FLAT TYPE HEAT PIPE

Abstract

A flat type heat pipe (10) is disclosed which includes a metal casing (12) and a wick structure (16) arranged inside the metal casing. The metal casing has an evaporating section (123) and a condensing section (124). The wick structure extends from the evaporating section towards the condensing section of the metal casing and has a first section in conformity with the condensing section of the metal casing and a second section in conformity with the evaporating section of the metal casing. The first section has a pore size larger than that of the second section of the wick structure. The wick structure includes a metal foam.

Description

FIELD OF THE INVENTION

The present invention relates generally to an apparatus for transfer or dissipation of heat from heat-generating components, and more particularly to a flat type heat pipe applicable in electronic products such as personal computers for removing heat from electronic components installed therein.

DESCRIPTION OF RELATED ART

Heat pipes have excellent heat transfer performance due to their low thermal resistance, and therefore are 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 fluid, which is employed to carry, under phase transitions between liquid state and vapor state, thermal energy from one section of the heat pipe (typically referring to as the “evaporating section”) to another section thereof (typically referring to as the “condensing section”). Preferably, a wick structure is provided inside the heat pipe, lining an inner wall of the casing, for drawing the working fluid back to the evaporating section after it is condensed at the condensing section. The wick structure currently available for heat pipes includes fine grooves integrally formed at the inner wall of the casing, screen mesh or bundles of fiber inserted into the casing and held against the inner wall thereof, or sintered powders combined to the inner wall of the casing by sintering process.

In operation, the evaporating section of the heat pipe is maintained in thermal contact with a heat-generating component. The working fluid contained at the evaporating section absorbs heat generated by the heat-generating component and then turns into vapor. Due to the difference of vapor pressure between the two sections of the heat pipe, the generated vapor moves and carries the heat simultaneously towards the condensing section where the vapor is condensed into condensate after releasing the heat into ambient environment by, for example, fins thermally contacting the condensing section. Due to the difference of capillary pressure developed by the wick structure between the two sections, the condensate is then brought back by the wick structure to the evaporating 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. Also, the wick structure is expected to provide a high permeability at the condensing section of the heat pipe in order for the condensate resulting from the vapor in that section to enter into the wick structure more easily. However, the wick structure provided in the conventional heat pipe generally has a uniform pore size distribution over its entire length. This uniform-type wick structure cannot satisfy these requirements. If the condensate is not timely brought back from the condensing section, the heat pipe will suffer a dry-out problem at the evaporating section.

Therefore, it is desirable to provide a heat pipe with a wick structure which can draw the condensate back from its condensing section to its evaporating section effectively and timely.

SUMMARY OF INVENTION

The present invention relates to a flat type heat pipe. The heat pipe includes a metal casing and a wick structure arranged inside the metal casing. The metal casing has an evaporating section and a condensing section. The wick structure extends from the evaporating section towards the condensing section of the metal casing and has a first section in conformity with the condensing section of the metal casing and a second section in conformity with the evaporating section of the metal casing. The first section has a pore size larger than that of the second section of the wick structure.

In the heat pipe, the first section of the wick structure generates a relatively low resistance for the condensate as it flows in the condensing section, and the second section of the wick structure is still capable of maintaining a relatively high capillary force for drawing the condensate back from the condensing section towards the evaporating section. Meanwhile, the condensate in the condensing section is capable of entering into the wick structure easily due to a relatively high permeability of the first section of the wick structure. As a result, the condensate is drawn back to the evaporating section rapidly and timely, thus preventing the potential dry-out problem occurring at the evaporating section.

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 DRAWINGS

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

FIG. 2 is a longitudinal cross-sectional view of the heat pipe of FIG. 1, taken along line II-II thereof;

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

FIG. 4 is a plan view of a portion of a metal casing of the heat pipe of FIG. 3, showing an interior of the metal casing; and

FIG. 5 is a transverse cross-sectional view of a heat pipe in accordance with a third embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a flat type heat pipe 10 in accordance with a first embodiment of the present invention. The heat pipe 10 has a plate-type configuration and includes a metal casing 12. The metal casing 12 includes a top plate 121 and a bottom plate 122 cooperating with the top plate 121 to define a chamber 14 in the metal casing 12. A wick structure 16 is provided inside the heat pipe 10, occupying a central region of the chamber 14. The wick structure 16 is so dimensioned as to fit between the top and bottom plates 121, 122 of the metal casing 12. The metal casing 12 is made of high thermally conductive material such as copper or aluminum. The heat pipe 10 is evacuated and hermetically sealed after a working fluid (not shown) is injected into the chamber 14 of the metal casing 12. The working fluid is saturated in the wick structure 16 and is usually selected from a liquid such as water or alcohol, which has a low boiling point and is compatible with the wick structure 16. The wick structure 16 is a porous structure and is in the form of a metal foam.

As shown in FIG. 2, the metal casing 12 has an evaporating section 123 and an opposing condensing section 124 along a longitudinal direction of the heat pipe 10. The evaporating and condensing sections 123, 124 occupy two end portions of the heat pipe 10, respectively. Although it is not shown in the drawings, it is well known by those skilled in the art that two ends of the heat pipe 10 are sealed. The wick structure 16 extends in the longitudinal direction of the heat pipe 10 and has a pore size that gradually increases from the evaporating section 123 towards the condensing section 124.

In operation, the evaporating section 123 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 fluid contained in the evaporating section 123 of the heat pipe 10 evaporates into vapor upon receiving the heat generated by the heat source. Then, the generated vapor moves, via the other region of the chamber 14 without being occupied by the wick structure 16, towards the condensing section 124 of the heat pipe 10. After the vapor releases the heat carried thereby and turns into condensate in the condensing section 124, the condensate is brought back by the wick structure 16 to the evaporating section 123 of the heat pipe 10 for being available again for evaporation.

In the present heat pipe 10, the capillary forces and the flow resistances generated by different sections of the wick structure 16 are different. The general rule is that the larger a pore size a wick structure has, the smaller a capillary force and the lower a flow resistance it provides. A first section of the wick structure 16 in conformity with the condensing section 124 of the heat pipe 10 has a pore size larger than that of a second section of the wick structure 16 in conformity with the evaporating section 123 of the heat pipe 10. Thus, the first section of the wick structure 16 generates a relatively low resistance for the condensate as it flows in the condensing section 124, and the second section of the wick structure 16 is still capable of maintaining a relatively high capillary force for drawing the condensate back from the condensing section 124 towards the evaporating section 123. Meanwhile, the condensate resulting from the vapor in the condensing section 124 is capable of entering into the wick structure 16 easily due to a relatively high permeability of the first section of the wick structure 16. As a result, the condensate is drawn back to the evaporating section 123 rapidly and timely, thus preventing a potential dry-out problem occurring at the evaporating section 123.

The metal foam used to form the wick structure 16 may be made of such materials as stainless steel, copper, copper alloy, aluminum alloy and silver. The wick structure 16 may be formed independently of the metal casing 12 and then inserted into the metal casing 12. Typically, the metal foam of the wick structure 16 is fabricated by expanding and solidifying a pool of liquid metal saturated with an inert gas under pressure. The porosity of the foam after solidification may be in a wide range, subject to the levels of pressure applied during the fabrication process. Electroforming is another typical method for fabricating the metal foam, which generally involves steps of providing one kind of porous material such as polyurethane foam, then electrodepositing a layer of metal over the surface of the polyurethane foam and finally heating the resulting product at a high temperature to get rid of the polyurethane foam to thereby obtain the porous metal foam. Still another fabrication method for the metal foam, called die-casting process, is also widely used, which generally includes steps of providing one kind of porous material such as polyurethane foam, filling ceramic slurry into the pores of the porous polyurethane foam and then solidifying the ceramic slurry therein, then heating the resulting product at a high temperature to get rid of the polyurethane foam to obtain a matrix of porous ceramic, thereafter filling metal slurry into the pores of the ceramic matrix and finally getting rid of the ceramic material after solidification of the metal slurry to thereby obtain the porous metal foam. However, there are still some other methods suitable for fabrication of the metal foam. Fox example, the metal foam can be made by steps of filling a kind of bubble-generating material such as metallic hydride into metal slurry to generate a large number of bubbles distributing randomly throughout the metal slurry and solidifying the metal slurry to thereby obtain the metal foam with a plurality of pores therein.

FIG. 3 illustrates a flat type heat pipe 20 in accordance with a second embodiment of the present invention. In addition to the wick structure 16 that is in the form of a metal foam, the heat pipe 20 also includes a plurality of fine grooves 201 longitudinally defined in an inner surface of the casing 22. These grooves 201 altogether function as another wick structure cooperating with the original wick structure 16 so as to obtain a higher capillary force inside the heat pipe 20. Furthermore, each of the grooves 201 may have a varying width throughout the heat pipe 20. As particularly shown in FIG. 4, each groove 201 has a width gradually increasing from the evaporating section 223 towards the condensing section 224 of the heat pipe 20. This particular design of the grooves 201 can reduce flow resistance to the condensate as it flows in the condensing section 224 of the heat pipe 20.

FIG. 5 illustrates a flat type heat pipe 30 in accordance with a third embodiment of the present invention. In this embodiment, two wick structures 16 are arranged inside the heat pipe 30 with each being located near a sidewall of the heat pipe 30. Thus, the central region of the chamber of the heat pipe 30 functions as a vapor channel for passage of vapor generated inside the heat pipe 30 from the evaporating section to the condensing section.

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 flat type heat pipe comprising:

a metal casing having an evaporating section and a condensing section; and

a wick structure made of a metal foam and arranged inside the metal casing, the wick structure extending from the evaporating section towards the condensing section of the metal casing and having a pore size gradually increasing from the evaporating section towards the condensing section of the metal casing.

2. The heat pipe of claim 1, wherein the wick structure extends along a longitudinal direction of the heat pipe and occupies a central region of an interior chamber defined in the metal casing.

3. The heat pipe of claim 1, wherein the wick structure extends along a longitudinal direction of the heat pipe and is located near a sidewall of the metal casing.

4. The heat pipe of claim 1, wherein the metal casing defines a plurality of grooves in an inner surface thereof.

5. The heat pipe of claim 4, wherein the grooves extend along a longitudinal direction of the metal casing and at least one of the grooves have a width gradually increasing from the evaporating section towards the condensing section of the metal casing.

6. A flat type heat pipe comprising:

a metal casing including a top plate and a bottom plate cooperating with the top plate to define a chamber inside the metal casing, the metal casing having an evaporating section and a condensing section; and

a wick structure located inside the casing and occupying a portion of the chamber, the wick structure having a first section in conformity with the condensing section of the metal casing and a second section in conformity with the evaporating section of the metal casing;

wherein the wick structure is sandwiched between the top and bottom plates of the metal casing and said first section has a pore size larger than that of the second section of the wick structure.

7. The heat pipe of claim 6, wherein the wick structure is in the form of a metal foam.

8. The heat pipe of claim 6, wherein the metal casing has a plurality of grooves formed in an inner surface thereof.

9. The heat pipe of claim 8, wherein the grooves each have a width gradually increasing from the evaporating section towards the condensing section of the metal casing.

10. The heat pipe of claim 6, wherein the wick structure occupies a central portion of the chamber.

11. The heat pipe of claim 6, wherein the wick structure occupies a side portion of the chamber.

12. A heat pipe, comprising:

an elongated casing having a flat plate, a plurality of grooves formed in an inner surface of the casing along a longitudinal direction thereof; and

a metal foam received in the casing and extending along the longitudinal direction thereof, the metal foam having a pore size which is gradually increased along the longitudinal direction of the casing, wherein the grooves and the metal foam cooperate as a wick structure for the heat pipe for moving a condensate in the heat pipe.

13. The heat pipe of claim 12, wherein at least one of the grooves has a width which is gradually increased along the longitudinal direction of the casing.