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].