GAS FOIL BEARINGS FOR OILFREE ROTATING MACHINERY

1
28th Turbomachinery Research Consortium Meeting
Rotordynamic Performance of Foil Gas Bearings
Tests and Analysis Part I Rotordynamic
Measurements of a High Temperature Rotor
Supported on Gas Foil Bearings (TRC-BC-2-08) Par
t II Thermohydrodynamic Analysis of Bump type
Gas Foil Bearings Model and Predictions
(TRC-BC-3-08) Tae-Ho Kim and Luis San Andrés
This material is based upon work supported by
NASA Research Announcement NNH06ZEA001N-SSRW2,
Fundamental Aeronautics Subsonic Rotary Wing
Project 2 32525/39600/ME and the Turbomachinery
Research Consortium
2
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 (large load capacity to 100 psi)
• 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

3
Gas Foil Bearings (/-)

• Increased reliability large load capacity (lt 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
  foil-bumps support (dry-friction damping)

4
Foil Bearing Research at TAMU
Reference DellaCorte (2000) Rule of Thumb

• Test Gas Foil Bearing (Bump-Type)
• Generation II. Diameter 38.1 mm
• 25 corrugated bumps (0.38 mm of height)

5
Rotordynamic test with side pressurization
2007 Effect of side pressurization, shimmed
GFBs, validation of predicted rotor responses
Rotor onset speed of subsync. whirl motions
increases as side feed pressure increases
agreement between predictions and
measurements!
6
Research objectives 07/08

• TRC-BC-1-08Rotordynamic Measurements of a High
  Temperature Rotor Supported on Gas Foil Bearings
• To revamp test rig using a cartridge heater for
  high temperature operation
• To determine the rotordynamic performance of a
  high temperature rotor supported on GFBs
• To quantify the effect of feed gas flow cooling
  GFBs
• TRC-BC-2-08
• Thermohydrodynamic Analysis of Bump type Gas Foil
  Bearings Model and Predictions
• To develop a computational model for design and
  performance prediction of GFBs operating in high
  temperature environments.
• (Gas film thermohydrodynamic model FE
  structural model)

7
Rotordynamic test rig setup GFBs
Rotordynamic test rig revamped with a cartridge
heater and instrumentation for operation at high
temperature.
Drive motor (25 krpm). Cartridge heater max.
temperature 300F Air flow meter (Max. 100 L/min
at 14 psig)
GFB housing
Driving motor
8
Schematic view of instrumentation setup
Ambient temp. (Ta) 22 C (71 F)
Speed sensor
Hollow test rotor
Flexible coupling
DC electric motor
Eddy current sensor (Max. 177 ºC)
Numbers in circles show locations of temperature
measurement.
9
Schematic representation of test GFB
at
Bump-type foil gas bearings with five (5)
thermocouples placed within machined axial slots.
10
Test cases for different heater temp. (Tc)
Heater temperature
38 hours
Test cases for increasing heater temperatures
(Tc) and cooling conditions
11
Bearing ID Temperature (TDBi -Ta)/Ta
Ambient temp. (Ta) 22 C (71 F)
Bearing support
Tc 132 C
Tc 93 C
Tc 22 C
Test rotor
Ta 22 C
Test GFB
Cross-section view of test rig (drive end)
Cases 1-3 without cooling flow, Cases 4-6 with
cooling flow (56 L/min)
As heater Tc and rotor speed O increase, bearing
temperature also increases. Axial cooling flow
decreases the bearing ID temperature.
12
Bearing OD Temperature (TDBo -Ta)/Ta
Ambient temp. (Ta) 22 C (71 F)
Bearing support
Test rotor
Tc 132 C
Tc 93 C
Tc 22 C
Ta 22 C
Test GFB
Cross-section view of test rig (drive end)
Cases 1-3 without cooling flow, Cases 4-6 with
cooling flow (56 L/min)
Bearing OD has lower temperature than bearing ID
(midplane) for Tc 93 C and 132 C due to heat
flow into the bearing support and to surrounding
ambient
13
Rotor speed up response Synchronous
Case 1-3 without cooling flow, Ta 22C, and Tc
22C, 93 C, and 132C
Rotor speed-up test
Drive end vertical plane
Above critical speed 14.5 krpm, 1X amplitude
drops suddenly, thus implying a nonlinearity. As
Tc increases, the peak amplitude decreases.
14
Rotor coastdown response Synchronous
Case 1-3 without cooling flow, Ta 22C, and Tc
22C, 93 C, and 132C
Rotor coastdown test
Drive end vertical plane
As Tc increases, critical speed increases by 2
krpm and the peak amplitude decreases.
15
Waterfall plots coastdown response, direct amp.
Case 1-3 without cooling flow, Ta 22C, and Tc
22C, 93 C, and 132C
Free end vertical plane
1X component dominant, i.e., no subsynchronous
motion, during coastdown tests
Cartridge temperature (Tc) increases
16
Prediction of pressure and temperature fields
GFB model
Convection of heat by fluid flow diffusion to
bounding surfaces compression work dissipated
energy
17
Heat flux paths in rotor - GFB system
Simple representation in terms of thermal
resistances within a GFB supporting a hot hollow
shaft Heat conducted into the bearing Cooling
gas stream carries away heat
Heat flow model
18
Geometry and operating conditions
GFB model Generation I GFB
with single top foil and bump strip layer
Gas viscosity density conductivity, foil
Youngs modulus, and clearance change with
temperature.
19
Predicted film pressure and temperature
Static load 5N 40 krpm
Heat convection
TshaftTbearing Tambient
25 C
0 lt ? lt 200 Temperature increases due to shear
induced mechanical energy. ? gt 200
Temperature drops due to gas expansion
(cooling gas film).
20
Film peak temperature versus static load
40 krpm
Heat convection to walls Adiabatic walls
TshaftTbearing Tambient
25 C
Film peak temperature increases as static load
increases. Adiabatic walls model predicts
higher temperature, in particular for large
static loads.
21
Journal eccentricity Film thickness
40 krpm
Heat convection or Adiabatic walls Isothermal
flow
TshaftTbearing Tambient
25 C
Isothermal flow model predicts largest journal
eccentricity and smallest minimum film
thickness Adiabatic walls model predicts
smallest journal eccentricity and largest minimum
film thickness
22
Temperature vs speed for increasing Tshaft
Static load 5N
Heat convection
Tshaft 25 C 150 C
Shaft temperature
Tbearing Tambient 25 C
150 C
100 C
As shaft temperature increases, peak film
temperature increases. Gas film has a lower
temperature than shaft since heat flows to the
outer bearing housing and due to the mixing of
the recirculation flow with fresh air.
50 C
25 C
23
Clearance vs Speed for increasing Tshaft
Static load 5N
Heat convection
Shaft temperature
Tshaft 25 C150 C
25 C
50 C
Tbearing Tambient 25 C
100 C
150 C
As shaft temperature thermal expansion increases
OD, thus reducing bearing clearance. As shaft
speed increases shaft centrifugal growth further
reduces bearing clearance.
24
Clearance vs Speed for increasing Tshaft
Static load 5N
Heat convection
Tshaft 25 C150 C
Shaft temperature
Tbearing Tambient 25 C
100 C
150 C
25 C
50 C
100 C
150 C
As shaft temperature increases, journal
eccentricity decreases.
Minimum film thickness increases with temperature
at low rotor speed but decreases with
temperature at high rotor speeds
25
Torque vs Speed for increasing Tshaft
Static load 5N
Heat convection
Shaft temperature
Tshaft 25 C150 C
150 C
Tbearing Tambient 25 C
100 C
50 C
25 C
Bearing drag torque is proportional to shaft
speed and shaft temperature
26
Effect of cooling flow
Heat convection
Rotor speed 40 krpm
Tshaft TbearingOD 150 C
Tcooling Ta 25 C
100 N
75 N
50 N
25 N
Cooling flow effectively decreases gas film
temperatures. As the static load increases, the
peak temperature increases
27
Conclusions

• As heater temperature increases and as rotor
  speed increases, bearing temperature increases.

• Axial cooling flow decreases bearing temperature

• As heater temperature increases to 132 C,
  system critical speed increases by 2 krpm and
  peak amplitude decreases.

• Developed GFB model with thermal energy
  transport, axial cooling flow, and thermoelastic
  deformation of top foil and bump strip layers

• Increase in shaft temperature causes significant
  thermal growth, thus reducing bearing clearance.
  Axial cooling flow effectively decreases gas film
  temperature.

• Foil Bearings continue to survive high
  temperature operation Still working !

28
TRC Proposal Rotordynamic Performance of a High
Temperature Rotor Supported on Foil Gas Bearings
Predictions Anchored to Test Data

• Validate model predictions to measurements
  (Static load parameters Dynamic force
  coefficients)
• Revamp test rig and Conduct experimentation.
• Replacements High temperature foil gas
  bearings (400 ºC), High temperature sensors (400
  ºC), High speed electric motor (50 krpm)
  budget from NASA GRC project.
• - Rotor lift off and touch down speeds, load
  capacity and drag power, identification of
  synchronous speed force coefficients and regime
  of stability.

TASKS
Budget from TRC for 2008/2009 Support for
graduate student (20 h/week) x 1,700 x 12
months 25,538 Fringe benefits (2.5) and
medical insurance (469/month) Tuition three
semesters (3,996 x 3), 11,988 Supplies
test rig (thermocouples and thermometers)
1,474 Total Cost


40,000
29

• Back up slides

30
Calibration of electric cartridge heater
Measured surface temperatures of heater and
hollow shaft vary along its axial length (Z).
31
Bearing ID Temperatures (TDBi)
Bearing ID temperature varies along bearing
circumference.
Heater temperature varies along heater cartridge
circumference. (Heater temp. with max. standard
deviation 10.7 C at Tc 93 C).
32
Shaft surface condition after heating
Color changes as the axial length (or the surface
temperature) increases. Careful inspection of
discoloration hints to rotor temperature profile.
33
Rotor surface Temperature (TDS -Ta)/Ta
Ambient temp. (Ta) 22 C (71 F)
Bearing support
Tc 132 C
Tc 93 C
Tc 22 C
Test rotor
Test GFB
Cross-section view of test rig (drive end)
Cases 1-3 without cooling flow, Cases 4-6 with
cooling flow (56 L/min)
Shaft temperature at rotor drive END.
Temperatures lower than those for ID and OD of
bearings.
34
End Bearing support temperature (TDBS -Ta)/Ta
Ambient temp. (Ta) 22 C (71 F)
Bearing support
Tc 132 C
Tc 93 C
Tc 22 C
Test rotor
Test GFB
Cross-section view of test rig (drive end)
Cases 1-3 without cooling flow, Cases 4-6 with
cooling flow (56 L/min)
Increase in bearing support temperature due to
heat flows through bearing sleeve. Temperatures
higher than those measured at rotor drive end.
35
Temperatures (Tk -Ta)/Ta k DBi, DBo, DS, and
DBS
Case 4 with cooling flow (56 L/min) and Tc Ta
22C
In general, all temperatures show similar values
with max differences of 0.1. Temperatures
increase as rotor speed increases.
36
Rotor coastdown response Phase angle
Case 1-3 without cooling flow, Ta 22C, and Tc
22C, 93 C, and 132C
Rotor coastdown test
As Tc increases, slope of phase angle becomes
mild when crossing 90 , thus implying increase
in damping