US3892273A - Heat pipe lobar wicking arrangement - Google Patents

Abstract

A heat pipe has the usual central wick for transporting a liquid, such as a cryogenic liquid, from end to end. A secondary wick transports liquid radially between the central wick and the wall of the heat pipe. This secondary wick is formed with lobes for contacting the heat pipe walls. In this manner, much of the wall surface is free of any contact with the wick, thereby providing improved heat transfer characteristics.

Description

United States Patent [1 1 Nelson HEAT PIPE LOBAR WICKING ARRANGEMENT [75] Inventor: Burke Edward Nelson, Ridgefield,

Conn.

[73] Assignee: The Perkin-Elmer Corporation,

Norwalk, Conn.

{22] Filed: July 9, 1973 [21] Appl. No.: 377,728

[52] U5. Cl 165/105; 29/1573 R; 138/40 [51] Int. Cl. F28d 15/00 [58] Field of Search 165/105; 138/40; 29/1573 R [56] References Cited UNITED STATES PATENTS 3,283,787 11/1966 Davis 138/148 [111 3,892,273 [451 July 1,1975

3,620,298 11/1971 Somerville et al. 165/[05 3,720,988 3/1973 Waters 165/105 X 3,734,173 5/1973 Mon'tz 165/105 Primary Examiner-Albert W. Davis, Jr. Attorney, Agent. or Firm-John K. Conant 5 7 ABSTRACT A heat pipe has the usual central wick for transporting a liquid, such as a cryogenic liquid, from end to end. A secondary wick transports liquid radially between the central wick and the wall of the heat pipe. This secondary wick is formed with lobes for contacting the heat pipe walls. In this manner, much of the wall surface is free of any contact with the wick thereby providing improved heat transfer characteristics.

5 Claims, 5 Drawing Figures HEAT PIPE LOBAR WICKING ARRANGEMENT BACKGROUND OF THE INVENTION This invention is a heat pipe for use with liquids, particularly cryogenic liquids, wherein the liquid evapo rates at one end of the pipe and condenses at the other. The liquid is moved from the condenser region to the evaporator region by capillary attraction along a central wick. A secondary wick in the condenser end transports liquid from the inner walls of the tube to the central wick and a similar secondary wick in the evaporator end transports the liquid from the central wick to the walls. In heat pipes constructed in accordance with the prior art, it is customary for the secondary wick to lie against the inner wall of the heat pipe. One example of such a construction will be found in US. Pat. No. 3,720,988 of Waters. The problem with such a construction arises from the fact that cryogenic liquids have very poor thermal conductivity. The heat of vaporization must traverse not only the wall of the heat pipe but also a layer of wick and a layer of the cryogenic liquid. This presents a path of high thermal resistance between the outer surface of the heat pipe and the point of evaporation on the inside of the screen wick. Accordingly, it is the primary object of the present invention to provide a construction which permits efficient wicking but reduces the thermal resistance of the heat path. Other objects, features, and advantages will be apparent from the following description and appended claims.

SUMMARY OF THE INVENTION A heat pipe is formed with a thermally conductive tube having threads or grooves on its inner surface and a liquid, particularly a cryogenic liquid, contained with the tube. A primary wick extends substantially axially through the tube to transport liquid therealong. A secondary wick is provided which has a plurality of substantially radial portions extending between the primary wick and the inner surface of the heat pipe. These portions contact the inner surface at circumferentially spaced discrete regions so as to transport iiquid between the primary wick and the inner surface.

BRIEF DESCRIPTION OF THE DRAWINGS With particular reference to the drawings:

FIG. I is a perspective view of the end of a heat pipe having a wick constructed in accordance with the present invention, portions thereof being broken away to better illustrate the construction;

FIG. 2 is an enlarged cross section taken through the outer wall of the heat pipe of FIG. 1;

FIG. 3 is a perspective view showing one manner in which the wick of the invention may be constructed;

FIG. 4 is a perspective view showing the wick of FIG. 3 positioned within a heat pipe; and

FIG. 5 illustrates another embodiment ofa wick constructed in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With particular reference to FIGS. 1 and 2 there is illustrated a heat pipe constructed in accordance with the present invention with the end cap removed. It comprises a metallic tube 10, having circumferential grooves or threads 12 formed on its internal surface. Extending axially the length ofthe heat pipe is a central wick [4 formed of a fine mesh stainless steel screen which has been spirally wrapped around a central core, such as a steel rod. In the illustrated heat pipe the central core is removed. However, it may be left in place if desired. Secondary wicking between the central wick and the side wall is provided by a similar fine mesh screen, spot welded to the central wick 14 so as to form lobes l6 a-d. The lobes are slightly longer than the distance from central wick 14 to the side wall and are, therefore, slightly deformed when inserted into tube 10. The four lobes engage the inside wall at discrete circumferentially spaced regions, thereby leaving large portions of the side wall out of contact with the wick. The natural resilience of the screen keeps the central wick l4 centrally located within the heat pipe. While the lobes l6 a-d may extend the entire length of the heat pipe, it is not necessary that they do so as their primary function is the transport of liquid between the central wick l4 and the tube 10 in the evaporator and condenser regions. Between these regions the lobes may be removed, if desired.

It will be understood that the materials and dimensions may vary with the proposed application and the liquid employed. However, in one successful embodiment utilizing liquid nitrogen as the liquid, the tube 10 had an external diameter of I inch and a wall thickness of 0.235 centimeters. There were 24 threads per inch with a depth of 0.0675 centimeters and a flat dimension at the root of 0.013 centimeters. The central wick 14 had a diameter of 0.3 inch and both it and the lobes were formed of stainless steel screen of 1,500 mesh.

Assuming FIG. 1 to illustrate the evaporator region of a heat pipe, the liquid nitrogen is transported from the remote condenser region by capillary attraction in the central wick 14. It is then transferred radially to the wall by the wicking action of the lobes 16. The threads 12 also provide wicking so that by capillary attraction the liquid 18 is pulled into the thread channels, as shown in FIG. 2, where it is evaporated. The fine channels formed by the threads prevent a continuous layer of the poor thermally conductive cryogenic liquid from forming on the walls. The flow is, of course, reversed in the condensing region of the heat pipe.

The four lobe arrangement illustrated in FIG. 1 is primarily for use in ground testing in a gravitational environment. As liquid will naturally tend to migrate to the bottom of the heat pipe, it is desirable to have a lobe there and it is also desired to have lobes on the side wall to pick up the fluid as it flows down under the influence of gravity. In the evaporator the opposite is true. In order for the upper quadrants of the wall to be wetted, liquid must be delivered to the upper edge so that it can flow down under the influence of gravity. For these reasons, lobes are desired at the top, bottom, and each side.

In a gravity-free space environment, three lobes may be more desirable and such a construction is illustrated in FIGS. 3 and 4. Furthermore, if desired, the central wick may have a mesh size differing from that of the lobes. FIG. 3 illustrates the construction of such a wick, wherein a screen 20 is spirally wrapped to form central wick 22. Another screen 24 of a different mesh size is interleaved with the outer layer of the central wick and is spot welded thereto leaving lobes 260-0. The lobes are then bent over and the entire wick inserted into a tube 28 with a rotating motion as shown in FIG. 4.

In the FIGS. 1 and 4 embodiments, the lobes are deformed into a swastika effect, However, they could also take the form of ellipses compressed along their major axes.

A different construction is illustrated in FIG. 5. In this embodiment a similar internally threaded tube 30 is employed. However, the wick32 is a solid formed of sintered metal having a pore: size suchf as to provide the required capillary action. The wick 32 is of cruciform cross-section and the ends of the cross arms have external threads 34 which match the threads in tube 30. The resulting criciform wick is threaded into the tube 30 and the arms function as lobes to radially transport the cryogenic liquid.

It is believed that the many advantages of this invention will now be apparent to those skilled in the art. It will also be apparent that a number of variations and modifications may be made therein without departing from its spirit and scope. Accordingly, the foregoing description is to be construed as illustrative only, rather than limiting. This invention is limited only by the scope of the following claims.

I claim:

l. A heat pipe comprising: a thermally conductive tube having circumferential grooves in its inner surface; a liquid contained within said tube; a primary wick extending substantially axially through said tube to transport said liquid therealong; and a secondary wick comprising a screen formed into radial lobes deformed between said primary wick and said inner surface, said lobes contacting said inner surface at circumferentially spaced, discrete regions thereof to transport said liquid between said primary wick and said inner surface.

2. The heat pipe of claim 1 wherein said central wick comprises a multi-layered screen.

3. The wick of claim 2 wherein the mesh size of the secondary wick is different from that of the central wick.

4. The heat pipe of claim 1 in which said secondary wick comprises four of said lobes, two extending radially from opposite sides of the primary wick, respectively, in a first plane and the other two extending from opposite sides of the primary wick in a second plane that is substantially normal to said first plane.

5. The heat pipe of claim I in which said primary wick comprises three of said lobes extending radially from the primary wick at points spaced substantially equal distances apart around the circumference of the primary wick.

Claims (5)

1. A heat pipe comprising: a thermally conductive tube having circumferential grooves in its inner surface; a liquid contained within said tube; a primary wick extending substantially axially through said tube to transport said liquid therealong; and a secondary wick comprising a screen formed into radial lobes deformed between said primary wick and said inner surface, said lobes contacting said inner surface at circumferentially spaced, discrete regions thereof to transport said liquid between said primary wick and said inner surface.

2. The heat pipe of claim 1 wherein said central wick comprises a multi-layered screen.

3. The wick of claim 2 wherein the mesh size of the secondary wick is different from that of the central wick.

4. The heat pipe of claim 1 in which said secondary wick comprises four of said lobes, two extending radially from opposite sides of the primary wick, respectively, in a first plane and the other two extending from opposite sides of the primary wick in a second plane that is substantially normal to said first plane.

5. The heat pipe of claim 1 in which said primary wick comprises three of said lobes extending radially from the primary wick at points spaced substantially equal distances apart around the circumference of the primary wick.

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