U.S. patent application number 13/216770 was filed with the patent office on 2011-12-15 for flat heat pipe.
This patent application is currently assigned to FUJIKURA LTD.. Invention is credited to Mohammad Shahed AHAMED, Yasuhiro HORIUCHI, Toshiaki MABUCHI, Koichi MASHIKO, Masataka MOCHIZUKI, Yuji SAITO.
Application Number | 20110303392 13/216770 |
Document ID | / |
Family ID | 42665510 |
Filed Date | 2011-12-15 |
United States Patent
Application |
20110303392 |
Kind Code |
A1 |
HORIUCHI; Yasuhiro ; et
al. |
December 15, 2011 |
FLAT HEAT PIPE
Abstract
A thin flat heat pipe capable of transporting heat even if it is
bent is provided. The flat heat pipe comprises: a working fluid to
be evaporated when heated and to be condensed when the heat
dissipates; and a wick, which is formed by bundling a plurality of
thin wires while twisting along a center axis thereof, and which is
adapted to create a capillary pressure for returning the liquid
phase working fluid to a portion where evaporation takes place. The
wick is arranged over the entire length of the flat container while
being in contact with both upper and lower inner faces of the
container or with an inner side face of the container in a manner
such that an inner space of the container for letting through an
evaporated working fluid is not closed, and a contact portion
between the wick and the container is fixed by sintering over the
entire length of the wick.
Inventors: |
HORIUCHI; Yasuhiro; (Tokyo,
JP) ; MABUCHI; Toshiaki; (Tokyo, JP) ; AHAMED;
Mohammad Shahed; (Tokyo, JP) ; MOCHIZUKI;
Masataka; (Tokyo, JP) ; SAITO; Yuji; (Tokyo,
JP) ; MASHIKO; Koichi; (Koto-ku, JP) |
Assignee: |
FUJIKURA LTD.
Tokyo
JP
|
Family ID: |
42665510 |
Appl. No.: |
13/216770 |
Filed: |
August 24, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2010/052696 |
Feb 23, 2010 |
|
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13216770 |
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Current U.S.
Class: |
165/104.26 |
Current CPC
Class: |
F28D 15/046 20130101;
F28D 15/0233 20130101 |
Class at
Publication: |
165/104.26 |
International
Class: |
F28D 15/04 20060101
F28D015/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 24, 2009 |
JP |
2009-041329 |
Claims
1. A flat heat pipe, which transports heat by a working fluid to be
evaporated when heated and to be condensed when the heat dissipates
therefrom, comprising: a container, which is flattened and in which
the working fluid is encapsulated; and a wick, which is formed by
bundling a plurality of thin wires while twisting along a center
axis thereof, and which is adapted to create a capillary pressure
when the liquid phase working fluid penetrates thereto; wherein the
wick thus formed of the thin wires is arranged over the entire
length of the container while being in contact with both an upper
inner face and a lower inner face of the container or with an inner
side face of the container in a manner not to close an inner space
of the container for letting through an evaporated working fluid;
and any of a contact portion between the wick and the container is
fixed by sintering over the entire length of the wick.
2. The flat heat pipe as claimed in claim 1, wherein the thin wire
includes a copper wire.
3. The flat heat pipe as claimed in claim 1, wherein the bundle of
thin wires is straightened before bundling by applying a thermal
treatment thereto.
4. The flat heat pipe as claimed in claim 3, wherein the thermal
treatment includes an annealing.
5. The flat heat pipe as claimed in any of claim 1 to 4 or 6,
wherein the container is formed by flattening a pipe holding the
bundle of thin wires in its width center.
6. The flat heat pipe as claimed in claim 2, wherein the bundle of
thin wires is straightened in before bundling by applying a thermal
treatment thereto.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a Continuation Application of International
Application No. PCT/JP2010/052696 filed Feb. 23, 2010. The contents
of the aforementioned application are incorporated herein by
reference.
TECHNICAL FIELD
[0002] This invention relates to a heat pipe configured to
transport heat by a working fluid encapsulated in a container, and
especially to a heat pipe, which is flattened entirely, and in
which a bundle of thin wires is used to form a wick for returning
the working fluid by a capillary pumping.
BACKGROUND ART
[0003] Basically, a heat pipe comprises: a container in which a
non-condensable gas such as air is evacuated therefrom; a working
fluid such as water, alcohol etc. which is evaporated and condensed
within a given temperature range; and a wick arranged in the
container to pump the liquid phase working fluid by capillary
action. In the heat pipe, therefore, the working fluid is
evaporated by external heat, and the evaporated working fluid flows
toward a low pressure side to dissipate heat therefrom.
Consequently, the evaporated working fluid is condensed again.
Thus, the heat is transported by the working fluid in the form of
latent heat. The working fluid thus condensed penetrates into the
wick and pumped by the capillary action of the wick to the portion
where the evaporation takes place.
[0004] Thus, in the heat pipe, the evaporated working fluid flows
from the evaporating portion to which the external hear is
transmitted toward the dissipating portion where the heat of the
evaporated working fluid is dissipated, and the liquid phase
working fluid flows in an opposite direction. Therefore, the heat
is transported continuously. In order to enhance a heat
transporting capacity of the heat pipe, that is, in order to reduce
a thermal resistance, it is necessary to flow both of evaporated
and condensed working fluids sufficiently and smoothly. In
addition, the heat pipe is used widely in many fields. For example,
in case of using the heat pipe to cool an electronic device, the
heat pipe has to be downsized in accordance with miniaturization of
an electron device and electronic circuit.
[0005] Therefore, various kinds of techniques to maintain a flow
passage for letting through evaporated working fluid, to improve
smoothness of returning liquid phase working fluid, to downsize the
heat pipe and so on have been developed in the prior art. For
example, Japanese Patent Laid-Opens No. 2004-53186, No. 2000-74579
and No. 2003-247791 disclose a technique to enhance capillary force
of a wick for pumping the working fluid by forming the wick by
bundling a plurality of thin wires of copper or carbon. The
capillary pumping is enhanced by reducing an effective capillary
radius of a meniscus formed on a surface of the working fluid.
Therefore, in order to enhance the capillary pumping, clearances
between wires forming the wick is reduced by bundling the wires. In
case of thus forming the wick, a flow passage for the working fluid
can also be smoothened and straightened so that flow resistance of
the working fluid can be reduced. For this reason, the reflux
characteristics of the working fluid can be further improved.
[0006] In case of thus forming the wick using the thin wires, the
flow path is formed between the wires. Therefore, those wires are
bundled without using an adhesive agent. For example, according to
the heat pipe taught by Japanese Patent Laid-Open No. 2004-53186, a
number of extrafine wires are bundled in a spiral elastic body such
as a coil spring. According to the teachings of Japanese Patent
Laid-Open No. 2000-74579, a plate member having a recessed groove
is arranged in a pipe, and wires are held in the groove. Further,
according to the teachings of Japanese Patent Laid-Open No.
2003-247791, a plurality of ultrafine wires are twisted into a
bundle, and the bundle of the wires is inserted into a grooved
pipe. Thus, according to the teachings of Japanese Patent
Laid-Opens No. 2004-53186 and No. 2000-74579, the thin wires are
kept in a bundle using the coil spring and the plate member.
Meanwhile, according to the teachings of Japanese Patent Laid-Open
No. 2003-247791, the ultrafine wires are bundled by twisting the
wires.
[0007] In addition, Japanese Patent Laid-Open No. 2001-208489
discloses a technique to form a sufficient steam passage for
letting through evaporated working fluid. According to the
teachings of Japanese Patent Laid-Open No. 2001-208489, a flat heat
pipe is formed by arranging a wire mesh longitudinally in a
container and fixing the wire mesh to the container by a seam
welding method, and by flattening the heat pipe thus formed.
Therefore, in case the heat pipe taught by Japanese Patent
Laid-Open No. 2001-208489 is bent, the wick is bent along the
container. For this reason, the wick will not be in contact with an
inner face of the container of an inner circumferential side in a
bending radius, so that the steam passages in the container can be
prevented from being closed.
[0008] Thus, according to the prior art, the heat pipe is flattened
by pushing the heat pipe in its radial direction as taught by
Japanese Patent Laid-Opens No. 2004-53186, No. 2000-74579, and No.
2001-208489. Specifically, according to the teachings of Japanese
Patent Laid-Open No. 2000-74579, the heat pipe is flattened to be
thinner than 1.5 mm. In addition, Japanese Patent Laid-Open No.
11-173777 discloses a heat pipe which can be flattened thinner than
1 mm.
[0009] In case of using the coil spring or the plate member to
bundle the wires as taught by Japanese Patent Laid-Opens No.
2004-53186 and No. 2000-74579, a total thickness of the wick or an
outer diameter of the wick has to be increased. Therefore, it is
not advantageous to use such a bundling member to reduce a
thickness of the flattened heat pipe. To the contrary, according to
the teachings of Japanese Patent Laid-Open No. 2003-247791, the
wires are bundled by twisting the wires without using a bundling
member. Therefore, the teachings of Japanese Patent Laid-Open No.
2003-247791 is advantageous to reduce an outer diameter of the
wick, that is, to flatten the heat pipe as taught by Japanese
Patent Laid-Opens No. 2001-208489 and No. 11-173777.
[0010] In case of forming the wick using a bundle of thin wires,
the wick is arranged in the container entirely in the length
direction. Therefore, in order to maintain a flow passage for the
vaporized working fluid sufficiently in the container even if the
heat pipe thus structured is deformed or bent, the wick is
preferably fixed as taught by Japanese Patent Laid-Opens No.
2000-74579 and No. 2001-208489. However, in case of holding the
bundled wires in the plate member as taught by Japanese Patent
Laid-Open No. 2000-74579, a number of elements is increased and the
thickness of the flattened heat pipe is increased. Alternatively,
in case of fixing the wick to the inner face of the container by a
seam welding method as taught by Japanese Patent Laid-Open No.
2001-208489, a manufacturing process has to be complicated and it
is very difficult to carry out such a demanding task.
DISCLOSURE OF THE INVENTION
[0011] The present invention has been conceived noting the
technical problems thus far described, and its object is to provide
a flat heat pipe capable of transporting heat efficiently even if
it is deformed or bent.
[0012] In order to achieve the above-mentioned object, according to
the present invention, there is provided a flat heat pipe, which is
configured to transport heat by a working fluid to be evaporated
when heated and to be condensed when dissipating the heat
therefrom. The flat heat pipe comprises: a container, which is
flattened and in which the working fluid is encapsulated; and a
wick, which is formed by bundling a plurality of thin wires while
twisting along a center axis thereof, and which is adapted to
create a capillary pressure when the liquid phase working fluid
penetrates thereto. According to the flat heat pipe of the present
invention, the wick thus formed of the thin wires is arranged over
the entire length of the flat container while being contacted with
both of an upper and a lower inner faces of the container or with
an inner side face of the container in a manner not to close an
inner space of the container for letting through an evaporated
working fluid. In addition, any of a contact portion between the
wick and the container is fixed by sintering over the entire length
of the wick.
[0013] The thin wires include a copper wire.
[0014] Preferably, the bundle of thin wires is straightened in
advance by applying a thermal treatment thereto. Specifically, the
bundle of thin wires is annealed in advance.
[0015] Specifically, the container is formed by flattening a pipe
holding the bundle of thin wires in its width center.
[0016] According to the present invention, the wick is thus formed
by bundling the thin wires. Therefore, an effective capillary
radius of a meniscus formed on a surface of the working fluid
penetrating into the wick can be reduced. As a result, a capillary
force for pumping the liquid phase working fluid can be enhanced.
In addition, smoothly extending flow passages for letting through
the liquid phase working fluid can be formed between the wires so
that a flow resistance of the working fluid can be reduced. As
described, the wick is formed by twisting the thin wires without
using any bundling member. Therefore, a number of construction
elements of the heat pipe can be reduced, and both of the vaporized
working fluid and the liquid phase working fluid are allowed to
flow through the flow passages without hindrance. As also
described, the bundle of thin wires is subjected to a thermal
treatment in advance to be straightened so that the flow of the
working fluid can be further smoothened. For this reason, refluxing
characteristics of the liquid phase working fluid can be improved.
Since the flow of the working fluid is thus smoothened, a heat
transporting characteristics of the heat pipe can also be improved
entirely. In addition to the above-explained advantages, since the
wick is formed of a plurality of thin wires without using a
bundling member, an outer diameter of the wick can be reduced
relatively. Therefore, the container can be flattened to be thinner
than the conventional flat container. In other words, the container
can be flattened without degrading the heat transporting
characteristic of the heat pipe. Further, a center portion of the
cylindrical pipe can be prevented from being deformed (or
depressed) excessively when flattened, by situating the bundled
wire in the width center of the pipe.
[0017] In addition, in case of bending the container of the heat
pipe thus structured, the wick fixed to the inner face of the
container is also bent together with the container. Therefore, an
intermediate portion of the wick will not be displaced to be in
contact with the inner face of the container in a manner that
closes the inner space of the container. For this reason, the flow
passages for letting through the vapor flow can be maintained in
the container so that the evaporated working fluid flowing
therethrough will not be blocked by the closed passage. In
addition, according to the present invention, the wick is fixed
entirely with the inner face of the container lengthwise by
inserting the wick into the container, and sintering the container
holding the wick therein. Thus, according to the present invention,
the heat pipe having excellent heat transporting capacity can be
manufactured easily.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic view showing a process of
manufacturing the wick of the heat pipe according to the present
invention.
[0019] FIG. 2 is a sectional view showing a cross-section of a
round shaped heat pipe before flattening during the manufacturing
process.
[0020] FIG. 3 is a sectional view showing a cross-section of one
example of the flat heat pipe according to the present
invention.
[0021] FIG. 4 is a sectional view showing a cross-section of
another example of the flat heat pipe according to the present
invention.
[0022] FIG. 5 is a view explaining a characteristic test carried
out on the example of the present invention and a comparative
example.
[0023] FIG. 6 is a graph indicating a relation between a thermal
input and a thermal resistance of the heat pipe according to the
example, and a relation between a thermal input and a thermal
resistance of the heat pipe of the second comparative example.
[0024] FIG. 7 is a schematic view showing a positional relation
between the container and the wick of a straight flat heat pipe of
the present invention, and a positional relation between the
container and the wick of a curved flat heat pipe of the present
invention.
[0025] FIG. 8 is a schematic view showing positional relation
between the container and the wick in a heat pipe in which the wick
is not subjected to a thermal treatment, in both cases in which the
heat pipe is not bent and the heat pipe is bent.
[0026] FIG. 9 is a schematic view showing positional relation
between the container and the wick in a heat pipe in which the wick
is fixed partially, in both cases in which the heat pipe is not
bent and the heat pipe is bent.
BEST MODE FOR CARRYING OUT THE INVENTION
[0027] Next, the present invention will be explained in more detail
hereinafter. The present invention relates to a heat pipe
comprising a wick having a specific structure. Specifically,
according to the present invention, the wick of the heat pipe is
formed by unifying a plurality of thin wires without using a
bundling member. For example, the wick is formed using a metal wire
such as a copper wire, a carbon fiber and so on having good
hydrophilicity with the working fluid encapsulated in a container.
In order to maintain the wires into a bundle, the unified wires are
twisted along a center axis thereof. Therefore, the wires have to
be capable of keeping their shape even after twisted. For this
purpose, a thin copper wire is especially suitable.
[0028] According to the present invention, the wick formed by thus
twisting a bundle of wires is arranged in the container, and the
wick is fixed with the container by a sintering method. Then, the
working fluid is filled in the container. Specifically, an airtight
hollow container is used to hold the wick, and in case of using the
heat pipe to transport the heat between sites away from each other,
a hollow tube is used as the container. In order to exchange an
internal heat and an external heat, the container is preferably
made of highly-heat conductive material such as a copper pipe. In
addition, narrow grooves can be formed optionally on an inner face
of the container thereby allowing the working fluid to flow
therethrough by capillary pumping.
[0029] The above-explained wick formed by twisting the thin wires
is fixed onto the inner face of the container. Specifically, the
container holding the wick therein is heated to a predetermined
temperature. As a result, the wick is bonded thermally with the
inner face of the container, and remaining space in the container
serves as a flow passage for letting through the evaporated working
fluid.
[0030] The working fluid is evaporated when heated and condensed
after dissipating the heat therefrom. Thus, the heat is transported
by the working fluid in the form of latent heat. For this purpose,
the working fluid is selected from water, alcohol,
hydrochlorofluorocarbon and so on, and encapsulated in the
container while evacuating non-condensable gas such as air from the
container.
[0031] Therefore, in case of heating a portion of the container
while cooling another portion of the container, the working fluid
is evaporated at the heated portion and flows toward the cooled
portion at which a temperature and a pressure are lower than the
heated portion. In this situation, a heat of the evaporated working
fluid is dissipated from the cooled portion, and then the
evaporated working fluid is condensed again. In the heat pipe of
the present invention, the wick is thermally fixed to the inner
face of the container, and a flow passage(s) for letting through
the evaporated working fluid is/are formed in the container along
the wick. Therefore, the flow passages for letting through the
evaporated working fluid can be maintained even if the heat pipe is
bent. For this reason, a sufficient vapor flow is allowed to flow
in the heat pipe so that the heat transporting capacity of the heat
pipe is enhanced.
[0032] Meanwhile, the condensed working fluid penetrates into the
wick and flows through the flow passages formed therein between the
wires forming the wick toward the portion where the evaporation
takes place. Specifically, when the working fluid is evaporated, a
level of the meniscus formed between the wires is lowered and a
capillary pumping force is thereby created. As a result, the liquid
phase working fluid is attracted toward the evaporating side by the
capillary pumping. According to the present invention, the narrow
clearances are formed between the wires forming the wick so that
the capillary pumping force can be enhanced. As a result, the
refluxing characteristics of the heat pipe can be improved. In
addition, the wick is formed by twisting the bundle of thin wires
without tightening at a specific point by a bundling member.
Therefore, the flow passages for letting through the liquid phase
working fluid formed between the wires can be smoothened so that
the flow resistance of the liquid phase working fluid can be
reduced. For this reason, the refluxing characteristics of the heat
pipe can be further improved. Further, the wick can be fixed easily
to the inner face of the container over the entire length by
inserting the wick formed of the thin wires into the container and
heating the container from outside. Therefore, manufacturability of
the heat pipe can be improved.
[0033] Next, an example of a method for manufacturing a heat pipe
according to the present invention will be explained hereinafter
with reference to the accompanying figures. In this example, a
copper wire whose diameter is approximately 0.05 mm is used as a
wire 1, and 100 to 400 pieces of the wires 1 are brought together
to form a bundle 2 as shown in FIG. 1 (a). Then, as shown in FIG. 1
(b), the bundle 2 is twisted along a center axis thereof. As a
result, the wires 1 form the bundle 2 without using any specific
bundling member. The bundle 2 thus formed is cut to a predetermined
length. In case of using an uncoiled wire as the wire 1, the wires
1 are straightened by a thermal treatment, thereby preventing the
wires 1 from being curled by residual stress.
[0034] A container 3 is prepared using a pipe whose thickness is
0.3 mm, and whose outer diameter is in a range of 3.0-6.0 mm. The
container 3 is subjected to degreasing and cut to a predetermined
length. In case of using copper wires to form a wick 4, a copper
pipe is also used to form the container 3. Then, the bundle 2 is
inserted into the container 3 to serve as the wick 4. Specifically,
the bundle 2 is laid linearly on a lower inner face of the
container 3 by gravity. The container 3 thus holding the wick 4
therein is set horizontally in a furnace (not shown) to be heated.
Provided that the container 3 and the wick 4 are made of copper,
the container 3 holding the wick 4 is heated at 1000.degree. C. As
a result, the wick 4 is fixed to the inner face of the container 3
over the entire length. In this situation, some of the wires 1 may
be fixed thermally with each other as shown in FIG. 2.
[0035] The container 3 in which the wick 4 is thus fixed thermally
thereto is ejected from the furnace and cooled. Then, one of the
end portions of the container is sealed by applying a swaging and
welding thereto, in other words, a bottom swaging and a bottom
welding are carried out. Meanwhile, a swaging is also applied to
other end portion of the container 3 (i.e., a top swaging). As a
result, a prototype of the container 3 is prepared.
[0036] As a result of thus applying the top swaging to said other
end portion of the container 3, a nozzle portion is formed thereon,
and the working fluid is filled in the container 3 from the nozzle
portion. In this situation, the non-condensable gas such as air is
evacuated from the container 3, and then the working fluid is
filled in the container 3. Alternatively, the working fluid is
filled in the container 3 in an amount larger than a required
amount, and then boiled to evacuate the non-condensable gas from
the container 3. Thus, the working fluid can be filled in the
container 3 by conventional methods. Then, the nozzle portion is
compressed and welded to be sealed. That is, a top welding is
carried out.
[0037] In this example, the pipe whose cross-section is circular is
used to form the container 3. Therefore, the round heat pipe thus
manufactured is then pressed in its radial direction to be
flattened. In case of forming a straight heat pipe, the round heat
pipe thus manufactured is flattened as is. Alternatively, in case
of forming a curved or bent heat pipe, the round heat pipe is
curved or bent into a desired shape, and then flattened in its
radial direction. In case of using a flat pipe as the container 3,
the flat heat pipe can be obtained without carrying out the
above-explained flattening step. However, in this case, it is
preferable to press the container 3 thereby contacting the wick 4
tightly with the inner face of the container 3. As described, the
wick 4 is formed by twisting the bundle 2 of the wires 1 and
thermally fixed to the inner face of the container 3. Therefore, in
case of curving or bending the flat heat pipe thus manufactured,
the wick 4 is curved or bent together with the container 3 without
closing the flow passage 5 for letting through the evaporated
working fluid formed along the wick 4.
[0038] As also described, according to the present invention, the
bundle 2 functioning as the wick 4 is formed by twisting the wires
1 without using a bundling member. Therefore, a thickness of the
flat heat pipe thus structured can be further reduced. In addition,
since the wick 4 is thus fixed thermally to the inner face of the
container 3, the flow passage 5 can be maintained without any
change. As shown in FIG. 3, the wick 4 of the heat pipe is formed
by twisting the bundle 2 of the plurality of wires 1, and the
working fluid is encapsulated in the container 3 made of copper and
flattened in its radial direction. Further, according to the
example shown in FIG. 3, a plurality of narrow grooves 11 are
formed on the inner face of the container 3 in the axial direction.
Those grooves 11 also serve as a wick, and a contact area between
the working fluid and the inner face of the container 3 is thereby
enlarged.
[0039] In this example, the wick 4 situated in the width center of
the container 3 is contacted with both upper and lower inner face
of the flattened container 3 and fixed thermally therewith over the
entire length. Since the wick 4 is contacted directly with the
inner face of the container 3 without interposing inclusion
therebetween, a total thickness of the wick 4 can be reduced. In
addition, an inner space of the container 3 is divided into two
chambers by the wick 4, and those chambers serve as the flow
passages 5 for letting through the evaporated working fluid. As
described, the straight container 3 and the straight wick 4 are
used to manufacture the heat pipe, therefore, the flow passages 5
are also straight when the heat pipe is manufactured. Since the
wick 4 is fixed with the inner face of the container 3, the wick 4
is bent together with the container 3 even if the container 3 is
bent. That is, the wick 4 will not be in contact with the opposite
inner side face of the container 3 even if the container 3 is bent.
Therefore, even if the container 3 is bent, the flow passage 5 for
letting through the evaporated working fluid can be maintained as
in the heat pipe just after manufactured so that the evaporated
working fluid is allowed to flow through the flow passages 5
without hindrance.
[0040] Thus, according to the present invention, the flat heat pipe
shown in FIG. 3 is manufactured by inserting the wick 4 formed by
twisting the bundle 2 of the wires 1 into the container 3.
Therefore, the thickness of the heat pipe can be reduced. In
addition, the flow passages for letting through the liquid phase
forking fluid can be formed narrowly and smoothly in the wick 4.
Therefore, the capillary pumping of the wick 4 can be enhanced.
Further, the flow passage 5 for letting through the evaporated
working fluid formed in the container 3 will not be closed by the
wick 4 even if the wick 4 is bent. Therefore, heat transporting
capacity of the heat pipe can be enhanced in comparison with that
of the conventional heat pipes.
[0041] Alternatively, according to the present invention, the wick
4 may also be situated on one of side ends of the flat heat pipe as
shown in FIG. 4, instead of arranging the wick 4 in the width
center of the container 3. The flat heat pipe shown in FIG. 4 is
also capable of deliver comparable performance as the heat pipe
shown in FIG. 3.
Example
[0042] Next, a non-limiting example of the present invention will
be explained by comparing with comparative examples.
Example
[0043] In this example, uncoiled thin copper wires were bundled and
twisted to form a wick. The wick thus formed was subjected to an
annealing to be straightened and then inserted into a copper pipe
serving as a container. Specifically, a thickness of the container
was 0.3 mm and an outer diameter of the container was 4.0 mm. In
addition, a plurality of thin grooves were formed on an inner face
of the container. Then, a heat pipe was manufactured by the
above-explained procedures. The heat pipe thus manufactured was
flattened to 1 mm thickness. The wick was situated at a width
center of the container, and fixed thermally with the container. A
total length of the flat heat pipe thus manufactured was 100 mm,
and water is encapsulated in the container to serve as working
fluid.
Comparative Example 1
[0044] In the first comparative example, a heat pipe was prepared
by the same manner as the above-explained example except for
applying an annealing to the wick in advance.
Comparative Example 2
[0045] In the second comparative example, the wick was formed by
arranging a plurality of copper wires around a spiral and pushing
the copper wires onto an inner face of the container using the
spiral, instead of twisting the wires. Thus, in the second
comparative example, only a structure of the wick was different
from the above-explained example, and remaining structures of the
heat pipe were identical to those of the heat pipe of the
example.
Comparative Example 3
[0046] In the third comparative example, the wick was formed by
bundling a plurality of straight copper wires using a spiral, and
the wick thus formed was inserted into a container. Thus, only a
structure of the wick was different from the second comparative
example, and remaining structures of the heat pipe were identical
to those of the second comparative example.
Test Procedure
[0047] As shown in FIG. 5, one of the end portions of a heat pipe
10 to be tested was contacted with a surface of an electric heater
15 (25 mm15 mm) and other end portion of the heat pipe 10 was
contacted with a dissipation plate 16 made of aluminum (64 mm40
mm1.5 mm). A lower face of the dissipation plate 16 was contacted
with a heat insulating plate 17. Said one of the end portions was
heated by energizing the electric heater 15. In this situation, an
electric energy (i.e., a thermal input Q), a temperature Th at a
contact point P1 between the electric heater 15 and the heat pipe
10, and a temperature Tc at the other end portion P2 of the heat
pipe 10 were measured. Then, a thermal resistance (.degree. C./W)
of each heat pipe and a maximum thermal input (W) thereof without
causing a dry out were obtained on the basis of the measured data.
Specifically, the thermal resistance R was calculated by the
following formula: R=(Th-Tc)/Q. In order to carry out the test, 30
pieces of the heat pipes of the Example and the Comparative Example
1 were prepared individually, and a number (or rate) of
non-defective products was also counted for the heat pipes of the
example and the comparison 1. Measurement result is shown in table
1. In addition, the measured thermal resistance and thermal input
of the heat pipes of the example and the comparison 1 are indicated
in FIG. 6.
TABLE-US-00001 TABLE 1 Bundling Member Twisted Wick Thermal Maximum
heat Using Without Straight Thermally Without Thermally Resistance
Transportation Non-Defective Spiral Spiral Wick Treated Treated
.degree. C./W W Rate Example .largecircle. .largecircle. 0.4 10
29/30 Comparative .largecircle. .largecircle. 0.8 10 3/30 Example 1
Comparative .largecircle. .largecircle. 3.0 5 -- Example 2
Comparative .largecircle. .largecircle. 1.0 7 -- Example 3
[0048] The following assessment can be derived from the measurement
results indicated in Table 1. As described above, according to the
heat pipe of the Example of the present invention, the wick is
formed by twisting the bundle of thin wires, and the wick thus
formed is straightened thermally in advance. As can be seen from
Table 1, the thermal resistance of the heat pipe according to the
present invention is 0.4.degree. C./W, and this is the smallest
value in the tested heat pipes. In addition, as indicated in FIG.
6, the thermal resistance of the heat pipe according to the present
invention is stabilized at approximately 0.4.degree. C./W under the
condition in which the thermal input is smaller than the maximum
thermal input thereof. Thus, the heat pipe according to the present
invention is capable of displaying excellent heat transportation
characteristics within an operating temperature limit. In addition,
a non-defective rate of the heat pipe according to the present
invention was higher than 90%, and this is also the highest rate in
the tested heat pipes.
[0049] According to the first comparative example, the thermal
treatment was not applied to the wick in advance. In this case, the
maximum thermal input was also 10 W. However, the thermal
resistance of the heat pipe according to the first comparative
example was 0.8.degree. C./W, that is, twice as much as that of the
heat pipe according to the present invention. In addition, only
three of the thirty heat pipes were non-defective. Thus, the heat
pipe according to the first comparative example was inferior to the
heat pipe according to the present invention in productivity.
[0050] According to the second comparative example, the wires were
arranged around the spiral and pushed onto the inner face of the
container using the spiral. In this case, the maximum thermal input
of the heat pipe was impaired to 5 W, that is, the maximum thermal
input was half as much as that of the heat pipe of the present
invention. In addition, the thermal resistance of the heat pipe
according to the second comparison was 3.0.degree. C./W. Thus, the
thermal resistance of the heat pipe according to the second
comparison was increased eightfold in comparison with that of the
heat pipe according to the present invention.
[0051] According to the third comparative example, the wick was
formed by bundling wires by the spiral. In this case, the maximum
thermal input was 7 W, and the thermal resistance was 1.degree.
C./W. That is, the maximum thermal input and the thermal resistance
of the heat pipe according to the third comparative example were
improved in comparison with those of the heat pipe according to the
second comparative example. However, the thermal resistance of the
heat pipe according to the third comparative example was more than
twice as much as that of the heat pipe according to the present
invention, but the maximum thermal input of the heat pipe according
to the third comparative example remained at 70% of that of the
heat pipe according to the present invention.
[0052] Thus, according to the heat pipe of the present invention,
the maximum thermal input was 10 W and the thermal resistance is
smaller than 1. This means that the heat pipe according to the
present invention has the most excellent heat transporting
capacity. In addition, according to the present invention, the
thickness of the heat pipe is only about 1 mm so that the heat pipe
can be installed easily even in a small electronic hardware.
Moreover, the heat pipe can be manufactured without variation so
that the productivity of the heat pipe can be improved. Further,
even if the heat pipe is flattened, the center part of the
container can be flattened neatly without being deformed
unevenly.
[0053] In addition, a plurality of modified heat pipes of the
example in which the wick was not fixed thermally to the container
were also prepared for the purpose of confirming the thermal
resistance thereof under the condition in which the heat pipe thus
structured is bent. As described, the heat pipe according to the
present invention in which the wick formed by twisting the bundled
wires was fixed thermally to the wick was stabilized within a range
from 0.4 to 0.6.degree. C./W. However, the range of the variation
in the thermal resistance of the heat pipe thus modified was
widened from 0.4 to 1.2.degree. C./W. Thus, the heat transporting
characteristic of the modified heat pipe was degraded.
[0054] Next, here will be explained advantages of straightening the
wires forming the wick by applying a thermal treatment in advance,
and to fix the wick thermally onto the inner face of the container
over the entire length. In most cases, the thin wires used to form
the wick are manufactured in the form of coil, and transported to a
production site of the heat pipe. Therefore, the uncoiled wires may
be curled by the residual stress even after bundling or at
subsequent processes. However, according to the present invention,
the wires used to form the wick are straightened in advance by
applying thermal treatment such as annealing thereby eliminating
the residual stress remaining in the wires. Therefore, the wires
forming the wick will not be curled during a process of inserting
the wick into the container, and the wick formed of straightened
wires can be fit to the interior configuration of the container as
shown in FIG. 7 (a). In addition, the wick thus arranged in the
container is heated to be fixed onto the inner face of the
container over the entire length. Therefore, in case of bending the
container 3 holding the wick 4 therein, the wick 4 is also bent
together with the container 3 as shown in FIG. 7 (b). For this
reason, the flow passage(s) 5 formed along the wick 4 will not be
closed by the wick 4 even after bending the container 3.
[0055] To the contrary, when the thermal treatment is not applied
to the wick 4 in advance, the wick 4 may be deformed by the
residual stress remaining therein. In this case, the curved wick 4
extends obliquely in the container 3 and is in partial contact with
the inner face of the container 3 as shown in FIG. 8 (a). As a
result, the flow passage 5 is closed by the curved wick 4 and the
evaporated working fluid flowing therethrough is blocked. For this
reason, the heat transporting capacity of the heat pipe is
degraded. As shown in FIG. 8 (b), the heat pipe thus structured
also suffers from this disadvantage even if it is bent.
[0056] Alternatively, in case of fixing the wick 4 onto the inner
face of the container 3 at both end portions, the wick 4 may be
arranged diagonally on the container 3 as shown in FIG. 9 (a). In
this case, therefore, a velocity of the evaporated working fluid
flowing through the flow passage 5 is changed by the wick 4 thus
extending diagonally. In this case, an intermediate portion of the
wick 4 is also in contact with the inner face of the container 3
when the heat pipe is bent, and the heat transporting capacity of
the heat pipe is thereby degraded. Especially, in the flat heat
pipe, the wick 4 is in contact with both of the upper and lower
inner faces of the container 3. Therefore, in case the
configuration of the wick 4 is not congruent with that of the inner
face of the container 3, the flow passage 5 for letting through the
evaporated working fluid is completely closed by the wick 4 as
shown in FIGS. 8 and 9. In this situation, the heat pipe is
disabled to transport the heat between the end portions of the
container 3.
* * * * *