U.S. patent number 3,840,069 [Application Number 05/245,821] was granted by the patent office on 1974-10-08 for heat pipe with a sintered capillary structure.
This patent grant is currently assigned to Brown, Boveri & Cie AG. Invention is credited to Wilfried Fischer, Gregor Gammel.
United States Patent |
3,840,069 |
Fischer , et al. |
October 8, 1974 |
HEAT PIPE WITH A SINTERED CAPILLARY STRUCTURE
Abstract
In a heat pipe, a sintered capillary structure is provided on
both of its heat-receiving and heat-delivering surfaces with the
sintered capillary structure including both coarse and fine pores.
In a preferred arrangement the fine and coarse pores are each
distributed in a range of sizes with a maximum range of fine pores
of certain sizes in the range of the fine pores and a maximum range
of coarse pores of certain sizes in the range of the coarse pores.
The coarse pores are formed in the sintering operation while the
fine pores can be formed in a number of different ways. In one
method the fine pores are formed by using different sizes and
shapes of grains in the metal powder which is sintered. In another
method, a chemical treatment of the sintered structure provides the
fine pores.
Inventors: |
Fischer; Wilfried
(Neckargemund, DT), Gammel; Gregor (Dossenheim,
DT) |
Assignee: |
Brown, Boveri & Cie AG
(Mannheim, DT)
|
Family
ID: |
5806009 |
Appl.
No.: |
05/245,821 |
Filed: |
April 20, 1972 |
Foreign Application Priority Data
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Apr 27, 1971 [DT] |
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2120475 |
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Current U.S.
Class: |
165/104.26;
29/890.032 |
Current CPC
Class: |
F28D
15/046 (20130101); Y10T 29/49353 (20150115) |
Current International
Class: |
F28D
15/04 (20060101); F28d 015/00 () |
Field of
Search: |
;165/105 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Davis, Jr.; Albert W.
Attorney, Agent or Firm: Toren, McGeady and Stanger
Claims
What is claimed is:
1. A heat pipe comprising a closed tube having a heat-receiving
surface and a heat-delivery surface, a layer of capillary structure
sintered to the inside surface of said tube at least on the
heat-receiving surface and the heat-delivery surface therein and
another capillary structure interconnecting the heat-receiving and
the heat-delivery surface for liquid transport therebetween, said
tube and layer of capillary structure defining an enclosed space in
the heat pipe for transporting vapor between the heat-receiving
surface and the heat delivery surface, wherein the improvement
comprises that the layer of capillary structure consists of pores
of different sizes, and a vaporizable liquid filled into said tube
in a quantity to fill only the smaller sized pores by capillary
force, said pores are of a size in which capillary force takes
effect and consists of a first group having a range of pores of
relatively small sizes and a second group having a range of pores
of relatively large sizes and within each of said first group and
said second group there is a broad range of a maximum number of
relatively small pore sizes and a broad range of a maximum number
of relatively large pore sizes and the two maximum ranges are
spaced apart by a range of pore sizes which fall between the pore
sizes represented by the two ranges of maximum sizes, and the
quantity of said working liquid being just sufficient to fill the
relatively small sized pores in said first group because of the
greater capillary force exerted by such pores.
2. A heat pipe, as set forth in claim 1, wherein said pores in said
first group have approximately the same total volume as said pores
in said second group.
3. A heat pipe comprising a closed tube having a heat-receiving
surface and a heat-delivery surface, a layer of capillary structure
sintered to the inside surface of said tube at least on the
heat-receiving surface and the heat-delivery surface therein and
another capillary structure interconnecting the heat-receiving and
the heat-delivery surface for liquid transport therebetween, said
tube and layer of capillary structure defining an enclosed space in
the heat pipe for transporting vapor between the heat-receiving
surface and the heat-delivery surface, wherein the improvement
comprises that the layer of capillary structure consists of pores
of different sizes, and a vaporizable liquid filled into said tube
in a quantity to fill only the smaller sized pores by capillary
force, said pores comprise a first group of pores and a second
group of pores and the pores in said first group being distinctly
smaller and of substantially the same size as compared to the pores
in said second group which are of substantially the same size and
both of said first and second groups having a maximum number of
pores as compared to similarly sized other said pores in said
capillary structure.
Description
SUMMARY OF THE INVENTION
The present invention is directed to a head tube having a capillary
structure formed at least on its heat-receiving and heat-delivering
surfaces and, more particularly, it is directed to a sintered
capillary structure containing an arrangement of both fine and
coarse pores within the capillary structure.
Heat pipes have the characteristic of transferring large quantities
of heat with only a small temperature difference occurring in the
surfaces between which the heat is transmitted. The temperature
difference .DELTA.T which, though small, is not to be neglected in
conventional heat pipes, and is composed of the temperature drop
due to the heat conduction through the heat pipe wall in the heat
receiving zone, .DELTA.T.lambda., the temperature drop within the
heat pipe .DELTA.T.alpha. occurring as a result of the heat
transfer from the heat pipe wall to the working medium within the
heat pipe, the corresponding values of .DELTA.T.lambda.' and
.DELTA.T.alpha.' at the heat-delivering surfaces of the heat pipe,
and the temperature gradient .DELTA.T.sub.D which occurs during the
transport of the vaporized working medium through the interior of
the heat pipe.
While .DELTA.T.sub.D is in many cases on the order of one-tenth of
a degree and .DELTA.T.lambda. + .DELTA.T.pi.'T.lambda.is on the
order of several degrees, .DELTA.T.alpha. + .DELTA.T.alpha.' often
amount to a multiple of 10.degree.. Accordingly, to improve the
"heat conductivity" of heat pipes, it is especially important to
make the sum of .DELTA.T.alpha. and .DELTA.T.alpha.' as small as
possible. This temperature difference, which is due to heat
transfer to the liquid working medium can be reduced by enlarging
the heat transfer area. The enlargement of the heat transfer area
is provided in the interior of the heat pipe by means of a
capillary structure at the surface where heat transfer takes place
so that the ratio of heat transfer area inside the heat pipe to the
corresponding outside area, from which the heat is removed, is
greater than one. To a degree the desired effect is obtained in any
heat pipe, because the transport of the liquid working medium
requires a capillary structure, that is, the inner surface of the
heat pipe is increased due to the capillary structure. In heat
pipes which use grooves or threads in the inner surface of the pipe
for effecting the liquid transport or where a cellular structure is
used, the increase in the inner surface is slight and, as a result,
the reduction of .DELTA.T.alpha. and .DELTA.T.alpha.' is at most
relatively small. An additional factor in heat pipes using cellular
structures is that the heat contact between the lattice or grid
forming the cellular structure and the heat pipe wall or between
different superposed lattices or grids is generally poor, and the
increased inner surface area cannot be fully utilized because the
passage of heat to it is accomplished only imperfectly.
Therefore, it is the primary object of the present invention, to
provide a heat pipe construction where a substantial increase in
the heat transfer area is afforded with a resultant reduction in
the temperature difference between the heat-receiving and
heat-delivering surfaces, and with the temperature drop within the
working medium itself being reduced. Furthermore, the disadvantages
of known capillary structures can be avoided.
Another object of the invention is to provide different methods for
the production of the capillary structure within the heat tube for
achieving the increased heat transfer area.
In accordance with the present invention, the problem experienced
in the past is solved by only partially filling the capillary
structure with a working liquid.
A sintered capillary structure formed of a metal powder of the same
grain size exhibits a pore size distribution curve in which the
maximum number of particular sizes covers a wide range. As a
result, there is a certain, if small, number of pores of a smaller
diameter. Accordingly, sufficient working medium is supplied into
the tube to fill the smaller pores, while the larger pores remain
free of the working medium. It should be noted, however, that if
the quantity of working medium is too small, there is the danger of
the heat-receiving surface drying out.
To avoid the problem of too small a quantity of the working medium
being located at the heat-receiving surface, the capillary
structure is formed, at least in the heat-receiving and the
heat-delivering surfaces, so that the pore sized distribution curve
exhibits two pronounced maximum ranges of pore sizes.
By filling a suitable quantity of the working medium into the heat
pipe it can be arranged that the fine pores are filled with the
working liquid while the coarse pores do not contain any of the
liquid. In this way it is possible to provide a large heat transfer
area in the heat pipe surface for the working medium while, at the
same time, affording a large transfer area for the liquid-vapor
working medium.
The advantage of the capillary structure formed in accordance with
the present invention exists in that the temperature drop due to
heat transfer in the working medium is greatly reduced. This
reduction is accomplished, on the one hand, in that a large contact
area between the heat pipe wall and the liquid working medium is
provided and, on the other, a large evaporation surface is
provided. In particular, this advantage occurs in the range of
ebullient boiling, that is, at the relatively great heat flow
density, where the vapor bubbles formed in known capillary
structures must first penetrate through the porous structure with a
resultant pressure loss. Because of the uniform vapor pressure, a
higher temperature corresponds to the higher pressure at which the
vapor bubbles are formed, that is, a temperature drop occurs in the
porous layer. This disadvantage existing in known heat pipe
constructions is avoided by means of the present invention.
Further, in accordance with the present invention, there are three
different methods for producing the desired capillary structure in
which the pore size distribution curve contains two maximum ranges
of pore sizes.
In one method the capillary structure is formed by a sintered metal
powder in which one portion of the metal powder consists of an
alloy of two metals while the other portion consists of one of the
metals forming the alloy. After the sintering operation has been
completed, a chemical treatment of the capillary structure effects
a dissolution of the metal which is present only in the alloy with
a resultant formation of fine pores. Therefore, in addition to the
coarse pores formed in the sintering operation, the chemical
treatment results in the formation of the desired fine pores.
In another method the capillary structure can be formed by
sintering with a metal powder in which the grains are all of the
same material and of the same size with the fine pores being formed
by a chemical treatment after the sintering operation, for example,
by providing alternating oxidizing and reducing conditions.
Moreover, in the third method the metal powder to be sintered
consists of grains of two different sizes with the larger of the
grains having an oblong configuration and the smaller ones having a
spherical configuration.
The various features of novelty which characterize the invention
are pointed out with particularity in the claims annexed to and
forming a part of this disclosure. For a better understanding of
the invention, its operating advantages and specific objects
attained by its use, reference should be had to the accompanying
drawings and descriptive matter in which there are illustrated and
described preferred embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWING
In the drawing:
FIG. 1 is a cross sectional view of a portion of a heat pipe
illustrating a sintered capillary structure formed thereon in
accordance with the present invention;
FIG. 2 is a pore size distribution curve of the pores formed in the
sintered capillary structure in FIG. 1;
FIG. 3 is a graph of the heat flow density in two comparable heat
pipes based as a function at the .DELTA.T, with one heat pipe
having a conventional capillary structure and the other having a
capillary structure formed in accordance with the present
invention; and
FIG. 4 is a cross sectional view of a heat pipe with a capillary
structure formed in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1 a sintered capillary structure 2 is shown formed on the
inside surface of a heat tube 1. The sintered capillary structure 2
contains large pores 3 and small pores 4. In the sintering
operation, two fractions of a metal powder formed of the same
material as forms the heat tube, are mixed and sintered quickly on
the inner surface of the heat pipe 1. One of the fractions of the
metal powder is coarser than the other and consists of particles
having a size of about 1 mm. The other fraction is made up of
particles having a size of about 50 microns. Preferably, the
coarser particles have an oblong configuration while the finer
particles have a spherical configuration. File shavings, for
example, are suitable for the coarse particles. It has been found,
if the coarse particles are spherically shaped, the spaces between
them become filled or almost completely filled with the fine
particles so that no or only relatively few coarse pores are
formed.
In FIG. 2 a pore sized distribution curve is shown, with the pore
size indicated along the abscissa and the range or number of pores
of a particular size shown along the ordinate. The distribution
curve in FIG. 2 is illustrative of the distribution of the pores
shown in the capillary structure in FIG. 1.
As can be seen in FIG. 2, two ranges of maximum quantities of pores
are indicated by the curve one maximum quantity or range being
located in the portion of the curve representing the finer pores
and the other maximum range being located in the portion of the
curve indicating the coarser pores. When a quantity of liquid
working medium is introduced into the heat pipe which corresponds
to the free pore volume of the small pores, then, because of the
greater capillary force, only the small or fine pores become filled
with the liquid while the larger pores remain free of the liquid.
In FIG. 2 the liquid within the small pores is indicated by the
hatching. To obtain the desired effect, it is not absolutely
necessary that two maximum ranges of pore sizes be provided in the
pore size distribution curve, as is indicated in FIG. 2, rather it
suffices if the pore size distribution curve indicates a single
broad maximum of small pores so that only the small pores are
filled and not the larger pores.
A similar capillary structure having the same pore size
distribution curve is obtained in the following manner.
The capillary structure is formed by sintering a metal powder of
two components on the inner surface of the heat tube. The heat
particles of the two components can be the same but they need not
be. One of the components is a metal powder and the other component
consists of an alloy formed of two metals one of which is the same
metal as in the metal powder forming the other component. After the
sintering operation has been completed, providing the coarse pore
arrangement in the capillary structure, the finer or smaller pores
can be provided by means of a chemical treatment. For example, if
the first component is a nickel powder then the second component is
a nickel-aluminum alloy, after the sintering operation, the
aluminum can be dissolved out of the alloy with potash lye The
dissolution of the aluminum forms finer or smaller pores in the
capillary structure than the coarse pores formed in the sintering
process so that a combination of fine and coarse pores are
provided. The capillary structure as shown in FIG. 1, achieves its
desired purpose only if care is taken that neither too much nor too
little of the liquid working medium is contained in the capillary
structure. The proper loading of the capillary structure with the
working medium can be achieved in either of two ways. In the
optimum case, all of the fine pores are filled with the liquid
working medium while all of the coarse pores are left free. The
simplest method for accomplishing this loading is to fill the
precise amount of working medium into the heat pipe so that the
desired effect is achieved. When this is done, the fine pores are
filled with the working medium in accordance with that shown in the
distribution curve in FIG. 2.
As an alternative to charging the heat pipe with the working medium
as indicated above, in a horizontally arranged heat pipe in which
heat is supplied into one end and removed from the opposite end, a
sufficient amount of working medium is introduced so that the
working medium stands in the bottom of the tube a little bit above
the pores present in its walls. At the end faces the liquid rises
into the fine pores, however, but not into the coarse pores due to
the capillary action.
In FIG. 4 a heat pipe having a capillary structure in accordance
with the present invention is shown. The excess liquid working
medium 5 stands in a pocket or recess below the heat-receiving
surface of the heat pipe. The capillary structure 8 lines the
entire interior surface of the heat pipe 7. The supply of heat into
the heat pipe is illustrated by arrows 9 and the removal of heat
from the heat pipe is shown by the arrows 10. In this arrangement,
the working medium rises only into the fine pores of the capillary
structure at the points where heat is either supplied or
removed.
The reason the pore size distribution should correspond to that
indicated in FIG. 2, is as follows: theorectically, any pore
distribution is suitable for achieving the effect described herein.
Even if the pore size distribution has a very limited maximum
range, at least by filling with a suitable quantity of the liquid
working medium, the fine pores, those pores whose radius or size is
only slightly less than that of the coarse pores, become filled
while the coarse pores remain open or free of the working medium.
However, in practice the filling of the pores in the heat-receiving
and heat-delivering or discharging surfaces depends not only on the
quantity of the liquid charged into the tube and on the capillary
action but also on the quantity of heat supplied. The dependence on
these factors is particularly noted in capillary structures where
the pore size distribution has a very narrow maximum range. To be
able to operate at all times in the vicinity of the minimum
.DELTA.T, even at variable heat flow, it is important to obtain the
pore size distribution shown in FIG. 2.
To demonstrate the improvements obtained with the capillary
structure formed in accordance with the present invention,
measurements have been taken on two heat pipes each formed of
copper and each using water as the liquid working medium. In each
of the heat pipes the heat flow density in the heating zone, that
is, heat-receiving zone, was great and in the cooling zone or the
heat-delivering zone was small, accordingly, it suffices to
describe the effects in the heating zone.
In heat pipe 1 a 2 mm sintered layer of copper particles having a
grain size range of 125 to 250 microns was formed. The pore size
distribution curve was relatively pointed, that is it did not have
a broad maximum size range. Just enough water was placed in the
heat pipe so that all of the pores were full of water.
In heat tube 2, in the heating zone, a 2 mm sintered layer was
formed consisting of copper filings having an oblong shape with a
length of about 1.5 mm and copper particles of spherical shape with
a diameter of about 50 microns. Before the sintering operation, the
coarse and fine particles were mixed in a ratio by weight of
2:1.
The quantity of water was varied until the temperature difference
between a pore in the heat-receiving surface of the heat tube and
the interior of the heat tube assumed a minimum at constant heat
supply.
In FIG. 3 the temperature differences measured on heat tube 1 and
heat tube 2 in the heat-receiving surface are plotted as a function
of the heat flow density at the heat-receiving surfaces. As can be
seen in FIG. 3, the heat tube 2 greater heat flow densities were
attained at equal temperature differences. As or perhaps more
important in some instances, is the fact that at equal heating flux
densities the temperature difference .DELTA.T can be reduced. In
FIG. 3 the range between the upwardly extending dashed lines
represents ebullient boiling while the space to the left of the
left hand dashed line represents surface evaporation. It will be
evident from FIG. 3, by changing over from heat tube 1 to heat tube
2 the passage from the range of ebullient boiling to surface
evaporation is facilitated. In the region of the change-over from
surface evaporation to ebullient boiling the reduction in .DELTA.T
is greater because in this region the curve is flatter.
* * * * *