U.S. patent application number 12/664037 was filed with the patent office on 2010-07-29 for loop heat pipe type heat transfer device.
Invention is credited to Hiroyuki Makino, Kazuyuki Obara.
Application Number | 20100186931 12/664037 |
Document ID | / |
Family ID | 40129676 |
Filed Date | 2010-07-29 |
United States Patent
Application |
20100186931 |
Kind Code |
A1 |
Obara; Kazuyuki ; et
al. |
July 29, 2010 |
LOOP HEAT PIPE TYPE HEAT TRANSFER DEVICE
Abstract
It is an object of the invention to provide a loop heat pipe
type heat transfer device that has reduced size, thickness and
weight and exhibits high heat transfer performance. The loop heat
pipe type heat transfer device is provided with an evaporator, a
steam pipe that conducts a gas phase working fluid from the
evaporator, a condenser connected to the steam pipe and a fluid
pipe that circulates the liquid phase working fluid from the
condenser to the evaporator, and is characterized in that a wick
composed of a fiber structure laminate comprising a laminated
nonwoven fabric is set inside the evaporator.
Inventors: |
Obara; Kazuyuki; (Tokyo,
JP) ; Makino; Hiroyuki; (Tokyo, JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Family ID: |
40129676 |
Appl. No.: |
12/664037 |
Filed: |
June 11, 2008 |
PCT Filed: |
June 11, 2008 |
PCT NO: |
PCT/JP2008/060699 |
371 Date: |
December 10, 2009 |
Current U.S.
Class: |
165/104.26 |
Current CPC
Class: |
F28F 3/12 20130101; H01L
2924/0002 20130101; G06F 1/20 20130101; H01L 2924/0002 20130101;
F28D 15/0266 20130101; F28D 15/046 20130101; H01L 2924/00
20130101 |
Class at
Publication: |
165/104.26 |
International
Class: |
F28D 15/04 20060101
F28D015/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 15, 2007 |
JP |
2007-158631 |
Claims
1-7. (canceled)
8. A loop heat pipe type heat transfer device provided with an
evaporator, a steam pipe that conducts a gas phase working fluid
from the evaporator, a condenser connected to the steam pipe and a
fluid pipe that circulates the liquid phase working fluid from the
condenser to the evaporator, the loop heat pipe type heat transfer
device being characterized in that a wick composed of a fiber
structure laminate comprising a laminated nonwoven fabric is set
inside the evaporator.
9. A loop heat pipe type heat transfer device according to claim 8,
characterized in that the fiber structure laminate has a mean flow
pore size of 0.1-30 .mu.m.
10. A loop heat pipe type heat transfer device according to claim 8
or 9, characterized in that the fiber structure laminate has a void
percentage of 65-95% and a void index (void percentage (%)/mean
flow pore size (.mu.m)) of 10-1000.
11. A loop heat pipe type heat transfer device according to claim 8
or 9, characterized in that the fiber structure laminate has a 10%
flow pore size that is 0-20 .mu.m larger than the mean flow pore
size.
12. A loop heat pipe type heat transfer device according to claim 8
or 9, characterized in that the nonwoven fabric of the fiber
structure laminate is bonded in a laminated state, and the bonding
area is 0.2-20% of a nonwoven fabric area, excluding the area
bonded at the periphery of the wick for maintenance of the
airtightness of the evaporator.
13. A loop heat pipe type heat transfer device according to claim 8
or 9, characterized in that the fiber structure laminate comprises
at least two different laminated nonwoven fabrics with different
void indexes (void percentage (%)/mean flow pore size (.mu.m)).
14. A loop heat pipe type heat transfer device according to claim
12, characterized in that some of the bonded sections of the fiber
structure laminate in the evaporator are used as sections of the
structure that maintain the airtightness of the evaporator.
Description
TECHNICAL FIELD
[0001] The present invention relates to a loop heat pipe type heat
transfer device, and particularly relates to a loop heat pipe type
heat transfer device suitable for use in fields that require small
and light, yet efficient, heat transfer devices, such as personal
computers and the like.
BACKGROUND ART
[0002] Loop heat pipe type heat transfer devices are known as
conventional heat transfer devices for use in space, industrial and
household appliances, and are described in, for example, Patent
document 1. In such loop heat pipe type heat transfer devices, heat
is absorbed by an evaporator from a heat-generating source,
evaporating a working fluid into a gas phase, and the obtained
steam is fed through a steam pipe to a condenser where heat is
dissipated at the heat sink, forming a liquid phase. Thus in space
applications, for example, the heat generated by internal devices
is absorbed by the evaporator and released out into space at the
condenser, thus allowing the temperature of the devices to be
controlled. Because no mechanically driven parts are used, the heat
transfer devices can be stably used for long periods in unmanned
spacecraft and the like.
[0003] Such a loop heat pipe type heat transfer device has a wick
composed of a porous material inside the evaporator. In the case of
a cylindrical evaporator, the wick divides the interior of the
evaporator into an interior liquid pool chamber that holds the
working fluid in its liquid phase, and an exterior steam chamber
which holds the working fluid in its gas phase. The liquid phase
working fluid migrates outward through the wick by capillary
action, and evaporation takes place on the surface sections of the
wick. The wick is a porous body composed of a material which may be
a metal such as copper, aluminum or nickel, a ceramic such as
alumina, titanium oxide or silica or a polymer such as stretched
porous polytetrafluoroethylene or polyethylene.
[0004] The pore size of the wick in the evaporator is uniform in
the radial direction, or the pore size of the inner perimeter of
the wick is larger than the outer perimeter. Patent document 1
teaches that by increasing the pore size at the inner perimeter, it
is possible to evaporate the working fluid uniformly across the
entire wick and thus improve the performance of the evaporator.
However, because the pore diameter is small at the outer perimeter
where the working fluid evaporates, evaporation does not occur
without application of a large quantity of heat. Therefore, 50 W or
more of heat must be applied to the evaporator in order to operate
the loop heat pipe type heat transfer device.
[0005] In order to solve this problem, Patent document 2 discloses
an invention wherein the pore size at the outer perimeter of the
wick is larger than the inner perimeter in the evaporator. As the
inner perimeter, sufficient capillary action is produced so that
the liquid phase working fluid is suctioned and evaporation is
accelerated at the surface of the wick. Because of the large pore
size at the outer perimeter however, movement of the liquid phase
working fluid by capillary action is inhibited and supply of the
working fluid to the evaporator can potentially pool, thus
interfering with the thermal conductivity of the loop heat pipe
type heat transfer device.
[0006] It has also been attempted to achieve high-efficiency heat
transfer performance with loop heat pipe type heat transfer devices
for use in personal computers (PCs) and especially laptop PCs.
Laptop PCs are small and lightweight while exhibiting high
performance because the clock frequency of the central (mobile)
processing unit MPU is increased and MPUs are also highly
integrated, but this also increases the heat density. In
particular, the heat density of high thermal elements is 100
W/cm.sup.2, which is a level close to that of a nuclear reactor.
Therefore, loop heat pipe type heat transfer devices must also be
reduced in size, thickness and weight while also exhibiting higher
thermal conductivity. Consequently, the wicks that determine the
thermal conductivity must have improved capillary action and
reduced resistance to fluid passage, in order to increase mobility
of liquid phase working fluids while promoting evaporation with
less application of heat.
[0007] In order to satisfy these requirements of performance, it
has been a goal to simultaneously achieve a reduced pore size to
increase capillary action, and a higher void percentage to lower
resistance to fluid passage in order to promote evaporation. Yet
porous materials cannot provide both reduction in pore size and
increase in void percentage, the void percentage being reduced with
smaller pore sizes, and this has limited performance
enhancement.
[0008] Patent document 3 discloses an ordinary heat pipe that
employs a fabric member as the wick. This document teaches that
fabric members have finer meshes than wire mesh and can provide
greater capillary action than wire mesh, while permitting smaller
thicknesses, and can therefore contribute to much smaller, thinner
and lighter cooling devices. However, Patent document 3 does not
disclose use of a fabric member in a loop heat pipe type heat
transfer device. Therefore, although document 3 relates to
capillary action it contains no description relating to the void
percentage. Furthermore, nothing is mentioned regarding the
properties of the fabric member, and it is not clear whether or not
the fabric member is suitable for use as a wick.
[0009] [Patent document 1] Japanese Unexamined Patent Publication
No. 10-246583
[0010] [Patent document 2] Japanese Unexamined. Patent Publication
No. 2002-181470
[0011] [Patent document 3] Japanese Unexamined Patent Publication
No. 2004-324906
DISCLOSURE OF INVENTION
Problems to be Solved by the Invention
[0012] The present invention has been accomplished in light of the
circumstances described above, and its object is to provide a loop
heat pipe type heat transfer device which is small, thin and light
while also exhibiting high heat transfer performance, by setting a
wick composed of a fiber structure laminate comprising a laminated
nonwoven fabric, inside an evaporator and optimizing the mean flow
pore size, void percentage and void index (void percentage/mean
flow pore size) of the fiber structure laminate.
Means for Solving the Problems
[0013] In order to achieve the object stated above, the present
invention employs a wick composed of a fiber structure laminate
comprising a laminated nonwoven fabric. The mean flow pore size,
void percentage and void index of the fiber structure laminate can
be easily controlled by the nonwoven fabric properties, the
laminate structure and the bonding method and conditions, and
therefore movement of the working fluid by capillary action can be
accelerated while minimizing the resistance to fluid passage of the
liquid phase working fluid flowing through the wick, and
evaporation can be promoted even with application of a small amount
of heat. As a result, it was found that this produces an enhancing
effect on the thermal conductivity of the evaporator, and the
invention was completed upon this finding.
[0014] Specifically, the present invention provides the
following.
[0015] (1) A loop heat pipe type heat transfer device provided with
an evaporator, a steam pipe that conducts a gas phase working fluid
from the evaporator, a condenser connected to the steam pipe and a
fluid pipe that circulates the liquid phase working fluid from the
condenser to the evaporator, the loop heat pipe type heat transfer
device being characterized in that a wick composed of a fiber
structure laminate comprising a laminated nonwoven fabric is set
inside the evaporator. This construction increases the capillary
action of the wick and lowers the resistance to fluid passage, thus
promoting movement of the working fluid and increasing the maximum
heat transport quantity.
[0016] (2) A loop heat pipe type heat transfer device according to
(1) above, characterized in that the fiber structure laminate has a
mean flow pore size of 0.1-30 .mu.m, a void percentage of 65-95%
and a void index (void percentage (%)/mean flow pore size (.mu.m))
of 10-1000. Setting the properties of the fiber structure laminate
within these ranges increases the capillary action of the wick
while lowering the resistance to fluid passage and accelerating
movement of the working fluid and increasing the maximum heat
transport quantity.
[0017] (3) A loop heat pipe type heat transfer device according to
(1) or (2) above, characterized in that the fiber structure
laminate has a 10% flow pore size that is 0-20 .mu.m larger than
the mean flow pore size. This will allow uniform movement of the
working fluid and improve the evaporation efficiency.
[0018] (4) A loop heat pipe type heat transfer device according to
any one of (1) to (3) above, characterized in that the nonwoven
fabric of the fiber structure laminate is bonded in a laminated
state, and the bonding area is 0.2-20% of the nonwoven fabric area.
This will result in satisfactory handleability without increasing
the resistance to fluid passage for the working fluid.
[0019] (5) A loop heat pipe type heat transfer device according to
any one of (1) to (4) above, characterized in that the fiber
structure laminate comprises at least two different laminated
nonwoven fabrics with different void indexes (void percentage
(%)/mean flow pore size (.mu.m)). This will accelerate movement of
the working fluid in the wick and allow the nonwoven fabric near
the evaporation surface to be suitable for evaporation, so that
evaporation can be promoted with low application of heat.
[0020] (6) A loop heat pipe type heat transfer device according to
(5) above, characterized in that some of the bonded sections of the
fiber structure laminate in the evaporator are used as sections
that maintain the airtightness of the evaporator. This can simplify
the construction of the evaporator and contribute to size,
thickness and weight reduction of the evaporator.
EFFECT OF THE INVENTION
[0021] By using the loop heat pipe type heat transfer device of the
invention it is possible to achieve high-efficiency heat transfer
in electronic devices such as PCs and servers, for which size,
thickness and weight reduction, as well as high performance, are
required.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 shows an example of a loop heat pipe type heat
transfer device according to the invention.
[0023] FIG. 2 shows an assembly drawing of the evaporator in FIG.
1.
EXPLANATION OF SYMBOLS
[0024] 1 Evaporator [0025] 2 Steam pipe [0026] 3 Condenser [0027] 4
Fluid pipe [0028] 5 Wick [0029] 6 Steam chamber [0030] 7 Liquid
pool chamber [0031] 10 Upper enclosure of evaporator [0032] 11
Lower enclosure of evaporator
BEST MODE FOR CARRYING OUT THE INVENTION
[0033] It is essential that the wick used for the invention must be
a nonwoven fabric with a laminated structure. The nonwoven fabric
is obtained by laminating fibers whose aspect ratio represented by
fiber length/diameter is 5 or greater, randomly and/or in an
oriented fashion within a plane and partially bonding and/or fixing
the fibers by any of various methods. Since nonwoven fabrics form
pores based on the large area-to-weight ratio of the fibers,
smaller pores can be formed compared to conventional porous
materials such as sintered metal that form pores using powder, even
when only small amounts of fibers are in the spaces. The pore size
can therefore be reduced and a larger void percentage can be
maintained. The diameter of the fibers composing the nonwoven
fabric may be selected in a range from submicron to several tens of
microns, thus allowing easy control of the pore size. The fabric
made of the fibers is a woven fabric or knitted fabric, and whereas
woven fabrics have weave textures and knitted fabrics have meshes,
such that large pore sections are present as the fabrics are
formed, these are not present in nonwoven fabrics. In other words,
only a nonwoven fabric can reduce the aperture size while
increasing the void percentage, as required for the wick.
[0034] There are no particular restrictions on the material used to
compose the nonwoven fabric used for the invention. Synthetic
resin-based fibers, cellulose-based fibers, metal fibers,
ceramic-based fibers, glass fibers or carbon fibers may be
used.
[0035] The form of the fibers may be long filaments or staple
fibers, or a mixture of the two.
[0036] As synthetic resin-based fibers there may be mentioned
fibers made of polyamides such as nylon 6, nylon 66 and
copolymerized polyamides, fibers made of polyolefins such as
polyethylene, polypropylene and copolymerized polypropylene, fibers
made of polyesters such as polyethylene terephthalate, polybutylene
terephthalate, polytetramethylene terephthalate and copolymerized
polyesters, acrylic fibers, acetal-based fibers and ethylene
tetrafluoride-based fibers.
[0037] As cellulose-based fibers there may be mentioned viscose
rayon, cupra rayon, pulp, cotton and hemp.
[0038] As metal fibers there may be mentioned copper, brass,
aluminum, aluminum alloys, SUS and titanium.
[0039] As ceramic-based fibers there may be mentioned alumina,
titanium oxide and silicon carbide.
[0040] Any of the above may be used alone, or two or more may be
used in admixture.
[0041] Synthetic resin-based fibers with low thermal conductivity
are preferred for easier control of the fiber diameter, and because
they allow the pore size to be freely varied and can suppress
evaporation of the working fluid inside the wick, and particularly
preferred are polyamide-based fibers, polyolefin-based fibers and
polyester-based fibers because of their excellent chemical
stability.
[0042] The fibers used are preferably made of a material with high
affinity with the working fluid. If a material with low affinity is
used, surface treatment after formation of the nonwoven fabric by a
known method is preferred to improve the affinity. For example,
when an aqueous fluid containing 80 wt % water is used as working
fluid, it is preferred to use a hydrophilic material such as a
cellulose-based fiber or polyamide-based fiber. When a hydrophobic
material such as polyolefin-based fiber or polyester-based fiber is
used, a preferred mode involves surface treatment with addition of
a surfactant or the like by a known method, to render the fiber
surfaces hydrophilic.
[0043] The fibers used may also be conjugate fibers in which the
sheath section may consist of polyethylene, polypropylene or a
copolymerized polyester while the core section consists of a
polyamide or polyester.
[0044] The cross-sectional shape of the fibers of the nonwoven
fabric used for the invention may be circular or a non-circular
irregular shape, but an irregular shape is preferred for an
increased area-to-weight ratio.
[0045] There are no particular restrictions on the process for
production of the nonwoven fabric used for the invention, and any
publicly known process may be employed. For example, there may be
mentioned spunlace methods, needle punching methods, spunbond
methods, melt blowing methods, flash spun methods and
electrospinning methods. Spunlace methods, spunbond methods, melt
blowing methods and flash spun methods are preferred since they
allow easier control of the pore size.
[0046] The properties of the nonwoven fabric used for the invention
are not particularly restricted so long as they are appropriately
selected so that the properties of the fiber structure laminate
obtained therefrom are within the ranges specified below.
[0047] The fiber structure laminate of the invention will now be
explained. The fiber structure laminate must be constructed by
laminating a nonwoven fabric. Also, the fiber structure laminate
preferably has a mean flow pore size of 0.1-30 .mu.m, a void
percentage of 65-95% and a void index (void percentage (%)/mean
flow pore size (.mu.m)) of 10-1000. The 10% flow pore size of the
fiber structure laminate is preferably 0-20 .mu.m larger than the
mean flow pore size. The nonwoven fabric of the fiber structure
laminate is bonded in a laminated state, and the bonding area is
preferably 0.2-20% of the nonwoven fabric area.
[0048] The fiber structure laminate used for the invention
preferably has a mean flow pore size of 0.1-30 .mu.m. It is more
preferably 0.1-10 .mu.m, even more preferably 0.2-10 .mu.m and most
preferably 0.2-5 .mu.m. A mean flow pore size of 0.1-30 .mu.m will
allow sufficient capillary action to be achieved and will permit
industrial production, while preventing very large resistance to
fluid passage through each pore.
[0049] Since the mean flow pore size does not change significantly
by lamination, a fiber structure laminate with the prescribed mean
flow pore size can be obtained by using a nonwoven fabric matching
that mean flow pore size for 30 wt % of the nonwoven fabric, and a
nonwoven fabric with a larger mean flow pore size for the rest. For
example, to obtain a fiber structure laminate with a mean flow pore
size of 2 .mu.m, the construction may consist of 100 wt % of a melt
blown nonwoven fabric with a mean fiber size of 1 .mu.m and a mean
flow pore size of 2 .mu.m.
[0050] The flow pore size according to the invention may be
determined by the airflow method described on p. 147 of "Maku Bunri
Gijutsu [Membrane Separation Technology] Manual", ed. by N. Kimura,
K. Sakai, T. Shirota and T. Ukai (published Aug. 10, 1990 by IPC
Shuppan). Specifically, the fiber structure laminate is first
immersed in a liquid with known surface tension, pressure is
applied to the laminate with all of the pores of the laminate
covered with a film of the liquid, and the pore size is calculated
from the pressure at which the liquid film breaks, and the surface
tension of the liquid. The following formula (1) is used for the
calculation.
d=C.times.r/P (1)
(d represents the pore size, r represents the surface tension of
the liquid, P represents the pressure at which the liquid film
breaks at that pore size, and C is a constant.)
[0051] The flow pore size of the fiber structure laminate will
usually have a distribution. The flow pore size distribution can be
determined by continuously varying the pressure and calculating the
amount of liquid film that breaks at each pressure, based on the
change in the flow rate through the laminate at that pressure. A
Palm Porometer by PMI, an apparatus conforming to ASTM E 1294-89,
may be used as the measuring apparatus.
[0052] Based on formula (1), when the pressure P applied to the
liquid-immersed laminate is continuously varied from low pressure
to high pressure, the initial pressure cannot break the liquid film
even on the largest pores, and therefore the flow rate is 0. As the
pressure is increased, the liquid film begins to break on the
largest pores, creating a flow (bubble point). As the pressure is
further increased, the liquid film begins to break on smaller
pores, and the flow rate approaches the flow rate without immersion
in the liquid (dry flow rate).
[0053] The mean flow pore size is the pore size broken with a
pressure such that the flow rate when the laminate is immersed in
the liquid (wet flow rate) is 50% of the flow rate without
immersion (dry flow rate).
[0054] The fiber structure laminate used for the invention
preferably has a 10% flow pore size which is 0-20 .mu.m larger than
the mean flow pore size. It is more preferably 0-10 .mu.m and most
preferably 0-5 .mu.m larger. The difference between the 10% flow
pore size and the mean flow pore size represents the distribution
of pore sizes, and a smaller value indicates a lower distribution.
A difference of 0 .mu.m indicates uniform pores. The 10% flow pore
size is preferably less than (mean flow pore size+20 .mu.m) in
order to allow uniform movement of the working fluid through the
entire evaporator and improve the evaporation efficiency, without
uneven flow of the working fluid through the large pores that have
low resistance to fluid passage.
[0055] The 10% flow pore size is the pore size broken with a
pressure such that the flow rate when the laminate is immersed in
the liquid (wet flow rate) is 10% of the flow rate without
immersion (dry flow rate). The 10% flow pore size and mean flow
pore size are determined by the methods described hereunder.
[0056] The fiber structure laminate used for the invention
preferably has a void percentage of 65-95%. It is more preferably
70-95% and even more preferably 80-95%. A void percentage of 65-95%
will lower the resistance to fluid passage of the laminate and
provide sufficient strength for handling, as well as sufficient
durability.
[0057] Since the void percentage tends to be slightly reduced by
the pressure during bonding, it is preferred to select a nonwoven
fabric with a void percentage of about 5% greater than the
prescribed Void percentage for the fiber structure laminate. A
known method may be used to set the appropriate bonding conditions
between the fibers, in order to obtain a nonwoven fabric with the
prescribed void percentage.
[0058] The void percentage, according to the invention, is the
proportion of voids per unit volume of the fiber structure
laminate. The void percentage is calculated by the following
formula (2), using the basis weight W (g/cm.sup.2) and thickness T
(cm) of the fiber structure laminate, and the true specific gravity
.rho. of the fibers composing the laminate.
Void percentage={1-W/(T.times..rho.)}.times.100 (2)
[0059] The basis weight and thickness of the fiber structure
laminate may be determined according to JISL-1096.
[0060] The fiber structure laminate used for the invention
preferably has a void index of 10-1000, as the ratio of the void
percentage (%) determined in the manner explained above, and the
mean flow pore size (.mu.m) (void percentage (%)/mean flow pore
size (.mu.m)). The lower limit for the void index is even more
preferably 20 and most preferably 25. The upper limit is even more
preferably 800 and most preferably 500.
[0061] The void index is the parameter most responsible for the
maximum heat transport quantity, which is an indicator of the
performance of the loop heat pipe type heat transfer device, and
generally a larger void index corresponds to a greater maximum heat
transport quantity. The optimal combination of the void percentage
and mean flow pore size, which are difficult to control separately,
produces the maximum void index, and therefore the void index is
effective as a parameter for optimizing both properties. A void
index of 10-1000 is optimal for a structure to be used as a wick
that has sufficiently powerful capillary action and low resistance
to fluid passage, and that can be industrially produced. By using a
nonwoven fabric with a void index of 10-1000 for at least 70 wt %
of the fabric, it is possible to ensure a void index of 10-1000 for
the fiber structure laminate. The void index of metal sintered
materials that are conventionally used as wicks is about 7.5 while
the void index of sintered ceramics is about 2, and these are
relatively small compared to the fiber structure laminate of the
invention. It has therefore been difficult to exhibit satisfactory
performance as a wick due to either poor capillary action or high
resistance to fluid passage.
[0062] According to the invention, it is possible to realize a
large void index by using as the material a nonwoven fabric that is
completely different from materials used in the prior art, while
simultaneously achieving sufficiently powerful capillary action and
low resistance to fluid passage, thereby increasing the maximum
heat transport quantity.
[0063] The fiber structure laminate used for the invention has the
nonwoven fabric bonded in a laminated state, and the bonding area
is preferably 0.2-20%, even more preferably 0.5-10% and most
preferably 1-10% of the nonwoven fabric area, excluding the area
bonded at the periphery of the wick for maintenance of the
airtightness of the evaporator, as explained hereunder. Within this
range, it is possible to obtain adhesive strength that can sustain
an integrated state during handling without increasing the
resistance to fluid passage of the working fluid. There are no
particular restrictions on the bonding method. There may be
employed, for example, emboss bonding that produces partial heat
sealing, partial ultrasonic fusion in various forms such as pin or
linear forms or combinations thereof, or thermal bonding with a
powdery or fibrous thermoplastic resin. Partial ultrasonic fusion
in various forms such as pin or linear forms or combinations
thereof is preferred for easier control of the bonding area
proportion and greater adhesive strength.
[0064] A single nonwoven fabric may be used for the invention, but
a preferred mode is a laminated composite of nonwoven fabrics with
different properties. Compositing of fabrics will allow improvement
in competing properties, that cannot be achieved with a single type
of fabric. It is particularly preferred to use a laminated
composite of nonwoven fabrics with different void indexes. The
method for measuring the void percentage and mean flow pore size
will be described below.
[0065] According to a preferred mode, a nonwoven fabric with a void
index of 5 is laminated on the front and back while 8 nonwoven
fabrics with a void index of 40 are laminated between them, for a
total of 10, to compose the fiber structure laminate. The nonwoven
fabrics with a void index of 40 constitute approximately 80 wt % of
this construction. Since nonwoven fabrics with small void indexes
are situated on the side in contact with the working fluid, the
resistance to fluid passage is very low and the working fluid that
circulates back can be rapidly and uniformly dispersed in the fiber
structure laminate. The working fluid that has been dispersed in
the first nonwoven fabric of the laminated body is suctioned by the
powerful capillary action in the layer of the next nonwoven fabric
which has a large void index. Also, since the layer in contact with
the evaporation surface has a small void index, the working fluid
evaporates more readily and evaporation is promoted with low
application of heat.
[0066] With conventionally employed metal or ceramic porous bodies,
it has been difficult to control the properties while adjusting
them. Stretched porous polytetrafluoroethylene obtained in sheet
form allows adjustment of the properties by using sheets with
different properties laminated as an integral whole. However, the
distinct border between the sheets causes pooling of the working
fluid between the sheets, potentially impeding the smooth flow of
the working fluid. The fibers on the surface of a nonwoven fabric,
however, become tangled and integrated with the fibers of the other
laminated nonwoven fabrics by the pressure applied during bonding,
and therefore no clear boundaries are present. Consequently, the
working fluid does not easily pool between the nonwoven fabrics and
smooth movement is promoted. Lamination and bonding of nonwoven
fabrics is also useful from this viewpoint.
[0067] The loop heat pipe type heat transfer device of the
invention is constructed with an evaporator, a steam pipe that
conducts a gas phase working fluid from the evaporator, a condenser
connected to the steam pipe and a fluid pipe that circulates the
liquid phase working fluid from the condenser to the evaporator,
wherein a wick composed of a fiber structure laminate with
specified properties is set inside the evaporator. Known structures
may be employed for the structure of the evaporator and condenser.
FIG. 1 shows an example of a loop heat pipe type heat transfer
device according to the invention, but there is no restriction to
this structure. In FIG. 1, 1 is a circular flat evaporator, 2 is a
steam pipe, 3 is a condenser and 4 is a fluid pipe. The condenser
has a construction with a cooling fan and copper fin integrated in
a cooling module, which is anchored by solder to the steam pipe.
The wick 5 composed of the fiber structure laminate is set so that
the interior of the evaporator 1 is split into an upper and lower
section, with the lower section constituting the steam chamber 6
and the upper section constituting the liquid pool chamber 7.
[0068] The wick composed of the fiber structure laminate may be set
in the evaporator in a manner that is known in the art, and it is
not particularly restricted. The fiber structure laminate has high
softness and flexibility, and is easily cut and bonded at the ends,
so that it is highly workable and may be set in the desired manner.
For example, if the evaporator is cylindrical, a groove may be
formed in the inner wall of the evaporator and the fiber structure
laminate shaped as a hollow cylindrical form may be set so as to
contact with the protrusions of the groove. The working fluid that
has returned from the fluid pipe flows into the liquid pool inside
the hollow section. If the evaporator is flat, a flat fiber
structure laminate may be set in the middle section with space
above and below, as shown in FIG. 1.
[0069] There are no particular restrictions on the manner in which
the wick composed of the fiber structure laminate is set inside the
evaporator, but preferably a part of the bonded section of the
fiber structure laminate is used as part of the structure
maintaining the airtightness of the evaporator. The evaporator must
have the non-condensable gas such as air in the container evacuated
before filling it with the working fluid, and must be airtight to
maintain a sufficient degree of vacuum of, for example, 0.1 Torr.
The airtightness must be such as to prevent leakage of the working
fluid steam generated by heating during use. Conventionally, the
enclosures of the evaporator have been welded or soldered together
for bonding to maintain airtightness. According to the invention,
however, some of the bonded sections of the wick composed of the
fiber structure laminate may also be used, in addition to the
conventional known method.
[0070] FIG. 2 shows an assembly drawing of the evaporator 1 in FIG.
1. A construction for maintaining airtightness of the evaporator is
shown in FIG. 2, but there is no restriction to this construction.
In FIG. 2, 10 is the upper enclosure of the evaporator which also
serves as a reservoir that stores the working fluid that has
returned from the fluid pipe. Numeral 11 is the lower enclosure of
the evaporator which absorbs heat from the device to be cooled and
evaporates the working fluid. The working fluid steam flows through
the grooves of the lower enclosure and reaches the steam pipe. The
wick 5 composed of the fiber structure laminate is bonded by fusion
at the peripheral sections. The bonding area proportion is 15%. The
upper enclosure 10 and lower enclosure 11 are assembled across the
wick 5 composed of the fiber structure laminate, to obtain an
evaporator. The method of assembly is not particularly restricted,
but a screw assembly is shown in this example. When the upper
enclosure 10 and lower enclosure 11 are assembled across the wick
composed of a porous material, airtightness cannot be maintained
and means must be provided to maintain airtightness, such as
welding of the enclosures together. When a wick composed of the
fiber structure laminate bonded by fusion is used, the pores become
crushed by the fusion, thus allowing airtightness to be maintained
so that convenient means such as screwing may be employed for
assembly.
[0071] As mentioned above, the nonwoven fabric may be integrated by
having sections thereof bonded beforehand, but preferably it is
bonded simultaneously with assembly of the evaporator. The bonding
method for simultaneous bonding is not particularly restricted, but
heat compression is preferred. With heat compression, the same
construction and assembly method described above may be employed to
achieve reduced size, thickness and weight while also lowering
assembly costs.
[0072] The working fluid used for the invention may be a fluid that
is commonly used for loop heat pipe type heat transfer devices.
Examples thereof include water, ammonia, alcohols such as ethanol,
hydrocarbons such as heptane, fluorocarbons such as Freon-11,
liquid oxygen, liquid nitrogen and the like, although there is no
restriction to these. Such fluids may be used either alone or in
combinations, and when combined, their types and mixing ratios are
preferably such as to form uniform solutions.
[0073] The working fluid is preferably an aqueous fluid containing
at least 80 wt % water, which has a large merit number as defined
by formula (3).
Merit number=(density.times.surface tension.times.heat of
vaporization)/viscosity (3)
[0074] A larger merit number is preferred since it represents a
greater maximum heat transport quantity. The merit numbers for
typical fluids are as follows: ammonia (1.1.times.10.sup.11),
Freon-11 (1.2.times.10.sup.10), Freon-113 (7.3.times.10.sup.9),
pentane (1.5.times.10.sup.10), acetone (3.times.10.sup.10),
methanol (4.8.times.10.sup.10), ethanol (4.1.times.10.sup.10),
heptane (1.3.times.10.sup.10), water (5.1.times.10.sup.11),
naphthalene (3.4.times.10.sup.10). Water is preferred as it has a
large merit number and therefore a high maximum heat transport
quantity. Water alone is preferred, but ketones such as acetone,
alcohols such as methanol or ethanol and surfactants may also be
added to the water. In this case, the water content is preferably
at least 80 wt % in order to maintain a large merit number.
EXAMPLES
[0075] The present invention will now be explained in greater
detail by examples and comparative examples, with the understanding
that the invention is in no way limited only to the examples.
[0076] The measurement methods used for the invention are the
following.
[0077] (1) Mean Flow Pore Size (.mu.m):
[0078] This was measured using a Palm Porometer (Model CFP-1200AEX)
by PMI. A Silwick by PMI with a surface tension of 20.1 dynes/cm
was used as the immersion liquid. The sample immersed in the
immersion liquid was pretreated by lowering it to 80 kPa below
atmospheric pressure and deairing so that no bubbles remained in
the sample. The measuring diameter was 20 mm. Dry air was passed
through the sample and the gas pressure was increased in stages,
measuring the gas flow rate at each point. The pressure P.sub.50
(PSI) at which the flow rate with the sample immersed in the liquid
(wet flow rate) was 50% of the flow rate when not immersed (dry
flow rate) was determined, and the mean flow pore size was
calculated by the following formula (4).
d.sub.50=C.times.r/P.sub.50 (4)
[0079] In the formula, d.sub.50 is the mean flow pore size (.mu.m),
r is the surface tension of the immersion liquid, which was 20.1
(dynes/cm), and the constant C is 0.451 (.mu.mcmPSI/dynes).
Measurement was performed 3 times and the average value was
calculated.
[0080] (2) 10% Flow Pore Size (.mu.m):
[0081] The pressure P.sub.10 (PSI) at which the flow rate with the
sample immersed in the liquid (wet flow rate) was 10% of the flow
rate when not immersed (dry flow rate) was determined, and
calculation was performed by the following formula (5) in the same
manner as (1) above. Measurement was performed 3 times and the
average value was calculated.
d.sub.10=C.times.r/P.sub.10 (5)
[0082] (3) Void Percentage (%):
[0083] The void percentage is calculated by the following formula
(2), using the basis weight W (g/cm.sup.2) and thickness T (cm) of
the sample, and the true specific gravity .rho. of the material
composing the sample.
Void percentage={1-W/(T.times..rho.)}.times.100 (2)
[0084] The basis weight and thickness of the sample was determined
according to JISL-1096.
[0085] (4) Bonding Area Proportion (%):
[0086] A 100 mm.times.100 mm sample was imaged with a CCD camera
and the image was analyzed with image analysis software, and upon
conversion to binary format, the bonding sections were extracted
and the bonding area (mm.sup.2) was determined using the image
analysis software. The proportion with respect to an imaged area of
10,000 mm.sup.2 was determined and the bonding area proportion was
calculated.
[0087] (5) Maximum Heat Transport Quantity (W):
[0088] The sample was set in a circular flat evaporator with an
outer diameter of 80 mm and a thickness of 20 mm made of SUS304, of
the type shown in FIG. 1. The condenser, steam pipe and fluid pipe
were all constructed of SUS316 pipes with outer diameters of 4 mm.
The condenser was cooled by forced air-cooling using a cooling
module having an integrated cooling fan and copper fins. The heat
transfer distance, i.e. the length of the steam pipe, was about 1
m. The working fluid used was ethanol. A heater was installed below
the evaporator, and the evaporator temperature T1 (K) and condenser
temperature T2 (K) were measured with thermocouples soldered to the
evaporator and condenser, while continuously increasing the input
power, recording the maximum heat transport quantity as the input
power at which T1-T2 was 50K.
Example 1
[0089] After laminating 10 polyethylene terephthalate melt blown
nonwoven fabrics (products of Asahi Kasei Fibers Corp., basis
weight: 40 g/m.sup.2, void percentage: 90%, mean flow pore size: 2
.mu.m; indicated as "MB" in Table 1), they were thermally bonded
with a pin-type ultrasonic fuser. The bonding area proportion of
the obtained fiber structure laminate was 1%. The mean flow pore
size, void percentage, void index and maximum heat transport
quantity of the laminate are shown in Table 1.
Examples 2-4
[0090] Fiber structure laminates were obtained in the same manner
as Example 1, except that thermal bonding was carried out with a
pin-type ultrasonic fuser having a different number of pins per
unit area, and the bonding area proportions were 5% (Example 2),
0.1% (Example 3) and 30% (Example 4). The fiber structure laminate
with a bonding area proportion of 0.1% in Example 3 had low
adhesive force, underwent peeling when set in the evaporator and
exhibited inferior handleability, but it was attached to the upper
enclosure of the evaporator and the maximum heat transport quantity
was measured. Example 4 which had a large bonding area proportion
of 30% had a low effective liquid flow area, and the maximum heat
transport quantity was smaller than Example 1.
Example 5
[0091] A fiber structure laminate was obtained in the same manner
as Example 1, except for laminating in order one polyethylene
terephthalate spunbond nonwoven fabric (ELTAS by Asahi Kasei Fibers
Corp., basis weight: 20 g/m.sup.2, void percentage: 85%, mean flow
pore size: 18 .mu.m, indicated "SB" in Table 1), 8 polyethylene
terephthalate melt blown nonwoven fabrics (product of Asahi Kasei
Fibers Corp., basis weight: 40 g/m.sup.2, void percentage: 90%,
mean flow pore size: 2 .mu.m) and one polyethylene terephthalate
spunbond nonwoven fabric (ELTAS by Asahi Kasei Fibers Corp., basis
weight: 20 g/m.sup.2, void percentage: 85%, mean flow pore size: 18
.mu.m), and the mean flow pore size, void percentage, void index
and maximum heat transport quantity were determined. The results
are shown in Table 1. The bonding area proportion of the obtained
fiber structure laminate was 1%. Laminating the spunbond nonwoven
fabric on the front and back sides resulted in more uniform
movement of the working fluid and further increased the maximum
heat transport quantity to facilitate evaporation.
Example 6
[0092] A fiber structure laminate was obtained in the same manner
as Example 1, except that two nonwoven fabrics obtained by water
stream interlacing of nylon 66 fibers with a fiber size of 5.5
.mu.m (basis weight: 80 g/m.sup.2, void percentage: 85%, mean flow
pore size: 5 .mu.m, indicated by "SL" in Table 1) were used. As
shown by the results in Table 1, the maximum heat transport
quantity was satisfactory.
Example 7
[0093] A fiber structure laminate was obtained in the same manner
as Example 1, except that two nonwoven fabrics obtained by water
stream interlacing and roll compression of nylon 66 fibers with a
fiber size of 5.5 .mu.m (basis weight: 150 g/m.sup.2, void
percentage: 60%, mean flow pore size: 5 .mu.m) were used. The
maximum heat transport quantity was lower than Example 6 due to a
smaller void percentage.
Example 8
[0094] A fiber structure laminate was obtained in the same manner
as Example 1, except that two nonwoven fabrics obtained by water
stream interlacing of nylon 66 fibers with a fiber size of 10.1
.mu.m (basis weight: 75 g/m.sup.2, void percentage: 80%, mean flow
pore size: 15 .mu.m) were used. The maximum heat transport quantity
was lower than Example 6 due to a larger mean flow pore size.
Example 9
[0095] A fiber structure laminate was obtained in the same manner
as Example 1, except that two nonwoven fabrics obtained by water
stream interlacing of nylon 66 fibers with a fiber size of. 5.5
.mu.m (basis weight: 120 g/m.sup.2, void percentage: 80%, mean flow
pore size: 6 .mu.m) were used. The difference between the 10% flow
pore size and mean flow pore size of this laminate was 12 .mu.m,
and a slightly larger pore size distribution resulted in a somewhat
lower heat transport quantity compared to Example 6.
Example 10
[0096] A fiber structure laminate was obtained in the same manner
as Example 1, except that two nonwoven fabrics obtained by water
stream interlacing of nylon 66 fibers with a fiber size of 7.9
.mu.m (basis weight: 150 g/m.sup.2, void percentage: 75%, mean flow
pore size: 8 .mu.m) were used. A void index of less than 10
resulted in a lower heat transport quantity compared to Example
6.
Comparative Example 1
[0097] The mean flow pore size, void percentage, void index and
maximum heat transport quantity were determined in the same manner
as Example 1, except that a nickel sintering porous material was
used as the wick. The results are shown in Table 1.
Comparative Example 2
[0098] The mean flow pore size, void percentage, void index and
maximum heat transport quantity were determined in the same manner
as Example 1, except that the wick was obtained by laminating 20
polytetrafluoroethylene porous materials (WP-500-100 POREFLON
membrane, product of Sumitomo Electric Fine Polymer, Inc.). The
results are shown in Table 1.
TABLE-US-00001 TABLE 1 Maximum heat Bonded area Mean flow 10% Flow
Void Void transport Laminated proportion pore size pore size
percentage index quantity structure (%) (.mu.m) (.mu.m) (%)
(%/.mu.m) (W) Example 1 MB (10) 1 2 9 88 44 216 Example 2 MB (10) 5
2 9 88 44 207 Example 3 MB (10)) 0.1 2 9 88 44 170 Example 4 MB
(10) 30 2 9 82 41 176 Example 5 SB(1)/MB(8)/SB(1) 1 2 10 89 45 235
Example 6 SL (2) 1 6 13 83 14 192 Example 7 SL (2) 1 7 15 58 8 168
Example 8 SL (2) 1 15 24 80 5 157 Example 9 SL (2) 1 6 18 78 13 180
Example 10 SL (2) 1 8 15 73 9 173 Comp. Ex. 1 -- -- 13 17 75 5.8
147 Comp. Ex. 2 -- -- 5 25 70 14 155
[0099] As clearly shown by the results for Example 1 and
Comparative Examples 1 and 2 in Table 1, using fiber structure
laminates according to the invention as wicks can increase the void
percentage while reducing the mean flow pore size, compared to
metal sintered materials or polytetrafluoroethylene porous
materials that are conventionally used as wicks, and can therefore
provide increased maximum heat transport quantity. As a result, it
is possible to enhance the performance of loop heat pipe type heat
transfer devices.
[0100] As shown by the results in Examples 6-10, limiting the
properties of the nonwoven fabrics and the fiber structure
laminates obtained by lamination of the nonwoven fabrics to the
specific range, can further increase the maximum heat transport
quantity and thus further enhance the performance of loop heat pipe
type heat transfer devices.
Example 11
[0101] The fiber structure laminate obtained in Example 1 was
punched into a circle with a diameter of 30 mm and then the
periphery was bonded at a width of 3 mm. The bonding was
accomplished by a method using a concentric circular die with an
outer diameter of 30 mm and an inner diameter of 24 mm, heated to
300.degree. C., for fusion by heat compression. Eight 1 mm-diameter
screw holes were formed at the bonded section at equal spacings to
form a wick. The wick was placed between a copper upper enclosure
with an outer diameter of 30 mm, a brim width of 3 mm, a height of
5 mm and a thickness of 1 mm and a copper lower enclosure with an
outer diameter of 30 mm, an edge width of 3 mm and a height of 5
mm, and 8 screws were used for assembly to obtain an evaporator.
Copper pipes with an outer diameter of 3 mm and an inner diameter
of 2 mm were soldered to 3.2 mm-diameter holes that had already
been formed in the upper enclosure and lower enclosure, to form a
fluid pipe and steam pipe, respectively. The fluid pipe was bent
into a U shape and soldered to the steam pipe, to form a loop. A
cooling module, comprising a cooling fan and 10 mm-high, 3 mm-pitch
copper fins integrated therewith was soldered to the steam pipe at
a position 100 mm from the lower enclosure, to obtain a condenser.
Upon evacuation using a vacuum pump through a copper pipe for
injection of the working fluid containing a 3-way cock separately
mounted on the upper enclosure, satisfactory airtightness was
exhibited and pressure reduction to 0.1 Torr was possible. After
evacuation to 0.1 Torr, the 3-way cock was switched for injection
of methanol. The copper pipe for injection of the working fluid was
closed by compression and then welded to obtain a loop heat pipe
type heat transfer device. Measurement of the maximum heat
transport quantity in the same manner as Example 1 yielded a value
of 80 W.
Example 12
[0102] The fiber structure laminate obtained in Example 1 was
punched into a circle with a diameter of 30 mm. After an upper
enclosure and lower enclosure having the same shapes as in Example
11 were heated to 300.degree. C., they were used to sandwich the
fiber structure laminate and compressed. A loop heat pipe type heat
transfer device was obtained in the same manner as Example 11,
except that the upper enclosure and lower enclosure were bonded by
the fused fiber structure laminate to obtain the evaporator.
Pressure reduction to 0.1 Torr was possible as in Example 11.
Measurement of the maximum heat transport quantity in the same
manner as Example 1 yielded a value of 80 W.
INDUSTRIAL APPLICABILITY
[0103] According to the invention it is possible to provide a loop
heat pipe type heat transfer device which is small and light while
also exhibiting high heat transfer performance, by setting a wick
composed of a fiber structure laminate comprising a laminated and
bonded nonwoven fabric, inside an evaporator and optimizing the
mean flow pore size, void percentage and void index (void
percentage/mean flow pore size) of the fiber structure laminate. It
can therefore be used as a high-efficiency heat transfer device in
electronic devices such as PCs and servers, for which size,
thickness and weight reduction, as well as high performance, are
required.
* * * * *