Abstract:
A method ( 50 ) for making a heat pipe ( 10 ) includes the following steps: a) providing a screen mesh ( 30 ) in the form of a multi-portion structure with at least one portion having an average pore size different from that of the other portions; b) rolling the screen mesh into a hollow column form; c) inserting the screen mesh into a hollow pipe body ( 22 ) of the heat pipe; d) sintering the screen mesh received therein at a predetermined temperature; and e) filling a working fluid into the pipe body and sealing the pipe body. The portion with large-sized pores is capable of reducing the flow resistance to a condensed fluid to flow back, whereas the portion with small-size pores is capable of providing a relatively large capillary pressure for drawing the condensed fluid from the condensing section to the evaporating section of the heat pipe.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to a heat pipe as a heat transfer device, and more particularly to a method for making a heat pipe with a wick structure of screen mesh.  
       DESCRIPTION OF RELATED ART  
       [0002]     As electronic industry continues to advance, electronic components such as central processing units (CPUs), are made to provide faster operation speeds and greater functional capabilities. When a CPU operates at a high speed, its temperature frequently increases greatly. It is desirable to dissipate the heat generated by the CPU quickly.  
         [0003]     To solve this problem of heat generated by the CPU, a cooling device is often used to be mounted on top of the CPU to dissipate heat generated thereby. It is well known that heat absorbed by fluid having a phase change is ten times more than that the fluid does not have a phase change; thus, the heat transfer efficiency by phase change of fluid is better than other mechanisms, such as heat conduction or heat convection. Thus a heat pipe has been developed.  
         [0004]     The heat pipe has a hollow pipe body receiving a working fluid therein and a wick structure disposed on an inner wall of the pipe body. During operation of the heat pipe, the working fluid absorbs the heat generated by the CPU or other electronic device and evaporates. Then the vapor moves to the condensing section to release the heat thereof. The vapor cools and condenses at the condensing section. The condensed working fluid returns to the evaporating section and evaporates into vapor again, whereby the heat is continuously transferred from the evaporating section to the condensing section. Thus, the heat generated by the CPU can be effectively dissipated.  
         [0005]     The movement of the condensed working fluid from the condensing section to the evaporating section depends on capillary pressure of the wick structure. Usually the wick structure has following four configurations: sintered powder, grooved, fiber and screen mesh. For the thickness and pore size of the screen mesh can be easily changed, the screen mesh is widely used in the heat pipe.  
         [0006]     It is well recognized that the capillary pressure of a screen mesh increases due to a decrease in pore size of the screen mesh. In order to obtain a relatively larger capillary pressure for a screen mesh, a screen mesh having small-sized pores is usually adopted. However, it is not always the best way to choose a screen mesh having small-sized pores, because the flow resistance to the condensed working fluid also increases due to the decrease in pore size of the screen mesh. The increased flow resistance reduces the speed of the condensed working fluid in returning back to the evaporating section and therefore limits the heat transfer performance of the heat pipe. As a result, a heat pipe with a screen mesh that has too large or too small pore size often suffers dry-out problem at the evaporating section as the condensed working fluid cannot be timely sent back to the evaporating section of the heat pipe.  
         [0007]     Therefore, there is a need for a heat pipe with a screen mesh which can provide simultaneously a relatively larger capillary pressure and a relatively lower flow resistance so as to effectively and timely bring condensed working fluid back from a condensing section to a evaporating section of a heat pipe and thereby to avoid the undesirable dry-out problem at the evaporating section.  
       SUMMARY OF INVENTION  
       [0008]     According to a preferred embodiment of the present invention, a method for making a heat pipe includes the following steps: a) providing a screen mesh in the form of a multi-portion structure with at least one portion thereof having an average pore size different from that of the other portions; b) rolling the screen mesh into column form; c) positioning the screen mesh into a pipe body of the heat pipe; d) sintering the screen mesh received in the pipe body at a predetermined temperature so that the screen mesh is bonded to an inner wall of the pipe body; e) filling a working fluid into the pipe body and sealing the pipe body. The portion with large-sized pores is capable of reducing the flow resistance to a condensed fluid to flow back, whereas the portion with small-size pores is capable of providing a relatively large capillary pressure for drawing the condensed fluid from the condensing section to the evaporating section of the heat pipe.  
         [0009]     Other objects, advantages and novel features of the present invention will be drawn from the following detailed description of a preferred embodiment of the present invention with attached drawings, in which: 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]      FIG. 1  is a longitudinal cross-sectional view of a heat pipe in accordance with the present invention;  
         [0011]      FIG. 2  is a flow chart showing a preferred method of making the heat pipe of  FIG. 1 ;  
         [0012]      FIG. 3  is a plain view of a screen mesh under an expanded condition for making a wick structure of the heat pipe of  FIG. 1 ;  
         [0013]      FIG. 4  is a perspective view of the screen mesh of  FIG. 3  rolled onto a mandrel along an end-to-end direction of the screen mesh;  
         [0014]      FIG. 5  is a cross-sectional view, showing the rolled screen mesh and the mandrel received in a part of a hollow pipe body of the heat pipe;  
         [0015]      FIG. 6  is similar to  FIG. 4 , but showing the screen mesh rolled onto the mandrel along a side-to-side direction of the screen mesh.  
         [0016]      FIG. 7  shows a screen mesh made by stacking several meshes together;  
         [0017]      FIG. 8  is similar to  FIG. 7 , but showing a second embodiment of the screen mesh;  
         [0018]      FIG. 9  shows a third embodiment of the screen mesh made from the same method as shown in  FIG. 7 ; and  
         [0019]      FIG. 10  shows an alternative embodiment of the screen mesh. 
     
    
     DETAILED DESCRIPTION  
       [0020]      FIG. 1  illustrate a heat pipe  10  formed in accordance with a method of the present invention. The heat pipe  10  is vacuumed and includes a pipe body  20  and a wick structure  30 ′ of a screen mesh arranged against an inner wall  22  of the pipe body  20 . The heat pipe  10  is divided into an evaporating section, an adiabatic section and a condensing section along an axial direction of the heat pipe  10 . The adiabatic section is located between the evaporating and condensing sections.  
         [0021]     The pipe body  20  is made of high thermally conductive material such as copper or aluminum. Although the pipe body  20  illustrated is in a round shape, it should be recognized that other shapes, such as polygon, rectangle, or triangle, may also be suitable. Although it is not shown in the drawings, it is well known by those skilled in the art that two ends of the pipe body  20  are sealed.  
         [0022]     The wick structure  30 ′ is saturated with a working fluid (not shown), which acts as a heat carrier when undergoing phase transitions between liquid state and vaporous state. The wick structure  30 ′ is in the form of a multi-layer structure, which includes in sequence an inner layer  32 ′, a middle layer  34 ′ and an outer layer  36 ′. These layers  32 ′,  34 ′,  36 ′ are stacked together along a radial direction of the pipe body  20  with the outer layer  36 ′ abutting the inner wall  22  of the pipe body  20 . Each layer of the wick structure  30 ′ has an average pore size different from that of the other layers, and these layers  32 ′,  34 ′,  36 ′ are stacked together in such a manner that the average pore sizes thereof gradually decrease along the radial direction from a central axis X-X of the pipe body  20  towards the inner wall  22  of the pipe body  20 .  
         [0023]     In the present invention, a method  50  as shown in  FIG. 2  is proposed to construct the heat pipe  10 . The method  50  includes a step providing a flat screen mesh  30 .  
         [0024]     As shown in  FIG. 3 , the screen mesh  30  is rectangular-shaped and formed by weaving a plurality of first wires  38  (i.e., woof) and a plurality of second wires  39  (i.e., warp) together. The wires  38 ,  39  are made of stainless steel, copper etc., which can coexist with the working fluid. The first wires  38  extend along a lateral direction, whereas the second wires  39  extend along a longitudinal of the screen mesh  30 . The distance between each two neighboring first wires  38  is constant. The distance between each two neighboring second wires  39  gradually decreases along the longitudinal direction of the screen mesh  30  from a bottom end to a top end thereof as viewed from  FIG. 3 . Along the longitudinal direction the screen mesh  30  can be generally divided into three portions, which includes in sequence, from the bottom end to the top end, a first portion  32 , a second portion  34  and a third portion  36 . Each portion of the screen mesh  30  has an average pore size different from that of the other portions. The first portion  32  has the largest average pore size, whereas the third portion  36  has the smallest average pore size. The screen mesh  30  has a length approximately the same as that of the pipe body  20 . Furthermore, the screen mesh  30  has a width approximately the same as a circumference of the inner wall  22  of the pipe body  20 ; accordingly, the screen mesh  30  can fully cover the inner wall  22  of the pipe body  20 .  
         [0025]     As shown in  FIG. 4 , the screen mesh  30  is then rolled onto an outer surface of a mandrel  100  along an end-to-end direction of the screen mesh  30 . The mandrel  100  may be a solid column made of stainless steel material. The shape of the mandrel  100  may vary according to the shapes or structures of the heat pipe  10  to be formed. In this embodiment, the mandrel  100  is column-shaped and thus the furled screen mesh  30 ″ has a shape of a hollow column. The three portions  32 ,  34 ,  36  of the screen mesh  30  are rolled to a three-layer form along a radial direction of the mandrel  100 , which in sequence includes an inner layer  32 ″, a middle layer  34 ″, and an outer layer  36 ″. The first portion  32  of the screen mesh  30  forms the inner layer  32 ″ of the furled screen mesh  30 ″ and abuts to the outer surface of the mandrel  100  directly, whereas the third portion  36  of the screen mesh  30  forms the outer layer  36 ″ of the furled screen mesh  30 ″.  
         [0026]     Then, the mandrel  100 , together with the furled screen mesh  30 ″ thereon is inserted into the hollow pipe body  20 , as shown in  FIG. 5 . The outer layer  36 ″ of the furled screen mesh  30 ″ is held against the inner wall  22  of the pipe body  20  by the mandrel  100 . The inner layer  32 ″ of the furled screen mesh  30 ″ abuts the outer surface of the mandrel  100 . The pipe body  20  and the furled screen mesh  30 ″ received therein are then heated under a predetermined temperature to thereby sinter the furled screen mesh  30 ″ to make the furled screen mesh  30 ″ and the pipe body  20  bonded together. Thus, the inner, middle, and outer layers  32 ′,  34 ′,  36 ′ of the wick structure  30 ′ of the heat pipe  10  of  FIG. 1  are constructed from the first, second, and third portions  32 ,  34 ,  36  of the screen mesh  30 , respectively. That is, the three layers  32 ′,  34 ′,  36 ′ of the wick structure  30 ′ are arranged in such a manner that the average pore sizes thereof gradually increase along the radial direction from the inner wall  22  of the pipe body  20  towards a central axis X-X of the pipe body  20  of  FIG. 1 .  
         [0027]     After this, the mandrel  100  is drawn out of the pipe body  20 . Finally, the pipe body  20  is vacuumed and a working fluid such as water, alcohol, methanol, or the like, is injected into the pipe body  20 , and then the pipe body  20  is hermetically sealed to form the heat pipe  10 .  
         [0028]     The inner layer  32 ′ and the middle layer  34 ′ of the wick structure  30 ′ of the heat pipe  10  have a relatively larger average pore size and therefore are capable of providing a relatively low resistance to the condensed working fluid to flow back. The outer layer  36 ′, however, has a relatively smaller average pore size and therefore is capable of having a relatively high capillary pressure for drawing the condensed working fluid back to the evaporating section. Thus, the three-layer construction of the wick structure  30 ′ is capable of providing between these layers, along the radial direction of the pipe body  20 , a gradient of capillary pressure gradually increasing from the central axis X-X of the pipe body  20  toward the inner wall  22  of the pipe body  20 , and a gradient of flow resistance gradually decreasing from the inner wall  22  of the pipe body  20  toward a central axis X-X of the pipe body  20 . Furthermore, the outer layer  36 ′ with small-sized pores is also capable of maintaining an increased contact surface area with the inner wall  22  of the pipe body  20 , as well as a large contact surface with the working fluid saturated in the wick structure  30 ′, to thereby facilitate heat transfer between the working fluid in the heat pipe  10  and a heat source outside the heat pipe  10  that needs to be cooled.  
         [0029]     As shown in  FIG. 6 , the method as shown above is also capable of producing a heat pipe with a multi-section wick structure along an axial direction thereof. In this embodiment, the screen mesh  30  is rolled onto the mandrel  100  along a side-to-side direction of the screen mesh  30 . Thus the three portions of the screen mesh  30  from three sections of a wick structure  31  along an axis direction of the mandrel  100 , which include in sequence a first section  33 , a second section  35  and a third section  37 . Finally the three sections  33 ,  35 ,  37  construct the wick structure  31  in the form of three sections along an axial of the pipe body  20 . The three sections  33 ,  35 ,  37  of the wick structure  31  correspond to the evaporating section, adiabatic section and condensing section of the heat pipe  10 , respectively. Accordingly, this three-section construction of wick structure  31  is capable of providing a capillary pressure gradually increasing from the condensing section toward the evaporating section, and a flow resistance gradually decreasing from the evaporating section toward the condensing section.  
         [0030]      FIG. 7  shows another method for forming a screen mesh for use in the present invention. In this method, a screen mesh  230  is formed by stacking three meshes  200 ,  210 ,  220  together. The three meshes  200 ,  210 ,  220  have average pore sizes different from each other, in which the mesh  200  has the smallest pore size while the mesh  220  has the largest pore size. The three meshes  200 ,  210 ,  220  have the same width and different lengths, wherein the mesh  220  is the longest and the mesh  200  is the shortest. The length of the mesh  200  is half of that of the mesh  210 , and one-third of that the mesh  220 . Thus these three meshes form the screen mesh  230  having an average pore size gradually increasing along a longitudinal direction thereof. When the screen mesh  230  is rolled side-by-side and mounted in the pipe body  20  of the heat pipe  10  of  FIG. 1 , a wick structure having a varied capillary force and flow resistance along a length of the heat pipe  10  can be obtained by the screen mesh  230 .  
         [0031]     Referring to  FIGS. 8-9 , the method shown in  FIG. 7  is also capable of producing a screen mesh in other structure. As shown in  FIG. 8 , a screen mesh  330  is constructed from stacking a first mesh  321  having a relatively larger average pore size, and a pair of second meshes  320  having a relatively smaller average pore size together. The length of the first mesh  321  is three times as that of the second mesh  320 . The second meshes  320  are arranged to overlap opposite upper and lower end portions of the first mesh  321 , respectively. Thus, the screen mesh  330  is in the form of three-portion, in which the upper and lower end portions  332  each have an average pore size smaller than that of a middle portion  334  located between the two end portions  332 .  FIG. 9  shows another form of a screen mesh for use in the present invention. A screen mesh  430  is constructed from a first mesh  420  having a relatively larger average pore size, and a second mesh  422  having a relatively smaller average pore size. The length of the first mesh  420  is three times of that of the second mesh  422 . The second mesh  422  is arranged to overlap a middle portion of the first mesh  40 . Thus the screen mesh  430  is in the form of three-portion, in which the two outer portions  432  have an average pore size larger than that of the middle portion  434  located between the two outer portions  432 .  
         [0032]     Each screen mesh as shown above has a rectangular shape; thus the thickness of the wick structure constructed by these screen meshes, when they are rolled side-by-side, is even. It is understood that the screen mesh can be in other form, such as trapezoid, as shown in  FIG. 10 . The pore size of the screen mesh  530  gradually decreases along a longitudinal direction thereof. Furthermore, the thickness of the wick structure constructed by the screen mesh  530  is not even when the screen mesh  530  is rolled to a mandrel  100  along a side-by-side direction of the screen mesh  530 . The wick structure formed by a lower end portion of the screen mesh  530  as viewed from  FIG. 10  has a larger thickness.  
         [0033]     It is understood that the invention may be embodied in other forms without departing from the spirit thereof. Thus, the present example and embodiment is to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given herein.