Contribution to the achievement of SDGs
by Furukawa Electric GroupThe Development of Heat-Pipe for Top Heat
Saki Takada*, Masahiro Uekubo*
Along with improvement of performance of electronic devices, thermal problems caused by the high heat generation and the high-density packaging havebeen widely recognized and the heat radiation measures have become important subjects. The
heat radiation design has been diversified and our improvement of the heat pipe performance atthe top heat condition will contribute to the improvement of flexibility in the heat radiation
design. We analyzed the pressure drop in heat pipes and applied copper powder which had
a high capillary pressure and copper short fiber which had a high transmission rate to their wick,
and we have successfully developed the heat pipe which improved its performance lowered bygravity to raise its heat transfer rate. In this paper, we will introduce our technology about theheat pipe for the top heat condition.

ABSTRACT

1.

explain the internal structure and the working principle ofthe heat pipe whose heat transfer rate under the top heatcondition has been improved.

INTRODUCTION

The heat radiation measures have become an important
subject in the design of electronic devices. Recently,
along with the improvement of performance of electronic
devices, thermal problems caused by the high heat generation and the high-density packaging have been widely
recognized and its heat radiation design has become
more difficult year after year. A heat pipe heatsink
installed with heat pipes which were highly efficient heattransfer devices has been applied to various products asa heat radiation device 1), 2).
However, we have a problem in the condition of “topheat”, where an evaporation portion of a heat pipe islocated higher than its condensation portion, so that theheat transfer rate of the heat pipe is largely reduced by
the effect of gravity. In other words, in the case where theheat generating source to be cooled is located at a higher
position compared with air-cooling fins or a water-coolingportion, it is difficult for a heat pipe to show its proper
heat transfer performance, and its application to devices
is limited.
With that in mind, we have developed the heat pipe
which is able to maintain a sufficient heat transfer rateeven under the top heat condition. In this paper, we will

Heat pipe (one half)

Inside the heat pipe as shown in Figure 1, its working
fluid is changing from liquid to vapor state at its evaporation portion and flows from the evaporation portion to thecondensation portion (cooling portion), and the working
fluid is changed from vapor to liquid state at the condensation portion and returns to the evaporation portion. In
such a way as mentioned above, the heat transfer bylatent heat in a tubular container is carried out between
Capillary force

Evaporation
portionVapor ﬂow pressure drop
Insulation
portion(3) Condensation(heat radiation)

Wick(4) Working ﬂuid reﬂux bywick capillary force

Evaporation pressure dropCounter ﬂow pressure drop
Liquid ﬂow pressure drop
Body force pressure drop

Cooling

Condensation
portionCondensation pressure drop

Container

Figure 1 (a) Sketch of the heat pipe operation.
*

WHAT IS THE TOP HEAT

(2) Stream movement(soaking)(1) Evaporation(endothermic)

Heating2.

Figure 1 (b) The pressure drops in a heat pipe.

Automotive Products & Electronics Laboratories R&D Division

Furukawa Electric Review, No. 54 2023

68

The Development of Heat-Pipe for Top Heat

the evaporation portion and the condensation portion inaccordance with the circulating flow formed between theevaporation portion and the condensation portion. In
case of the operation in the top heat condition, the refluxof the working fluid is carried out by the capillary pressure
and its maximum heat transfer rate is determined by thefollowing balance equation shown below 3).
Capillary pressure = Evaporation pressure drop
+ Vapor flow pressure drop
+ Condensation pressure drop
+ Liquid flow pressure drop
+ Counter flow pressure drop
+ Body force pressure drop
caused by gravity
The performance of a wick is a determining factor of
the capability of a heat pipe, because the capillary pressure and each of the pressure drops are determined by
the wick structure inside the heat pipe. Such a wick has
been applied to a traditional heat pipe used under the top
heat condition where a copper powder was sintered overits full length as shown in Figure 2 (a). The sintered copper powder was able to transport the heat against thepressure drop of the body force caused by the gravity
even when applied under the top heat condition, because
it had a large capillary pressure. On the other hand, themaximum value of heat transfer (maximum heat transferrate) of the sintered copper powder is small, because it
has a smaller transmission factor of liquid to have larger
pressure drop of liquid flow. We had a way to reduce thepressure drop of liquid flow by thickening the sinteredcopper powder, however, if the sintered copper powder
was thickened when keeping the diameter of the heatpipe at the same size, the internal vapor space was
reduced, and the vapor flow pressure drop wasincreased. Therefore, no proper countermeasures were
found.
(a)Figure 2

3.

RESEARCH FOR THE WICK STRUCTURE

4.1

The Performance of a Wick for Sucking up the
Liquid and the Expected Structure of the Wick
We prepared samples of heat pipes with the sinteredcopper powder and the sintered copper short fiber, eachof which was different in its position and length to checkany changes in the performance of sucking up the liquidin accordance with the wick structure. The suction heightof the liquid at room temperature is shown in Figure 3 forthe sintered copper powder and the sintered copper
short fiber. The sintered copper powder was more effective for the higher suction height of the liquid than theone of the sintered copper short fibers. Because the capillary pressure is proportional to the suction height, theperformance of such a structure was expected to be
improved that sucks up the fluid through the sinteredcopper powder which had a larger capillary pressure after
sucking up the fluid through the sintered copper shortfiber which had a smaller liquid flow pressure drop. For
reference, we also prepared samples which had the sintered copper short fiber at the evaporation portion.

Container(b)
Evaporation
portion4.

Insulation portion+
Condensation portion

1.2
Relative value ofthe suction height

Copper short ﬁber
Copper powder

nation of two kinds of sintered bodies was applied in thelongitudinal direction. The sintered copper powder bodywhich was conventionally in use was installed at theevaporation portion and the copper short fiber sinteredbody was installed at the insulation portion through thecondensation portion. Because a higher capillary pressure was necessary to overcome the body force pressure
drop caused by gravity, the copper powder which wasconventionally in use was adopted to the evaporation
portion. On the other hand, in the range from the insulation portion to the condensation portion, the influence ofa pressure drop caused by the liquid flow was larger thanthe capillary pressure. Adopting the copper short fiber
which had a higher transmission factor, we were able toreduce the liquid flow pressure, and as a result we foundthe heat transfer rate improved.

The internal structure of the heat pipe is shown in
Figure 2 (b), which was just developed to improve theheat transfer rate under the top heat condition. A combi-

0.6
0.4
0.2
0

(a) Structure of a conventional heat pipe.
(b) S tructure of a newly developed heat pipe
(sintered copper powder + sintered copper shortfiber).

INTERNAL STRUCTURE OF A HEAPPIPE FOR THE TOP HEAT

1.0
0.8

ΔPc =

Copper powder

2δcosθ

rc

Copper short ﬁber

= ρl gh = ρl ghave

ΔPc : Capillary pressure (Pa), h: Suction height (m),have: Average suction height (m), ρl : Density of pure water (kg/m3 )σ : Surface tension (N/m), θ: Contact angle (rad)γc : Diameter of a pore (m), g : Gravitational acceleration (m/s2 )Figure 3

The equation to calculate the suction height and thecapillary force of a wick.
The suction height with the sintered copper powder
is set at 1.0.

Furukawa Electric Review, No. 54 2023

69

The Development of Heat-Pipe for Top Heat

4.2

The Preparation of Heat Pipes to Examine theWick Configuration
We installed a sintered body made of a copper powder
and that of a copper short fiber in a copper pipe of φ8 mm
in outer diameter and 200 mm in full length to make thestructure shown in Figure 4 (a). Injecting water as a working fluid, heat pipe samples for our examination were prepared. We specified a sample with the sintered copper
powder over its full length as (1) shown in Figure 4 (a),and that with the sintered copper short fiber over its fulllength as (6). And because the combination of the sintered copper powder and the sintered copper short fiber,
which sucked the liquid through the copper short fiber
first and the copper powder next was expected to
improve the performance further, we specified samples
as (2) and (3), and those which sucked the liquid throughthe sintered copper powder first and the sintered copper
short fiber next as (4) and (5) for your reference.
4.3 The Measurement Condition of Heat Pipes to
Examine the Wick Configuration
Applying grease to the surface of the evaporation portionof a heat pipe, we installed it on a heater block of 40 mm
in length made of copper. Non-working portion of theheat pipe at the evaporation portion was 15 mm in length.
The heater block was heated through a thermal contact
with a cartridge heater through grease. And the condensation portion was installed on a cooling block of 100 mm
(a)

(1)

(2)

(3)

(4)

(5)

in length made of aluminum after applying grease on thesurface of the heat pipe. The non-working portion of theheat pipe at the condensation portion was 15 mm inlength same as that at the evaporation portion. The cooling block was cooled through a thermal contact with awater-cooling jacket made of copper through grease. The
sample was positioned as the top heat position shown in
Figure 4 (b) and its verticality was checked with an angle
meter. The measurement was carried out at the fixedworking temperature of 50°C. The maximum heat transfer
rate was measured as the value of the entering heat
quantity just before the thermal resistance sharply
increased, which was calculated from the difference oftemperature between the heater block and the insulation
portion of heat pipe.
4.4 The Measurement Results of Heat Pipes
Samples to Examine the Wick Configuration
The measurement results of each heat pipe are shown in
Table 1. In comparison to the heat pipe (1) with the sintered copper powder in its full length or that (6) with thesintered copper short fiber in its full length against thosewith the sintered copper powder + the sintered copper
short fiber, we have found that (2) and (3) which wereapplied with the sintered copper powder over the evaporation portion had improved the heat transfer rate but (4)and (5) which were applied with the sintered copper shortfiber over the evaporation portion had reduced the heat
(b)

(6)

Evaporation portion
40 mm in length70
100

Non-working portion 15 mm in length

100
130

Eﬀectivelength100 mmCondensation portion
100 mm in length

130
100

Copperpowder100

70

Non-working portion 15 mm in length

Copper powder over Copper short ﬁber over Copper
the evaporation portion the evaporation portion short ﬁber

Figure 4 (a) The structure of heat pipes to examine the wick configuration.
Table 1Heat pipe length
200 mmFigure 4 (b) The measuring condition of heat pipes.

The experimental result of maximum heat transfer rate of heat pipes (200 mm in full length) in which the wick proportion
was varied.
The maximum heat transfer rate of theoretical value is set as 1.0 when covering the full length of wick with the sinteredcopper powder ((1)).

Relative values ofmaximum heattransfer rate

(1)

(2)

(3)

(4)

(5)

(6)

Experimental
values

1.4

2.0

2.8

0.7

0.9

1.4

Theoreticalvalues

1.0

1.7

2.3

0.3

0.4

1.1

Wick configuration

CopperpowderCopper powder + Copper short fiber(Copper powder over theevaporation portion)

Copper powder + Copper short fiber(Copper short fiber over theevaporation portion)

Coppershort fiber

Furukawa Electric Review, No. 54 2023

70

The Development of Heat-Pipe for Top Heattransfer rate. Considering (2) and (3), the performance
was improved because the liquid flow was sucked up
through the sintered copper powder with a large capillarypressure after being sucked up through the sintered copper short fiber with a small liquid flow pressure drop as
we had expected. Considering (4) and (5), we estimatedthat the performance became worse as a result of theinsufficient capillary pressure of the sintered copper shortfiber.
And, as for the sintered copper powder + the sinteredcopper short fiber, either (3) or (5) which had a lower proportion in length of the sintered copper powder (a higher
proportion of the sintered copper short fiber), as a result
showed a larger heat transfer rate than either (2) or (4)respectively. We estimated that it was caused by theeffect of the lower liquid flow pressure drop reduced byan increased proportion of the sintered copper short fiber
which had a larger transmission factor. It was confirmedthat the value of the maximum heat transfer rate corresponded to the value estimated from theoretical calculations overall. That of either (4) or (5), which had the evaporation portion covered with the sintered copper shortfiber, was found to deviate from the theoretical value. We
estimated that the actual value of the heat transfer rate
was found to be larger than the theoretical value because
only the capillary pressure of the sintered copper shortfiber was used in the calculation but actually not only thecapillary pressure from the sintered copper short fiber but
also that from the sintered copper powder, worked.

5.

EXAMINATION FOR THE OPTIMUM
SOLUTION OF THE WICK CONFIGURATION

5.1

Preparation of Heat Pipe Samples to Examine
the Optimum Solution of the Wick Configuration
According to the measurement results shown in Chapter 4,
we have found that such a structure was effective toimprove the performance under the top heat condition(a)(7)

that the evaporation portion was covered with the sintered copper powder and the condensation portion wascovered with the sintered copper short fiber. So, we haveexamined the optimum proportion of the sintered copper
powder and the sintered copper short fiber based on the
structure in which the evaporation portion was covered
with the sintered copper powder.
When maintaining the capillary pressure through thesintered copper powder at the level free from any problems, a longer sintered copper short fiber was preferable
to improve the heat transfer rate by the reduction of flowpressure drop. On the other hand, there was a possibility
that the liquid was not able to return to the position of thesintered copper powder, when the sintered copper shortfiber was too long, because the height from the bottom,
to which the liquid can reflux through the sintered bodywas determined according to the capillary pressure.
Then, the length with which the sintered copper shortfiber was able to reflux the liquid when the heat pipe waspositioned at the top heat position (45 degree) was set asa standard (the sintered copper short fiber 300 mm, (9)shown in Figure 5 (a)). In order to check the change ofperformance caused by the change of proportion inlength of the sintered copper powder against the sinteredcopper short fiber, we prepared samples respectively foreach side of the proportion standard ((8) and (10)) as well.
Specifically, we installed the sintered copper powder andthe sintered copper short fiber respectively in a copper
pipe of φ8 mm in outer diameter and 400 mm in fulllength in a configuration such as shown in Figure 5.
Injecting water as a working fluid, heat pipe samples forour examination were prepared. Further, we preparedsuch a heat pipe that had only the copper powder sintered over its longitudinal full length to check improvements by applying the sintered copper short fiber ((7)).

(b)
(8)

(9)
60

(10)
100

140

Non-working portion
15 mm in length
Eﬀective length
300 mm

Evaporation portion
40 mm in length
Mounting plate

Heat pipe length
400 mmHeat pipe
340

300

260

Lab jack

CopperpowderCopper powder+ Copper short ﬁber

Figure 5 (a) The structure of a heat pipe to examine the optimum
wick configuration.

45°

Non-working
portion15 mm in length
Fixative block

Condensation portion 100 mm in length
Figure 5 (b) The measuring condition of heat pipes.

Furukawa Electric Review, No. 54 2023

71

The Development of Heat-Pipe for Top Heat

The Measurement Condition of Heat Pipe
Samples to Examine the Optimum Solution of
the Wick Configuration
How to provide the angle for the top heat position is
explained as follows. A set of a heat pipe and a block
were placed on a plate and fixed to secure its stable position and to provide more efficient handling. As shown in
Figure 5 (b), the evaporation portion was positioned at the
top side of the heat pipe, and it was tilted to the position
at an angle of 45 degree. The tilt angle was adjusted bychanging the height with a lab-jack placed under themounting plate. The total structure was fixed by placing a
metal block at the condensation portion.
Other measurement conditions were same as thoseshown in Clause 4.2.
5.3 The Measured Results of Heat Pipe Samples to
Examine the Optimum Solution of the Wick
Configuration
The measured results were shown in Figure 6. As a result,any of the heat pipes ((8), (9) and (10)) which had the wickmade of the sintered copper powder + the sintered copper short fiber, had a better heat transfer rate in the top
heat condition by approximately two times than the heatpipe which had the wick made of the sintered copper
powder only over its full length. We estimated that the liquid flow pressure drop was reduced with the effect ofincreased length of the sintered copper short fiber, which
had a larger transmission factor similar to the examination
result of Clause 4.3. Considering the effect of the proportion of the sintered copper powder and the sintered copper short fiber to the performance, the heat transfer rate
was more improved as the length of the sintered copper
powder became shorter based on the calculations, however, as an actual result, the performance remained at thesame level independently from the proportion. In comparison against the results of (2) and (3) in Clause 4.3, where
the experimental values had agreed with the calculation
values, we estimated it was because the changed proportion of the length of the sintered copper powder
against the full length was smaller.

Relative values ofmaximum heat transfer rate

5.2

Figure 66.

CONCLUSION

We have found that the heat transfer rate in the top heatcondition was improved when we installed the sinteredcopper powder with a high capillary pressure at the evaporation portion and the sintered copper short fiber with asmall liquid flow pressure drop both at the insulation portion and at the condensation portion.
In response to increasing development of electronic
devices, we are going to improve the performance of heat
pipes and heat pipe heatsinks further and continue tocontribute to the development of electronic devices.

REFERENCES
1) Masahiro Uekubo: “Development of Copper Short Fiber Wick
Heat-pipe for Data Center”, Furukawa Electric Review, No.51(2020), 40-43.
2) Masahiro Uekubo: “Heat Pipes” Japan PATENT JP 6827117, 2021.
3) Koichi Oshima: Heat Pipe Engineering, Asakura Publishing(1979), 14-61. (in Japanese)

3.0
CopperpowderCopper powder+ Copper short ﬁber

2.0

1.0

0

(7)

(8)

Wick

(9)

(10)

The experimental result of maximum heat transfer
rate of heat pipes in which the wick proportion wasvaried.
The maximum heat transfer rate of theoretical value
is set as 1.0 when covering the full length of the wickwith the sintered copper powder ((7)).

Furukawa Electric Review, No. 54 2023

72