The mechanism of heat transfer in the evaporator zone of the heat pipe

By combination of mass, energy, and momentum balances over a planar heat pipe wick, an equation to predict the critical heat flux of capillary flow limited heat pipes can be developed. This equation has been demonstrated to predict accurately the critical heat flux of liquid metal filled wicks and to overpredict the critical heat flux of wicks used with a nonmetallic working fluid above a heat flux of 94,600 J/sq m.sec. A revised equation taking into account vapor friction in the wick structure was considered and used to predict the critical heat flux of wicks filled with water as working fluid. The mode of evaporative heat transfer in the evaporator zone of the heat pipe was found to be dependent upon the type of working fluid. When liquid metal working fluids are used, heat is transferred by conduction across a liquid-saturated wick to the wick outer surface where vaporization of the working fluid takes place. When nonmetallic working fluids are used, heat is transferred by conduction to a liquid-vapor interface which exists within the wick structure. The wicks studied were fabricated from beds of packed beads or planar slabs of sintered metal fibers and powders. Liquid potassium at a saturation temperature of 866 K (16,800 N/sq m) and water at atmospheric pressure were utilized as working fluids.