HEAT TRANSPORT CHARACTERISTICS OF CAPILLARY PUMPED LOOP WITH PLATE-TYPE EVAPORATOR

Atsushi TSUJIMORI*, Kosuke HIRATA**, Kentaro NAKAGUCHI** and Maiko UCHIDA*

*Department of Mechanical Engineering Kanto Gakuin University

**Graduate School of Kanto Gakuin University 1-50-1 Mutsuura-cho, Kanazawaku, Yokohama, Kanagawa236-8501, JAPAN

Phone: +81(45) 786-7842 Fax: +81(45) 786-7842

Email: tujimori@kanto-gakuin.ac.jp

ABSTRACT A tube-type evaporator requires capillary force in order to lift

liquid refrigerant to circumference direction under a gravity field, which reduces thermal performance. Therefore, a capillary pumped loop with a plate-type evaporator was newly manufactured to improve the performance and reduce the thickness of the evaporator. The plate-type evaporator is flat disk shaped with an effective diameter of 46mm. The wick with equivalent diameter of 5μm and 2mm thickness is embedded in it. The vapor line and liquid line are both 750mm in length, and the condenser is 1000mm in length. The condenser is soaked in a water bath set to a constant temperature by the refrigerator. The total heat transport length is 2500mm. Using HFC134a as the working fluid, the heat transport characteristics were experimentally investigated by changing the heat load to the evaporator from 8 to 24.5W. The plate-type evaporator achieved a higher evaporative heat transfer coefficient with higher heat flux into the evaporator than the tube-type evaporator.

KEY WORDS: capillary pumping, cooling device, heat transfer, heat transport, thermal resistance, evaporator, wick

NOMENCLATURE Greek symbols q: heat flux to evaporator [W/m2]Q: heat load to evaporator [W] R: total thermal resistance in the loop [K/W] Tl: condenser outlet temperature [K] (point 3) Tr: reservoir temperature [K] (point 4) Tv: evaporated vapor temperature [K] (point 2) Tvs: saturated vapor temperature [K] Tw: wall temperature of evaporator [K] (point 1)

INTRODUCTION Cooling fan/heat sink combinations have been widely used

for the cooling of electronic devices such as personal computers. Recently, despite the fact that CPU heat flux is increasing, the packaging size has been minimized, making it

difficult to mount the cooling devices adjacent to the heat source. To meet the cooling requirement of tower-type personal computers, a capillary pumped loop was applied in this study as a cooling device. The capillary pumped loop has been investigated and

developed for space thermal control devices by Butler et al. [1], Maidanik et al. [2], and Antoniuk [3]. This device works on the principle of capillary pumping driven by heat without any mechanical driving part for circulating a working fluid to remove heat from a high heat source, transport it and release it to a low heat source. The capillary pumped loop with a flat evaporator is also considered to be suitable for cooling micro-processors in notebook PCs [4],[5]. The heat transport characteristics of such capillary pumped loop under a gravity field were investigated by Kobayashi et al. [6]. In our previous study, a capillary pumped loop with a tube-

type evaporator was manufactured as a cooling device for the tower-type PC, and its static characteristics under a gravity field were investigated by changing the thickness of the wick, the refrigerant enclosed rate, the heat flux and the height of the condenser above the evaporator [7],[8]. However, heat load of a micro-processor in a computer differs according to the usage conditions of the application software and varies hourly. Thus, thermal controllability accompanying the changes of heat load is an important factor to consider for computer cooling devices. So, in our next study, the heat transport characteristics of the capillary pumped loop in dynamic conditions including the start-up process were experimentally investigated [9]. However, a problem remained in that the tube-type evaporator requires capillary force in order to lift the liquid refrigerant to the circumference direction under a gravity field, which decreases thermal performance. A plate-type evaporator is one of the promising candidates

for improving the heat transfer performance of evaporators. Figure 1 shows the performance comparison between a tube-type evaporator and a plate-type evaporator as calculated by the simulation model in our previous study [7]. There are two plate-type evaporators having a flat surface in order to demonstrate good thermal contact to a heat source: one is the evaporator having a cylindrical wick, which is classified as the tube-type evaporator in this paper, and the other is the evaporator having a flat wick, which is defined in this study as

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the plate-type evaporator. Use of this plate-type evaporator improves the heat transport performance, increasing the heat transport rate about 10 to 18% compared with the tube-type evaporator. Therefore, in this study, a capillary pumped loop with a plate-type evaporator was manufactured and its heat transport characteristics in static conditions were investigated.

A plate-type evaporator (width: 70 mm, length: 200 mm) introduced by Katsuta et al. for space use [10] piled up 10 stainless meshes without deposition and it was used as the wick for capillary pumping. For cooling notebook PCs under a gravity field, Singh et al. [4],[5] developed a sintered Nickel wick for the plate-type evaporator in order to provide thermal contact in the wick superior to that achieved with the piled mesh wick. However, use of a metal wick can not prevent heat flow into a reservoir (heat leak), which largely affects the heat transport performance, and also makes it difficult to maintain good contact between the metal wick and the heat transfer plate. Thus, the present study introduced a porous Teflon wick in order to reduce the heat leak and achieve smaller contact thermal resistance.

0.4

0.6

0.8

1

1.2

1.4

0 0.002 0.004 0.006 0.008 0.01

Pore diameter de (mm)

Hea

trat

eQ

(�)

Fig. 1 Performance comparison between tube-type evaporator and plate-type evaporator

EXPERIMENTAL DESCRIPTION Experimental equipment

The experimental equipment shown in Fig. 2 consists of the evaporator, the condenser, the liquid line and the vapor line. The evaporator and the condenser are connected by the liquid line and the vapor line. The outer diameter of the evaporator is 50mm, and the flat wick with effective diameter of 46mm and equivalent pore diameter of 5μm was embedded in it. The length of the condenser is 1000mm. Both the liquid line and the vapor line are 750mm in length. The diameters of the

Fig. 2 Test configuration

(a) Plate-type

(b) Tube-type

Fig. 3 Cross section of the evaporator

Heat transfer tube

Wick

Reservoir

Vapor path

��

��

Pressure transducer

Heater

Evaporator

Condenser

� �

�

�

�h

Liquid line

Vapor line

Plate-type evaporator

Tube-type evaporator

Gravity

Reservoir Wick Vapor path

Heat transfer plate

Gravity

1

4

tw

Attachment

Flange

13

condenser and the liquid line are both 3mm, and the diameter of the vapor line is 6mm. Heat load is applied to the evaporator using a resistance heater, and heat is released from the condenser to cooling water. Points 1 to 4 are temperature measuring points monitored by Type-K thermocouples: point 1 shows the wall temperature of the heat transfer plate in the evaporator; point 2 shows the evaporated refrigerant vapor temperature at the evaporator outlet; point 3 shows the liquid refrigerant temperature at the condenser outlet; and point 4 shows the refrigerant reservoir temperature. Vapor pressure is measured by the pressure transducer at the evaporator outlet.

Figure 3 shows the cross section of the evaporator. Figure 3(a) is the cross section of plate-type evaporator with the flat wick manufactured in this study, and Fig. 3(b) is the cross section of tube-type evaporator with the cylindrical wick manufactured in our previous study [7]. The evaporator consists of three parts: the heat transfer plate (or tube) with rectangular grooves; the capillary wick; and the reservoir. The plate-type evaporator is flat disk shaped, so the liquid refrigerant in the reservoir uniformly penetrates into the wick. In order to maintain good thermal contact between the flat wick and the heat transfer plate, an attachment made of porous material is inserted. The tube-type evaporator is cylindrical in shape and the wick is embedded in it. The inside space is used as the refrigerant reservoir to compensate the refrigerant circulation rate in the loop.

In the experiments, heat load is applied to the evaporator and the liquid refrigerant penetrating the wick evaporates forming the meniscus at the wick-wall interface. Then the evaporated refrigerant vapor flows through the vapor path in the evaporator and the vapor line, and condenses releasing heat to the cooling water in the condenser. The liquid refrigerant then flows back to the reservoir. Experimental conditions

The experimental conditions are shown in Table 1. The thickness of the wick is that shown in Fig. 3(a). The equivalent diameter of the wick was measured using a capillary flow pore meter. The parameters in this experiment were heat flux, cooling water temperature and condenser height above the evaporator. The heat flux applied to the evaporator was changed from 4800 to 14700 W/m2 (heat load: 8 to 24.5W). Cooling water temperature was set to 20, 25 and 30�.

The condenser height above the evaporator was 0mm, -50mm, -100mm and -150mm. It is very important to understand the effect of condenser height above the evaporator on heat transport performance for cooling tower-type PCs because the top heat mode (i.e. the condenser is set below the evaporator) requires capillary force in order to lift the liquid refrigerant from the condenser height to the evaporator height, as well as to circulate the refrigerant around the loop.

HFC134a was used as the refrigerant. For practical application, water with higher surface tension than is used here is needed, but this study focuses on a comparison of the performance between the plate-type evaporator manufactured in this study and the tube-type evaporator manufactured in previous study. Consequently, the heat flux in this experiment was lower than that of the targeted zone.

Heat load was applied to the evaporator using the resistance heater and heat was released to cooling water from the condenser which was soaked in a water bath. The water bath was set to a constant temperature by the refrigerator.

Table 1 Experimental conditions Thickness of wick

tw [mm] 2

Equivalent diameter of wickDe [μm] 5

Heat flux q [W/m2](Heat load [W])

480014700 (824.5) plate-type 19006000 (3095) tube-type [7]

Cooling water temperatureTc [�] 20, 25, 30

Condenser height above evaporator �h [mm] 0mm, -50mm, -100, -150mm

Refrigerant HFC134a

RESULTS AND DISCUSSION Start-up profile

The start-up profile is shown in Fig. 4. As soon as heat load was applied to the evaporator, circulation of the working fluid started and the liquid refrigerant was cleared from the vapor path in the evaporator and the vapor line. The evaporator wall temperature Tw, the refrigerant vapor temperature Tv and the reservoir temperature Tr increased, and the liquid refrigerant temperature Tl decreased after heat load was applied to the evaporator. It took 5 to 10 minutes to reach the steady state.

Fig.4 Temperature profiles during start-up to steady state (�h=0mm, Tc=20�, q=4800W/m2)

0

5

10

15

20

25

30

35

0 100 200 300 400 500 600Time (s)

Tem

pera

ture

(�)

Evaporator wall Tw

Refrigerant vapor TvReservoir Tr

Liquid refrigerant Tl

14

Across the entire heat flux range from 4800 to 14700W/m2,the start-up processes were stable without any preconditioning.

Effect of cooling water temperatureFigure 5 shows the temperature differences in the loop. Such

temperature difference can be determined based on the experimental measurements of the wall temperatures of the evaporator Tw and the condenser out temperatures Tl.

lw TTT −=Δ (1)

It was observed that the temperature differences in the loop increased as heat flux to the evaporator increased, and varied from 3.8 to 34.3K. Also, the higher the cooling water temperature, the lower the temperature difference. Cooling devices for microprocessors in tower-type computers require a temperature difference of less than 20K, so improvement in performance is expected in a heat flux range of more than 10000W/m2.

Figure 6 shows the evaporative heat transfer coefficients defined by Equation (2),

vsw TTq−

=α (2)

where Tvs is the saturated vapor temperatures calculated from the evaporated vapor pressure. The super heat of evaporated vapor (Tv Tvs ) was 4.8 to 11.9K in these experiments. The heat transfer phenomenon including the wick structure is very complex, because the heat transfer is made from the heat transfer plate to the liquid refrigerant through the wick and enough liquid refrigerant must be supplied to satisfy the evaporation rate through the wick.

0

5

10

15

20

25

30

35

40

0 5000 10000 15000 20000

Heat flux q (W/m2)

Tem

pera

ture

diff

eren

ceΔ

T(K

)

Fig.5 Temperature difference in the loop (�h=0mm, Tc=20 to 30�)

0

500

1000

1500

2000

2500

3000

0 5000 10000 15000 20000

Heat flux q (W/m2)

Hea

ttra

nsfe

rcoe

ffici

entα

(W/m2 K

)

Fig.6 Evaporative heat transfer coefficient (�h=0mm, Tc=20 to 30�)

The evaporative heat transfer coefficient decreased as the heat flux to the evaporator increased. It is considered that the liquid-vapor interface receded from the surface of the wick and the thermal resistance including the heat conduction of the dried wick increased in the high heat flux region. Further, the higher the cooling water temperature, the higher

the heat transfer coefficient. The evaporative heat transfer coefficients varied from 1200 to 2490W/m2K. The evaporative heat transfer coefficients of the plate-type evaporator were 5 to 6 times higher than those of the tube-type evaporator in the previous study [7], although the heat flux range and the size of heat transfer area were not the same between the two types of evaporator. Generally the tube-type evaporator requires lift of the liquid refrigerant to circumference direction under a gravity field, which may affect the heat transfer performance. Figure 7 shows the effect of heat flux on working pressure.

The working pressure increased as the heat flux increased. In these experiments, the condenser has enough heat transfer area to adapt the entire heat flux range; therefore the liquid refrigerant temperatures at the condenser outlet are almost constant in all experimental conditions. A possible explanation for the change of working pressure is the difference of heat leak accompanied by heat flux change. In the plate-type evaporator the heat flow (heat leak) yields from the heat transfer plate to the reservoir not only through the wick (internal heat leak) but also through the flange, which may increase the pressure in the reservoir. Figure 8 shows the total thermal resistances in the loop, which is defined by

QTT

R lw −= (3)

where Q is the heat load to the evaporator. Total thermal resistances in the loop increased as the heat flux increased. Usually the total thermal resistance of conventional

� 20�� 25��30�

Plate-type evaporator

Tube-type evaporator

� 20�� 25��30�

Tem

pera

ture

diff

eren

ce�

T(K

)

Hea

ttra

nsfe

rcoe

ffic

ient

(W/m

2 K)

15

heat pipes and capillary pumped loops tend to decrease as heat flux increases. However, in these experiments, use of the plate-type evaporator increased the heat flow from the heat source to the reservoir and raised the temperature of the evaporator. Total thermal resistances varied from 0.48 to 1.37K/W. Also, the higher the cooling water temperature, the lower the total thermal resistance.

0.5

0.6

0.7

0.8

0.9

1

0 5000 10000 15000 20000

Heat flux q (W/m2)

Pres

sure

P(k

Pa)

Fig.7 Working pressure (�h=0mm, Tc=20 to 30�)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5000 10000 15000 20000

Heat flux q (W)

Ther

mal

resis

tanc

eR

(K/W

Fig.8 Total thermal resistance in the loop (�h=0mm, Tc=20 to 30�)

Effect of condenser height above the evaporatorFor practical application of a cooling device in tower-type

computers, the heat transport characteristics in the top heat mode must be investigated. In the top heat mode, the condenser is set below the evaporator and therefore liquid refrigerant condensed in the condenser must be lifted to the evaporator level by capillary pumping in order to return the refrigerant to the reservoir and wick in the evaporator.

The working heat flux ranges of the experimental results by changing the condenser height above the evaporator are shown in Table 2. The working heat flux range was reduced when the condenser height was below -100mm. Figure 9 shows the effect of the condenser height above the evaporator on heat transport performance which was calculated by the simulation model developed in our previous study [7]. A condenser height of -100mm is equivalent to 18% of the maximum liquid head by capillary pumping of the wick applied in this study, and decreased the heat transport performance by approximately 15%. The calculation result indicates that it is possible to operate the capillary pumped loop under the top heat condition of -200mm. However, experimentally, it was difficult to start up with a high liquid head of the refrigerant and the temperature of the evaporator rapidly increased. To reduce the effect of the liquid head, a wick with smaller

pore diameter is needed. In this case, the pressure drop in the wick accompanying the refrigerant penetration also increased, which decreased the heat transport performance shown in Fig.1.

Table 2 Working range

Condenser height [mm] Working heat flux range

[W/m2]

0 4800 to 14700

-50 4800 to 14700

-100 4800 to 10800

-150 4800

0

0.2

0.4

0.6

0.8

1

1.2

-300 -250 -200 -150 -100 -50 0

Condenser height Δh mm

Hea

trat

eQ

(�)

0

0.2

0.4

0.6

0.8

Liqu

idhe

ad(�

)

Fig.9 Effect of condenser height on heat transport performance

� 20�� 25��30�

Heat transport rate

Liquid head of condenser

� 20�� 25��30�

Ther

mal

resi

stanc

eR

(K/W

)

16

Figure 10 shows the effect of the condenser height above the evaporator on temperature differences in the loop. The effect of the condenser height on the temperature difference in the loop was small, although the working heat flux range was reduced in the top heat conditions. Similarly, the effects of the condenser height on the evaporative heat transfer coefficient (Fig.11) and total thermal resistance (Fig. 12) were also small, as compared to the effect of cooling water temperature. Total thermal resistance depends on all the variables of the working conditions, such as the heat flux, the working temperature and the nature of working fluid motion. Generally total thermal resistance of conventional heat pipes tends to increase as the inclination angle increase (i.e. the evaporator zone is set

0

5

10

15

20

25

30

35

40

0 5000 10000 15000 20000

Heat flux q (W/m2)

Tem

pera

ture

diffe

renc

eΔT

(�)

Fig.10 Temperature difference in the loop (�h=0 to -150mm, Tc=20�)

0

500

1000

1500

2000

2500

3000

0 5000 10000 15000 20000

Heat flux q (W/m2)

Hea

ttra

nsfe

rcoe

ffici

entα

(W/m2 K

)

Fig.11 Evaporative heat transfer coefficient (�h=0 to -150mm, Tc=20�)

above the condenser zone). However, the capillary pumped loop used in this study showed almost constant performance even when the condenser height above the evaporator was changed.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5000 10000 15000 20000

Heat flux q (W/m2)

Ther

mal

resis

tanc

eR

(K/W

Fig.12 Total thermal resistance in the loop (�h=0 to -150mm, Tc=20�)

CONCLUSION In this study, a capillary pumped loop with a plate-type

evaporator was manufactured and a Teflon wick was introduced in order to reduce the contact thermal resistance between the heat transfer plate and the wick, and decrease the heat flow from heat transfer plate to the reservoir. The heat transport characteristics were then investigated. The following results were obtained.

1) The temperature differences in the loop varied from 3.8

to 34.3K. 2) The evaporative heat transfer coefficients varied from

1200 to 2490W/m2K. The evaporative heat transfer coefficients of the plate-type evaporator were 5 to 6 times higher than that of the tube-type evaporator manufactured in our previous study.

3) Total thermal resistances increased as the heat flux increased and varied from 0.48 to 1.37K/W.

4) The effect of condenser height on the temperature difference in the loop, the evaporative heat transfer coefficient and total thermal resistance was small, although the working heat flux range was reduced under the top heat conditions.

REFERENCES [1] D. Butler, L. Ottenstein and J. Ku, �Flight Testing of the

Capillary Pumped Loop Flight Experiment�, SAE Technical Paper, Series 951566, pp1-15, 1995.

[2] Y.F. Maidanik, Y.G. Fershatater and V.G. Pastukhov, �Design and Investigation of a Capillary Pumped Loop for Advanced Thermal Control Systems of Space

� 0mm � -50mm � -100mm � -150mm

� 0mm � -50mm � -100mm � -150mm

� 0mm � -50mm � -100mm � -150mm Th

erm

alre

sista

nce

R(K

/W)

Hea

ttra

nsfe

rcoe

ffic

ient

(W/m

2 K)

17

Vehicles�, SAE Technical Paper, Series 951509, pp1-9, 1995.

[3] D. Antoniuk, �An Investigation of the CAPL Flight Experiment Thermal Anomalies�, SAE Technical Paper, Series 951717, pp1-9, 1995.

[4] Singh R., Akabarzadeh A., Saito Y., Nguyen T., Kao B., Sataphan T., Takenaka E. & Wuttijumnong V., �Experimental Investigation of the Miniature Loop Heat Pipe with Flat Evaporator�, InterPACK2005 (2005).

[5] Singh R., Akabarzadeh A., Saito Y., Nguyen T., Kiyooka F. & Wuttijumnong V., �Thermal Performance of Miniature Loop Heat Pipe Operating under Different Heat Modes�, ITherm06 (2006).

[6] Kobayashi T., Ogushi T., Haga S., Ozaki E. and Fujii M., �Heat Transfer Performance of a Flexible Looped Heat Pipe Using R134a as a Working Fluid (A Proposal of a Method for Predicting the Maximum Heat Transfer Rate of FLHP)�, Transactions of the JSME, Vol 66, No 654 (2000).

[7] Tsujimori A., Kato M., Morita H. and Uchida M., �Heat Transport Characteristics of the Capillary Pumped Loop for Cooling the Tower-type computer�, InterPACK�05 (2005).

[8] Tsujimori A., Kato M., Morita H. & Uchida M, �Heat Transport Characteristics of the Capillary Pumped Loop for Cooling the Tower-type computer -2nd report: Considering the Inclination of the Evaporator�, ITherm06 (2006).

[9] Tsujimori A., Kato M. and Uchida M., �Dynamic Characteristics of the Capillary Pumped Loop for Cooling the Tower-type Computer�, InterPACK �07 (2007).

[10] Katsuta M., Matsushita M., Murase M., Nagata K. and Tanaka K., �Study on Capillary Pump Loop (CPL) for Space Use � Characteristics and Performance Test of CPL Using Two-Flat-Plate-Type Evaporators)�, Transactions of the JSME, Vol 62, No 597 (1996).

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