U.S. patent application number 13/772879 was filed with the patent office on 2013-06-27 for loop heat pipe and electronic apparatus.
This patent application is currently assigned to FUJITSU LIMITED. The applicant listed for this patent is FUJITSU LIMITED. Invention is credited to Hiroki UCHIDA.
Application Number | 20130160974 13/772879 |
Document ID | / |
Family ID | 45938001 |
Filed Date | 2013-06-27 |
United States Patent
Application |
20130160974 |
Kind Code |
A1 |
UCHIDA; Hiroki |
June 27, 2013 |
LOOP HEAT PIPE AND ELECTRONIC APPARATUS
Abstract
An evaporator of a loop heat pipe includes a case having a
liquid flow inlet and a vapor flow outlet, and at least one porous
body disposed inside the case and configured to guide liquid-phase
working fluid inward of the case. The evaporator further includes a
liquid supply duct disposed inside the case and configured to guide
the working fluid into the porous body from the liquid flow inlet.
The liquid supply duct is made of a material having lower heat
conductivity than a material of the case. The working fluid having
flowed into the evaporator is prevented from evaporating before
reaching the porous body.
Inventors: |
UCHIDA; Hiroki; (Isehara,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FUJITSU LIMITED; |
Kawasaki-shi |
|
JP |
|
|
Assignee: |
FUJITSU LIMITED
Kawasaki-shi
JP
|
Family ID: |
45938001 |
Appl. No.: |
13/772879 |
Filed: |
February 21, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2010/068041 |
Oct 14, 2010 |
|
|
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13772879 |
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Current U.S.
Class: |
165/104.21 |
Current CPC
Class: |
F28D 15/0266 20130101;
F28D 15/043 20130101; F28D 15/0233 20130101; F28D 15/046
20130101 |
Class at
Publication: |
165/104.21 |
International
Class: |
F28D 15/02 20060101
F28D015/02 |
Claims
1. A loop heat pipe comprising: a liquid transport line; an
evaporator; a vapor transport line; and a condenser, the liquid
transport line, the evaporator, the vapor transport line, and the
condenser being connected to one another to circulate a working
fluid, the evaporator including a case including a liquid flow
inlet and a vapor flow outlet, at least one porous body disposed
inside the case and configured to guide the working fluid in a
liquid phase inward of the case, and a liquid supply duct disposed
inside the case and configured to guide the working fluid in the
liquid phase into the at least one porous body from the liquid flow
inlet, wherein the liquid supply duct is made of a material having
lower heat conductivity than a material of the case.
2. The loop heat pipe as claimed in claim 1, wherein the material
of the case is a metal or an alloy, and the material of the liquid
supply duct has a heat conductivity of 1 W/mK or lower.
3. The loop heat pipe as claimed in claim 1, wherein the material
of the case is a metal or an alloy, and the material of the liquid
supply duct is a resin.
4. The loop heat pipe as claimed in claim 3, wherein the material
of the liquid supply duct is selected from a group consisting of a
fluorine resin, a nylon resin, a polyether ether ketone resin, a
polypropylene resin, and a polyacetal resin.
5. The loop heat pipe as claimed in claim 1, wherein the at least
one porous body is made of a porous resin.
6. The loop heat pipe as claimed in claim 5, wherein the porous
resin is selected from a group consisting of a fluorine resin, a
polyether ether ketone resin, a polypropylene resin, and a
polyacetal resin.
7. The loop heat pipe as claimed in claim 5, wherein the at least
one porous body and the liquid supply duct are made of a same
porous resin.
8. The loop heat pipe as claimed in claim 1, wherein the liquid
supply duct extends continuously from the liquid flow inlet to the
at least one porous body.
9. The loop heat pipe as claimed in claim 1, wherein the liquid
supply duct includes a tubular part extending inside the liquid
transport line.
10. The loop heat pipe as claimed in claim 9, wherein the tubular
part is disposed in close contact with an inner wall of the liquid
transport line.
11. The loop heat pipe as claimed in claim 1, wherein the at least
one porous body is provided as plural of the porous bodies, and the
liquid supply duct is a manifold for distributing the working fluid
in the liquid phase to the plural porous bodies.
12. The loop heat pipe as claimed in claim 1, wherein the at least
one porous body includes, on an outer periphery, a vapor discharge
groove running an entire length of the at least one porous body
from a side closer to the liquid transport line to a side closer to
the vapor transport line, and an end portion of the vapor discharge
groove on the side closer to the liquid transport line is
terminated by a wall surface of the liquid supply duct.
13. The loop heat pipe as claimed in claim 1, wherein the case has
a plate-like outer shape.
14. The loop heat pipe as claimed in claim 1, wherein the case
includes a first part disposed in contact with the at least one
porous body and a second part disposed on a side closer to the
liquid supply duct and housing at least part of the liquid supply
duct, and the second part is made of a material having lower heat
conductivity than a material of the first part.
15. The loop heat pipe as claimed in claim 14, wherein the material
of the first part is selected from a group consisting of
oxygen-free copper, a copper alloy, aluminum, and an aluminum
alloy.
16. The loop heat pipe as claimed in claim 14, wherein the material
of the second part is selected from a group consisting of an iron
based alloy and a titanium alloy.
17. An electronic apparatus comprising: the loop heat pipe as
claimed in claim 1; and an electronic component thermally bonded to
the evaporator of the loop heat pipe.
18. The electronic apparatus as claimed in claim 17, wherein on a
bonded plane between the electronic component and the case of the
evaporator, a center of the electronic component is offset, in
relation to a center of the case, in a direction opposite from a
direction to the liquid flow inlet.
19. The electronic apparatus as claimed in claim 18, wherein the
case of the evaporator includes a first part disposed in contact
with the at least one porous body and a second part disposed on a
side closer to the liquid supply duct and made of a material having
lower heat conductivity than a material of the first part, and the
electronic component is bonded to the first part.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation application filed
under 35 U.S.C. 111(a) claiming benefit under 35 U.S.C. 120 and
365(c) of PCT International Application No. PCT/JP2010/068041,
filed on Oct. 14, 2010, the entire contents of which are
incorporated herein by reference.
FIELD
[0002] The embodiments discussed herein are related to a loop heat
pipe and an electronic apparatus.
BACKGROUND
[0003] Loop heat pipes (LHP) are known as devices for cooling
various types of heat generating elements. In a loop heat pipe, an
evaporator and a condenser are connected in a loop using a vapor
transport line and a liquid transport line. Liquid-phase working
fluid evaporates in the evaporator due to heat supplied from a heat
generating element, and the vaporized working fluid is transported
to the condenser in which the vapor condenses back to liquid by
heat dissipation. The evaporator vaporizes the working fluid to
transfer heat from the heat generating element (the heat of
vaporization), and also functions as a pump for driving the
circulation of the working fluid.
[0004] FIGS. 1A through 1C illustrate a conventional evaporator 10.
FIG. 1A is a schematic cross-sectional view of the evaporator 10
along the direction of working fluid flowing from a liquid
transport line to a vapor transport line. FIGS. 1B and 1C are
schematic cross-sectional views taken along the A-A' line of FIG.
1A, and illustrate two conventional evaporator structures, a
cylindrical type and a plate type, respectively.
[0005] The evaporator 10 includes a metal case 20 connected to a
liquid transport line 50 and a vapor transport line 55; and a wick
30 which is a porous body disposed inside the metal case 20.
Working fluid 60a supplied through the liquid transport line 50
flows into a liquid supply path 31 disposed substantially in the
center of the wick 30 and is guided to the inner wall of the metal
case 20 by capillary force in pores of the wick 30, which is a
driving force for the working fluid 60a. Subsequently, the working
fluid 60a is vaporized by heat transferred to the metal case 20
from the heat generating element, to thereby form vapor 60b. The
vapor 60b then passes through vapor discharge grooves 32 provided
on the outer periphery of the wick 30, or on the inner wall of the
metal case 20, and flows into the vapor transport line 55.
[0006] Application of loop heat pipes to cooling of electronic
components, for example, central processing units (CPU) of computer
systems, has been studied in recent years. Many electronic
components have a flat heat dissipation surface, as represented by
large scale integrated (LSI) packages. In the case of a cylindrical
evaporator 10' as illustrated in FIG. 1B, a flat plate 28 serving
as a heat receiving surface is mounted on the case 20 of the
evaporator 10' in order to enhance contact of the case 20 with the
heat dissipation surface. On the other hand, in the case of a
plate-type evaporator 10'' as illustrated in FIG. 1C, one surface
29 of the case 20 generally having a rectangular parallelepiped
shape is used as a heat receiving surface.
[0007] In general, in order to improve cooling performance of the
loop heat pipe, increasing the internal volume of the evaporator is
effective. On the other hand, the evaporator needs to be as compact
as possible for producing a smaller and lighter electronic
apparatus. In order to increase the internal volume while
maintaining a compact, especially thin configuration, a plate-type
evaporator, as illustrated in FIG. 1C, is considered to be
preferable. To improve the cooling performance, using the
evaporator case made of a metal, especially a metal with high heat
conductivity such as copper, is effective. This is because the
metal evaporator case facilitates heat transfer from the heat
generating element to the entire outer periphery of the wick, which
in turn facilitates vaporization of the working fluid. The metal
evaporator case is preferable also in terms of providing sealing
reliability to prevent the working fluid contained in the sealed
evaporator case from leaking out.
[0008] However, downsizing of the evaporator may cause a problem as
illustrated in FIG. 2. Heat is transferred from a heat generating
element 70 to a part of the evaporator case 20, adjacent to the
liquid transport line 50 (hereinafter, simply referred to as the
"adjacent part"). This may heat the working fluid 60a before the
working fluid 60a reaches the wick 30 after flowing out of the
liquid transport line 50. Due to the heat, the working fluid 60a
may come to a boil at the adjacent part, which produces air bubbles
60c. As illustrated in an enlarged schematic diagram of FIG. 2, the
air bubbles 60c enter the liquid supply path 31 of the wick 30,
which leads to a condition where a gas phase exists on both sides
of the wick 30. This creates a surface tension force 36 that
cancels out a usual surface tension force 35 toward the outer
periphery of the wick 30. The cancelling out of the surface tension
forces 35 and 36 results in a loss of capillary force of the wick
30. In addition, the occurrence of the air bubbles 60c raises the
internal pressure of the liquid supply path 31, which may block the
inflow of the working fluid 60a from the liquid transport line 50.
As a result, the circulation of the working fluid 60a is reduced or
stopped, which in turn leads to reduced cooling performance and/or
operational instability of the loop heat pipe. These problems are
likely to occur not only in downsizing of the evaporator but also
in the use of a plate-type evaporator having a case, a part of
which is used as a heat receiving surface, and/or in the use of an
evaporator case made of a metal with high heat conductivity.
[0009] In view of the above-described problems, a technology has
been proposed in which an end face of a cylindrical evaporator
case, at which a liquid transport line is connected to the
evaporator case, is made of a metal with relatively low heat
conductivity or a resin to thereby prevent heat of the evaporator
case from being directly transferred to the liquid transport line.
However, using a resin as a material of the evaporator case
presents problems in terms of pressure resistance and long-term
sealing reliability. In the case of using a metal with low heat
conductivity, the heat conductivity of such a metal is still
several tens to several hundreds times that of a resin.
Accordingly, a low-heat-conductive metal evaporator case does not
provide sufficient heat insulation and, therefore, has limitations
to sufficiently control a reduction in cooling performance. [0010]
[Patent Document 1] Japanese Laid-open Patent Application
Publication No. 2004-218887 [0011] [Patent Document 2] Japanese
Laid-open Patent Application Publication No. 2009-115396 [0012]
[Patent Document 3] Japanese Patent No. 3591339
[0013] Consequently, demand is still being raised for technology
capable of preventing the working fluid from evaporating before
reaching the wick and avoiding a reduction in cooling performance
and/or operational instability of the loop heat pipe.
SUMMARY
[0014] According to one aspect, there is provided a loop heat pipe
with an evaporator which includes a case provided with a liquid
flow inlet and a vapor flow outlet; and at least one porous body
disposed inside the case and configured to guide liquid-phase
working fluid inward of the case. The evaporator further includes a
liquid supply duct disposed inside the case and configured to guide
the working fluid into the porous body from the liquid flow inlet.
The liquid supply duct is made of a material having lower heat
conductivity than a material of the case.
[0015] According to another aspect, there is provided an electronic
apparatus including the above-described loop heat pipe; and an
electronic component thermally bonded to the evaporator of the loop
heat pipe.
[0016] The object and advantages of the disclosure will be realized
and attained by means of the elements and combinations particularly
pointed out in the claims.
[0017] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are not restrictive of the invention, as
claimed.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1A is a schematic cross-sectional view of a
conventional evaporator;
[0019] FIG. 1B is a schematic cross-sectional view of a
conventional cylindrical evaporator;
[0020] FIG. 1C is a schematic cross-sectional view of a
conventional plate-type evaporator;
[0021] FIG. 2 is a schematic cross-sectional view illustrating a
problem of the conventional evaporator;
[0022] FIG. 3A is a perspective view of components of an evaporator
included in a loop heat pipe according to an embodiment;
[0023] FIG. 3B is a perspective view of a manifold of FIG. 3A,
viewed in a different direction;
[0024] FIG. 3C is a cross-sectional view of the evaporator
including the components of FIG. 3A, along a flow direction of
working fluid;
[0025] FIG. 3D is a cross-sectional view of the evaporator taken
along a B-B' line of FIG. 3C;
[0026] FIG. 4 is a perspective view of components of an evaporator
included in a loop heat pipe according to another embodiment;
[0027] FIG. 5 is a cross-sectional view of the evaporator included
in the loop heat pipe according to the other embodiment;
[0028] FIG. 6A is a perspective view illustrating an electronic
apparatus according to an embodiment;
[0029] FIG. 6B is a cross-sectional view taken along a C-C' line of
FIG. 6A;
[0030] FIG. 6C is a cross-sectional view taken along a D-D' line of
FIG. 6A;
[0031] FIG. 7 provides schematic cross-sectional views illustrating
structures of application examples; and
[0032] FIG. 8 is a graph of evaluation results of the structures of
FIG. 7.
DESCRIPTION OF EMBODIMENTS
[0033] Several embodiments are described in detail below with
reference to the accompanying drawings, wherein like reference
numerals refer to like elements throughout. Note that, in the
drawings, the relative size ratio of various components may not be
accurate, and dimensions are not to be scaled from the
drawings.
[0034] Referring to FIGS. 3A through 3D, the following describes an
evaporator 110 included in a loop heat pipe according to one
embodiment. FIG. 3A is an exploded view of main components of the
evaporator 110, and FIG. 3B illustrates a manifold 140 of FIG. 3A,
viewed in a different direction. FIG. 3C is a cross-sectional view
of the evaporator 110 including the components of FIG. 3A, along a
flow direction of working fluid, and FIG. 3D is a cross-sectional
view taken along the B-B' line of FIG. 3C. It should be noted that
the cross-sectional view of FIG. 3C does not depict a section made
by one plane cutting the evaporator 110.
[0035] In the example of FIGS. 3A through 3D, the evaporator 110 is
a plate-type evaporator. The evaporator 110 includes a first case
part 121 communicating with a vapor transport line 155 at a vapor
discharge outlet 126; a second case part 122 communicating with a
liquid transport line 150 at a liquid flow inlet 125; two porous
bodies (wicks) 130; and the branch tube (manifold) 140. The first
and the second case parts 121 and 122 are coupled to each other to
form a single evaporator case 120 for housing the wicks 130 and the
manifold 140. The flat dimensions of the evaporator case 120 (i.e.,
dimensions of a heat receiving surface) are determined based on the
size of a heat generating element, which is a cooling target. The
thickness of the evaporator case 120 may be limited according to
the packaging density of an electronic apparatus in which the
evaporator case 120 is used. For example, in the case of applying
the evaporator case 120 to an electronic apparatus with
high-density packaging, such as a server and a personal computer
(PC), the evaporator case 120 may need to be as thin as about 10 mm
or less.
[0036] Two holes 123 for housing the two wicks 130 are provided in
the first case part 121, as seen from the second case part 122
side. The shape of each of the holes 123 is determined by the
profile of the wick 130 to be inserted thereto, but is typically
round or oval. Between the two holes 123, a separation wall 124 is
provided, connecting the top and bottom faces of the first case
part 121. The second case part 122 may house the manifold 140.
However, in this embodiment, no particular limitation is imposed on
the structures of the two case parts 121 and 122 to form the
evaporator case 120 as long as the case parts 121 and 122 can be
coupled to each other after the wicks 130 and the manifold 140 are
housed therein. For example, one of the case parts 121 and 122 may
house both the wicks 130 and the manifold 140, and the other case
part 121 or 122 may have a plate-like shape to form an end face of
the evaporator case 120. Alternatively, the first and the second
case parts 121 and 122 may have structures formed by splitting the
evaporator case 120 into two in the thickness direction of the
plate-type evaporator 110.
[0037] Each of the wicks 130 roughly has a cup-like shape, and has
an opening defined by the inner periphery of the wick 130. The
opening serves as a liquid supply path 131 for supplying the
working fluid to the wick 130. The wicks 130 individually have
multiple vapor discharge grooves 132 on the outer periphery. The
grooves 132 may run the entire length of the wicks 130 along the
flow direction of the working fluid. In that case, end portions of
the grooves 132 on the liquid transport line 150 side may be
terminated by the manifold 140, as illustrated in FIG. 3C. Note in
FIG. 3C that the wick 130 and the evaporator case 120 are not in
contact with each other because the cross-section of FIG. 3C is
taken where the groove 132 is present. In a cross section of where
no groove 132 is present, the wick 130 and the evaporator case 120
are in contact with each other. The wicks 130 are preferably resin
wicks to be described later, and are formed to be individually
larger than the internal dimensions of the corresponding holes 123
in the first case part 121 so as to be compressed when inserted
into the holes 123. This enhances contact of the outer surfaces of
the resin wicks 130 with the inner wall of the evaporator case 120,
which may facilitate evaporation of the working fluid at the
contact site of the wicks 130 and the evaporator case 120.
[0038] The average pore diameter of the wicks 130 is preferably 15
.mu.m or less, and more preferably 5 .mu.m or less, in order to
obtain a sufficiently large capillary force. The wicks 130
preferably have high porosity so that the working fluid does not
become insufficient at the contact site of the wicks 130 and the
evaporator case 120. The porosity is set in the range of 30% or
more and less than 90%.
[0039] The manifold 140 (to be described in detail later)
insulates, from the evaporator case 120, working fluid 160a having
flowed in from the liquid transport line 150, and also serves as a
liquid supply duct for supplying the working fluid 160a to the
individual wicks 130. The manifold 140 includes an inlet 143
provided corresponding to the liquid flow inlet 125 of the
evaporator case 120; and two outlets 144 (see FIG. 3B) for
supplying the working fluid 160a to the two wicks 130. The manifold
140 has diverging flow paths 145 internally (FIG. 3D).
[0040] The manifold 140 is preferably formed to be in contact with
the wicks 130 when housed in the evaporator case 120 so that the
working fluid 160a supplied from the liquid transport line 150 does
not come in contact with the evaporator case 120 before reaching
the wicks 130. However, covering a large part of the stretch from
the liquid flow inlet 125 of the evaporator case 120 to the wick
housing part of the evaporator case 120 by the manifold 140 may be
enough to prevent the working fluid 160a from coming to a boil
before reaching the wicks 130. Accordingly, it may be acceptable to
have a gap between the manifold 140 and the wicks 130.
[0041] The manifold 140 includes a main body 141 disposed inside
the evaporator case 120 and may also include, on an as-needed
basis, a tubular part (inner pipe) 142 extending inside the liquid
transport line 150, as illustrated. The inner pipe 142 is
preferably formed to be in close contact with the inner wall of the
liquid transport line 150 so that the working fluid 160a does not
enter between the inner pipe 142 and the inner wall of the liquid
transport line 150. It is preferable to integrally form the inner
pipe 142 with the main body 141.
[0042] Materials of the evaporator case 120 provided with the first
and the second case parts 121 and 122 preferably include a metal or
an alloy so as to ensure the strength and sealing reliability. The
first and the second case parts 121 and 122 are joined and fixed to
each other by a method selected from among various methods capable
of ensuring the sealing reliability, such as welding, brazing, and
resin adhesion.
[0043] Further, the materials of the evaporator case 120 preferably
include a metal or an alloy with high heat conductivity, such as
oxygen-free copper, a copper alloy, aluminum, and an aluminum
alloy, in order to transfer heat from the cooling-target heat
generating element to the entire evaporator case 120. However, the
evaporator case 120 may be made of a metal or an alloy with
relatively low heat conductivity (for example, an iron based alloy
such as stainless steel, or a titanium alloy) according to, for
example, the size and/or required cooling capacities of the
evaporator case 120.
[0044] Materials of the manifold 140 need to have lower heat
conductivity than the evaporator case 120 so as to obtain heat
insulation. Although the manifold 140 having lower heat
conductivity is more preferable, the manifold 140 having a heat
conductivity of 1 W/mK or lower achieves a significant heat
insulation effect. The heat conductivity of 1 W/mK or lower is one
to several orders of magnitude less compared to, for example, the
case where the evaporator case 120 is made of copper (about 380
W/mK) and the case where the evaporator case 120 is made of
stainless steel (about 16 W/mK). This may create a significant
temperature difference between the outer wall and the inner wall of
the manifold 140. As a result, the working fluid 160a having flowed
into the evaporator case 120 is effectively insulated from the
evaporator case 120, which prevents the working fluid 160a from
evaporating before reaching the wicks 130.
[0045] For example, the materials of the manifold 140 may include a
resin, such as a fluorine resin, a nylon resin, a PEEK (polyether
ether ketone) resin, a polypropylene resin, and a polyacetal resin.
As an example, MC nylon (registered trademark of Quadrant Polypenco
Japan Ltd.) has a heat conductivity of about 0.2 W/mK, which is
about 1/1900 that of copper and about 1/80 that of stainless steel,
and therefore, MC nylon (registered trademark) with a thickness of,
for example, even 1 to several mm achieves heat insulation. The
manifold 140 may be a porous body made of a resin selected from the
above-mentioned resins.
[0046] Although being possibly selected from various types of
porous bodies, such as metal wicks, carbon wicks, and resin wicks,
the wicks 130 are preferably resin wicks because they are easy to
ensure close contact with the evaporator case 120 and have lower
heat conductivity than other types of wicks. If the wicks have high
heat conductivity, heat may be transferred to the inner periphery
of the wicks at which air bubbles are generated and may have a
similar effect as the generation of air bubbles before the working
fluid reaches the wicks. Therefore, using resin wicks may prevent
the generation of air bubbles on the inner periphery side of the
wicks. Preferable materials of resin wicks include, for example, a
fluorine resin, a PEEK resin, a polypropylene resin, and a
polyacetal resin.
[0047] The wicks 130 and at least part of the manifold 140 may be
made of the same porous resin. In that case, for example, the wicks
130 and the at least part of the manifold 140 are integrally molded
and the remaining part of the manifold 140 is designed to be a
simple structure, allowing easy manufacturing.
[0048] Even in the case of being applied to a small plate-type
evaporator, the above-described configuration reduces or prevents
generation of air bubbles due to evaporation of the working fluid
before reaching the wicks, which enables stable operation of the
loop heat pipe to maintain the cooling performance.
[0049] The evaporator 110 of FIGS. 3A through 3D includes two wicks
130, however, three or more wicks may be provided. According to the
number of wicks, the number of outlets 144 and the internal
branching structure of the manifold 140 need to be changed.
[0050] In addition, for an evaporator including a single wick, a
liquid supply duct with low heat conductivity, corresponding to a
manifold may be provided. FIG. 4 illustrates an evaporator 210 of a
loop heat pipe including a single wick, according to another
embodiment. In the following description of the evaporator 210, a
detailed description of those features common to the evaporator 110
of FIGS. 3A through 3D is omitted.
[0051] The evaporator 210 includes a first case part 221
communicating with a vapor transport line 255; a second case part
222 communicating with a liquid transport line 250; a single wick
230; and a liquid supply duct 240. The first and the second case
parts 221 and 222 are coupled to each other to form a single
evaporator case for housing the wick 230 and the liquid supply duct
240.
[0052] A hole 223 for housing the wick 230 is provided in the first
case part 221. The second case part 222 may house the liquid supply
duct (manifold) 240. However, in this embodiment, no limitation is
imposed on the structures of the two case parts 221 and 222 to form
the evaporator case as long as the case parts 221 and 222 can be
coupled to each other after the wick 230 and the liquid supply duct
240 are housed therein.
[0053] The wick 230 has an internal opening to serve as a liquid
supply path 231 for supplying working fluid to the wick 230, and
has multiple vapor discharge grooves (simply "grooves") 232 on the
outer periphery. The grooves 232 may run the entire length of the
wick 230 along the flow direction of the working fluid.
[0054] The liquid supply duct 240 insulates the working fluid
before reaching the wick 230 from the evaporator case (221 and
222), and also supplies, to the wick 230, the working fluid having
flowed in from the liquid transport line 250. The liquid supply
duct 240 includes a main body 241 disposed inside the evaporator
case and may also include, on an as-needed basis, an inner pipe 242
extending inside the liquid transport line 250, as illustrated. The
main body 241 of the liquid supply duct 240 may include an outer
wall disposed along the inner wall of the evaporator case and an
opening defined by the outer wall. Alternatively, the liquid supply
duct 240 may include one or more piping systems for distributing
the working fluid to the entire wick 230.
[0055] Materials of the first case part 221, the second case part
222, the wick 230, and the liquid supply duct 240 may be the same
as those of the corresponding components (121, 122, 130, and 140,
respectively) of the evaporator 110. For example, the materials of
the first and the second case parts 221 and 222 include a metal or
an alloy, the materials of the wick 230 include a porous resin, and
the materials of the liquid supply duct 240 include a resin.
[0056] As is the case with the above-described evaporator 110, the
evaporator 210 also reduces or prevents generation of air bubbles
due to evaporation of the working fluid before reaching the wick
230, which enables stable operation of the loop heat pipe to
maintain the cooling performance.
[0057] Note however that the configuration of the evaporator 210
that includes the single wick 230 results in a reduction in the
number of components and simplification of component processing
and/or assembly, which in turn reduces the manufacturing costs. On
the other hand, the configuration of the evaporator 110 that
includes the multiple wicks 130 and the multiple holes 123 for
individually housing the corresponding wicks 130 results in an
increase in the contact area between the wicks 130 and the
evaporator case 120. In addition, the separation wall 124 between
the multiple holes 123 acts as a heat transfer path, and therefore,
heat received from the heat generating element is further uniformly
transferred to the entire evaporator case 120. As a result, the
evaporator 110 has an advantage over the evaporator 210 in the
cooling performance of the evaporator itself and, therefore, in the
cooling performance of the loop heat pipe.
[0058] Next, an evaporator 310 of a loop heat pipe according to
another embodiment is described with reference to FIG. 5. FIG. 5
illustrates a cross-sectional view of the evaporator 310, as in
FIG. 3C. In the following description of the evaporator 310, a
detailed description of those features common to the evaporator 110
of FIGS. 3A through 3D is omitted.
[0059] The evaporator 310 includes a first case part 321
communicated with a vapor transport line 355; a second case part
322 communicated with a liquid transport line 350; at least one
wick 330; and a liquid supply duct 340. The first and the second
case parts 321 and 322 are coupled to each other to form a single
evaporator case 320 for housing the wick 330 and the liquid supply
duct 340.
[0060] The wick 330 has an internal opening to serve as a liquid
supply path 331 for supplying working fluid 360a to the wick 330,
and has multiple vapor discharge grooves (simply "grooves") 332 on
the outer periphery.
[0061] The liquid supply duct 340 insulates, from the evaporator
case 320, the working fluid 360a having flowed in from the liquid
transport line 350, and also supplies the working fluid 360a to the
wick 330. In the case where the evaporator 310 has multiple wicks
330, the liquid supply duct 340 takes the form of a manifold. The
liquid supply duct 340 may include, on an as-needed basis, an inner
pipe (not illustrated) extending inside the liquid transport line
350.
[0062] The first case part 321 and the second case part 322 may be
made of different materials. Materials of the first case part 321
for housing the wick 330 therein preferably include a metal or an
alloy with high heat conductivity, such as oxygen-free copper, a
copper alloy, aluminum, and an aluminum alloy, in order to transfer
heat from the cooling-target heat generating element to the entire
evaporator case 320. Materials of the second case part 322 for
housing the liquid supply duct 340 therein have lower heat
conductivity than the materials of the first case part 321. In
addition, the second case part 322 is preferably made of a metal or
an alloy in terms of sealing reliability of the evaporator case
320. For example, the second case part 322 may be made of a metal
or an alloy with relatively low heat conductivity, for example, an
iron based alloy such as stainless steel, or a titanium alloy.
[0063] It is preferable that the boundary between the first case
part 321 and the second case part 322 of the evaporator case 320 be
substantially aligned to the boundary between the liquid supply
duct 340 and the wick 330. This is in order to achieve heat
transfer to the entire contact area between the wick 330 and the
evaporator case 320 and obtain heat insulation for the working
fluid 360a before reaching the wicks 330.
[0064] The first and the second case parts 321 and 322 are joined
to each other by a method selected from among various methods
capable of ensuring the sealing reliability, such as welding,
brazing, and resin adhesion.
[0065] Materials of the wick 330 and the liquid supply duct 340 may
be the same as those of the corresponding components (130 and 140,
respectively) of the evaporator 110. For example, the materials of
the wick 330 include a porous resin, and the materials of the
liquid supply duct 340 include a resin.
[0066] As is the case with the above-described evaporator 110, the
evaporator 310 also reduces or prevents generation of air bubbles
due to evaporation of the working fluid 360a before reaching the
wick 330, which enables stable operation of the loop heat pipe to
maintain the cooling performance. Note however that because the
second case part 322 is made of materials with lower heat
conductivity than those of the first case part 321, the evaporator
310 enhances the effect of reducing the evaporation of the working
fluid 360a before reaching the wick 330 and, therefore, further
stabilizes the operation of the loop heat pipe. Note that since the
first case part 321 is made of materials with high heat
conductivity, the cooling performance of the loop heat pipe is not
impaired.
[0067] Next, an electronic apparatus 400 according to an embodiment
is described with reference to FIGS. 6A through 6C. FIGS. 6B and 6C
are cross-sectional views taken along the C-C' line and the D-D'
line, respectively, of FIG. 6A, illustrating an example of mounting
an evaporator on a heat generating element of the electronic
apparatus 400. Note that the D-D' cross section of FIG. 6C is
selected as a cross section passing substantially through the
center of one of wicks and including neither a liquid transport
line nor a vapor transport line.
[0068] The electronic apparatus 400 includes an electronic
component (hereinafter, sometimes referred to as the "heat
generating element") 470 which is a heat generating element; and a
loop heat pipe 405 for cooling the electronic component 470.
[0069] The loop heat pipe 405 includes an evaporator 410 which is,
for example, any one of the above-described evaporators 110, 210,
and 310; and a condenser 461 for dissipating heat and condensing
vaporized working fluid generated by the evaporator 410 into liquid
working fluid. The condenser 461 is cooled, for example, by sending
air 462 from an air blower to heat dissipating fins of the
condenser 461 or by placing the condenser 461 into a liquid cooled
to ambient temperature or lower. The vaporized working fluid is
supplied to the condenser 461 from the evaporator 410 via a vapor
transport line 455. The working fluid from the condenser 461 is
supplied to the evaporator 410 via a liquid transport line 450. The
loop heat pipe 405 typically has a reservoir tank 463 in the middle
of the liquid transport line 450, disposed before the evaporator
410. The reservoir tank 463 stores therein working fluid needed at
startup. The working fluid may be, for example, water, ethanol,
R141B, n-Pentane, acetone, butane, or ammonia.
[0070] The heat generating element 470 of the electronic apparatus
400 is, for example, a semiconductor device (such as a CPU), and is
mounted on a wiring substrate 475 (such as a mother board) of the
electronic apparatus 400. The evaporator 410 may be mounted and
fixed to the heat generating element 470 by, for example, screwing
a pressing fastener (not illustrated) onto the wiring substrate
475. A high heat conductive material 480, such as a thermal grease,
may be disposed between the heat generating element 470 and the
evaporator 410. Note that the single evaporator 410 may be used to
cool multiple heat generating elements.
[0071] As illustrated in FIG. 6C, the heat generating element 470
may be disposed offset to the vapor transport line side (right-hand
side in FIG. 6C) relative to the evaporator 410. That is, the
evaporator 410 may be mounted on the heat generating element 470 in
such a manner that the center of the heat generating element 470 is
located on the vapor transport line side away from the center of
the evaporator case 420. Such an offset results in an increase in
the distance between the heat generating element 470 and working
fluid 460a before reaching the wicks, which may in turn prevent the
working fluid 460a from evaporating before reaching the wicks. For
example, if dimensional constraints are not so much an issue, the
evaporator 410 is disposed in such a manner that the manifold 440
and the heat generating element 470 do not overlap one another.
[0072] Next described are several application examples used to cool
a CPU (heat generating element) with a package size of about 30
mm.times.30 mm.
[0073] An evaporator case used is made up of two split parts, a
first case part disposed on the vapor side and a second case part
disposed on the liquid side. The first case part is made of
oxygen-free copper, and the second case part is made of oxygen-free
copper or stainless SUS304. The outer size of the evaporator case
with the first and the second case parts coupled together is about
40 mm.times.40 mm in planar size and about 8 mm in thickness. The
small dimensions allow the evaporator case to be mounted on a CPU
in a computer system with high-density packaging, such as a server
and a personal computer. Two oval holes are provided parallel to
each other on the inner side of the first case part. Each of the
holes is about 18 mm in width (major axis) and about 6 mm in height
(minor axis). A porous resin body (a resin wick) is inserted into
each of the two holes.
[0074] Each of the wicks used is an about 30 mm long porous body
made of PTFE (polytetrafluoroethylene). The average pore diameter
of the resin wicks is about 2 .mu.m and the porosity is about 40%.
Both the thickness and width of the wicks are set to be about 100
to 200 .mu.m larger than the dimensions of the corresponding holes
on the first case part. Because the PTFE porous body is elastic,
the wick having an outer size slightly larger than the wick
insertion hole allows the inner wall of the first case part and the
outer periphery of the wick to be in close contact with each other.
On the inner periphery of each of the resin wicks, an oval hole
about 2 mm in height and about 14 mm in width is provided to serve
as a liquid supply path for receiving working fluid supplied from a
liquid transport line via a manifold. In addition, multiple
grooves, each 1 mm in depth and 1 mm in width, are formed on the
outer periphery of the wicks. Vaporized working fluid is released
from the surface of the grooves and flows through the grooves, and
is then discharged to a vapor transport line.
[0075] A resin manifold made of MC nylon (registered trademark) is
disposed in the evaporator case with no space between the manifold
and the resin wicks. The manifold distributes the working fluid
having flowed in from the liquid transport line to the two resin
wicks without letting the working fluid run out of the manifold.
That is, the working fluid having flowed into the evaporator is
guided into the resin wicks via the resin manifold, without coming
in contact with the metal evaporator case. As a result, heat
transfer from the metal evaporator case to the working fluid is
reduced, which may prevent generation of air bubbles. The resin
manifold has a wall thickness of about 1 mm. Since the heat
conductivity of MC nylon (registered trademark) is 0.2 W/mK, which
is a fraction of several tens to several thousands of that of
copper (380 W/mK) and that of SUS304 (16 W/mK), such a thin MC
nylon material is still effective as a heat insulator.
[0076] For some of the application examples, the heat insulation
resin of the manifold is extended to the liquid transport line side
to thereby form an inner pipe, which is inserted into the liquid
transport line.
[0077] To assemble the evaporator, the resin wicks and the resin
manifold are inserted into the first and the second case parts.
Subsequently, the first and the second case parts are sealed
together to thereby complete the assembly. For the application
examples, the first and the second case parts are sealed by laser
welding.
[0078] After the evaporator is assembled in the above-described
manner, the evaporator, the vapor transport line, a condenser unit
provided with heat dissipating fins, and the liquid transport line
are connected one to the other in a loop by welding, and then
working fluid is enclosed therein. As an example, copper pipes
having an outer diameter (.phi.) about 4 mm and an inner diameter
(.phi.) about 3 mm may be used as the vapor transport line and the
liquid transport line. The entire length of the copper pipes may
be, for example, about 900 mm. The working fluid used is n-Pentane.
The condenser is cooled by sending air from an air blower to the
heat dissipating fins of the condenser unit.
[0079] Subsequently, the evaporator is thermally bonded onto the
CPU via a thermal grease (for example, W4500 produced by COSMO OIL
Co., Ltd.). The evaporator is fixed onto the CPU by screwing a
pressing fastener. At this point, in order to provide a longer
distance between the working fluid having flowed into the
evaporator and the CPU, the evaporator is disposed in such a manner
that the center of the CPU is located on the vapor transport line
side away from the center of the evaporator case.
[0080] An experiment was carried out to examine the operation of a
loop heat pipe configured in the above-described manner. FIG. 7
illustrates application examples (a) to (c) used for the operation
examination.
[0081] An evaporator 510 of application example (a) has a structure
in which PTFE wicks 530 and an MC nylon manifold 540 are disposed
inside a metal case 520 including a first case part 521 and a
second case part 522 both made of oxygen-free copper. An evaporator
510' of application example (b) includes a manifold 540' formed by
integrally molding the MC nylon manifold 540 of application example
(a) and an MC nylon inner pipe 542. The 20 mm long MC nylon pipe
542 (outer diameter (.phi.) 4 mm and inner diameter (.phi.) 3 mm)
is inserted into the end portion of the liquid transport line
(outer diameter (.phi.) 5 mm and inner diameter (.phi.) 4 mm). An
evaporator 510'' of application example (c) includes an evaporator
case 520'' which is identical to the evaporator case 520 of
application example (a) except for a second case part 522'' made of
stainless SUS304 in place of oxygen-free copper. The SUS304 second
case part 522'' corresponds to about 8 mm of the 40-mm-long
evaporator case 520''.
[0082] As for the application examples (a) through (c), the offset
between the CPU 570 and the corresponding evaporators 510, 510',
and 510'' is about 4 mm. The offset is provided in order to
prevent, in application example (c), the 30-mm-long CPU 570 and the
8-mm-long second case part 522'' from overlapping each other.
[0083] By way of comparison, comparison examples (d) and (e) having
no resin manifold (both not illustrated) were prepared. Comparison
examples (d) and (e) have structures identical to those of
application examples (a) and (c), respectively, except for simply
not having the resin manifold 540.
[0084] For these examples (a) through (e), confirmation of
operation capabilities of the loop heat pipe as well as measurement
of heat transfer resistance of the loop heat pipe were carried out
under fixed conditions, using the amount of heat generated by the
CPU as a parameter (FIG. 8). The heat transfer resistance is
calculated as follows. The average temperature of the condenser (an
average value of the inlet and outlet temperatures) is subtracted
from the temperature of the heat receiving surface of the
evaporator to obtain a temperature difference. Then, the
temperature difference is divided by the amount of heat generated
by the CPU to obtain the heat transfer resistance.
[0085] As for comparison examples (d) and (e) having no resin
manifold, the working fluid came to a boil and evaporated around a
part of the evaporator at which the liquid transport line is
connected to the evaporator case and the working fluid flows in. As
a result, the circulation of the working fluid was unstable, and
the loop heat pipe did not operate properly.
[0086] On the other hand, as for application examples (a) through
(c) having a resin manifold, the circulation of the working fluid
was stable, and the loop heat pipe operated properly. FIG. 8
illustrates evaluation results in terms of the heat transfer
resistance of application examples (a) through (c). Based on the
results illustrated in FIG. 8 and the fact that comparison examples
(d) and (e) did not operate properly, it is understood that the
manifold with low heat conductivity contributes largely to
stabilizing operation of the loop heat pipe. It has been also found
that the combination of the manifold and the inner pipe
(application example (b)) and the combination of the manifold and
the second case part with relatively low heat conductivity
(application example (c)) may further improve the cooling
performance of the loop heat pipe. These results indicate that it
is possible to make the evaporator of the loop heat pipe even
smaller and thinner, which expands the possibility of cooling
design for high-heat-generating electronic components mounted on
electronic apparatuses, such as computer systems with high-density
packaging.
[0087] As in application examples (a) through (c) described above,
a structure with the evaporator case made of a metal has good
pressure resistance and prevents leakage of the working fluid in
the long term and, therefore, largely contributes to offering a
reliable cooling system.
[0088] While the embodiments have been described in detail, it
should be understood that the present invention is not limited to
these specific embodiments, and various changes and modification
may be made to the particular examples without departing from the
scope of the claims appended hereto. For example, although the
embodiments above are described based on the plate-type evaporator,
also in other types of evaporators such as a cylindrical
evaporator, the working fluid may be supplied to a single or
multiple wicks via a liquid supply duct with low heat conductivity,
on an as-needed basis.
[0089] The above-described embodiments have the following
advantageous effects. Heat transfer from the evaporator case to the
working fluid having flowed into the evaporator is reduced, which
prevents the working fluid from evaporating before reaching the
wicks. As a result, the capillary force of the wicks is maintained,
allowing stable circulation of the working fluid. This in turn
achieves efficient cooling of the electronic component in the
electronic apparatus.
[0090] All examples and conditional language recited herein are
intended for pedagogical purposes to aid the reader in
understanding the invention and the concepts contributed by the
inventor to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions, nor does the organization of such examples in the
specification relate to a showing of the superiority or inferiority
of the invention. Although the embodiments of the present
disclosure have been described in detail, it should be understood
that various changes, substitutions, and alterations could be made
hereto without departing from the spirit and scope of the
invention.
* * * * *