Study of Two-Metal (Cu-Ag) Micro Heat Pipe of Square Cross Section Using Different Liquids of Low Boiling Point

Abstract

The efficiency-to-size ratio of the electronic machines has been increasing rapidly while leaving the electronics highly thermally stressed. In an attempt to solve the overheating issue, a comparative study is conducted between a two-metal (Cu-Ag) micro heat pipe and the contemporary single-metal one (Cu) which is currently being installed in the electronic equipment. Thermal effects of a two-metal square cross section micro heat pipe (MHP) at steady state are experimentally investigated. It is a 150 mm long square hollow tube having each side of 3.6 mm and thickness of 0.3 mm which holds a 3.0 mm hollow space axially. The evaporator and condenser sections are made of pure copper and silver respectively while the adiabatic section is made of both copper and silver. Water and three low boiling point liquids ─ ethanol, methanol and iso-propanol ─ are used as working fluids. In view of the usage of the actual equipment, tests are conducted by placing the heat pipe at three different inclinations of 0 o , 45 o and 90 o . To provide heat flux, an electric heater-coil has been wrapped around the evaporator simulating the heat-generation within an actual device, and simultaneously the condenser section is directly cooled by ambient water in an annular space. Internal fluid-flow is considered to be one dimensional. Ten K-type thermocouples are installed at different locations ─ five of them are to measure the temperatures of internal fluid and five are used to measure the surface temperatures of the tube. Temperatures are monitored by digital electronic thermometers. Unlike in the single-metal micro heat pipe (SMMHP), it is found that the super heater effect in the evaporator of TMMHP simplify the complex two-phase flow into a superheated vapor. Thus, it increases TMMHP's heat transfer capability multiple times higher than that of SMMHP.