Experimental Analysis of Wire Sandwiched Micro Heat Pipes

Experimental Analysis of Wire Sandwiched Micro Heat Pipes Rag, R. L. Department of Mechanical Engineering, John Cox Memorial CSI Institute of Technology, Thiruvananthapuram 695 011, India Abstract Micro heat pipes are a promising option for the thermal management of microelectronic systems with high heat flux dissipation rates. An experimental analysis is performed on wire-sandwiched micro heat pipes, a relatively new design of micro heat pipes, which utilizes an array of wires sandwiched between metallic plates to produce the flow channels. The temperature distribution which results in the calculation of effective thermal conductivity, obtained in the computational analysis is compared with that obtained from the experiments. The theoretical performance of the heat pipe is verified using experiments in a micro heat pipe array. The analysis provides guidelines for the geometric design of wire-sandwiched micro heat pipes for heat dissipation from microelectronic chips, based on the results corresponding to the thermal management conditions encountered in such applications. Nomenclature A = area of cross section, m 2 k = thermal conductivity, W/m K L = length of the heat pipe, m P w = pitch of the wires (center to center distance of the wire), m Q = heat, W q = heat flow rate, W/m 2 R w = radius of the wire, m T = temperature, K T = temperature difference, T-T amb, K Subscripts amb = ambient ave = average a = adiabatic section c = condenser section e = evaporator section eff = effective I. INTRODUCTION The micro heat pipe is a heat transfer device of high effective conductance, which utilizes phase change of the working fluid and the capillary action at the corners of flow passages for its operation, as described extensively in the literature [1-5]. Though the micro heat pipe also utilizes similar thermal phenomena as a conventional heat pipe [6], and consists of the evaporator, adiabatic and condenser sections, the absence of a wick structure and the smaller size make the micro heat pipe physically different from the conventional heat pipe. The sharp corners of the noncircular cross sections of the micro heat pipes provide the liquid arteries for the transport of the liquid from its condenser section to the evaporator section [2, 3]. Schematic of a typical micro heat pipe channel of the design analyzed in the present paper is given in Fig. 2, which gives the configuration of the sections of the device. The first attempt to utilize an array of micro heat pipes for thermal management of electronic devices was made by Cotter [2] who proposed the fabrication of such devices as an integral part of microelectronic devices. In a typical design of micro heat pipes, heat addition at the evaporator section generates vapor from the liquid phase of the working fluid, which is at its saturation condition, and makes the vapor flow from the evaporator section to the condenser section. If sufficient capillary pressure difference is provided by the corners of the passages, the liquid is transported back to the evaporator section from the externally cooled condenser section, thus setting up a fluid circuit in the device [5]. Theoretical and experimental investigations on micro heat pipes with trapezoidal, triangular and rectangular cross sections can be found in the literature. The wire sandwiched micro heat pipes, which is analyzed in this paper, is a novel design of micro heat pipes [3, 6], constructed by sandwiching an array of wires in between two plates to provide flow passages. 205

A one dimensional steady state model for the evaporator and the adiabatic sections of a triangular micro heat pipe was developed by Longtin et al. [7] assuming uniform vapor temperature throughout its length, and incorporating the interfacial shear stress and body force terms in the momentum equation. A steady state mathematical model for the maximum heat transport capability of a micro heat pipe was developed by Peterson and Ma [8], which considered the governing equations for fluid flow and heat transfer in the evaporating thin film region. A transient model for a triangular micro heat pipe consisting of an evaporator section and a condenser section was developed by Sobhan et al. [9]. Fig. 1.Schematic of the wire-sandwiched micro heat pipe construction [3, 7] In this model, the differential forms of the momentum and energy equations were solved, along with the interfacial mass conservation equations across the meniscus, and the Laplace-Young equation containing the meniscus radius. Sobhan and Peterson [10] further explored this model solving the governing equations to analyze the performance of the heat pipe for a much wider range of parametric variations. A comprehensive review of the analysis of micro heat pipes found in the literature, and a quantitative comparison of the performance of various designs are presented in a recent publication by Sobhan et al. [11]. 206

The wire-sandwiched (wire-bonded) micro heat pipe [3] is one of the latest innovations in micro heat pipes, proposed for use in conventional electronic cooling applications and also in advanced applications such as in spacecrafts. This device can be fabricated by sintering an array of wires between two thin metal plates. The sharp corners formed between the plate and the wires serve as liquid arteries [3, 6]. A few publications dealing with the theoretical analysis of the flow and heat transfer process in wire-sandwiched micro heat pipes can be found in the literature [6, 12]. The wire-bonded micro heat pipe was conceptualized, modeled and analyzed by Wang and Peterson [6] to obtain the effects of various parameters on its maximum heat transport capacity. Launay et al. [12] investigated the effects of contact angle, fluid type, corner angle and fill charge on the performance of a wire-plate micro heat pipe array. However, these investigations have not incorporated the fluid cross-sectional area variation along the heat pipe as a function of the position, in a differential formulation. In the present study, an experimental study is performed on the wire-sandwhiched micro heat pipe, with an objective to understand and quantify its performance. Fig. 2. Configuration of a single micro heat pipe in the array The temperature distribution in the flow of the working fluid is computed from the governing equations, which is used for calculating the effective thermal conductivity which is the performance indicator of the heat pipe. The experimental analysis is useful both for validation and getting the actual performance of the wire-sandwiched micro heat pipe under consideration. II. CONFIGURATION OF WIRE-SANDWICHED MICRO HEAT PIPE The wire-sandwiched micro heat pipe essentially consists of an array of channels, formed by sandwiching an array of wires between two metal plates. Each channel in the array acts as an individual micro heat pipe as shown in Fig.1. The configuration of a single micro heat pipe in the array is shown in fig. 2. An externally heated evaporator, an adiabatic section with no external heat transfer and a condenser section subjected to convective cooling are the distinct sections of the micro heat pipe. The cross sectional areas of the liquid and the vapor change longitudinally from the evaporator end to the condenser end, as represented in Fig 1(c). The mathematical formulation, the solution procedure and the computational results are discussed in literature [9, 13, 14] in detail. 207

Fig. 3 Schematic of the apparatus for fabricating the wire-bonded micro heat pipe array III. FABRICATION OF WIRE SANDWICHED MICRO HEAT PIPES For fabricating the wire-bonded micro heat pipe by sandwiching an array of copper wires between two thin copper plates and bonding them together, first a fabrication apparatus was made. The arrangement is shown in Fig. 3. This provided methods to have the wires stretched at precisely equal distances in between the plates, and to join the edges of the plates, keeping the wire-plate assembly under pressure. As shown in the figure, and end block A with slots fabricated at a spacing of the pitch of the wires is used to hold the wires in place. It is possible to keep the wires under tension, by using the screws provided for this purpose B in order to ensure that the wires are held in position without the possibility of cross-communication between the channels, a very thin film of epoxy is used on the plate surfaces. The schematic of the specimen obtained is shown in Fig. 4. Keeping the plates on both sides of the wire-array, the plates are held under pressure and three edges of the assembly (C, D and E) are hard-soldered. IV. THE EXPERIMENTAL SETUP The set up had the facility to hold the fabricated micro heat pipe specimen, the vacuum pump and vacuum lines to evacuate it, the charging line with a precision syringe to transfer the pre-calculated amount of fluid charge (1.008 ml), crimping devices to seal the micro heat pipe after charging, thermocouples attached to the surface, and the data-logging system. A procedure to evacuate and charge the device was also evolved, as will be described. A schematic of the components of the experimental set up is shown in Fig. 5. Fig. 4. Schematic diagram of the fabricated specimen 208

Fig. 5. Schematic diagram of the experimental setup showing the test specimen and the evacuation and charging arrangements. The dimensions of the evaporator section are 20 mm x 20mm, the adiabatic section is 85 mm x 20mm and the condenser section is 20 mm x 20 mm. The heating element wounded over the evaporator section is insulated externally to avoid significant heat leakage to the ambient. The electrical input to this heating element can be controlled, using multi-power unit. The adiabatic section is insulated externally using a fiber insulation material of low thermal conductivity. The condenser section is covered by a water jacket, for cooling this section externally. V. EXPERIMENTAL PROCEDURE After charging the calculated quantity (1.008 ml) of deionized water to the micro heat pipe, the cooling water is circulated in the cooling jacket of the condenser, and electrical power is supplied to the heater coil at the evaporator section. The voltage and current are adjusted in the multi-power unit to obtain a heater output corresponding to 0.42 W/cm 2 [6] applied to the surface of the evaporator. The cooling water flow is adjusted so that the surface temperature at the condenser end is maintained at 303 K. VI. COMPARISON OF RESULTS In order to make a comparison between the computed values and measured values of temperatures, the vapor temperature in the experimental channels were estimated through a simple conduction calculation based on Fourier s law across the plate. The temperature drop across the plate is found negligibly small as per this calculation. The temperature distribution along the micro heat pipe obtained from the experiment is shown compared with computational results for identical values of heat input and condenser temperature applied to an identical case of geometric dimensions. As is seen in Fig. 6 the results are found to agree within a maximum deviation of 6.9% at the mid point of the evaporator. The comparison is found to give a fairly good indication of the acceptability of the computational results. 209

Temperature (K) International Journal of Emerging Technology and Advanced Engineering 322 320 318 316 314 312 310 308 306 304 302 0.0 0.2 0.4 0.6 0.8 1.0 Normalised Length (x/l) Fig. 6 Comparison of computed temperature distribution [13,14] with temperatures obtained from the experiment for an input heat flux of 0.415 W/cm 2 and condenser temperature of 303K. VII. RESULTS AND DISCUSSIONS For benchmarking the computational approach [13,14], the calculated values were compared with the maximum evaporator temperature (Te max ), the average Temperature of the adiabatic section (Ta ave ) and the minimum condenser temperature (Tc min ) on the surface, as reported in the literature for the case of a wire sandwiched micro heat pipe [6]. The reported values for the input heat flux (0.42 W/cm 2 ), wire diameter (0.813 mm) and condenser end temperature (313K) were utilized in the computation [13, 14]. VIII. CONCLUSIONS Computational Result Experimental Result An experimental analysis is performed for exploring the heat transfer in a wire sandwiched micro heat pipe, which is a novel design in micro heat pipes. An array of wire bonded micro heat pipes was fabricated, identical to the base-line case utilized in the computational study. An experimental set up which also comprised of an evacuation and charging facility for the micro heat pipe was developed. The temperature distribution obtained from computation for corresponding conditions and the results obtained from the experiment were compared and found to agree within a maximum deviation of 6.9%, indicating the acceptability of the computational results. REFERENCES [1] Garimella, S. V., and Sobhan, C. B., 2001, Recent Advances in the Modeling and Applications of Nonconventional Heat Pipes, Advances in Heat Transfer, Vol 35, pp. 249-308. [2] Cotter, T. P., 1984, Principles and Prospects of Micro Heat Pipes, Proceedings of the 5th International. Heat Pipe Conference, Tsukuba, Japan,, pp. 328-335. [3] Peterson, G. P., and Sobhan, C. B., 2005, Applications of Microscale Phase Change Heat Transfer: Micro Heat Pipes and Micro Heat Spreaders, The MEMS Handbook, 2 nd ed., Vol. 3 Applications, CRC Press Inc., USA. [4] Babin, B. R., Peterson, G. P., and Wu, D., 1990, Steady-state Modeling and Testing of a Micro Heat Pipe, ASME Journal of Heat Transfer, Vol. 112, pp. 595-601. [5] Sobhan, C. B., and Peterson, G. P., 2008, Microscale and Nanoscale Heat Transfer Fundamentals and Engineering Applications, Taylor and Francis/CRC Press. [6] Wang, Y. X., and Peterson, G. P., 2002, Analysis of Wire Bonded Micro Heat Pipe Arrays, AIAA. Journal of Thermophysics and Heat Transfer, Vol. 16, No. 3, pp. 346 355. [7] Longtin, J. P., Badran, B., and Gerner, F.M., 1994, A One- Dimensional Model of a Micro Heat Pipe During Steady-State Operation, ASME Journal of Heat Transfer, Vol. 116, pp. 709-715. [8] Peterson, G. P., and Ma, H. B., 1999, Temperature Response and Heat Transfer in a Micro Heat Pipe, ASME Journal of Heat Transfer, Vol. 121, pp. 438-445. [9] Sobhan, C. B., Xiaoyang, H., and Yu, L. C., 2000, Investigations on Transient and Steady State performance of micro heat pipe, AIAA Journal of. Thermophysics and Heat transfer, Vol. 14, pp. 161-169 [10] Sobhan, C. B., and Peterson, G. P., 2004, Modeling of the Flow and Heat Transfer in Micro Heat Pipes, Second International conference on Microchannels and Minichannels, Rochester, NY. [11] Sobhan, C. B., Rag, R. L., and Peterson, G. P., 2007, A Review and Comparative Study of the Investigations on Micro Heat Pipes, International Journal of Energy Research, Vol. 31, pp. 664 688. [12] Launay, S., Sartre, V., and Lallemand, M., 2004, Investigation of a Wire Plate Micro Heat Pipe Array, International Journal of Thermal Sciences,Vol. 43, pp. 499-507. [13] Rag, R. L., and Sobhan C. B., 2009,Computational Analysis of Fluid Flow and Heat Transfer in Wire-Sandwiched Micro Heat Pipes, AIAA J. Thermophysics and Heat Transfer, Vol.23, No. 4, pp. 741-751. [14] Rag, R. L., and Sobhan, C. B., 2010,Computational Analysis and Optimization of Wire Sandwiched Micro Heat Pipes, International Journal of Micro-Nano Scale Transport, Multi-Science Publishing, Vol.1, No.1, pp. 57 78. 210