U.S. patent application number 10/163831 was filed with the patent office on 2003-07-31 for heat pipe and method of manufacturing the same.
This patent application is currently assigned to SAMSUNG ELECTRO-MECHANICS CO., LTD.. Invention is credited to Oh, Se Min, Vasiliev, Leonard.
Application Number | 20030141045 10/163831 |
Document ID | / |
Family ID | 27607050 |
Filed Date | 2003-07-31 |
United States Patent
Application |
20030141045 |
Kind Code |
A1 |
Oh, Se Min ; et al. |
July 31, 2003 |
Heat pipe and method of manufacturing the same
Abstract
Disclosed is a heat pipe comprising: an evaporating section, a
heat insulating section, a condensing section and a porous sintered
powder wick structure, in which the wick structure comprises
sub-structures different from one another in at least one selected
from group including material, shape and particle size, each of the
sub-structures being arranged into each of the evaporating, heat
insulating and condensing sections, in which the wick structure has
a biporous distribution made through sintering of a powder mixture
having various particle sizes to increase porosity and permeability
of the wick structure, and in which the heat pipe has an asymmetric
cross sectional shape in a radial direction. Powder having a large
particle size is readily inserted into the heat pipe to simplify
manufacture of the heat pipe while thermal conductivity of the heat
pipe is not degraded compared to a conventional structure which is
not eccentric.
Inventors: |
Oh, Se Min; (Suwon-Shi,
KR) ; Vasiliev, Leonard; (Minsk, KR) |
Correspondence
Address: |
DARBY & DARBY P.C.
805 Third Avenue
New York
NY
10022
US
|
Assignee: |
SAMSUNG ELECTRO-MECHANICS CO.,
LTD.
Suwon-Shi
KR
|
Family ID: |
27607050 |
Appl. No.: |
10/163831 |
Filed: |
June 5, 2002 |
Current U.S.
Class: |
165/104.26 |
Current CPC
Class: |
F28D 15/046
20130101 |
Class at
Publication: |
165/104.26 |
International
Class: |
F28D 015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 2002 |
KR |
2002-5405 |
Claims
What is claimed is:
1. A wick structure composed of a porous sintered powder wick and
arranged into a heat pipe which has functional sections including
an evaporating section, a heat insulating section and a condensing
section, the wick structure comprising: a method disposing
sub-structures different from one another in at least one selected
from group including material, shape and particle size, each of the
sub-structures being arranged into each of the evaporating, heat
insulating and condensing sections, in order to elevate thermal
conductivity, amount of heat transport and temperature-controlling
performance of the heat pipe.
2. The method in accordance with claim 1, further comprising adding
an additive such as Co(NH.sub.2).sub.2 inputted into sintering
powder to generate a gas through thermal decomposition of the
additive during sintering of a wick to increase porosity and
permeability of the wick structure.
3. The method in accordance with claim 1, further comprising an
arrangement of a biporous distribution in a radial direction of the
heat pipe asymmetrically through sintering of a powder mixture
having various particle sizes, to increase porosity and
permeability of the wick structure.
4. The method in accordance with claim 1, further comprising
manufacturing porous sintered powder wick composed of a powder
mixture which contains materials including copper, nickel,
graphite, carbon and diamond, each of the materials having shape
and thermal conductivity different from one another, to improve a
heat transfer ability of the heat pipe in a radial direction.
5. The method in accordance with claim 1, further comprising an
absorptive coating applied to the surface of the wick structure or
particles constituting the wick structure to increase an ability of
the wick structure absorbing a working fluid.
6. The method in accordance with one of claim 1 to 5, further
comprising an absorptive coating for increasing an ability of the
wick structure for absorbing a working fluid, the absorptive
coating is made of one selected from group including hydrates,
hydroxides, carbonates and LiBr.
7. The method in accordance with one of claim 1 to 5, wherein the
wick structure and a coating applied to the wick structure are
planar or cylindrical.
8. The method in accordance with one of claim 1 to 5, further
comprising an absorptive coating applied to the surface of the wick
sub-structure of the evaporating section of the heat pipe or
particles constituting the wick sub-structure of the evaporating
section of the heat pipe.
9. A heat pipe comprising an evaporating section, a heat insulating
section, a condensing section and a porous sintered powder wick
structure, wherein the wick structure comprises sub-structures
different from one another in at least one selected from group
including material, shape and particle size, each of the
sub-structures being arranged into each of the evaporating, heat
insulating and condensing sections, wherein the wick structure has
a biporous distribution made through sintering of a powder mixture
having various particle sizes to increase porosity and permeability
of the wick structure, and wherein the heat pipe has an asymmetric
cross sectional shape in a radial direction.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a heat pipe in use for heat
transfer, cooling and heat radiation, and more particularly, to an
internal structure of a Miniature Heat Pipe (MHP) and a method of
manufacturing the same.
[0003] 2. Description of the Related Art
[0004] Well known to those skilled in the art, a heat pipe
functioning to efficiently transfer heat from one place to another
is used as a critical part of a heat transporting apparatus. In
particular, an MHP can be effectively used for heat transport and
thermal diffusion for cooling a high density electronic circuit or
an electronic chip.
[0005] FIG. 1 is a sectional view illustrating an internal
structure of a heat pipe. As shown in FIG. 1, the heat pipe is
constituted of a wall in the outer side thereof, a channel for
flowing a working fluid for performing heat transport and a porous
wick structure provided between the wall and the channel for
regulating the working fluid to continuously perform heat
transport.
[0006] Further, the heat pipe is divided into an evaporating
section, a heat insulating section and a condensing section in
length as shown in FIG. 1.
[0007] The operational principle of the heat pipe is as follows:
When the working fluid saturated in the wick in the evaporating
section evaporates due to heat from an external heat source, vapor
moves toward the condensing section due to the difference of vapor
pressure to perform heat transport, and then cools and condenses in
the condensing section again to perform heat radiation. In this
case, condensed the working fluid is absorbed into the condensing
wick and then returns to the condensing section due to the
difference of capillary pressure between the condensing and
evaporating sections. Such motion and returning processes are so
repeated that heat continuously transfers from the evaporating
section into the condensing section.
[0008] In general, movement of the working fluid mainly depends on
the amount of heat transfer, the capillary pressure of the wick and
permeability as resistance against flow of the working fluid in the
wick.
[0009] The capillary pressure P.sub.c is determined according to
Equation 1: 1 P c = 4 cos d 0 , Equation 1
[0010] wherein d.sub.0 is mean hydraulic diameter, .sigma. is
surface tension coefficient and .theta. is wetting angle of the
wick.
[0011] The capillary pressure has the following relation as in
Equation 2:
P.sub.c=P.sub.v+P.sub.l+P.sub.g Equation 2,
[0012] wherein, P.sub.l, P.sub.v and P.sub.g mean pressure loss of
liquid channel, pressure loss of vapor channel and gravity
resistance, respectively.
[0013] The pressure loss of liquid channel and the pressure loss of
vapor channel are expressed, respectively, as in Equations 3 and 4
according to the Darcy's Law and the Equation of Poiselle: 2 P l =
Q l l ef l LS d 0 v , and Equation 3 P v = 128 Q v l ef D ch 4 v L
. Equation 4
[0014] Further, the pressure loss due to gravity resistance is
expressed as in Equation 5:
P.sub.g=.rho..sub.lgl sin .phi. Equation 5,
[0015] wherein g means gravitational constant.
[0016] Further, permeability k determining the migration resistance
of the working fluid in the wick has the following relation with
the porosity of the wick as in Equation 6: 3 k = f ( .PI. ) D 2 ,
Equation 6
[0017] wherein D means particle diameter.
[0018] Further, the quantity of thermal transport Q.sub.max due to
flow of thermal fluid is obtained on the following assumption.
[0019] Characteristic parameters of the wick in the heat pipe are
uniform, the sintered powder wick is saturated with the working
fluid, the evaporating and condensing sections have uniform heat
flux, saturated vapor having a temperature T.sub.s moves through a
vapor channel, liquid and gas have non-compressive fluid flow
expressed with the Navier-Stocks equation, vapor has no heat source
or cooling source, liquid flow within the porous wick follows the
Darcy's Law, frictional force at the vapor-liquid interface is very
small compared to liquid pneumatic resistance within the wick so as
to be disregarded, and the working fluid evaporates at the surface
of the evaporating section.
[0020] On the basis of the foregoing assumption, the amount of heat
transport Q is calculated as in Equation 7: 4 Q = L 4 l ef 4 cos d
0 - g l sin l l ( D p 2 - D ch 2 ) d 0 v + 32 v D 4 v . Equation
7
[0021] In the meantime, the heat pipe is restricted in the
performance thereof by viscous limit, capillary limit, entrain or
flooding limit, sonic limit and boiling limit.
[0022] Therefore, design parameters are determined considering the
foregoing working limits in designing the heat pipe. The viscous
limit and the boiling limit are considered, in particular, in a low
temperature heat pipe used at or under 200.degree. C. When the
evaporating section of the heat pipe undergoes dry out due to
overheating in order to improve working ability at thermal limit
conditions of the heat pipe, ability and time of the heat pipe for
recovering from the dry out are also considered.
[0023] The foregoing "dry out" means that the amount of heat
inputted into the heat pipe exceeds the maximum heat transport
Q.sub.max so that the amount of the working fluid evaporating at
the evaporating section exceeds the amount of the working fluid
returning to the evaporating section from the condensing section,
thereby leaving the evaporating section completely dry for a
certain time. The temperature of the evaporating section rises
rapidly and drops again as the working fluid returns to the wick so
that the heat pipe recovers the ability thereof. However, if a
function for recovering this ability is slow, the temperature
controlling ability of the heat pipe is disabled. Then, the
corresponding heat pipe cannot be used at or over the amount of
heat which is being inputted.
[0024] As shown in FIG. 1, the heat pipe has a longitudinal section
divided into a evaporating section, a heat insulating section and a
condensing section. In this case, it is preferred that the heat
pipe has the first partial wick structure which may elevate
capillary pressure and thermal conductivity, the heat insulating
section has the second partial wick structure having high
permeability, and the condensing section has the third partial wick
structure which may elevate the permeability and the thermal
conductivity.
[0025] In order to satisfy the foregoing requirements, a typical
heat pipe has a wick structure constituted into the following four
configurations, or combined configurations or variations thereof.
The configurations have the following characteristics together with
advantages and disadvantages.
[0026] Metal sintered powder wicks have an excellent fluid
transport ability against gravitational resistance due to a large
value of capillary head, excellent thermal conductivity due to a
fin effect of porous metal sintered powder. Further, rapid
temperature elevation rarely takes place since the viscous limit
gradually takes place. However, the metal sintered powder wicks
have a large amount of pressure loss occurring in movement of the
working fluid due to a small value of permeability.
[0027] Grooved wicks have a small pressure loss in movement of the
working fluid due to a large permeability. In particular, it is
advantageous in price since a simple grooved wick can be integrally
manufactured in manufacture of a heat pipe envelope or container.
However, the simple groove wick has drawbacks that capillary
pressure is small due to a large capillary diameter, working
ability is inferior in a partially superheated-dry state, and
viscous limit occurs abruptly thereby resulting in rapid
temperature growth.
[0028] Fine fiber bundle wicks have characteristics that capillary
pressure is large, but permeability is small and thus pressure loss
is large in movement of the working fluid, and working ability is
inferior in a partially superheated-dry state.
[0029] In mesh screen wicks, capillary pressure is about in the
middle and permeability is small so that pressure loss is large in
movement of the working fluid as well as thermal resistance is
large.
[0030] The foregoing basic wick structures cannot be compared on
the basis of a single criterion since they have their own
advantages and disadvantages and can be modified into structures
which can complement the disadvantages. However, the sintered
powder wicks are preferred to other wicks with regard to heat
transport ability and ability against gravitational resistance
which are the basic performances of the heat pipes. The sintered
powder wicks have a very dense inter-particle structure causing the
capillary pressure thereof to be larger than that of the grooved
wicks or the mesh screen wicks and the thermal conductivity to be
higher than that of the mesh screen wicks thereby to show a
relatively large heat flux.
[0031] The sintered powder wicks have a more excellent working
ability against gravitational resistance compared to other wick
structures such as the grooved or mesh screen wicks. However, the
sintered powder wicks are not superior in the maximum heat transfer
due to a large amount of liquid pneumatic resistance.
[0032] Further, those structures applied to the conventional
sintered powder wicks generally have a single porous structure.
Therefore, the MHP requires metal powder having a relatively large
particle size in order to increase the permeability of the wick.
However, the pore size of the wick is not optimized due to problems
of the internal structure and a manufacturing process of the wick
so that the basic relative superiority of the sintered powder wick
is not sufficiently utilized.
[0033] Therefore, for the purpose of obtaining the optimized shape
of the wick as above, the U.S. Pat. No. 6,056,044 proposes a wick
structure which uses microscopic multi-capillary tubes via the MEMS
to have different particle sizes so as to improve the capillary
pressure and the permeability.
[0034] However, in the foregoing structure, the manufacturing
process is sophisticated and accordingly the manufacturing cost is
elevated. In other words, after a bonding agent is coated on
underlying mesh screens, another mesh screens are scrolled into the
multiple pipes thereby making the manufacture of the multiple pipes
difficult.
[0035] In order to overcome the foregoing problems, it is proposed
that wick structures belonging to functional components have
difficulties from one another with pore size, pore shape, thermal
conductivity and absorbing ability of the working fluid. However, a
powder mixture having different particle sizes is hardly
constructed into a biporous wick in practice.
[0036] The above problem is caused due to the fact that powder
having a large particle size can be hardly inserted into the MHP
considering that the inside diameter of the outer wall of the
conventional MHP is limited with size.
SUMMARY OF THE INVENTION
[0037] Accordingly the present invention is proposed to solve the
foregoing problems in regard to a conventional MHP having a single
wick or a multi-capillary tube structure, and it is an object of
the invention to provide a porous sintered powder wick structure
having sub-structures, which are different in material, shape or
particle size from one another adequate to requirements of
evaporating, heat insulating and condensing sections, so as to
increase porosity and permeability of the wick structure.
Therefore, in order to arrange the wick sub-structures different in
material, shape or particle size from one another, a mixture of
such powder undergoes sintering to have a biporous structure
thereby providing a heat pipe of an eccentric structure having a
radially asymmetric sectional shape.
[0038] Further, the invention proposes a method for improving
ability and time for recovering from dry out due to overheating in
an evaporating section of the heat pipe in order to improve working
ability in occurrence of thermal limit conditions.
[0039] Moreover, the invention proposes a method for minimizing the
pore size of the sintered powder wick by rapidly recovering the
heat pipe function from such an superheated dry state and forming
an absorptive coating on the surface of the wick in the evaporating
section by adding a hydrate into the working fluid.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 illustrates the structure of a heat pipe in the
related art;
[0041] FIG. 2 is a graph illustrating the mean hydraulic pore
diameter and permeability;
[0042] FIG. 3 is a graph illustrating the optimum size of wick
pores in respect to the working temperatures of a heat pipe for a
fixed amount of heat transport thereof;
[0043] FIG. 4 is a graph illustrating the relation between vapor
channel diameters D.sub.ch and working temperatures;
[0044] FIG. 5 is a graph illustrating the relation between the
maximum amounts of heat transport Q.sub.max and working
temperatures;
[0045] FIG. 6 is a graph illustrating the relation between the mean
pore size and the wick pore diameter;
[0046] FIG. 7 is a graph illustrating a wick structure having an
asymmetric radial cross section;
[0047] FIG. 8 is a graph illustrating thermal conductivities
radially measured in the outer diameter surface of the heat pipe
having the wick structure of the asymmetric cross section shown in
FIG. 7;
[0048] FIG. 9 illustrates a biporous structure of main pores;
[0049] FIG. 10 illustrates a wick structure in which a liquid or
solid compound is added into copper powder having fine particle
size;
[0050] FIG. 11 illustrates a wick structure made of metal powder
and carbon fiber;
[0051] FIG. 12 illustrates a hydrate which is coated on surface
particles of an evaporating section wick of a heat pipe having a
rectangular cross section;
[0052] FIG. 13 illustrates a sintered wick structure made of a
powder mixture in which copper powder is mixed with crystal powder
of nickel, graphite or diamond;
[0053] FIG. 14 is a sectional view illustrating a heat pipe having
sections with their own wick structures different from one another
in length; and
[0054] FIG. 15 illustrates a planar wick structure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0055] In order to obtain the foregoing objects, the present
invention proposes a heat pipe comprising an evaporating section, a
heat insulating section, a condensing section and a porous sintered
powder wick structure, in which the wick structure comprises
sub-structures different from one another in at least one selected
from group including material, shape and particle size, each of the
sub-structures being arranged into each of the evaporating, heat
insulating and condensing sections, in which the wick structure has
a biporous distribution made through sintering of a powder mixture
having various particle sizes to increase porosity and permeability
of the wick structure, and in which the heat pipe has an asymmetric
cross sectional shape in a radial direction.
[0056] Further, the optimum condition for improving performance
within the wick will be presented to preferably carry out the
invention and a method thereof will be described.
[0057] The application will design and analyze the relation between
the capillary head, permeability and pore capacity of a porous
medium and the thermal flow of a working fluid in order to improve
the performance of the sintered powder wick, and accordingly derive
the optimum pore size and the particle size of the metal powder for
realizing the optimum pore size. The optimum design policy will be
described as follows for the optimum conditions for carrying out
wick sintering of each functional component of the heat pipe.
[0058] In other words, although the sintered powder wick
advantageously has a capillary pressure larger than that of a
grooved wick or mesh screen wick and a thermal conductivity higher
than that of the mesh screen wick thereby showing a relatively
large heat flux, it is necessary to optimize working parameters of
the wick in order to design a heat pipe with excellent performance
by utilizing those advantages while compensating disadvantages such
as a lower permeability.
[0059] Examples of pore parameters for constructing the optimized
porous wick may include the size and shape of particles, the
specific volume, pore diameter and porosity of the porous wick,
which cooperate with one another to influence the design of the
heat pipe.
[0060] The present invention represents that the permeability k in
Equation 6, which is an important design parameter of the heat
pipe, can be experimentally obtained according to Equation 8:
k=0.00144d.sub.0.sup.179 Equation 8,
[0061] wherein d.sub.0 indicates mean hydraulic diameter.
[0062] Other thermal fluid parameters can be obtained through
experiments as follows.
[0063] The capillary pressure can be obtained through a test of a
porous medium specimen of an equivalent pore diameter.
[0064] The liquid hydraulic head can be obtained through
measurement in the wick.
[0065] The permeability can be obtained through the liquid
hydraulic head measurement and the Darcy's Law.
[0066] The heat flux can be obtained by evaluating mass flux in an
evaporating process through calculation of two-phase pressure
loss.
[0067] The wick porosity can be obtained through
measurement/evaluation of the thermal conductivity of the wick
saturated with liquid.
[0068] The heat flux determining the amount of heat transport of
the heat pipe mainly depends on conditions for applying the heat
pipe as follows. Examples of the conditions may include the
distance between the evaporating section and the condensing
section, superheating of a wall of the heat pipe, subcooling of a
working fluid, a thermal contact status between a heat source and
the wick.
[0069] As described above, working parameters of a specific heat
pipe can be designed on the basis of the working parameters of the
wick and information about the conditions for applying the heat
pipe.
[0070] For example, when a cylindrical MHP has a length l, an outer
diameter D.sub.p of 4 mm, an inner diameter or vapor channel
diameter D.sub.ch of 2 mm and a sintered wick cross section of S,
in which the length of an evaporating section is l.sub.e, the
length of a heat insulating section is l.sub.t, and the length of a
condensing section is l.sub.c, the maximum heat transport Q.sub.max
can be obtained according to Equation 7.
[0071] Although the amount of heat transport Q of the invention
mainly depends on the vapor channel diameter D.sub.ch in a vapor
channel of the actual heat pipe and the mean hydraulic pore
diameter in a liquid channel, the maximum heat transport Q.sub.max
is varied according to a temperature in the heat insulating section
of the heat pipe T.sub.sat (or working temperature) due to
temperature dependency of thermal-physical characteristics of the
working fluid. Further, Q.sub.max is varied by a large amount in
respect to the inclination angle of the heat pipe installed about
the gravitational field.
[0072] In general, on the basis of horizontal installation
(.PHI.=0.degree.), .PHI. is expressed--when the evaporating section
is arranged over the condensing section but+ when the former is
arranged under the latter. When .PHI. is -90.degree., Q.sub.max is
restricted by the largest amount from gravitational resistance.
[0073] Based upon the foregoing principles, design/analysis results
about main design parameters of a sintered copper powder wick about
the MHP can be expressed as in FIGS. 2 to 6.
[0074] From the relation between the mean hydraulic pore diameter
and the permeability in FIG. 2, it can be seen that the
permeability increases due to increase in the pore size of the
wick. However, since the capillary pressure decreases due to
increase in the pore size, the invention employs a wick structure
using a metal powder having a biporous distribution or different
particle shapes or mixed with fiber in order to prevent degradation
of the capillary head. By using the method as above, reduction of
the capillary pressure can be minimized while the permeability of
the wick is enlarged.
[0075] FIG. 3 is a graph illustrating the optimum size of the wick
pores in respect to the working temperatures of the heat pipe for a
fixed amount of heat transport thereof. It can be understood that
the optimum pore size is 100 to 160 .mu.m from FIG. 3.
[0076] FIG. 4 is a graph illustrating the relation between the
vapor channel diameters D.sub.ch and the working temperatures, and
FIG. 5 is a graph illustrating the relation between the maximum
amounts of heat transport Q.sub.max and the working
temperatures.
[0077] FIG. 6 is a graph illustrating that the wick pores sized as
above can be made through sintering of copper powder having
particle sizes of 300 to 500 .mu.m. However, copper powder having
such a large particle size can be hardly filled between a copper
envelope or container having an outer diameter of 4 mm and an iron
core having an outer diameter of 2 mm installed in the center of
the copper envelope in manufacture of the heat pipe. Therefore, it
has been difficult to optimize the pores in the porous sintered
powder wick applied to the MHP in the related art.
[0078] Therefore, the invention proposes the first method for
realizing the optimum hydraulic diameter of the wick pore, in which
the iron core is asymmetrically installed from the radial center of
the hollow copper pipe and copper powder filled therebetween
undergoes sintering. In particular, the radial cross section is
provided asymmetric, as shown in FIG. 7, to optimize the wick
pore.
[0079] FIG. 8 is a graph illustrating thermal conductivities
radially measured in the outer diameter surface of the heat pipe
having the wick structure as above. As shown in FIG. 8, it can be
seen that heat transfer heavily takes place at a portion having a
relatively smaller wick thickness. When applied to the heat pipe,
this selectively applies a contact surface between the heat source
and a heat sink in the evaporating section and the condensing
section thereby providing an additional function of raising a heat
transfer efficiency.
[0080] As the second method of realizing the optimum hydraulic
diameter of the wick pore, the invention proposes a wick having
biporous structure which is obtained through sintering of mixed
copper powders having different particle sizes.
[0081] FIG. 9 illustrates a biporous structure of main pores.
[0082] As the third method for realizing the optimum hydraulic
diameter of the wick pore, the invention proposes a method for
adding a liquid or solid additive into particulate copper powder
and enlarging the pore size among copper powder particles by using
a gas which is generated when the additive undergoes thermal
reaction or thermal decomposition at a temperature lower than a
sintering temperature of copper powder in a sintering process of
the wick. The additive for enlarging the permeability of the wick
is sufficiently melted and cleared but may reside by a very small
amount. Therefore, it is required that the additive does not
generate gas through thermal reaction with components of the wick
and the working fluid. Examples of the additive satisfying the
above characteristic may include Co(NH.sub.2).sub.2. The shape of
the wick manufactured according to the above method is shown in
FIG. 10.
[0083] As a method for increasing the permeability, the capillary
pressure and the heat transfer rate of the heat pipe wick, the
invention proposes a wick structure which is manufactured by using
a powder mixture made of copper powder and smashed copper
(graphite) or cellulose (coconut shells, peach pits and the like)
or a powder mixture of copper powder and Polyvinylidene Chloride
(PVDC) as a kind of non-cellulose.
[0084] FIG. 11 illustrates a wick structure made of metal powder
and carbon fiber. As shown in FIG. 11, various sizes of pores are
distributed in the wick thereby improving the capillary pressure
and the permeability of the wick while the carbon fiber enhancing
the thermal conductivity.
[0085] As a method for reducing a recovery time of the heat pipe
from a partial dry out status of the wick due to increase of input
heat into the evaporating section of the heat pipe, the invention
proposes a method for adding an absorptive or absorbent material
into the working fluid for absorbing the same. When the working
fluid is water, examples of the material having the above
capability may include hydrate such as MnCl.sub.2, NiCl.sub.2,
CaCl.sub.2, BaCl.sub.2 and LiBr. Such hydrate exists in the form of
a water solution of the working fluid such as water at a room
temperature until the wick in the evaporating section is heated.
When the wick in the evaporating section is heated, the hydrate is
separated from the water solution and coated on the particle
surface of the evaporating section wick as shown in FIG. 12. Then,
the hydrate absorbs water again to assist return or supply of the
working fluid into the evaporating section wick. As described
above, almost of the hydrate ingredient added into the working
fluid is coated on the surface of the evaporating wick thereby to
accelerate reflow of the working fluid toward the evaporating
section from the condensing section.
[0086] FIG. 12 illustrates a hydrate which is coated on surface
particles of the evaporating section wick of the heat pipe having a
rectangular cross section. Such an additive accelerates a recovery
time of the evaporating section from a dried status due to
overheating, thereby to enhance temperature controlling features
and working limits of the heat pipe.
[0087] The wick can undergo sintering by using a powder mixture of
different metals in order to raise the thermal conductivity of the
wick.
[0088] FIG. 13 illustrates a sintered wick structure made of a
powder mixture in which copper powder is mixed with crystal powder
of nickel, graphite or diamond. In such a wick, the evaporating and
condensing sections have thermal conductivities elevated in the
radial direction thereby improving the heat exchange performance of
the heat pipe.
[0089] Further, in order to maximize the heat transport ability of
the heat pipe while maximizing the heat transfer performance
thereof with the outside, the invention optimizes the
characteristics of the heat pipe by applying a wick having
different sub-structures, each of which is adequate to a function
of each functional component of the heat pipe. Therefore, the
invention applies the first sub-structure for elevating the
capillary pressure and the thermal conductivity to the evaporating
section, the second sub-structure having a high permeability to the
heat insulating section and the third sub-structure for elevating
the permeability and thermal conductivity to the condensing section
as shown in FIG. 14.
[0090] The above structures can apply the metal powder mixtures
having different particle sizes or different kinds such as copper
and nickel or carbon fiber to the each functional component of the
heat pipe through sintering while utilizing the above
characteristics.
[0091] Further, the above wick structures and coats can employ any
of planar and cylindrical structures.
[0092] FIG. 15 illustrates a planar wick structure which has a
rectangular cross section.
[0093] As described above, the inventive heat pipe has the biporous
structure which is optimized to the sintered powder wick so that
the maximum heat transport is improved by a large amount. For
example, when the inventive structure is applied to a heat pipe
having an outer diameter of 4 mm, the heat transport ability is
improved for 1.3 times over a conventional sintered powder wick and
two times over a conventional grooved wick.
[0094] Further, the maximum heat transport ability and the ability
of resisting against gravity are enhanced to increase the
difference from conventional products.
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