Introduction
Micro heat pipe is a heat transferring device based on the phase change
phenomenon of fluid contained in it. Before filling with the fluid, the
container must be vacuumed to below the atmospheric pressure. MHP is
considerably of small diameter, usually not over 3.0 mm [6, 18 and
25]. The micro heat pipe receives heat at one end to vaporize the
fluid which is evaporator, and then travels through the next section
losing no heat called adiabatic section, and terminally the condenser
part through which the carried away heat dissipates to the atmosphere.
Usually, micro heat pipe is made with good heat conducting metal, i.e.
copper, stainless steel, nickel etc. Depending on the operating
temperature range, selected working fluids may be water or hydrocarbon
compounds or it can be cesium, bismuth, sodium, lithium etc. Fluids of
low boiling point (LBP) indicate here the fluids that have boiling
points below the water at atmospheric pressure. A wick is shaped
accordingly, and inserted within the heat pipe spanning end to end to
let the condensate crawl back to the evaporator by capillary action. The
wick can be made of stainless steel mesh, sintered metal powder, fiber,
wire braid etc. In a micro heat pipe, the presence of sharp or non
circular edges, and in other case, radially etched micro grooved inner
wall of the MHP are also replacing wick that provides the capillary
service. Comparing with solid metal, heat transport ability of a heat
pipe of same geometry is found to be many times high at a small
temperature difference. MHP’s applications are widely endorsed in
cooling microelectronics, nuclear reactors as far as in space
satellites. Globally many researchers have been engaged in improving the
MHP concepts for the last several decades; however, few of their works
are cited bellow.
Study on heat pipe has been in practice since 1942 when R. S. Gaugler of
General Motors, USA proposed [1]. However, heat pipe did not receive
a target oriented attention until 1963 when Grover et al. [2]
directed the heat pipe’s condensate-returning mechanism from its
confined gravitation-fed state to the simple capillary-force action of
wick structure inserted in it. By the U.S. government funding, between
1964 and 1966, RCA was the first corporation to undertake research and
development of heat pipes for commercial applications [3]. Starting
in the 1980s Sony began incorporating heat pipes into the cooling
schemes for some of its commercial electronic products instead of the
more traditional finned heat sink with and without forced convection.
But, it was twenty years later in 1984 when T. P. Cotter first
introduced the idea of “micro” heat pipes [1]. Sreenivasa et al.
[4] determined the optimum fill ratio in miniature heat pipe which
indicates the same performance as the evaporator section was half filled
rather than filling in full. Akhanda et al. [5] tested an air cooled
condenser to investigate the thermal performance of MHPs charged with
different fluids and oriented a different inclinations. Sakib Lutful
Mahmood [6] at Islamic University of Technology (IUT), OIC has
performed tests on different cross sections of MHP of the same hydraulic
diameter charged with water at different inclinations. It was found that
the best heat transfer coefficient at the circular cross section was at
an angle of 90o. Further observation was made as the
thermal resistance of micro heat pipe increases with increasing of
flatness ratio and its heat transfer coefficient decreases also with
increasing of flatness ratio. Finally, Sakib developed an empirical
equation from the experimental data and correlated all his findings
which showed ± 7% nearness with the developed equation. Moon [9]
used a miniature heat pipe which was squeezed in the Notebook PC to cool
which may be heated up to 1000 C. From the output of
the experiment using miniature heat pipe with woven wire wicks was found
to be quiet viable candidate for a stable cooling unit of Notebook P. C.
Bai et al [20] experimented on loop heat pipes (LHP) under gravity
assisted operation based on two driving modes: gravity driven mode and
capillarity-gravity co-driven mode, determined by a defined transition
heat load. Then they compared the results with the established
steady-state mathematical model. The results show the steady-state
operating temperature is much lower under the gravity driven mode, and
is in similar values under capillarity-gravity co-driven mode. Li et al
[21] studied a ultrathin flattened heat pipe with sintered wick. The
effects of each processing parameter on the thermal performance of the
UTHP samples were analyzed and compared with a mathematical model
incorporating effects of the evaporation and condensation heat transfer
in a copper-water wick. Results indicate that the most critical factor
for thermal performance of UTHP is flattened thickness, as it decreases,
the heat transport capability drastically decreases and the thermal
resistance increases. Babin et al. [10] developed the model that
analyzes the heat transport behavior of micro-heat pipe, and presented
the model of micro-heat pipe based on the analysis by Chi [11] in a
steady-state operation. Longtin et al. [12] presented the improved
prediction results, considering partially the shear stress in
liquid-vapor interface of groove in a micro heat pipe. Swanson and
Peterson [13] analyzed thermo-dynamically the heat transport
phenomena in the liquid-vapor interface of heat pipe, and Wu and
Peterson [14] studied the thermal performance of micro-heat pipe in
an unsteady state. Le Berre et al. [15] studied experimentally the
performance of a micro heat pipe array for various filling charges under
various experimental conditions. The results showed that the performance
of the micro heat pipe array is favored by decreasing the input heat
flux or increasing the coolant temperature. Wu et al [22]
investigated the use of sintered PTFE (polytetrafluoroethylene)
particles as the wick material of loop heat pipe (LHP), taking advantage
of PTFE’s low thermal conductivity to reduce the heat leakage problem
during LHP’s operation. Thermal performance of a miniature loop heat
pipe using water–copper nanofluid. The results of this study shows
that, for high heat transfer capacity cooling devices, PTFE wicks
possess great potential for applications to LHPs. Kole and Dey [16]
investigated thermal performances using Cu-distilled water nano-fluid
which enhanced thermal conductivity by 15% at 30o C.
Chiang et al. [17] developed a magnetic-nanofluid (MNF) heat pipe
(MNFHP) with magnetically enhanced thermal properties. The results
showed that an optimal thermal conductivity exists in the applied field
of 200 Oe.
Throughout this survey, it has been found that only a single metal or
bimetal alloy has been used to manufacture the heat pipes including its
varieties of isometric geometry. In these cases, heat transfers occur
only at constant heat conductivity at both ends of MHP for being it a
single metal or an alloy. No individual or company has been found to
have attempted on doing investigation on a variable heat conductivity
micro heat pipe. Thus, a two-metal micro heat pipe (TMMHP) made with two
different metals (i.e. Cu and Ag) of closer heat conductivity (i.e. 398
W/m-K for copper and 429 W/m-K for silver) for variable heatconductivity has been selected by the author Iqbal [8] in his
doctoral thesis. A series of heat inputs ranging from 2W to 16 W have
been supplied to the evaporator keeping the MHP at 0oto study the heat transfer behavior of pure water along with ethanol,
methanol and iso-propanol. Then it was reexamined at
45o and 90o positions (evaporator
uphill) while the condenser was being cooled by ambient water at a
constant flow-rate of 400 ml/min. At the end, the fluid temperatures
within the TMMHP as well as the surface temperature at designated
locations at steady state have been recorded to compare with other
researchers’ experimental data. To confirm the reproducibility of the
data, the experiments were repeated and found to be on the same trend
line. Iqbal and Akhanda [23] studied the same on
convergent-divergent geometry where the heat transfer coefficient is
found two times higher than that of single metal (Cu) heat pipe. Similar
results are also found in the square cross section geometry [24].