Heat Pipes

Abstract

The specification and drawings disclose two embodiments of heat pipe structures arranged to readily conduct heat in one direction and conduct only a limited amount of heat in the opposite direction. In both embodiments, the heat pipe is shown as comprising a hollow, sealed tube having wick material positioned in engagement with selected portions of the inner surface. A vaporizable fluid is placed in the tube in an amount sufficient to wet the wick. In one embodiment, the wick extends over and is in engagement with only those portions of the wall surface from which it is desired to transfer heat. The other portions of the wall surface are bare. Consequently, fluid in the wick can be vaporized and flow to the bare wall portion for condensation. However, if the bare wall portions are at a higher temperature, heat cannot be transferred to the wicked portion except by conduction through the tube wall. In the second embodiment, the entire inner surface is covered by a wick but in certain areas, the wick is spaced from the surface. The spaced areas function in the manner of the bare areas of the first embodiment but allow the unit to be used in any orientation since droplet formation in the spaced wick areas permit the droplets to engage the spaced wick for capillary flow to the heated wick portion.

Description

The present invention is directed toward the heat transfer art and, more particularly to improved heat pipes which provide controlled heat flow characteristics.

The invention will be described with reference to certain preferred embodiments formed from specified materials; however, as will become apparent, the heat pipes constructed in accordance with the invention could have many different structural arrangements and be made from many different materials or combination of materials.

In the past few years, heat pipes have become an important method of transferring heat. Their high efficiency and capacity have made them ideal for use in cooling electronic components and the like. Further, since orientation and presence or absence of gravity have no effect on a heat pipe's ability to function, they are particularly suited for outer space applications.

The typical heat pipe comprises a hollow, fluid impervious tube with a tubular wick member positioned in its interior closely in engagement with the inner wall surface. A vaporizable fluid is placed in the tube in an amount generally slightly in excess of that required to completely wet the wick. Thereafter, the tube is partially evacuated and sealed.

The operation of a heat pipe is relatively simple. When the tube is subjected to uneven heat conditions, liquid in the wick at the hot points is vaporized. The vapor moves to cooler points on the tube and condenses giving up its latent heat of vaporization. The condensed liquid then moves back to the hot points by capillary action in the wick.

As can be appreciated, since the latent heat of vaporization is carried by movement of vapor from the point of vaporization to the point of condensation, heat is transferred down the pipe with little or no temperature drop along the length of the pipe. Further, heat can be transferred in either direction along the pipe.

Generally, the fact that heat is transferred in either direction is a decided advantage. However, in certain installations, it would be much more preferable to have a heat pipe which is effective to conduct heat only in one direction. For example, when it is desired to maintain a component at a high temperature, it would be desirable to use heat pipes which would not conduct heat away from the component during periods when the heat source is disconnected.

The subject invention provides a heat pipe structure in which heat flow can take place predominately only in one direction. Although some heat can flow in the other direction, it must take place primarily by conduction through the outer wall of the tube. In accordance with the invention, the heat pipe comprises a hollow, fluid impervious body having a relatively thin wall. Wick material is positioned in engagement with the inner surface of the wall throughout only a portion of the wall's total inner surface. The portions engaged constitute those areas from which it is desired to conduct heat. The remaining portions of the inner wall surface are spaced from the wick material and constitute areas to which it is desired to conduct heat but from which it is desired to impede the flow of heat. Additionally, a vaporizable fluid is placed in the body in an amount preferably slightly greater than that required to fully wet the wick.

The functioning of the device is readily apparent. Note that heat applied to the wick engage area will cause vaporization of the fluid and the vapor will travel to the cooler, non-wicked areas and condense. However, if the high heat level occurs at a non-wicked area, any liquid on these areas will be vaporized and passed to the wicked areas. Further heat conduction cannot take place since liquid in the wicked areas cannot return to the non-wicked areas. Thus, any heat transfer from the non-wicked areas can only take place by conduction through the heat pipe wall. This can, of course, be controlled by making the body or portions thereof from insulating material or material having a low conductivity.

In accordance with another aspect of the invention, the wick extends over all inner surfaces of the wall. However, in those areas from which it is not desired to have heat transfer, the wick is spaced outwardly from the surface a distance substantially equal to the droplet forming capabilities of the liquid. Thus, the wick cannot conduct fluid to the wall in the portion in the areas where it is outwardly spaced. It can, however, conduct liquid from the spaced wall back to the wall surfaces with which it engages. Note that as droplets are formed, they will build up to a size where they can engage the spaced wick.

As can be appreciated, this form of the invention can operate in any orientation. Further, heat pipes can be formed with controlled transfer characteristics so that heat will be conducted equally from several spaced points while intermediate points can only receive heat.

Accordingly, the primary object of the invention is the provision of a simple heat pipe structure wherein selected areas of the pipe can have different heat transfer characteristics.

Another object is the provision of a heat pipe with selective heat transfer characteristics which is simple to construct.

Still another object is the provision of a heat pipe of the general type described which can control the direction of heat flow without complicated internal valving or other moving parts.

A still further object of the invention is the provision of a heat pipe structure that can have substantially any desired heat transfer characteristics.

The above and other objects and advantages will become apparent from the following description when read in conjunction with the accompanying drawings wherein:

FIG. 1 is a longitudinal cross-sectional view through a heat pipe formed in accordance with the invention;

FIGS. 2 and 3 are cross-sectional views taken on lines 2--2 and 3--3 of FIG. 1, respectively;

FIG. 4 is a longitudinal cross-section through a second embodiment of heat pipe formed in accordance with the invention; and,

FIG. 5 is a cross-sectional view taken on line 5--5 of FIG. 4.

Referring more particularly to FIG. 1, the overall arrangement of the inventive heat pipe is shown as comprising a tubular outer body 10 formed from any desired fluid impervious material such as hard copper tubing, stainless steel tubing, or the like. The opposite ends of the tubing are sealed in any convenient manner such as through the use of metallic discs 12 and 14 soldered or bonded into the ends of the tube 10. Positioned within the tube and closely in engagement with the inner wall 16 throughout a selected portion of the tube 10, is a cylindrical wick member 18. The wick member 18, in the embodiment under consideration, is formed from a fine wire screen (shown diagramatically in a somewhat larger than actual size). It should be appreciated that any desired type of wicking material can be used and, for example, metal felt, fiber glass and the like is often used.

As is customary, a vaporizable fluid is placed within the tube in an amount sufficient to slightly more than wet the entire wick 18. Additionally, the interior of the tube is evacuated and/or filled with a non-condensible gas to provide selected heat transmitting characteristics in the resulting tube.

According to the invention, the inner wick member is arranged so that it engages only those portions of the tube wall which are desired to function as heat transmitting or conducting portions, That is, only those areas of the tube from which it is desired to conduct heat are covered by the wick. As shown in FIG. 1, the wick extends approximately half way up the tube from the end 14. The inner surface of the end 14 is likewise covered with wick material.

The FIG. 1 form of the invention is adapted to function primarily when the end 12 is at an elevation higher than end 14. To appreciate the operation of the FIG. 1 embodiment, assume that the end coated with wicking material is at a higher temperature level than the opposite end. The fluid in the wick 18 is thus caused to evaporate and moves to the opposite end where it will condense giving up its heat of vaporization. The condensed liquid will form droplets and flow back to the wick material for conduction to the particular spots at which vaporization is taking place. It should be appreciated that the area covered by wick soon becomes relatively uniform in temperature in the manner of a conventional heat pipe. Likewise, the heat flow to the opposite end is rapid as is the case with a standard heat pipe. However, if the unwicked end should become warmer than the wicked end, very little heat can flow to the wicked end. Note that the unwicked end cannot maintain a supply of fluid in engagement with the walls. If any moisture droplets remain on the walls, they will be quickly vaporized and condensed on the wick portion. The lack of wick in the higher temperature end thus prevents the return of the fluid to the hotter portion of the tube. If any heat is to be transferred from the unwicked to the wicked end, it travels by conduction through the walls of tube 10. Thus, the rate of heat flow from the unwicked end to the wicked end is extremely low when compared to heat flow in the opposite direction.

It should be appreciated that many variations could be made in the arrangement of the internal wick surfaces. For example, the central section of the tube could be provided with wick and both ends left bare so that heat could only flow from the center of the tube to the outer ends. Additionally, since the only heat flow which can take place from the unwicked end to the wicked end is through conduction, intermediate insulating rings could be inserted to reduce this heat flow if desired.

Although the FIGS. 1 through 3 embodiment is limited to orientations in which the unwicked portions are at an elevation slightly above the wicked end, the embodiment of FIGS. 4 and 5 shows how the same general principals can be utilized to provide a heat pipe which permits flow of heat substantially in only one direction but is not limited by orientation. As shown in FIGS. 4 and 5, the heat pipe includes an outer tubular body or housing member 30 which can be formed from any desired type of fluid impervious material having characteristics required. The opposite ends of the tube 30 are closed and sealed by end plates 32 and 34 soldered or otherwise bonded therein. Positioned within the tube 30 is a wick member 36 having a first portion 38 which engages the inner wall 40 of tube 30. The remaining portion 42 of the wick 36 is spaced inwardly of the wall surface 40 a predetermined distance. In this embodiment, the wick portions 38 and 42 are continuous and join through a tapered section 44; however, as will become apparent hereafter, the wick could have many shapes and configurations and be formed from a plurality of different materials and in independent sections merely connected by some capillary portion to provide a flow path between them. It is important that the tapered section 44 between the wick sections have openings of other means for permitting vapor flow between the annular space 51 and the inner space 53.

As previously mentioned, portions of the wick are spaced inwardly from the inner wall 40 of the tube. The particular areas which are spaced inwardly can be located as desired. Those areas to which heat is to be conducted, but from which heat is not to be conducted, should have a substantial spacing from the wall. For example, in the FIG. 4 embodiment, the tube is arranged so that heat can be conducted from area A to area B. To explain, assume that the wick has been saturated with the desired vaporizable fluid and the assembly is subjected to heating along area A. The liquid within the wick portion 38 is quickly vaporized and travels to the cooler wall of the tube 10 in area B where it condenses. As shown, the fluid will tend to form droplets, for example, droplets 50, on the inner wall of the tube. When the droplets have formed to a relatively large size, they will contact the wick portion 42 and be conducted by capillary action back to the heated area of the tube. Note that this action will take place irrespective of the orientation of the tube. For example, assume that the end 32 is at an elevation higher than the end 34. The droplets or the condensed fluid will either flow by gravity along the wall to the wick portion 38 or, alternately, they will engage the spaced wick portion 42 and be conducted back to wick portion 38 by capillary action.

Assume however, that the tube is heated to a higher level at some point on portion B. In such case, the fluid which happens to be on the inner wall 40 will be vaporized and will pass to the wick or cooler wall portion in area A. Quickly, however, all the fluid on the wall in the portion of area B will be vaporized. Thereafter, no further fluid is available for vaporization and heat can only be conducted to area A by conduction through the wall of the tube 30. A substantial increase in temperature would be required to cause any vaporization of the fluid in the wick in portion 42. Thus, the heat pipe can readily conduct heat from portion A to portion B but substantially no heat is conducted from B to A.

As can be appreciated, the heat pipe of FIGS. 4 and 5 can have any desired configuration and the internal wick can be spaced at whatever location is desired to be a non-heat transmitting portion. Further, the internal wick arrangement can vary widely.

The invention has been described in great detail sufficient to enable one of ordinary skill in the heat transfer art to make and use the same. Obviously, modifications and alterations of the preferred embodiment will occur to others upon a reading and understanding of the specification and it is my intention to include all such modifications and alterations as part of my invention insofar as they come within the scope of the appended claims.

Claims

What is claimed is:

1. A heat pipe assembly adapted to transmit heat primarily in one direction comprising:

a sealed, fluid impervious body having a hollow interior;

a wick member adapted to conduct fluid by capillary action positioned within said body with a portion of said wick in close engagement with the interior surfaces of said inner walls and the remaining portion being spaced from said wall a distance such that fluid condensing on the wall adjacent said spaced portion will not engage the spaced wick until it has formed substantial sized droplets.

2. The heat pipe as defined in claim 1 wherein said wick member is substantially continuous throughout the interior of said body and spaced from the inner wall of said body throughout an extent of greater than 10% of the internal wall surface.

3. A heat pipe as defined in claim 1 wherein said body comprises a first sealed tubular member and wherein said wick member comprises a second tubular member having portions of at least two different outer diameters including a first outer diameter to closely engage the interior surface of said body and a second outer diameter to be spaced from interior surface of said body.

4. A heat pipe as defined in claim 1 wherein said body is formed from metal having a relatively thin wall.