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Thermal & Fluids Analysis Workshop TFAWS 2011 August 15-19, 2011 NASA Langley Research Center Newport News, VA

Variable Conductance Heat Pipe for a Variable Thermal Link C. J. Peters, J. R. Hartenstine, C. Tarau, & W. G. Anderson Advanced Cooling Technologies, Inc. Bill.Anderson@1-act.com. TFAWS Paper Session. Presented By Calin Tarau. Thermal & Fluids Analysis Workshop TFAWS 2011 August 15-19, 2011

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Thermal & Fluids Analysis Workshop TFAWS 2011 August 15-19, 2011 NASA Langley Research Center Newport News, VA

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  1. Variable Conductance Heat Pipe for a Variable Thermal LinkC. J. Peters, J. R. Hartenstine,C. Tarau, & W. G. Anderson Advanced Cooling Technologies, Inc.Bill.Anderson@1-act.com TFAWS Paper Session Presented By Calin Tarau Thermal & Fluids Analysis WorkshopTFAWS 2011August 15-19, 2011 NASA Langley Research CenterNewport News, VA

  2. Design Targets Variable Thermal Links Variable Conductance Heat Pipe Design Testing Conclusions and Recommendations Presentation Outline ISO:9001-2000 / AS9100-B Certified

  3. International Lunar Network Trade Study • Objective: Develop Variable Thermal Link designs to be used for Thermal Management of the Warm Electronics Box (WEB) on the International Lunar Network (ILN) Anchor Node mission • Remove ~ 60 W during the lunar day • Conserve heat to keep the electronics and battery warm during the lunar night ISO:9001-2000 / AS9100-B Certified

  4. Design Targets Solar power controls, Maximum Day and Minimum Night ISO:9001-2000 / AS9100-B Certified

  5. Design Targets • Minimizing power usage at night is extremely important • 1 W power = 5 kg Batteries! • 20° tilt means that conventional grooved aluminum/ammonia CCHPs can not be used in the WEB to isothermalize the system • Maximum Adverse Elevation: 13.3 inch ISO:9001-2000 / AS9100-B Certified

  6. Variable Thermal Link • Three basic elements to the WEB thermal control system • A method to isothermalize the electronics and battery during the lunar night, and to remove heat to a second, variable conductance thermal link during the day (Constant Conductance Heat Pipes (CCHPs)). • A variable thermal link between the WEB and the Radiator • A radiator to reject heat • Possible Thermal Links • Variable Conductance Heat Pipes (VCHPs) • Loop Heat Pipes (LHPs) • LHPs with a Thermal Control Valve ISO:9001-2000 / AS9100-B Certified

  7. LHP Shut-Down • Need to shut down LHP during the Lunar night • Minimize Heat Losses from the WEB • Standard method uses a heater on the compensation chamber • During normal operation, the Compensation Chamber runs at a lower temperature than the LHP evaporator • Activate heater to shut down • Increase saturation temperature and pressure of LHP • Cancels the pressure difference required to circulate the sub-cooled liquid from the condenser to the evaporator • Standard method validated in spacecraft • 1 W = 5 kg • Develop variable thermal links with no power requirement • LHP with Thermal Control Valve – discussed in separate presentation • VCHP ISO:9001-2000 / AS9100-B Certified

  8. VCHP Design Constraints • VCHP differs from normal VCHP in 5 different ways • Need to operate in space, and on the Lunar surface • Need to operate with fairly large tilts in the evaporator • Slope can vary from -20° to +20° • ~13 inch adverse elevation across the WEB • Grooved CCHPs operate with 0.1 inch adverse tilt • Requires non-standard wick • Tight temperature control not required • Have a ~40°C range versus ±1°C for conventional VCHPs • No power available for reservoir temperature control • 1 W = 5 kg • External reservoir will cool down to ~140 K • Need to minimize heat leak when shut down ISO:9001-2000 / AS9100-B Certified

  9. VCHP Design • Develop a VCHP design with three novel features • Hybrid-Wick • Screen wick in evaporator, grooved wick in condenser • Allows operation on the Lunar surface, and during transit • Reservoir Near Evaporator • Keeps the reservoir warm at night • Minimizes the reservoir size • Bimetallic Adiabatic Section • Grooved stainless steel section in the adiabatic section acts as a thermal dam to minimize heat leak during shutdown ISO:9001-2000 / AS9100-B Certified

  10. Standard VCHPs use grooved wick - not suitable for Moon 0.1 inch against gravity Tilt range for lunar surface: ±14° VCHP evaporator needs to operate against gravity Maximum adverse elevation: (9 inch) × sin(14°) = 2.2 inch Screen wick in evaporator; Grooved wick in condenser Grooves and screen pump in space Screen pumps on lunar surface ILN Anchor Node – Hybrid Wick +14°, Evaporator Gravity Aided -14°, Evaporator Works Against Gravity 0°, Puddle Flow in Evaporator ISO9001-2008 & AS9100-B Certified

  11. Overall Design – NCG Reservoir Adjacent to Evaporator • Reservoir is located near evaporator instead of condenser • Placing near condenser is standard for most spacecraft VCHPs with electric heaters • Condenser is too cold • Would require oversized reservoir • Location of reservoir inside WEB ensures that its temperature will be regulated • NCG tube connects reservoir to condenser NCG Tube Radiator Condenser NCG Tube NCG Reservoir Condenser Al SS Al NCG Reservoir Bimetallic Transition Evaporator Adiabatic Evaporator ISO9001-2008 & AS9100-B Certified

  12. VCHP Design – NCG Reservoir Adjacent to Evaporator Radiator WEB Enclosure Condenser (Grooves) Adiabatic (Grooves) NCG Reservoir Evaporator (Screen) ISO9001-2008 & AS9100-B Certified

  13. ILN Anchor Node – NCG Reservoir Adjacent to Evaporator • Standard condenser location gives much higher mass • Vertical Asymptotes • Reservoir Near Evaporator: 0 K • Reservoir Near Condenser: ≈29.78 K ISO9001-2008 & AS9100-B Certified

  14. VCHP with Internal Reservoir Adiabatic Section Condenser NCG Reservoir Evaporator ISO:9001-2000 / AS9100-B Certified

  15. VCHP with Internal Reservoir NCG Reservoir Evaporator Condenser Adiabatic Section Heating Block Cooling Block ISO:9001-2000 / AS9100-B Certified

  16. VCHP Testing – Objectives • Lunar Surface Operations (1/6 g) • Freeze tolerance; conductance of “on” versus “off” states • Performance in adverse gravity orientations • Space Operations • Thermal diode behavior • Thermal Performance ISO9001-2008 & AS9100-B Certified

  17. VCHP Testing – Instrumentation TC Locations ISO9001-2008 & AS9100-B Certified

  18. Task 3. VCHP Testing – Lunar Freeze/Thaw • Purpose • Demonstrate ability to shut down • Demonstrate ability to startup and operate for brief periods of time when cold • Determine overall thermal conductance • Procedure • Vary sink conditions to simulate lunar cycle • -60ºC (liquid) and -177ºC (frozen) • Several orientations • -2.3º & +2.3º • Condenser nearly vertical • Adiabatic and condenser sections gravity aided • 25ºC initial evaporator temperature • Measure performance • Evaluate temperature gradients across heat pipe; conductances; input power ISO9001-2008 & AS9100-B Certified

  19. Task 3. VCHP Testing – Lunar Freeze/Thaw Results TC27 (Cond) TC1 (Gas) TC26 (Cond) TC10 (Evap) TC30 (Cond) TC23 (Cond) Power ISO9001-2008 & AS9100-B Certified

  20. Task 3. VCHP Testing – Lunar Freeze/Thaw Results VCHP Operation (25 °C, 95 W, -2.3°) ISO9001-2008 & AS9100-B Certified

  21. Task 3. VCHP Testing – Lunar Freeze/Thaw Results VCHP Cold Shutoff (-60 °C, 0.2 W, -2.3°) ISO9001-2008 & AS9100-B Certified

  22. Task 3. VCHP Testing – Lunar Freeze/Thaw Results VCHP Very Cold Shutoff (-177 °C, 0.1 W, -2.3°) ISO9001-2008 & AS9100-B Certified

  23. Task 3. VCHP Testing – Lunar Freeze/Thaw Results • VCHP can undergo freeze/thaw cycles without performance degradation • Effectively shuts off at cold temperatures and reduces heat transfer • VCHP can experience short-duration full-power bursts during -60 °C and -177 °C cold shutdown • Evaporator stays within -10 °C to +50 °C temperature range with no power except heat in-leak ISO9001-2008 & AS9100-B Certified

  24. Task 3. VCHP Testing – Lunar Performance • Purpose: Demonstrate thermal performance in a simulated lunar environment • Test Procedure – 1 temperature, 3 elevations • -2.3º, 0º and +2.3º inclinations • Condenser nearly vertical • Adiabatic and condenser sections gravity aided • 25ºC evaporator temperature • Test Results Summary • 220W @ -2.3º • 212W @ 0º • 220W @ +2.3º • Dryout was not demonstrated, test was terminated based on elevated temperature on TC9 • Possible gap between evaporator wall and screen wick resulting in a “hot spot” • 2 × target power ISO9001-2008 & AS9100-B Certified

  25. Task 3. VCHP Testing – Lunar Performance Results Temperature Profile (25 °C, 220 W, -2.3°) ISO9001-2008 & AS9100-B Certified

  26. Task 3. VCHP Testing – Lunar Performance Results • Powers demonstrated are twice maximum target power • Pipe can operate against lunar gravity • Evaporator stays within -10 °C to 50 °C target temperature range ISO9001-2008 & AS9100-B Certified

  27. Task 3. VCHP Testing – Space • Thermal diode • Backward operation (reverse heat input/output & elevation) • Measure heat transport in reverse direction • Thermal Performance • Determine dryout • Extrapolate dryout power to 0-g • For All Space Tests • Near horizontal • Vary adverse elevation of evaporator (0.1 in, 0.2 in, 0.3 in) ISO9001-2008 & AS9100-B Certified

  28. Task 3. VCHP Testing – Space Thermal Diode • The purpose of this test is to demonstrate that the pipe can behave as a diode in space • Test Procedure – 3 elevations, 1 evaporator temperature • 0.1”, 0.2” and 0.3” adverse elevation • 25ºC evaporator temperature • 20ºC ΔT between evaporator and condenser • Determine reverse heat transfer rate required to meet 20ºC ΔT requirement • Conservative NCG charge • Test Results • 0.1 inch, 4.3 watts, -0.0195 W/°C • 0.2 inch, 3.2 watts, -0.0157 W/°C • 0.3 inch, 3.2 watts, -0.0160 W/°C ISO9001-2008 & AS9100-B Certified

  29. Task 3. VCHP Testing – Space Thermal Diode Results Thermal Diode Temperatures (Evaporator at 25 °C, 0.1 Inch Adverse) ISO9001-2008 & AS9100-B Certified

  30. Task 3. VCHP Testing – Space Thermal Diode Results • Pipe is an effective thermal diode • Pipe has very low thermal conductance • Pipe reduces reverse heat transfer (transports only 4 % of maximum power) ISO9001-2008 & AS9100-B Certified

  31. Task 3. VCHP Testing – Space • Purpose: Demonstrate thermal performance in a simulated space environment • Test Procedure – 3 elevations, 1 temperature • 0.1”, 0.2” and 0.3” adverse elevation • 25ºC evaporator temperature • Pipe operation with NCG and without NCG • Possible asymmetry • Flipped pipe 180º • Marked improvement in performance ISO9001-2008 & AS9100-B Certified

  32. Task 3. VCHP Testing – Space Performance Results (25 °C, No NCG) Zero-g power extrapolated ISO9001-2008 & AS9100-B Certified

  33. Task 3. VCHP Testing – Space Performance Results • Pipe carries approximately 72 % of the zero-gravity target power • Possible contributing factors causing asymmetry and lower than expected thermal performance • Screen attachment resulting in a gap between the wall and wick • Interface between screen and grooves resulting in a larger than designed hydraulic joint ISO9001-2008 & AS9100-B Certified

  34. Conclusions and Recommendations • Variable Thermal Link can be provided by • Loop Heat Pipe • LHP with Thermal Control Valve • Variable Conductance Heat Pipe • VCHP has the following benefits • No power to shutdown • Least expensive • However, lowest TRL level • VCHP was developed with the following • Hybrid-Wick, to allow the VCHP to operate with a tilt • Reservoir Near Evaporator, to minimize the reservoir size • Bimetallic Adiabatic Section, to minimize axial heat leak to the cold radiator during shutdown ISO:9001-2000 / AS9100-B Certified

  35. Conclusions and Recommendations • Simulated Lunar performance testing demonstrated • Shuts off at cold temperatures and reduces heat transfer • Freeze/thaw cycles without performance degradation and accommodated short-duration full-power bursts during -60 °C and -177 °C cold shutdown • Design can meet target power at adverse elevations • Demonstrated start up with frozen condenser and can operate briefly at low condenser temperatures • Simulated 0-g testing demonstrated • Effective thermal diode operation • Performance shortfalls encountered in testing indicated potential hybrid wick design and fabrication issues • Currently examining sintered wick insert • Eliminate hot spots • Better wick/groove interface ISO:9001-2000 / AS9100-B Certified

  36. Acknowledgements • The trade study was sponsored by NASA Marshall Space Flight Center under Purchase Order No. 00072443. The VCHP was sponsored by NASA Marshall Space Flight Center under Purchase Order No. NAS802060. Jeffery Farmer was the Technical Monitor • Kara Walker was the engineer on the Variable Thermal Link trade study. Tim Wagner as the technician at ACT. We would like to thank Kyle Van Riper for technical discussions about the VCHP. • Any opinions, findings, and conclusions or recommendations expressed in this presentation are those of the authors and do not necessarily reflect the views of the National Aeronautics and Space Administration. ISO:9001-2000 / AS9100-B Certified

  37. Variable Conductance Heat Pipe for a Variable Thermal LinkC. J. Peters, J. R. Hartenstine,C. Tarau, & W. G. Anderson Advanced Cooling Technologies, Inc.Bill.Anderson@1-act.com TFAWS Paper Session Presented By Calin Tarau Thermal & Fluids Analysis WorkshopTFAWS 2011August 15-19, 2011 NASA Langley Research CenterNewport News, VA

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