US20160091258A1

US20160091258A1 - Heat pipe

Info

Publication number: US20160091258A1
Application number: US14/869,350
Authority: US
Inventors: Mohammad Shahed AHAMED; Yuji Saito
Original assignee: Fujikura Ltd
Current assignee: Fujikura Ltd
Priority date: 2014-09-30
Filing date: 2015-09-29
Publication date: 2016-03-31
Also published as: JP5759606B1; JP2016070593A

Abstract

A heat pipe for improving heat transport performance by reducing heat resistance in an evaporating section is provided. The heat pipe comprises a groove wick extending in the longitudinal direction from the evaporating section to the condensing section through the insulated section in the container. A metal powder layer formed of metal powder adhering to the inner wall of the groove wick is arranged in the evaporating section. The metal powder layer is formed into the groove wick having a predetermined thickness.

Description

The present invention claims the benefit of Japanese Patent Application No. 2014-200493 filed on Sep. 30, 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 pipe having a wick structure.
2. Discussion of the Related Art
Heat pipes have been widely used to transport heat in the form of latent heat of working fluid. The conventional heat pipe is comprised of a tubular container sealed at both ends, and phase changeable working fluid encapsulated in the container. The working fluid is evaporated by a heat of a heat generating element and flows toward a low temperature site.
In general, one end of the heat pipe is brought into contact to a heat generating element, and other end of the heat pipe is connected to a radiation member. The end portion of the heat pipe brought into contact to the heat generating element serves as an evaporation portion at which an evaporation of the working fluid in the liquid phase takes place, and the other end serves as a condensing portion at which the working fluid in the vapor phase is condensed again by radiating heat from the radiation member. In order to return the working fluid thus condensed at the condensing portion to the evaporating section, a wick structure adapted to perform a capillary pumping is arranged in the conventional heat pipe.
JP-A-04-098093 discloses a heat pipe having a wick structure wherein a first metallic tube serving as a condensing section is connected to a second metallic tube serving as an evaporating section through an electrical insulating tube. A thread groove is formed on an inner surface of each metallic tube to reflux working fluid. In the heat pipe taught by JP-A-04-098093, a perforated layer made of sintered copper powders is formed in the thread grooves of the second metallic tube serving as the evaporating section.
In a thin flat heat pipe, an inner space of the container is used as a vapor passage. However, since a thickness of the container is reduced, an inner volume of the flat container is rather small and hence it is difficult to improve a flow performance of the working fluid.
If a large wick structure is arranged inside of the flat container to return the working fluid to the evaporating section efficiently, the wick structure shares the most space in the container and hence it is difficult to ensure a space serving as the vapor passage. If the working fluid cannot be returned to the evaporating section smoothly, the evaporating section would be dried out.
In the flat heat pipe taught by JP-A-04-098093, the inner space for the vapor passage can by ensured owing to the groove formed on the inner face of the heat pipe.
However, in the flat heat pipe taught by JP-A-04-098093, the groove of the second metallic tube serving as the evaporating section is buried by the perforated layer of sintered metallic powders having the grain size of 100 to 400 μm. For this reason, capillary pumping of the groove may be weakened by the perforated layer formed in the groove. In addition, while the working fluid can be evaporated through a thin portion of the perforated layer, the working fluid may not be evaporated sufficiently through a thick portion of the perforated layer.

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 heat pipe having enhanced evaporation performance and heat transport performance.
According to one aspect of the preferred example, the heat pipe is provided with: a container sealed at both longitudinal ends; a working fluid encapsulated in the container; a wick that pulls the working fluid by a capillary pumping; an evaporating section situated on one of the longitudinal ends of the container at which evaporation of the working fluid takes place; and a condensing section situated on the other longitudinal end of the container at which condensation of the working fluid takes place. Specifically, the wick includes a groove wick having a plurality of grooves extending on an inner wall of the container between the condensing section and the evaporating section via the insulated section while keeping predetermined intervals. The heat pipe is further provided with a metal powder layer formed at least on an inner wall of the groove wick and at least within the evaporating section. In addition, a thickness of the metal powder layer falls within a predetermined range.
According to another aspect of the present invention, the metal powder layer may be formed at least on the inner wall of the groove wick and only within the evaporating section.
The inner wall of the groove wick includes a pair of side walls and a bottom wall connecting the side walls so that each groove of the groove wick is individually shaped to have a U-shaped cross-section. The metal powder layer includes a first portion formed on the side wall having a thickness less than one fifth of a width of each groove; and the metal powder layer includes a second portion formed on the bottom wall having a thickness less than one third of a depth of each groove.
The container is made of metal material. The metal powder includes copper particles whose grain size falls within a range from 1 to 5 μm, and the metal powder is sintered to the inner wall. The thickness of the metal powder layer is restricted within a range from one to five times of grain size of the metal powder in such a manner that the thickness of the first portion is restricted to be less than one third of the width of each groove, and that the thickness of the second portion is restricted to be less than one fifth of the depth of each groove.
The metal powder may be sprinkled sparsely on the inner wall in the evaporating section in such a manner that a surface of the inner wall is exposed partially and finely textured.
The metal powder layer may also be formed into a porous wick by sintering the deposited metal powder.
Optionally, the metal powder layer may also be formed on the inner surface of the container between the grooves.
Specifically, a thickness of the metal powder layer formed on the inner surface of the container between the grooves is less than one fifth of a width of the each groove.
In addition, the container includes a flat container; and the groove wick is formed entirely on the inner wall of the container.
As described, according to the present invention, the groove wick is not filled completely with the metal powder layer so that the working fluid can be returned effectively by the capillary pumping performed by the groove wick. In addition, since the metal powder layer is formed on the inner wall of the groove in the evaporating section, the working fluid in the liquid phase returned thereto can be spread entirely within the evaporating section. Consequently the evaporation area can be enlarged so that the working fluid can be evaporated efficiently. In addition, the working fluid flowing through the groove wick can be returned to the evaporating section efficiently by an enhanced capillary pumping of the inner section of the metal powder layer. Further, since the thickness of the metal powder layer is thus restricted within the above-mentioned range, the heat resistance in the evaporating section can be reduced and heat transport capacity of the heat pipe can be enhanced. Furthermore, the space serving as the vapor passage can be ensured sufficiently so that the working fluid in the vapor phase is allowed to flow through the vapor passage smoothly.

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 schematically showing a preferred example of the heat pipe;

FIG. 2( a) is a cross-sectional view of a the heat pipe in the evaporating section along A-A line in FIG. 1 showing grooves formed on the inner surface of the container, and FIG. 2( h) is a perspective view showing the grooves;

FIG. 3( a) is an enlarged cross-sectional view of the region B in FIG. 2 (a) showing the metal powder adhering to the inner wall of the groove in an evaporating section, and FIG. 3( b) is a perspective view along C-C line in FIG. 1 showing the metal powder in the evaporating section;

FIG. 4( a) shows a condition where the groove wick is filled fully with the working fluid in the evaporating section when the heat pipe is not activated, FIG. 4( h) shows a condition where the working fluid in the groove wick is decreased when the heat pipe is activated, FIG. 4( c) shows a condition where the working fluid in the groove wick is further decreased so that a water layer is formed in the metal powder; and FIG. 4( d) shows a condition where the groove wick 10 in the condensing section is filled fully with the working fluid;

FIG. 5 is an enlarged cross-sectional view showing an example of forming the metal powder layer into a porous wick;

FIG. 6( a) is an enlarged cross-sectional view showing an example of forming the metal powder layer also on the inner surface of the container, and FIG. 6( h) is an enlarged cross-sectional view showing the metal powder layer formed into a porous wick;

FIG. 7( a) is a top view of a testing equipment, and FIG. 7( h) is a front view of the testing equipment;

FIG. 8( a) is a schematic illustration showing a level of the working fluid in the container of the heat pipe of the preferred example in activation; and FIG. 8( b) is a schematic illustration showing a level of the working fluid in the container of the heat pipe of the comparative example in activation; and

FIG. 9 is a bar chart showing measurement results of the preferred example and the comparative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Hereinafter, a preferred example of the 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 structure of a heat pipe 1 according to the preferred example. The heat pipe 1 is a heat transfer device utilizing a latent heat of phase changeable working fluid encapsulated in a sealed container 2 as a straight rectangular tube.
The container 2 is a hollow flattened rectangular tube having a thickness narrower than a width, and both longitudinal ends thereof are sealed. In an inner space of the container 2, a width dimension is accordingly larger than a thickness dimension. The container 2 is made of metal material having excellent heat conductivity such as copper, steel and aluminum.
For example, the container 2 is formed by pressing a metal tubular material into a flat shape. Specifically, the container 2 thus formed is comprised of a first flat wall 21 as a lower wall, a second flat wall 22 as an upper wall, and a pair of side walls 23. Both the first and second flat walls 21 and 22 are rectangular plates having smooth outer surfaces, and connected to each other through the curved side walls 23 extending along each width end of the container 2.
The working fluid is a known phase changeable heat transfer medium such as water, alcohol, ammonia etc. encapsulated in the container 2.
As illustrated in FIG. 1, one of longitudinal ends of the container 2 serves as an evaporating section 3, other longitudinal end serves as a condensing section 4, and an intermediate portion therebetween serves as an insulated section 5, The insulated section 5 is insulated from an external heat so that the working fluid is allowed to flow therethrough without causing a phase change.
According to the preferred example, an outer surface of the first flat wall 21 is brought into contact to a (not shown) heat generating electronic component such as a CPU at the evaporating section 3 of the heat pipe 1 in such a manner to receive heat therefrom.
On the other hand, an outer surface of the first flat wall 21 is brought into contact to a (not shown) radiation member such as a metal fin array or a metal heat sink at the condensing section 4 in such a manner to radiate heat of the evaporated working fluid therefrom.
Thus, in the heat pipe 1, the working fluid is evaporated at the evaporating section 3 by the heat of the heat generating element H contacted to the first flat wall 21. The vapor of the working fluid flows toward the condensing section 4 through the insulated section 5, and the heat of the vapor is radiated to the external air from the condensing section 4. Consequently, the working fluid is condensed into the liquid phase. The working fluid thus condensed again at the condensing section 4 is returned to the evaporating section 3 by a capillary pumping of a groove wick 10 shown in FIG. 2 formed on an inner surface of the container 2.
A structure of the groove wick 10 will be explained in more detail with reference to FIG. 2. As illustrated in FIGS. 2( a) and 2(b), the groove wick 10 is formed on an inner surface 2 a of the container 2 throughout the flat walls 21 and 22 and the side walls 23. Specifically, the groove wick 10 is formed of a plurality of grooves extending in the longitudinal direction parallel to one another at predetermined intervals, Each groove of the wick 10 has a U-shaped cross-sectional shape, and a width W of each groove is 80 μm and a depth D of each groove is 50 μm.
The inner surface 2 a of the container 2 is entirely smoothened, and inner surface 2 a includes a first inner surface 21 a of the first flat wall 21, a second inner surface 22 a of the second flat wall 22, and side inner surfaces 23 a of the side walls 23. As described, the groove wick 10 is formed throughout the inner surfaces 21 a, 22 a and 23 a and hence the inner surface 2 a of the container 2 is knurled entirely in the circumferential direction.
Next, a metal powder layer 12 formed in the container 2 will be explained with reference to FIG. 3, In FIGS. 3( a) and 3(b), the groove wick 10 formed on the first flat wall 21 at the evaporating section 3 is individually shown as an example. In the evaporating section 3, remaining portion of the groove wick 10 on the second flat wall 22 and on the side wall 23 has the same structure as that on the first flat wall 21.
In the example shown in FIG. 3( a), a metal powder layer 12 is formed of metal powder P adhering to an inner wall 11 of the groove wick 10. Specifically, the metal powder P is a metal particle as exemplified by copper powder, having a grain size of 1 to 5 μm.
The metal powder layer 12 may be formed into not only a single layer but also multi-layers. Specifically, in the example illustrated in FIG. 3( a), all of the metal powder P adheres to the inner surface 2 a to form the metal powder layer 12. That is, the metal powder layer 12 is formed into a single layer in which a thickness thereof from the inner surface 2 a is identical to the grain size of the metal powder P.
The inner wall 11 forming the U-shaped cross-sectional groove is comprised of a bottom wall 11 a and a pair of side walls 11 b, and the metal powder P adheres to the inner wall 11 at the evaporating section 3.
As illustrated in FIG. 3( h), the metal powder layer 12 is formed only within the evaporating section 3, but not in the condensing section 4 and the insulated section 5.
In the example shown in FIGS. 3( a) and 3(b), the metal powder P is sprinkled sparsely on the inner wall 11 of the groove wick 10 in such a manner that the inner wall 11 is not covered completely by the metal powder layer 12 to be exposed partially so that a surface of the inner wall 11 is finely textured in the evaporating section 3. According to the preferred example, the container 2 is made of copper and the metal powder P is sintered to the inner wall 11 of the groove wick 10.
Next, an example of manufacturing method of the heat pipe 1 will be explained hereinafter. According to the preferred example, a metal tubular material is used to form the container 2. Specifically, a plurality of longitudinal grooves are formed on the inner surface 2 a of the tubular material to form the groove wick 10. First of all, a portion of the tubular material to serve as the evaporating section 3 is filled with the metal powder P using a jig or the like in such a manner that the remaining portions to serve as the condensing section 4 and the insulated section 5 are prevented from being filled with the metal powder P.
Then, the tubular material is tapped or vibrated while situating an opening end of the evaporating section 3 downwardly to discharge excess metal powder P therefrom. Consequently, the metal powder P whose grain size is only several μm is attached to the inner wall 11 by van der Waals force only in an amount of covering the inner wall 11 sparsely. At this step, for example, the tubular material may be tapped by a hammer or shaken by a vortex mixer for a few second.
Thereafter, the metal powder P is sintered to the inner wall 11 of the groove wick 10. As described, the grain size of each metal powder P falls within a range from 1 to 5 μm, and hence the metal powder P may be sintered at a comparatively lower temperature (e.g., 500° C.) than a conventional sintering temperature (e.g., 1000° C.).
Thereafter, one of the end portions of the tubular material is sealed by a swaging method or the like, and water is poured into the tubular material while evacuating air therefrom. Then, the other end portion of the tubular material is welded to be closed.
Thereafter, the tubular material is flattened by a conventional method to form the first flat wall 21, the second flat wall 22 and the side walls 23.
Next, a principle of heat transport in the heat pipe 1, and a change in the amount of the working fluid in the evaporating section 3 will be explained hereinafter with reference to FIG. 4. As shown in FIG. 4( a), before the heat pipe 1 is activated, the working fluid has not yet been evaporated and hence the groove wick 10 is filled fully with the working fluid L. Specifically, the groove wick 10 is filled fully with the working fluid L until a temperature of the working fluid L is raised in the evaporating section 3 by a heat generating element H to the evaporating point.
Then, when the temperature of the working fluid L is raised to the evaporating point by the heat of the heat generating element H, evaporation of the working fluid L is caused in the evaporating section 3. Consequently, as shown in FIG. 4( b), a water level in the groove wick 10 in the evaporating section 3 starts lowering in such a manner to form a concave meniscus.
As described, the metal powder P is sprinkled sparsely on the inner wall 11 of the groove wick 10 so that a surface roughness of the inner wall 11 is increased to enhance hydrophilicity. For this reason, in an area E enclosed by a dashed line in FIG. 4( b), the working fluid L is still allowed to adhere to the side wall 11 b from an upper end even after the water level starts lowering by the evaporation of the working fluid L. Consequently, a thin water layer of the working fluid L is formed on the side wall 11 b from the upper end of the side wall 11 b to the bottom wall 11 a within the evaporating section 3. Since a thickness of the working fluid L forming the water layer on the side wall 11 b is thus reduced, heat resistance of the water layer is reduced so that evaporation of the working fluid L is expedited within the area E. That is, an evaporating area of the working fluid L is enlarged in the groove wick 10.
As shown in FIG. 4( c), an amount of the working fluid L in the groove wick 10 is further decreased with an increment of the heat generated by the heat generating element H and a progression of the evaporation of the working fluid L. In this situation, the water layer of the working fluid L is formed not only on the side walls 11 b but also on the bottom wall 11 a. Thus, since at least the water layer remains on the inner wall 11 of the groove wick 10 even when most of the working fluid L has been evaporated, the working fluid L can be prevented from being dried out within the evaporating section 3.
The vapor generated in the evaporating section 3 flows through a vapor passage toward a condensing section 4 where pressure and temperature are lower than those in the evaporating section 3. The metal powder P is not arranged on the surfaces of the insulated section 5 and the condensing section 4 so that the vapor of the working fluid L is allowed to flow smoothly through those sections. That is, a flow resistance of the vapor flow can be reduced within the insulated section 5 and the condensing section 4. In addition, a wick structure is not arranged in the space enclosed by the inner surfaces 21 a, 22 a and 23 a so that a cross sectional area of the vapor passage can be ensured sufficiently and hence heat transport capacity can be enhanced.
The working fluid L in the vapor phase reaches the condensing section 4 eventually, and condensed into the liquid phase again. Consequently, as illustrated in FIG. 4( d), the groove wick 10 in the condensing section 4 is filled with the working fluid L thus condensed. Meanwhile, the working fluid L is evaporated in the evaporating section 3 while forming the above-mentioned concave meniscus so that the working fluid L in the liquid phase is returned from the condensing section 4 to the evaporating section 3 by a capillary pumping. Consequently, in the condensing section 4, the working fluid L outside of the groove wick 10 is aspirated into the groove wick 10 and pulled toward the evaporating section 3. Thus, the working fluid L condensed at the condensing section 4 is returned to the evaporating section 3 through the groove wick 10 by the capillary pumping via insulated section 5.
The working fluid L returned to the evaporating section 3 through the groove wick 10 is evaporated by the heat of the heat generating element H again, and above-described heat transport cycle is repeated.
As described, according to the preferred example, the metal powder layer 12 is formed into single layer, and the thickness thereof is identical to the grain size of the metal powder P However, the metal powder layer 12 may also be formed into multiple layers as long as the surface of the groove wick 10 is not covered completely with the metal powder P. In this case, as illustrated in FIG. 5, the metal powder P may be heaped within a range from one to five times of grain size thereof and sintered to form a porous wick having clearances among particles of the metal powder P In this case, some of particles of the metal powder P adhere only to the inner wall 11, but the remaining particles of the metal powder P adhere only to the other particles without adhering to the inner wall 11.
As described, the grain size of the metal powder P is 5 μm at the maximum, and on the other hand, the width W of the groove wick 10 is 80 μm and the depth D of the groove wick 10 is 50 μm respectively According to the preferred example, since the metal powder layer 12 is formed on the inner wall 11 in such a manner that the thickness thereof falls within the range from one to five times of grain size of the metal powder P (i.e., 25 μm at the maximum), the groove wick 10 will not be filled completely with the metal powder P In the example in which the metal powder P is thus heaped to form the porous wick, the surface of the inner wall 11 is not covered completely with the porous layer so that the surface of the inner wall 11 is still exposed partially to the air.
In the evaporating section 3, the metal layer 12 may be formed not only on the inner wall 11 but also on the inner surface 2 a including flat faces 21 a, 22 a and 23 a.
FIG. 6( a) illustrates an example in which the metal powder P adheres to the inner surface 2 a entirely in the evaporating section 3. In the example shown in FIG. 6( a), specifically, the metal powder layer 12 is formed not only on the inner wall 11 of the groove wick 10 but also on the inner surface 2 a of the container 2 in the evaporating section 3. That is, the metal powder layer 12 includes an inner section 13 formed on the inner wall 11 of the groove wick 10, and an outer section 14 formed on the inner surface 2 a of the container 2 (i.e., on the first inner surface 21 a in FIG. 6( a)), Here, it is to be noted that the metal powder layer 12 may be formed into not only a single layer but also porous structure.
According to the example shown in FIG. 6, the metal powder layer 12 is formed into a porous wick, and thickness of each portion of the metal powder layer 12 will be explained with reference to FIG. 6( b). In FIG. 6( b), “D” represents a depth of each groove of the groove wick 10 between an opening end to the bottom wall 11 a, and “W” represents a width of each groove of the groove wick 10 between side walls 11 b. According to the example shown in FIG. 6, specifically, a thickness t1 of a first portion 13 a of the inner section 13 of the metal powder layer 12 formed on the bottom wall 11 a is less than one third of the depth D, and a thickness t2 of a second portion 13 b of the inner section 13 of the metal powder layer 12 formed on each side wall 11 b is less than one fifth of the width W On the other hand, a thickness t3 of the outer section 14 of the metal powder layer 12 formed on the inner surface 21 a (or 22 a, 23 a) of the container 2 is also less than one fifth of the width W.
According to the example shown in FIG. 6, the metal powder layer 12 may also be formed in such a manner that the thickness of each portion falls within the range from one to five times of grain size of the metal powder P. Given that the width W is 80 μm, the depth D 50 μm, and the grain size of the metal powder P is 1 μm, the thickness of the metal power layer 12 will be 5 μm at the maximum. In this case, the thickness t1 will be less than one third of the depth D (approx. 16.7 μm), and the thickness t2 and the thickness t3 will be less than one fifth of the width D (i.e., less than 16 μm). However, given that the grain size of the metal powder P is 5 μm, the maximum thickness of the metal powder layer 12 will be 25 μm and exceed the above-explained thicknesses. In this case, therefore, an amount of the metal powder P is adjusted to maintain the thickness t1 thinner than 16.7 μm, and to maintain the thicknesses t2 and t3 thinner than 16 μm respectively. For these reasons, the groove wick 10 will not be filled completely with the metal powder P.
In the heat pipe shown in FIG. 3 (a) or 5, only the inner section 13 of the metal powder layer 12 is formed on the inner wall 11 of the groove wick 10 so that the vapor of the working fluid L is allowed to flow through the vapor passage in the container 2 smoother in comparison with the heat pipe shown in FIGS. 6( a) and 6(b) in which the outer section 14 of the metal powder layer 12 is also formed on the inner surface 2 a of the container 2. For his reason, a heat transport capacity of the heat pipe shown in FIG. 3( a) or 5 is larger than that of the heat pipe shown in FIGS. 6( a) and 6(b). In addition, since the required amount of the metal powder P to form the metal powder layer 12 in the heat pipe shown in FIG. 3 (a) or 5 is smaller, a production cost of the heat pipe shown in FIG. 3( a) or 5 is cheaper than that of the heat pipe shown in FIGS. 6( a) and 6(b).
In addition the metal powder layer 12 may also be arranged not only in the evaporating section 3 but also entirely in the length direction of the heat pipe from the evaporating section 3 to the condensing section 4 through the insulated section 5. In this case, the metal powder layer 12 may be formed not only on the inner wall 11 but also on the inner surfaces 21 a, 22 a and 23 a.
In the heat pipe in which the metal powder layer 12 is formed also in the condensing section 4 and the insulated section 5, flow resistance of the working fluid L flowing through the inner space of the container 2 is increased by the metal powder layer 12, irrespective of existence of the outer section 14 of the metal powder layer 12. For this reason, a heat transport capacity of the heat pipe in which the metal powder layer 12 is formed only in the evaporating section 3 is larger than that of the heat pipe in which the metal powder layer 12 is formed also in the condensing section 4 and the insulated section 5.
Next, experimental results about the heat transport capacity will be explained hereafter. In the heat pipe 1 of the preferred example, the metal powder layer 12 including the inner section 13 and the outer section 14 were arranged entirely from the evaporating section 3 to the condensing section 4 through the insulated section 5. The metal powder P having the above-explained grain size was used to form the metal powder layer 12, and the thickness of the metal powder layer 12 was adjusted to the above-explained range. On the other hand, in the heat pipe 100 of the comparative example, the metal powder layer 12 was not arranged so that the working fluid is returned only by the groove wick 10. The container 2 was formed by pressing a tubular material whose outer diameter is 6.0 mm to have a length of 100 mm, a thickness of 0.55 mm and a width of 2.85 mm, and the container 2 thus prepared was used respectively in the heat pipes 1 and 100. Here, the width W of the groove wick 10 was 0.08 mm and the depth D of the groove wick 10 was 0.05 mm in both heat pipes 1 and 100.
As illustrated in FIG. 7( a), a 10 mm square electric heater was used as the heat generating element H. The heat generating element H is thermally connected individually to each heat pipe 1 and 100 through a metal plate S of 100 mm length and 50 mm width. A heat input Q applied from the electric heater to the heat pipe is governed by the electric power of the electric heater.
As shown in FIG. 7( b) and FIGS. 8 (a) and (h), the outer face of the first flat wall 21 was entirely brought into contact to the metal plate 5, and a lower face of the metal plate S was brought into contacted to the heat generating element H in the evaporating section 3. The heat pipes 1 and 100 are individually attached to a thermocouple thermometer in such a manner that first flat portion 21 was situated horizontally to measure a surface temperature Th of the electric heater. Specifically, the surface temperature Th of the electric heater was measured at a contact portion between the outer face of the electric heater and the lower face of the metal plate S. In FIGS. 8 (a) and (b), upward arrows represent the heat input to the heat pipe and downward arrows represent the heat radiation from the heat pipe.
The surface temperature of the electric heater Th of each case in which the heat input Q was 3W, 4W and 5W was measured respectively while heating the evaporating section 3 by energizing the electric heater under the room temperature. The surface temperature Th stabilized after applying a predetermined heat input Q to the heat pipe for a predetermined period of time is shown in FIG. 9.
In FIG. 9, measurement results of surface temperature Th in the heat pipe 1 according to the preferred example are represented by hatched bars, and measurement results of the surface temperature Th in the heat pipe 100 according to the comparative example are represented by the outlined bars. When the heat input Q was 3W the surface temperature Th in the heat pipe 100 is 79.2° C. and the surface temperature Th in the heat pipe 1 was 76.3° C. When the heat input Q was 4W, the surface temperature Th in the heat pipe 100 was 88.8° C. and the surface temperature Th in the heat pipe 1 was 85.3° C., When the heat input Q is 5W the surface temperature Th in the heat pipe 100 was 105.1° C. the surface temperature Th in the heat pipe 1 was 101.1° C.
As can be seen from the experimental results, the surface temperature Th at each heat input Q in the heat pipe 1 according to the preferred example was respectively lower than that in the heat pipe 100 according to the comparative example. This means that the heat transport capacity of the heat pipe 1 having the metal powder layer 12 according to the preferred example is higher than that of the heat pipe 100 according to the comparative example.
As described, according to the preferred example, the groove wick is not filled completely with the metal powder layer, Therefore, the working fluid can be returned effectively by the capillary pumping performed not only by the metal powder layer but also by the groove wick. In addition, since the metal powder layer is formed in the evaporating section 3, the working fluid in the liquid phase returned thereto can be spread entirely within the evaporating section. Consequently, the evaporation area can be enlarged so that the working fluid can be evaporated efficiently, as compared to the heat pipe having the groove wick without the metal powder layer.
As also described, a thickness of the first portion in the inner section of the metal powder layer formed on the bottom wall is less than one third of the depth of each groove, and a thickness of the second portion in the inner section of the metal powder layer formed on the side wall is less than one fifth of the thickness of each groove. Therefore, an increment in the heat resistance resulting from an increment in the thickness can be avoided while reducing a pumping loss.
Given that the metal powder layer arranged in the evaporating section is formed into the porous wick having a predetermined thickness, the working fluid flowing through the groove wick can be returned to the evaporating section efficiently by an enhanced capillary pumping established by the clearances in the porous wick.
In addition, since the thickness of the metal powder layer is thus restricted to the above-mentioned range, the space serving as the vapor passage can be ensured sufficiently in the flat container while reducing the heat resistance, Therefore, the working fluid in the vapor phase is allowed to flow through vapor passage smoothly to enhance heat transport capacity of the heat pipe.
Preferably, the heat resistance in the heat pipe can be further reduced while enhancing the hydrophilicity on the inner wall of the groove wick by forming the metal powder layer only on the groove wick within the evaporating section by sparsely sprinkling the metal powder thereon. In this case, the heat transport capacity of the heat pipe can be further enhanced, and the working fluid is allowed to spread all over the evaporating section to prevent occurrence of drying-out in the evaporating section. In addition, the evaporating area in the evaporating section can be enlarged to evaporate the working fluid efficiently. Further, since a required amount of the metal powder to form the metal powder layer is reduced, weight of the heat pipe can be lightened and manufacturing cost thereof can be reduced.
As also described, the grain size of the metal powder falls within a range from 1 to 5 μm. Since the grain size is not too small, the metal powder can be prevented from being completely melted when sintering to the container. In addition, since the grain size is not too large, the groove wick can be prevented from being covered completely by the metal powder. If the grain size of the metal powder is smaller than 1 μm, porous volume in the porous wick would be reduced and hence the flow resistance of the working fluid has to be increased. By contrast, if the grain size of the metal powder is larger than 5 μm, a capillary pressure of the groove wick may be damped, and hence a larger space for the vapor passage and for the groove wick would be required to ensure required performance.
The heat pipe according to the present invention should not be limited to the foregoing preferred example, and may be modified within the spirit of the present invention.
For example, the heat generating element may also be attached to either one of outer faces of the first flat wall 21 and second flat wall 22, In addition, the metal powder P may adhere to at least the inner surface 2 a in the evaporating section 3. That is, the metal powder P may sparsely adhere to the inner wall 11 or to the inner surfaces 21 a, 22 a and 23 a.
In order to form the metal powder layer only on the inner surface of the groove wick, a jig having an outer diameter slightly smaller than an inner diameter of the tubular material may be inserted into the tubular material, and then a preferable amount of the metal powder is poured into the groove wick. Alternatively, in order to form the metal powder layer entirely on the inner surface 2 a, the metal powder may be poured in the tubular container without using the jig.

Claims

What is claimed is:

1. A heat pipe, comprising:

a container sealed at both longitudinal ends;

a working fluid encapsulated in the container;

a wick that pulls the working fluid by a capillary pumping;

an evaporating section situated on one of the longitudinal ends of the container at which evaporation of the working fluid takes place; and

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

wherein the wick includes a groove wick having a plurality of grooves extending on an inner wall of the container between the condensing section and the evaporating section via the insulated section while keeping predetermined intervals;

the heat pipe further comprising a metal powder layer formed at least on an inner wall of the groove wick and at least within the evaporating section; and

wherein a thickness of the metal powder layer falls within a predetermined range.

2. The heat pipe as claimed in claim 1,

wherein the inner wall of the groove wick includes a pair of side walls and a bottom wall connecting the side walls:

wherein each groove of the groove wick is individually shaped to have a U-shaped cross-section;

wherein the metal powder layer includes a first portion formed on the side wall having a thickness less than one fifth of a width of each groove; and

wherein the metal powder layer includes a second portion formed on the bottom wall having a thickness less than one third of a depth of each groove.

3. The heat pipe as claimed in claim 2,

wherein the container is made of metal;

wherein the metal powder includes copper particles whose grain size falls within a range from 1 to 5 μm, and the metal powder is sintered to the inner wall; and

wherein the thickness of the metal powder layer is restricted within a range from one to five times of grain size of the metal powder in such a manner that the thickness of the first portion is restricted to be less than one third of the width of each groove, and that the thickness of the second portion is restricted to be less than one fifth of the depth of each groove.

4. The heat pipe as claimed in claim 3, wherein the metal powder is sprinkled sparsely on the inner wall in the evaporating section in such a manner that a surface of the inner wall is exposed partially and finely textured.

5. The heat pipe as claimed in claim 3, wherein the metal powder layer is formed into a porous wick by sintering the deposited metal powder.

6. The heat pipe as claimed in claim 1, wherein the metal powder layer is also formed on the inner surface of the container between the grooves.

7. The heat pipe as claimed in claim 6, wherein a thickness of the metal powder layer formed on the inner surface of the container between the grooves is less than one fifth of a width of the each groove.

8. The heat pipe as claimed in claim 1,

wherein the container includes a flat container; and

wherein the groove wick is formed entirely on the inner wall of the container.

9. The heat pipe as claimed in claim 1, wherein the metal powder layer is formed only within the evaporating section.

10. A heat pipe, comprising:

a metal container sealed at both longitudinal ends;

a working fluid encapsulated in the container;

a wick that pulls the working fluid by a capillary pumping;

the heat pipe further comprising a metal powder layer that is formed of copper particles whose grain size fall within a range from 1 to 5 μm, and that is formed at least on an inner wall of the groove wick and at least within the evaporating section; and

wherein a thickness of the metal powder layer falls within a predetermined range;

wherein the inner wall of the groove wick includes a pair of side walls and a bottom wall connecting the side walls;

wherein the metal powder layer includes a first portion formed on the side wall having a thickness less than one fifth of a width of each groove and within a range from one to five times of grain size of the metal powder; and

wherein the metal powder layer includes a second portion formed on the bottom wall having a thickness less than one third of a depth of each groove and within a range from one to five times of grain size of the metal powder.

11. The heat pipe as claimed in claim 10, wherein the metal powder is sprinkled sparsely on the inner wall in the evaporating section in such a manner that a surface of the inner wall is exposed partially and finely textured.

12. The heat pipe as claimed in claim 10, wherein the metal powder layer is formed into a porous wick by sintering the deposited metal powder.

13. The heat pipe as claimed in claim 10, wherein the metal powder layer is also formed on the inner surface of the container between the grooves.

14. The heat pipe as claimed in claim 13, wherein a thickness of the metal powder layer formed on the inner surface of the container between the grooves is less than one fifth of a width of the each groove.

15. The heat pipe as claimed in claim 10,

wherein the container includes a flat container; and

wherein the groove wick is formed entirely on the inner wall of the container.