Management of the residual heat generated during power conversion in space is a critical stage for a successful space exploration mission. Under the Prometheus program, an exploratory design of space radiators using intermediate-temperature (450 to 700 K) heat pipe systems to efficiently distribute the heat within radiator panels was studied at the NASA Glenn Research Center. A heat pipe is a self-contained two-phase sealed device that can transport large quantities of heat with minimal temperature drop. This is accomplished by the evaporation of a liquid at the hotter end of the device and the condensation of the vapor at the cooler end. Specially designed wicks lined against the device wall return the condensed liquid to the evaporator section. Hundreds of these devices are planned to be major components of radiator panels for Moon, Mars, and deep-space missions.
Although water has been tested to about 550 K, its vapor pressure rises to a level that will require very thick container walls, resulting in heavier components. Thus, a study was conducted at Glenn to identify alternative fluids with the desired thermodynamic properties along with envelope materials that have sufficient chemical compatibility, thermal conductance, and mechanical integrity. This study resulted in the selection of the halides aluminum bromide (AlBr3), antimony bromide (SbBr3), and titanium chloride (TiCl4). The halide choices were based on several factors, including vapor pressure, melting and boiling points, cost, availability, and handling safety. The envelope and wick metals chosen for the study were two aluminum alloys (Al-5052 and Al-6061) and commercial pure titanium grade 2 (Ti CP2) because of thermodynamics compatibility predictions and the availability, low density, and wide use of these materials in aerospace technologies. The next step was to conduct experiments to determine the chemical compatibility of selected metals for the envelope and wick with the selected halides.
View of the Ti (CP2) and Al-alloys testing capsules and testing station.
Commercial pipes and rods were procured, machined, and welded to fabricate the experimental capsules. Then, assembled capsules were tested at 500 K in a specially designed test station that mimics actual heat pipe conditions. A total of five fluid/envelope combinations, with duplicates, were evaluated experimentally. The internal pipe surfaces and metallographically polished cross sections were observed by optical and field-emission scanning electron microscopy. Chemical and x-ray analyses were performed on the products and layers that formed on the inner diameter of the capsule. The compatibility studies have shown that AlBr3 is not compatible with Ti (CP2), Al-6061, or Al-5052 alloys. Although thermodynamic calculations indicated that pure-Al is compatible with AlBr3, the influence of alloying additions was not considered analytically. Although the results show that AlBr3 was more stable than Ti (CP2) in the Al-6061 envelope, microstructural characterization revealed severe intergranular corrosion on both metal systems. Among the successful combinations, a Ti (CP2) capsule containing TiCl4, and an Al-6061 capsule containing SbBr3 have reached over 4000 hr of exposure at 500 K with no sign of degradation.
Electron microscopy images showing intergranular corrosion attack produced on the wall of the Al-6061 alloy by AlBr3 after 1100 hr of testing at 500 K. Left: Internal surface of tube midsection. Right: Cross-section view of the evaporator.
In summary, it was found that several advanced heat pipe fluids can be incorporated into realistic intermediate temperature heat pipes with good compatibility with the proposed envelope and wick materials.
Anderson, William G., et al.: Evaluation of Heat Pipe Working Fluids in the Temperature Range 450 to 700 K. Space Technology and Applications International Forum-STAIF 2004, 2004, pp. 20-27.
Devarakonda, A.; and Olminsky, J.K.: An Evaluation of Halides and Other Substances as Potential Heat Pipe Fluids. AIAA-2004-5575, 2004.
Locci, Ivan E., et al.: Analytical and Experimental Thermo-Chemical Compatibility Study of Potential Heat Pipe Materials. AIAA-2005-5666, 2005.
Find out more about materials research at Glenn: http://www.grc.nasa.gov/WWW/5000/MaterialsStructures/
University of Toledo contact:
Dr. Ivan E. Locci, 216-433-5009, Ivan.E.Locci@nasa.gov
Glenn contact:
Duane E. Beach, 216-433-6285, Duane.E.Beach@nasa.gov
Authors:
Dr. Ivan E. Locci, Dr. Angirasa Devarakonda, Dr. Evan H. Copland, Jami K. Olminsky, and Duane E. Beach.
Headquarters program office:
Space Exploration
Programs/Projects:
Project Prometheus
Last updated: October 16, 2006
Responsible NASA Official:
Gynelle.C.Steele@nasa.gov
216-433-8258
Point of contact for NASA Glenn's Research & Technology reports:
Cynthia.L.Dreibelbis@nasa.gov
216-433-2912
SGT, Inc.
Web page curator:
Nancy.L.Obryan@nasa.gov
216-433-5793
Wyle Information Systems, LLC
