Keeping Cool With Heat Pipes on the Space Station

What happens when electronics overheat? The short answer is: nothing good! In microgravity, natural convection does not occur, which makes cooling equipment a challenge. So how do you keep electronic and computer components from overheating in space?

In satellites used for communications, global positioning systems, and defense purposes, a heat pipe is the device used to regulate temperature and keep the overall systems operating reliably. A heat pipe is a simple device that can efficiently transfer heat from a hot spot to a cooler remote location without the use of a mechanical pump.

To further insights into the operation of a heat pipe in space, scientists launched an investigation called the Constrained Vapor Bubble, or CVB, to the International Space Station. The Constrained Vapor Bubble is the prototype for a wickless heat pipe and is the first full-scale fluids study in the Fluids Integrated Rack or FIR facility flown on the U.S. module of the space station. The experiment completed on March 1, 2011, when the crew removed the fourth module for return on STS-135.

A heat pipe is usually a sealed tube in which all the air is removed and a small amount of liquid is introduced under a partial vacuum. A portion of the liquid in contact with a hot surface evaporates into a vapor as it absorbs heat from the hot surface.

The vapor condenses back to a liquid when the vapor comes in contact with a cool surface, thus releasing its stored or latent heat to the cold surface. The liquid then draws back toward the hot surface, due to the interaction of the individual liquid molecules and their attraction to the surface of the container -- a process called capillary action. The whole liquid and vapor cycle requires no moving parts and the heat transfer process can repeat indefinitely.

Nearly all heat pipes contain wicks or grooves that enhance capillary action and promote the pumping of the working liquid from the cool liquid pool back to the hot surface where the liquid can evaporate again. "The wicks or grooves are complex to fabricate inside the tube and add weight to the heat pipe. Heat pipes built without wicks hold open the possibility of significant weight savings for space flight," says Constrained Vapor Bubble Project Manager Ronald Sicker, NASA's Glenn Research Center in Cleveland.

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