US20070240858A1

US20070240858A1 - Heat pipe with composite capillary wick structure

Info

Publication number: US20070240858A1
Application number: US11/309,493
Authority: US
Inventors: Chuen-Shu Hou; Tay-Jian Liu; Qian-Hua He; Chih-Hsien Sun; Chao-Nien Tung
Original assignee: Foxconn Technology Co Ltd
Current assignee: Foxconn Technology Co Ltd
Priority date: 2006-04-14
Filing date: 2006-08-11
Publication date: 2007-10-18
Also published as: CN101055158A

Abstract

A heat pipe includes a metal casing (100) and a composite capillary wick received in the casing. The metal casing has an evaporating section (400), a condensing section (600) and a central section (500) between the evaporating section and the condensing section. First type of capillary wick (200) is provided on an inner all of the metal casing at all of the condensing, central and evaporating sections, while second type of capillary wick (220) is provided on the first type of capillary wick at the central and evaporating sections only. A capillary pore size of the first type of capillary wick is larger than that of the second type of capillary wick.

Description

1. FIELD OF THE INVENTION

The present invention relates generally to a heat transfer apparatus, and more particularly to a heat pipe having a composite capillary wick structure.

2. DESCRIPTION OF RELATED ART

As a heat transfer apparatus, heat pipes can transfer heat rapidly and therefore are widely used in various fields for heat dissipation purposes. Fox example, in electronic field, heat pipes are commonly applied to transfer heat from heat-generating electronic components, such as central processing units (CPUs), to heat dissipating devices, such as heat sinks, to thereby remove the heat away. A heat pipe in accordance with related art generally includes a sealed casing made of thermally conductive material and a working fluid contained in the casing. The casing includes an evaporating section at one end and a condensing section at the other end. The working fluid is employed to carry heat from the evaporating section to the condensing section of the casing. Specifically, when the evaporating section of the heat pipe is thermally attached to a heat-generating electronic component, the working fluid receives heat from the electronic component and evaporates. Then, the generated vapor moves towards the condensing section of the heat pipe under the vapor pressure gradient between the two sections. In the condensing section, the vapor is condensed to liquid state by releasing its latent heat to, for example, a heat sink attached to the condensing section. Thus, the heat is removed away from the electronic component.
In order to rapidly return the condensed liquid back from the condensing section to the evaporating section to start a next cycling of evaporation and condensation, a capillary wick is generally provided on an inner surface of the casing in order to accelerate the return of the liquid. In particular, the liquid is drawn back to the evaporating section by a capillary force developed by the capillary wick. The capillary wick may be a plurality of fine grooves defined in a lengthwise direction of the casing, a fine-mesh wick, or a layer of sintered metal or ceramic powders. However, the capillary force derived from each type of these wicks is generally different, and meanwhile, the liquid flow resistance provided by each type of wick is also different. The general rule is that the larger average capillary pore size a wick has, the smaller capillary force it develops and the lower liquid flow resistance it provides.
FIG. 5 shows an example of a heat pipe in accordance with related art. The heat pipe includes a metal casing 10 and a single-type, uniform capillary wick 20 on an inner surface of the casing 10. The casing 10 includes an evaporating section 40 at one end and a condensing section 60 at the other end. An adiabatic section 50 is provided between the evaporating and condensing sections 40, 60. The adiabatic section 50 is typically used for transport of the generated vapor from the evaporating section 40 to the condensing section 60. The wick 20 is uniformly arranged against the inner surface of the casing 10 from the evaporating section 40 to the condensing section 60. However, this single-type, uniform wick 20 generally cannot provide optimal heat transfer effect for the heat pipe because it cannot simultaneously obtain a large capillary force and a low flow resistance.
In view of the above-mentioned disadvantage of the heat pipe, there is a need for a heat pipe having a good heat transfer effect.

SUMMARY OF THE INVENTION

A heat pipe in accordance with a preferred embodiment of the present invention includes a metal casing and a composite capillary wick arranged on an inner surface of the casing. The casing has an evaporating section, a condensing section and an adiabatic central section between the evaporating section and the condensing section. A first type of capillary wick is provided on an inner wall of the casing at the evaporating, condensing and central sections of the casing. A second type of capillary wick is provided on the first type of capillary wick at the evaporating and central sections of the casing. A capillary pore size of the first type of capillary wick is larger than that of the second type of capillary wick.
Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiment when taken in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present apparatus and method can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present apparatus and method. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a longitudinally cross-sectional view of a heat pipe in accordance with a first embodiment of the present invention;

FIG. 2 is a transversely cross-sectional view of the heat pipe in accordance with the first embodiment of the present invention;

FIG. 3 is a longitudinally cross-sectional view of a heat pipe in accordance with a second embodiment of the present invention;

FIG. 4 is a longitudinally cross-sectional view of a heat pipe in accordance with a third embodiment of the present invention; and

FIG. 5 is a longitudinally cross-sectional view of a heat pipe in accordance with related art.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a heat pipe in accordance with a first embodiment of the present invention. The heat pipe comprises a casing 100 and a composite capillary wick (not labeled) arranged on an inner wall of the casing 100. The casing 100 comprises an evaporating section 400 at one end and a condensing section 600 at an opposite end thereof, and a central section (i.e., adiabatic section) 500 located between the evaporating section 400 and the condensing section 600. The casing 100 is made of highly thermally conductive materials such as copper or copper alloys and filled with a working fluid (not shown), which acts as a heat carrier for carrying thermal energy from the evaporating section 400 to the condensing section 600. Heat that needs to be dissipated is transferred firstly to the evaporating section 400 of the casing 100 to cause the working fluid therein to evaporate. Then, the heat is carried by the working fluid in the form of vapor to the condensing section 600 where the heat is released to ambient environment; thus, the vapor condenses into liquid. The condensed liquid is then brought back via the composite capillary wick to the evaporating section 400 where it is again available for evaporation.
The composite capillary wick comprises a first type of capillary wick 200 made by a plurality of fine grooves and a second type of capillary wick 220 formed by a layer of meshed wick. The first type of capillary wick 200 is formed on the inner wall of the casing 100 through all of the evaporating, central and condensing sections 400, 500, 600 of the casing 100, and the second type of capillary wick 220 is arranged to be attached on the first type of capillary wick 200 at the evaporating and central sections 400, 500 of the casing 100 only. Accordingly, the condensing section 600 only has the first type of capillary wick 200, i.e., a plurality of fine grooves, while the central and evaporating sections 500, 400 have both the first and second types of capillary wicks 200, 220, i.e., a plurality of fine grooves and a layer of meshed wick, wherein the fine grooves surround the layer of meshed wick. The meshed wick is consisted of a fine mesh. The first type of capillary wick 200 extends in the lengthwise direction of the casing 10 and may be formed by mechanical machining. The capillary pore size of the first type of capillary wick 200 is larger than that of the second type of capillary wick 220 (see FIG. 2) so that the first type of capillary wick 200 at the central section 500 can provide a smaller flow resistance than the second type of capillary wick 220 at the central section 500. In addition, the second type of capillary wick 220 can provide a larger capillary force than the first type of capillary wick 200. By such design of a composite capillary wick at the evaporating section 400 and the central section 500, the condensed liquid can be quickly drawn back to the evaporating section 400 from the condensing section 600 via the central section 500.
In this embodiment, the composite capillary wick has different types of capillary wick disposed in different sections of the heat pipe. The first type of capillary wick 200 has a relatively large average capillary pore size and therefore provides a relatively low flow resistance to the condensed liquid to flow therethrough, and meanwhile, the second type of capillary wick 220 has a relatively small average capillary pore size and accordingly develops a relatively large capillary force to the liquid. As a result, the first type of capillary wick 200 reduces the flow resistance the condensed liquid encounters when flowing through the condensing and central sections 600, 500, and the second type of capillary wick 220 has a large capillary force and therefore the liquid is then rapidly drawn back to the evaporating section 400 from the central section 500. The condensed liquid is returned back from the condensing section 600 in an accelerated manner. After the condensed liquid is returned back to the evaporating section 400, a next phase-change cycling will then begin. Thus, as a whole, the cycling of the working fluid is accelerated and therefore the total heat transfer capacity of the heat pipe is enhanced.
FIG. 3 illustrates a heat pipe in accordance with a second embodiment of the present invention. Main differences between the first and second embodiments are that in the second embodiment a sintered-type wick 210 is arranged on the first type of capillary wick 200 at the evaporating section 400 of the casing 100 instead of the second type of capillary wick 220. The first type of capillary wick 200 surrounds the sintered-type wick 210. Metal or ceramic powders are filled into the grooves of the first type of capillary wick 200 at the evaporating section 400 and sintered to form the sintered-type wick 210. Filling the powders into the grooves of the first type of capillary wick 200 at the evaporating section 400 can change the capillary pore size of the first type of capillary wick 200 so as to form a small pore size structure in the first type of capillary wick 200 at the evaporating section 400. The combined first type of capillary wick 200 and the sintered-type wick 210 at the evaporating section 400 can develop a very large capillary force to draw the condensed liquid from the condensing and central sections 600, 500 back to the evaporating section 400.
FIG. 4 illustrates a heat pipe in accordance with a third embodiment of the present invention. Main differences between the third and second embodiments are that in the third embodiment the second type of capillary wick 220 is arranged on the sintered-type wick 210 at the evaporating section 400 of the casing 100 and connects with the second type of capillary wick 220 at the central section 500. An arrangement of the first type of capillary wick 200, the sintered-type wick 210 and the second type of capillary wick 220 is that the first type of capillary wick 200 surrounds the sintered-type wick 210, which in turn surrounds the second type of capillary wick 220. The capillary pore size of the composite capillary wick at the evaporating section 400 is gradually increased along an inward direction from the first type of capillary wick 200 toward the second type of capillary wick 220.
In the present invention, it is feasible that any other type of capillary wick with smaller pore size, such as a beehive-type capillary wick can be disposed on the first type of capillary wick 200 at the central and the evaporating sections 500, 400 of the casing 100 to form the composite wick structure.
It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Claims

1. A heat pipe comprising:

a metal casing containing a working fluid therein, the casing comprising an evaporating section at one end and a condensing section at an opposite end thereof, and a central section located between the evaporating section and the condensing section;

a first type of capillary wick arranged in an inner wall of the casing at the condensing, central and evaporating sections of the casing and a second type of capillary wick disposed on the first type of capillary wick at the central and evaporating sections; wherein

a capillary pore size of the first type of capillary wick is larger than that of the second type of capillary wick.

2. The heat pipe of claim 1, wherein the first type of capillary wick is a plurality of fine grooves defined in the inner wall of the metal casing, and the second type of capillary wick is a fine mesh.

3. The heat pipe of claim 1, wherein the first type of capillary wick is a plurality of fine grooves defined in the inner wall of the metal casing, and the second type of capillary wick comprises a fine mesh at the central section and a sintered-type wick at the evaporating section.

4. The heat pipe of claim 3, further comprising an additional fine mesh arranged on the sintered-type wick at the evaporating section of the metal casing, the additional fine mesh connecting with the fine mesh at the central section of the metal casing.

5. The heat pipe of claim 4, wherein the sintered-type wick is formed by one of sintered metal powders and sintered ceramic powders and the sintered-type wick fills in the fine grooves.

6. The heat pipe of claim 4, wherein the additional fine mesh has a capillary pore size larger than that of the sintered-type wick.

7. The heat pipe of claim 5, wherein a capillary pore size of the first type of capillary wick, the sintered-type of capillary wick and the additional fine mesh at the evaporating section is gradually increased in an inward direction from the first type of capillary wick toward the additional fine mesh.

8. The heat pipe of claim 1, wherein the first type of capillary wick has a lower liquid flow resistance than that of the second type of capillary wick.

9. A heat pipe comprising:

a hollow metal casing containing a working fluid therein, the casing comprising an evaporating section, a condensing section and a central section between the evaporating section and the condensing section; and

first capillary wick applied to an inner wall of the metal casing at the central section and second capillary wick applied on the first capillary wick, the first capillary wick generating a smaller flow resistance for the working fluid than the second capillary wick.

10. The heat pipe of claim 9, wherein the first capillary wick is also arranged in the inner wall of the metal casing at the evaporating and condensing sections.

11. The heat pipe of claim 10, wherein the first capillary wick is a plurality of fine grooves, and the second capillary wick is a fine mesh.

12. The heat pipe of claim 11, wherein an additional fine mesh is located on the first capillary wick at the evaporating of the casing and connects with the second capillary wick at the central section of the metal casing.

13. The heat pipe of claim 11, wherein a sintered-type wick is located on the first capillary wick at the evaporating section of the metal casing.

14. The heat pipe of claim 13, wherein an additional fine mesh is located on the sintered-type wick at the evaporating section of the casing and connects with the second capillary wick at the central section of the metal casing, a capillary pore size of the first capillary wick, the sintered-type wick and the additional fine mesh at the evaporating section of the metal casing is gradually increased in an inward direction from first capillary wick to the additional fine mesh.