Study of Two-metal (Cu-Ag) Micro Heat Pipe of Convergent-Divergent Circular Cross Section Using Different Working Liquids of Low Boiling Point
With the modernization of electronic machines, the size or weight to efficiency ratio is getting small enough to leave the equipment highly thermally stressed. To manage this stress, an experiment on thermal effects of a two-metal (Cu-Ag) micro heat pipe (TMMHP) of convergent-divergent cross section, which provides variable conductivity and pressure gradient, at steady state are investigated to compare with a single metal micro heat pipe (SMMHP). Internal fluid flow is considered as one dimensional which is axially flowing. It is a 150 mm long convergent-divergent hollow circular tube having a hydraulic diameter of 3.74 mm and thickness of 0.3 mm. 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 real situation of machines usage, tests are conducted by placing the heat pipe at three different inclinations of 0o, 45o and 90o. 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. Ten calibrated 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 SMMHP, it is found that the super heater effect in the evaporator of TMMHP simplifies the initially developed complex two-phase flow into a single-phase of super heated vapor. Thus, it increases TMMHP’s heat transfer capability several times higher than that of a SMMHP.
