Ultra Thin Heat Pipe and its Application

Transcription

1 1th IHPS, Taipei, Taiwan, Nov. 6-9, 11 Ultra Thin Heat Pipe and its Appliation Hirofumi Aoki, Masami Ikeda and Yuihi Kimura FURUKAWA ELECTRIC Co., Ltd., Higashi-yawata 5-1-9, Hiratsuka, Kanagawa, , Japan Tel : , Fax: , ABSTRACT Thermal performane of heat pipe beomes worse rapidly when heat pipe is flattened in order to be mounted on eletroni equipments. This is beause maximum heat transfer rate and thermal resistane of heat pipe are strongly related to inner vapor and liquid flow spae, so heat pipe does not work well when these spae are narrowed. In order to overome this issue, we have developed ultra thin heat pipe whih has both high maximum heat transfer rate and thin thikness by optimizing inner struture. In this paper, we report development of ultra thin heat pipe and some appliations of it. Keywords: Ultra Thin Heat Pipe,Maximum Heat Transfer Rate, Thermal Resistane, Thermal Solution 1. INTRODUCTION Reently, most eletroni equipments tends to be thinner and lighter, so heat pipe, whih is used in thermal solution to ool down heat soure suh as semiondutor devie, needs to be thinner and lighter too. But heat pipe has unfavorable feature that thermal performane beomes worse when heat pipe is flattened. When heat pipe is flattened, pressure drop originated in vapor and liquid flow are inreased remarkably beause of narrow spae for vapor and liquid flow. Heat pipe does not work when inner pressure drop ross over apillary pumping head of wik struture. Therefore, in order to realize heat pipe whih has both thin thikness and high performane, optimization of inner struture is needed. Figure 1 shows relationship of maximum heat transfer rate and heat pipe thikness. Maximum heat transfer rate of onventional groove heat pipe is shown in huthed area. Groove heat pipe has large maximum heat transfer rate when heat pipe thikness is larger than 3mm. However, when heat pipe thikness is 2mm or less than, maximum heat transfer rate drops to almost zero. In order to overome this problem, we utilize mesh wik, whih is optimized by theoretial alulation, and the ontainer with thin wall thikness. Then, we have developed the ultra thin heat pipe, shown by dotted area in Fig.1, whih has high maximum heat transfer rate. Maximum heat transfer rate [W] Horizontal working, T a =5 o C Ultra thin heat pipe Groove heat pipe Flattened Heat pipe thikness [mm] Round Figure 1. Heat pipe thikness dependene of maximum heat transfer rate 2. THEORETICAL STUDY OF THIN HEAT PIPE 2.1. Pressure balane in heat pipe Heat pipe for ommon use onsists of opper ontainer, deionized water as working liquid, and wik struture. There are several types of wik suh as groove, sintered metal, wire, and mesh, whih are seleted depending on speifiation. Though groove wik is generally adopted beause of its simple struture and high reliability, it is unfavorable for thin heat pipe beause apillary pumping head of it is very small. In order for heat pipe to operate normally, the apillary pumping head P should be larger than total pressure drop in heat pipe. Though there are several kinds of pressure drop in heat pipe, it is vapor flow pressure drop P v and liquid flow pressure drop P l that determine thermal performane of heat pipe mainly. Thus, formula (1) is approved. P P v + P l (1) If this inequality is not satisfied, working liquid annot return to evaporator setion from ondenser setion and then dry-out will our. In

2 ase of thin heat pipe, both vapor and liquid flow pressure drop beome larger beause their flow hannels are restrited. Figure 2 shows inner struture of flattened heat pipe. In suh struture, ross setion of vapor hannel approahes to retangle, then vapor flow pressure drop is expressed by next equation [1]. 12µ Q v Pv = L (2) eff s π ρvabd λ(1 + s) (1 4 tanh ) 5 π 2s On the other hand, P l is deided by permeability of wik aording to Dary s law [3]. µ Q l Pl = L (3) eff KAw ρlλ Then, we tried to develop ultra thin heat pipe by optimizing both vapor and liquid hannel in order to minimize total of vapor and liquid pressure drop. Vapor flow hannel Liquid flow hannel Figure 2. Inner struture of flattened heat pipe 2.2. Optimization of thin heat pipe In this setion, we show how to optimize inner struture of thin heat pipe. As mentioned above, beause vapor and liquid pressure drop are very high in thin heat pipe, large apillary pumping head is needed to drive working fluid irulations. Capillary pumping head of wik struture is estimated from following formula. 2δ P = osθ (4) r Table 1 shows property of typial wiks whih are ommonly used [2]. From table 1, we an see that groove wik is unfavorable for thin heat pipe beause of low apillary pumping head. Then, we onsidered improving thermal performane of heat pipe by using mesh wik or sintered metal wik. Table 1. Property of typial wik struture [2] r [m] P [Pa] K [m 2 ] 1 mesh mesh Sintered metal Groove Though we mentioned that high apillary pumping head is needed, it is not enough for thin heat pipe. When the effetive pore radius of wik is redued and then apillary pumping head is inreased, liquid flow pressure drop is inreased beause permeability is dereased. In this ase, maximum heat transfer rate is restrited by liquid flow pressure drop. From formula (3), it is needed that liquid flow hannel is enlarged in order to redue liquid flow pressure drop. However, when liquid flow hannel is enlarged, vapor flow hannel is narrowed and vapor flow pressure drop restrits thermal performane in turn. Therefore, it is neessary to onsider the balane of vapor and liquid flow pressure drop. Figure 3 shows wik width dependene of maximum heat transfer rate in sintered metal heat pipes whose thikness is 1 mm and length is 15 mm. From Fig. 3, we an see that high apillary pumping head does not always yield high performane. Maximum heat transfer rate [W] Wik width [mm] r 1 =1 miron r 1 =1 miron Figure 3. Maximum heat transfer rate in various wik width. (sintered metal ; r =1 m. 1 m) Between 1 m and 1 m, the position of peak is different and it depends on wik width whih heat pipe has higher maximum heat transfer rate. The differene aused by pressure balane in heat pipes. Pressure balane normalized by apillary pressure head in the heat pipes is shown in Fig

3 (a) 1.2 P/ P P/ P (b) Wik width [mm] Wik width [mm] P Pv v Pl l P Pv v Pl l Figure 4. Pressure balane normalized by P in thin heat pipe. ((a)r =1m, (b)r =1m) As for r =1 m, influene of liquid flow pressure drop is very large beause permeability is low. In this ase, wik width is preferable to be larger in order to extend liquid flow hannel. So, the peak of maximum heat transfer rate exists in the side of large width. On the other hand, with respet to r =1 m, relationships between superior and inferior of liquid and vapor flow pressure drop hange plaes depending on wik width. When wik width is narrow, liquid flow pressure drop is larger than vapor flow pressure drop. The opposite is approved too. In general, the former type of wik is preferable for thinner heat pipe, and the latter yields high performane in thiker heat pipe shown in Fig.5. Maximum heat transfer rate [W] r =1 miron 1 r =1 miron Groove Heat pipe thikness [mm] Figure 5. Thikness dependene of maximum heat transfer rate in sintered metal wik heat pipe. 2.3 Maximum heat transfer rate of ultra thin heat pipe Considering from the above, we utilized mesh wik whih has moderate apillary pressure head and moderate permeability for ultra thin heat pipe. Figure 6 shows theoretial value of maximum heat transfer rate in ultra thin heat pipe. Maximum heat heat transfer tranfer rate [W] Heat pipe thikness [mm] Figure 6. Thikness dependene of maximum heat transfer rate in ultra thin heat pipe. When heat pipe thikness is 1 mm, maximum heat transfer rate is expeted to be 21.4W. 2.4 Thermal resistane of ultra thin heat pipe Thermal resistane is also important parameter whih indiates thermal performane of heat pipe and it an be also expeted theoretially [4]. In this paper, only alulated result is shown in Fig. 7, beause of spae limitation. Thermal resistane [ o [C/W] Heat pipe thikness [mm] Figure 7. Thermal resistane of ultra thin heat pipe (numerial value) When heat pipe thikness is thinner, thermal resistane inreases remarkably beause vapor flow pressure drop beomes large exessively. 3. EXPERIMENTAL EQUIPMENT AND PROCEDURE Experimental apparatus is shown in Fig. 8. Copper blok heater was used for heat soure whose size was 4 mm 1 mm. For ondenser setion, opper blok whih was ooled by water ooling system was used. Condenser size was 85 mm 1 mm. Heat pipe was attahed to opper bloks at both evaporator and ondenser setion

4 on ontat with thermal grease. Personal Computer Digital multimeter Power Supply A V Data Aquisition Heater Thermoouples Heat Pipe Insulator Cooling plate Water ooling system Figure 8. Shemati of experimental setup Temperature distribution was measured by thermoouples. Figure 9 shows thermoouples loation on heat pipe. In addition, temperature of heater blok was measured. For thermal performane test, temperature on adiabati setion was kept 5 o C and heat pipe was settled in horizontal position. T e1 4mm mm Evaporator Adiabati T a T 1 T 2 T 3 T 4 T 5 85mm Condenser Figure 9. Thermoouple Loation Experimental proedure of thermal performane test is given below. 1. Attah thermoouples to heat pipe. 2. Attah heat pipe to opper bloks of both evaporator and ondenser setion on ontat with thermal grease. 3. Clamp heat pipe on opper bloks and adjust test fixture of heat pipe to required angle. 4. Turn on power of heater and water ooling system and reord temperature of all thermoouples. 5. Adjust water temperature so that adiabati temperature will be 5 o C. 6. Wait until eah temperature reah steady state and reord temperature whih is averaged for the last 5 minutes. 7. Inrease heat load and repeat proedure 4 to 6 until dry-out is ourred. From obtained result, maximum heat transfer rate and thermal resistane will be known. Here, thermal resistane of evaporator R e-a, and heat pipe R hp is defined as below equations. R e-a = (T e T a ) / Q (2) R hp = (T e T ) / Q (3) When dry-out is ourred in heat pipe, R e-a will inrease suddenly. We defined maximum heat transfer rate as adjaent heat input at whih dry-out was ourred. 4. RESULT AND DISCUSSION 4.1 Thermal performane of ultra thin heat pipe Figure 1 shows thermal performane of ultra thin heat pipe whose thikness is 1mm and length is 15 mm. As diagram indiates, maximum heat transfer rate is 22.5 W and thermal resistane of heat pipe is.13 o C/W. We an onfirm that experimental result agrees with alulation well (Fig.6 and Fig. 7). (a) T [oc] [ o (b) Thermal Resistane [ o [C/W] Te T e Ta T a T1 T T2 T T3 T T4 T T T Measurement Point Heat Input [W] 1W 12.5W 15W 17.5W W 22.5W 25W Re-a R Rhp R Figure 1. Thermal performane of ultra thin heat pipe. (a)temperature distribution, (b)thermal resistane In order to adopt ultra thin heat pipe to eletroni equipment, it is neessary to be bended. In the present, minimum bending radius is 25 mm and performane degradation is not found when heat pipe is bended. So, ultra thin heat pipe an be applied to most of thermal solutions in eletroni equipments. Figure 11 shows maximum heat transfer rate of ultra thin heat pipe in various thikness. In the area where thikness is 1mm or less, experimental value and alulated value are orresponding well. However, there is about % error margin in these values at 1.5mm thikness. Though vapor flow hannel is approximated to retangle when vapor flow pressure drop is alulated by formula (2), vapor flow hannel geometry at 1.5 mm thikness goes away from retangle shape. Then, the error margin omes into being. Therefore, it is neessary to alulate vapor flow pressure drop by other methods in suh area

5 Maximum heat heat transfer tranfer rate [W] experimental alulated Heat pipe thikness [mm] Figure 11. Maximum heat transfer rate of ultra thin heat pipe in various thikness 4.2 Appliation of ultra thin heat pipe In this setion, some appliations of ultra thin heat pipe are introdued. Figure 12 shows an appliation of remote heat sink. The heat sink onsists of 1mm thikness heat pipe,.5mm thikness attah plates and 5mm height fin units, so maximum thikness of this heat sink is only 6mm. All parts of heat sink are bonded with solder. 1mm heat pipe heater 1 heater 2 our up to total 27W. Here, maximum heat transfer rate of this heat sink is more than the value whih is shown in last setion, beause effetive length of heat pipe is shorter and working temperature is larger than 5 o C. Thus, ultra thin heat pipe an be used for remote heat sink whose thikness needs to be thin suh as mobile PC. 7mm 115mm Heater 1 1 mm 2.8mm Heat Pipe 6mm.2mm Aluminum Plate Figure 14. Heat spreader apparatus 6mm Next, we introdue another appliation of ultra thin pipe as heat spreader [5]. Figure 14 shows manufatured heat sink whih onsists of.8mm thikness heat pipe and.2mm thikness nikel plated aluminum plate. These parts are soldered for assembly. (a) fored onvetion maximum thikness ~ 6mm.5mm attah plate Figure 12. Remote heat sink apparatus There are two heat soures whih are attahed to eah attah plates with thermal grease. Fin units are ooled by entrifugal fan (not illustrated) on ondition that fan voltage is onstant. Measurement is onduted in open air and same heat input is added to eah heat soure from 3W to 13.5W respetively. In that ondition, thermal resistane between heat soures and room temperature is measured. R C-A [ o C/W] heater1 heater Heat input [W] Figure 13. Experimental result of remote heat sink Experimental result of remote heat sink is shown in Fig. 13. We an see that heat pipe works normally in measured area and dry-out does not (b) Figure 15. Temperature distribution of heat sinks (a) graphite sheet (b) heat pipe + aluminum plate We ompared the heat sink and.3mm thikness graphite sheet whih has same shape. Cerami heater was used for heat soure and attahed onto heat pipe or graphite sheet diretly with thermal grease. Then, we monitored temperature distribution of heat spreaders. Figure 15 shows experimental results on ondition that heat input is 5W and ooling is natural onvetion

6 We an see that heat sink suppressed heat spot better than graphite sheet and lowered heat soure temperature by 17 o C. From what has been disussed above, we an onlude that ultra thin heat pipe an be applied to various thermal solutions, suh as remote heat sink and heat spreader, et., whih is demanded to be thin for mounting to eletroni equipments. 5. CONCLUSION Theoretial study and experimental researh of ultra thin heat pipe was done. Conlusion of this paper is as follows. 1. We have developed ultra thin heat pipe whih has high maximum heat transfer rate by optimizing inner struture. 2. In the range whih heat pipe thikness is 1mm or less, experimental value and alulated value are orresponding well in thermal performane. But, it is neessary to improve alulation method of vapor flow pressure drop in the area where vapor flow hannel geometry goes away from the retangle. 3. Ultra thin heat pipe an be used for various thermal solutions whose thikness is needed to be thin. ρ l density of liquid [kg/m 3 ] θ ontat angle [ o ] REFERENCES [1] Masafumi Katsuta, Tomoya Shindo, Koihiro Saito, Junji Sotani, Yuihi Kimura and Yoshio Nakamura, Heat transfer steady and transient harateristis in flat palate miro heat pipe, Proeedings of the 13 th International Heat Pipe Conferene, 36, 4. [2] Bienert, W. B. and Skrabek, E. A. Heat Pipe Design Handbook, Contrat Rept. No.NAS , [3] Seo, J. K., A generalized mathematial model to aount for the effet of liquid-vapor international shear stress of a heat pipe with a groove wik struture: modified shah method, Proeedings of the 7 th International Heat Pipe Symposium, 3. [4] Ohshima K., Matsushita T., and Murakami M., Hear Pipe Engineering, Asakura Shoten Ltd., 1981 (in Japanese) [5] 9/ele_971.htm NOMENCLATURE A on area of ondenser setion [m 2 ] A eva area of evaporator setion [m 2 ] A w wik ross setion of area [m 2 ] D Hydrauli diameter [m] K Permeability [m 2 ] L eff effetive length [m] Q heat input [W] r effetive pore radius [m] R e-a thermal resistane between evaporator and ondenser setion [ o C/W] R hp thermal resistane of heat pipe [ C/W] s aspet ratio T a adiabati temperature [ o C] T temperature of ondenser setion [ o C] T e temperature of evaporator setion [ o C] δ surfae tension [N/m] P apillary pumping head [Pa] P l liquid pressure drop [Pa] P v vapor pressure drop [Pa] λ latent heat of evaporation μ v absolute visosity of vapor [Ns/m 2 ] μ l absolute visosity of liquid [Ns/m 2 ] ρ v density of vapor [kg/m 3 ] - 4 -