Vapor flow considerations in conventional and gravity-assist heat pipes

Terrestrial applications for heat pipes have grown rapidly in recent years, particularly in areas involving permafrost stabilization and energy conservation. Ordinarily, these applications do not require heat pipes that are conventional in the sense that circulation of working fluid is maintained exclusively by capillary forces. Instead, they usually permit heat-pipe operation with the condenser elevated so that condensate is returned to the evaporator with gravity assistance. When operated in this manner, relatively high rates of heat transfer have been achieved even with heat pipes containing simple and inexpensive wicks and working fluids having low surface tensions. When the performance of a heat pipe is improved substantially by gravity assistance or by employing some type of composite wick, heat transfer is often limited primarily by vapor flow effects as opposed to liquid pressure losses. Five separate effects and their associated limitations were encountered in experiments with liquid-metal working fluids. All are identified.