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 Luis San Andrés Mast-Childs Professor Fellow ASME Texas A&M University Keun Ryu Research Assistant Texas A&M University Tae Ho Kim Senior Research Scientist Korea Institute of Science and Technology ASME TURBO EXPO 2010, Glasgow, Scotland, UK ASME paper GT Thermal Management and Rotordynamic Performance of a Hot Rotor-Gas Foil Bearings System Part 1: Measurements J. Eng. Gas Turbines Power (in press)

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 with adequate load capacity and high temperature capability Tolerant to misalignment and debris, Need coatings to reduce friction at start-up & shutdown Damping from dry-friction and operation with limit cycles Gas Foil Bearings – Bump type

Gas Foil Bearings Issues 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

Heshmat et al (2005): demonstrates hot (650C) GFB operation in a turbojet engine to 60 krpm. Cooling flow rates to 570 L/min still give large axial thermal gradients (13ºC/cm) Dykas (2006): investigates thermal management in foil thrust bearings. Cooling flow rates, to 450 L/min, increase bearing load capacity at high rotor speeds. Inadequate thermal management can give thermo-elastic distortions affecting load capacity of test FB Overview – Thermal management Radil et al (2007): evaluate effectiveness of three cooling methods (axial cooling, direct and indirect shaft cooling) for thermal management in a hot GFB environment San Andrés et al (2009): forced cooling flow has limited effectiveness at low rotor temperatures. At high test temperatures, large cooling flows (turbulent) remove heat more efficiently Gases have limited thermal capacity, hence (some) bearings demand large cooling flows to remove heat from hot rotor sections.

To develop a detailed, physics- based computational model of gas-lubricated foil journal bearings including thermal effects to predict bearing performance. The result of this work shall include a fully tested and experimentally verified design tool for predicting gas foil journal bearing torque, load, gas film thickness, pressure, flow field, temperature distribution, thermal deformation, foil deflections, stiffness, damping, and any other important parameters. Main Objective Agreement NASA NNH06ZEA001N-SSRW2

TAMU Foil Bearing research highlights yearTopic Metal Mesh Foil Bearings: construction, verification of lift off performance and load capacity, identification of structural stiffness and damping coefficients, identification of rotordynamic force coefficients Performance at high temperatures, temperature and rotordynamic measurements Thermoelastohydrodynamic model for prediction of GFB static and dynamic forced performance at high temperatures Integration of Finite Element structure model for prediction of GFB static and dynamic forced performance Effect of feed pressure and preload (shims) on stability of FBS. Measurements of rotordynamic response Rotordynamic measurements: instability vs. forced nonlinearity? Model for ultimate load capacity, Isothermal model for prediction of GFB static and dynamic forced performance Measurement of static load capacity, Identification of structural stiffness and damping coefficients. Ambient and high temperatures

Test foil bearings Generation II. Diameter: 38.1 mm 25 corrugated bumps (0.38 mm of height) Reference: DellaCorte (2000) Rule of Thumb UNCOATED TOP Foil for HT operation

Objectives & tasks Quantify effect of rotor temperature on performance of rotor-GFBs system Use cartridge heater to heat hollow rotor to high temperature (up to 360C) Measure bearing & rotor surface temperatures & measure rotordynamic performance during shaft speed coast downs (from 30 krpm) for increasing shaft temperatures Quantify effect of side air flow on cooling bearings (max. 150 L/min per bearing)

TAMU Hot rotor-GFB test rig Insulated safety cover Infrared thermometer Flexible coupling Drive motor Cartridge heater Test GFBs Test hollow shaft (1.1 kg, 38.1mm OD, 210 mm length) Tachometer Eddy current sensors Hot heater inside rotor spinning 30 krpm Gas flow meter (Max. 500 LPM). Drive motor (max. 50 krpm) ) Instrumentation for high temperature. Insulation casing Max. 360°C

Hot rotor-GFB test rig Dimensions FOIL BEARINGS Heater

Temperatures in test rig Foil bearings Cartridge heater Heater stand T14 T10 T12 T13 Coupling cooling air Drive motor T15 T16 Th Hollow shaft T5 T11 Tamb T1 T4 T3 T2 45º Free end (FE) GFB g 45º 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)

Top foil thickness,Bump foil thickness,Bump height,Gas Constant,Viscosity,1.73Conductivity,Density, Test Foil Bearings Parameter [mm] Bearing cartridge Outer diameter50.85 Inner diameter39.36 Top foil and bump strip layer Top foil axial length38.2 Top foil thickness0.100 Bump foil thickness0.100 Number of Bumps25 × 5 axial Bump pitch4.581 Bump length3.742 Bump height0.468 Bump arc radius5.581 Bump arc angle [deg]36.5 Elastic Modulus 214 GPa, Poisson ratio=0.29 Foster-Miller FB uncoated (Gen II)

Significant temperature gradient along rotor axis Heat source warms (unevenly) rotor and its bearings Rotor OD Temperature at increasing heater temperature T11T11T FEB TcT DEB T12T13 Cartridge heater location Ambient temp. (T a ) ~ 22 °C Rotor out of its bearings Cartridge heater lose inside hollow rotor

AIR SUPPLY Gas pressure Max. 100 psi Cooling flow needed for thermal management : to remove heat from shear drag or to reduce thermal gradients from hot to cold engine sections Cooling gas flow into GFBs heater

AIR SUPPLY Gas pressure Max. 100 psi Heater warms unevenly hollow rotor. Cooling gas flow into GFBs heater Side forced flows cool unevenly rotor. Ultimate effect is to also cool the cartridge heater! High T Cold T

Video: Operation of hot rotor-GFB test rig

Top foil thickness,Bump foil thickness,Bump height,Gas Constant,Viscosity,1.73Conductivity,Density, Test Conditions Test condition # Rotor speed Axial cooling flow per bearing (L/min) Heater temperature T hs ( ºC) Test time (minutes) Cond. 1 Coast down from 30 krpm No cooling No heating, 100, 200, 300, Cond. 2 Fixed rotor speed = 30 krpm No cooling 60 Cond. 350 Cond No heating, 100, 200 Cond No heating, 100, 165

Verification of rotor-bearing system response FH FV DH DV g OP Cond: Room temp. Baseline, No cooling, No heating Mass m i (g), In phase Displacement u (µm) DEFEDEFE U1U U2U U3U Normalized rotor synchronous response amplitudes (coastdown) Response is proportional to mass imbalance Rotordynamic model with linearized GFB force coefficients predicts rotor behavior correctly Drive End Free End

Baseline. No cooling Similar responses – free of sub synchronous whirl motions Waterfalls of rotor motion Drive End (Horizontal) (a) Heater off (a) Heater on, T hs =360 C trace 1 X_left WF X_left Xaxis Frequency [Hz] Amplitude [ µ m, 0 - pk] 1X 2X 30krpm 2 14krpm Test #1

Baseline imbalance. No forced cooling Rotor 1X motions for cold and hot conds No heatingT hs =360C Flexible rotor mode at 29 krpm (480 Hz): Soft coupling and connecting rod Critical speed (Rigid body mode) ~ 13 krpm No major differences in responses between cold and hot Test #1

DH Heater up to 360C. Baseline. No cooling Cartridge temperature (T hs ) increases As T hs increases to 360ºC, peak motion amplitudes between 7~15 krpm decrease. natural frequency Rotor 1X motions for cold and hot conds DH Drive End (Horizontal) Test #1

Time for rotor coast down Baseline, Heater up to 360C. No forced cooling Coastdown time reduces as rotor heats (lesser clearance) Cartridge temperature (T hs ) increases Exponential decay Long time to coastdown : very low viscous drag (no contact between rotor and bearings) Test #1 Effect of temperature Coastdown time (s) Speed (krpm)

Steady increase (proportional to HEATER T hs ) No forced cooling flow Temperatures in rotor and bearings Test #1 Heater up to 360C Rotor speed : 30 krpm Heater temperature increases

Heating of rotor Baseline. No cooling flow & 50 L/min T 1 -Tamb T 11 -Tamb T 6 -T amb T 12 -T amb 10 krpm20 krpm30 krpm : Temp. drop due to 50L/min cooling flow Bearing cartridge and rotor temperatures increase steadily with time Speed makes rotor and bearings hotter Cooling flow removes heat from shear dissipation in rotor, most effective at high speed Heater OFF Tests#2,3 Rotor speed : 10, 20, 30 krpm Test time (min) Effect of rotor speed and side cooling Temperature rise (C)

Heating of rotor at steady state Baseline. No side flow and 50 L/min T 1 -T amb T 11 -T amb T 6 -T amb T 12 -T amb : Temp. drop due to 50L/min cooling flow Side flow removes heat from shear drag energy in rotor. Thermal gradient Hot to cold FE rotor >FEB > DEB > DE rotor Heater cartridge OFF Tests#2,3

Heater power is limited Cooling>100 LPM cools both rotor & HEATER! 21C100C200C 300C 360C No cooling & 50L/min 100L/min 150L/min Effect of cooling flow Heater temperature Tests #2-5 Heater up to 360C Rotor speed : 30 krpm w/o & w cooling flows Heater temperature increases Test time (min)

High temp. (heater up to 360C). Cooling flow up to 150 L/min rotor speed : 30 krpm Cooling effective > 100 LPM, when heater at highest temperature T 1- T amb T 6- T amb 21C100C200C 300C 360C Cooling flow increases Effect of cooling flow Bearings OD temperatures Tests #2-5 Heater temperature increases Test time (min)

Effect of cooling flow Cooling flow increases T 1- T amb T 6- T amb Cartridge temperature (T hs ) increases 200C 100C No heating Bearing OD temperature decreases with cooling flow Turbulent flow > 100 LPM Bearing cartridge temperature Tests 2-5 Cooling flow (LPM) High temp. (heater up to 360C). Cooling flow to 150 LPM rotor speed : 30 krpm Temperature rise (C)

No heating 360C 50 L/min No cooling Overall coastdown time reduces by 20% (13 s) with cooling flow of 50 LPM. Time for rotor coast down Effect of cooling rate Tests #2,3

Post-test condition of rotor and bearings Before operation FEDE After extensive heating with rotor spinning After extensive hearing with rotor spinning tests FEDE UNCOATED top foil ! Wear marks on top foils are at side edges Rotor shows polishing marks at bearing locations. Deep wear marks at outboard edges Static load direction

 Foil Bearings continue to survive high temperature operation – Still working ! Conclusions  Amplitudes of rotor synchronous motion are proportional to added imbalance masses.  Thermal management with axial cooling streams is beneficial at high temperatures and with large flow rates that ensure turbulent flow conditions.  As rotor and bearing temperatures increase, air becomes more viscous and bearing clearances decrease; hence coastdown time somewhat decreases.  For operation with hot shaft, amplitude of rotor motion drops while crossing (rigid body mode) critical speed.

Acknowledgments Thanks support of NASA GRC ( ) & Dr. Samuel Howard NSF ( ), TRC ( ) Foster-Miller (FBs) Learn more Questions ? Next presentation shows predictions benchmarked to test data

Rotordynamic tests for hot rotor operation 2008: Effects of hot rotor & axial cooling flow As T c increases, critical speed increases by ~ 2 krpm and the peak amplitude decreases. Heater temp. 22 °C Heater temp. 93 °C Heater temp. 132 °C Cartridge temperature (T c ) increases Coastdown Rotor coastdown test ASME J. Tribol., Vol. 132 DE – vertical plane