Abstract:
A heat pipe ( 10 ) includes a hollow pipe body ( 20 ) for receiving a working fluid therein and a screen mesh ( 30 ) disposed in the pipe body. The screen mesh includes at least two layers. One of the two layers is in the form of a planar layer ( 50 ) and the other is in the form of a wave layer ( 40 ). A plurality of flowing channels ( 48 ) is formed by the wave layer. The channels formed by the wave layer of the screen mesh are capable of reducing the flow resistance for the condensed fluid to flow back while pores in the screen mesh provide a relatively large capillary pressure for drawing the condensed fluid to flow back.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to heat pipes, and more particularly to 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 operational 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. Accordingly, 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. Generally the heat pipe is divided into an evaporating section, an adiabatic section and a condensing section along a longitudinal direction thereof. During operation of the heat pipe, the working fluid absorbs the heat generated by the CPU or other electronic device and evaporates into vapor. The vapor moves from the evaporating section to the condensing section to dissipate the heat, whereby the vapor cools and condenses at the condensing section. The condensed working fluid returns to the evaporating section via a capillary force generated by the wick structure. From the evaporating section, the fluid is evaporated again to thereby repeat the heat transfer from the evaporating section to the condensing section.  
         [0005]     In general, movement of the working fluid depends on the capillary pressure (force) of the wick structure. Usually the wick structure has following four configurations: sintered powders, grooves, fiber and screen mesh. Since 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 the pore size of the screen mesh. In order to obtain a relatively large capillary pressure, a mesh screen having a small-sized pore 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 a decrease in the 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 a 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 large capillary pressure and a relatively low flow resistance so as to effectively and timely bring the condensed working fluid back from the condensing section to the evaporating section of the heat pipe and thereby to avoid the undesirable dry-out problem at the evaporating section.  
       SUMMARY OF THE INVENTION  
       [0008]     According to a preferred embodiment of the present invention, a heat pipe comprises a hollow pipe body for receiving a working fluid therein and a screen mesh disposed in the pipe body. The screen mesh comprises at least two layers. One of the two layers is in the form of a planar layer and the other of the two layers is in the form of a wave layer. The wave layer forms a plurality of flowing channels for the working fluid to flow from a condensing section to an evaporating section of the heat pipe. The channels formed by the wave layer of the screen mesh is capable of reducing the flow resistance to the condensed fluid to flow back while pores in the screen mesh are capable of providing a relatively large capillary pressure for drawing the condensed fluid to flow back.  
         [0009]     Other 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 transverse cross-section view of a heat pipe in accordance with a preferred embodiment of the present invention;  
         [0011]      FIG. 2  is an isometric, unfurled view of a planar layer of a mesh screen of the heat pipe of  FIG. 1 ;  
         [0012]      FIG. 3  is an isometric, unfurled view of a wave layer of the mesh screen of the heat pipe of  FIG. 1 ;  
         [0013]      FIG. 4  is a transverse cross-section view of the heat pipe in accordance with a second embodiment of the present invention;  
         [0014]      FIG. 5  is an isometric, unfurled view of the wave layer of the mesh screen of the heat pipe of  FIG. 4 ;  
         [0015]      FIG. 6  is a transverse cross-section view of the heat pipe in accordance with a third embodiment of the present invention;  
         [0016]      FIG. 7  is an isometric, unfurled view of the wave layer of the mesh screen of the heat pipe of  FIG. 6 ;  
         [0017]      FIG. 8  is a transverse cross-section view of the heat pipe in accordance with a fourth embodiment of the present invention;  
         [0018]      FIG. 9  is a transverse cross-section view of the heat pipe in accordance with a fifth embodiment of the present invention, and  
         [0019]      FIG. 10  is a transverse cross-section view of the heat pipe in accordance with a sixth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]     Referring to  FIG. 1 , a heat pipe  10  according to a preferred embodiment of the present invention comprises a hollow pipe body  20  and a screen mesh  30  disposed on an inner wall  22  of the pipe body  20 . The heat pipe  10  comprises an evaporating section and a condensing section at respective opposite ends thereof, and an adiabatic section located between the evaporating section and the condensing section. The heat pipe  10  is vacuumed and two ends of the heat pipe  10  are sealed.  
         [0021]     The pipe body  20  is made of high heat conductivity material such as copper or copper alloys. The screen mesh  30  has a plurality of pores and is saturated with a working fluid (not shown). The working fluid may be water, alcohol or other material having a low boiling point; thus, the working fluid can easily evaporate to vapor during operation when the evaporating section receives heat from a heat-generating electronic device, such as a CPU.  
         [0022]     The screen mesh  30  comprises a wave layer  40  and a planar layer  50  arranged along circumferential and axial directions of the pipe body  20 . The wave layer  40  is staked on the inner wall  22  of the pipe body  20  while the planar layer  50  is stacked on the wave layer  40  along a radial direction of the heat pipe  10  from a center to a periphery thereof. The wave layer  40  is directly attached to the inner wall  22  of the pipe body  20 . The planar layer  50  is disposed on an inner side of the wave layer  40 .  
         [0023]     As best seen in  FIG. 2 , which shows the planar layer  50  in an unfurled state, outer surfaces of the planar layer  50  are flat.  
         [0024]      FIG. 3  shows the wave layer  40  in an unfurled state. The wave layer  40  is square-wave shaped and comprises alternate upper and lower horizontal sections  42  and vertical sections  46  between the horizontal sections  42 . When the wave layer  40  is rolled and inserted into the pipe body  20 , the upper horizontal sections  42  abut against the inner wall  22  of the pipe body  20 . Thus, a flow channel  48  is formed between two adjacent vertical sections  46  of the wave layer  40  of the screen mesh  30  and the inner wall  22 . Each flow channel  48  extends along the longitudinal direction and entire length of the pipe body  20 , and has a trapezoid-shaped cross section (as shown in  FIG. 1 ).  
         [0025]     During operation of the heat pipe  10 , when the working fluid saturated in the screen mesh  30  at the evaporating section of the heat pipe  10  evaporates to vapor due to heat absorbed from the CPU, the vapor moves toward the condensing section of the heat pipe  10  due to the difference of vapor pressure to perform heat transport. The vapor then cools and condenses at the condensing section to perform heat dissipation. In this case, the condensed working fluid is absorbed into the screen mesh  30  at the condensing section, and then returns to the evaporating section through the screen mesh  30 . The pores of the screen mesh  30  can provide a relatively large capillary pressure to the working fluid while the flow channels  48  can provide a relatively small flow resistance to the working fluid. The screen mesh  30  accordingly can increase the speed of the condensed working fluid in returning back to the evaporating section and therefore promotes the heat transfer performance of the heat pipe  10 . As a result, a dry-out problem of the heat pipe  10  can be avoided.  
         [0026]     Referring to  FIGS. 4-5 , they illustrate the heat pipe  410  in accordance with a second embodiment of the present invention. Similar to the first embodiment, the heat pipe  410  also comprises a pipe body  20  and a screen mesh  430  arranged in the pipe body  20 . The screen mesh  430  comprises a wave layer  440  directly attached to the pipe body  20  and a planar layer  50  disposed on an inside of the wave layer  440 . Flow channels  448  are formed by the wave layer  440  between it and the pipe body  20  and between it and the planar layer  50 . The difference of the second embodiment over the first embodiment is that the wave layer  440  is consisted of a plurality of continuous serrations as viewed from the transverse cross-sectional view of the heat pipe  410 . Thus, each flow channel  448  has a triangle-shaped cross section. Upper tips of the serrations of the wave layer  440  abut against the inner wall of the pipe body  20 , while lower tips thereof abut against the planar layer  50 .  
         [0027]      FIGS. 6-7  illustrate the heat pipe  610  in accordance with a third embodiment of the present invention. Except for the screen mesh  630  and flow channels  648 ,  648 ′, other parts of the heat pipe  610  in accordance with the third embodiment are substantially the same as the heat pipe  410  of the previous embodiment. The screen mesh  630  also comprises a wave layer  640 . The wave layer  640  comprises a plurality of horizontal sections  642  and a plurality of serrate sections  646  each interconnecting two neighboring horizontal sections  642 . The serrate sections  646  are equally spaced from each other. When the screen mesh  630  is rolled and installed in the pipe body  20 , the wave layer  640  defines triangle-shaped first flow channels  648  and trapezoid-shaped second flow channels  648 ′ alternately arranged along the circumferential direction of pipe body  20 . Tips of the serrate sections  646  abut against the inner wall of the pipe body  20 , and the horizontal sections  642  abut against planar layer  50 .  
         [0028]     It is to be understood that the screen mesh  30 ,  430 ,  630  is used to provide capillary pressure to force the working fluid returning back to the evaporating section. The screen mesh  30 ,  430 ,  630  may be in the form of a multi-layer structure more than two layers. Referring to  FIG. 8 , the screen mesh  830  has three layers stacked on each other along the radial direction of the pipe body  20 . The three layers comprise a wave layer  40  and two planar layers  50  sandwiching the wave layer  40  therebetween.  FIG. 9  shows a screen mesh  930  also having three layers stacked on each other along the radial direction of the pipe body  20 . These three layers comprise a planar layer  50  and two wave layers  40  sandwiching the planar layer  50  therebetween.  
         [0029]      FIG. 10  also illustrates the heat pipe having a screen mesh  130  comprising three layers. The three layers comprise a planar layer  150  and two wave layers  140 ,  140 ′ sandwiching the planar layer  150  therebetween. The difference of this embodiment over that of  FIG. 9  is that the planar layer  150  has a pore size different from that of the wave layers  140 ,  140 ′. In this embodiment, the wave layers  140 ,  140 ′ have the same pore size. Although it is not shown in the drawings, it is apparent to those skilled in the art that the embodiment of  FIG. 10  can be further modified that the two wave layers  140 ,  140 ′ have different pore sizes, whereby the heat pipe can be used in an environment with a broader range of parameters regarding heat-dissipation requirement.  
         [0030]     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.