U.S. patent application number 11/291879 was filed with the patent office on 2006-07-27 for heat pipe with sintered powder wick.
This patent application is currently assigned to Foxconn Technology CO., LTD.. Invention is credited to Ching-Tai Cheng, Chu-Wan Hong, Chang-Ting Lo, Jung-Yuan Wu.
Application Number | 20060162907 11/291879 |
Document ID | / |
Family ID | 36695489 |
Filed Date | 2006-07-27 |
United States Patent
Application |
20060162907 |
Kind Code |
A1 |
Wu; Jung-Yuan ; et
al. |
July 27, 2006 |
Heat pipe with sintered powder wick
Abstract
A heat pipe (10) includes a casing (12) and a sintered powder
wick (14) arranged at an inner surface of the casing. The sintered
powder wick is in the form of a multi-layer structure of at least
three layers and each layer has an average powder size different
from that of the other layers, wherein the layer (143) with
large-sized powders is capable of reducing the flow resistance to
the condensed liquid to flow back while the layer (141) with
small-sized powders is still capable of providing a relatively
large capillary force for the wick.
Inventors: |
Wu; Jung-Yuan; (Tu-Cheng,
TW) ; Hong; Chu-Wan; (Tu-Cheng, TW) ; Lo;
Chang-Ting; (Tu-Cheng, TW) ; Cheng; Ching-Tai;
(Tu-Cheng, TW) |
Correspondence
Address: |
MORRIS MANNING & MARTIN LLP
1600 ATLANTA FINANCIAL CENTER
3343 PEACHTREE ROAD, NE
ATLANTA
GA
30326-1044
US
|
Assignee: |
Foxconn Technology CO.,
LTD.
Tu-Cheng City
TW
|
Family ID: |
36695489 |
Appl. No.: |
11/291879 |
Filed: |
December 1, 2005 |
Current U.S.
Class: |
165/104.26 ;
165/146 |
Current CPC
Class: |
F28D 15/046
20130101 |
Class at
Publication: |
165/104.26 ;
165/146 |
International
Class: |
F28D 15/02 20060101
F28D015/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 21, 2005 |
TW |
94101783 |
Claims
1. A heat pipe comprising: a casing; and a sintered powder wick
arranged at an inner surface of the casing; wherein the sintered
powder wick is in the form of a multi-layer structure of at least
three layers and each layer has an average powder size different
from that of each the other layers.
2. The heat pipe of claim 1, wherein said at least three layers are
stacked in such a manner that the average powder sizes thereof
increase along a predetermined direction.
3. The heat pipe of claim 2, wherein said direction is a radial
direction of the casing.
4. The heat pipe of claim 2, wherein said direction is a
longitudinal direction of the casing.
5. The heat pipe of claim 1, wherein the sintered powder wick is
one of a sintered metal powder wick and a sintered ceramic powder
wick.
6. A method for manufacturing a heat pipe comprising steps of:
providing a hollow casing and at least three groups of powder with
each group having an average powder size different from that of
each of the other groups; filling said at least three groups of
powder sequentially into said casing at a location adjacent to an
inner surface of the casing with the later filled group of powder
being stacked on the earlier filled group of powder; and conducting
sintering process to the casing and the filled powder, whereby a
sintered powder wick with a multi-layer structure is formed inside
the casing.
7. The method of claim 6, wherein said at least three groups of
powder are filled into said casing in such an order that the
average powder size of the later filled group of powder is larger
than that of the earlier filled group of powder.
8. The method of claim 6, wherein a mandrel is used to be inserted
into said casing with a space formed between the casing and the
mandrel, and said at least three groups of powder are filled into
said space to be stacked along a longitudinal direction of the
casing.
9. The method of claim 6, wherein said at least three groups of
powder are stacked along a radial direction of the casing, and when
filling each group of powder, a mandrel is used to control a
thickness thereof.
10. The method of claim 9 further comprising a step of
pre-sintering each group of powder before a corresponding mandrel
used to control a thickness thereof is drawn out of the casing.
11. A heat pipe comprising: a metal casing having an outer surface
and an inner surface; a wick formed on the inner surface of the
metal casing; and working fluid received in the metal casing and
capable of becoming into vapor upon receiving heat at a first place
of the heat pipe and liquid upon releasing the heat at a second
place of the heat pipe, the liquid being drawn by the wick from the
second place of the heat pipe to the first place of the heat pipe;
wherein the wick forms capillary pressure in drawing the liquid,
the capillary pressure being in gradient along one of radial and
longitudinal directions of the heat pipe.
12. The heat pipe of claim 11, wherein the wick is formed by
sintering powders having different powder sizes.
13. The heat pipe of claim 12, wherein the powders have three
different sizes, with the powders having the largest size being
located near the second place of the heat pipe and the powders
having the smallest size being located near the first place of the
heat pipe.
14. The heat pipe of claim 12, wherein the powders have three
different sizes, with the powders having the largest size being
located close to the casing and the powders having the smallest
size being located close to a center of the heat pipe.
15. The heat pipe of claim 12, wherein the powders have three
different sizes, with the powders having the largest size being
located close to a center of the heat pipe and the powders having
the smallest size being located close to the casing.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to heat pipes as
heat transfer/dissipating device, and more particularly to a heat
pipe with a sintered powder wick.
BACKGROUND
[0002] Heat pipes have excellent heat transfer performance due to
their low thermal resistance, and therefore are an effective means
for heat transfer or dissipation from heat sources. Currently, heat
pipes are widely used for removing heat from heat-generating
components such as central processing units (CPUs) of computers. A
heat pipe is usually a vacuum casing containing therein a working
fluid, which is employed to carry, under phase transitions between
liquid state and vapor state, thermal energy from one section of
the heat pipe (typically referring to as "evaporating section") to
another section thereof (typically referring to as "condensing
section"). Preferably, a wick structure is provided inside the heat
pipe, lining the inner walls of the casing, for wicking the working
fluid back to the evaporating section after it is condensed at the
condensing section. Specifically, as the evaporating section of the
heat pipe is maintained in thermal contact with a heat-generating
component, the working fluid contained at the evaporating section
absorbs heat generated by the heat-generating component and then
turns into vapor. Due to the difference of vapor pressure between
the two sections of the heat pipe, the generated vapor moves
towards and carries the heat simultaneously to, the condensing
section where the vapor is condensed into liquid after releasing
the heat into ambient environment by, for example, fins thermally
contacting the condensing section. Due to the difference of
capillary pressure developed by the wick structure between the two
sections, the condensed liquid is then wicked back by the wick
structure to the evaporating section where it is again available
for evaporation.
[0003] The wick structure currently available for heat pipes
includes fine grooves integrally formed at the inner walls of the
casing, screen mesh or bundles of fiber inserted into the casing
and held against the inner walls thereof, or sintered powder
combined to the inner walls by sintering process. Among these
wicks, the sintered powder wick is preferred to the other wicks
with respect to heat transfer ability and ability against gravity
of the earth.
[0004] The primary function of a wick is to draw condensed liquid
back to the evaporating section of a heat pipe under the capillary
pressure developed by the wick. Therefore, the capillary pressure
is an important parameter affecting the performance of the wick.
Since it is well recognized that the capillary pressure of a wick
increases due to a decrease in pore size of the wick, the sintered
powder wick generally has a capillary pressure larger than that of
the other wicks due to its very dense structure of small particles.
In order to obtain a relatively large capillary pressure for a
sintered powder wick, small-sized powder is often used so as to
reduce the pore size formed between the particles of the powder.
However, it is not always the best way to choose a sintered powder
wick based on the size of powder, because the flow resistance to
the condensed liquid also increases due to a decrease in pore size
of the wick. The increased flow resistance reduces the speed of the
condensed liquid in returning back to the evaporating section and
therefore limits the heat transfer performance of the heat pipe. As
a result, a heat pipe with a wick that has too large or too small a
pore size often suffers dry-out problem at the evaporating section
as the condensed liquid cannot be timely sent back to the
evaporating section of the heat pipe.
[0005] Therefore, there is a need for a heat pipe with a sintered
powder wick which can provide simultaneously a relatively large
capillary force and a relatively low flow resistance so as to
effectively and timely bring the condensed liquid back from its
condensing section to its evaporating section and thereby to avoid
the undesirable dry-out problem at the evaporating section.
SUMMARY
[0006] A heat pipe in accordance with a preferred embodiment of the
present invention includes a casing and a sintered powder wick
arranged at an inner surface of the casing. The sintered powder
wick is in the form of a multi-layer structure of at least three
layers and each layer has an average powder size different from
that of each of the other layers.
[0007] The present invention in another aspect, relates to a method
for manufacturing a sintered heat pipe. The preferred method
includes steps of: (1) providing a hollow casing and at least three
groups of powder with each group having an average powder size
different from that of each of the other groups; (2) filling said
at least three groups of powder sequentially into said casing at a
location adjacent to an inner surface of the casing with the later
filled group of powder being stacked on the earlier filled group of
powder; and (3) conducting sintering process to the casing and the
filled powder, whereby a sintered powder wick with a multi-layer
structure is formed inside the casing.
[0008] Other advantages and novel features of the present invention
will become more apparent from the following detailed description
of preferred embodiment when taken in conjunction with the
accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a longitudinal cross-sectional view of a heat pipe
in accordance with a first embodiment of the present invention;
[0010] FIG. 2 is a longitudinal cross-sectional view of a heat pipe
in accordance with a second embodiment of the present
invention;
[0011] FIG. 3 is a longitudinal cross-sectional view of a heat pipe
in accordance with a third embodiment of the present invention;
[0012] FIGS. 4-5 are longitudinal cross-sectional views showing the
steps of a preferred method in manufacturing the heat pipe of FIG.
1; and
[0013] FIGS. 6-8 are longitudinal cross-sectional views showing the
steps of a preferred method in manufacturing the heat pipe of FIG.
2.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0014] FIG. 1 illustrates a heat pipe 10 in accordance with a first
embodiment of the present invention. The heat pipe 10 includes a
casing 12 and a capillary wick 14 arranged at an inner surface of
the casing 12. The casing 12 includes an evaporating section 121
and a condensing section 123 at respective opposite ends thereof,
and a central section 122 located between the evaporating section
121 and the condensing section 123. The casing 12 is typically made
of high thermally conductive materials such as copper or copper
alloys. The wick 14 is saturated with a working fluid (not shown),
which acts as a heat carrier for carry thermal energy from the
evaporating section 121 toward the condensing section 123 when
undergoing a phase transition from liquid state to vaporous state.
In more detail, heat that needs to be dissipated is transferred
firstly to the evaporating section 121 of the heat pipe 10 to cause
the working fluid saturated in the wick 14 to evaporate. Then, the
heat is carried by the working fluid in the form of vapor to the
condensing section 123 where the heat is released to ambient
environment and the vapor is condensed into liquid. The condensed
liquid then is brought back, via the wick 14, to the evaporating
section 121 where it is again available for evaporation.
[0015] The capillary wick 14 is a sintered powder wick which is
formed by sintering small-sized powder, such as metal powder
including copper and aluminum, or ceramic powder under high
temperature. Along a longitudinal direction of the casing 12, the
wick 14 has a multi-layer structure, which includes in sequence a
first layer 141, a second layer 142 and a third layer 143. In this
embodiment, the first, second and third layers 141, 142, 143
correspond to the evaporating, central and condensing sections 121,
122, 123 of the casing 12, respectively. Each layer of the wick 14
has an average powder size different from that of each of the other
layers. The first layer 141 has the smallest average powder size,
whereas the third layer 143 has the largest average powder size.
That is, the three layers 141, 142, 143 are stacked together in
such a manner that the average powder sizes thereof gradually
increase along the longitudinal direction from the evaporating
section 121 toward the condensing section 123.
[0016] Since the particle size of the powder also determines the
pore size formed between the particles of the powder, the average
pore sizes of these layers 141, 142, 143 also increase along the
longitudinal direction from the evaporating section 121 toward the
condensing section 123. According to the general rule that the
capillary pressure of a wick and its flow resistance to the
condensed liquid increase due to a decrease in pore size of the
wick, the multi-layer construction of the wick 14 is thus capable
of providing a capillary pressure gradually increasing from the
condensing section 123 toward the evaporating section 121, and a
flow resistance gradually decreasing from the evaporating section
121 toward the condensing section 123. Specifically, the third
layer 143 and the second layer 142 have a large average pore size
and therefore provide a relatively low resistance to the condensed
liquid to flow back, thereby effectively reducing the barriers the
condensed liquid encounters in returning back from the condensing
section 123 toward the evaporating section 121. The first layer
141, however, is constructed to have a small average pore size in
order to still maintain a relatively high capillary force for the
wick 14. Therefore, the wick 14 is capable of providing
simultaneously a relatively low flow resistance to the condensed
liquid when it flows at the condensing and central sections 123,
122 and a relatively high capillary pressure for drawing the
condensed liquid back from the condensing and central sections 123,
122 toward the evaporating section 121. As a result, the condensed
liquid is timely brought back to the evaporating section 121 in an
accelerated manner, thereby effectively avoiding dry-out problem
happening at the evaporating section 121.
[0017] FIG. 2 illustrates a heat pipe 20 according to a second
embodiment of the present invention. The heat pipe 20 includes a
casing 22 and a capillary wick 24 arranged at an inner surface of
the casing 22. The wick 24 is in the form of a multi-layer
structure which includes an outer layer 241, an intermediate layer
242 and an inner layer 243. These layers 241, 242, 243 are stacked
together along a radial direction of the casing 22 with the outer
layer 241 being connected to the inner surface of the casing 22.
Each layer of the wick 24 has an average powder size different from
that of the other layers, and these layers 241, 242, 243 are
stacked in such a manner that the average powder sizes thereof
gradually increase along the radial direction from the inner
surface of the casing 22 towards a central axis X-X of the casing
22. Since powder size defines pore size between particles of the
powder, the average pore sizes of these layers 241, 242, 243 also
gradually increase from the inner surface of the casing 22 towards
the central axis X-X of the casing 22. According to the
above-mentioned general rule that the capillary pressure of a wick
and its flow resistance to the condensed liquid increase due to a
decrease in pore size of the wick, the inner layer 243 and the
intermediate layer 242 have a large average pore size and therefore
are capable of providing a relatively low resistance to the
condensed liquid to flow back. The outer layer 241, however, has a
small average pore size and therefore is still capable of
maintaining a relatively high capillary force for the wick 24.
Thus, the multi-layer construction of the wick 24 is capable of
providing between these layers 241, 242, 243 along the radial
direction of the casing 22 a gradient of capillary pressure
gradually increasing from the central axis X-X of the casing 22
toward the inner surface of the casing 22, and a gradient of flow
resistance gradually decreasing from the inner surface of the
casing 22 toward a central axis X-X of the casing 22. Furthermore,
the small-sized outer layer 241 of the wick 24 is also capable of
maintaining an increased contact surface area with the inner
surface of the casing 22, as well as a large contact surface with
the working fluid saturated in the wick 24, to thereby facilitate
heat transfer between the heat pipe 20 and a heat source outside
the heat pipe 20 that needs to be cooled.
[0018] FIG. 3 illustrates a heat pipe 30 according to a third
embodiment of the present invention. Similar to the second
embodiment, the heat pipe 30 also has a multi-layer powder-based
capillary wick 34 arranged at an inner surface of the heat pipe 30.
The wick 34 includes an outer layer 341, an intermediate layer 342
and an inner layer 243 which are stacked along a radial direction
of the heat pipe 30 with the outer layer 341 being connected to the
inner surface of the heat pipe 30. These layers 341, 342, 343 have
different powder sizes to each other. In contrary to the second
embodiment, these layers 341, 342, 343 are arranged in an order
that the powder sizes thereof gradually decrease from the inner
surface of the heat pipe 30 towards a central axis Y-Y of the heat
pipe 30. Therefore, the large-sized outer layer 341 has a
relatively large pore size and accordingly develops a relatively
low resistance to the condensed liquid to return back. However,
this construction of the wick 34 is suitable for heat pipes with
relatively short lengths, in comparison with the wick 24.
[0019] The heat pipe 10 as disclosed in the first embodiment can be
made by using the method as illustrated in FIGS. 4-5. In order to
form the multi-layer capillary wick 14, three groups of powder are
prepared in advance with each group having an average powder size
different from that of the other groups. Firstly, a mandrel 40 is
inserted into the casing 12 with a space 50 formed between the
casing 12 and the mandrel 40. The three groups of powder are
sequentially filled into the space 50 with the later filled group
of powder being stacked on the earlier filled group of powder along
the longitudinal direction of the casing 12. The three groups of
powder are filled into the casing 12 in such an order that the
powder size of the later filled group of powder is larger than that
of the earlier filled group of powder so as to prevent the
particles of the later filled group of powder from falling into the
spaces formed between the particles of the earlier filled group of
powder. After all of these groups of powder are filled into the
casing 12, the casing 12 with the three groups of powder is subject
to heat with a high temperature, fox example, about 950 degrees
Celsius when the powder is copper powder, to thereby sinter the
three groups of powder together whereby the heat pipe 10 with the
multi-layer sintered powder wick 14 arranged along the inner
surface of the casing 12 is obtained.
[0020] The heat pipe 20 as disclosed in the second embodiment can
be made by using the method as illustrated in FIGS. 6-8. Three
groups of powder with different powder sizes from each other are
filled sequentially into the casing 22 at a location adjacent to
the inner surface of the casing 22 with the later filled group of
powder being stacked on the earlier filled group of powder along
the radial direction of the casing 22. The three groups of powder
are filled into the casing 22 in the order that the powder size of
the later filled group of powder is larger than that of the earlier
filled group of powder. When filling each group of powder, a
mandrel is used to control the thickness thereof. For example, as
illustrated in FIG. 6, a first mandrel 40a is inserted into the
casing 22 to control the thickness of the first group of powder,
which is to be formed as the outer layer 241 of the wick 24.
Likewise, second and third mandrels 40b, 40c with smaller diameters
than that of the first mandrel 40a are respectively and
successively used to control the thicknesses of the second and
third groups of powder, which are to be constructed as the
intermediate and inner layers 242, 243 of the wick 22,
respectively. In order to keep the powder in place and prevent,
after the corresponding mandrel 40a, 40b or 40c is drawn out, the
powder from dropping into the hollow space that is originally
occupied by the corresponding mandrel 40a, 40b or 40c, each group
of powder is pre-sintered at a suitable temperature, for example,
about 630 degrees Celsius in the context of copper powder, before
the corresponding mandrel 40a, 40b or 40c used to control the
thickness thereof is drawn out of the casing 22. Finally, all of
these groups of powder filled into the casing 22 together with the
casing 22 are sintered under a high temperature, for example, about
950 degrees Celsius to thereby obtain the heat pipe 20 with the
multi-layer sintered powder wick 24 arranged in the casing 22. It
is apparent that, if the filling order of the three groups of
powder into the casing 22 is reversed, this method is also suitable
for manufacturing the heat pipe 30 of the third embodiment. In this
situation, in order to prevent the later filled small-sized powder
from falling into the spaces defined between particles of the
former filled large-sized powder, partitioning means such as a
layer of polymeric bonding agent can be applied between every two
adjacent groups of powder. However, the bonding agent can be
decomposed by subsequently applying heat thereto.
[0021] It is to be understood, however, that even though numerous
characteristics and advantages of the present invention have been
set forth in the foregoing description, together with details of
the structure and function of the invention, the disclosure is
illustrative only, and changes may be made in detail, especially in
matters of shape, size, and arrangement of parts within the
principles of the invention to the full extent indicated by the
broad general meaning of the terms in which the appended claims are
expressed.
* * * * *