Controllable Heat Pipe

Abstract

In a heat pipe comprising three heat-transfer regions, a first capillary structure is disposed at, and extends between, two of the regions; and a second capillary structure, hydrodynamically isolated from the first capillary structure, is disposed at the third region. Thermal switching between the first two regions, one of which serves as the condenser region of the heat pipe, is controlled by a heat flow between the third region and the condenser region.

Description

BACKGROUND OF THE INVENTION

This invention relates to a novel heat pipe and particularly to an improved controllable heat pipe.

Controllable heat pipes, i.e., heat pipes capable of functioning as thermal switches, have been prepared for various applications, including the temperature regulation of electronic equipment. Heretofore, such devices have comprised separate evaporator and condenser chambers interconnected by separate vapor-flow and liquid-flow conduits. See, for example, U.S. Pat. No. 3,543,839, issued on Dec. 1, 1970, to A. P. Schlosinger; and U.S. Pat. No. 3,614,981, issued on Oct. 26, 1971, to J. B. Coleman et al. To effect thermal switching, the devices disclosed in the former patent necessarily include a control valve, preferably pneumatically-actuated, in the vapor-flow conduit; and the devices disclosed in the latter patent necessarily include a wick-compression means, preferably magnetically-controlled, in the liquid-flow conduit.

Thus, in addition to being complicated in structure, these prior-art devices must employ internal moving parts and external mechanical control means. These disadvantages could be severely limiting in space-type environments.

SUMMARY OF THE INVENTION

The novel heat pipe comprises a sealed envelope including three heat-transfer regions. Adjacent to the inner wall of the envelope, at two of the regions, is a first capillary structure extending therebetween. Adjacent to the inner wall of the envelope, at another of the regions, is a second capillary structure hydrodynamically isolated from the first capillary structure. A vaporizable working fluid is also disposed within the envelope. Preferably, the two heat-transfer regions interconnected by the first capillary structure are included in a first chamber of the envelope, and the heat-transfer region in contact with the second capillary structure is included in a second chamber of the envelope, the second chamber being in communication with the first chamber. Also preferably, a thermoelectric (Peltier) device is connected between one of the heat-transfer regions of the first chamber and the heat-transfer region of the second chamber.

The two heat-transfer regions interconnected by the first capillary structure serve as the evaporator and condenser regions of the heat pipe. By having the second capillary structure hydrodynamically isolated (i.e., isolated by a non-wetting barrier zone) from the first capillary structure, thermal switching between the evaporator and condenser regions can be controlled merely by varying the direction of the heat flow between the third heat-transfer region and the condenser region. Thus, the novel heat pipe eliminates the need for internal moving parts and external mechanical-control means. Including the evaporator and condenser regions in a first chamber of the heat pipe, while including the third heat-transfer region in a communicating second chamber thereof, serves to thermally-insulate the third heat-transfer region from the condenser region and thereby effect better switching control. By connecting a thermoelectric (Peltier) device between the condenser region and the third heat-transfer region, the direction of the heat flow between these two regions can be varied simply by varying the direction of a small direct current through the device. Hence, the novel heat pipe is simpler to construct and also to operate than are the prior art controllable heat pipes. Moreover, it is better suited to space applications.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal sectional view of an example of the novel heat pipe; and

FIG. 2 is a longitudinal sectional view of an alternative form of a portion of the novel heat pipe of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the example shown in FIG. 1, the novel heat pipe 10 comprises a sealed envelope 12 containing a first chamber 14 in communication with a second chamber 16. The wall of the envelope 12 is made of a metal which may be copper; and the first and second chambers 14 and 16, respectively, may each be tubular in shape. The first chamber 14 includes a first heat-transfer region 18, which serves as teh evaporator region of the heat pipe 10, and a second heat-transfer region 20, which serves as the condenser region thereof. Adjacent to the inner wall of the envelope 12, within the first chamber 14, is a first capillary structure 22 disposed at, and extending between, the first and second heat-transfer regions 18 and 20, respectively. The first capillary structure 22 may be a tubular mesh made of copper wire.

The second chamber 16 includes a third heat-transfer region 24, which serves as the control region of the heat pipe 10 (the operation of which is described below). Adjacent to the inner wall of the envelope 12, within the second chamber 16, is a second capillary structure 26 disposed at the third heat-transfer region 24. The second capillary structure 26, which also may be a tubular mesh made of copper wire, is hydrodynamically isolated from the first capillary structure 22. That is, the second capillary structure 26 is separated from the first capillary structure 22 by a non-wetting barrier zone which prevents any liquid flow therebetween. The envelope 12 also contains a quantity of vaporizable working fluid (not shown), such as water, preferably insufficient to saturate either the first or the second capillary structure 22 or 26, respectively.

Connected between, and thermally coupled to, the second and third heat transfer regions 20 and 24, respectively, is a typical thermoelectric (Peltier) device 28. As is known, the junction of two dissimilar thermoelectric materials can be cooled or heated by passing a direct current in either of two directions therethrough. Two of these junctions can be connected in series with a direct current source so that one junction can be cooled while the other is being heated, i.e., a heat flow can be produced between the two junctions. See, for example, McGraw-Hill Encyclopedia of Science and Technology, Vol. 13, pp. 577-585 (1960). The device 28 employs two such junctions (not shown), one junction in thermal contact with the second heat-transfer region 20 and the other junction in thermal contact with the third heat-transfer region 24. The direction and magnitude of the heat flow between the second and third heat-transfer regions 20 and 24, respectively, can then be controlled by a suitable direct current supply (not shown) electrically connected to the device 28 by means of two input terminals 30. Thermoelectric (Peltier) devices, as well as power supplies therefor, are discussed by R. R. Heikes and R. W. Ure, Jr. in Thermoelectricity: Science and Engineering, Interscience Publishers, N. Y. (1961).

To effect normal operation of the heat pipe 10, a direct current is passed through the terminals 30 in a first direction such that the thermoelectric device maintains a heat flow from the second heat-transfer region 20 to the third heat-transfer region 24. As a result, the working fluid in the second capillary structure 26 is evaporated therefrom and condensed onto the first capillary structure 22, at the second heat-transfer region 20.

Liquid flow in the first capillary structure 22 transfers the condensate from the second heat-transfer region 20 to the first heat-transfer region 18. Heat into the first heat-transfer region 18 is then conducted out of the second heat-transfer region 20 by typical heat pipe action. That is, the working fluid at the first heat input region 18 is vaporized by absorption of the thermal-energy input; the pressure difference between the first and second heat-transfer regions 18 and 20, respectively, causes the vapor to flow from the former to the latter; the thermal energy is released by condensation of the vapor at the second heat-transfer region 20; and the condensate is returned to the first heat-transfer region 18 by liquid flow in the first capillary structure 22.

When it is desired to inhibit normal operation of the heat pipe 10, a direct current is passed through the terminals 30 in a second direction, opposite to the first, such that the thermoelectric device 28 maintains a heat flow from the third heat-transfer region 24 to the second heat-transfer region 20. As a result, the vaporized working fluid at the first heat input region 18 flows to the third heat-transfer region 24, at which condensation occurs. The condensate is not returned to the first heat-transfer region 18, because of the hydrodynamic isolation of the first and second capillary structures 22 and 26, respectively, but is instead retained at the third heat-transfer region 24. Such action continues until all of the working fluid is "stored" in the second capillary structure 26 and complete turn-off of the heat pipe 10 is effected. To resume normal operation as described above, the current is again reversed through the terminals 30, whereby the stored working fluid is evaporated from the third heat-transfer region 24 and condensed at the second heat-transfer region 20.

A typical application for the heat pipe 10 is the temperature regulation of electronic equipment. For example, an electronic component may serve as the heat source (not shown) at the first heat-transfer region 18. When the temperature of the component rises above a given value, the heat pipe 10 can be switched "on"; and when it falls below another given value, the latter can be switched "off"--all preferably automatically.

An alternative form of the envelope 12 of the heat pipe 10 is shown in FIG. 2, wherein the former comprises a first metal portion 12a sealed to a second glass or ceramic portion 12b. The first envelope portion 12a encloses the first chamber 14 and includes the first and second heat-transfer regions 18 and 20, respectively. The second envelope portion 12b encloses the second chamber 16 and includes the third heat-transfer region 24. Thus, the alternative form of the envelope 12 serves to further thermally-insulate the second and third heat-transfer regions 20 and 24, respectively, so as to effect better switching control of the heat pipe, 10. Metal-to-glass or ceramic envelopes for heat pipes are disclosed in U.S. Pat. No. 3,543,841, issued on Dec. 1, 1970, to G. Y. Eastman.

General Considerations

It should be understood that the invention is not limited to the embodiments described above. For example, various combinations of envelope, capillary structure, and working fluid materials may be employed by the novel heat pipe. Thus, for sodium or potassium working fluids, the envelope and capillary structures may be made of nickel; for mercury working fluids, the latter may be made of stainless steel. Also, various envelope and capillary-structure geometries may be employed. Thus, the envelope may comprise an annular, angular, or flat-plate design; and the capillary structures may be of the porous-fiber or groove type. See G. Y. Eastman, "The Heat Pipe--A Progress Report," Proceedings of the 4th Intersociety Energy Conversion Engineering Conference, pp. 873-878 (September 1969); see also U.S. Pat. Application Ser. No. 38,323, filed on May 18, 1970, by R. A. Freggens.

The thermoelectric (Peltier) device may be internal to the heat pipe envelope or it may be entirely eliminated from the design, where appropriate. Thus, other means may be employed to control the direction of the heat flow between the control and condenser regions of the heat pipe. Also, these two heat-transfer regions may be thermally insulated from one another (as well as from the evaporator region) by further envelope design, external means, etc. Finally, the heat source may be a mechanical, as well as electrical, component whose temperature is to be controlled manually or automatically.

Claims

What is claimed is:

1. A heat pipe comprising:

a. a sealed envelope containing a first chamber in communication with a second chamber;

b. said first chamber including two heat-transfer regions in contact with a first capillary structure extending therebetween;

c. said second chamber including a third heat-transfer region in contact with a second capillary structure, said second capillary structure being hydrodynamically isolated from said first capillary structure;

d. a vaporizable working fluid disposed within said envelope; and

e. means for producing a heat flow between said third heat-transfer region and one of said other two heat-transfer regions.

2. The heat pipe of claim 1, wherein said means comprises a thermoelectric device.

3. The heat pipe of claim 1, wherein said third heat-transfer region is thermally insulated from said other two heat-transfer regions.

4. The heat pipe of claim 3 wherein a first portion of said envelope is made of metal and a second portion thereof is made of glass or ceramic.

5. The heat pipe of claim 4, wherein said first envelope portion contains said first chamber and said second envelope portion contains said second chamber.