US20160010927A1

US20160010927A1 - Heat transport device

Info

Publication number: US20160010927A1
Application number: US14/797,956
Authority: US
Inventors: Mohammad Shahed AHAMED; Yuji Saito; Phan THANHLONG
Original assignee: Fujikura Ltd
Current assignee: Fujikura Ltd
Priority date: 2014-07-14
Filing date: 2015-07-13
Publication date: 2016-01-14

Abstract

A heat transport device having enhanced heat transport capacity is provided. The heat transport device comprises an evacuated and sealed container 2, and phase changeable working fluid encapsulated in the container to transport heat a fiber wick 3 is laid on an inner face of the container 2 in a longitudinal direction, and a powder wick is formed on a surface of the fiber wick 3. A thickness of the powder wick is five to ten times smaller than a diameter of the fiber.

Description

The present invention claims the benefit of Japanese Patent Application No. 2014-143877 filed on Jul. 14, 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 heat transport device in which a phase-changeable working fluid in encapsulated in a sealed container.
2. Discussion of the Related Art
Various kinds of heat pipes and thermosiphon are known in the art. For example, JP-A-2013-002640 describes a flat heat pipe formed by flattening a tubular container holding working fluid therein. In the flattened container, the working fluid is evaporated when it is heated and condensed when it is cooled, and the working fluid in the liquid phase is pulled by a capillary pumping of a wick laid on an inner flat face.
According to the teachings of JP-A-2013-002640, a grooved pipe in which a plurality of rows of grooves is formed on an inner face, and a bear pipe having an unprocessed smooth inner face can be used as the container. The wick is comprised of a layer of carbon fibers or copper fibers laid on the inner surface, and a metal powder layer covering the fiber layer.
In the heat pipe taught by JP-A-2013-002640, menisci of the liquid phase working fluid are formed among the powers of the powder layer, and a liquid level in the powder wick lowered as a result of evaporation of the working fluid is raised by a capillary pumping of menisci. Such capillary pumping force is enhanced by narrowing clearances among the powders. In the heat pipe of this kind, the working fluid in the liquid phase is allowed to flow back smoothly to an evaporation site through clearances among the metal fibers where a flow resistance is small. However, the working fluid has to flow back over the long distance to a heated site at one end of the container, and then the working fluid evaporated by the heat generating device contacted to the heated site has to flow upwardly through the powder layer having a certain thickness to escape from an outer surface of the powder layer. This increases a thermal resistance between the heated site and the working fluid in the heat pipe taught by JP-A-2013-002640. Thus, conventional heat pipes have to be improved to enhance heat conductivity by reducing thermal resistance to transport heat by the working fluid.

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 enhance heat transport capacity of a heat transport device by reducing a thermal resistance to transfer heat to working fluid.
The present invention is applied to a heat transport device having an evacuated and sealed container, and phase changeable working fluid is encapsulated in the container to transport heat. In order to achieve above-mentioned objective, according to the present invention, a fiber wick formed of fibers is laid on an inner face of the container in a longitudinal direction, and a powder wick is formed on a surface of the fiber wick. Grain diameters of the particles fall within a range between 5 to 10 μm that is five to ten times smaller than a diameter of the fiber, and a thickness of the powder wick is one to five times larger than the grain diameter of the particle.
The sealed container includes a heating site heated by an external heat, and a cooling site from which the heat is radiated to outside. According to one aspect of the present invention, the powder wick may be formed from the surface of the fiber wick to an inner surface of the heating site of the container.
According to another aspect of the present invention, the powder wick may also be formed entirely on the inner face of the sealed container except for a portion on which the fiber wick is laid.
Specifically, a diameter of the fiber is 50 μm at smallest, and a grain diameter of the particle is 10 μm at largest.
The sealed container may be made of metal material, and metal fibers and metal particles may be used to form the fiber wick and the powder wick. In this case, the metal fibers and the metal particles are fixed to an inner face of the container by a sintering method.
Optionally, the sealed container may be formed into a flat container having a larger width than a thickness.
In the heat transport device of the present invention, longitudinal clearances among the fibers serve as straight flow passages of the working fluid in the liquid phase, and diameters of passages are not varied partially. For this reason, the working fluid is allowed to flow smoothly back to the heating site through the fiber wick so that the thermal resistance in the heat transport device can be reduced to enhance heat transporting performance. Further, the powder wick on the fiber wick is formed of fine particle to create strong pumping forces at menisci to pump up the working fluid to the surface of the powder wick where the evaporation of the working fluid takes place. For this reason, the working fluid is also allowed to flow back smoothly to the heating site so that thermal resistance in the heat transport device can be further reduced to enhance heat transporting performance. In addition, since the thickness of the powder wick is significantly thinner than fibers of the fiber wick, the thermal resistance between the working fluid penetrating into the powder wick and an external heat source can be reduced so that heat transport performance of the heat pipe can be further 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 perspective view showing a preferred example of the heat transport device formed into a flat heat pipe;

FIG. 2 is a cross-sectional view of the flat heat pipe;

FIG. 3 is a schematic illustration showing a surface of the fiber wick and particles adhering thereto;

FIG. 4 is a schematic illustration showing a situation of measuring a thermal resistance of the heat pipe; and

FIG. 5 is a graph indicating measurement results of thermal resistance in the heat pipe according to the preferred example and the conventional heat pipe according to the comparative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Hereinafter, a preferred example of the heat transport device according to the present invention will be explained in more detail with reference to the accompanying drawings. Referring now to FIG. 1, there is schematically shown a heat pipe 1 as a heat transfer device adapted to transport heat by the known principle. A sealed container 2 of the heat pipe 1 is formed by flattening a metal pipe to have a wider width than a height, and working fluid is encapsulated therein liquid-tightly. The working fluid is evaporated when it is heated at a predetermined temperature, and condensed when it is cooled by a predetermined temperature. In order to pull the condensed working fluid to a heating site by capillary pumping, a fiber wick 3 is laid on an inner flat surface of the flattened container 2. For example, a steel pipe, an aluminum alloy pipe, a copper pipe can be used to form the container 2, and the copper pipe having the highest heat conductivity is most suitable. Specifically, a bear metal pipe in which an inner surface thereof is unprocessed is used to form the container 2.
The wick 3 is formed by bundling fibers. For example, metal fibers such as copper fibers or inorganic fibers such as carbon fibers can be used to form the wick 3. According to the preferred example, a diameter of each fiber 3 a is larger than 50 μm, and several hundreds of fibers 3 a are bundled to form the wick 3. The fibers may be twisted to be prevented from being loosened, however, it is preferable to bundle the straight fibers without twisting to reduce pressure loss in flow passages formed among the fibers. Optionally, the fibers may be bundled by a not shown banding band. Given that the copper fibers are used to form the wick 3, the copper fibers may be sintered together and to be fixed onto an inner face of the container 2 in a longitudinal direction.
As depicted in FIGS. 2 and 3, particles 4 are deposited on a surface of the fiber wick 3. According to the preferred example, copper powers whose maximum grain diameter is smaller than an average diameter of the fiber 3 a (50 μm at minimum) are used as the particles 4. Specifically, diameters of the particles 4 fall within a range from 5 to 10 μm, and other metal powders may also be used instead of copper powder. The particles 4 thus covering the surface of the fiber wick 3 serves as the claimed powder wick. A thickness of the powder wick formed of the particles 4 is determined in such a manner to ensure capillary pumping force to pull the working fluid and specific surface area to expedite evaporation of the working fluid. To this end, specifically, one to five layers of the particles 4 is/are formed on the top surface of the fiber wick 3. That is, a thickness of the powder layer is one to five times larger than the grain diameter of the particles 4. The thickness of the powder layer is adjusted to a desired value by pouring the particles 3 into the container 2 in an amount sufficient to cover the surface of the fiber wick 3, and then discharging surplus particles 4 from the container 2. Consequently, clearances among the fibers 3 a forming the fiber wick 3 are filled with the particles 4, and a top surface of the powder wick is formed into a smooth surface.
The powder wick may be formed not only on the top face of the fiber wick 3 but also entirely on the inner face of the container 2 (except for the portion on which the fiber wick 3 is laid). Especially, it is preferable to cover the inner surface of the container 2 from a heating site 2H heated by the heat generating element to the top surface of the fiber wick 3 by the powder wick. In this case, a thickness of the portion of the powder wick covering the inner face of the container 2 may not only be equal to that of the portion covering the top surface of the fiber wick 3 but also be different from that of the portion covering the top surface of the fiber wick 3. The particles 4 may be fixed to the top surface of the fiber wick 3 and the inner face of the container 2 by an appropriate method. For example, the particles 4 may be fixed to the top surface of the fiber wick 3 and the inner face of the container 2 by a sintering method. In this case, since the grain diameter of the particle 4 is smaller the diameter of the fiber 3 a, a sintering temperature of the particles 4 is lower than that of the fibers 3 a.
An example of fixing the particles 4 onto the top surface of the fiber wick 3 will be explained hereinafter. As described, a copper tubular pipe in which both ends are opened is used to form the sealed container 2. Appropriate number of the copper fibers 3 a to form the fiber wick 3 is laid on the inner face of the tubular pipe in the longitudinal direction using a predetermined jig (e.g., a center rod) to situate the bunch of the fibers 3 a at a desired position while keeping into a desired configuration. Then, the tubular pipe thus holding the fibers 3 a is sintered to fix the fibers 3 a to one another and to the inner face of the tubular pipe at approximately 1000 degrees C. under an inert atmosphere. After the copper fibers 3 a are fixed to the inner surface of the tubular pipe, the jig is withdrawn from the tubular pipe. In this situation, both ends of the tubular pipe are still opened. Then, one of the end portions of the tubular pipe is closed using a predetermined tool, and the copper particles 4 is fed into the tubular pipe from the other opening end in an amount sufficient to form the powder wick. Thereafter, surplus of the copper particles 4 is discharged from the tubular pipe by tapping or knocking the closed end thereof while situating the opening end thereof downwardly. In this situation, however, a necessary amount of the copper powders 4 to form an unsintered powder layer is still adsorbed to the top surface of the fiber wick 3 by a surface energy of the copper powder 4 or by an attraction between the copper pipe and the copper powder. In this situation, the tubular pipe is sintered again at approximately 600 degrees C. under an inert atmosphere to fix the copper powder layer to the top surface of the fiber wick 3 and to the inner face of the tubular pipe.
The working fluid is selected from water, ammonia, alcohol etc. having good hydrophilic property to the copper tube and the copper powder 4, and according to the preferred example, water is used as the working fluid. For example, the container 2 can be filled with the working fluid by evacuating air therefrom, and pouring an appropriate amount of the working fluid thereinto. Then, the opening end of the container 2 is closed. Alternatively, the container 2 may also be filled with the working fluid by pouring an excessive amount of the working fluid into the tubular pipe while boiling the working fluid by heating the container 2 to evacuate air from the container 2, and then closing the opening end of the container 2.
In the heat pipe 3 shown in FIGS. 1 to 3, the working fluid in the liquid phase penetrates into the fiber wick 3. The working fluid in the heating site 2H is heated by the heat “Q” of the heat generating element that is brought into contact to an outer face of the heating site 2H. The vapor of the working fluid flows toward a cooling site 2C while transporting the heat “Q” in the form of latent heat, and the heat “Q” is then radiated from the cooling site 2H. Consequently, the working fluid is condensed again into the liquid phase. Optionally, a portion between the heating site 2H and the cooling site 2C may be insulated from an external heat. To this end, the powder wick is formed on the inner face of the container 2 between the heating site 2H and the cooling site 2C through the heat insulating portion.
The working fluid is condensed on the inner face of the container 2. Given that the powder wick of the copper powder 4 is formed on the inner face of the container 2, the working fluid is allowed to spread throughout the inner surface through the powder wick so that hydrophilicity of the inner surface can be enhanced. Consequently, the condensed working fluid is allowed to penetrate into the fiber wick 3 efficiently to be pulled toward the heating site 2H. In addition, the condensed working fluid is also allowed to flow back partially to the heating site 2H through the powder wick formed of the fine copper powder 4.
The condensed working fluid is returned to the heating site 2H to be evaporated mainly through the fiber wick 3. In the heating site, the working fluid is evaporated and hence an amount of the working fluid in the liquid phase is decreased. However, the working fluid in the liquid phase is returned continuously from the cooling site 2C through the fiber wick 3, and pumped up to a surface of the powder wick formed of the finer copper powder 4 where the evaporation of the working fluid takes place by the capillary pumping of menisci among the powder copper powder 4. Thus, the working fluid is circulated efficiently within the heat pipe 1 while changing phase to enhance heat transporting performance of the heat pipe 1. Especially, according to the preferred example, the thickness of the powder wick is five to ten times thinner than the average diameter of the copper fibers 3 a so that the thermal resistance to transport heat to the working fluid at the evaporating site can be reduced. For this reason, the thermal resistance in the heat pipe 1 can be reduced entirely so that the heat transporting performance of the heat pipe 1 can be enhanced.
Here will be explained a comparison result of thermal resistance between the heat pipe 1 according to the preferred example shown in FIGS. 1 to 3 and a conventional heat pipe according to a comparison example. Specifically, the heat pipe according to a comparison example is provided with the fiber wick 3 but not provided with the powder wick. The remaining structures of the heat pipes used in the measurement are similar to each other. Lengths, widths and thicknesses of both heat pipes are 150 mm, 9.1 mm and 1.0 mm respectively. In the measurement, a square heater 5 (15 mm each side) adapted to change a temperature thereof electrically is attached to the heating site 2 of each heat pipe, and a portion of each heat pipe within 50 mm from a leading end of the cooling site 2C is attached to a copper radiator plate 6 to be cooled. During the measurement, the heating site 2C of each heat pipe were heated at an operating temperature of 60 degrees C. while changing a thermal input thereto, and a surface temperature Th of the heating site 2C of each heat pipe and a surface temperature of the cooling site Tc of each heat pipe were measured respectively by a same sensor.
Measurement results are shown in FIG. 5. As can be seen from FIG. 5, the thermal resistance ((Th−Tc)/W(° C./W)) of the heat pipe according to the preferred example falls within a range from 0.5 (° C./W) to 0.6 (° C./W) under a condition that the thermal input is 10 to 18 W. By contrast, the thermal resistance of the heat pipe according to the comparison example is larger than that of the heat pipe according to the preferred example over the entire range of the thermal input. In addition, a dry out of the heat pipe according to the preferred example is caused at around 18 W of thermal input. By contrast, a dry out of the heat pipe according to the comparison example is caused at around 16 W of thermal input. Thus, thermal resistance of the heat pipe according to the preferred example is reduced significantly to enhance heat transporting performance.
It is understood that the invention is not limited by the exact construction of the foregoing preferred example, but that various modifications may be made without departing from the spirit of the inventions.

Claims

What is claimed is:

1. A heat transport device, comprising:

a sealed container from which air is evacuated;

a working fluid that is encapsulated in the container, and that circulates within the container while being evaporated by being heated and condensed by removing heat therefrom;

a fiber wick that is formed by bundling a plurality of fibers, and that is laid on an inner face of the container in a longitudinal direction; and

a powder wick that is formed on a surface of the fiber wick by depositing particles whose grain diameters fall within a range between 5 to 10 μm that is five to ten times smaller than a diameter of the fiber;

wherein a thickness of the powder wick is one to five times larger than the grain diameter of the particle.

2. The heat transport device as claimed in claim 1,

wherein the sealed container includes a heating site heated by an external heat, and a cooling site from which the heat is radiated to outside, and

wherein the powder wick is formed from the surface of the fiber wick to an inner surface of the heating site of the container.

3. The heat transport device as claimed in claim 1, wherein the powder wick is formed entirely on the inner face of the sealed container except for a portion on which the fiber wick is laid.

4. The heat transport device as claimed in claim 1, wherein a diameter of the fiber is 50 μm at smallest, and a grain diameter of the particle is 10 μm at largest.

5. The heat transport device as claimed in claim 3,

wherein the sealed container is made of metal material;

wherein the fibers and the particles include metal fibers and metal particles; and

wherein the metal fibers and the metal particles are fixed to an inner face of the container by a sintering method.

6. The heat transport device as claimed in claim 1, wherein the sealed container includes a flat container having a larger width than a thickness.