"To further insights into the operation of a heat pipe in space, NASA 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.
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 Constrained Vapor Bubble experiment represents state-of-the-art heat pipe research. It uses a cuvette pipe, which is a rectangular-shaped glass tube made of quartz, filled with a fluid called pentane. This design allows for temperature measurement along the Constrained Vapor Bubble with great accuracy. The transparency also enables observation of the fluid flow to allow scientists to measure the size and shape of the menisci."
For a journal article on this research see: Chatterjee, A., et al., "The
Constrained Vapor Bubble Fin Heat Pipe in Microgravity," Industrial &
Engineering Chemistry Research. http://pubs.acs.org/doi/full/10.1021/ie102072m