Loop Heat Pipe Schematic 100 px spacer

Loop Heat Pipe Background

Loop heat pipes (LHPs) are a class of capillary pumped devices used for transporting heat across a distance of up to several meters in both gravity and microgravity environments. They were originally developed for the terrestrial solar energy market by Y. F. Maidanik and his colleagues in the former Soviet Union in the early 1980's (Maidanik et al, 1985, Wolf et al, 1994).

LHPs consist of a heat-accepting evaporator, a heat-rejecting condenser, a fluid reservoir (compensation chamber) and tubing to connect the pieces. There are no moving parts, only moving fluid. Capillary pumping is produced by a small-pore wick inside the evaporator. The typical annular structure of the wick is shown above, in which fluid is drawn by capillary forces from the inside to the outside surface where it evaporates due to the addition of heat. This vapor then flows to the condenser, rejects heat to the environment and returns to the reservoir as a liquid.

The efficient nature of phase change heat transfer allows for the transport of kilowatts of heat with only a few degrees of temperature drop. The pressure head developed by the capillary action is inversely proportional to the square of the pore diameter in the wick material. Current devices use micron-sized pores and generate 15,000 to 30,000 Pa (2-4 psi) of head with ammonia as the working fluid. The approximate temperature range of ammonia-based devices is -40 degrees C to +65 degrees C.

The advantages of LHPs over conventional heat pipes include: self-priming during startup transients, diode-action capability, increased tolerance for non-condensable gas, ability for ground testing in any orientation, and reduced mass by the elimination of wicks in the adiabatic and condenser sections. These advantages, combined with the inherent heat pipe properties of high heat transfer rates and no moving parts, make loop heat pipes attractive for many applications of interest to JPL and NASA.

 
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