Heat pipe having multiple integral wick structures

This invention relates generally to the field of heat pipes, and is specifically concerned with an internal wicking structure to promote the capillary flow of a system liquid from the condensor section to the evaporator section, such that the evaporator section is uniformly saturated. This phenomena is achieved through the use of multiple diverse wicking patterns, formed integrally in the interior walls of the heat pipe.

BACKGROUND OF THE INVENTION 
In general, heat pipes, and the concepts which surround their use, are well 
recognized by the prior art. A heat pipe is a closed, constant mass 
heating system in which the system liquid coexists in equilibrium with its 
vapor during normal operating temperatures. It consists of an evaporator 
section, in which the system liquid is heated and vaporized; an adiabatic 
section, in which both vapor and condensed liquid flow with no heat being 
externally transferred; and a condensor section, in which the latent heat 
of vaporization is transferred to the external surroundings, and the 
condensed liquid flows via capillary action back towards the evaporator. 
The axial flow of the vapor and the capillary flow of the returning system 
liquid are both produced by pressure gradients, that are created by the 
interaction between naturally-occurring pressure differentials within the 
heat pipe. These pressure gradients eliminate the need for external 
pumping of the system liquid. In addition, the existence of liquid and 
vapor in equilibrium, under vacuum conditions, results in higher thermal 
efficiencies. 
Heat pipes, as mentioned above rely for their operation upon the existence 
of the induced pressure gradients, which work to force vapor flow toward 
the condenser, and capillary liquid flow back toward the evaporator. 
In order to increase the efficiency of heat pipes, various wicking 
structures have been developed to promote liquid transfer between the 
condensor and evaporator sections. They have included longitudially 
disposed parallel grooves and the random scoring of the internal pipe 
surface. In addition, the prior art also discloses the use of a wick 
structure which is fixidly attached to the internal pipe wall. The 
compositions and geometries of these wicks have included, a uniform fine 
wire mesh, and circumferentially disposed fine wire hoops of varied 
spacing. 
All of the geometries, either integral to the internal pipe wall or 
integral to the affixed wick, are chiefly designed to promote liquid and 
vapor flow while maintaining high thermal efficiencies through the pipe 
wall to the ambient surroundings. 
In general, the wick structures disclosed in the prior art provide grooves 
for condensate return to the evaporator, such that evaporator "dryout" is 
minimized. However, these internal structures fail to accomodate the 
different requirements and functions of each particular section of the 
heat pipe, and therefore they do not produce the optimum output available 
from structures of this type. 
Examples of some of the aforementioned prior art devices may be seen by 
reference to U.S. Pat. Nos. 4,109,709; 4,116,266; 4,058,159; 4,274,479; 
and 4,186,796. 
These prior art devices, while adequate for their intended purpose, suffer 
from the common deficiency, in that they do not fully realize the optimum 
inherent heat transfer potential available from a given heat pipe. 
To date, no one has devised a wick structure for a heat pipe, which is 
simple to produce, and yet provides optimum heat transfer characteristics 
for the heat pipe in which it is utilized. 
SUMMARY OF THE INVENTION 
An object of this invention is the provision of an improved heat pipe 
internal wicking structure which promotes liquid transfer from the 
evaporator section to the condensor section and vice versa. 
Another object of the present invention is the provision of an improved 
heat pipe having a wicking structure which is both integral and diverse. 
A further object of the present invention is the provision of an improved 
heat pipe, wherein integral and diverse wicking structures are formed in 
the interior walls by knurling. Broaching, thread rolling, cutting, 
extruded and cold forming can also be used. 
A still further object of the present invention is the provision of an 
improved heat pipe, wherein the process used to form an integral and 
diverse wicking structure, increases the internal surface area of the heat 
pipe. 
Yet another object of the present invention is the provision of an improved 
heat pipe structure, which optimizes the inherent heat transfer available 
from the heat pipe. 
A yet further object of the present invention is the provision of an 
improved heat pipe structure, which will insure that the evaporator 
section remains saturated, thereby eliminating evaporator "dryout", which 
causes "hot spots" on the heat pipe surface. 
Another object of the present invention is the ability to select the 
optimum wick structure for the heat pipe. This optimum wick structure is 
achieved by matching wick and liquid characteristic requirements for each 
section of each heat pipe. 
These and other objects, advantages, and novel features of the invention 
will become apparent from the detailed description which follows when 
considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As can be seen by reference to FIG. 1, the improved heat pipe, which forms 
the basis for this invention is designated generally as 10, and comprises 
a tubular element 11 having a plurality of fins, or external projections 
12 disposed on its exterior wall, in a well recognized fashion. 
The heart of this invention, however, resides in the revolutionary integral 
and diverse wicking structures 50, which have been formed in the internal 
walls of the tubular element 11. 
Since every heat pipe 10 is divided into three distinct sections, these 
sections have been labeled A thru C, and represent the condensor, 
adiabatic and evaporator sections respectively. As will be explained 
below, each of these sections performs their own unique function. 
A heat pipe 10, basically consists of three elements; a sealed container 
formed by closing the ends of the tubular element 11; a capillary wick 
structure 50, integral to the interior surface of the container; and 
sufficient working fluid 100, to saturate the wick structure. 
Because the container is sealed under vacuum, the working fluid, is in 
equilibrium with its own vapor. Heating any part of the external surface 
causes the instantaneous evaporation of the working fluid near the 
surface, which becomes an evaporator region C, and the latent heat of 
vaporization is absorbed by the vapor formed. 
The rapid generation of vapor at any point on the tube wall area creates a 
pressure gradient within the heat pipe, which forces the excess vapor to a 
remote area of the heat pipe having lower pressure and temperature, where 
it condenses on the tube wall and the latent heat of vaporization is 
released. 
Heat is removed from the surface at the point of condensation by 
conduction, convection or radiation. A continuous process is established 
by the capillary pumping forces within the wick structure that returns the 
fluid from the condenser region A (point of heat removal) to the 
evaporator region C (point of heat addition). Each heat pipe theoretically 
has a thermal efficiency approaching 99%. 
The evaporation-condensation heat transfer process causes liquid to be 
depleted in the evaporator section C and accumulated in the condensor 
section A, which must be equalized by the surface-tension pumping of the 
liquid in the wicking structure 50. In order for the wicking structure to 
function properly it is necessary for the liquid to wet the surfaces of 
the integral wick structure of the evaporator section C. Proper wetting 
occurs when the liquid is drawn into, and saturates the integral wick 
structure by the surface-tension forces of the liquid. The surface tension 
forces exist along the free surface interface, between the liquid in the 
integral saturated wick, and the adjacent vapor space. As heating causes 
liquid to be depleted from the evaporator section C and the local vapor 
pressure to increase, the free surface interface becomes depressed (see 
FIG. 3), into the integral wick structure of the evaporator. 
In the condensor section A, the accumulation of liquid and lower vapor 
pressure causes the free surface interface to become nearly flat (see FIG. 
2). 
The difference in the shape of the free surface interface and the integral 
wick design between the evaporator and the condensor section creates, a 
pressure gradient in the liquid. This pressure gradient is sustained by 
the different surface tension forces, and the diverse integral wick 
configurations of the respective sections. 
Referring back to FIG. 1, it can be seen that the condensor section A is 
provided with a plurality of axial grooves 61, which form the condensor 
section wicking structure 60. These axial grooves or channels 61 can be 
formed by any suitable means such as extrusion, broaching, cutting, cold 
forming, knurling, rolling, etc. It should be noted at this point; 
however, that knurling is the preferred method of forming all of the 
wicking structures described in the specification, for reasons which are 
set forth in our co-pending patent application Ser. No. 357,102 filed 
3/11/82. 
For the purpose of this disclosure, it should also be understood that axial 
grooves or channels are defined as grooves or channels, with or without a 
spiral, that are parallel to each other, and extend along the centerline 
of the pipe. 
Since different working fluids 100, have different flow characteristics, 
the dimensions of the axial grooves or channels 61, will be selected to 
produce a low liquid flow resistance to the return of the condensed vapor, 
to the evaporation section C of the heat pipe from the condensor section 
A. The existence of these grooves or channels 61, further increases the 
surface area integral to the heat pipe wall in the condensor section, and 
thereby improves the thermal efficiency in that section. 
In the preferred embodiment illustrated in FIG. 1, the evaporator section 
C, obviously has entirely different operating parameters than the 
condensor section A, since this particular heat pipe is used in 
unidirectional applications. In order to increase the efficiency of this 
section, it is desirable to increase the surface area integral with the 
wall, where the fluid can be evaporated into the vapor phase. While this 
increase in surface area in the condensor section had been accomplished by 
the use of axial grooves 61, the evaporator section requires that the 
fluid be distributed circumferentially, to expose the maximum fluid 
surface area, to the elevated temperatures in this region. 
The evaporator section C is therefore provided with a plurality of 
circumferential grooves 81, which form the evaporator wicking structure 
80. These circumferential grooves 81 can assume various alignments with 
respect to the centerline of the heat pipe, as long as they are parallel, 
and have a vertical orientation. 
As can be seen by reference to FIGS. 1 and 4 thru 7, the circumferential 
grooves 81 can be arranged in a variety of ways (i.e. perpendicular to the 
centerline of the pipe, angled to the left or to the right, or a 
combination thereof). The primary criteria for the circumferential 
grooves, are that they aid in the lifting of heat pipe fluid by way of 
surface tension forces, thereby improving the internal surface wetting of 
the evaporator section wall, around its circumference. An added benefit 
obtained by the circumferential grooves is the increase in internal 
surface area integral to the wall, which improves the evaporator section 
performance for obvious reasons. 
These circumferential grooves can be formed by any suitable means such as 
cutting, cold forming, knurling, rolling, etc. It should be noted at this 
point; however, that knurling is the preferred method of forming all of 
the wicking structures described in the specification, for reasons which 
are set forth in our copending patent application Ser. No. 357,102 filed 
3/11/82. 
The adiabatic section B functions as a transition zone or interface between 
the evaporator section C and condensor section A, and it is in this 
section B, that theoretically no heat transfer is taking place. However, 
as a practical matter, it is almost impossible to pinpoint the exact 
location of the adiabatic zone, for a given heat pipe, using various 
working fluids, and under different operating conditions. The only thing 
that is certain about its location, is that it is somewhere between the 
condensor section end, and the evaporator section end. 
Given this situation, the adiabatic section should have an adiabatic 
wicking structure 90, which combines features found, in both the 
condenser, and the evaporator sections. The adiabatic wicking structure 
90, therefore comprises a combination of axial grooves or channels 61, and 
circumferential grooves or channels 81. Examples of proposed wicking 
structures 90, for the adiabatic section are illustrated in FIGS. 4 thru 
6. 
The operation of this improved heat pipe containing the multiple and 
diverse wicking structures proceeds as follows: The working fluid (liquid 
and vapor phases) fills the interior of the heat pipe. As heat is applied 
to the evaporator section C, the liquid which has been drawn up into the 
circumferential grooves or channels, via capillary pumping and wick 
wetting action, is exposed to the heat over a much greater surface area, 
due to the presence of the integral wicking structure. As the working 
liquid evaporates, it allows returning condensed liquid to take the place 
of the evaporated liquid, via capillary pumping action, to keep the 
evaporator section saturated. The heated vapor then migrates to the 
cooler, lower pressure, condensor section, due to the pressure 
differential created with the heat pipe, by the application of heat to the 
evaporator section. The heated vapor will then condense on the enlarged 
surface area created by the axial channels and grooves. The condensed 
liquid will then be collected in the axial channels and fed by capillary 
and/or gravitational action back to the evaporator section. 
The direction of capillary flow through the condensor, adiabatic, and 
evaporator sections of the preferred embodiment are indicated by the 
arrows shown in FIG. 1, and are axial, axial-circumferential, and 
circumferential respectively. FIGS. 4 thru 6, illustrated different 
capillary flow patterns available in the adiabatic section, by varying the 
arrangement of the circumferential grooves or channels; and FIG. 7, 
illustrates in detain the preferred flow pattern through the evaporator 
section. 
As mentioned supra, the heat pipe illustrated in FIG. 1, is used for 
uni-directional applications. The heat pipe shown in FIG. 8; however, can 
be used in bi-directional applications by combining all of the diverse, 
integral, capillary wicking structures throughout the heat pipe interior. 
It should by appreciated, that not only does this arrangement 
substantially increase the internal surface area integral with the heat 
pipe wall, but also allows axial flow in the axial grooves or channels, in 
either direction with respect to the heat pipe axis, depending on whether 
the heat pipe is being used for heating, cooling, or other bi-directional 
applications (i.e. process applications may require bidirectional use such 
as pollution control, condensing, etc.). 
It should be noted at this point, that a good condenser wick geometry is 
one which has: a low pressure drop of returning condensate; no temperature 
gradient between wick and wall; a high condensing surface area to absorb 
latent heat of vaporation; compatability with the container and the 
liquid; high capillary pumping forces; the ability to return the condensed 
liquid uniformly for maximum condensate return; the ability to reduce 
evaporator wick dryout; the ability to reduce vapor/liquid shear; the 
ability to compliment different liquid characteristics; the ability to 
compliment the evaporator and adiabatic wick geometries; a smooth surface 
to reduce wetting and pressure drop; the ability to provide flow paths for 
condensate return; and the ability to match the operating temperature and 
heat flux. 
A good evaporator wick geometry is one which has: a small pore radii; no 
temperature gradient between wick and wall; a high evaporating surface 
area to increase latent heat of vaporization; compatibility with container 
and liquid; the ability to provide maximum wetting; the ability to 
distribute the liquid uniformly throughout the evaporator for maximum 
operation in evaporating the liquid; the ability to compliment different 
liquid characteristics; the ability to lift the liquid; the ability to 
compliment the adiabatic and condenser wick geometries, no nucleate 
boiling within its structure; and the ability to match the operating 
temperatures and heat flux. 
A good adiabatic wick geometry is one which has: the ability to provide a 
separation between the vapor/liquid interface where the vapor/liquid shear 
is at its highest concentration; a transistion between condensor and 
evaporator wick geometries; the ability to provide a path for the 
condensate return; a low pressure drop of returning condensate; 
compatibility with the container and the liquid; and the ability to 
compliment different liquid characteristics. 
The reason that different wick structures are needed can be summarized as 
follows: 
The pore radii of the evaporator wick must be smaller than the pore radii 
of the condensor wick, to increase available surfact tension pumping 
forces. The pore radii of the evaporator which must be smaller than the 
pore radii of the condensor wick to improve wetting, by reducing meniscus 
radius in the evaporator region, while the larger pore radii in the 
condensor wick reduces wetting by increasing meniscus radius in the 
condensor region. The surface area, in the evaporator wick must be greater 
than the surface area in the condensor wick, due to the difference of the 
vapor state in the condensor as opposed to the liquid state in the 
evaporator. Vapor is more easily distributed in the condensor than liquid 
is distributed in the evaporator. The evaporator wick geometry 
characteristics, must be different than the condensor wick 
characteristics, due to the lifting requirements in the evaporator wick as 
opposed to the axial transport requirements in the condensor wick. Lifting 
requirements in the evaporator are to provide maximum wetting and 
evaporation. Axial transport requirements in the condensor are to provide 
maximum condensate return. 
Evaporator wick geometries compliment condensor wick geometries because of 
the ability to select specific, but different evaporator and condensor 
wick designs, within a single heat pipe. Specific design requirements of 
the evaporator and condensor sections must match the specific operating 
temperatures and heat fluxs. Therefore, different wick designs in the 
evaporator and condensor sections can be selected. The axial transport 
characteristics of the condensor requires minimum liquid pressure drop of 
returning condensate over a longer distance; whereas, the lifting 
characteristics of the evaporator wick requires an insignificant pressure 
drop over a shorter distance. Finally, the vapor/liquid shear conditions 
are different in each section of the heat pipe. Thus requiring multiple 
and different wick geometries for each. 
As can be seen by reference to FIGS. 9 and 10, not only does this invention 
contemplate different axial and circumferential wick geometries, but it 
also envisions wicking structures having the same geometries, but 
different dimensions for the individual groups of parallel grooves. 
Having thereby disclosed the subject matter of this invention, it would be 
obvious that many substitutions, variations and modifications are possible 
in light of the above teachings. It is therefore to be understood that the 
invention as taught and disclosed is only to be limited by the breadth and 
scope of the appended claims.