U.S. patent application number 14/799719 was filed with the patent office on 2016-01-21 for flat heat pipe.
This patent application is currently assigned to FUJIKURA LTD.. The applicant listed for this patent is FUJIKURA LTD.. Invention is credited to Mohammad Shahed AHAMED, Yuji SAITO.
Application Number | 20160018166 14/799719 |
Document ID | / |
Family ID | 53887591 |
Filed Date | 2016-01-21 |
United States Patent
Application |
20160018166 |
Kind Code |
A1 |
AHAMED; Mohammad Shahed ; et
al. |
January 21, 2016 |
FLAT HEAT PIPE
Abstract
A heat pipe a flat heat pipe having enhanced heat transport
capacity is provided. The flat heat pipe 1 comprises a working
fluid encapsulated in a container 2, a first wick 11 that returns
the working fluid from a condensing portion 4 to an evaporating
portion 3 by a capillary pumping, and a second wick 12 that spreads
the working fluid returned thereto over an inner face of the
container by a capillary pumping. The second wick is formed only in
the evaporating portion 3 to extend in a circumferential direction
on the inner face of the container 2.
Inventors: |
AHAMED; Mohammad Shahed;
(Tokyo, JP) ; SAITO; Yuji; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FUJIKURA LTD. |
Tokyo |
|
JP |
|
|
Assignee: |
FUJIKURA LTD.
Tokyo
JP
|
Family ID: |
53887591 |
Appl. No.: |
14/799719 |
Filed: |
July 15, 2015 |
Current U.S.
Class: |
165/104.26 |
Current CPC
Class: |
F28D 15/0233 20130101;
F28D 15/04 20130101; F28D 15/046 20130101 |
International
Class: |
F28D 15/04 20060101
F28D015/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 16, 2014 |
JP |
2014-146261 |
Claims
1. A flat heat pipe, comprising: a sealed container having a pair
of upper and lower flat walls extending in a length direction; a
working fluid encapsulated in the container; an evaporating portion
that is situated on one of the longitudinal ends of the container
at which evaporation of the working fluid takes place; a condensing
portion that is situated on the other longitudinal end of the
container at which condensation of the working fluid takes place; a
first wick formed of a bundled fibers or a porous structure that
extends in the length direction of the container to return the
working fluid condensed at the condensing portion to the
evaporating portion by a capillary pumping; and a second wick that
is formed by forming a groove on an inner face of the container
only in the evaporating portion to spread the working fluid
returned thereto over the inner face within the evaporating portion
by a capillary pumping; wherein the first wick is formed on at
least one of the upper flat wall or the lower flat wall; and
wherein the second wick transversely extends underneath the first
wick to be closed partially by the first wick.
2. The flat heat pipe as claimed in claim 1, wherein the second
wick is formed on the inner face of the container in the
evaporating portion in a spiral manner.
3. The flat heat pipe as claimed in claim 1, wherein the second
wick is formed on the inner face of the container in the
evaporating portion in a circular manner.
4. The flat heat pipe as claimed in claim 1, wherein the second
wick is formed only on the inner face of one of the upper flat face
and the lower flat face on which the first wick is formed.
5. The flat heat pipe as claimed in claim 1, wherein the second
wick includes an opening portions extending from both sides of the
first wick in the circumferential direction.
6. The flat heat pipe as claimed in claim 1, wherein the porous
structure is formed by sintering metal powders.
7. The flat heat pipe as claimed in claim 1, wherein the first wick
is formed of copper fibers.
8. The flat heat pipe as claimed in claim 1, wherein the first wick
is formed of carbon fibers.
9. The flat heat pipe as claimed in claim 1, wherein the first wick
is formed of a mixture of carbon fibers and copper fibers.
Description
[0001] The present invention claims the benefit of Japanese Patent
Application No. 2014-146261 filed on Jul. 16, 2014 with the
Japanese Patent Office, the disclosure of which is incorporated
herein by reference in its entirety.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to an art of a flat heat pipe
having a wick structure.
[0004] 2. Discussion of the Related Art
[0005] Heat pipe adapted to transport heat utilizing latent heat of
fluid has been widely used as a heat transport device. The
conventional heat pipe comprises a tubular sealed container and
working fluid encapsulated therein, and both ends of the container
are closed. The working fluid is vaporized by a heat of the heat
generating element transmitted to one end of the heat pipe, and
flows toward the other end to be cooled.
[0006] In general, one of the end portions of the heat pipe is
brought into contact to the heat generating element to evaporate
the working fluid, and the other end portion is brought into
contact to a radiation member to condense the working fluid by
radiating heat through the radiation member. The condensed working
fluid is returned to the heated site through a wick by capillary
pumping of the wick.
[0007] For example, JP-A-2001-296093 describes a heat pipe in which
spiral or loop minute grooves are formed on an inner face of a
tubular container. In the heat pipe taught by JP-A-2001-296093, the
working fluid condensed at a radiation site is returned
gravitationally to a heated site.
[0008] In the flat heat pipe, an internal space thereof serving as
a vapor passage is narrow in its height direction but can be
ensured sufficiently in its width direction. However, since an
inner volume of the thin flat heat pipe is rather small, it is
difficult to circulate the phase changeable working fluid.
[0009] Given that a large wick structure is arranged in the
container to enhance capillary pumping to pull the working fluid to
a heating site, most part of the internal space would be occupied
by the wick structure, and hence it is difficult to ensure the
vapor passage sufficiently. If the working fluid cannot be returned
to the heating site sufficiently, the heating site would be dried
out.
[0010] One possible solution to ensure the vapor passage in the
flat container is to form the minute grooves taught by
JP-A-2001-296093 on an inner face of the flat container.
[0011] However, the minute grooves taught by JP-A-2001-296093 is
adapted to return the working fluid to the heating site
gravitationally. That is, in the heat pipe having the minute
grooves taught by JP-A-2001-296093, an efficiency to return the
working fluid to the heating site would be varied significantly
depending on an orientation or posture of the heat pipe. For
example, given that the flat heat pipe is situated horizontally, it
would be difficult to return the working fluid to the heating site
efficiently by the gravity. Consequently, heat transport
performance of the flat heat pipe will be worsened.
SUMMARY OF THE INVENTION
[0012] The present invention has been conceived nothing the
foregoing technical problems, and it is therefore an object of the
present invention is to provide a flat heat pipe having enhanced
heat transport capacity that can be manufactured easily.
[0013] The heat pipe according to the present invention comprises:
a sealed container having a pair of upper and lower flat walls
extending in a length direction; a working fluid encapsulated in
the container; an evaporating portion that is situated on one of
the longitudinal ends of the container at which evaporation of the
working fluid takes place; a condensing portion that is situated on
the other longitudinal end of the container at which condensation
of the working fluid takes place; a first wick formed of a bundled
fibers or a porous structure that extends in the length direction
of the container to return the working fluid condensed at the
condensing portion to the evaporating portion by a capillary
pumping; and a second wick that is formed by forming a groove on an
inner face of the container only in the evaporating portion to
spread the working fluid returned thereto over the inner face
within the evaporating portion by a capillary pumping. The first
wick is formed on at least one of the upper flat wall or the lower
flat wall, and the second wick transversely extends underneath the
first wick to be closed partially by the first wick.
[0014] For example, the second wick may be formed on the inner face
of the container in the evaporating portion in a spiral manner.
[0015] Instead, the second wick may also be formed on the inner
face of the container in the evaporating portion in a circular
manner.
[0016] Further, the second wick may also be formed only on the
inner face of one of the upper flat face and the lower flat face on
which the first wick is formed.
[0017] The second wick includes an opening portions extending from
both sides of the first wick in the circumferential direction.
[0018] Specifically, the porous structure may be formed by
sintering metal powders.
[0019] Thus, in the flat heat pipe of the present invention, the
groove wick is formed on the inner face of the container in the
evaporating portion. According to the present invention, therefore,
hydrophilicity on the inner face of the container in the
evaporating portion can be improved to spread the working fluid
over the inner face smoothly. Consequently, the working fluid in
the evaporating portion can be evaporated efficiently so that the
working fluid can be circulated smoothly within the container. For
these reasons, the thermal resistance to transport heat can be
reduced so that the heat transporting performance of the heat pipe
can be enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Features, aspects, and advantages of exemplary embodiments
of the present invention will become better understood with
reference to the following description and accompanying drawings,
which should not limit the invention in any way.
[0021] FIG. 1 is a schematic illustration showing the flat heat
pipe according to the preferred example;
[0022] FIG. 2 is a perspective view showing the sealed container in
which a spiral groove wick is formed on an inner face;
[0023] FIG. 3 (a) is a cross-sectional view showing a cross-section
of the evaporating portion of the heat pipe along the line A-A in
FIG. 1, FIG. 3 (b) is a cross-sectional view showing a
cross-section of the heat pipe at a boundary between the
evaporating portion and the insulating portion along the line B-B
FIG. 1, and FIG. 3 (c) is a schematic illustration showing the
groove wick and the fiber wick formed on a lower flat wall;
[0024] FIG. 4 (a) is a top view of a testing device, and FIG. 4 (b)
is a front view of a testing device;
[0025] FIG. 5 is a graph indicating testing result of the heat
pipes according to the preferred example and the comparison
example;
[0026] FIG. 6 is a schematic illustration showing the flat heat
pipe in which circular groove wick is formed on the inner face;
[0027] FIG. 7 (a) is a cross-sectional view showing a cross-section
of the heat pipe in which the groove wick is formed only on the
lower flat wall of the evaporating portion, and FIG. 7 (b) is a
cross-sectional view showing a cross-section of the heat pipe in
which a transverse groove wick is formed only on the lower flat
wall; and
[0028] FIG. 8 (a) is a cross-sectional view showing a cross-section
of the heat pipe in which the fiber wicks are formed on the width
centers of both upper and lower flat wall, FIG. 8 (b) is a
cross-sectional view showing a cross-section of the heat pipe in
which the fiber wicks formed on the upper and lower flat walls are
displaced from each other, and FIG. 8 (c) is a cross-sectional view
showing a cross-section of the heat pipe in which the fiber wick is
formed while brought into contact to both upper and lower flat
walls.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0029] Hereinafter, preferred examples of the flat heat pipe
according to the present invention will be explained in more detail
with reference to the accompanying drawings.
[0030] Referring now to FIG. 1, there is shown a heat pipe 1
according to the preferred example. The heat pipe 1 shown therein
is a heat transport device adapted to transport heat in the form of
latent heat of phase changeable working fluid encapsulated in a
sealed container 2.
[0031] The container 2 is flattened to have flat longitudinal
walls, and both ends are closed. In the heat pipe 1, one of the end
portions serves as an evaporating portion 3 and the other end
serves as a condensing portion 4, and an insulated portion 5
therebetween is insulated from an external heat.
[0032] The evaporating portion 3 is brought into contact to a heat
generating element, and the working fluid held therein is heated by
a heat of the heat generating element. According to the preferred
example, approximately third part of the container 2 serves as the
evaporating portion.
[0033] The condensing portion 4 is brought into contact to a not
shown radiation member such as a fin assembly. The working fluid
evaporated at the evaporating portion 3 is aspirated toward the
condensing portion 4 where a temperature and a pressure are low
through the insulated portion 5 to radiate heat thereof through the
external radiation member. Consequently, the working fluid is
condensed into liquid phase again at the condensing portion 4.
[0034] In order to return the working fluid condensed at the
condensing portion 4 to the evaporating portion 3 by a capillary
pumping, a fiber wick 11 and a groove wick 12 are arranged in the
container 2.
[0035] According to the preferred example, the fiber wick 11 is
formed by bundling a plurality of copper fibers, and situated on a
width center of the flat wall of the container 2. In the fiber wick
11 thus formed, each longitudinal clearance among the fibers
individually serves as a flow passage to pull the working fluid
condensed at the condensing portion 4 toward the evaporating
portion 3 through the insulated portion 5 by a capillary
pumping.
[0036] In order to guide the working fluid returned to the
evaporating portion 3 also in a circumferential direction, a groove
wick 12 is formed on an inner face of the container 2 in the
evaporating portion. According to the preferred example, the groove
wick 12 is formed into a spiral groove on the inner face of the
container 2
[0037] An internal structure of the flat heat pipe 1 will be
explained in more detail with reference to FIGS. 2 and 3. Turning
first to FIG. 2, there is schematically shown the container 2
according to the preferred example. For example, the container 2
can be formed by flattening a metal tube in such a manner to form a
lower flat wall 21 and an upper flat wall 22 in a longitudinal
direction.
[0038] Turning now to FIG. 3, FIG. 3 (a) shows a cross-section of
the evaporating portion 3 of the heat pipe 1 along the line A-A in
FIG. 1. As depicted in FIG. 3 (a), an inner face 2a of the
container 2 includes a lower inner face 21a of the lower flat wall
21, and an upper inner face 22a of the upper flat wall 22. FIG. 3
(b) shows a cross-section of the heat pipe 1 at a boundary between
the evaporating portion 3 and the insulating portion 5 along the
line B-B FIG. 1. As depicted in FIG. 3 (b), the groove wick 12 is
formed on the inner face 2a only in the evaporating portion 3, and
the remaining portion of the inner face 2a in the insulated portion
5 and the condensing portion 4 has a smooth surface. As depicted in
FIG. 3 (c), the groove wick 12 is formed on the inner face 2 in a
spiral manner.
[0039] Specifically, the groove wick 12 is a depressed minute
groove formed on the inner face 2a in a spiral manner from the
boundary between the insulated portion 5 and the evaporating
portion 3 to one of the end portions of the container 2.
[0040] Optionally, in order to enhance the capillary pumping force
of the spiral groove wick 12, a plurality of the spiral groove wick
12 may be formed on the inner surface 2 of the evaporating portion
3.
[0041] The fiber wick 11 is laid on the lower inner face 21a of the
lower flat wall 21 in a length direction of the container 2. That
is, the lower inner face 21a of the lower flat wall 21 serves as an
installation surface. Specifically, as shown in FIG. 3 (a), the
copper fibers are heaped on the lower inner face 21a to form the
fiber wick 11 in such a manner to have a semi-oval cross section
while keeping a space between a peak of the fiber wick 11 and the
upper inner face 22a of the upper flat wall 22. The fiber wick 11
is sintered to fix the copper fibers to one another, and to be
fixed to the lower inner face 21a of the lower flat wall 21.
[0042] As shown in FIGS. 3 (a), (b) and (c), in the evaporating
portion 3, the groove wick 12 formed on the inner face 2a of the
container 2 passes underneath a lower face 11a of the fiber wick 11
in the transverse direction repeatedly at predetermined intervals.
That is, the lower face 11a of the fiber wick 11 is brought into
contact to the lower inner face 21a of the lower flat wall 21
intermittently. In other words, in the evaporating portion 3, the
groove wick 12 transversely crossing the fiber wick 11 is partially
closed underneath the fiber wick 11 by the lower face 11a of the
fiber wick 11. In the following description, the portion of the
lower face 11a of the fiber wick 11 thus covering the groove wick
12 will be called the "lid portion" 11b, and the portion of the
groove wick 12 thus closed by the lid portion 11b will be called
the "closed portion" 12a. In FIG. 3 (c), the closed portion of the
groove wick 12 is illustrated by dashed lines. In the evaporating
portion 3, the condensed working fluid flowing through the fiber
wick 11 is pulled by the capillary force of the groove wick 12
through the lid portion 11b so that the working fluid is allowed to
be spread entirely over the inner face 2a of the container 2
through the groove wick 12. That is, the remaining opening portion
12b of the groove wick 12 illustrated by solid lines in FIG. 3 (c)
serves as a flow passage for the working fluid in the liquid phase,
and the working fluid evaporated in the opening portion 12b is
allowed to ascend therefrom.
[0043] Here will be explained a heat transport cycle in the heat
pipe 1. In the heat pipe 1, the working fluid flowing through the
fiber wick 11 and the groove wick 12 is evaporated at the
evaporating portion 3 by the heat of the not shown heat generating
element. The working fluid vaporized at the evaporating portion 3
flows toward the condensing portion 4 where an internal pressure
and a temperature are lower than those in the evaporating portion 3
through an internal space of the container 2. The vapor of the
working fluid is cooled to be liquefied at the condensing portion 4
and penetrates into the fiber wick 11. Then, the working fluid in
the liquid phase is returned to the evaporating portion 3 through
the fiber wick 11, and spread over the inner face 2a of the
container 2 through the groove wick 12. The working fluid thus
returned to the evaporating portion 3 is again heated to be
vaporized.
[0044] According to the preferred example, the working fluid
flowing through the fiber wick 11 in the length direction is
allowed to flow into the groove wick 12 through the lid portions
11b of the fiber wick 11 to be spread on the inner face 2a of the
container 2 in the circumferential direction. Since the wick groove
12 is thus formed on the inner face 2a of the container 2, a
surface roughness of the inner face 2a is increased to enhance
hydrophilicity thereof. For this reason, the thermal resistance in
the evaporating portion 3 can be reduced to enhance the heat
transporting performance of the heat pipe 1. In addition, since the
groove wick 12 is thus formed in such a manner to cross the fiber
wick 11 diagonally, the copper fibers of the fiber wick 11 can be
prevented from falling into the groove wick 12.
[0045] Next, here will be explained test result of heat transport
capacities of the heat pipes according to the preferred example and
the comparison example with reference to FIGS. 4 and 5.
[0046] Turning now to FIG. 4, there is shown specifications of the
heat pipe 1 of the preferred example and the heat pipe 100 of the
comparison example used in the test. As shown in FIGS. 4 (a) and
(b), lengths, widths and thicknesses of both heat pipes were 150
mm, 9.1 mm and 1.0 mm respectively, and the containers of both heat
pipes were prepared by flattening metal tubular pipes whose
external diameter was 6.0 mm. In the evaporating portion 3 of the
heat pipe 1 according to the preferred example, a plurality of the
groove wicks 12 whose width and depth were 0.15 mm and 0.02 mm were
formed on the inner face 2a of the container 2 at intervals of 0.45
mm. By contrast, the heat pipe 100 of the comparison example was
not provided with the groove wick.
[0047] As shown in FIG. 4 (a), an electric heater H whose length
and width were respectively 15 mm was attached to one end of the
heat pipe to serve as the heat generating element, and a radiating
member S whose length and width were respectively 50 mm was
attached to the other end of the heat pipe. In addition, each heat
pipe 1 and 100 is individually flexed to a substantially right
angle at its intermediate portion.
[0048] As shown in FIG. 7 (b), an outer face of the lower flat wall
21 of the evaporating portion 3 is brought into contact to the
heater H, and an outer face of the lower flat wall 21 of the
condensing portion 4 is brought into contact to the radiating
member S. In the test, each heat pipe of the preferred example and
the comparison example was individually attached horizontally to a
test equipment.
[0049] Temperatures of each heat pipe and the heater H was measured
by a conventional thermocouple sensor. Specifically, as shown in
FIGS. 4 (a) and (b), a surface temperature Th of the heater H
contacted to the lower flat wall 21 of the evaporating portion 3, a
surface temperature Ti of the upper flat wall 22 of the insulating
portion 5, and a surface temperature Tc of the upper flat wall 22
of the condensing portion 4 were measured.
[0050] The evaporating portion 3 of each heat pipe was heated by
energizing the heater H under room temperature, and the surface
temperatures Th, Tc, and Ti were measured respectively while
changing a heat input Q to the evaporating portion 3. Then, a
thermal resistance R of each heat pipe was calculated under the
condition that a temperature Ti of the insulating portion was
stabilized at 60 degrees C. as expressed by the following
expression:
R=(Th-Tc)/Q.
[0051] The calculation results of the thermal resistance R of the
heat pipes of the preferred example and the comparison example are
plotted in FIG. 5. In FIG. 5, a line penetrating through round dots
represents the thermal resistance R of the heat pipe 1 according to
the preferred example, and a dot-and-dash line represents the
thermal resistance R of the heat pipe 100 according to the
comparison example.
[0052] As can be seen from FIG. 5, a maximum heat transporting
quantity QMAX of the heat pipe 1 according to the preferred example
was achieved by 22 W of the heat input, and the thermal resistance
R thereof at the maximum heat transporting quantity QMAX was 0.50.
By contrast, a maximum heat transporting quantity QMAX of the heat
pipe 100 according to the comparison example was achieved by 16 W
of the heat input, and the thermal resistance R thereof at the
maximum heat transporting quantity QMAX was 0.58.
[0053] If the heat input Q to the evaporating portion 3 exceeds the
limitation value, the working fluid in the evaporating portion 3
would be dried out and the thermal resistance R of the heat pipe
would be increased significantly. That is, the maximum heat
transporting quantity QMAX of the heat pipe is increased with an
increment of the limitation value of the heat input to the
evaporating portion 3.
[0054] Specifically, the heat pipe 1 according to the comparison
example causes the dry-out if the thermal input thereto exceeds 16
W, but the heat pipe 100 according to the preferred example will
not cause the dry-out until the thermal input thereto exceeds 22
W.
[0055] That is, the maximum heat transporting quantity QMAX of the
heat pipe 1 according to the preferred example was larger than that
of the heat pipe 100 according to the comparison example.
[0056] Thus, hydrophilicity of the inner face 2a of the container 2
in the evaporating portion 3 can be improved by forming the groove
wick 12 thereon so that heat transporting capacity of the heat pipe
1 can be enhanced.
[0057] The structure of the heat pipe 1 according to the preferred
examples may be modified according to need within the spirit of the
present invention. For example, as shown in FIG. 6, a plurality of
unconnected circular groove wicks 12 may be formed on the inner
face 2a of the container 2 in the evaporating portion 3 instead of
the foregoing spiral groove wick.
[0058] Instead, as shown in FIGS. 7 (a) and 7 (b), the groove wick
may also be formed only on the inner face 2a of the lower flat wall
21. In this case, in the evaporating portion 3, a plurality of
straight grooves extending perpendicular to the fiber wick 11 are
formed as the groove wick 12 on the inner face 2a of the lower flat
wall 21, and a length of each straight groove is respectively
longer than the width of the fiber wick 11 to ensure the opening
portions 12b on both sides of the fiber wick 11.
[0059] In case of thus forming the groove wick 12 only on the inner
face 2a of the lower flat wall 21, a configuration of the groove
wick 12 may be altered arbitrarily according to need. For example,
a plurality of rectangular grooves individually having a width
wider than that of the fiber wick 11 may be formed on the inner
face 2a of the lower flat wall 21.
[0060] In addition, the fiber wick 11 may also be formed of carbon
fibers instead of the copper fibers to reduce thermal resistance to
transport heat in the length direction. In this case, the fiber
wick 11 may also be formed not only of the carbon fibers but also
of a mixture of the carbon fibers and the copper fibers.
[0061] Instead, a sintered porous wick made of metal powers may be
employed instead of the fiber wick 11 to return the working fluid
in the liquid phase from the condensing portion 4 to the
evaporating portion 3. In this case, stronger capillary force can
be achieved to pull the working fluid so that the working fluid can
be returned to the evaporating portion more smoothly.
[0062] Further, as shown in FIG. 8 (a), a second fiber wick 13 may
be formed also on the width center of the upper inner face 22a of
the upper flat wall 22 to be opposed to the fiber wick 11.
Alternatively, the second fiber wick 13 may be displaced widthwise
as shown in FIG. 8 (b). In any of those cases, the fiber wick 11
and the second fiber wick 13 are isolated away from each other so
as to ensure the vapor passage in the internal space of the
container 2. In those cases, the heat transporting performance of
the heat pipe 1 will not be changed even if the heat pipe 1 is
reversed. In addition, the fiber wick 11 may also be brought into
contact to both the upper inner face 22a of the upper flat wall 22
and the lower inner face 21a of the lower flat wall 21.
* * * * *