Design, fabrication and performance tests for a ...

Energy Conversion and Management 70 (2013) 10–19

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Design, fabrication and performance tests for a polymer-based flexible flat heat pipe Shou-Shing Hsieh ⇑, Ya-Ru Yang Department of Mechanical and Electromechanical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC

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Article history: Received 17 October 2012 Accepted 11 February 2013 Available online 19 March 2013 Keywords: Flexible heat pipe Charge ratio Thermal resistance

a b s t r a c t In this paper, we report on the novel design, fabrication and performance tests for a polymer-based flexible flat heat pipe (FHP) with a bending angle in the range of 15–90°. Each heat pipe is 4 mm thick, 20 mm wide and 80 mm long, with two layers of No. 250 copper mesh as the wicking material. A copper/silicone rubber hybrid structure is designed and fabricated to achieve the flexibility of the heat pipe. Thermal characteristics are measured and studied for de-ionized water under different working conditions. Experimental results reveal that a bending angle of 15° on the vertical plane has a better thermal performance than those of heat pipes with/without bending. In addition, a higher power of 12.67 W can be transferred/ delivered. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction As electronic equipment, such as personal and laptop computers, becomes smaller and more compact, the CPUs generate increasing amounts of heat which can cause serious problems. With the ever-increasing heat dissipation demand of high-power electronic devices, cooling devices must be highly efficient, low in weight, compact in size and flexible enough to meet the cooling requirements. Generally, due to the space limitations for cooling and heat dissipation, flat heat pipes (FHPs) have been widely used for more than two decades [1], and performance tests have been carried out for many different types of micro-FHPs [2]. The working mechanism of an FHP is the same as that of a conventional heat pipe with cylindrical geometries [3]. An FHP for use in electronic equipment normally consists of a container made of copper, with water as the working fluid and the wick serving as the flow path of the working fluid. For mounting on a laptop computer, most heat pipes are flattened with a rectangular cross section. So far, most FHPs have been constructed using thermally conductive metals due to their high thermal conductivity and high strength. In addition, these metals have the added merits of low thermal resistance, easy fabrication and tightening. Flexible and non-electronically conductive heat pipes for special applications are very limited. Polymers have recently received attention as a possible candidate for thermal management applications. The major advantages of using a polymer for the casing material are its flexibility, chemical resistance and electrical insulating characteristics. Gillot et al. ⇑ Corresponding author. Tel.: +886 75252000x4215; fax: +886 75254215. E-mail address: [email protected] (S.-S. Hsieh). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.02.011

[4] fabricated a silicon-based heat pipe with rectangular capillary grooves formed with deep plasma etching, and tested its performance by measuring the temperature difference between the evaporator and condenser. In 2001, McDaniels and Peterson [5] proposed a flexible polymer-based micro-heat pipe applicable to space applications, based on a performance analysis. Based on their study, an effective thermal conductivity of 740 W/m K, a length of 16.4 cm and a groove width of 30 lm were exhibited. Lim et al. [2] reported a heat transfer capacity of 8 W under adverse gravity conditions in an experiment where the evaporator was situated vertically above the condenser. Most recently, Oshman et al. [6] presented the fabrication and application of a micro-hybrid wicking structure in a flat polymer heat pipe heat spreader with a size of 80 40 1.2 mm3 under adverse acceleration. The performance was found to be 31–61 W/cm2, and the capillary pumping capability of the wick structure was assessed from zero to ten times the force of gravity. Lefèvre et al. [7] reported an experimental investigation of flat plate heat pipes with screen meshes or grooves covered with screen meshes; they found that Q = 1.9–72.8 W with the thermal resistance decreasing slowly from 0.2 K/W to 0.04 K/W when the grooves were covered by two screen mesh layers, which was less promising compared to that of one or two screen mesh layers. Wang [8] investigated the startup behavior in axial grooved heat pipes with different bending angles. They found that the effect of inclination on the bent heat pipes was significant. Yang et al. [9] presented a paper regarding recent developments of lightweight, high-performance heat pipes; their extensive review of the literature determined that micro-flat heat pipes are widely applied in electrical device cooling with a smaller pressure drop needed across the wick structure. Grooved heat pipes have been found to

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S.-S. Hsieh, Y.-R. Yang / Energy Conversion and Management 70 (2013) 10–19

Nomenclature A P Ra Rc Re Rt

surface area, m2 pressure, kg/m2 thermal resistance of adiabatic region, °C/W thermal resistance of condenser, °C/W thermal resistance of evaporator, °C/W total thermal resistance (=Ra + Rc + Re), °C/W

have a large heat transfer capacity when the pipe has a round/circular cross section. However, with an FHP, the maximum heat transport capacity is dramatically decreased [1]. In order to overcome this problem, the aim of this study was to take advantage of the present configuration using the mesh-type pillar, which was used for strengthening the heat pipe to avoid shrinkage during the air-extraction process and so increase the capillary force. In this study, a novel design, fabrication and thermal performance tests were completed for a flexible polymer-based FHP with bending angles on the vertical plane in the range of 15–90°. Silicone rubber was used as a substrate for the longitudinal liquid gas flow channels; copper-filled thermal vias were embedded in the evaporator and condenser parts of the FHP to increase the thermal conduction through the casing. Two layers of brass mesh (No. 250) were bonded to the side wall with the mesh serving to increase the capillary pressure as well as the surface area of the evaporator and condenser. Thermal characteristics, including startup performance, temperature distribution and thermal resistance of the heat pipe, were extensively studied (see Tables 1 and 2). 2. Design and fabrication

Q q T Te h

total heat input, W heat flux (=Q/A), W/m2 temperature, °C evaporator average temperature, °C bending angle, °

condensing areas. The thickness, width and total length of the FHP were 4 mm, 20 mm and 80 mm, respectively. A mesh-type pillar (size 2 1.6 mm2) was positioned at the center of each cross section. The measurements were in accordance with the general design considerations of a conventional heat pipe, such as boiling limit, capillary limit and sonic limit. In order to meet the cooling needs of modern small and compact electronic devices, the proposed micro-heat pipes had the dimension characteristics of rc/rh P 1, where rc is the capillary radius and rh is the hydraulic radius. In addition, as stated previously, in order for the heat pipe to operate normally, the capillary pressure DPc should be larger than the total pressure drop (gravitational losses, DPg; pressure drop of vapor flow, DPv; and liquid flow, DPl). Hence, DPc P DPg + DPv + DPl [9]. The wick was composed of a two-layer woven copper mesh (No. 250) bonded to the top and bottom. Also, such a wick was applied for the pillar. Before bonding, impurities were removed by cleaning the upper/lower half of the heat pipe using an ultrasonic vibrator and organic solvent detergent. After completing the FHP device, air extraction (both air and non-condensable gas) was carried out for at least 10 min; the vacuum was then maintained via a vacuum pump and the working fluid was ejected into the FHP.

2.1. Copper/silicone-rubber hybrid structure design 2.2. Fabrication of flexible FHP The proposed FHP in the present study was developed, as shown in Fig. 1. The FHP consisted of an evaporator (right end), an adiabatic section (middle part) and a condenser (left end). The heat pipe works through the use of a two-phase heat transfer loop. The material of the FHP was silicone rubber, except for parts of the evaporator and condenser. To improve thermal conductance, copper-filled thermal vias were designed and fabricated for the evaporating and Table 1 Characterization of heat pipe and experimental parameters.

The major fabrication steps for the present flexible heat pipe are depicted in Fig. 2a–e. Due to the special requirements regarding the channel wall and flow compatibility, materials such as silicone rubber, brass sheeting and brass wire mesh were used to fabricate the heat pipe. The fabrication steps, briefly described below, were as follows: (1) Brass wire mesh cutting

Characterization of heat pipe Heat pipe length, L (mm) Heat pipe width, W (mm) Heat pipe height, H (mm) Evaporator length, Le (mm) Condenser length, Lc (mm) Effective length, Leff (mm) Evaporator area (cm2) Condenser area (cm2) Vapor channel dimension, Lv Wv Hv (cm3) Wick material Wick structure Layers Screen wire diameter, d (mm) Spacing of the layer, s (mm) Pillar’s wick material Pillar’s wick structure Fiber diameter, d0 , (mm)

80 20 4 20 10 55 11 22 70 10 2 Cu #250 screen 4 0.04 0.16 Cu Fiber 0.1

Experimental parameters Input power (W) Working medium Room temperature, T1 (°C) Bending angle (°) Filling ratio (%)

1–12 DI water 25 15, 30, 45, 60, 90 25–50

The inner dimensions were measured at 72 72 cm2; 250mesh network brass was cut to the desired dimensions. (2) Brass sheet cutting The brass sheet was cut into two pieces, with the dimensions of 1 1 cm2 and 2 2 cm2, and used as the base material for the evaporation and condensation ends of the heat pipe, respectively. (3) Preheating A dynamic mold and a static mold were placed in the oven for preheating at 270 °C for 45 min. Table 2 Properties of silicone rubber and copper sheet.

Thermal conductivity (W/m K) Melting point (°C) Boiling point (°C) Tensile strength (MPa) Density (g/cm3)

Silicone rubber

Cu sheet

2.00 45 200 12 1.13

391 1083.4 2567 196 8.97

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Fig. 1. Schematic of the present heat pipe.

(4) Spraying interface agent and release agent The dynamic and static molds were covered with a release agent; in addition, an interface agent was applied to the brass sheet to allow a tight connection between the brass sheet and the silicone rubber. (5) Material preparation (Fig. 2a) The molds were preheated until they reached a uniform temperature distribution; the silicone rubber and brass sheet were then placed in the mold cavity. (6) Embossing (Fig. 2b) The dynamic and static molds were connected and adjusted to a certain strength (10 kg/cm2) to allow for the formation of an opposite micro-structure pattern. (7) Heating (Fig. 2b) After the molds were pressed, they were put in the oven and baked at 270 °C for 30 min. (8) Holding (Fig. 2b) After baking, the molds were cooled. The pressure of the dynamic and static molds was maintained at a specific level (1 kg/cm2). (9) Opening the mold (Fig. 2c) After the cooling stage was finished, the dynamic mold was raised in order to remove the finished product. Further cooling was required for final shrinking and shaping. (10) Adhesion of the brass wire mesh (Fig. 2d) First, four pieces of brass wire mesh cut in advance were placed on the heat pipe, with two on the top and two on the bottom. During this process, the brass wire mesh adheres to the silicone

rubber with immature silicone gel, thereby connecting the brass sheet with the heat pipe. (11) Adhesion of copper fiber (Fig. 2d) Copper fiber was cut in the pillar as a fiber wick. (12) Assembly of top half and bottom half of the heat pipe (Fig. 2e) The top and bottom of the heat pipe were tightened together with silicone gel. Upon completing the fabrication process, proper cleansing was carried out.

3. Experiment 3.1. Experimental apparatus and parameters The structure of the experimental apparatus is shown in Fig. 1. The proposed heat pipe was tested through water cooling with various bending angles on a vertical plane (see Fig. 3 for details) at different heating powers to measure its thermal performance. Fig. 4 shows three images (a–c) of the present system with 0°, 30° and 90° bending angles, respectively, tested. The experimental apparatus, shown in Fig. 4d, consisted of the main test section, an electrical heater, a cooling bath and data acquisition with a PC. Temperature measurements were in two parts: 15 thermocouple (T type, 80 lm) measurements along the heat pipe wall, and then another 5 thermocouples (T type, 80 lm) for lateral position measurements. All of the 20 thermocouples were embedded in the heat pipe wall, with a 0.1 mm thickness, by drilling tiny holes (dia ’ 100 lm). Details of the placements and positioning are shown in Fig. 5. The entire flow loop for the test apparatus is shown in Fig. 6. A copper block heater was used as the power supply. The size of the heater was 1 mm 1 mm with a 1 mm thickness. The condenser was cooled by water flow. A variaccontrolled AC power supply, a current shunt (15 X with 1% accuracy) and two precision multimeters, one for current and one for


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Fig. 2. Fabrication process of the heat pipe.

voltage measurement and control of the input electric power with an accuracy of ±1%, were employed. The optimum charge of the working medium (DI water) was examined and obtained by varying its charge ratio in the heat pipe without bends for different heat inputs, as shown in Fig. 7. The total volume of the heat pipe was measured to be about 2000 ml. The minimum evaporation temperature could be acquired based on this examination, which indicated an optimum charge ratio of about 40% for a power input of 1–12.7 W to avoid system dry out. After several trial runs, it was found that the maximum power input was about 12.8 W for all the cases under study. Unless otherwise stated, the optimum charge ratio was used throughout this study, even for heat pipes with bends. 3.2. Experimental conditions and data reduction The thickness, width and total length of the flexible flat heat pipe were 4 mm, 20 mm and 80 mm, respectively. The brass mesh number was 250 with a thickness of 0.25 mm. The porosity of the brass mesh was 60%, the effective pore radius 10 lm, the perme-

ability 6.09 1012 m2 and the thermal conductivity 1.5 W/m K. The input power varied in the range of 1–12.7 W. The fill ratio of the working liquid varied. All the tests were conducted in a normal environment with the condenser cooled by forced convection with a mass flow rate of 1 ml/s at an ambient temperature of 25 ± 1 °C. In order to monitor the heat loss through the insulation surface, thermocouples were also installed on both the inner and outer surfaces of the polystyrofoam thermal insulation. The prepared heat pipes were cleaned with chlorinol and water, and then acetone. After clamping the heat pipe on the copper block with thermal grease and adjusting the test fixture of the heat pipe to the required angle on a vertical plane, based on the illustration shown in Fig. 3 (evaporator section is setup below), air-extraction began and, thereafter, the working fluid was fed. It took about 30– 40 min for the temperature curve to reach steady state. Generally speaking, the theoretical aspects of a heat pipe consist of the fundamental processes of fluid dynamics and heat transfer. Fluid mechanics usually includes the axial liquid pressure drop in the wick structure, the maximum capillary pumping force and the vapor flow in the vapor core of the channel. Heat transfer is

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Fig. 3. Bending angle on vertical plane (in clockwise) with the corresponding radius of curvature.

Fig. 4. Images of three test heat pipes and experimental apparatus setup.


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Fig. 5. Detailed temperature measuring positions and thermocouple placement with a thermal resistance model.

Fig. 6. Schematic of heat pipe experimental setup.

Fig. 7. Evaporator temperature (without bending) vs working medium charge ratio.

the transfer of heat in and out of the heat pipe. The physical behavior of a heat pipe covers conjugate heat conduction between the wall and wick, evaporation and condensation at the liquid–vapor

interface and forced convection in the vapor core and in the wick. In this study, to avoid an increase in contact resistance, a very thin film of thermal grease was applied to the interface of the heat pipe wall at the evaporator region; the heat pipe wall at the condenser region was in direct contact with the cooling water. In order to model the physical behavior of a heat pipe and estimate the heat pipe performance, a 1-D simple thermal resistance model similar to that in Fig. 5c was used in the present study [10]. For simplicity, by neglecting Rww, Rec and Rcc and considering Ra = Rv in Fig. 5c, the present simplified model could be considered as a 1-D series network rather than a combined parallel and series configuration. Note that the transfer heat of the present FHPs was being tested two dimensionally but, for convenience, the 1-D simple total thermal resistance model was used instead for data presentation without loss of generality [6]. Axial conduction through the heat pipe wall and wick structure was also neglected because the heat pipe wall thickness was small compared to that of the flow channel, and because the wick structure had a high thermal conductivity. With the proper results obtained, the maximum heat transfer capacity and thermal resistance could be calculated. The thermal resistance of evaporator Re (including Rel, Rewi and Rew), the adiabatic region Ra (only Rv) and condenser Rc (including Rcl, Rcwi and Rcw) were defined based on the equations below:

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Re ¼ ðT e T a Þ=Q_ in

ð1Þ

Ra ¼ ðT e T c Þ=Q_ in

ð2Þ

Rc ¼ ðT e T a Þ=Q_ in

ð3Þ

Here, Q_ in ¼ Q_ out and total thermal resistance Rt = Re + Ra + Rc, assuming that each component is connected in series, and neglecting the thermal contact resistance between the heat pipe and the water bath and the evaporator [1]. The maximum heat transfer capacity is defined according to where dry out occurs. Upon its occurrence, the total thermal resistance Rt increases suddenly and the maximum heat transport capacity is found. 3.3. Uncertainty analysis Uncertainty estimates were made considering the errors of the instruments, the measurement variance, geometric uncertainty and calibration errors for the heat flux and temperature measurements. The maximum variation of the 15 measured wall temperatures was ±0.3 °C at the maximum power input. The uncertainty for the saturation temperature in the core of the heat pipe was estimated to be less than ±0.1 °C. Wall conduction heat losses were quantified at different power inputs. This loss varied between 15% and 3% for heat input between 1 W and 12.7 W, respectively. The other primary contributor to heat flux uncertainty was the condenser or evaporator surface area. Combining these effects led to an overall uncertainty estimate in the heat flux of 15% at the lowest heat input. 4. Results and discussion Unless otherwise stated, the angular effect was on the vertical plane; namely, the angle was from the horizontal plane, as it is known [11] that the heat transfer performance of a bent heat pipe becomes poorer as the bending angle increases when considering the angular effect on the horizontal plane. 4.1. Thermal performance for heat pipe without bend (0°) Fig. 8a illustrates the steady-state temperature distributions along the heat pipe wall near the front side in the longitudinal direction for the evaporator, adiabatic and condenser regions. Three different temperature scales were noted. Generally, the longitudinal temperature decreased along the pipe wall from the evaporator to the condenser. For each region, the temperature seemed to be uniform, independent of the longitudinal direction; the differences were small for the evaporator and condenser regions, but a bit greater in the adiabatic region, as also evidenced by Jiang et al. [12]. In the case of a larger heat flux (e.g., 12.7 W, 12 W and 10 W), the longitudinal (axial) temperature distribution had an almost uniform temperature distribution (i.e., isothermal) at the different measuring points (15 points), as shown in Fig. 8a. The spanwise (radial) heat pipe wall temperature distributions at the evaporator, adiabatic and condenser were also plotted against the width of the heat pipe, as shown in Fig. 8b. The five temperature distributions measured also showed a nearly uniform distribution, indicating that a 1-D approximation for the longitudinal direction was possible. In addition, the variations of local temperature, as compared to the average value, were quite small (