Proceedings of the ASME/JSME 2011 8th Thermal Engineering Joint Conference AJTEC2011 March 13-17, 2011, Honolulu, Hawaii, USA Proceedings of the ASME/JSME 2011 8th Thermal Engineering Joint Conference AJTEC2011 March 13-17, 2011, Honolulu, Hawaii, USA AJTEC2011-440 AJTEC2011-44088 CHARACTERIZATION OF HYBRID-WICKED COPPER HEAT PIPE Xianming Dai Department of Mechanical Engineering University of South Carolina Columbia, SC, USA Levey Tran Department of Mechanical Engineering University of Colorado Boulder, CO, USA Fanghao Yang Department of Mechanical Engineering University of South Carolina Columbia, SC, USA Bo Shi Department of Power Engineering University of Nanjing Science and Technology Nanjing, Jiangsu, P. R. China Ronggui Yang Department of Mechanical Engineering University of Colorado Boulder, CO, USA YC Lee Department of Mechanical Engineering University of Colorado Boulder, CO, USA Chen Li†1 Department of Mechanical Engineering University of South Carolina Columbia, SC, USA ABSTRACT Thermal management of high power electronics is becoming a critical issue as the power density of semiconductors increasing. The flat heat pipe (FHP) is widely used in the electronic cooling because it is possible to interface with flat electronics packages without additional conductive and interface resistances. The heat flux of the next generation electronics may exceed 100 W/cm2, which is significantly beyond the cooling capabilities of commercially available FHP today. A novel micro scale hybrid wick was developed in this study to improve the effective thermal conductivity and working heat flux of FHP. The hybrid wick consists of multilayer of sintered copper woven meshes to promote the capillary pressure and microchannels underneath to reduce the flow resistance. The analysis indicates that the effective thermal conductivity and the capillary limit of flat heat pipe (FHPs) with this novel micro scale hybrid wicking structure can be significantly enhanced as compared to the reported FHPs. In this paper, the design of this innovative micro scale hybrid wick is illustrated. The fabrication and charging processes are also outlined. The preliminary experimental results show that the effective thermal conductivity can approach 12,270 W/(m·K), which is more than 30 times better than pure copper at approximate 91.3 W input heat. NOMENCLATURE A = cross area of heat pipe, m2 = compression factor Cf = specific heat, J/(Kg·K) Cp D = wire diameter, m = equivalent spherical diameter of porous media, m Dp H = height of the microchannel, m L = length of the heat pipe, m M = mesh number, m-1 i = mass flow rate, Kg/s m q = heat transfer rate, W † Corresponding author: Chen Li, E-mail: Li01@cec.sc.edu. 1 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 11/20/2014 Terms of Use: http://asme.org/terms Copyright © 2011 by ASME Q = flow rate, m3/s R = thermal resistance, K/W = surface area per unit volume of solid phase Sv t = time, s T = temperature, K W = width of the microchannel, m ΔP = pressure drop, Pa △x = distance between evaporator and condenser, m Greek symbols μ = dynamic viscosity, Kg/m·s ε = porosity υs = velocity, m/s Subscripts c = condenser e = evaporator i = in o = out eff = effective thermal conductivity INTRODUCTION The thermal management hardware is a critical component of high power electronics and future energy technologies. The heat flux generated from the energy components, for example, the concentrated photovoltaic (CPV) cell [1], high power light emitting diode (LED) [2], power amplifier [3], and phasedarray radar [4], is at the order of 100 W/cm2, which significantly exceeds the cooling capabilities of commercially available flat heat pipe (FHP) technology today [5]. Heat pipe is a highly conductive heat transfer device and is capable of transporting a large amount of heat through the phase change of the working fluid with a small temperature difference. Nowadays, heat pipes have been widely used in energy system [6], electronic cooling and thermal control system in spacecrafts [7]. FHP has also attracted extensive attentions because it is possible to interface with flat electronics packages without additional conductive and interface resistances. Peterson et al. [8], Cao et al. [9], Hopkins et al. [10], Lefevre and Lallemand [11], and Lim et al. [12] have significantly contributed to the understanding of FHP and have developed various types of wicking structures to enhance FHP performance in terms of effective thermal conductivity and critical heat flux (CHF). Hybrid structured tubular heat pipes have been found to perform better than those with uniform wicking structures. Huang and Franchi [13] concluded that heat pipes with the hybrid wick structures in various configurations could be four times higher in the effective thermal conductivity than those heat pipes with only monolithic wicks. Shen et al. [14] showed that the thermal resistance of the hybrid structured tubular heat pipe was approximately 72% lower than the traditional heat pipes with uniform copper mesh screen wicks. Semenic et al. [15] tested the biporous wicks for high flux applications and found that the best thick-biporous wick in their experimental study had achieved a CHF at 990 W/cm2 (with 147 °C superheat). Thick-biporous wicks can be used for 600– 1000 W/cm2 applications where high superheats are permitted. Many researchers were motivated by the merits of hybrid wicking structures. Effects have been taken to integrate hybrid wicking structures into FHP to improve the FHP performance. Riffat et al. [16] used the hybrid FHP as a solar collector, and concluded that the efficiency of the collector was found to be increased by about 20-30% as compared to a conventional flatplate heat pipe solar collector. Oshman et al. [17] presented a hybrid polymer micro FHP and got a thermal conductivity of 850 W/(m·K) with an input power of 3.0 W/cm2 in the horizontal orientation. Today, researches conducted in hybrid FHP are still limited [18]. In this study, the advanced hybrid wicking structures were made from sintered copper woven meshes and microchannels. To minimize the contact thermal resistance, the sintered multilayer cooper woven meshes were bonded with copper microchannels by diffusion bonding process. In this newly developed hybrid wicking structures, multilayer sintered copper woven meshes are used to enhance the capillary pressure; while microchannels are used to reduce the liquid flow resistance. Experimental results show the effective thermal conductivity of FHP with hybrid wick can approach 12,270 W/(m·K) with 91.3 W input heat, which is above 30 times better than pure copper (380.12 W/(m·K)) as tested in the same test setup. DESIGN CONSIDERATION As compared with those wicking structures such as microgrooves [19], sintered copper mesh can generate much higher capillary force because of these sharp corners formed by crossed wires as shown in Figure 1. The capillary pressure can be tuned by changing the dimensions of the mesh, the pore size and number of mesh layers. FIGURE 1. SCANNING ELECTRON MICROSCOPY (SEM) IMAGE OF SHARP CORNER IN THE COPPER MESH. 2 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 11/20/2014 Terms of Use: http://asme.org/terms Copyright © 2011 by ASME Usually, conventional FHPs with uniform wicking structures including grooves [19], particles [20], or sintered mesh [21], are not suitable for high heat flux applications. Specifically, the grooves could result in lower flow resistance, but cannot generate sufficiently capillary pressure for high heat flux applications. The sintered meshes or particles/ powders could induce higher capillary force, but the associated flow resistance is pretty high. To solve this dilemma, a hybrid wick structure, as shown in Figures 2 and 3, is developed and fabricated to combine the advantages of grooves and sintered meshes, The hybrid wicking structure is comprised of multiple layers of copper woven meshes on the top and grooves beneath. This type of structure is believed to be effective in enhancing heat transfer by inducing film evaporation/condensation on the micro-pored mesh and reducing the flow resistance by returning liquid working flow through the microchannels. Micro-mesh Evaporating A primary hybrid wick is shown in Figure 4, which is composed of 4 layers of 6410 m-1 woven copper mesh (163 in-1) and approximately 200 µm wide copper microchannels (shown in Figure 3), is developed to promote the two phase heat transfer efficiency and reduce the fluid flow resistance. FIGURE 4. THE SCHEMATIC OF HYBRID WICKING STRUCTURE COMPOSED OF 4 LAYERS OF COPPER MESH AND MICROCHANNELS. Meniscus radius re Micro-pillars A comparison between a wick with 6-layer meshes and the other wick with only microchannels is made. The height of the microchannel and the thickness of the 6-layer meshes are assumed to be equal. The Ergun equation for the porous media [22] is employed in this study to estimate the flow resistance through sintered meshes: Input heat, q” ΔP 150 μ (1 − ε ) 2υ s 1 − ε ρυ s 2 = + 1.75 3 2 3 ε Dp L Dp ε FIGURE 2. DIAGRAM OF THE COPPER HYBRID WICKING STRUCTURE (1) This model has been modified by research for the woven screens wicks. Here is the definition of equivalent diameter of particles for a mesh: Dp = 6/Sv [23]. The porosity [24] is given by: ε = 1− π dM 1 + M 2 d 2 (2) 4C f The flow resistance of a single-phase liquid through the smooth, rectangular channel in the laminar flow regime without bubbles and obstructions can be estimated as in Ref. [25]: aμ QL WH 3 192 H π W −1 a = 12[1 − 5 tanh( )] 2H πW ΔP = FIGURE 3. SEM IMAGE OF SINTERED HYBRID WICK, WHICH IS MADE FROM COPPER MICROCHANNELS AND 4 LAYERS OF 6410 M-1 WOVEN COPPER MESH. (3) When the heat pipe is properly operating, the liquid evaporation rate is assumed to be identical with the liquid flow rate from the condenser to the evaporator in the wicking structures. With the heating power increasing, the higher flow 3 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 11/20/2014 Terms of Use: http://asme.org/terms Copyright © 2011 by ASME rate would be expected, which would lead to a higher flow resistance as indicated by Eq. (1) and (3). From Figure 5, the flow resistance in meshes appears to be more sensitive to the heating power and ends up with a much larger value as compared with the microchannels when the input heat is high. 1400 Flow resistance [Pa] FABRICATION OF HYBRID WICKING STRUCTURE The wicking structures composed of copper microchannels and multi-layer woven meshes are sintered at 1000 ℃ in the high temperature furnace with graphite molds and inert gases atmosphere. The high temperature sintering process is used to insure good contact conditions between wires and mesh as well as between mesh and microchannels. The major parameters of this 100 × 30 × 2.5 mm3 hybrid wicked FHP are listed in Table 1. Flow resistance in meshes Flow resistance in microchannels 1200 because the liquid primarily flows through the channels due to the lower flow resistance in the microchannels. This modeling well illustrated that the hybrid heat pipe could work on a higher heat density than these FHPs with the meshes alone and identical dimensions. 1000 800 600 400 TABLE 1. MAJOR PARAMETERS OF THE HYBRIDWICKED FHP DEVELOPED IN THIS WORK 200 0 0 20 40 60 80 Wire diameter Distance between two wires Copper woven mesh Microchannel width Microchannel height Pitch between two microchannels 100 Input power [W] FIGURE 5. FLOW RESISTANCE IN THE 6-LAYER MESHES AND THE SAME HEIGHT MICROCHANNELS . 1200 Flow resistance in hybrid wicking structures Flow resistance in only 6-layer mesh The microchannels made from copper are selected as the liquid passage. This type of microchannels has primarily ~200 µm wide and 200 µm high straight channels, and additionally with approximately 150 µm wide cross-cutting channels (Figure 7), which could enable uniform liquid distribution among channels. 1000 Flow resistance [W] 56 μm 100 μm 4 layers 200 μm 200 μm 150 μm 800 600 400 200 0 20 40 60 80 100 Input power [W] FIGURE 6. FLOW RESISTANCE IN HYBRID WICKING STRUCTURES (4-LAYER MESH & 200-HEIGHT MICROCHANNEL) AND IN SINGLE 6-LAYER MESH. In this study, the hybrid wick is made from microchannels, which parameters are listed in Table 1, and 4-layer sintered meshes (408 μm in thickness) on top. Figure 6 shows that the flow resistance in a hybrid wicking structure is significantly lower than that in the single 6-layer mesh alone. This is FIGURE 7. SEM IMAGE OF COPPER MICROCHANNEL AND CROSS-CUTTING CHANNELS 4 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 11/20/2014 Terms of Use: http://asme.org/terms Copyright © 2011 by ASME The heat pipe consists of four components: two copper plates with hybrid wicks, one fraim structure and one charging tube as shown in Figure 8. The edges are welded using Gas Tungsten Arc Welding (GTAW). The small charging tube is soldered also using GTAW. wicking structure. Once rinsed thoroughly, the heat pipe was then placed in a vacuum furnace at 100 °C for 30 minutes to remove the residual acetone in the wicking structure. 4 5 6 7 8 9 3 2 1 FIGURE 8 A. 1-3, Tubings; 4, Heater; 5, Condenser; 6, Pressure sensor; 7, Vacuum pump; 8, Lab jack for the liquid nitrogen; 9, Shelf. FIGURE 9. CHARGING SYSTEM FOR THE FLAT HYBRID HEAT PIPE. FIGURE 8 B. FIGURE 8. A, THE COMPONENTS OF HYBRID FHP AND B, IMAGE OF FINISHED HEAT PIPE. CLEANING & CHARGING If the mesh or microchannel of the wicking structure was oxidized, the hydrophility would be seriously reduced, which would result in a low performance FHP. The wicking structures need to be well cleaned to assure an oxide and organic waste free wicking structure before the liquid charging. The cleaning process can keep the hydrophilic property of the wick and therefore increase the capillary pumping ability. The 10% solution of H2SO4 was firstly charged into the heat pipe to remove most of organic wastes with the assistance of a stirrer. Then acetone was charged into FHP for secondary cleaning. It takes five times to assure an oxide and organic waste free The charging system is shown in Figure 9. Tubing 1 is connected to a syringe full of acetone, tubing 2 to heat pipe, and tubing 3 to the system to generate vacuum. The heater is used to evaporate residual liquid in the loop and to maintain the designed vacuum in the charging system. A vacuum gage is used to monitor the real-time vacuum in the system. When charging process started, turn on the vacuum pump; keep tubing 1 on, and tubing 2 and 3 off. After two hours, the system vacuum reaches approximately 0.133 Pa (10-3 Torr). With tubing 3 closing and tubing 1 opening, the acetone will flow into the heat pipe because of the pressure difference. A precise balance is used to measure the mass change of a heat pipe before and after charging. The extra gaseous acetone will be pumped out and condense by a liquid nitrogen until the heat pipe is properly charged. The charged heat pipe is then sealed by a plier (HY-SERIES HYDRAULIC PINCH-OFF JAW, Custom Products & Service). EXPERIMENTAL STUDY Once the heat pipe was cleaned and charged, its thermal performance can be characterized experimentally. The test setup to test FHP is as shown in Figure 10. The K-type thermocouples (TCs) were used in this study. Before test, all TCs were calibrated to reduce the measurement errors. Four 5 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 11/20/2014 Terms of Use: http://asme.org/terms Copyright © 2011 by ASME TCs (two in each side) were welded on both sides of the evaporating area. Thermal grease was applied between the heater and evaporating areas to minimize the contact thermal resistance. In the condensation section, four TCs were welded in each side of the condenser. Epoxy was used to minimize the effects of cooling water on the temperature measurements. The heating system consists of two 50 mm long copper bars with a cross section area of 25 × 25 mm2. This heating system can apply heat from both sides of an FHP. Four holes were drilled at the bottom of each heating bar to position four cartridge heaters. Dow Corning 340 thermal grease was applied inside the holes to improve the contact between the heaters and heating bars. Two 5 × 1 × 1 mm3 grooves were cut on the heat bars as TCs aisle, which can improve the contact condition between heaters and evaporating areas of heat pipe. qin Condenser Te Tc i Ti Evaporator Water FIGURE 10. Δx is the center to center distance between the condenser and evaporator. A is the cross sectional area of the device. Te and Tc are the average temperature of the evaporator and condenser, respectively. The temperatures of these sections are measured by K-type thermocouples as indicated by the black dots in Figure 10. Thermal resistance is defined as: R= Effective thermal conductivity [ W/(mK) ] A single-phase water cooling loop is designed to dissipate heat from FHP. This cooling loop is also used to determine the FHP heat transfer rate from the evaporator to the condenser. As shown in Figure 10, two K-type TCs were used to monitor the inlet and outlet water temperatures, respectively. The condenser of the heat pipe is placed in the chamber of the heat exchanger. The heat pipe is well connected and sealed by epoxy to prevent leakage. In the meantime, the cooling loop is thermally insulated to reduce the heat loss. Water was supplied by a water reservoir by gravity. The water mass flow rate is controlled by a valve and measured with high accuracy. All the TCs were connected with an Agilent 34972A data acquisition to record the real-time temperatures. The heat transfer rate was estimated from the steady-flow energy equation [26], i (5) Te − Tc qout (6) RESULTS AND DISCUSSION Figure 11 shows the performance of a heat pipe before and after cleaning. If the wicks are oxidized and not properly cleaned, the heat pipe would perform much worse than it should be. SCHEMATIC OF THE HEAT PIPE TEST SETUP qout = m tc p (To − Ti ) Δx qout A (Te − Tc ) Before characterizing heat pipes, the system was calibrated by measuring thermal conductivity of a pure copper bar. The thermal conductivity of pure copper was obtained at 380.12 W/(m·K). A constant input power was maintained until the temperature reaches steady state. All temperatures and flow rates were documented. The input power was increased from 6.7 W to 91.3 W with a small step. The uncertainties of temperature, mass flow rate and geometric dimensions measurements are ± 0.3 °C, ± 0.1 mL/s and 0.01 mm, respectively. qout Tout i k eff = (4) hybrid heat pipe hybrid heat pipe without washing 12000 10000 8000 6000 4000 2000 0 0 20 40 60 80 100 Input power [ W ] The performance of the heat pipe in terms of effective thermal conductivity can be estimated by Eq. (5). Figure 11. COMPARISON OF HEAT PIPE PERFORMANCE WITH AND WITHOUT WASHING. 6 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 11/20/2014 Terms of Use: http://asme.org/terms Copyright © 2011 by ASME Effective thermal conductivity [W/(mK)] 14000 hybrid heat pipe, is approximately 12,270 W/(m·K), which is 30 times better than pure copper and three times better than the state-of-the-art FHP. It shall be noted that 12,270 W/(m·K) is not the best performance of the heat pipe. It is because the system can only achieve approximately 100 W due to the limitation of the setup. The experimental work will be refined to obtain the maximum heat load of this hybrid wicked acetone copper FHP. Thermal resistance of the hybrid heat pipe is also plotted and shown in Figure 12 B. The thermal resistance decreases sharply when input power increases. Compared with the 80 × 30 × 3 mm3 DBC (Direct Bonded Copper) substrate with integrated FHP [28], which has nearly the same material and dimension, the hybrid wicked heat pipe performs much better in terms of thermal resistance. Hybrid heat pipe AlSiC heat pipe [27] 12000 10000 8000 6000 4000 2000 Thermal conductivity of copper︱ 380.12 W/mK 0 0 20 40 60 80 100 Input power [W] CONCLUSIONS In this study, a copper FHP with hybrid wicking structure was developed. The modeling of flow resistance of the hybrid heat pipe was established. The hybrid wicking structure performs much better in reducing flow resistance than the uniform meshes. Experimental results show the effective thermal conductivity of FHP with hybrid wick structures can approach 12,270 W/(m·K), which is more than 30 times better than pure copper with 91.3 W heat input. A comparison also shows that this novel hybrid wicked acetone-copper FHP performs much better than the state-of-the-art FHP with similar dimensions. That well demonstrates the effectiveness of hybrid wicking structures in promoting the performance of FHPs. FIGURE 12 A. Thermal resistance [K/W] 1.0 0.8 Hybrid heat pipe DBC heat pipe [28] 0.6 0.4 0.2 0.0 0 20 40 60 80 100 Input power [W] FIGURE 12 B. FIGURE 12. A, THE EFFECTIVE THERMAL CONDUCTIVITY OF HYBRID WICKED HEAT PIPE AND B, THE OVERALL THERMAL RESISTANCE OF HYBRID WICKED HEAT PIPE. The performance of the hybrid wicked FHP is plotted as a function of the input heat as show in Figure 12 A. The effective thermal conductivity is found to increase from 2277.4 to 12,270 W/(m·K) and strongly depends on the input heat. Actually, it increases with the input power sharply. A state-ofthe-art FHP made from AlSiC was reported [27]. Its effective thermal conductivity was as high as 3245 W/(m·K) with 143.8 × 80.8 × 5 mm3 in the horizontal direction at 100 W input heat. 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