Heat pipe

A heat pipe is disclosed which carries an evaporable working fluid and which includes a wick. More particularly, in accordance with the invention the wick comprises a first layer having a small-pore structure and disposed adjacent the vapor space within the pipe and a second layer having a large-pore structure and disposed adjacent the first layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a heat pipe and, in particular, a heat pipe which 
carries an evaporable working fluid and which includes a wick. 
2. Description of the Prior Art 
In a known heat pipe of the above type, the wick is in the form of a 
hollow, cylindrical member having an outer surface which rests against the 
inside diameter of the wall of the pipe and an inner surface which is 
adjacent a vapor space extending through the central portion of the 
interior of the pipe. Additionally, the pipe is evacuated and filled with 
a small amount of an evaporable working fluid, such as, e.g., water or 
alcohol. In use, one end of the pipe is brought into contact with a heat 
source from which heat is to be removed and, simultaneously, therewith the 
opposite end of the pipe is cooled. At the end adjacent the heat source an 
evaporation section or region is created where the working fluid in the 
wick evaporates and the resultant vapor enters into the vapor space. In 
turn, at the other end of the pipe a condensation section is formed. Since 
the vapor pressure in the region of the evaporation section is higher than 
in the region of the condensation section, the vapor molecules move 
through the vapor space from the evaporation section toward the 
condensation section. In the latter section the evaporated working fluid 
is condensed and is drawn back into the wick through capillary action 
along the wick surface adjacent the vapor space. The wick then carries the 
fluid back to the evaporation section where the cycle of operation is 
again repeated. 
In the above known heat pipe, the wick is typically comprised of netting, 
felt or sintered layers, which have a homogeneous structure with 
substantially uniform pore size over the entire layer thickness. As a 
result of employing a wick with a uniform pore size, one is faced with 
having to select a single pore size which best satisfies two contradictory 
requirements. Small pores, on the one hand, permit large capillary 
pressure differences and, therefore, good absorption of the condensed 
vapor back into the wick. On the other hand, small pores offer increased 
resistance to the reflow of the condensed working fluid back through the 
wick, which counteracts the good absorption capacity. Large pores have 
just the opposite effect, i.e., offer low resistance to reflow of the 
condensed working fluid, but provide the small capillary pressure 
differences. In this known heat pipe, therefore, selection of the wick 
pore size necessarily involves a compromise between achieving maximum 
reflow and maximum capillary pressure differences. 
In another known heat pipe an attempt has been made to overcome the latter 
disadvantage by providing separate, free canals, so-called "arteries" for 
the backward flow of the working fluid. For working fluids which boil 
quickly such as, for example, water or alcohol, these so-called "artery 
heat pipes" have not proved satisfactory, as the backward flow of the 
working fluid in the free canals is blocked by the formation of steam 
bubbles. 
It is an object of the present invention to provide a heat pipe having an 
increased heat removing capacity. 
SUMMARY OF THE INVENTION 
In accordance with the principles of the present invention the above and 
other objectives are realized in a heat pipe of the above type by 
including therein a wick which includes a first layer which is disposed 
adjacent the vapor space in the pipe and has a small-pore structure and a 
second layer which is disposed adjacent the first layer and has a 
large-pore structure. Preferably, the pore diameter of the pores of the 
first layer should be less than one-half the pore diameter of the pores of 
the second layer. 
With the heat pipe so configured, the return of the working fluid is 
improved by the large capillary force of the fine-pore layer and the low 
flow resistance of the large-pore layer. The amount of heat that can be 
removed is thereby increased. 
In order to ensure the lack of steam bubble formation the wick may be 
further provided with another layer having a small-pore structure and 
disposed adjacent the second layer and the inner wall of the pipe. 
Additionally, to further facilitate the return of the evaporated liquid 
the thickness of the fine-pore layer may be substantially smaller than the 
thickness of the large-pore layer. 
In one embodiment of the heat pipe to be disclosed herein the large-pore 
layer of the wick comprises several layers of a wide-mesh net and the 
small-pore layer comprises a fine-mesh net. In another embodiment to be 
disclosed, the large-pore layer of the wick is in the form of a hollow, 
cylindrical, sintered layer and the small-pore layer a thin sintered layer 
or fine-mesh net.

DETAILED DESCRIPTION 
FIG. 1 shows a heat pipe in accordance with the principles of the present 
invention. As shown, the heat pipe includes a hollow, cylindrical wick 1 
having an outer surface which rests against the inside diameter of the 
wall 2 of the pipe and an inner surface which is adjacent a vapor space 3 
extending through the central portion of the interior of the pipe. 
Additionally, the pipe is evacuated and filled with a small amount of an 
evaporable working fluid, such as, for example, water or alcohol. One end 
of the heat pipe is brought into contact with a heat source, for instance, 
a hot component 4, from which heat is to be removed. The opposite end of 
the heat pipe is simultaneously cooled, via the cooling fins 5. 
As can be appreciated, with the heat pipe so constructed an evaporation 
section is formed in the region of the hot component 4 where the working 
fluid in the wick evaporates and the vapor enters into the vapor space 3. 
As can be also appreciated, a condensation section is formed in the region 
of the cooling fins 5. Since the vapor pressure in the region of the 
evaporation section is higher than that in the region of the condensation 
section, the evaporated working fluid moves from the evaporation section 
toward the condensation section. In the latter section, the evaporated 
fluid is condensed and is drawn radially back into the wick through 
capillary action along the wick surface adjacent the vapor space. The wick 
then carries the fluid axially back to the evaporation section where it is 
again evaporated. 
In accordance with the principles of the present invention, the wick 1 is 
formed so as to include a first layer which is adjacent the vapor space 3 
and which has a small-pore structure and a second layer which is adjacent 
the first layer and has a large-pore structure. Preferably, the pore 
diameter of the small pores of the first layer should be less than 
one-half the pore diameter of the large pores of the second layer. 
With the wick so formed, the backward transport (axial flow) of the 
condensed working fluid takes place in the large-pore layer of the wick. 
These large pores prevent the transport path from getting blocked by 
formation of steam bubbles. Additionally, the large pores form a 
substantially free flow cross section which offers little resistance to 
the axially backward flow of the condensed working fluid. As a result, 
maximum backward flow can be realized. Also, the absorption of the 
condensed vapor into the wick at the condensation region is now controlled 
by the small-pore layer. The small pores of the latter layer provide 
maximum capillary action (radial flow) and, hence, maximum absorption of 
the condensed fluid radially back into the wick is also achieved. 
The amount of heat which can be removed by the present heat pipe can be 
increased over heat pipes having wicks with homogeneous pore structures by 
a factor which corresponds approximately to the ratio of the pore 
diameters of the large-pore layer to the small-pore layer. The ratio of 
the pore diameters can, therefore, be determined by the desired capacity 
increase over a heat pipe whose wick has a homogeneous, small-pore 
structure. Additionally, as compared to the latter type heat pipe, the 
present heat pipe can be, for the same amount of heat capacity, longer 
and/or thinner and work better against the force of gravity. Also, in the 
present heat pipe there is more freedom as to the choice of the working 
fluid. 
As capillary action takes place only at the boundary surface between the 
first layer of the wick and the vapor space 3, the thickness of the 
fine-pore first layer may be substantially smaller than the thickness of 
the large-pore second layer. In such case, the large-pore second layer 
serves as a carrier or support for the very thin small-pore first layer. 
The choice of the suitable pore diameter of the first and second layers of 
the wick depends particularly on the physical properties of the working 
fluid. The pores in the large-pore layer should be as large as possible. 
The maximum size of the pores is limited by the start of steam bubble 
formation due to the delay in boiling of the working fluid. When water is 
used as the working fluid, a pore diameter between 0.1 mm and 1 mm, and 
preferably about 0.5 mm, is found to be advantageous for the large-pore 
second layer. 
The pores of the small-pore first layer should be as small as possible to 
produce a capillary force as large as possible. The minimum size of the 
pores is limited by the producibility of the small-pore layer. When water 
is used as the working fluid, a pore diameter between 5 .mu.m to 100 .mu.m 
and, preferably, a pore diameter of about 25 .mu.m, is found to be 
advantageous for the small-pore layer. 
Within the limits mentioned, the choice of layers with suitable pore 
diameters will also be determined by their producibility. It is essential, 
however, that the ratio of the pore diameter of the large-pore layer to 
the small-pore layer be as large as possible. 
The right half of FIG. 2 shows an embodiment of the wick of FIG. 1 in which 
the second large-pore layer is wound of several layers of a wide-mesh net 
6 and the first small-pore layer consists of a fine-mesh net 7. To produce 
such a wick one or both ends of a wide-mesh net in tape form are attached 
to one or two pieces of a fine-mesh net. The entire tape is then wound on 
a mandrel, the diameter of which is smaller than the inside diameter of 
the heat pipe. The wound net is then placed inside the pipe and makes 
close contact with the pipe wall 2. When water is used as the working 
fluid, netting of phosphor bronze is found to be particularly corrosion 
resistant. Such phospor bronze netting can also be made with a very large 
number of meshes per unit of area. 
In the left-hand portion of FIG. 2 the wick of the right-hand portion has 
been modified to include a third layer 10, which is adjacent the second 
layer 6 and the inner wall 2. The third pore layer has a small-pore 
structure similar to that of first layer 7. The presence of the layer 10 
further inhibits any tendency of the wick to form steam bubbles, which 
would prevent backward passage of the condensed liquid. 
FIG. 3 shows another embodiment of the wick of FIG. 1 in which the layers 
thereof comprise sintered material. More particularly, a thick layer 8 of 
large-pore structure sintered material is lined at its inside surface with 
a thin layer 9 of small-pore structure sintered material. As can be 
appreciated, the inner layer 9 may be replaced by a fine-mesh net having a 
small-pore structure. 
It should be also pointed out that the first and second layers of the wick 
of FIG. 1 can be constructed of steel wool or felt having the required 
pore structure.