U.S. patent number 3,681,843 [Application Number 05/017,117] was granted by the patent office on 1972-08-08 for heat pipe wick fabrication.
This patent grant is currently assigned to Westinghouse Electric Corporation. Invention is credited to Frank G. Arcella, Russell E. Brumm.
United States Patent |
3,681,843 |
Arcella , et al. |
August 8, 1972 |
HEAT PIPE WICK FABRICATION
Abstract
An economical heat pipe wick fabrication technique that yields
wicks with fine pores at the liquid/vapor interface and
unrestricted fluid flow beneath this interface. The resulting wick
may be employed with either high or low thermal conductivity
fluids.
Inventors: |
Arcella; Frank G. (Bethel Park,
PA), Brumm; Russell E. (Pittsburgh, PA) |
Assignee: |
Westinghouse Electric
Corporation (Pittsburgh, PA)
|
Family
ID: |
21780821 |
Appl.
No.: |
05/017,117 |
Filed: |
March 6, 1970 |
Current U.S.
Class: |
29/423;
29/890.032; 165/104.26 |
Current CPC
Class: |
F28D
15/046 (20130101); B21C 37/151 (20130101); Y10T
29/4981 (20150115); Y10T 29/49353 (20150115) |
Current International
Class: |
F28D
15/04 (20060101); B21C 37/15 (20060101); B23p
017/00 () |
Field of
Search: |
;29/423,424,DIG.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Campbell; John F.
Assistant Examiner: Rooney; Donald P.
Claims
We claim:
1. A process for fabricating a heat pipe wick having channels just
beneath the fine pore surfaces, which comprises winding several
turns of fine mesh screen over a mandrel, placing a plurality of
fine wires axially along said mandrel over said fine mesh screen
and inserting the resulting assembly into an outer tube which
closely receives said assembly in the center thereof, constricting
the final assembly so that the mesh screen conforms to the shape of
the axially laid wires and outer tube at the wire/mesh and
tube/mesh interface respectively and then dissolving away said
mandrel and said wires.
2. The process for fabricating a heat pipe wick of claim 1 wherein
said mandrel and said wires are formed from copper and said fine
mesh material and said outer tube are chosen to be insoluble in
nitric acid.
3. The process for fabricating a heat pipe wick of claim 2 wherein
said dissolving step comprises etching away said copper wires and
said copper mandrel in nitric acid.
4. The process for fabricating a heat pipe wick of claim 1 wherein
said mandrel and said wires are formed from aluminum and said fine
mesh material and said outer tube are chosen to be insoluble in
sodium hydroxide.
5. The process for fabricating a heat pipe wick of claim 4 wherein
said dissolving step comprises etching away said aluminum wires and
said aluminum mandrel in sodium hydroxide.
6. The process for fabricating a heat pipe wick of claim 1 wherein
said fine mesh screen is constructed from a material selected from
the group consisting of 304 stainless steel and pressed and
sintered felt metal.
7. The process for fabricating a heat pipe wick of claim 1 wherein
said constricting step comprises swaging said final assembly.
8. The process for fabricating a heat pipe wick of claim 1 wherein
said constricting step comprises expanding said mandrel against
said rigid outer tube.
9. The process for fabricating a heat pipe wick of claim 1
including winding several turns of fine mesh screen over said
axially laid wires, constricting said final assembly so that the
mesh screen closely conforms to the axially laid wire surface at
the wire/mesh interface and forms a continuous mesh cross-section
at the mesh/mesh interface and dissolving away said outer tube
concurrently with said wires and said mandrel.
10. The process for fabricating a heat pipe wick of claim 9 wherein
said mandrel, said wires and said outer tube are formed from copper
and said fine mesh material is chosen to be insoluble in nitric
acid.
11. The process for fabricating a heat pipe wick of claim 10
wherein said dissolving step comprises etching away said copper
wires, said copper mandrel and said copper outer tube in nitric
acid.
12. The process for fabricating a heat pipe wick of claim 9 where
said mandrel, said wires and said outer tube are formed from
aluminum and said fine mesh material is chosen to be insoluble in
sodium hydroxide.
13. The process for fabricating a heat pipe wick of claim 12
wherein said dissolving step comprises etching away said aluminum
wires, said aluminum mandrel and said aluminum outer tube in sodium
hydroxide.
14. The process for fabricating a heat pipe wick of claim 9 wherein
said fine mesh screen is constructed from a material selected from
the group consisting of 304 stainless steel and pressed and
sintered felt metal.
Description
BACKGROUND OF THE INVENTION
This invention pertains to heat pipe wicks, and more particularly
to a new economical heat pipe wick fabrication technique.
Heat pipe wicks of the prior art are fabricated in many ways. For
liquid metal working fluids, several critical properties must be
retained. The wick liquid/vapor interface must possess extremely
small pore sizes for optimum capillary drawing forces. This can be
seen from the following equation:
.DELTA.P.sub.cap .DELTA.P.sub.vap + .DELTA.P.sub.liq ,
Where .DELTA.P.sub.cap is the pressure differential due to
capillary action, .DELTA.P.sub.vap is the pressure drop in the
vapor region and .DELTA.P.sub.liq is the liquid pressure drop
experienced in the wick. Thus, for effective heat pipe operation,
the pressure differential due to capillary action,
.DELTA.P.sub.cap, must be equal to or exceed the sum of the vapor
and liquid pressure drops experienced in the vapor region and the
wick respectively. The greater the difference, the greater the heat
transfer capability of the heat pipe. If the wick were totally
manufactured from fine pore material, the liquid friction flow
factor, .DELTA.P.sub.liq, would be excessively large due to the
restrictions to flow.
The prior art has retained the aforementioned critical properties
by fabricating channels into the heat pipe walls by a broaching
process. The channels, which permit unrestricted fluid flow from
the heat pipe condenser to the evaporator section, are covered by a
fine mesh screen to establish greater capillary wicking forces.
Composite wicks have also been fabricated by placing layers of
heavy mesh screen (30 to 60 mesh) beneath the liquid/vapor
interface layers of fine mesh screen (200 to 400 mesh). Another
technique comprises the fabrication of open annulus wicks by
swaging several turns of screen wound between two copper tubes. The
copper tubes are then etched away and the porous rigid wick is
sinter bonded. Upon insertion into a heat pipe with an open annulus
between the wick and the heat pipe walls, this wick presents an
optimum arrangement for liquid metal charged heat pipes. The first
two of the aforementioned techniques have the disadvantages of
being both uneconomical and time consuming and the latter technique
is only suitable for liquid metal working fluids, since a low
thermal conductivity fluid would boil beneath the capillary drawing
free wick. Although other wick structures have been fabricated, the
three mentioned above are the ones most frequently employed.
SUMMARY OF THE INVENTION
Briefly, this fabrication technique comprises winding several turns
of fine mesh screen onto a cylindrical mandrel. Fine wires are then
positioned axially along the mandrel over the fine mesh screen.
Additional layers of fine mesh screen are wound over the assembly
and it is inserted into an outer tube. The final assembly is swaged
and the mandrel, wires and outer tube are dissolved away.
This process may also be used to replace the costly broaching
process used to fabricate channels in a heat pipe container tube. A
similar wick can be fabricated by utilizing a fine mesh
screen/mandrel assembly which is covered with axial wires and
inserted into an outer tube and swaged. Dissolving away the mandrel
and wires leaves a tight homogeneous, channeled wick.
Thus, the present fabrication technique provides fine pore sizes at
the wick liquid/vapor interface and channels immediately below this
interface for unrestricted fluid flow (low friction flow). The
resulting process is economical and produces a wick that can be
employed with either high or low thermal conductivity fluids.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of an exemplary embodiment of this
invention, reference may be had to the accompanying drawings, in
which:
FIG. 1 is an isometric view of a heat pipe, having a portion
thereof cut away for clarity, and including an enlarged
longitudinal sectional view of a portion of a heat pipe wick
constructed in accordance with the teachings of this invention;
FIG. 2 is an isometric view of a heat pipe wick assembly broken
away in layers, and is illustrative of an interim stage of the wick
assembly during fabrication of the heat pipe wick of FIG. 1;
FIG. 3 is a cross-sectional view of the heat pipe wick assembly of
FIG. 2 and is taken along the lines III--III thereof;
FIG. 4 is a cross-sectional view of the heat pipe wick of FIG.
1;
FIG. 5 is an isometric view of a heat pipe wick assembly broken
away in layers and is illustrative of an interim stage of the wick
assembly during fabrication of another embodiment of a heat pipe
wick constructed in accordance with the teachings of this
invention; and
FIG. 6 is a cross-sectional view of another embodiment of a heat
pipe wick constructed in accordance with the teachings of this
invention and is illustrative of the final wick derived from the
interim assembly illustrated in FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the heat pipe illustrated in FIG. 1, it will be
appreciated that a heat pipe 10 constructed in accordance with the
principles of this invention includes an evacuated chamber 12 whose
inside walls are lined with a capillary structure, or wick 30, that
is saturated with a volatile fluid. The operation of a heat pipe
combines two familiar principles of physics; vapor heat transfer
and capillary action. Vapor heat transfer serves to transport the
heat energy from the evaporator section 14 at one end of the pipe
to the condenser section 16 at the other end. Capillary action
returns the condensed working fluid back to the evaporator section
14 to complete the cycle.
The working fluid absorbs heat at the evaporator section 14 and
changes from its liquid state to a gaseous state. The amount of
heat necessary to cause this change of state is the latent heat of
vaporization. As the working fluid vaporizes, the pressure at the
evaporator section 14 increases. The vapor pressure sets up a
pressure differential between the ends of the heat pipe, and this
pressure differential causes the vapor, and thus the heat energy,
to move towards the condenser section 16. When the vapor arrives at
the condenser section 16, it is subjected to a temperature slightly
lower than that of the evaporator section 14 and condenses, thereby
releasing the thermal energy stored in its heat of vaporization at
the condenser section 16 of the heat pipe. As the vapor condenses
the pressure at the condenser section 16 decreases so that the
necessary pressure differential for continued vapor heat flow is
maintained.
Movement of the fluid from the condenser section 16 back to the
evaporator section 14 is accomplished by capillary action within
the wick 30 which connects the condenser 16 to the evaporator
14.
As is known, the driving force that causes the liquid to move
through the capillary is the surface tension of the liquid. When a
fluid is placed in a compatible vessel, that is a vessel composed
of a material that the fluid wets well, there is an attractive
force between the fluid and the walls of the vessel. This force
combines with the surface tension in such a way as to move the
liquid towards the unfilled portion of the vessel. If the vessel is
a capillary of small diameter, such as the wick 30, this force,
called capillary attraction, can be large compared with the mass of
fluid in the capillary. The resulting forces will thus cause the
liquid to pump itself through the wick indefinitely in the absence
of other forces. It is the use of vapor pressure and capillary
action that enables the heat pipe to operate as a self-contained
heat pump.
For liquid metal working fluids the wick 30 must retain several
critical properties. For example the wick liquid/vapor interface 32
must possess extremely small pore sizes for optimum capillary
drawing forces. This can be seen from the following equation;
.DELTA.P.sub.cap .DELTA.P.sub.vap + .DELTA.P.sub.liq ,
where .DELTA.P.sub.cap is the pressure differential due to
capillary action, .DELTA.P.sub.vap is the pressure drop in the
vapor region and .DELTA.P.sub.liq is the liquid pressure drop
experienced in the wick. Thus, for effective heat pipe operation,
the pressure differential due to capillary action,
.DELTA.P.sub.cap, must equal or exceed the sum of the vapor,
.DELTA.P.sub.vap, and liquid, .DELTA.P.sub.liq, pressure drops
experienced in the vapor region and wick respectively. If the wick
were totally manufactured from fine pore material, the liquid
friction flow factor, .DELTA.P.sub.liq, would be excessively large
due to the restrictions to flow. A more detailed explanation of
heat pipe operation is presented in the May, 1968 issue of
"Scientific American," in an article entitled "The Heat Pipe," by
Y. Eastman.
This invention provides an economical heat pipe wick fabrication
technique that retains the aforementioned critical properties by
providing fine pore sizes at the liquid/vapor interface 32 and
channels 34 immediately below this interface for unrestricted fluid
flow (low friction flow).
Referring now to the heat pipe wick assembly illustrated in FIGS. 2
and 3, it will be appreciated that a heat pipe wick assembly 35
constructed in accordance with the principles of this invention
includes several turns of fine mesh screen 36, from approximately
200 to 500 mesh (or higher mesh), desirably constructed from a
material that is relatively resistant to being etched away in the
chosen etchant specified below, such as 304 stainless steel or
pressed and sintered felt metal, which are wound onto a central
mandrel 38, desirably constructed from a material that can be
etched away in the chosen etchant. The mandrel 38 may be designed
so as to assume any desired shape depending upon the desired shape
of the wick being fabricated, but it is to be understood that the
shape of the mandrel is not to be limited to the hollow circular
cylindrical configuration illustrated by reference character 38.
Fine wires 40, desirably constructed from a material that will
dissolve in the chosen etchant, are laid axially along the mandrel
38 over the fine mesh screen 36. The size of the wires will depend
upon the working fluid being employed in the heat pipe. For
example, for a working fluid such as sodium the diameter of the
wires may vary approximately from 15 to 25 mills. Additional layers
of fine mesh screen 42, approximately from 200 to 400 mesh,
desirably formed from the same material as the mesh screen 36, are
wound over the assembly 35 and inserted into an outer tube 44,
which is constructed from a material having the same
characteristics as that of the central mandrel 38, and which
closely receives the assembly 35 in the center thereof. The
assembly is then shaped by swaging or by any other process that
uniformly constricts the interface between the two mesh layers 36
and 42 so that they closely conform to the axially laid wire
surface at the wire/mesh interface and form a continuous mesh
cross-section at the mesh/mesh interface. Such a process may also
be accomplished by expanding the central mandrel 38 against the
fixed outer tube 44 by internally pressurizing and/or heating the
central mandrel 38. The mandrel 38, wires 40 and outer tube 44 are
then dissolved away in a suitable etching solution such as nitric
acid where the components to be etched away are constructed from
copper, or sodium hydroxide may be used where the components to be
etched away are constructed from aluminum. It is to be understood
that any other etchant may be used that will suitably react with
the components to be dissolved away without dissolving the mesh
screen.
Referring now to FIG. 4, it will be observed that a free standing
wick 46 with channels 48 just beneath the evaporator surface 50
results from this process. It can also be seen that this process
can easily replace the costly broaching process previously used to
fabricate channels in a heat pipe container tube. This may be
accomplished by the assembly illustrated in FIG. 5, which is
similar to the assembly illustrated in FIG. 2. This assembly
includes several turns of fine mesh screen 52, formed from a
material having the same characteristics as the mesh screen 36,
which are wound around a central mandrel 54, having the same
characteristics as the mandrel 38. Fine wires 56, which are formed
from a material having the same characteristics as that of the
wires 40, are laid axially along the mandrel 54 over the fine mesh
screen 52. The assembly is then inserted into an outer tube 58,
constructed from a material that is relatively resistant to the
chosen etchant mentioned below. The entire assembly is then swaged,
or by any other process uniformly constricted, so that the mesh
screen 52 conforms to the shape of the axially laid wires 56 and
outer tube 58 at the wire/mesh and tube/mesh interface
respectively. The mandrel 54, and wires 56 are then dissolved away
in a suitable etching solution such as nitric acid where the
components to be etched away are constructed from copper, or sodium
hydroxide where the components to be etched away are constructed
from aluminum. It is to be understood that any other etchant may be
used that will suitably react with the components to be dissolved
away without dissolving the mesh screen 52 and outer tube 58.
Referring now to FIG. 6 it will be observed that the resulting
structure is a tight homogeneous, channeled wick similar to the
wick fabricated by the aforementioned broaching process.
* * * * *