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Presented By Scott Hansen

TFAWS Active Thermal Paper Session. Water Phase Change Heat Exchanger System Level Analysis for Low Lunar Orbit Eugene Ungar, Moses Navarro, Scott Hansen, and Rubik Sheth (NASA: JSC). Presented By Scott Hansen. Thermal & Fluids Analysis Workshop TFAWS 2016 August 1-5, 2016

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Presented By Scott Hansen

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  1. TFAWS Active Thermal Paper Session Water Phase Change Heat Exchanger System Level Analysis for Low Lunar OrbitEugene Ungar, Moses Navarro, Scott Hansen, and Rubik Sheth (NASA: JSC) Presented ByScott Hansen Thermal & Fluids Analysis WorkshopTFAWS 2016August 1-5, 2016 NASA Ames Research Center Mountain View, CA

  2. Phase Change Heat Exchangers • In cyclic thermal environments, manned spacecraft often require heat rejection beyond their radiator capacity so a supplemental heat rejection device (SHReD) is needed • Evaporants can be used - Gemini capsule, Apollo Command Module, and the Space Shuttle Orbiter all used water evaporators • The International Space Station has no supplemental heat rejection • Skylab used a tridecane wax-filled aluminum honeycomb thermal capacitor downstream of its radiator • Orion EFT 1 used an ammonia boiler • NASA plans to use its new Orion crewed vehicle in low lunar orbit (LLO) where thermal environment varies between near absolute zero during eclipse to warmer than room temperature near the subsolar point • The baseline Orion active thermal control system (ATCS) includes an n‑pentadecane wax phase change material heat exchanger (PCM HX) for use in LLO • The melting point of n‑pentadecane is 9.9°C, which is enveloped by the nominal internal active thermal control system setpoint of 8.3°C and allowable excursion limit of 12.2°C • Water is an attractive replacement for wax • Higher heat of fusion (333 kJ/kg vs. 200 kJ/kg) and greater density have the potential to reduce PCM HX mass by 38% and its volume by 50%. • Current work is a study to assess use of a water PCM HX in LLO on the Orion spacecraft TFAWS 2016 – August 1-5, 2016

  3. Orion Active Thermal Control System (ATCS) Operation • ITCS supplies an 8.3°C 50/50 mixture of inhibited propylene glycol and water (PGW) to the cabin air heat exchanger and internal coldplates at 106 kg/hr • The temperature is controlled by an internal bypass valve • The ITCS fluid is cooled by interface heat exchanger (IFHX) by cold ETCS HFE 7200 flowing at 578 kg/hr • A radiator bypass valve maintains a mixed radiator return temperature of ‑6.7°C, this value was chosen based on the low temperature pumping limit of the PGW • In cold environments and low heat loads, the ETCS setpoint of ‑6.7°C can be maintained, allowing the PCM to solidify • In warm environments and/or high heat loads, the radiator return setpoint of ‑6.7°C cannot be maintained • As the radiator return temperature rises, the ITCS temperature at the wax PCM HX rises • If the PCM HX wax is solid, its PGW exit temperature will remain near 9.9°C – the melting point of n‑pentadecane • Once the PCM HX is fully melted, its outlet temperature will exceed 9.9°C, causing the ITCS to lose setpoint control • However, as long as the PCM HX exit temperature is 12.2°C or lower, the ITCS maintain allowable system setpoint TFAWS 2016 – August 1-5, 2016

  4. Orion ATCS with Water PCM HX – Case 1&2 • Several different configurations were considered in the performance analysis • Case 1 was a simple replacement of the wax PCM HX with a water PCM HX • A PCM bypass valve was added to maximize use of the ice • Case 2 replaced used the Case 1 configuration but replaced the 50/50 PGW mixture with 30/70 water/PG • The decreased viscosity allows a lower radiator setpoint of ‑12.0°C TFAWS 2016 – August 1-5, 2016

  5. Orion ATCS with Water PCM HX – Case 3 • Case 3 placed the water PCM HX on the external loop to take advantage of the lower available temperatures • A PCM bypass valve was used to maximize use of the ice TFAWS 2016 – August 1-5, 2016

  6. Thermal Math Models • Thermal Desktop models • Simulated half of Orion’s active thermal control system • one ITCS loop with mass • one ETCS loop with mass • half of the radiator area and mass • IFHX effectiveness of 0.95 • PCM HXs were sized to provide 3830 kJ of latent energy capacity and were assumed to have a structural mass equal to the PCM mass • Wax PCM had 19.1 kg of n‑pentadecane plus 19.1 kg of aluminum housing • Water PCM HX was contained 11.5 kg of water surrounded by 11.5 kg of stainless steel (for corrosion resistance) • Thermal conductance (UA) of 10,000 W/K used for all PCM HXs • The model was exercised for a 100 km circular low lunar orbit with a 120 minute period. Four system configurations were assessed: • Case 0: The baseline Orion ATCS with a wax PCM HX – the ETCS setpoint temperature was -6.7°C • Case 1: The baseline Orion system with an equivalent water PCM - the ETCS setpoint temperature was -6.7°C • Case 2: The Orion system with 30/70 PGW, a water PCM HX on the ITCS - the ETCS setpoint temperature was -12.0°C • Case 3: The baseline Orion system with a water PCM HX at the radiator outlet - the ETCS setpoint temperature was -6.7°C TFAWS 2016 – August 1-5, 2016

  7. Radiator Environment • Hot design case radiator sink temperatures were based on a sphere in a 100km circular lunar orbit at b=0 • Lunar surface optics were a/e = 0.90/0.90 • Solar constant was 1426 W/m2 - the maximum insolation in low lunar orbit • Radiator optics were Orion Z‑93 end-of-life spec, a/e = 0.20/0.90 • An additional uniform background sink temperature of -173°C was included to account for structure and other hardware in view of the radiators. TFAWS 2016 – August 1-5, 2016

  8. Analysis • The models were operated at various heat loads to evaluate the system performance and assess the heat load limits • The heat load was applied to the ITCS in the cabin heat exchanger and coldplate location • Heat loads were incrementally raised to find the performance limit of the baseline wax system • Heat load was found to be 1.6 kW • The water PCM HX systems were assessed at that heat load • The radiator area was adjusted as needed to obtain successful operation TFAWS 2016 – August 1-5, 2016

  9. Case 0 (Orion Baseline) at 1.6 kW • Wax PCM system maintains ITCS setpoint at 1.6 kW heat load TFAWS 2016 – August 1-5, 2016

  10. Case 0 and Case 1 (Direct Replacement) Setpoints at 1.6 kW • Water PCM maintain ITCS setpoint while rejecting 1.6 kW • Radiator area must be increased by >60% for successful operation TFAWS 2016 – August 1-5, 2016

  11. Results for All Cases • None of the water PCM HX cases could maintain setpoint at 1.6 kW heat load • Additional radiator area was required in all cases • The water PCM HX is not competitive with Orion’s wax PCM HX • Because the n‑pentadecane freezing point is enveloped by Orion’s ITCS setpoint requirements, the wax PCM HX performs well • A water PCM HX • Requires a controlled bypass to maximize its performance • Even then, the lower freezing potential of the water PCM HX results in poorer performance than the wax PCM HX TFAWS 2016 – August 1-5, 2016

  12. Conclusion • A system level study was completed to investigate the integration of a water PCM HX in Orion’s ATCS for a circular low lunar orbit mission. • Three system configurations were compared to the baseline Orion configuration with a wax PCM HX: • Water PCM HX on Orion ITCS, • Water PCM HX on Orion ITCS with a lowered ETCS radiator return setpoint, • Water PCM HX on Orion ETCS. • All configurations fell short of the performance of the wax PCM HX in Orion’s system • The system with the water PCM HX on the external loop performed best, but still required 15% additional radiator area to maintain the ITCS setpoint temperature within the specified limits • On the component level, water PCM HXs are attractive owing to their lower mass and smaller volume • However, when integrated into an ATCS, their performance does not match that of a more traditional wax PCM HX • System level effects must be carefully considered before advocating the use of water PCM HXs in future human spacecraft TFAWS 2016 – August 1-5, 2016

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