US20160018166A1

US20160018166A1 - Flat heat pipe

Info

Publication number: US20160018166A1
Application number: US14/799,719
Authority: US
Inventors: Mohammad Shahed AHAMED; Yuji Saito
Original assignee: Fujikura Ltd
Current assignee: Fujikura Ltd
Priority date: 2014-07-16
Filing date: 2015-07-15
Publication date: 2016-01-21
Also published as: JP5759600B1; JP2016023821A

Abstract

A heat pipe a flat heat pipe having enhanced heat transport capacity is provided. The flat heat pipe 1 comprises a working fluid encapsulated in a container 2, a first wick 11 that returns the working fluid from a condensing portion 4 to an evaporating portion 3 by a capillary pumping, and a second wick 12 that spreads the working fluid returned thereto over an inner face of the container by a capillary pumping. The second wick is formed only in the evaporating portion 3 to extend in a circumferential direction on the inner face of the container 2.

Description

The present invention claims the benefit of Japanese Patent Application No. 2014-146261 filed on Jul. 16, 2014 with the Japanese Patent Office, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Invention
The present invention relates to an art of a flat heat pipe having a wick structure.
2. Discussion of the Related Art
Heat pipe adapted to transport heat utilizing latent heat of fluid has been widely used as a heat transport device. The conventional heat pipe comprises a tubular sealed container and working fluid encapsulated therein, and both ends of the container are closed. The working fluid is vaporized by a heat of the heat generating element transmitted to one end of the heat pipe, and flows toward the other end to be cooled.
In general, one of the end portions of the heat pipe is brought into contact to the heat generating element to evaporate the working fluid, and the other end portion is brought into contact to a radiation member to condense the working fluid by radiating heat through the radiation member. The condensed working fluid is returned to the heated site through a wick by capillary pumping of the wick.
For example, JP-A-2001-296093 describes a heat pipe in which spiral or loop minute grooves are formed on an inner face of a tubular container. In the heat pipe taught by JP-A-2001-296093, the working fluid condensed at a radiation site is returned gravitationally to a heated site.
In the flat heat pipe, an internal space thereof serving as a vapor passage is narrow in its height direction but can be ensured sufficiently in its width direction. However, since an inner volume of the thin flat heat pipe is rather small, it is difficult to circulate the phase changeable working fluid.
Given that a large wick structure is arranged in the container to enhance capillary pumping to pull the working fluid to a heating site, most part of the internal space would be occupied by the wick structure, and hence it is difficult to ensure the vapor passage sufficiently. If the working fluid cannot be returned to the heating site sufficiently, the heating site would be dried out.
One possible solution to ensure the vapor passage in the flat container is to form the minute grooves taught by JP-A-2001-296093 on an inner face of the flat container.
However, the minute grooves taught by JP-A-2001-296093 is adapted to return the working fluid to the heating site gravitationally. That is, in the heat pipe having the minute grooves taught by JP-A-2001-296093, an efficiency to return the working fluid to the heating site would be varied significantly depending on an orientation or posture of the heat pipe. For example, given that the flat heat pipe is situated horizontally, it would be difficult to return the working fluid to the heating site efficiently by the gravity. Consequently, heat transport performance of the flat heat pipe will be worsened.

SUMMARY OF THE INVENTION

The present invention has been conceived nothing the foregoing technical problems, and it is therefore an object of the present invention is to provide a flat heat pipe having enhanced heat transport capacity that can be manufactured easily.
The heat pipe according to the present invention comprises: a sealed container having a pair of upper and lower flat walls extending in a length direction; a working fluid encapsulated in the container; an evaporating portion that is situated on one of the longitudinal ends of the container at which evaporation of the working fluid takes place; a condensing portion that is situated on the other longitudinal end of the container at which condensation of the working fluid takes place; a first wick formed of a bundled fibers or a porous structure that extends in the length direction of the container to return the working fluid condensed at the condensing portion to the evaporating portion by a capillary pumping; and a second wick that is formed by forming a groove on an inner face of the container only in the evaporating portion to spread the working fluid returned thereto over the inner face within the evaporating portion by a capillary pumping. The first wick is formed on at least one of the upper flat wall or the lower flat wall, and the second wick transversely extends underneath the first wick to be closed partially by the first wick.
For example, the second wick may be formed on the inner face of the container in the evaporating portion in a spiral manner.
Instead, the second wick may also be formed on the inner face of the container in the evaporating portion in a circular manner.
Further, the second wick may also be formed only on the inner face of one of the upper flat face and the lower flat face on which the first wick is formed.
The second wick includes an opening portions extending from both sides of the first wick in the circumferential direction.
Specifically, the porous structure may be formed by sintering metal powders.
Thus, in the flat heat pipe of the present invention, the groove wick is formed on the inner face of the container in the evaporating portion. According to the present invention, therefore, hydrophilicity on the inner face of the container in the evaporating portion can be improved to spread the working fluid over the inner face smoothly. Consequently, the working fluid in the evaporating portion can be evaporated efficiently so that the working fluid can be circulated smoothly within the container. For these reasons, the thermal resistance to transport heat can be reduced so that the heat transporting performance of the heat pipe can be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and advantages of exemplary embodiments of the present invention will become better understood with reference to the following description and accompanying drawings, which should not limit the invention in any way.

FIG. 1 is a schematic illustration showing the flat heat pipe according to the preferred example;

FIG. 2 is a perspective view showing the sealed container in which a spiral groove wick is formed on an inner face;

FIG. 3 (a) is a cross-sectional view showing a cross-section of the evaporating portion of the heat pipe along the line A-A in FIG. 1, FIG. 3 (b) is a cross-sectional view showing a cross-section of the heat pipe at a boundary between the evaporating portion and the insulating portion along the line B-B FIG. 1, and FIG. 3 (c) is a schematic illustration showing the groove wick and the fiber wick formed on a lower flat wall;

FIG. 4 (a) is a top view of a testing device, and FIG. 4 (b) is a front view of a testing device;

FIG. 5 is a graph indicating testing result of the heat pipes according to the preferred example and the comparison example;

FIG. 6 is a schematic illustration showing the flat heat pipe in which circular groove wick is formed on the inner face;

FIG. 7 (a) is a cross-sectional view showing a cross-section of the heat pipe in which the groove wick is formed only on the lower flat wall of the evaporating portion, and FIG. 7 (b) is a cross-sectional view showing a cross-section of the heat pipe in which a transverse groove wick is formed only on the lower flat wall; and

FIG. 8 (a) is a cross-sectional view showing a cross-section of the heat pipe in which the fiber wicks are formed on the width centers of both upper and lower flat wall, FIG. 8 (b) is a cross-sectional view showing a cross-section of the heat pipe in which the fiber wicks formed on the upper and lower flat walls are displaced from each other, and FIG. 8 (c) is a cross-sectional view showing a cross-section of the heat pipe in which the fiber wick is formed while brought into contact to both upper and lower flat walls.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Hereinafter, preferred examples of the flat heat pipe according to the present invention will be explained in more detail with reference to the accompanying drawings.
Referring now to FIG. 1, there is shown a heat pipe 1 according to the preferred example. The heat pipe 1 shown therein is a heat transport device adapted to transport heat in the form of latent heat of phase changeable working fluid encapsulated in a sealed container 2.
The container 2 is flattened to have flat longitudinal walls, and both ends are closed. In the heat pipe 1, one of the end portions serves as an evaporating portion 3 and the other end serves as a condensing portion 4, and an insulated portion 5 therebetween is insulated from an external heat.
The evaporating portion 3 is brought into contact to a heat generating element, and the working fluid held therein is heated by a heat of the heat generating element. According to the preferred example, approximately third part of the container 2 serves as the evaporating portion.
The condensing portion 4 is brought into contact to a not shown radiation member such as a fin assembly. The working fluid evaporated at the evaporating portion 3 is aspirated toward the condensing portion 4 where a temperature and a pressure are low through the insulated portion 5 to radiate heat thereof through the external radiation member. Consequently, the working fluid is condensed into liquid phase again at the condensing portion 4.
In order to return the working fluid condensed at the condensing portion 4 to the evaporating portion 3 by a capillary pumping, a fiber wick 11 and a groove wick 12 are arranged in the container 2.
According to the preferred example, the fiber wick 11 is formed by bundling a plurality of copper fibers, and situated on a width center of the flat wall of the container 2. In the fiber wick 11 thus formed, each longitudinal clearance among the fibers individually serves as a flow passage to pull the working fluid condensed at the condensing portion 4 toward the evaporating portion 3 through the insulated portion 5 by a capillary pumping.
In order to guide the working fluid returned to the evaporating portion 3 also in a circumferential direction, a groove wick 12 is formed on an inner face of the container 2 in the evaporating portion. According to the preferred example, the groove wick 12 is formed into a spiral groove on the inner face of the container 2
An internal structure of the flat heat pipe 1 will be explained in more detail with reference to FIGS. 2 and 3. Turning first to FIG. 2, there is schematically shown the container 2 according to the preferred example. For example, the container 2 can be formed by flattening a metal tube in such a manner to form a lower flat wall 21 and an upper flat wall 22 in a longitudinal direction.
Turning now to FIG. 3, FIG. 3 (a) shows a cross-section of the evaporating portion 3 of the heat pipe 1 along the line A-A in FIG. 1. As depicted in FIG. 3 (a), an inner face 2 a of the container 2 includes a lower inner face 21 a of the lower flat wall 21, and an upper inner face 22 a of the upper flat wall 22. FIG. 3 (b) shows a cross-section of the heat pipe 1 at a boundary between the evaporating portion 3 and the insulating portion 5 along the line B-B FIG. 1. As depicted in FIG. 3 (b), the groove wick 12 is formed on the inner face 2 a only in the evaporating portion 3, and the remaining portion of the inner face 2 a in the insulated portion 5 and the condensing portion 4 has a smooth surface. As depicted in FIG. 3 (c), the groove wick 12 is formed on the inner face 2 in a spiral manner.
Specifically, the groove wick 12 is a depressed minute groove formed on the inner face 2 a in a spiral manner from the boundary between the insulated portion 5 and the evaporating portion 3 to one of the end portions of the container 2.
Optionally, in order to enhance the capillary pumping force of the spiral groove wick 12, a plurality of the spiral groove wick 12 may be formed on the inner surface 2 of the evaporating portion 3.
The fiber wick 11 is laid on the lower inner face 21 a of the lower flat wall 21 in a length direction of the container 2. That is, the lower inner face 21 a of the lower flat wall 21 serves as an installation surface. Specifically, as shown in FIG. 3 (a), the copper fibers are heaped on the lower inner face 21 a to form the fiber wick 11 in such a manner to have a semi-oval cross section while keeping a space between a peak of the fiber wick 11 and the upper inner face 22 a of the upper flat wall 22. The fiber wick 11 is sintered to fix the copper fibers to one another, and to be fixed to the lower inner face 21 a of the lower flat wall 21.
As shown in FIGS. 3 (a), (b) and (c), in the evaporating portion 3, the groove wick 12 formed on the inner face 2 a of the container 2 passes underneath a lower face 11 a of the fiber wick 11 in the transverse direction repeatedly at predetermined intervals. That is, the lower face 11 a of the fiber wick 11 is brought into contact to the lower inner face 21 a of the lower flat wall 21 intermittently. In other words, in the evaporating portion 3, the groove wick 12 transversely crossing the fiber wick 11 is partially closed underneath the fiber wick 11 by the lower face 11 a of the fiber wick 11. In the following description, the portion of the lower face 11 a of the fiber wick 11 thus covering the groove wick 12 will be called the “lid portion” 11 b, and the portion of the groove wick 12 thus closed by the lid portion 11 b will be called the “closed portion” 12 a. In FIG. 3 (c), the closed portion of the groove wick 12 is illustrated by dashed lines. In the evaporating portion 3, the condensed working fluid flowing through the fiber wick 11 is pulled by the capillary force of the groove wick 12 through the lid portion 11 b so that the working fluid is allowed to be spread entirely over the inner face 2 a of the container 2 through the groove wick 12. That is, the remaining opening portion 12 b of the groove wick 12 illustrated by solid lines in FIG. 3 (c) serves as a flow passage for the working fluid in the liquid phase, and the working fluid evaporated in the opening portion 12 b is allowed to ascend therefrom.
Here will be explained a heat transport cycle in the heat pipe 1. In the heat pipe 1, the working fluid flowing through the fiber wick 11 and the groove wick 12 is evaporated at the evaporating portion 3 by the heat of the not shown heat generating element. The working fluid vaporized at the evaporating portion 3 flows toward the condensing portion 4 where an internal pressure and a temperature are lower than those in the evaporating portion 3 through an internal space of the container 2. The vapor of the working fluid is cooled to be liquefied at the condensing portion 4 and penetrates into the fiber wick 11. Then, the working fluid in the liquid phase is returned to the evaporating portion 3 through the fiber wick 11, and spread over the inner face 2 a of the container 2 through the groove wick 12. The working fluid thus returned to the evaporating portion 3 is again heated to be vaporized.
According to the preferred example, the working fluid flowing through the fiber wick 11 in the length direction is allowed to flow into the groove wick 12 through the lid portions 11 b of the fiber wick 11 to be spread on the inner face 2 a of the container 2 in the circumferential direction. Since the wick groove 12 is thus formed on the inner face 2 a of the container 2, a surface roughness of the inner face 2 a is increased to enhance hydrophilicity thereof. For this reason, the thermal resistance in the evaporating portion 3 can be reduced to enhance the heat transporting performance of the heat pipe 1. In addition, since the groove wick 12 is thus formed in such a manner to cross the fiber wick 11 diagonally, the copper fibers of the fiber wick 11 can be prevented from falling into the groove wick 12.
Next, here will be explained test result of heat transport capacities of the heat pipes according to the preferred example and the comparison example with reference to FIGS. 4 and 5.
Turning now to FIG. 4, there is shown specifications of the heat pipe 1 of the preferred example and the heat pipe 100 of the comparison example used in the test. As shown in FIGS. 4 (a) and (b), lengths, widths and thicknesses of both heat pipes were 150 mm, 9.1 mm and 1.0 mm respectively, and the containers of both heat pipes were prepared by flattening metal tubular pipes whose external diameter was 6.0 mm. In the evaporating portion 3 of the heat pipe 1 according to the preferred example, a plurality of the groove wicks 12 whose width and depth were 0.15 mm and 0.02 mm were formed on the inner face 2 a of the container 2 at intervals of 0.45 mm. By contrast, the heat pipe 100 of the comparison example was not provided with the groove wick.
As shown in FIG. 4 (a), an electric heater H whose length and width were respectively 15 mm was attached to one end of the heat pipe to serve as the heat generating element, and a radiating member S whose length and width were respectively 50 mm was attached to the other end of the heat pipe. In addition, each heat pipe 1 and 100 is individually flexed to a substantially right angle at its intermediate portion.
As shown in FIG. 7 (b), an outer face of the lower flat wall 21 of the evaporating portion 3 is brought into contact to the heater H, and an outer face of the lower flat wall 21 of the condensing portion 4 is brought into contact to the radiating member S. In the test, each heat pipe of the preferred example and the comparison example was individually attached horizontally to a test equipment.
Temperatures of each heat pipe and the heater H was measured by a conventional thermocouple sensor. Specifically, as shown in FIGS. 4 (a) and (b), a surface temperature Th of the heater H contacted to the lower flat wall 21 of the evaporating portion 3, a surface temperature Ti of the upper flat wall 22 of the insulating portion 5, and a surface temperature Tc of the upper flat wall 22 of the condensing portion 4 were measured.
The evaporating portion 3 of each heat pipe was heated by energizing the heater H under room temperature, and the surface temperatures Th, Tc, and Ti were measured respectively while changing a heat input Q to the evaporating portion 3. Then, a thermal resistance R of each heat pipe was calculated under the condition that a temperature Ti of the insulating portion was stabilized at 60 degrees C. as expressed by the following expression:
R=(Th−Tc)/Q.
The calculation results of the thermal resistance R of the heat pipes of the preferred example and the comparison example are plotted in FIG. 5. In FIG. 5, a line penetrating through round dots represents the thermal resistance R of the heat pipe 1 according to the preferred example, and a dot-and-dash line represents the thermal resistance R of the heat pipe 100 according to the comparison example.
As can be seen from FIG. 5, a maximum heat transporting quantity QMAX of the heat pipe 1 according to the preferred example was achieved by 22 W of the heat input, and the thermal resistance R thereof at the maximum heat transporting quantity QMAX was 0.50. By contrast, a maximum heat transporting quantity QMAX of the heat pipe 100 according to the comparison example was achieved by 16 W of the heat input, and the thermal resistance R thereof at the maximum heat transporting quantity QMAX was 0.58.
If the heat input Q to the evaporating portion 3 exceeds the limitation value, the working fluid in the evaporating portion 3 would be dried out and the thermal resistance R of the heat pipe would be increased significantly. That is, the maximum heat transporting quantity QMAX of the heat pipe is increased with an increment of the limitation value of the heat input to the evaporating portion 3.
Specifically, the heat pipe 1 according to the comparison example causes the dry-out if the thermal input thereto exceeds 16 W, but the heat pipe 100 according to the preferred example will not cause the dry-out until the thermal input thereto exceeds 22 W.
That is, the maximum heat transporting quantity QMAX of the heat pipe 1 according to the preferred example was larger than that of the heat pipe 100 according to the comparison example.
Thus, hydrophilicity of the inner face 2 a of the container 2 in the evaporating portion 3 can be improved by forming the groove wick 12 thereon so that heat transporting capacity of the heat pipe 1 can be enhanced.
The structure of the heat pipe 1 according to the preferred examples may be modified according to need within the spirit of the present invention. For example, as shown in FIG. 6, a plurality of unconnected circular groove wicks 12 may be formed on the inner face 2 a of the container 2 in the evaporating portion 3 instead of the foregoing spiral groove wick.
Instead, as shown in FIGS. 7 (a) and 7 (b), the groove wick may also be formed only on the inner face 2 a of the lower flat wall 21. In this case, in the evaporating portion 3, a plurality of straight grooves extending perpendicular to the fiber wick 11 are formed as the groove wick 12 on the inner face 2 a of the lower flat wall 21, and a length of each straight groove is respectively longer than the width of the fiber wick 11 to ensure the opening portions 12 b on both sides of the fiber wick 11.
In case of thus forming the groove wick 12 only on the inner face 2 a of the lower flat wall 21, a configuration of the groove wick 12 may be altered arbitrarily according to need. For example, a plurality of rectangular grooves individually having a width wider than that of the fiber wick 11 may be formed on the inner face 2 a of the lower flat wall 21.
In addition, the fiber wick 11 may also be formed of carbon fibers instead of the copper fibers to reduce thermal resistance to transport heat in the length direction. In this case, the fiber wick 11 may also be formed not only of the carbon fibers but also of a mixture of the carbon fibers and the copper fibers.
Instead, a sintered porous wick made of metal powers may be employed instead of the fiber wick 11 to return the working fluid in the liquid phase from the condensing portion 4 to the evaporating portion 3. In this case, stronger capillary force can be achieved to pull the working fluid so that the working fluid can be returned to the evaporating portion more smoothly.
Further, as shown in FIG. 8 (a), a second fiber wick 13 may be formed also on the width center of the upper inner face 22 a of the upper flat wall 22 to be opposed to the fiber wick 11. Alternatively, the second fiber wick 13 may be displaced widthwise as shown in FIG. 8 (b). In any of those cases, the fiber wick 11 and the second fiber wick 13 are isolated away from each other so as to ensure the vapor passage in the internal space of the container 2. In those cases, the heat transporting performance of the heat pipe 1 will not be changed even if the heat pipe 1 is reversed. In addition, the fiber wick 11 may also be brought into contact to both the upper inner face 22 a of the upper flat wall 22 and the lower inner face 21 a of the lower flat wall 21.

Claims

What is claimed is:

1. A flat heat pipe, comprising:

a sealed container having a pair of upper and lower flat walls extending in a length direction;

a working fluid encapsulated in the container;

an evaporating portion that is situated on one of the longitudinal ends of the container at which evaporation of the working fluid takes place;

a condensing portion that is situated on the other longitudinal end of the container at which condensation of the working fluid takes place;

a first wick formed of a bundled fibers or a porous structure that extends in the length direction of the container to return the working fluid condensed at the condensing portion to the evaporating portion by a capillary pumping; and

a second wick that is formed by forming a groove on an inner face of the container only in the evaporating portion to spread the working fluid returned thereto over the inner face within the evaporating portion by a capillary pumping;

wherein the first wick is formed on at least one of the upper flat wall or the lower flat wall; and

wherein the second wick transversely extends underneath the first wick to be closed partially by the first wick.

2. The flat heat pipe as claimed in claim 1, wherein the second wick is formed on the inner face of the container in the evaporating portion in a spiral manner.

3. The flat heat pipe as claimed in claim 1, wherein the second wick is formed on the inner face of the container in the evaporating portion in a circular manner.

4. The flat heat pipe as claimed in claim 1, wherein the second wick is formed only on the inner face of one of the upper flat face and the lower flat face on which the first wick is formed.

5. The flat heat pipe as claimed in claim 1, wherein the second wick includes an opening portions extending from both sides of the first wick in the circumferential direction.

6. The flat heat pipe as claimed in claim 1, wherein the porous structure is formed by sintering metal powders.

7. The flat heat pipe as claimed in claim 1, wherein the first wick is formed of copper fibers.

8. The flat heat pipe as claimed in claim 1, wherein the first wick is formed of carbon fibers.

9. The flat heat pipe as claimed in claim 1, wherein the first wick is formed of a mixture of carbon fibers and copper fibers.