C

Due to higher expense and long lead times, these will be evaluated in Phase 2

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Bearing Performance

Project focused on developing a journal bearing Diameter: 63.5 mm (2.50 inches) Length: 44.5 mm (1.75 inches) Speed: 60,000 rpm D⋅N: 3.81 million

Phase 1 testing was performed in air

Estimated Hydrodynamic Load Capacity

0

10

20

30

40

50

60

70

80

0 10000 20000 30000 40000 50000 60000

Load

Cap

acity

(lbf

)

Speed (RPM)

Journal Bearing Load Capacity vs. Speed in Air

2193204316427538649760

Temp(°C)

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

0 10000 20000 30000 40000 50000 60000

Stiff

ness

(lbf

/inch

)

Speed (RPM)

Journal Bearing Stiffness vs. Speed in Air

KxxKyy

Estimated Stiffness under Constant Load

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Hydrodynamic Performance

Phase 2 testing will be in sCO2

Estimated Hydrodynamic Load Capacity

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Hydrostatic Performance

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Adding a hydrostatic component is one method of enhancing a gas foil bearing

Pressurized gas is injected directly into the bearing cavity

Hydrodynamic load capacity often limits gas foil bearing use in some equipment, particularly larger machines running at lower speeds

Supplementing load capacity and stiffness could enable broader use of gas foil bearings

3. Kumar, M., "Analytical and Experimental Investigation of Hybrid Air Foil Bearings," A Thesis submitted to the Office of Graduate Studies ofTexas A&M University, August 2008.

Source: Texas A&M University (Kumar3)

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Hydrostatic Performance

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MSI is studying a proprietary design using CFX (ANSYS, Inc.)

CFD Results of a Single Nozzle

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Hydrostatic Bearing Performance

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Generated Hydrostatic Force and Stiffness vs. Differential Pressure for the Journal Bearing

0

20

40

60

80

100

120

0 10 20 30 40 50 60

Hyd

rost

atic

For

ce (l

b)

Differential Pressure (psi)

Hydrostatic Force in Air

0

5000

10000

15000

20000

25000

30000

35000

0 10 20 30 40 50 60St

iffne

ss (l

bf/in

ch)

Differential Pressure (psi)

Hydrostatic Stiffness in Air

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Hydrostatic Bearing Performance

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Same CFD model used to predict performance in sCO2 Nature of sCO2 required the use of a Real Gas

Properties (RGP) table Table consists of a matrix of fluid properties as a

function of pressure and temperature Unlike the air case, high flow velocities are not present

due to much higher density, tending to keep the Mach number low.

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Validation Testing

Static Testing – Hydrostatic BearingDynamic Testing – Hybrid Bearing

Coating Evaluation

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Static Testing – Hydrostatic Bearing

Prototype hybrid bearings were constructed for both static (zero speed) and high-speed (50,000 rpm) testing

The bearings were installed in MSI’s high-speed test rig Initial static load testing was conducted by varying the load,

applying supply pressure, and measuring the vertical levitation of the shaft

Stiffness was measured by incrementally changing the load and recording the change in shaft position

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0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.0014

0 5 10 15 20 25

Vert

ical

Lift

(inc

h)

Differential Static Pressure (psi)

6 lbf13.5 lbf21 lbf

AppliedLoad

Hydrostatic Lift vs. Applied Static Pressure

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Static Testing – Hydrostatic Bearing

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Air Supply Line

Test Load

Hybrid BearingHousing

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Dynamic Testing – Hybrid Bearing

The same test rig was used to perform high-speed testing to 50,000 rpm

A hybrid foil bearing was installed on the drive end A conventional foil bearing was installed on the opposite end Rotor weight: 17 kg (37 lbm) Rig is driven by an air turbine

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AIR TURBINEDRIVE

FOIL BEARINGHOUSINGS

17 kgTEST SHAFT

TRADITIONALFOIL BEARING

RADIAL BUMPERS

HYDROSTATIC-ASSISTSUPPLY PORTS

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Dynamic Testing – Hybrid Bearing

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MSI’s High Speed Foil Bearing Test Rig Installed in Test Cell

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Dynamic Testing – Results

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Synchronous Vibration

At 30k rpm the rig experienced very stable operation

Vibration levels were very low

30krpm, 0 psi.05

0

MilsRMS

Turbine End: X & Y Probes Thrust End: X & Y Probes

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0

MilsRMS

Turbine End: X & Y Probes Thrust End: X & Y Probes30krpm, 10 psi

Dynamic Testing – Results

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Sub-Synchronous Vibration

At 30k rpm the rig experienced very stable operation

Vibration levels were very low

As hydrostatic air pressure was added (10 psi), a small sub-synchronous vibration (~79 Hz) was present

The sub-synchronous vibration grew as pressure was increased to 15 psi, but diminished at 20 psi

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Advanced Coating Testing

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High Temperature Start/Stop Cycle Testing

DRIVE MOTOR

PILLOW BLOCKBEARINGS

TEST SHAFT

HOT BOX

TORQUE ARMW/ LOAD CELL

TEST BEARING

LOAD

Rig Capabilities Automatic start/stop cycling Constant applied load 17,000 RPM 650°C Continuous torque monitoring

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Start/Stop Cycle Testing

Sample Bearings and Test Shaft After Cycling

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100 Cycles, 20°C 500 Cycles, 540°C 500 Cycles, 650°C

100 Cycles, 20°C 500 Cycles, 540°C 500 Cycles, 650°C

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Start/Stop Cycle Testing - Results

Coating successfully passed the cycle testing Test shaft shows evidence of some coating transfer Shaft has appearance of a typical burnishing

operation No evidence of adhesive wear was found Test shaft measured 8 microns (0.0003 inch) of

wear (10% of bearing clearance) after initial room temperature run

No measurable wear after high temperature cycles Wear of the top foil was 3-5 microns

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Ongoing Work

Phase 2 Continue to optimize design and refine analytical

models incorporating both hydrostatic and hydrodynamic bearing behavior

Include intra-bearing interaction among the bearing foils

Generate both optimized and practical (in sCO2) journal and thrust bearing designs

Conduct bearing validation tests in sCO2environment at Sandia National Labs

Continue evaluation and improvement of high temperature, low-wear coatings

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Acknowledgment

This material is based upon work supported by the U.S. Department of Energy,Office of Science, Office of Fossil Energy, under Award Number DE-SC0013691.

Disclaimer

This report was prepared as an account of work sponsored by an agency of theUnited States Government. Neither the United States Government nor anyagency thereof, nor any of their employees, makes any warranty, express orimplied, or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privately owned rights.Reference herein to any specific commercial product, process, or service bytrade name, trademark, manufacturer, or otherwise does not necessarilyconstitute or imply its endorsement, recommendation, or favoring by the UnitedStates Government or any agency thereof. The views and opinions of authorsexpressed herein do not necessarily state or reflect those of the United StatesGovernment or any agency thereof.

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Stop By Our PosterPeter Chapman 973-326-9920 ext. 144; pac@mechsol.com