Flexible Heat Pipe

Abstract

A heat pipe comprising a flexible sealed envelope and an internal wick structure comprising two sets of cross-members in transverse relation to each other, the cross-members being transverse to the flexible axis of the envelope. The wick structure may be a cylindrical mesh screen consisting of orthogonal metal wires disposed within a cylindrical metal bellows such that each of the cross wires is at an angle with respect to the flexible axis of the bellows.

Description

BACKGROUND OF THE INVENTION

This invention relates to a novel heat pipe and particularly to a flexible heat pipe having increased flexibility.

A prior flexible heat pipe has a sealed envelope comprising a hollow cylindrical bellows which is flexible along its longitudinal axis. Such a heat pipe is disclosed by G. Y. Eastman in "The Heat Pipe --A Progress Report," Proceedings of Fourth Intersociety Energy Conservation Engineering Conference, Sept. 1969. Disposed within the envelope is a flexible wick structure, which is saturated by a suitable working fluid. Usually, the wick structure comprises a porous mesh screen consisting of two orthogonal sets of interwoven cross wires. The screen in wound into a cylinder and then inserted within the bellows such that one set of cross wires is parallel to the longitudinal axis of the bellows.

Wick structures made in this manner tend to be very stiff along the desired axis of heat pipe flexibility. This is because the wires which are parallel to the longitudinal axis of the bellows cannot readily change their lengths when the heat pipe is flexed or bent. Thus, prior heat pipes have had poor flexibility. Also, the bending of such heat pipes has often resulted in mechanical failure of the wick structures, e.g., broken cross wires.

SUMMARY OF THE INVENTION

The novel heat pipe comprises a flexible sealed envelope having a longitudinal axis and an internal wick structure comprising a plurality of first crossmembers in transverse relation to a plurality of second crossmembers. Both the first and the second crossmembers are transverse to the flexible axis. The wick structure is saturated by a suitable working fluid.

By being transverse relative to one another and transverse to the flexible axis, the first and second crossmembers can readily change their effective lengths, in bending regions of the wick structure, when the heat pipe is bent. Thus, the novel heat pipe is more flexible than are prior heat pipes. Also, the problems of mechanical failure of the wick structures are greatly reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal view, partly in axial section, of an example of the novel heat pipe.

FIG. 2 is a sectional view, along the line 2--2, of the heat pipe of FIG. 1.

FIG. 3 is an enlarged perspective view of a portion of the wick structure of FIG. 1, and

FIG. 4 illustrates a method of shaping the wick structure of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the example shown in FIGS. 1 and 2, the novel heat pipe comprises an envelope 11 consisting of a long cylindrical stainless steel bellows 13 having flexibility along its longitudinal axis 14. The bellows 13 is sealed at each of its opposite ends to a cup-shaped end member 15 made of copper. Disposed within the envelope 11 and adjacent the inner walls thereof is a cylindrical wick structure 17 made of stainless steel wire mesh. The envelope 11 also contains a quantity of acetone working fluid sufficient to saturate the wick structure 17.

The wick structure 17, shown in detail in FIG. 3, comprises a plurality of first cross wires 19 orthogonal to and interwoven with a plurality of second cross wires 21. The cross wires 19 and 21 are not bonded at their cross over points, so that the wires are slidable with respect to each other. The wick structure 17 is disposed within the envelope 11 such that each of the cross wires 19 and 21 is at an angle of about 45.degree. with respect to the longitudinal axis 14 of the bellows 13. Thus, when the heat pipe is flexed or bent along the flexible axis 14, the cross wires 19 and 21 can slide with respect to each other and thereby change their effective lengths in the direction of the axis 14, in the bending regions of the wick structure 17.

FIG. 4 illustrates a method of shaping the wick structure 17. The starting structure is a square mesh screen comprising a plurality of first stainless steel cross wires 19' orthogonal to and interwoven with a plurality of second stainless steel cross wires 21'. The cross wires 19' are parallel to two parallel sides of the square, and the cross wires 21' are parallel to the other two parallel sides of the square. Each side of the square has a length equal to "(a+ b)/ 2", where "a" is the length of the wick structure 17 and "b " is the rolled-out width of the wick structure 17. That is, "b" is equal to "n .pi.d," where "d " is the effective diameter of the wick structure 17 and "n " is the number of wraps of the wire mesh constituting the wick structure 17. The square mesh screen is first shaped into a rectangular mesh screen having two parallel sides of length "a " and two parallel sides of length "b, " by suitable cutting or slicing the former at 45.degree. angles as indicated in FIG. 4. The rectangular mesh screen is then rolled or wound about its longitudinal axis to effect the cylindrical wick structure 17. Thus, the cross wires 19' and 21' of the rectangular mesh screen become the cross wires 19 and 21, respectively, of the wick structure 17.

The heat pipe may be assembled by the following procedure. The wick structure 17 is inserted within the open bellows 13. One end of the bellows 13 is welded to the open end of one of the end members 15. The other end of the bellows 13 is welded to the open end of the other end member 15, which has an exhaust tabulation (not shown). The exhaust tubulation is connected to a suitable vacuum exhaust system and also to a source of acetone working fluid, by valve means (none of which is shown). The assembled heat pipe is evacuated and then filled with the acetone working fluid. The completed heat pipe is sealed by pinching off the exhaust tubulation.

A heat pipe as described above was constructed to conduct 20 watts at 40.degree. C, over a distance of 18 inches. The wick structure had an effective diameter of about 0.5 inch and comprised 2 wraps of 120 .times.120 stainless steel wire mesh screen. The flexibility of the heat pipe along its longitudinal axis was grater than that previously obtainable, and the heat pipe operated successfully in a space environment for more than 100 hours.

GENERAL CONSIDERATIONS

There are various configurations embodying the invention. The bellows may be other than cylindrical in shape; for example, it may have a substantially square, rectangular, or oval cross section. Also, the cross section may be nonuniform over its length. Where acetone is employed as the working fluid, the bellows may be made of copper, copper alloy, or ferrous alloy, instead of stainless steel. Where another working fluid, such as water, is employed, the bellows may be made of any of various materials not corroded thereby.

The wick structure may also be other than cylindrical in shape; and it may be spaced from, rather than adjacent to, the inner walls of the heat pipe envelope. Depending upon the working fluid employed, the wick structure may be made of a material other than stainless steel, such as copper, copper alloy, or ferrous alloy. The crossmembers comprising the wick structure may be strips, rather than wires, made of the suitable material, and the sizes and spacings of the crossmembers may vary, instead of remaining constant. Also, the number of first crossmembers may be different from, rather than the same as, the number of second crossmembers.

The two sets of crossmembers may be other than orthogonal (90.degree.) to each other; in general, they may be in transverse relation. Experiments have shown that heat pipe flexibility is near maximum when the angle between the two sets of crossmembers is in the range from about 30.degree. (90.degree.-60.degree.) to about 150.degree. (90.degree.+60.degree.). Also, the wick structure need not be disposed such that the crossmembers are each at an angle of 45.degree. with respect to the longitudinal axis of the bellows. Each of the crossmembers need merely be transverse to the flexible axis.

The crossmembers may be interconnected (e.g., interlinked), rather than interwoven, so as to be slidable with respect to one another. However, the crossmembers need not be slidable with respect to one another. If they are not too stiff, the crossmembers may be bonded (e.g., spot welded) at their crossover points. The transverse relationship of the two sets of crossmembers to each other and also to the flexible axis will permit the bonded crossmembers to change their effective axial lengths in the bending regions of the wick structure. For example, a square capillary pore defined by four bonded cross wires may be stretched or compressed into a diamond-shaped opening by suitably bending the wick structure.

While the wick structure may be guided by the walls of the bellows, it is not supported thereby as a coating would be. This self-supporting quality is necessary to leave the crossmembers free to adjust their effective lengths when the heat pipe is flexed or bent. However, the wick structure may be attached to one or both of the end members. Depending upon the heat conductivity and other requirements thereof, the end members may be other than cup-shaped and made of a material other than copper.

Claims

What is claimed is:

1. A heat pipe comprising:

a. a flexible sealed envelope having a longitudinal axis;

b. a wick structure within said envelope, comprising a plurality of first crossmembers in transverse relation to a plurality of second crossmembers, said first crossmembers and said second crossmembers each being transverse to said axis; and

c. a working fluid saturating said wick structure within said envelope.

2. The heat pipe of claim 1, wherein said first crossmembers are in slidable relation to said second crossmembers.

3. The heat pipe of claim 1, wherein said first crossmembers are bonded to said second crossmembers.

4. The heat pipe of claim 1, wherein the angle between each of said first crossmembers and each of said second crossmembers is in the range from about 30.degree. to about 150.degree..

5. A heat pipe comprising:

a. a cylindrical flexible metal bellows;

b. means sealing said bellows at the opposite ends thereof;

c. a cylindrical wick structure within said bellows, comprising a plurality of first metal crossmembers in slidable transverse relation to a plurality of second metal crossmembers, said first crossmembers and said second crossmembers each being transverse to the longitudinal axis of said bellows; and

d. a working fluid saturating said wick structure within said bellows.

6. The heat pipe of claim 5, wherein said first crossmembers are interwoven with said second crossmembers.

7. The heat pipe of claim 5, wherein each of said first crossmembers is orthogonal to each of said second crossmembers.

8. The heat pipe of claim 5, wherein each of said first and second crossmembers is at an angle of about 45.degree. with respect to said longitudinal axis.

9. The heat pipe of claim 5, wherein said first crossmembers are substantially equally spaced constant-sized wires and said second crossmembers are substantially equally spaced constant-sized wires.

10. The heat pipe of claim 9, wherein the spacing and size of said first cross wires are substantially equal to the spacing and size, respectively, of said second cross wires.