Thermal performances of Two-Metal (Cu-Ag) Micro Heat Pipe of different
conductivity of circular cross section using different liquids of low
boiling point
Abstract
Electronic machines are rapidly being developed with the increasing
benefits while getting smaller in sizes resulting in more thermal
stress. To manage this thermal stress, a comparative study was conducted
between a two-metal (Cu-Ag) micro heat pipe (TMMHP) and the presently
utilized single-metal (Cu) micro heat pipe (SMMHP). Thermal effects of
TMMHP of circular cross section at steady state are experimentally
investigated. The tube possesses three common basic dimensions – 150 mm
long hollow axial space, 3.0 mm hydraulic diameter and 0.3 mm thickness.
The evaporator and condenser section is made of pure copper and silver.
The adiabatic section is made of two parts – first half is made with
copper and the second half is made with silver. Water and three low
boiling point liquids – ethanol, methanol and iso-propanol – are used
as working liquids. Tests are conducted by placing the heat pipe at
three different orientations – horizontal, vertical and at 45o
inclination. To provide heat flux electric heater-coil is spiraled
around the evaporator and simultaneously the condenser section is
directly cooled by water in an annular space. Internal fluid-flow is
considered one dimensional. Ten calibrated K-type thermocouples are
installed at different locations. Temperatures are recorded by digital
electronic thermometers. Unlike in the SMMHP, it is found that the
boiling and super heat effects in the evaporator of TMMHP transform the
two-phase flow into a single phase superheated vapor flow, which
increases TMMHP’s heat transfer capability to three and half times the
capacity of SMMHP. Such an enhanced heat transfer coefficient may be
possible from the improved convection which is developed from the
different heat conductivity of metals that enables the TMMHP to reject
heat at a higher rate through its condenser than the rate it can take
heat in SMMHP through its evaporator.