US20090020269A1

US20090020269A1 - Heat pipe with composite wick structure

Info

Publication number: US20090020269A1
Application number: US11/858,080
Authority: US
Inventors: Chang-Shen Chang; Chao-Hao Wang; Juei-Khai Liu; Hsien-Sheng Pei
Original assignee: Foxconn Technology Co Ltd
Current assignee: Foxconn Technology Co Ltd
Priority date: 2007-07-18
Filing date: 2007-09-19
Publication date: 2009-01-22
Also published as: CN101349519A

Abstract

A heat pipe (10) includes a metal casing (12) having an evaporator section (121) and a condenser section (122). A major wick structure (14) is disposed on an inner wall of the casing. At least one assistant wick structure (16) is disposed in and contacts with the major wick structure, and working medium fills in the casing and saturates the major wick structure and the at least one assistant wick structure. An average pore size of the major wick structure corresponding to the evaporator section is smaller than that of the major wick structure corresponding to the condenser section. A diameter of a cross section of the at least one assistant wick structure is smaller than a diameter of a cross section of the major wick structure, and thus the at least one assistant wick structure has a linear contact with the major wick structure.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates generally to a heat transfer apparatus, and more particularly to a heat pipe having composite capillary wick structure.
2. Description of Related Art
Heat pipes have excellent heat transfer performance due to their low thermal resistance, and are therefore an effective means for transfer or dissipation of heat from heat sources. Currently, heat pipes are widely used for removing heat from heat-generating components such as central processing units (CPUs) of computers.
A heat pipe is usually a vacuum casing containing therein a working medium, which is employed to carry, under phase transitions between liquid state and vapor state, thermal energy from an evaporator section to a condenser section. Preferably, a wick structure is provided inside the heat pipe, lining an inner wall of the casing, for drawing the working medium back to the evaporator section after it is condensed at the condenser section. In operation, the evaporator section of the heat pipe is maintained in thermal contact with a heat-generating component. The working medium contained at the evaporator section absorbs heat generated by the heat-generating component and then turns into vapor and moves towards the condenser section where the vapor is condensed into condensate after releasing the heat into ambient environment. Due to the difference in capillary pressure which develops in the wick structure between the two sections, the condensate is then brought back by the wick structure to the evaporator section where it is again available for evaporation.
In order to draw the condensate back timely, the wick structure provided in the heat pipe is expected to provide a high capillary force and meanwhile generate a low flow resistance for the condensate. In ordinary use, the heat pipe needs to be flattened to enable the miniaturization of electronic products incorporating the heat pipe, which may result in damage to the wick structure of the heat pipe. When this happens, the flow resistance of the wick structure is increased and the capillary force provided by the wick structure is decreased, which in turn reduces the heat transfer capability of the heat pipe. If the condensate is not quickly brought back from the condenser section, the heat pipe will suffer a dry-out problem at the evaporator section.
Therefore, it is desirable to provide a heat pipe with improved heat transfer capability; wick structure of the heat pipe will not be damaged and still can have a satisfied wicking force when the heat pipe is flattened.

SUMMARY OF THE INVENTION

The present invention relates to a heat pipe for removing heat from heat-generating components. The heat pipe includes a longitudinal casing having an evaporator section and a condenser section; a major wick structure is disposed on an inner wall of the casing; at least one assistant wick structure is disposed in and contacts with the major wick structure, and extends from the evaporator section to the condenser section. Working medium fills in the casing and saturates the major wick structure and the at least one assistant wick structure. An average pore size of the major wick structure corresponding to the evaporator section is smaller than that of the major wick structure corresponding to the condenser section. A diameter of a cross section of the at least one assistant wick structure is smaller than a diameter of a cross section of the major wick structure, and thus the at least one assistant wick structure has a linear contact with the inner wall of the major wick structure.
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 invention 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 invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views:

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

FIG. 2 is a transverse cross-sectional view of a condensing section of the heat pipe of FIG. 1 taken along line II-II;

FIG. 3 is a transverse cross-sectional view of an evaporating section of the heat pipe of FIG. 1 taken along line III-III;

FIG. 4 is similar to FIG. 3, but shows the evaporating section according to a second embodiment of present invention;

FIG. 5 is a transverse cross-sectional view of the evaporating section according to a third embodiment of the present invention;

FIG. 6 is a transverse cross-sectional view of the evaporating section according to a fourth embodiment of the present invention;

FIG. 7 is a transverse cross-sectional view of the evaporating section according to a fifth embodiment of the present invention;

FIG. 8 is a transverse cross-sectional view of the evaporating section according to a sixth embodiment of the present invention; and

FIG. 9 is a transverse cross-sectional view of the evaporating section according to a seventh embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-2 illustrate a heat pipe 10 in accordance with a first embodiment of the present invention. The heat pipe 10 includes a round tube-like metal casing 12, and a variety of elements enclosed in the metal casing 12, i.e., a major wick structure 14, an assistant wick structure 16, and a working medium (not shown).
The metal casing 12 is made of high thermally conductive material such as copper or aluminum. The metal casing 12 has an evaporator section 121, an opposing condenser section 122 along a longitudinal direction of the heat pipe 10, and an adiabatic section 123 disposed between the evaporator and condenser sections 121, 122. Two ends of the heat pipe 10 are sealed. The working medium is saturated in the major and assistant wick structures 14, 16, and is usually selected from a liquid such as water, methanol, or alcohol, which has a low boiling point and is compatible with the major and assistant wick structures 14, 16. Thus, the working medium can easily evaporate to vapor when it receives heat at the evaporator section 121 of the heat pipe 10. The metal casing 12 of the heat pipe 10 is evacuated and hermetically sealed after the working medium is injected into the metal casing 12 and saturated in the major and assistant wick structures 14, 16.
The major wick structure 14 is provided around an inner wall of the metal casing 12 and extends along the longitudinal direction of the metal casing 12 of the heat pipe 10. The major wick structure 14 is tube-shaped in profile, and usually selected from a porous structure such as grooves, sintered powder, screen mesh, or bundles of fiber, which enable the major wick structure 14 to provide a capillary force to drive condensed working medium at the condenser section 122 of the heat pip 10 to flow towards the evaporator section 121 thereof. In this embodiment, the major wick structure 14 includes a first portion 142 applied to the evaporator section 121 of the metal casing 12 of the heat pipe 10 and a second portion 141 applied to the condenser section 122 and the adiabatic section 123 of the metal casing 12. The first portion 142 of the major wick structure 14 is sintered powder wick, and the second portion 141 of the major wick structure 14 is groove wick. An average pore size of the first portion 142 of the major wick structure 14 is smaller than that of the second portion 141 of the major wick structure 14. According to the general rule, the capillary pressure of the wick structure and its flow resistance to the condensed fluid increase due to a decrease in pore size of the wick structure; the first portion 142 of the major wick structure 14 corresponding to the evaporator section 121 of the heat pipe 10 is thus capable of providing a capillary pressure gradually increasing from the condenser section 122 toward the evaporator section 121, and a flow resistance gradually decreasing from the evaporator section 121 toward the condenser section 122.
The assistant wick structure 16 is an elongated hollow tube, which is attached to an inner wall of the major wick structure 14 and extends along the longitudinal direction of the metal casing 12. The assistant wick structure 16 is formed by weaving a plurality of metal wires, such as copper or stainless steel wires. Alternatively, the assistant wick structure 16 may be formed by sintering an amount of powders. A channel 163 is defined in an inner space of the assistant wick structure 16 for passage of condensed working medium. The channel 163 has a diameter ranging from 0.5 mm to 2 mm. A plurality of pores 161 are formed in a peripheral wall of the assistant wick structure 16, which provides a capillary action to the working medium and communicates an inside (not labeled) of the assistant wick structure 16 with an inside (not labeled) of the major wick structure 14. A composite wick structure is thus formed in the metal casing 12 of the heat pipe 10. The assistant wick structure 16 has a ring-like transverse cross section. A diameter of the assistant wick structure 16 is much smaller than a diameter of the major wick structure 14. The assistant wick structure 16 has an adjacent portion contacting with the inner wall of the major wick structure 14, and a distal portion spaced a distance from the inner wall of the major wick structure 14 along a radial direction of the heat pipe 10.
In operation, the evaporator section 121 of the heat pipe 10 is placed in thermal contact with a heat source (not shown), for example, a central processing unit (CPU) of a computer, that needs to be cooled. The working medium contained in the evaporator section 121 of the heat pipe 10 is vaporized into vapor upon receiving the heat generated by the heat source. Then, the generated vapor moves via a space between the major and assistant wick structures 14, 16 of the heat pipe 10. After the vapor releases the heat carried thereby and it is condensed into condensate in the condenser section 122, the condensate is brought back by the major wick structure 14 to the evaporator section 121 of the heat pipe 10 for being available again for evaporation. Meanwhile, the condensate resulting from the vapor in the condenser section 122 is capable of entering into the assistant wick structure 16 easily due to the capillary action of the assistant wick structure 16 and then can move through the channel 163 to the evaporator section 121. As a result, the condensate is drawn back to the evaporator section 121 rapidly and timely, thus preventing a potential dry-out problem occurring at the evaporator section 121. In addition, the working medium can not be accumulated in a bottom portion of the major wick structure 14 of the heat pipe 10 under an action of gravity. This prevents the increase of the flow resistance of the heat pipe 10, which is caused by the accumulation of the working medium in a specific place of the heat pipe. The heat transfer capability of the heat pipe 10 is thus increased.
In the present invention, the assistant wick structure 16 cooperates with the major wick structure 14 to form the composite wick structure, which increases the capillary force inside the heat pipe 10. Thus, the heat transfer capability of the heat pipe 10 is increased. The assistant wick structure 16 is distributed along the longitudinal direction of the heat pipe 10 and has a smaller diameter than that of the major wick structure 14. As a result, the assistant wick structure 16 can not easily be damaged by the flattening process of the heat pipe 10. On the other hand, the average pore size of the first portion 142 of the major wick structure 14 corresponding to the evaporator section 121 of the heat pipe 10 is smaller than that of the second portion 141 of the major wick structure 14 corresponding to the condenser section 122 and the adiabatic section 123 of the heat pipe 10. The major wick structure 14 provides a capillary pressure gradually increasing from the condenser section 122 toward the evaporator section 121, and a flow resistance gradually decreasing from the evaporator section 121 toward the condenser section 122.
FIGS. 4-9 schematically show the evaporator section 121 of the heat pipe 10 in accordance with some additional embodiments of the present invention. Except for the first portion of the major wick structure 14, other parts of these embodiments are the same as the first embodiment. In FIG. 4, the first portion 142 a of the major wick structure corresponding to the evaporator section 121 is screen mesh wick which is planar-shaped in an unfurled state. FIGS. 5-7 show that the first portion of the major wick structure has two stacked layers. Referring to FIG. 5, a first layer 141 b of the first portion is groove wick formed on the inner wall of the casing 12, and a second layer 142 b is sinter powder wick contacting with an inner surface of the first layer 141 b. A portion of the second layer 142 b is filled in the grooves of the first layer 141 b, and the other portion forms a cylindrical-shaped inner surface. The assistant wick structure 16 contacts the inner surface of the second layer 142 b of the first portion of the major wick structure. As shown in FIGS. 6 and 8, the major wick structure has a first layer 141 c being groove wick and a second layer 142 c being screen mesh wick. The second layer 142 c is planar shaped in an unfurled state. Thus, the second layer 142 c contacts to the first layer 141 c in part, and is spaced from the grooves of the first layer 141 c of the major wick structure. In FIGS. 7 and 9, the major wick structure also has a first layer 141 d being groove wick and a second layer 142 d being screen mesh wick. The difference of these two embodiments over the third embodiment of FIG. 5 is that the second layer 142 d is square-wave shaped in an unfurled state. The second layer 142 d is received in the grooves of the first layer 141 d. Thus, the second layer 142 d forms an inner surface defining a plurality of recesses (not labeled) along a circumferential direction of the heat pipe.
Although, as shown in these embodiments, only the major wick structure 14 at the evaporator section 121 has various types of configuration, it is understood that the major wick structure 14 at the condenser section 122 and the adiabatic section 123 also can have various types of configuration. In addition, the heat pipe 10 may include more than one assistant wick structure 16. Theses assistant wick structures 16 may be evenly and tidily attached to the inner wall of the major wick structure 14 (as shown in FIGS. 8-9) or randomly arranged on the inner wall of the major wick structure 14. In addition, these assistant wick structures 16 may be spaced by a distance (as shown in FIG. 9) or intimately contact with each other (as shown in FIG. 8). In the above embodiments from FIG. 1 to FIG. 9, the assistant wick structure 16 has a linear contact with the major wick structure 14.
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:

an elongated casing having an evaporator section and a condenser section;

a major wick structure disposed on an inner wall of the casing, an average pore size of the major wick structure corresponding to the evaporator section being smaller than that of the major wick structure corresponding to the condenser section;

at least one assistant wick structure disposed in and contacting with an inner wall of the major wick structure and extending from the evaporator section to the condenser section, a diameter of a cross section of the at least one assistant wick structure being smaller than a diameter of a cross section of the major wick structure, the at least one assistant wick structure having a linear contact with the inner wall of the major wick structure; and

working medium filling in the casing and saturating the major wick structure and the at least one assistant wick structure.

2. The heat pipe of claim 1, wherein the at least one assistant wick structure is a hollow tube made of sintered powder.

3. The heat pipe of claim 1, wherein the at least one assistant wick structure is a hollow tube made of a plurality of woven metal wires, and the metal wires are selected from a group consisting of copper wires and stainless steel wires.

4. The heat pipe of claim 1, wherein the at least one assistant wick structure comprises a plurality of assistant wick structures spacing a distance from each other.

5. The heat pipe of claim 1, wherein the at least one assistant wick structure comprises a plurality of assistant structures contacting with each other.

6. The heat pipe of claim 1, wherein the at least one assistant wick structure is a hollow tube with an inner diameter ranging from 0.5 mm to 2 mm.

7. The heat pipe of claim 1, wherein the major wick structure is selected from the group of grooves, sintered powder, fiber and screen mesh.

8. The heat pipe of claim 7, wherein the major wick structure corresponding to the evaporator section of the heat pipe is one of sintered powder and screen mesh, and the major wick structure corresponding to the condenser section of the heat pipe is grooves.

9. The heat pipe of claim 7, wherein the major wick structure corresponding to the evaporator section of the heat pipe has two layers stacked along a radial direction of the heat pipe, one layer being grooves and the other layer being one of sintered powder and screen mesh.

10. A heat pipe comprising:

a metal casing having an evaporator section and a condenser section;

a porous major wick structure disposed on an inner wall of the casing, an average pore size of the major wick structure corresponding to the evaporator section being smaller than that of the wick structure corresponding to the condenser section;

at least one assistant wick structure disposed in an inner wall of the major wick structure, the at least one assistant wick structure defining a plurality of pores communicating an inside of the at least one assistant wick structure with an inside of the major wick structure and a channel in an inner space thereof; and

working medium filling in the casing and saturating the major wick structure and the at least one assistant wick structure;

wherein vapor in the metal casing flows from the evaporator section to the condenser section via a space between the major wick structure and the at least one assistant wick structure, and condensate in the metal casing flows from the condenser section to the evaporator section via the pores of the major and the assistant wick structures and the channel of the assistant wick structure.

11. The heat pipe of claim 10, wherein a diameter of the at least one assistant wick structure ranges from 0.5 mm to 2 mm.

12. The heat pipe of claim 10, wherein the at least one assistant wick structure is made of a plurality of woven metal wires selected from a group consisting of copper and stainless steel wires.

13. The heat pipe of claim 10, wherein the at least one assistant wick structure comprises a plurality of assistant wick structures spaced from each other.

14. The heat pipe of claim 13, wherein the plurality of assistant wick structures are evenly spaced from each other.

15. The heat pipe of claim 10, wherein the at least one assistant wick structure comprises a plurality of assistant wick structures contacting with each other.

16. The heat pipe of claim 10, wherein the major wick structure is selected from the group of grooves, sintered powder, fiber and screen mesh.

17. The heat pipe of claim 16, wherein the major wick structure corresponding to the evaporator section of the heat pipe is one of sintered powder and screen mesh, and the major wick structure corresponding to the condenser section of the heat pipe is grooves.

18. The heat pipe of claim 16, wherein the major wick structure corresponding to the evaporator section of the heat pipe has two layers stacked along a radial direction of the heat pipe, one layer being grooves and the other layer being one of sintered powder and screen mesh.