On the problem of heat transfer in low-temperature heat pipes

The processes in metal-screen wicks were observed visually on their internal and near-wall surfaces in a transparent heat pipe with Freon-113 and water. Vapor-filled spaces were noted in poor-contact areas. Heat transfer and critical heat flux densities were measured in a vertical setup with wick of copper and stainless-steel screens, and water, ethyl alcohol and Freon-113 as working fluids. In a comparison with a plain-surface, an enhanced heat transfer was noted in the presence of a wick, up to a certain value of the heat flux, followed by a decrease of the heat transfer for further heating. Both the heat transfer and the critical heat flux are higher for higher efficient conductivites and improved vapor evacuation conditions. A perforated press-screen used to fix the wick at the pipe wall had no significant effects on the heat transfer coefficient, but gave a 20 to 30% decrease in the critical heat flux density. A general relation was derived for the heat transfer.