Thermal analysis of reservoir structure versus capillary pumped loop

Thermal analysis of reservoir structure versus capillary pumped loop

Available online at www.sciencedirect.com Applied Thermal Engineering 29 (2009) 186–194 www.elsevier.com/locate/apthermeng Thermal analysis of reser...

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Available online at www.sciencedirect.com

Applied Thermal Engineering 29 (2009) 186–194 www.elsevier.com/locate/apthermeng

Thermal analysis of reservoir structure versus capillary pumped loop Hung-Wen Lin a,*, Wei-Keng Lin b,1 a

Energy and Environment Research Laboratories, Industrial Technology Research Institute, 408A, Building 64, 195, Section 4, Chung Hsing Road, Chutung, Hsinchu 31040, Taiwan, ROC b Department of Engineering and System Science, National Tsing-Hua University, 101, Section 2 Kuang Fu Road, Hsinchu 30013, Taiwan, ROC Received 20 November 2006; accepted 13 February 2008 Available online 20 February 2008

Abstract Capillary pumped loop (CPL) was already used in man-made satellites and space aircrafts with proven heat control technology. However, small-sized CPL had not yet made a breakthrough application in electronic components owing to poor heat-absorption capacity of evaporator structure. Hence, a small-scale CPL was designed for server in this research. The evaporator was designed with a circular groove and embedded with a high density polyethylene (HDPE) as a capillary structure to absorb working fluid. The influence of reservoir upon thermal resistance was also analyzed. The experimental results showed that, under a filling level of 72%, CPL with optimized design could remove 110 W energy while maintaining its temperature at 80 °C. Comparison of CPL with/without reservoir, the loop thermal resistance Rth,loop was reduced by 0.14 °C/W and was able to increase the stability of CPL, too, the results confirmed that reservoir could enhance CPL performance and this technology will probably find application in electronics cooling for electronic devices. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Capillary pumped loop; Reservoir; Thermal resistance; 1U server

1. Introduction Microprocessor manufacturing process speeds up with growing component density, performance and higher power removal. To guarantee the cost, weight, stability and reliability, it is necessary to solve the problem of heat radiation in the future. Meanwhile, there is not an enough space for heat consumption in existing design. Thus, a heat transfer device shall be designed with the assistance of CPL development. According to the technology reports, the microprocessor’s operating power for frequency at 3.73 GHz will reach to 130 W, and the surface temperature could not higher than 75 °C to make sure CPU is running in a stable state. Similar to a heat pipe, thermal loading is absorbed by CPL and is transferred to condenser through vapor–liquid phase change process of working fluid, and then removed *

1

Corresponding author. Tel.: +886 3 5914880; fax: +886 3 5820250. E-mail address: [email protected] (H.-W. Lin). Tel.: +886 3 5715131x42664; fax: +886 3 5165707.

1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.02.015

by means of forced convection of fin fans thermal module or water-cooling. The difference is that CPL has a separated vapor–liquid channel in a closed circulating loop. The working fluid absorbs heat and generates vapor in the evaporator, and transfers to condenser via a vapor line. After heat exchange with cold air, vapor is cooled down to liquid and returned to the evaporator via a liquid line. This is called capillary pumped loop that allows driving loop with surface tension of working fluid and helping solve the problem of heat radiation for electronic products, such as computers, mobile telephones and fuel cells, etc. Many experts had recently shown keen interest on the applications of CPL and loop heat pipe (LHP) into smallsized electronic components [1–3]. The concept of CPL was initiated by Stenger [4]. At NASA/Lewis research center in 1966, a two-phase heat transfer mechanism proved to transfer energy efficiently under slight temperature difference, without needing of pumping power. It was primarily used for heat radiation in military and space industry. Nikitkin and Cullimore [5] once studied the difference of LHP and CPL. LHP presented better stability, ease-of-start and

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187

Nomenclature Qin Rth,sys Rth,loop THS,i Tcpu Te,o

Tc,i Tc,o Te,i Tres Tsys

input power (W) system thermal resistance (°C/W) loop thermal resistance (°C/W) fan inlet temperature (°C) CPU temperature (°C) evaporator outlet temperature (°C)

stronger ability of adapting automatically operating temperature; CPL featured better temperature control, making it easier to change the position of reservoir for system integration. Lin and Chen [6] designed three types of miniature CPLs, with evaporator of 30  40  10 mm, vapor line and liquid line of 360 and 80 mm respectively. The loop inner diameter was 3.5 mm. The maximum heat transfer of CPL was computed using system pressure drop and capillary pressure drop. This loop could provide the energy of 25 W, and maintain the heat source at 100 °C. A flat-type CPL was developed by Liu and Liu [7], which had a flat groove similar to evaporator in terms of condenser design in order to reduce the pressure vibration. Liu designed a porous structure onto condenser and presented that the maximum heat transfer could reach 200 W, provided that both evaporator and condenser were sized by 250 mm  150 mm  1000 mm. Earth Observing System of NASA Goddard Space Flight Center designed three types of two-phase reservoirs of different capillary structures [8,9], and made discharge rate test and CPL control test with anhydrous ammonia. The reservoir was internally designed with a capillary structure to prevent vapor into CPL, as illustrated by the test

condenser inlet temperature (°C) condenser outlet temperature (°C) evaporator inlet temperature (°C) reservoir temperature (°C) system operating temperature (°C)

results. Ke [10] designed a two-phase reservoir suitable for space micro-gravity environment. The reservoir was placed onto support frame with adjustable angle, the working fluid might gather at different positions of reservoir, but capillary structure could sent the fluid out of reservoir, and prevent expulsion of vapor and overheat. To analysis the CPL loop performance and stability, Lin and Lin [11] reported a simple axial heat transfer model to predict the CPL condenser temperature distributions. The condenser inlet temperature and condenser outlet temperature could be calculated by this model via the mass conservation and energy conservation. The relative error between experimental results and simulation values was less than 8%. Despite of well-proven applications of CPL in space aircrafts, etc, and continuous cooling study on micro-gravity flight vehicles by NASA, the applications of CPL on electronic components (such as computer CPU, mobile telephone chip) are still in experiment phase, primarily owing to shortage of complete theoretical models and significant difference of CPL with regard to surface tension and flow resistance. This research aims to address the heat problem of electronic components based on the experimental model of 1U

Reservoir Liquid line

Tres Liquid head

Te,i

TLL Tsys

Tevap

Evaporator (heat source)

Te,o Vapor head

Condenser (heat sink)

Vapor line

Tc,i

Tc,o THS,i

Fig. 1. Schematic diagram of the CPL and K-type thermocouples position.

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server. The evaporator was internally designed with a semiarc structure and a vapor guide groove, such that vapor could flow smoothly into vapor line. Thermostatic control was performed for large-scale loops through thermoelectric cooler (TE) or other sources of heat. Meanwhile, electronic products shall prevent excessive sources of heat. Thus, attentions shall be paid to heat control of CPL without excessive sources of heat at reservoir. Reservoir was connected to liquid head, with a bigger volume than total volume of loop, and slightly higher than evaporator. 2. Experimental design for CPL As illustrated in Fig. 1, CPL included evaporator, vapor head, vapor line, condenser, liquid line and liquid head as well as reservoir. Due to rectangular structure of chips after packaging, evaporator was externally designed with a flat structure to reduce the contact thermal resistance between evaporator bass and CPU die, and internally with a semiarc radius and a semi-circular groove. The semi-arc structure and the inner shape of aluminum 6061 evaporator were shown in Fig. 2a and b, respectively. Considering the existing encapsulation technique and structural design of heat-radiating modules, the external structure of evaporator was sized by 60 mm  60 mm  20 mm. Working fluid flowed from liquid head, then it absorbed heat and changed phase to vapor, and escaped from the vapor groove. The idea was generated by groove heat pipe. Seven grooves were designed on the surface guiding the vapor to the outlet of evaporator. Fig. 2c was the scheme of the reservoir. The reservoir, a gas/liquid two-phase device, could control the system temperature and supplement rapidly working fluid to the evaporator for a declined deprime effect. The design principles were: (1) outlet pressure drop of reservoir was reduced; (2) system temperature was controlled, (3) working fluid flowing out of reservoir was in liquid phase. Reservoir was a component of achieving a balance between evaporator and source of heat, so, total loop volume must be slightly bigger than total volume of vapor line and condenser [12]. In this study, the volume of reservoir was bigger than total volume of evaporator and pipeline to ensure enough fluid in the loop. Capillary structure was made of high density polyethylene, with a permeability of kw = 6.84  1013 m2 and pore radius was equal to 9.34  105 m2. At inlet channel of capillary structure, a rectangular groove enabled the condensed working fluid to flow smoothly into the evaporator. Alternatively, the condenser might be bent in S-shape to increase the condensation length and area. The length of vapor head and liquid head of the experimental loop was 8 cm; and the length of condenser and liquid line was 12 cm and 37 cm, respectively. For the same server system, the heating components should be arranged properly, and condenser was placed optimally at left side of evaporator as the vapor channel. The condenser structure was composed of red brass fins and two heat-radiating fans. Fin diameter and thickness

Fig. 2. Inner shape of the CPL: (a) vapor guide, (b) evaporator crosssection view of evaporator and (c) reservoir.

were 40 and 28 mm, respectively. The flow rate at operating point for each fan was 0.5 cubic meter per minute. In this research, there were two fans blowing the condenser, hence, the volume flow rate was equal to 1 cubic meter per minute. A guide plate was designed between fan and fin to guide air into fin for cooling. The operating temperature of working fluid was closely related to the vapor pressure and loop length. In this experiment, the working fluid was a coolant of HCFC 141b. The chemical components contain Dichlorofluoroethane 96 wt%, Methanol 3.7 wt%, and Nitromethane 0.3 wt%. It was found from literature [7] that, loop pressure vibration was smaller in the case of bigger pipe diameter and smaller flow resistance. The pipe diameter was related to the vapor amount at the outlet. Numerous heats could be taken away from evaporator with bigger vapor amount.

H.-W. Lin, W.-K. Lin / Applied Thermal Engineering 29 (2009) 186–194 Table 1 CPL geometry size

189

PC

Data Recorder

Evaporator Length  width  height (mm) Effective inner volume (cm3) Groove number Groove radius (mm) Semi-arc radius(mm)

60  60  20 4.438 7 Rg = 1.5 R = 18

Porous Mean radius (mm) Permeability (mm2)

rc = 9.34  102 Kw = 6.84  107

Reservoir Length  width  height (mm) Effective inner volume (cm3)

50  50  20 38.42

Condenser Condenser length (mm) Inner/outlet diameter (mm) Fin length  width  height (mm)

120 4/6 120  40  30

Vapor head/liquid head Length (mm) Inner/outlet diameter (mm)

80 6/8

CPL volume No reservoir (cm3) With reservoir (cm3)

20.7 59.12

Power Supply

Programmable Logic-Controller

Power

South Bridge

3. Experimental equipment The experimental equipment included heat controller, measuring system and vacuum system. To remove efficiently Non-Condensed Gas (NCG) in the loop, CPL was pumped smoothly to 7.52  108 Pa with turbo pump. The filling level had great influence upon the operation of CPL. Insufficient filling level of working fluid would lead to shortage of liquid in the evaporator and dry-out phenomenon; excessive filling level likely led to abnormal periodic operation of loop. The optimum filling level of different working fluids must be provided to obtain optimum filling level of CPL. The calculation of filling level covered effective total volume of evaporator, vapor line and liquid line and condenser section. In the presence of reservoir, the effective volume of reservoir should be added. Therefore, filling level was defined as ratio of the volume for working fluid to the total effective loop volume. Lee et al. [13] reported the LHP optimum filling level and the heat flux conditions. The filling level ranged from 40 vol% to 60 vol% and the heat flux was varied from 1.5 to 5.9 W/cm2, and the best performance was occurred at 51.3 vol%. Boo and Wang [14] presented a LHP with methanol as the working fluid and polypropylene as wick structure. From the experimental

DRAM

North Bridge

CD-R

The inner diameter of vapor head and liquid head was 6 mm; that of vapor line, liquid line and condenser pipe was 4 mm. Table 1 was a geometry size of CPL, which includes evaporator, reservoir, capillary structure, condenser, loop channel and evaporator guide channel.

Thermocouple

H.D.D

Floppy

Fig. 3. Scheme diagram of the 1U server simulate cabinet.

results, the optimum filling level ranged from 40 vol% to 50 vol%. However, according to the previous experiences in the laboratory, the filling level of 65%, 72% and 85% should be analyzed. On the other hand, the filling level of 64%, 72% and 82% was to be considered in the absence of reservoir. The experimental system of 44.45 mm in height was based on 1U server, which comprised CPU, CD-ROM, Hard Disk, Floppy, D-RAM, South Bridge and North Bridge. A simulation cabinet of the same dimension was designed to simulate the performance of CPL in the server and was shown in Fig. 3. The test machine was equipped with power supply units, power meters, temperature indicators and PLC of heating components for unified test control. Copper heat source, made of red brass, was available with a heating surface of 31 mm  31 mm  2 mm, and was insulated by bakelite at bottom and surrounding parts, with a maximum heating power of 300 W. The test method was shown in Fig. 1, where 7 K-type thermocouples were placed onto CPL, i.e. evaporator surface Tevap, evaporator outlet Te,o, condenser inlet Tc,i, condenser outlet Tc,o, liquid line TLL, evaporator inlet Te,i and reservoir Tres. Along with simulation cabinet temperature Tsys, fan inlet temperature THS,i, and copper heat source Tcpu, there were 10 temperature points totally. According to the analytical method of experimental uncertainties was reported by Moffat [15]. One can get the Ktype thermal couple uncertainties were about ±0.6 °C.

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4. Results and discussions Thermal resistance was a benchmark of judging the quality of loop. System thermal resistance generally included contact thermal resistance between copper heat source and evaporator bottom, working fluid’s flow thermal resistance, pipe thermal resistance and contact thermal resistance between condenser pipe and fins. For analysis of steady heat-transfer thermal resistance, thermal resistance Rth,sys of CPL system was defined as the temperature difference of copper heat source and fan inlet divided by heating power: DT T cpu  T HS;i ¼ Qin Qin

ð1Þ

High temperature vapor flowed via the loop, if the high vapor flow could not be transferred by pipe, the Rth,sys value would be large. For this reason, one could obtain the CPL performance from Rth,sys. To obtain the system stability value from CPU to condenser in the process of heat removal, a loop thermal resistance Rth,loop was defined as the thermal resistance from CPU temperature to condenser outlet temperature: DT T e;o  T c;o ¼ Qin Qin

90

ð2Þ

Vapor out

Tcpu Te,o Tc,i

80 70

Tc,o Te,i THS,i

60 50 40 30 20 30

40

50

60

70 80 Power (W)

90

100

110

120

90 Tcpu Te,o Tc,i Tc,o

80 70

Temperature (οC)

System stability was an important element in device. And the characteristic of the CPL was uniform temperature. Hence, the system was stability when the value of Rth,loop was low. The experimental analysis included two parts: 1. Different filling levels of loop and estimate thermal resistance; 2. The influence of thermal resistance with/without reservoir. To find the best condenser section, a condensation position test for various parts of CPL was made by Lin et al. [16]. The scheme of CPL condenser position was shown in Fig. 4, Section A was located near the exit of the vapor head, section B was located on the exit of the vapor line, and section C was located on the liquid line opposite to the evaporator. The experimental results displayed heat sink modules on the section C were even not better than sections A and B. It was implied that vapor line (sections A and B) could be the best choice for the heat sink location under high power loading.

ο

Rth;loop ¼

Loop Temperature ( C)

Rth;sys ¼

Fig. 5a depicted the temperature distribution of loop without reservoir, under different heating powers at a filling level of 72%. Whenever thermal load increased by 5 W, CPU temperature rose about 1.5 °C; however, after exceeding 60 W, CPU temperature grew about 2.5 °C whenever thermal load increased by 5 W. Under different thermal loads, the temperature difference between evaporator surface, evaporator outlet and condenser inlet was maintained at 2–3 °C, and the mean temperature difference at inlet and outlet of condenser was about 10 °C. CPU temperature was 80.3 °C in the case of 110 W. If thermal load ranges between 50 and 65 W, condenser outlet temperature fluctuated with time passage. On the contrary, CPU temperature rose only about 0.4 °C during the period of thermal load between 50 and 55 W, possibly owing to the fact that vapor flowed rapidly out of evaporator, and fluid was filled back immediately for a lower temperature of evaporator. To compare the influence of performance with/without reservoir, a reservoir channel was connected at 4 cm to left side of evaporator inlet. The reservoir was mounted onto CPL at a 10° included angle to overcome the viscosity effect of evaporator and reservoir on the same plane and the flow

Te,i Tres THS,i

60 50 40

A 30

B

20 30

C Fig. 4. The scheme of CPL condenser position.

40

50

60 70 Power (W)

80

90

100

Fig. 5. Thermal load and loop temperature distribution (a) without reservoir, filling level of 72%; (b) with reservoir, filling level of 85%.

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an optimal heat removal if heating power was less than 60 W; filling level 72% may provide a better performance for the loop if exceeding 60 W. In the case of higher power, filling level 72% could help reduce efficiently CPU temperature. For the filling level of 82% and 100%, the temperature of copper heat source was over 80 °C in the case of 35 W at the beginning. For CPL with reservoir, the dependence of the CPU temperature vs. thermal load for different values of fluid inventory is shown in Fig. 6b. From this figure, clearly, 85% inventory had well performance and it could solve 95 W and kept CPU temperature at 76.2 °C before dry out. CPL could solve 60 and 40 W with 75% and 65% filling level, respectively. With the period of the heating, there was no additional heat source to maintain reservoir temperature at the constant values. Evaporator vapor pressure increased as thermal load increasing. When the evaporator pressure was higher than the reservoir, there was no more working fluid flowing into evaporator, after then dry out occurred. Most of the working fluid gathered at the reservoir at the same time via the visualization experiment. Fig. 7a was the thermal resistance with respect to different thermal loading; CPL without reservoir, three fluid

110

2.0

100

1.8

90

1.6

Thermal Resistance ( C/W)

80 70

64% Rth,sys 64% Rth,loop 72% Rth,sys 72% Rth,loop 82% Rth,sys 82% Rth,loop

1.4

o

CPU Temperature (oC)

resistance caused by vapor pressure. Fig. 5b depicted the loop temperature distribution diagram under the CPL with reservoir and 85% filling levels. Whenever thermal load was increased by 5 W, CPU temperature rose about 2.5–3 °C. The temperature difference between evaporator surface and outlet was only about 1 °C, and that between evaporator outlet and condenser inlet was less than 1 °C, and increased linearly with growing thermal load. The temperature difference at condenser inlet and outlet was also less than 1 °C during the experimental period. CPU temperature was 76.2 °C in the case of 95 W. With the period of the heating, there was no extra heating element to control reservoir temperature at the constant values. Evaporator vapor pressure increased as thermal loading increase. When the evaporator pressure was higher than the reservoir pressure, there was no more working fluid flowing into evaporator, after then dry out was occurred. Most of the working fluid gathered at the reservoir at the same time. For this reason CPU temperature rose abruptly by 30 °C or more when the thermal load reached 100 W. Fig. 6a depicted CPU temperature and heating power distribution diagram without reservoir, at four filling levels of 64%, 72%, 82% and 100%. CPL of filling level 64% had

191

60 50 64% 72% 82% 100%

40 30 20 10 0

1.2 1.0 0.8 0.6 0.4 0.2 0.0

30

40

50

60

70

80

90

100

110

120

30

40

50

60

Power (W)

70

80

90

100

110

120

Power (W)

100 0.6

90

Thermal Resistance ( C/W)

70

o

CPU Temperature (oC)

80

60 50 40

65% 72% 85%

30 20 10

0.5

65% Rth,sys 65% Rth,loop 72% Rth,sys 72% Rth,loop 85% Rth,sys 85% Rth,loop

0.4 0.3 0.2 0.1 0.0

0 30

40

50

60

70

80

90

100

110

120

Power (W) Fig. 6. Various inventories, CPU temperature with different power load. (a) without reservoir; (b) with reservoir.

30

40

50

60

70

80

90

100

110

120

Power (W) Fig. 7. CPL filling levels versus thermal resistance distribution: (a) without reservoir; (b) with reservoir.

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Fig. 8. Temperature distribution of CPL thermal image: (a) evaporator and (b) condenser.

o

inventories 64%, 72% and 82% were discussed in this figure. In the case of high heating power, thermal resistance at filling level 64% and 72% had a similar trend. In the case of low heating power, both evaporator temperature and vapor pressure of working fluid would decline. Thus, vapor flowed slowly in the loop. In the case of high heating power, working fluid with higher vapor pressure flowed more quickly. For this reason, CPL performance was improved, and then thermal resistance was kept steady if heating power was over 65 W. In the case of filling level 72%, the mean system thermal resistance Rth,sys was 0.57 °C/W, loop thermal resistance Rth,loop was 0.2 °C/W; in the case of filling level 64%, the mean system thermal resistance Rth,sys and loop thermal resistance Rth,loop was 0.59 °C/W and 0.08 °C/W, respectively. If CPL was applied to server, filling level 72% was more suitable for heat exchange between CPL and ambient environment. CPL with reservoir, thermal resistance with respect to different thermal loading were discussed in Fig. 7b. The mean system thermal resistances Rth,sys were equal to 0.55 and 0.5 with the 85% and 72% filling level, respectively. CPL with 85% inventory had the highest Rth,sys, however, it also had the lowest values in the mean loop thermal resistance Rth,loop. It seems that CPL with reservoir had close temperatures with 85% inventory compared with others. Fig. 8 depicted the images taken by a far infrared ray heat-imaging thermoscope. Firstly, took off thermocouple and insulating tape adhered on the loop, and atomized the surface of CPL, such that CPL surface radiation was simulated into blackbody radiation. For correction of heat-imaging thermoscope, the CPL evaporator surface temperature measured by K-type thermocouple was compared with the temperature of heat-imaging device. The loop with filling level 72% was also analyzed at 90 W, and temperature was divided by the image color. As illustrated in Fig. 8a, the heat received by CPL was rapidly transferred from vapor head to condenser via a vapor line, and residual heat was fed to condenser line for heat exchange with server. The ambient temperature within server system rose slightly with the growing heating power in the test process. The evaporator structure shall be designed in a manner to make vapor flow along the direction of vapor head, rather than liquid head. This was determined by the temperature of vapor head higher than that of liquid head. Fig. 8b depicted the temperature chart of condenser fins of the same dimension. In this experiment, when high-temperature vapor flowed to the first fin, it released latent heat for heat exchange with fin. On the other hand, when the residual heat flowed to the second fin, the temperature declined dramatically. Thermal resistance distribution of CPL with/without reservoir was shown in Fig. 9. In the presence of reservoir with a filling level of 72%, system thermal resistance Rth,sys was 0.5 °C/W; loop thermal resistance Rth,loop was 0.05 °C/W, and maximum allowable thermal load of CPL was Qlimit = 60 W. When heating power exceeded 60 W, the temperature

Thermal Resistance ( C/w)

192

72% w/o Reser, Rth,sys 72% w/o Reser, Rth,loop 72% with Reser, Rth,sys 72% with Reser, Rth,loop 85% with Reser, Rth,sys 85% with Reser, Rth,loop

1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0. 1 30

40

50

60

70

80

90

100

110

120

Power (W)

Fig. 9. Thermal resistance distribution diagram with/without reservoir.

of copper heat source grew rapidly to 110 °C due to failure of loop. In the presence of reservoir with a filling level of 85%, system thermal resistance Rth,sys was 0.55 °C/W; loop

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of [18] was 88 °C, and CPL of semi-arc structure was 73 °C, semi-arc capillary structure showed a better CPL performance.

120 110 100

o

CPU Temperature ( C)

193

90

5. Conclusions

80 70 60 50 40

3

flat, 30x40x10 mm [6] 3 flat, 42x38x8 mm [17] 3 flat, 77x63x16.4 mm [18] 3 semi-arc, 60x60x20 mm

30 20 10 0 0

20

40

60

80

100

120

Power (W) Fig. 10. Performance comparison of semi-arc structure and flat structure. (See above-mentioned references for further information.)

thermal resistance Rth,loop was 0.01 °C/W, and maximum allowable thermal load of CPL was Qlimit = 95 W. In this test, reservoir could maintain a constant temperature without additional sources of heat. When critical heat transfer was reached, CPL was continuously heated up. The evaporator generates a high vapor pressure in relation to reservoir, so the liquid under higher thermal load flowed toward reservoir with relatively low pressure. If working fluid cannot flow smoothly into evaporator, dry-out effect would occur, CPL performance declines rapidly, and thermal resistance rises quickly. In Fig. 8, thermal resistance was reduced to a stable value with growing input power, for this reason, CPL performance was improved with growing input power. Reservoir was of crucial importance to CPL. When critical heat transfer was not reached and evaporator was not supplemented rapidly with liquid, the reservoir released liquid into evaporator to minimize the occurrence of deprime effect. Conversely, in the case of backflow of excessive fluid, the reservoir could absorb surplus liquid to avoid the negative impact of excessive working fluid upon the performance. By comparing filling level 72% without reservoir and 85% with reservoir, Rth,sys reduce only by 0.03 °C/W in the case of high thermal load and reservoir over 70 W. To the contrary, Rth,loop reduce by 0.14 °C/W. With induction of reservoir, working fluid circulation mechanism became more flexible under the same heating power. The temperature difference at various points of loop was only 0.1–1 °C, such that the temperature difference between CPU and condenser outlet was reduced and Rth,loop was dropped too. Conversely, the temperature difference and Rth,loop was bigger without reservoir. The evaporator versus heating power diagram of semiarc capillary structure and flat structure without reservoir was shown in Fig. 10. The evaporators were available with four dimensions and two structures. Under the experiment conditions with optimized design, CPU temperature curvature was higher than semi-arc structure, notwithstanding bigger space for evaporator. In the case of 100 W, CPL

Thermal resistance and maximum heat transport capability are the two factors at CPL design. However, the structural design of evaporator often has influence upon CPL performance, especially in a limited space system. Electronic products focus on small volume and convenient to carry, nevertheless, heat dissipation space is obviously insufficient. For this reason, powerful heat dissipation component must be come out in time. With the induction of semi-arc structure in evaporator, CPL performance could be improved efficiently. No matter what CPU temperature distributions or maximum heat transport capability. CPL could solve efficiently 110 W at the temperature of copper heat source was controlled under 80 °C. As an extension of the concept of grooved heat pipe, semi-arc structure represented an excellent thermal performance by minimizing the thermal resistance. Reservoir played a key role in temperature control design, the CPL reservoir was designed at outside the evaporator generally; on the contrary, the compensation camber of LHP was integrated in evaporator. Hence, there should be a well bridge between reservoir and evaporator for CPL. The present literature review has shown that the reservoir should have an additional heat source controlling it. However, this was a big breakthrough in this research that no additional heat source was supported in the reservoir. Comparing the CPL which had filling level 72% without reservoir and 85% with reservoir, the experimental results showed that the mean deviation of system thermal resistance Rth,sys was 14%. The thermal resistance of CPL with reservoir was much lower than that without reservoir, while CPL stability also improved. Thus, reservoir could improve CPL performance in an efficient way. In the presence of reservoir, thermal resistance of CPL was maintained at a constant value from low to high thermal load. Otherwise, the thermal resistance became stable at higher power, so reservoir was helpful to maintain a stable CPL performance. Acknowledgements This work was supported by the National Science Council of Taiwan under the contract no. of NSC94-2212-E-007050 and NSC-95-2221-E-007-117. References [1] J. Yun, D. Wolf, E. Kroliczek, T. Hoang, Multiple evaporator loop heat pipe, in: 30th International Conference on Environmental Systems, Toulouse, France, July 10–13, 2000. [2] T. Hoang, T. O’Connell, J. Ku, D. Butler, T. Swanson, Miniature loop heat pipes for electronic cooling, in: International Electronic Packaging Technical Conference, July 2003.

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