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T0014-P

Heat Pipe Limits in Reduced Gravity Environments

PI: Marc A. Gibson, NASA/Glenn Research Center

Fission power provides game-changing solutions for powering advanced NASA missions. Radiators are needed to reject waste heat from Fission Power Systems (FPS). Titanium – water heat pipes are being considered for use in the radiators of a fission power system option for lunar exploration. Embedded in the radiators and deployed on the surface, heat pipes would be oriented vertically and would operate as thermosyphons, a subset of heat pipes that have no wick in their condenser. Their design is determined in part by the flooding limit which is attributed to the interfacial shear force at the boundary between liquid and vapor, and occurs when concurrent vapor flow is so severe that liquid flow is prevented. Flooding is determined by the thickness of the fluid film on the walls and the interaction of fluid flow with concurrent vapor counter flow, both inversely proportional to gravity. The planned test objective of this project is to validate the gravity-dependent flooding limit model for thermosyphons.

Technology Areas (?)
  • TA02 In-Space Propulsion Technologies
  • TA03 Space Power & energy Storage
  • TA09 Entry, Descent and Landing Systems
  • TA14 Thermal Management Systems
Problem Statement

Evaluating thermosyphon performance in 1g may lead to overestimating thermosyphon performance in 0.16g. Heat pipes have been studied thoroughly in 0 g and thermosyphons have been studied in 1g. However, the study of thermosyphons operating in a 0.16g environment is absent from the literature. Indeed, parabolic flights in the low-g aircraft are the only practical means of achieving Technology Readiness Level (TRL) 6, the demonstrating of thermosyphon performance in a relevant 0.16 g environment. The reciprocal dependence on acceleration due to gravity must be incorporated in the design of thermosyphons for FPS radiators and the proposed activity will validate the model needed to predict gravity-dependent flooding.

Technology Maturation

The TRL for modeling of thermosyphons will mature from one validated in a 1g laboratory environment (TRL 4) to a model validated in a 0.16g relevant environment (TRL 6).

Future Customers

Thermosyphons are included in the baseline radiator technology for the Technology Demonstration Unit ground test currently under way at NASA Glenn Research Center. Envisioned to operate for 8 years on the lunar surface, thermosyphons will be needed in the FPS radiator to provide redundancy for micrometeoroid impact risk mitigation. The commercial applications or other characteristics that may be benefit from knowledge gained include other lunar or Martian assets (such as landing craft, habitat modules, and lunar rovers) requiring radiators equipped with heat pipes operating as thermosyphons.

Flight Experiment Objectives

The planned test process is to monitor temperature in an array of twelve identical low-mass thermosyphons where a sudden increase in evaporator temperature serves as the indicator for flooding.

Payload Description

The test rig consists of three basic components. First, a thermosyphon array contains twelve identical thermosyphons (red vertical bars in below image). Each thermosyphon has a heater block and four electric cartridge heaters. Next, the rig contains a subsystem of twelve power supplies. Each power supply is controlled independently to deliver a range of wattages to the thermosyphon array. Finally, the rig contains a Data Acquisition and Control (DAQ) system. The DAQ system monitors thermocouples placed at strategic locations on each thermosyphon and controls power supplied to the twelve thermosyphons. The DAQ runs on LabView Realtime software.

Technology Maturation Progress

The NASA Glenn Research Center completed its technology demonstration of titanium water heat pipes after a series of successful parabolic test campaigns. The GRC team supports the Game Changing Development Nuclear Systems Project and the Radioisotope Power System Technology Advancement Project with the goal of bringing titanium water heat pipes to a Technology Readiness Level six (TRL6). The heat pipes are the main component of the heat rejection system used on nuclear power systems necessary to effectively radiate waste heat and cool the power convertors that provide electricity.

The research started in 2011 with the need to evaluate the gravity dependence of thermosyphons in reduced gravity environments that could only be provided via parabolic flight. NASA’s Flight Opportunities Program (FOP) and Reduced Gravity Office (RGO) provided the necessary flights for the research, which led to new experimental data and modeling correlations. The new information allowed the researchers to better understand the thermal limits of thermosyphons for use with lunar and Martian surface nuclear power systems that could one day provide electrical power for a long term human presence.

In 2013, the team conducted follow-on research on advanced heat pipe prototypes specifically designed for cooling the Advanced Stirling Convertor (ASC). The ASC is a power convertor that uses nuclear heat and converts it to electricity for spacecraft consumption. The flight campaigns allowed the team to prove the thermal performance of two specific designs that could be configured for Plutonium-based Radioisotope Power Systems (RPS) and Uranium-based Fission Power Systems (FPS) used in deep space science missions. Both heat pipe technologies used titanium as the structural envelope and water as the working fluid with the requirement of transferring 150W of thermal power from the ASC to the heat rejection radiator. The technologies met all thermal requirements in their relevant environment achieving TRL6. This technology advancement directly addresses NASA’s need to provide higher power spacecraft for future deep space science missions.

Technology Details

  • Selection Date
    AFO1 (May 2011)
  • Program Status
    Testing Complete
  • Current TRL (?)
    TRL 6
    Successful FOP Campaigns
  • 3 Parabolic

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