Heat Pipes

Abstract

A heat pipe comprising a fluid-tight container 10 for transferring heat from a source 13 adjacent to an evaporation region 14 to a sink 15 adjacent to a condenser region 16, a passage 17 for transferring vapor from the evaporator region to the condenser region, and a wick 18 having high heat conductivity for transferring condensate from the condenser region back to the evaporator region by capillary pumping and for conducting heat from the container in the evaporator region to the evaporation sites 19 and from the condensation sites 20 to the container in the condenser region. The wick comprises a bundle-like arrangement of substantially direct, parallel, substantially uniform capillary channels 21, each about 10.sup..sup.-4 to 10.sup..sup.-1 square millimeter in cross-sectional area and having a low rugosity factor, from the condenser region to the evaporator region. Each end 24,26 of the wick forms an angle 25,27 of about 15.degree. to 60.degree. with the adjacent capillary channels, and may have porous surfaces, to provide substantial areas for evaporation and condensation at the ends of the channels.

Description

BACKGROUND OF THE INVENTION

This invention relates to heat transfer devices of the type commonly referred to as heat pipes.

The conventional heat pipe consists of a sealed container lined with a "wicking" structure that is saturated with a liquid or working fluid at a temperature corresponding to liquidus temperature for a given saturation pressure. Thus, the operating temperature is a function of the heat sink temperature and the pressure-temperature characteristic of the selected working fluid. Heat is added to one region of the heat pipe, referred to as the evaporator region, and the liquid at the exposed end of the wick structure evaporates, generally into a central vapor channel. Thus, the thermal input is assimilated in the evaporator section of the heat pipe via the latent heat of vaporization of the working fluid. This vaporization phenomena results in a vapor pressure gradient which causes the generated vapor to flow from the evaporator region through a vapor channel to a condenser region where the vapor condenses onto the opposite end of the wick, giving up its latent heat of vaporization. This phase change phenomena, which occurs at very nearly constant temperature, provides highly efficient transport of thermal energy.

The evaporation of liquid at the vapor-liquid interface in the evaporator region causes the residual liquid to retreat into the capillary structure producing a meniscus radius of curvature and a contact angle that are smaller in the evaporator region than in the condenser region where the condensate is deposited. This difference effects a pressure gradient which pumps liquid through the wicking structure from the condenser region back to the evaporator region to complete the working fluid transfer cycle.

In order to be an effective heat transfer device, the heat pipe must be optimized to properly merge the physical characteristics of the working fluid with the geometric constraints and the desired operational temperature range. The maximum thermal power per unit temperature difference between the extreme end points of the heat pipe that can be transferred in a heat pipe of fixed dimensions, is determined by

A. the pumping capability of the wick structures,

b. the thermophysical properties, particularly the thermal conductivity of the materials of construction employed for the wick and containment vessel and methods of attachment (i.e., thermal impedances between shell and wick in the evaporator and condenser sections),

c. the physical properties of the working fluid, such as surface tension, contact angle, latent heat of vaporization, viscosity of the liquid and gas phases, density of the liquid and gas phases, and vapor pressure, over the temperature range of interest,

d. the onset of boiling of the fluid in the evaporator regions, due to superheating of the fluid induced by high heat fluxes,

e. the onset of entrainment, i.e., the counterflow shear between the liquid on the wick and the vapor in the vapor passage, and

f. the vapor phase sonic limit, i.e., the "upper-limit" velocity at which vapor can be transferred from the evaporator to the condenser regions of the heat pipe.

The key component of the heat pipe is the wick structure, which performs the following four basic functions:

1. Liquid pumping. Results from surface tension forces developed in wick pores at the liquid-vapor interface; small pores are desirable, particularly in the evaporator region.

2. Liquid-flow path. Liquid drawn from the condenser to the evaporator flows in wick channels; large, smooth-wall channels are desirable for low hydrodynamic losses.

3. Radial heat-flow path. Thermal energy transferred by evaporation or condensation is conducted through a liquid-wick composite structure; high thermal conductivity of both wick and liquid is desirable.

4. Liquid-vapor flow separation. At high-performance conditions, the counterflow shear between the liquid and vapor phases becomes important; fine pores or even a solid separation layer is desirable at the liquid-vapor interface in the adiabatic regions of high-performance heat pipes.

Capillary wick structures of the prior art include (1) woven cloth, fiberglass, or metal mats, or screens, (2) porous metal or ceramic tubes (generally made from sintered powders), and (3) parallel grooved channels (e.g., as shown by U.S. Pat. No. 3,402,767). These wick structures are generally positioned contiguous to the inside wall of the sealed container and are limited in performance by the tortuous liquid-flow paths in the case of mats, screens or porous tubes, and by channel shape and size in the case of the parallel grooved channels.

The parallel capillary channel wick structure of the present invention provides significant increases in the heat pumping capacity over the conventional wick structures. A multiplicity of parallel capillary channels provide a significant reduction in hydrodynamic losses when compared with the tortuous channels of typical sintered powder wicks, or the metal felt or screen wicks commonly used. The parallel capillary channel wick structure of the present invention has the following advantages over conventional wick designs:

a. high fluid conductance of the wick flow passages,

b. reproducible wick structure (in terms of porosity and pore shape and size),

c. higher effective thermal conductivity between the container, the working fluid, and the wick, and

d. wide variety of materials that can be used in the fabrication of the wick, such as nickel, copper, and stainless steel.

The wick structure can be fabricated with extremely uniform pore diameters ranging from 10 to 300 microns and having porosities in the range of 40 to 80 percent. The advantages of the wick structure of the present invention result in an order of magnitude increase in the heat pumping capacity as compared with conventional sintered powder wick structures.

In the present heat pipe diffusion-bonded, and optionally braze-bonded, wick-container composite structures serve to minimize the thermal impedance associated with conduction heat transfer across the container-wick interface and through the wick structure leading to the sites of evaporation. In addition, a surface porous coating is applied to the parallel capillary channel wick in the evaporator region. This porous coating serves to increase the number of evaporation sites and, hence, decreases the thermal impedance associated with the transfer of heat from the wick to the center region of the capillary meniscus where evaporation is actually taking place.

One of the advantages of the parallel capillary channel wick is its "restart capability," the capability of resaturating the wick and any optional arteries or grooves once the working fluid in the liquid state has been removed. For example, composite wicks (such as the artery-containing wick and the grooved-channel wick with screen) are difficult to "restart" since the capillary pumping action required is inversely proportional to the effective capillary diameter. Hence, in the case of the wick containing an "enlarged" artery, the capillary pumping action is usually inadequate to resaturate the wick, particularly if the heat pipe is oriented such that capillary pumping must work against gravity. The "temporary removal" of working fluid (in the liquid state) from portions of the wick may result from (1) shock- or vibration-induced loss, (2) "burn-out" of wick under excessive heat load conditions, (3) exceeding the critical temperature of the working fluid, and (4) freeze-out in condenser region of heat pipe before vaporization ceases in evaporator region. If any of the above or other adverse conditions cause liquid displacement from the wick to below some critical level, viz, the capillary/artery interface, then the composite wick may be difficult if not impossible to restart in situ. Since the capillaries are of uniform size in the present concept, the restarting, or resaturating of the wick, is readily achieved.

SUMMARY OF THE INVENTION

A typical heat pipe according to the present invention comprises a fluid-tight container for transferring heat therethrough from a source adjacent to an evaporation region thereof to a sink adjacent to a condenser region thereof, a passage for transferring vapor from the evaporator region to the condenser region, and a wick for transferring condensate from the condenser region back to the evaporator region by capillary pumping and for conducting heat from the container in the evaporator region to the evaporation sites and from the condensation sites to the container in the condenser region, the wick comprising a bundle-like arrangement of substantially direct, parallel, substantially uniform capillary channels, each about 10.sup.-.sup.4 to 10.sup.-.sup.1 square millimeter in cross-sectional area and having a low rugosity factor, from the condenser region to the evaporator region, the end of the wick in the evaporator region forming an acute angle with the adjacent capillary channels to provide substantial areas for evaporation of the condensate at the ends of the channels.

The cross-sectional area of the channels typically is about 40 to 80 percent of the cross-sectional area of the wick, and the end of the wick in the evaporator region may have a porous surface thereon for conveying condensate from the ends of the adjacent capillary channels to provide additional areas for evaporation of the condensate.

The end of the wick in the condenser region preferably also forms an acute angle with the adjacent capillary channels to provide substantial areas for condensation of the vapor at the ends of the channels, and each angle preferably is about 15.degree. to 60.degree..

The wick should comprise a material having high heat conductivity, and should be in tight thermal contact with the container over substantial areas in the evaporation region and in the condenser region. Adjacent surfaces in the wick between the evaporation site and the evaporator region of the container and between the condensation sites and the condenser region of the container should also be in tight thermal contact over substantial areas to minimize the thermal impedance therebetween. The tight thermal contact typically is provided by diffusion-bonded or braze-bonded surfaces.

Where nonmetallic liquids are used in the heat pipes, the cross-sectional area of each capillary channel should be about 10.sup.-.sup.4 to 2.times.10.sup.-.sup.2 square millimeter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of a typical heat pipe according to the present invention.

FIG. 2 is a transverse cross-sectional view taken in the plane 2--2 of FIG. 1.

FIG. 3 is an enlarged fragmentary section of the encircled area 3 in FIG. 1.

FIG. 4 is a longitudinal cross-sectional view of another typical heat pipe according to the present invention.

FIGS. 5-9 are transverse cross-sectional views of other typical embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a typical preferred embodiment of a heat pipe according to the present invention comprising a fluid-tight container 10 formed of a hollow cylindrical body member 11 sealed at each end with an end plate 12. The container 10 preferably is made of a material having high thermal conductivity, such as nickel, for transferring heat therethrough from a source, indicated schematically at 13, adjacent to an evaporation region 14 of the container 10 to a sink, indicated schematically at 15, adjacent to a condenser region 16 of the container 10. A passage 17 is provided for transferring vapor from the evaporator region 14 to the condenser region 16, and a wick 18 is provided for transferring condensate from the condenser region 16 back to the evaporator region 14 by capillary pumping and for conducting heat from the container 10 in the evaporator region 14 to the evaporation sites 19 at the evaporator end 24 of the wick 18 and from the condensation sites 20 at the condenser end 26 to the container 10 in the condenser region 16.

The wick 18 comprises a bundle-like arrangement of substantially direct, parallel, substantially uniform capillary channels 21 each about 10.sup.-.sup.4 to 10.sup.-.sup.1 square millimeter in cross-sectional area and having a low rugosity factor (approaching) unity), from the condenser region 16 to the evaporator region 14. As used herein, "bundle-like arrangement" means that in an end view or cross-sectional view, as in FIGS. 2 and 5-9, the channels appear as an array (rows and columns as in FIG. 5) or a substantially similar two-dimensional arrangement (as in FIGS. 6-9). A bundle-like arrangement provides substantial advantages over a single row or circle of channels such as those along the inside of the tube in the heat pipe of United States Pat. No. 3,402,767 of Bohdansky. In Bohdansky's arrangement it is easy to provide a large evaporation area simply by having the channels open along the inner side. However, the number of channels that can be provided in a single layer is limited. Where many layers are used, as in the bundle-like arrangement in the present invention, many more channels are available and a much larger quantity of condensate can be transported in a given time. As used herein, "having a low rugosity factor (approaching unity)" means that impedance to the flow of fluid is substantially the minimum that is possible for the cross-sectional area, which requires primarily that the cross-sectional shape be substantially circular, square, or other shape of low eccentricity such that the distance across is nearly the same in all directions. Also, it is required that the inner surfaces of the capillary channels be reasonably smooth in order to minimize viscous losses.

The end 24 of the wick 18 in the evaporator region 14 forms an acute angle 25 with the adjacent capillary channels 21 to provide substantial areas 19 for evaporation of the condensate at the evaporator ends of the channels 21. This is important to provide a large enough total evaporation area in a plurality of layers of channels. The end 26 of the wick 18 in the condenser region 16 preferably also forms an acute angle 27 with the adjacent capillary channels 21 to provide substantial areas 20 for condensation of vapor at the condenser ends of the channels 21. Each angle 25,27 preferably is about 15.degree. to 60.degree.. The cross-sectional area of the channels 21 preferably is about 40 to 80 percent of the entire cross-sectional area of the wick 18.

As shown in the FIG. 3, the end 24 of the wick 18 in the evaporator region 14 may be provided with a porous surface, as indicated at 28, for conveying condensate from the ends 19 of the adjacent capillary channels 21 to provide additional areas at 28 for evaporation of the condensate.

Like the container 10, the wick 18 preferably is made of a material having high heat conductivity. The wick 18 should be in tight thermal contact with the container 10 over substantial areas in the evaporator region 14 and in the condenser region 16. Where the container 10 and the wick 18 are formed separately, the tight thermal contact may be provided by a very tight press fit, as by heating the container 10 to a substantially higher temperature than that of the wick 18 when they are placed together, or by diffusion bonding the contiguous surfaces, or by brazing them together.

The wick 18 may comprise a substantially solid member, except for the channels 21 therein, as in FIGS. 2, 7, and 8, or it may be formed from more than one piece, as in FIGS. 5, 6, and 9, with adjacent surfaces in the end 24 of the wick 18 between the evaporation sites 19 and the evaporator region 14 of the container 10, and in the end 26 between the condensation sites 20 and the condenser region 16 of the container 19, in tight thermal contact over substantial areas to minimize the thermal impedance between the evaporation sites 19 and the container 10 and between the condensation sites 20 and the container 10. The tight thermal contact may be provided by diffusion bonding or brazing the contiguous surfaces.

Where the channels 21 are substantially circular in cross-section, the diameter of each should be about 10 to 300 microns (about 10.sup.-.sup.4 to 10.sup.-.sup.1 square millimeter in cross-sectional area) to provide the optimum capillary pumping action. Where nonmetallic liquids are to be transported through the channels 21, the diameter should be in the low portion of the range, about 10 to 130 microns (about 10.sup.-.sup.4 to 2.times. 10.sup.-.sup.2 square millimeters), for optimum capillary pumping action. The size of the angle 25 at the evaporator end 24 of the wick 18 and the size of the angle 27 at the condenser end 26 of the wick 18 depend primarily on the lengths of the adjacent head source 13 and heat sink 15 since the object of the tapered sections is to distribute the evaporation and condensation sites over the available surfaces of the heat source and sink. As shown in FIG. 4, at 30 and 31, the end region may be curved so that the angle varies in size along the end region 24 or 26. Within the range of about 15.degree. to 60.degree. the exact size of the angle is not critical.

FIG. 4 also shows another variation that can be made in the heat pipe of FIG. 1, namely that in part or all of the adiabatic region 32, between the evaporator region 14 and the condenser region 16, the wick 18 may be omitted, and the condensate from the condenser region 16 may return from the condenser end wick 26 through the annular open space 33 to the evaporator end wick 24. However, a heat pipe as shown in FIG. 4 is not self-starting. The space 33 must be substantially full of fluid immediately before and continually during operation of the heat pipe.

The annular space 33 in FIG. 4 may contain a wick 18 as in FIG. 1. Where desired, especially where the region 32 is longer than the wick 18 can be made conveniently, several sections of wick 18 may be arranged in tandem along the length of the annular space 33. Preferably each section of wick 18 should be spaced a fraction of a millimeter from the adjacent section so as not to block the passage of condensate through the heat pipe as might happen if the channels 21 in successive sections of the wick 18 did not substantially register with one another.

FIGS. 2 and 5-9 illustrate ways in which the wick 18 may be formed. In FIGS. 2, 7, and 8, the channels 21 may be formed by rods that are embedded in the wick 18 when it is cast or formed in any other convenient way, the rods being subsequently removed by any convenient method such as melting or chemical dissolution. The passages 17 in FIGS. 2 and 7 may be formed in similar ways or by using cores of sand or other appropriate material in the forming process.

FIG. 8 illustrates the fact that the passage 17 need not be surrounded by the wick 18. A longitudinal cross-sectional view of the heat pipe in FIG. 8 is the same as FIG. 1 with the lower part of the wick 18 omitted.

The heat pipe in FIG. 9 is similar to that in FIG. 2 except that the channels 21 comprise the spaces between adjacent rods or wires 34 arranged in a bundle between the container 10 and the passage 17.

The heat pipe in FIG. 6 is similar to that in FIG. 9 except that the passages 21 comprise the spaces between adjacent portions of a thin corrugated member 35 and a contiguous thin flat member 36 wound in a spiral around the passage 17 and mounted inside the container 10. In FIG. 5 also the channels 21 comprise the spaces between the adjacent surfaces of a thin corrugated member 35 and a thin flat member 36 but with the members 35 and 36 stacked alternately inside the container 10 and around the passage 17. In FIGS. 5 and 6 the flat members 36 may be omitted if the corrugated members 35 are arranged in such a manner as to assure that adjacent peaks and valleys are sufficiently out of registry to provide the spaces required for the channels 21.

Of course the drawings are not to scale or even roughly in proportion. The container 10 typically has an outside diameter of less than one-half inch and a length of 2 inches or greater. One typical heat pipe as in FIGS. 1 and 6 using ammonia as the heat transfer fluid is about 0.19 inch in outside diameter and 6 inches long. Another, as in FIGS. 1 and 2 and using liquid nitrogen, is about 0.25 inch in outside diameter and 3 inches long.

While the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. It is not intended herein to mention all of the possible equivalent forms or ramifications of the invention. It is to be understood that the terms used herein are merely descriptive rather than limiting, and that various changes may be made without departing from the spirit or scope of the invention.

Claims

I claim:

1. A heat pipe comprising

a fluid-tight container for transferring heat therethrough from a source adjacent to an evaporation region thereof to a sink adjacent to a condenser region thereof,

a passage for transferring vapor from the evaporator region to the condenser region, and

a wick for transferring condensate from the condenser region back to the evaporator region by capillary pumping and for conducting heat from the container in the evaporator region to the evaporation sites and from the condensation sites to the container in the condenser region,

the wick comprising a bundle-like arrangement of substantially direct, parallel, substantially uniform capillary channels, each about 10.sup.-.sup.4 to 10.sup.-.sup.1 square millimeter in cross-sectional area and having a low rugosity factor, from the condenser region to the evaporator region,

the end of the wick in the evaporator region forming an acute angle with the adjacent capillary channels to provide substantial areas for evaporation of the condensate at the ends of the channels, and

the end of the wick in the evaporator region having a porous surface thereon for conveying condensate from the ends of the adjacent capillary channels to provide additional areas for evaporation of the condensate.

2. A heat pipe as in claim 1, wherein the cross-sectional area of the channels is about 40 to 80 percent of the cross-sectional area of the wick.

3. A heat pipe as in claim 1, wherein the angle is about 15.degree. to 60.degree..

4. A heat pipe as in claim 1, wherein the end of the wick in the condenser region forms an acute angle with the adjacent capillary channels to provide substantial areas for condensation of the vapor at the ends of the channels.

5. A heat pipe as in claim 4, wherein each angle is about 15.degree. to 60.degree..

6. A heat pipe as in claim 1, wherein the wick comprises a material having high heat conductivity.

7. A heat pipe as in claim 6, wherein the wick is in tight thermal contact with the container over substantial areas in the evaporation region and in the condenser region.

8. A heat pipe as in claim 7, wherein adjacent surfaces in the wick between the evaporation site and the evaporator region of the container and between the condensation sites and the condenser region of the container are in tight thermal contact over substantial areas to minimize the thermal impedance therebetween.

9. A heat pipe as in claim 8, wherein the tight thermal contact is provided by diffusion-bonded surfaces.

10. A heat pipe as in claim 8, wherein the tight thermal contact is provided by braze-bonded surfaces.

11. A heat pipe as in claim 1, wherein the cross-sectional area of each capillary channel is about 10.sup.-.sup.4 to 2.times.10.sup.-.sup.2 square millimeter.