U.S. patent application number 13/591397 was filed with the patent office on 2012-12-13 for loop heat pipe.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Shigenori AOKI, Hideaki NAGAOKA, Susumu OGATA, Takeshi SHIOGA, Hiroki UCHIDA.
Application Number | 20120312506 13/591397 |
Document ID | / |
Family ID | 44711595 |
Filed Date | 2012-12-13 |
United States Patent
Application |
20120312506 |
Kind Code |
A1 |
UCHIDA; Hiroki ; et
al. |
December 13, 2012 |
LOOP HEAT PIPE
Abstract
A loop heat pipe includes an evaporator to vaporize a working
fluid due to heat supplied from an external heat source; a
condenser to cause the vaporized working fluid to condense; and
connecting lines to connect the evaporator and the condenser in a
loop, wherein the evaporator includes a first space defined by a
set of walls including a contact wall that comes into contact with
the external heat source, a second space provided adjacent to at
least one of the walls other than the contact wall, and a
through-hole formed in a dividing wall separating the first space
and the second space to allow the first space and the second space
to communicate with each other.
Inventors: |
UCHIDA; Hiroki; (Kawasaki,
JP) ; SHIOGA; Takeshi; (Kawasaki, JP) ; AOKI;
Shigenori; (Kawasaki, JP) ; OGATA; Susumu;
(Kawasaki, JP) ; NAGAOKA; Hideaki; (Kawasaki,
JP) |
Assignee: |
FUJITSU LIMITED
Kawasaki-shi
JP
|
Family ID: |
44711595 |
Appl. No.: |
13/591397 |
Filed: |
August 22, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2010/066329 |
Sep 21, 2010 |
|
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|
13591397 |
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Current U.S.
Class: |
165/104.21 |
Current CPC
Class: |
F28F 2265/12 20130101;
H01L 2924/0002 20130101; F28D 15/046 20130101; H01L 2924/0002
20130101; F28D 15/0266 20130101; H01L 23/427 20130101; F28D
2021/0028 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
165/104.21 |
International
Class: |
F28D 15/00 20060101
F28D015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2010 |
JP |
2010-075443 |
Claims
1. A loop heat pipe comprising: an evaporator to vaporize a working
fluid due to heat supplied from an external heat source; a
condenser to cause the vaporized working fluid to condense; and
connecting lines to connect the evaporator and the condenser in a
loop, wherein the evaporator includes a first space having set of
walls including a contact wall that comes into contact with the
external heat source, a second space provided adjacent to at least
one of the walls other than the contact wall, and a through-hole
formed in a dividing wall separating the first space and the second
space to allow the first space and the second space to communicate
with each other.
2. The loop heat pipe according to claim 1, wherein the second
space is provided adjacent to a wall opposite to the contact
wall.
3. The loop heat pipe according to claim 2, wherein a vapor
pressure of the working fluid is higher than an atmospheric
pressure, and wherein a thickness of an outer wall separating the
second space and the atmosphere is less than a thickness of the
dividing wall between the first space and the second space.
4. The loop heat pipe according to claim 2, wherein a vapor
pressure of the working fluid is higher than an atmospheric
pressure, and wherein the dividing wall between the first space and
the second space is swelled toward the first space.
5. The loop heat pipe according to claim 3, wherein the working
fluid is selected from a group of pentane, butane and ammonia.
6. The loop heat pipe according to claim 4, wherein the working
fluid is selected from a group of pentane, butane and ammonia.
7. The loop heat pipe according to claim 1, wherein a porous
material is provided along an inner wall of the first space, and a
flow path is formed in the porous material through which the
working fluid supplied from one of the connecting lines passes.
8. The loop heat pipe according to claim 1, wherein the evaporator
is made of a material with a thermal conductivity higher than that
of stainless.
9. A loop heat pipe comprising: an evaporator to vaporize a working
fluid due to heat supplied from an external heat source; a
condenser to cause the vaporized working fluid to condense; and
connecting lines to connect the evaporator and the condenser in a
loop, wherein the evaporator includes a first space having a first
set of walls including a contact wall that comes into contact with
the external heat source, and a second space provided adjacent to
at least one of said walls other than the contact wall and defined
by a second set of walls, the second space being filled with a
second fluid that has a saturated vapor pressure higher than that
of the working fluid at a same temperature.
10. The loop heat pipe according to claim 9, wherein at least a
part of the second fluid is in a liquid phase when the loop heat
pipe is not activated.
11. The loop heat pipe according to claim 9, wherein a thickness of
a dividing wall separating the first space and the second space is
less than a thickness of an outer wall separating the second space
from an atmosphere.
12. The loop heat pipe according to claim 9, wherein a porous
material is provided along an inner wall of the first space, and a
flow path is formed in the porous material through which the
working fluid supplied from one of the connecting lines passes.
13. The loop heat pipe according to claim 9, wherein the evaporator
is made of a material with a thermal conductivity higher than that
of stainless.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of
International Application PCT/JP2010/066329 filed on Sep. 21, 2010
designating the United States, which International application
further claims the benefit of the earlier filing date of Japanese
Patent Application No. 2010-075443 filed in Japan on Mar. 29, 2010,
the entire contents of the International Application and the
priority Japanese Application being incorporated herein by
reference.
FIELD
[0002] The embodiments discussed herein relate to a loop heat pipe
used to cool heat generating components such as electronic
devices.
BACKGROUND
[0003] A loop heat pipe is known as a device for cooling various
types of heat generating components, in which an evaporator and a
condenser are connected in a loop via a vapor transport line and a
liquid transport line. Liquid-phase working fluid evaporates in the
evaporator due to heat supplied from an external heat source, and
the vaporized working fluid is transported via the vapor transport
line to the condenser, in which the vapor condenses back to a
liquid by releasing heat. See, for example, Japanese Laid-open
Patent Publication No. 2004-218887.
[0004] FIG. 1A through FIG. 1C illustrate a conventional evaporator
1000, where FIG. 1A is a cross-sectional view of the evaporator
along the direction of flow of the working fluid, and FIG. 1B and
FIG. 1C are cross-sectional views taken along the A-A' line of FIG.
1A. A heat generating component 1010 such as an electronic device
1010 is generally shaped flat, and accordingly, the heat-receiving
face 1002 of the evaporator 1000 of a loop heat pipe is made flat
so as to be kept in stable contact with the heat generating
component 1010. To improve the cooling ability of the loop heat
pipe, it is desired to increase the internal volume of the
evaporator 1000 as much as possible. On the other hand, there is
another demand for shaping the evaporator to be as compact as
possible. To satisfy both demands, a flat plate loop heat pipe is
used.
[0005] In order to efficiently remove heat from the heat generating
component 1010 during operation, it is desired to cause the working
fluid 1006 supplied through the liquid transport line 1003 to the
evaporator 1000 to vaporize in an efficient manner. In this regard,
a wick 1007 is provided in an evaporator case 1001 so as to be in
close contact with the inner wall of the evaporator case 1001. Heat
is transferred promptly from the evaporator case 1001 to the wick
1007 and it allows the working fluid 1006 penetrating the wick 1007
to vaporize quickly. The vaporized working fluid is guided through
grooves 1005 toward the vapor transport line 1004. However, as the
heat is transferred to the evaporator 1000, the temperature of the
internal working fluid rises and the adhesion between the
evaporator case 1001 and the wick 1007 is degraded. This state is
illustrated in FIG. 1C.
[0006] In FIG. 1C, when the saturated vapor pressure of the working
fluid exceeds the atmospheric pressure at the operating temperature
of the loop heat pipe, the internal surface of the evaporator case
1001 is pressed outward by the internal pressure of the working
fluid. If the loop heat pipe is placed at ordinary temperature and
pressure, and if the boiling point of the working fluid (such as
pentane, R141B, butane, or ammonia) used in the loop heat pipe is
above room temperature under standard atmospheric pressure, then
the flat evaporator case 1001 deforms outward. If the evaporator
has a cylindrical shape, the internal pressure is equally
distributed in the circumferential direction and expansion of the
evaporation case is less. In contrast, with the flat plate loop
heat pipe, the internal pressure is applied toward the top face
with a large area size, which causes the top wall to swell as
illustrated in FIG. 1C. Especially when a flat plate evaporator is
employed for the purpose of reducing the size and the weight of
electric equipment, the evaporator body is made as thin as
possible. In this circumstance, it is difficult to guarantee a
thickness of the evaporator case 1001 enough to provide rigidity to
resist the internal pressure. When the evaporator case 1001 swells
due to the increasing internal pressure, adhesion between the
evaporator case 1001 and the inner wick 1007 is degraded. This
issue becomes more conspicuous at the top face of the evaporator
case 1001 than the bottom face secured to the heat generating
component 1010 (such as a CPU). Besides, a gap 1020 is produced
between the evaporator case 1001 and the wick 1007 at a higher
temperature. In this state, sufficient heat cannot be transferred
from the evaporator case 1001 to the wick 1007, and the working
fluid is prevented from vaporizing from the surface of the wick
1007. Consequently, the cooling capacity is lowered.
[0007] It is desired for the loop heat pipe to maintain thermal
contact between the evaporator case 1001 and the wick 1007 during
operation to ensure the thermal performance even if the temperature
and the pressure of the working fluid are increasing in the
evaporator.
SUMMARY
[0008] According to an aspect of the embodiments, a loop heat pipe
includes:
[0009] an evaporator to vaporize a working fluid due to heat
supplied from an external heat source;
[0010] a condenser to cause the vaporized working fluid to
condense; and
[0011] connecting lines to connect the evaporator and the condenser
in a loop,
[0012] wherein the evaporator includes
[0013] a first space having a set of walls including a contact wall
that comes into contact with the external heat source,
[0014] a second space provided adjacent to at least one of the
walls other than the contact wall, and
[0015] a through-hole formed in a dividing wall separating the
first space and the second space to allow the first space and the
second space to communicate with each other.
[0016] The object and advantages of the invention 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 to the invention as
claimed.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1A is a cross-sectional view of a conventional flat
plate evaporator used in a loop heat pipe along a direction of flow
of working fluid;
[0019] FIG. 1B is a cross-sectional view of the evaporator taken
along the A-A' line of FIG. 1A, illustrating the non-operating
state;
[0020] FIG. 1C is a cross-sectional view of the evaporator taken
along the A-A' line of FIG. 1A, illustrating an issue arising in
the conventional flat plate evaporator under application of
heat;
[0021] FIG. 2 is a general view of a loop heat pipe to which the
present invention is applied;
[0022] FIG. 3A is a cross-sectional view of an evaporator according
to the first embodiment of the invention, along a direction of flow
of working fluid;
[0023] FIG. 3B is a cross-sectional view of the evaporator taken
along the A-A' line of FIG. 3A;
[0024] FIG. 4 is a diagram illustrating saturated vapor pressure
curves of several working fluids as a function of temperature;
[0025] FIG. 5A is a schematic cross-sectional diagram of the
evaporator in the non-operating state, for explaining the advantage
of the first embodiment;
[0026] FIG. 5B is a schematic cross-sectional diagram of the
evaporator under application of heat, for explaining the advantage
of the first embodiment;
[0027] FIG. 6A illustrates an example of mounting the evaporator
according to the first embodiment;
[0028] FIG. 6B is a perspective view of the mounted evaporator of
FIG. 6A;
[0029] FIG. 7 is a graph illustrating an advantage of the loop heat
pipe using the evaporator according to the first embodiment;
[0030] FIG. 8A illustrates a first modification of the evaporator
of the first embodiment, which evaporator is in the non-operating
state;
[0031] FIG. 8B illustrates the evaporator of the first modification
illustrated in FIG. 8A, which is in the operating (heat absorbing)
state;
[0032] FIG. 9A illustrates a second modification of the evaporator
of the first embodiment, which evaporator is in the non-operating
state;
[0033] FIG. 9B illustrates the evaporator of the second
modification illustrated in FIG. 9A, which is in the operating
(heat absorbing state);
[0034] FIG. 10A is a cross-sectional view of an evaporator
according to the second embodiment along the direction of flow of
the working fluid;
[0035] FIG. 10B is a cross-sectional view taken along the A-A' line
of FIG. 10A;
[0036] FIG. 11A is a schematic diagram for explaining an advantage
of the evaporator of the second embodiment, which evaporator is in
the non-operating state;
[0037] FIG. 11B is a schematic diagram for explaining the advantage
of the evaporator of the second embodiment, which is in the
operating (heat absorbing) state;
[0038] FIG. 12A illustrates a modification of the evaporator of the
second embodiment, which evaporator is in the non-operating
state;
[0039] FIG. 12B illustrates the evaporator of the modification,
which is in the operating (heat absorbing) state;
[0040] FIG. 13A illustrates an example of mounting the evaporator
according to the second embodiment;
[0041] FIG. 13B is a perspective view of the mounted evaporator of
FIG. 13A; and
[0042] FIG. 14 is a graph illustrating an advantage of the loop
heat pipe using the evaporator of the second embodiment.
DESCRIPTION OF EMBODIMENTS
First Embodiment
[0043] FIG. 2 illustrates an overall structure of a loop heat pipe
1 to which the present invention is applied. The loop heat pipe 1
includes an evaporator 10 which vaporizes a liquid-phase working
fluid due to heat supplied from a heat generating component (e.g.,
an electronic component), and a condenser 11 that causes a
vapor-state working fluid to condense by removing the heat. The
evaporator 10 and the condenser 11 are connected in a loop by a
vapor line 14 for transporting the vaporized working fluid from the
evaporator 10 to the condenser 11 and a liquid line 13 for
transporting the liquid-state working fluid from the condenser 11
to the evaporator 10. The liquid line 13 and the vapor line 14 form
the connecting lines. In the example of FIG. 2, a blast fan 12 is
provided near the condenser 11 to enhance removal of heat.
[0044] The working fluid in the vapor line 14 or the liquid line 13
is not necessarily 100% vapor or 100% liquid, and it is in a vapor
phase and a liquid phase mixed with each other. During operation of
the loop heat pipe 1, for the most part the working fluid inside
the vapor line 14 is in the vapor phase, while for the most the
working fluid inside the liquid line 13 is in the liquid phase. In
this regard, the connecting lines are named the "vapor line" and
the "liquid line" for the sake of convenience.
[0045] FIG. 3A and FIG. 3B illustrate an evaporator 10 according to
the first embodiment of the invention, where FIG. 3A is a
cross-sectional view of the evaporator 10 along a direction of flow
of the working fluid and FIG. 3B is a cross-sectional view taken
along the A-A' line in FIG. 3A. In the first embodiment, the
evaporator 10 has a vaporization chamber (a first space) 40A having
a liquid supply path 46, and a pressure adjusting chamber (a second
space) 40B for adjusting the pressure in the vaporization chamber
40A. A pressure adjusting hole 55 is formed in a dividing wall 51
that separates between the vaporization chamber 40A and the
pressure adjusting chamber 40B to allow the vaporization chamber
40A and the pressure adjusting chamber 40B to communicate with each
other.
[0046] In the example illustrated in FIG. 3A and FIG. 3B, the
bottom face of an evaporator case 40 is a heat-receiving surface
42. The evaporator 10 is mounted on a heat generating component
such that the heat receiving surface 42 comes into contact with the
heat generating component such as an electronic component (see FIG.
6A) to receive heat from the electronic component. A wick (porous
material) 47 is provided in the vaporization chamber 40A so as to
be kept in mechanical and thermal contact with the inner wall of
the vaporization chamber 40A. The (liquid-state) working fluid is
supplied through the liquid line 13 into the vaporization chamber
40A and penetrates into the wick 47. The liquid absorbed in the
wick 47 is heated by heat transferred from the evaporator case 40
to the wick 47. The inner space of the evaporator 10 is maintained
at a saturated vapor pressure of the working fluid. When the
temperature of the working fluid has reached the boiling point
under the saturated vapor pressure, the working fluid is vaporized.
During vaporization, the working fluid takes in latent heat. The
vapor that has taken in the latent heat passes through grooves
(vapor discharge grooves) 45 and flows into the vapor line 14.
Simultaneously, a portion of the vapor passes through the pressure
adjusting hole 55 and flows into the pressure adjusting chamber
40B. Consequently, the pressures in the vaporization chamber 40A
and the pressure adjusting chamber 40B become almost the same. The
saturated vapor pressure within the utilized temperature range of
the working fluid 49 is at or above the atmospheric pressure in the
environment in which the loop heat pipe 1 is used.
[0047] Explanation is made of the exemplified structure of the
evaporator 10 illustrated in FIG. 3A and FIG. 3B. The evaporator
case 40 is a flat plate case with an entire height of 18 mm, a
width of 60 mm and a length of 70 mm. A double chamber structure is
employed in which the pressure adjusting chamber 40B is provided on
the top of the vaporization chamber 40A. The pressure adjusting
chamber 40B has a space with the dimensions 66 mm length.times.56
mm width.times.1 mm height. The pressure adjusting chamber 40B and
the vaporization chamber 40A are separated from each other by a
dividing wall 51 with a thickness of 2 mm. A pressure adjusting
hole 55 with a diameter of 1 mm is formed in the dividing wall 55
so as to allow the pressure adjusting chamber 40B to communicate
with the vapor side of the vaporization chamber 40A. The inner
dimensions of the vaporization chamber 40A are 66 mm
length.times.56 mm width.times.11 mm height. The thickness of the
walls defining the vaporization chamber 40A is 2 mm on the
whole.
[0048] The material of the vaporization case 40 and the dividing
wall 51 is oxygen-free copper in the first embodiment. Conventional
flat evaporators are often made of a rigid material such as
stainless so as to be tolerant of high internal pressure. In
contrast, the evaporator of the first embodiment does not
necessarily use a rigid material, as will be described below.
Rather, a material with a higher thermal conductivity than
stainless is used such that the temperature distribution of the
evaporator case 40 becomes uniform. For example, aluminum alloy can
be used for reducing weight.
[0049] The wick 47 arranged inside the vaporization chamber 40A is
made of sintered nickel.
[0050] The porous diameter is about 10 .mu.m, and the porosity is
about 50%. The outer dimensions of the wick 47 are 50 mm
length.times.56 mm width.times.11 mm height. Especially, the height
of the wick 47 is set precisely such that the wick 47 is held in
the vaporization chamber 40A in close contact with the inner wall
thereof. Fifteen grooves (vapor passages) 45 with a width of 1 mm
and a depth of 2 mm are formed at a pitch of 3 mm in the top face
and the bottom face (which come into contact with the ceiling and
the bottom of the vaporization chamber 40A, respectively). In the
center of the wick 47 is formed a liquid supply path 46 with a
height of 3 mm, a width of 40 mm and a length of 40 mm to take the
working fluid 49 supplied from the liquid line 13 into the wick
47.
[0051] The vapor line 14 and the liquid line 13 connecting the
evaporator 10 and the condenser 11 are copper pipes with an outer
diameter of 6 mm, an inner diameter of 5 mm, and a length of 300
mm. The condenser 11 is also a copper pipe, like the vapor line 14
and the liquid line 13, with an outer diameter of 6 mm, an inner
diameter of 5 mm and a length of 400 mm. Radiation fins are
thermally connected to the circumference of the pipe, and are
cooled by the blast fan 12 (see FIG. 2).
[0052] Although in the first embodiment n-pentane is used as the
working fluid 49, other fluids with high saturation pressures
including butane and ammonia can be used.
[0053] FIG. 4 is a graph of saturation pressure curves of various
fluids. When n-pentane is used as the working fluid 49, the boiling
point at atmospheric pressure is about 36.degree. C. During
operation of the loop heat pipe 1, the temperature of the working
fluid 49 becomes near 50-70.degree. C. If butane or pentane is used
as the working fluid 49, the saturation pressure of the working
fluid exceeds the atmospheric pressure in the temperature range of
50-70.degree. C. With the conventional evaporator illustrated in
FIG. 1A, the top wall of the case 1001 swells due to the internal
pressure of the working fluid as illustrated in FIG. 10. The
contact between the evaporation case 1000 and the wick 1007 is
degraded and the cooling performance lowers. In contrast, with the
evaporator 10 of the first embodiment with the double chamber
structure, a pressure adjusting chamber 40B is provided on the top
of the vaporization chamber 40A, and a pressure adjusting hole 55
is formed in the dividing wall 51 to allow the vapor coming from
the surface of the wick 47 to flow into the pressure adjusting
chamber 40B. The internal pressures in the vaporization chamber 40A
and the pressure adjusting chamber 40B become equal.
[0054] FIG. 5A and FIG. 5B are diagrams to explain an advantage of
the first embodiment. When butane is used as the working fluid 49,
the vapor pressure inside the vaporization chamber 40A increases as
the working fluid absorbed in the wick 47 is heated by heat
transferred from the electronic component 20. Because the vaporized
working fluid flows into the pressure adjusting chamber 40B through
the pressure adjusting hole 55, the vapor pressure applied to the
dividing wall 51 from the vaporization chamber 40A becomes equal to
the vapor pressure applied to the dividing wall 51 from the
pressure adjusting chamber 40B. Accordingly, the dividing wall 51
with a surface which is in contact with the wick 47 is prevented
from deforming due to the internal pressure. On the other hand, the
top wall 53 of the evaporator case 40 (which is also the top wall
of the pressure adjusting chamber 40B in the first embodiment)
expands and bends outward because the saturated vapor pressure of
butane is higher than the atmospheric pressure. Even if the
internal pressure in the vaporization chamber 40A becomes high due
to the increasing vapor pressure of the working fluid 49, the
thermal contact between the vaporization chamber 40A and the wick
47 can be maintained satisfactorily because of no deformation in
the dividing wall 51.
[0055] FIG. 6A and FIG. 6B illustrate a structure in which the
evaporator 10 of the first embodiment is mounted over a heat
generating component. The evaporator 10 of the loop heat pipe 1 is
placed, via thermal grease 21, over the electronic component 20 on
a printed circuit board 30 and secured to the printed circuit board
30 using attachment screws 31.
[0056] The amount (rate) of heat absorption of the evaporator 10 is
about 60 W in the first embodiment. At this time, the condenser 11
(not shown in FIG. 6A and FIG. 6B) is cooled at the room
temperature (25.degree. C.) using a blast fan 12 (90 mm diameter,
12-volt driving voltage).
[0057] FIG. 7 is a diagram illustrating the cooling ability of the
loop heat pipe of the first embodiment, with a comparison example a
loop heat pipe using a conventional evaporator illustrated in FIG.
1. The horizontal axis of the graph represents amount of heat
generated by a heater (i.e., the electronic component), and the
vertical axis represents thermal resistance [.degree. C./W] between
the evaporator 10 and the condenser 11. The thermal resistance
indicates a difference between the temperature of the
heat-receiving surface 42 of the evaporator 10 and the average
temperature of the condense 11 per watt (divided by the quantity of
heat generated by the electronic component 20). The smaller the
thermal resistance, that is, the smaller the temperature difference
between the heat-receiving surface 42 and the condenser 11, the
greater is the heat transfer rate from the evaporator 10 to the
condenser 11. Consequently, the cooling ability is improved.
[0058] With the conventional loop heat pipe, the internal pressure
in the evaporator increases as the quantity of heat increases, and
the gap between the evaporator case 1001 and the wick 1007 spreads
as illustrated in FIG. 1C. In this situation, the thermal
resistance increases, and the cooling ability is impaired. In
contrast, the loop heat pipe 1 of the first embodiment can maintain
the cooling ability at the satisfactory level (by keeping the
thermal resistance low). This is because the thermal contact
between the dividing wall 51 of the evaporator case 40 and the wick
47 is maintained in the satisfactory state even if the temperature
of the evaporator rises along with the increase in the quantity of
heat generated from the electronic component.
[0059] FIG. 8A and FIG. 8B illustrate a first modification of the
evaporator of the first embodiment. In this modification, an outer
wall 63 (e.g., the top wall 63) of the evaporator that defines the
pressure adjusting chamber 60B is made thinner than the dividing
wall 61 separating the vaporization chamber 60A and the pressure
adjusting chamber 60B. For example, the thickness of the dividing
wall 61 is 2 mm, and the thickness of the top wall 63 of the
evaporator case 40 is 1 mm. In the non-operating state, there is no
deformation of the pressure adjusting chamber 60B occurring as
illustrated in FIG. 8A. In operation (during heat absorption), the
pressure adjusting chamber 60B expands as illustrated in FIG. 8B.
Because the outer wall (top wall) 63 is made thinner than the
internal dividing wall 61, the outer wall 63 swells outward (toward
the atmosphere) due to the increased pressure of the vapor flowing
into the pressure adjusting chamber 60B through the pressure
adjusting hole 65, while little deformation occurs in the internal
dividing wall 61. This arrangement is advantageous to maintain the
adhesion between the internal dividing wall 61 and the wick 47
constant. Although in FIG. 8A and FIG. 8B, the thickness of the
outer wall 63 is set half the thickness of the dividing wall 61,
the invention is not limited to this example. The outer wall 63 is
designed with an appropriate thickness as long as the outer wall 63
is deformable without affecting the shape of the dividing wall 61.
The thickness of the outer wall 63 can be set to one fifth to two
third of the thickness of the dividing wall 61, depending on the
type of the working fluid used in the loop heat pipe 1.
[0060] FIG. 9A and FIG. 9B illustrate a second modification of the
evaporator of the first embodiment. In the second modification, the
thicknesses of the top wall 73 and the internal dividing wall 71 of
the evaporator 70 are similar to each other, but the dividing wall
71 is slightly curved toward the vaporization chamber 70A in which
the wick 47 is provided. In the non-operating state, there is no
deformation in the pressure adjusting chamber 70B as illustrated in
FIG. 9A. In operation (during heat absorption), the pressure
adjusting chamber 70B expands as illustrated in FIG. 9B. The outer
wall (top wall) 73 of the evaporator case 70 swells outward due to
the vapor flowing into the pressure adjusting chamber 70B through
the pressure adjusting hole 75. Simultaneously, the internal
dividing wall 71 also deforms toward the wick 47 so as to increase
the curvature. A compressive force acts on the dividing wall 71 so
as to press it against the wick 47. Consequently, adhesion between
the dividing wall 71 and the wick 47 is enhanced and the cooling
ability of the loop heat pipe 1 is improved.
[0061] According to the arrangements of the first embodiment, the
cooling ability of the loop heat pipe 1 is improved and settled
with a simple structure, and stable operation of electronic
equipment is realized.
Second Embodiment
[0062] FIG. 10A and FIG. 10B illustrate an evaporator 80 according
to the second embodiment of the invention, where FIG. 10A is a
cross-sectional view along a direction of flow of the working fluid
and FIG. 10B is a cross-sectional view taken along the A-A' line of
FIG. 10B. In the second embodiment, the evaporator 80 has a
vaporization chamber (first space) 90A with a liquid supply path 86
and a second fluid chamber (second space) 90B with an airtight
structure. The second fluid chamber 90B is filled with a second
fluid 100 that has a saturated vapor pressure higher than that of
the working fluid supplied to the vaporization chamber 90A at the
same temperature. At least a portion of the second fluid 100 is in
a liquid phase 100b. Referring to the graph in FIG. 4, when ethanol
is used as the working fluid, the second fluid can be selected from
the group of ethanol, pentane, butane, ammonia and so on. The
selected fluid is introduced in the second fluid chamber 90B with a
portion thereof in a liquid phase. If the working fluid is pentane,
then the second fluid is selected from the group of pentane,
butane, ammonia and so on, and introduced in the second fluid
chamber 90B with a portion thereof in a liquid phase.
[0063] In the example illustrated in FIG. 10A and FIG. 10B, the
bottom face of the evaporator case 90 is the heat receiving face
82. The evaporator 80 is mounted over a heat generating component
20 such that the heat receiving face 82 comes into contact with the
heat generating component 20 (such as an electronic component 20)
to receive heat from the electronic component 20 (see FIG. 11A and
FIG. 11B). A wick (a porous material) 47 is provided in the
vaporization chamber 90A so as to be mechanically and thermally in
contact with the inner surface of the vaporization chamber 90A. The
working fluid 89 supplied via the liquid line 83 to the
vaporization chamber 90A penetrates in the wick 47 and is vaporized
by heat transferred from the evaporation case 40 to the wick 47.
The vaporized fluid flows through the grooves 45 formed in the wick
47 into the vapor line 84. A portion of the second fluid
encapsulated in the second fluid chamber 90B is also vaporized by
heat in the evaporation case 90 during operation of the electronic
component. In this state, a vapor phase 100a and a liquid phase
100b coexist.
[0064] Explanation is made of the exemplified structure of the
evaporator 80 illustrated in FIG. 10A and FIG. 10B. The evaporator
case 90 is a flat plate case with an entire height of 18 mm, a
width of 60 mm and a length of 70 mm. A double chamber structure is
employed in which the second fluid chamber 90B is provided on the
top of the vaporization chamber 90A. The second fluid chamber 90B
is an airtight space with the dimensions 66 mm length.times.56 mm
width.times.1 mm height. The second fluid chamber 90B and the
vaporization chamber 90A are separated from each other by a
dividing wall 91 with a thickness of 2 mm. The inner dimensions of
the vaporization chamber 90A are 66 mm length.times.56 mm
width.times.11 mm height. The thickness of the walls of the
vaporization chamber 90A is 2 mm on the whole.
[0065] The material of the vaporization case 90 and the dividing
wall 91 is oxygen-free copper in the second embodiment.
Conventional flat evaporators are often made of a rigid material
such as stainless so as to be tolerant of the high internal
pressure. In contrast, the evaporator of the second embodiment does
not necessarily use a rigid material, as will be described below.
Rather, a material with a higher thermal conductivity than
stainless is used such that the temperature distribution of the
evaporator case 90 becomes uniform. For example, aluminum alloy can
be used for reducing weight.
[0066] The wick 47 arranged inside the vaporization chamber 90A is
made of sintered nickel. The porous diameter is about 10 .mu.m, and
the porosity is about 50%. The outer dimensions of the wick 47 are
50 mm length.times.56 mm width.times.11 mm height. Especially, the
height of the wick 47 is set precisely such that the wick 47 is
held in the vaporization chamber 90A in close contact with the
inner wall thereof. Fifteen grooves (vapor passages) 45 with a
width of 1 mm and a depth of 2 mm are formed at a pitch of 3 mm in
the top face and the bottom face (which come into contact with the
ceiling and the bottom of the vaporization chamber 90A,
respectively). In the center of the wick 47 is formed a liquid
supply path 86 with a height of 3 mm, a width of 40 mm and a length
of 40 mm to take the working fluid 89 supplied from the liquid line
13 into the wick 47.
[0067] The vapor line 84 and the liquid line 83 connecting the
evaporator 80 and the condenser 11 (see FIG. 2) are copper pipes
with an outer diameter of 6 mm, an inner diameter of 5 mm, and a
length of 300 mm. The condenser 11 is also a copper pipe, like the
vapor line 84 and the liquid line 83, with an outer diameter of 6
mm, an inner diameter of 5 mm and a length of 400 mm. Radiation
fins are thermally connected to the circumference of the pipe, and
are cooled by the blast fan 12.
[0068] In the second embodiment, n-pentane is used as the working
fluid 89. The boiling point of pentane under the atmospheric
pressure is 36.degree. C. The temperature of the working fluid 89
reaches around 50-70.degree. C. during the operation of the loop
heat pipe 1, and accordingly, the vapor pressure of pentane becomes
at or above the atmospheric pressure. The second fluid chamber 90B
contains 1 cc of butane serving as the second fluid in advance.
Butane is introduced in the second fluid chamber 90B by evacuating
the air from the second fluid chamber 90B and inletting only
butane, using the same method as introducing the working fluid in
the loop heat pipe 1. The vapor phase become dominant in the second
fluid during operation of the heat generating component (i.e., the
electronic component) 20, and at least a portion of the second
fluid is in a liquid phase throughout the operating state and
non-operating state.
[0069] FIG. 11A and FIG. 11B are schematic diagrams for explaining
an advantage of the second embodiment. When butane is used as the
second fluid 100, the saturated vapor pressure of butane is higher
than that of n-pentane used as the working fluid 89 at the same
temperature. There is no deformation in the second fluid chamber
90B in the non-operating state. Assuming that the temperatures on
the working fluid side (in the vaporization chamber 90A) and the
second fluid side (in the second fluid chamber 90B) of the
evaporator case 90 are substantially the same during operation,
then the dividing wall 91 is pressed toward the lower pressure
side, that is, toward the vaporization chamber 90A in which the
wick 47 is provided. As the temperature rises, the pressure
difference between the working fluid 90 and the second fluid 100
becomes large. As the temperature of the evaporator case 90 rises
due to heat transferred from the heat generating component 20, the
dividing wall 91 is brought into closer contact with the wick 47.
In this state, the top wall 93 of the second fluid chamber 90B
swells outward because the pressure difference between the second
fluid chamber 90B and the atmospheric pressure is greater than the
pressure difference between the vaporization chamber 90A and the
second fluid chamber 90B. At this time, the dividing wall 91 also
tends to swell toward the vaporization chamber 90A, and the
adhesion between the dividing wall 91 and the wick 47 is
enhanced.
[0070] FIG. 12A and FIG. 12B illustrate a modification of the
evaporator 80 of the second embodiment. In the above-described
example, the thickness of the dividing wall 91 is 2 mm, which is
the same as the thickness of the evaporator case 90. In the
modification, the thickness of the dividing wall 91a separating the
vaporization chamber 90A and the second fluid chamber 90B of an
evaporator 80a is made less than the wall thickness of the
evaporator case 90, and it is, for example, 1 mm. With this
arrangement, there is no deformation in the second fluid chamber
90B in the non-operating state (FIG. 12A), and the dividing wall
91a is more deformable during operation or heat absorption.
Consequently, the dividing wall 91a and the wick 47 come into tight
contact with each other under higher compressive force (FIG.
12B).
[0071] FIG. 13A and FIG. 13B are schematic diagram illustrating a
structure in which the evaporator 80 of the second embodiment is
mounted over a heat generating component. The evaporator 80 of the
loop heat pipe 1 is placed, via thermal grease 21, over the
electronic component 20 on a printed circuit board 30 and secured
to the printed circuit board 30 using attachment screws 31. The
amount (rate) of heat absorption of the evaporator 80 is about 60 W
in the second embodiment. At this time, the condenser 11 (not shown
in FIG. 13A and FIG. 13B) is cooled at room temperature (25.degree.
C.) using a blast fan 12 (90 mm diameter, 12-volt driving voltage).
Heat transferred from the electronic component 20 to the evaporator
case 90 vaporizes the working fluid 89 penetrating in the wick 47.
Simultaneously, the second fluid with a saturated vapor pressure
higher than that of the working fluid 89 and encapsulated in the
second fluid chamber 90B also vaporizes. The dividing wall 91 is
pressed against the wick 47 in the vaporization chamber 90A.
[0072] FIG. 14 is a diagram illustrating the cooling ability of the
loop heat pipe 1 of the second embodiment, with a comparison
example a loop heat pipe in which a conventional wick structure
illustrated in FIG. 1A through FIG. 1C is incorporated. The
horizontal axis of the graph represents the amount (rate) of heat
generated by a heater (i.e., the electronic component), and the
vertical axis represents thermal resistance [.degree. C./W] which
is determined by dividing the difference between the average
temperatures of the evaporator 10 and the condenser 11 by the
quantity of heat generated by the electronic component 20. The
smaller the thermal resistance, that is, the smaller the
temperature difference between the heat-receiving surface 82 and
the condenser 11, the greater is the heat transfer rate from the
evaporator 80 to the condenser 11. Consequently, the cooling
ability is improved.
[0073] With the conventional loop heat pipe, the temperature of the
evaporator rises as the quantity of heat increases, and the gap
between the evaporator case 1001 and the wick 1007 spreads as
illustrated in FIG. 1C. In this situation, the thermal resistance
increases, and the cooling ability is impaired. In contrast, the
loop heat pipe 1 of the second embodiment can maintain the cooling
ability in the satisfactory state (by maintaining the thermal
resistance low). This is because the thermal contact between the
dividing wall 91 (or 91a) of the evaporator case 90 and the wick 47
is maintained in the satisfactory state even if the temperature of
the evaporator rises along with the increase in the quantity of
heat transferred from the electronic component.
[0074] To support the above-described advantage, the deformation of
a copper (Cu) evaporator case 90 of 56 mm width is calculated when
using pentane as the working fluid. With the conventional structure
illustrated in FIGS. 1A-1C, the difference between the atmospheric
pressure and the internal pressure of the evaporator case is 0.2
MPa at the LHP operating temperature (near 70.degree. C.) as
illustrated in FIG. 4. Under this condition, the evaporator case
expands and deforms outward by 95 .mu.m. This state impairs thermal
contact between the evaporator case and the wick and the thermal
resistance increases. In contrast, when butane is introduced in the
second fluid chamber 90B illustrated in FIG. 11, the internal
pressure in the vaporization chamber 90A is lower than the internal
pressure in the second fluid chamber 90B by 0.5 MPa. If the wick 47
is not arranged in the vaporization chamber 90A, the dividing wall
91 of the evaporator case 90 will swells toward the vaporization
chamber 90A by 140 .mu.m. However, because the wick 47 is provided
in the vaporization chamber 90A, the dividing wall 91 is pressed
against the wick 47 and tight contact is produced between the
dividing wall 91 and the wick 47.
[0075] According to the comparison between the graphs of FIG. 14
and FIG. 7, it is understood that the evaporator configuration of
the second embodiment can further improve the cooling efficiency,
compared to the first embodiment.
[0076] In the first and second embodiments, the second space is
provided only on the top of the evaporator case, opposite to the
heat-receiving face, to define a double chamber structure because
the top face has a large area size of a thermally conductive
surface. However, the second space may be provided so as to cover
at least one of the side walls of the vaporization chamber (first
space). If the second space is provided so as to cover the top face
and a pair of side faces of the vaporization chamber (first space),
the double chamber structure is applied to three sides of the
evaporator, except for the heat receiving surface. In this case,
thermal adhesion between the wick and the vaporization chamber is
further enhanced.
[0077] 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 superiority or inferiority of
the invention. Although the embodiments of the present inventions
have been described in detail, it should be understood that the
various changes, substitutions, and alterations could be made
hereto without departing from the spirit and scope of the
invention.
* * * * *