Patent Publication Number: US-7594537-B2

Title: Heat pipe with capillary wick

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to apparatuses for transfer or dissipation of heat from heat-generating components such as electronic components, and more particularly to a heat pipe having a capillary wick with graduated thickness. 
     DESCRIPTION OF RELATED ART 
     Heat pipes have excellent heat transfer properties, and therefore are an effective means for the transference or dissipation of heat from heat sources. Currently, heat pipes are widely used for removing heat from heat-generating components such as the central processing units (CPUs) of computers. A heat pipe is usually a vacuum casing containing a working fluid therein, which is employed to carry thermal energy from one section of the heat pipe (typically referred to as an evaporating section) to another section thereof (typically referred to as a condensing section) under phase transitions between a liquid state and a vapor state. Preferably, a wick structure is provided inside the heat pipe, lining an inner wall of the casing, drawing the working fluid back to the evaporating section after it is condensed in the condensing section. Specifically, as the evaporating section of the heat pipe is maintained in thermal contact with a heat-generating component, the working fluid contained at the evaporating section absorbs heat generated by the heat-generating component and then turns into vapor. The generated vapor flows towards the condensing section under the influence of the difference of vapor pressure between the two sections of the heat pipe. The vapor is then condensed into liquid after releasing the heat into ambient environment, for example by fins thermally contacting the condensing section, where the heat is then dispersed. Due to the difference in capillary pressure developed by the wick structure between the two sections, the condensed liquid can then be drawn back by the wick structure to the evaporating section where it is again available for evaporation. 
       FIG. 5  shows an example of a heat pipe in accordance with related art. The heat pipe includes a metal casing  10  and a single layer capillary wick  20  of uniform thickness attached to an inner surface of the casing  10 . The casing  10  includes an evaporating section  40  at one end and a condensing section  60  at the other end. An adiabatic section  50  is provided between the evaporating and condensing sections  40 ,  60 . The generated vapor flows from the evaporating section  40  through the adiabatic section  50  to the condensing section  60 . The thickness of the capillary wick  20  is uniformly arranged against the inner surface of the casing  10  from its evaporating section  40  to its condensing section  60 . However, this singular and uniform-type wick  20  generally cannot provide optimal heat transfer for the heat pipe because it cannot simultaneously produce a large capillary force and a low thermal resistance. The evaporating and condensing sections  40 ,  60  of the heat pipe have different demands due to their different functions. The thermal resistance between the working fluid and the condensing section  60  of the heat pipe increases due to the uniform thickness of the capillary wick  20 . The increased thermal resistance significantly reduces the heat-dissipating speed of the working fluid in the condensing section  60  of the heat pipe to ambient environment and ultimately limits the heat transfer performance of the heat pipe. 
     Therefore, it is desirable to provide a heat pipe with wick of graduated thickness that can provide a satisfactory rate of heat dissipation for the working fluid in the condensing section of the heat pipe and a reduced thermal resistance to the condensed liquid. 
     SUMMARY OF THE INVENTION 
     A heat pipe in accordance with a preferred embodiment of the present invention includes a casing containing a working fluid therein and a capillary wick arranged on an inner wall of the casing. The casing includes an evaporating section at one end thereof and a condensing section at an opposite end thereof, and a central section located between the evaporating section and the condensing section. The capillary wick formed at the evaporating section is thinner than the capillary wick formed at the central section. The capillary wick is capable of reducing thermal resistance between the working fluid and the casing. 
     Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiment when taken in conjunction with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present apparatus and method can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present apparatus and method. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a longitudinal cross-sectional view of a heat pipe in accordance with a first embodiment of the present invention; 
         FIG. 2  is a longitudinal cross-sectional view of a heat pipe in accordance with a second embodiment of the present invention; 
         FIG. 3  is a longitudinal cross-sectional view of a heat pipe in accordance with a third embodiment of the present invention; 
         FIG. 4  is a longitudinal cross-sectional view of a heat pipe in accordance with a fourth embodiment of the present invention; and 
         FIG. 5  is a longitudinal cross-sectional view of a heat pipe in accordance with related art. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a heat pipe in accordance with a first embodiment of the present invention. The heat pipe comprises a casing  100  and a capillary wick  200  arranged to attach on an inner surface of the casing  100 . The casing  100  comprises an evaporating section  400  and a condensing section  600  at an opposite end thereof, and a central section (i.e., adiabatic section)  500  located between the evaporating section  400  and the condensing section  600 . The casing  100  is made of highly thermally conductive materials such as copper or copper alloys and filled with a working fluid (not shown), which acts as a heat carrier for carrying thermal energy from the evaporating section  400  to the condensing section  600 . Heat that needs to be dissipated is transferred firstly to the evaporating section  400  of the casing  100  to cause the working fluid to evaporate. Then, the heat is carried by the working fluid in the form of vapor to the condensing section  600  where the heat is released to ambient environment, thus condensing the vapor into liquid. The condensed liquid is then brought back via the capillary wick  200  to the evaporating section  400  where it is again available for evaporation. 
     The capillary wick  200  can be a groove-type wick, a sintered-type wick or a meshed-type wick. Pore sizes of the capillary wick  200  gradually increase from the evaporating section  400  to the condensing section  600  of the casing  100 . The capillary wick  200  comprises a first capillary wick  240  formed at the evaporating section  400  of the casing  100 , a second capillary wick  250  formed at the central section  500  of the casing  100  and a third capillary wick  260  formed at the condensing section  600  of the casing  100 . A thickness of the first capillary wick  240  gradually increases towards the condensing section  600  along a lengthwise direction of the casing  100 . The first capillary wick  240  has a graduated thickness along a radial direction of the casing  100 . The thickness of the first capillary wick  240  is arranged so that the working fluid may be evaporated rapidly through heat absorption. The thicknesses of the second and third capillary wick  250 ,  260  in the radial direction of the casing  100  are equal, and equal to the thickest point of the first capillary wick  240  in the radial direction of the casing  100 , which is located at an end edge of the first capillary wick  240  immediately adjacent to the second capillary wick  250 . 
       FIG. 2  illustrates a heat pipe in accordance with a second embodiment of the present invention. The heat pipe comprises an evaporating section  410  at an end thereof, a condensing section  610  at an opposite end thereof, and a central section  510  located between the evaporating section  410  and the condensing section  610 . First, second and third capillary wicks  241 ,  251  and  261  are formed at the evaporating, central and condensing sections  410 ,  510  and  610  respectively. The third capillary wick  261  is designed to have a changeable section in a radial direction of the heat pipe on the base of the first embodiment of the present invention. The third capillary wick  261  gradually decreases in thickness towards an end of the condensing section  610  remote from the evaporating section  410  in a lengthwise direction of the heat pipe. The closer the third capillary wick  261  is to the end of the heat pipe at the condensing section  610 , the thinner the third capillary wick  261  is and even no the third capillary wick  261  is arranged in the end of the heat pipe at the condensing section  610  so as to reduce thermal resistance between the inner wall of the heat pipe at the condensing section  610  and the vaporous working fluid. An average thickness of the third capillary wick  261  at the condensing section  610  is thinner than that of the first capillary wick  241  in the evaporating section  410 . The thickness of the thickest point of the first capillary wick  241  at the evaporating section  410  and the third capillary wick  261  at the condensing section  610  is the same and is also equal to the thickness of the second capillary wick  251  formed at the central section  510 . 
       FIG. 3  illustrates a heat pipe in accordance with a third embodiment of the present invention. The heat pipe comprises an evaporating section  420  at one end thereof, a condensing section  620  at an opposite end thereof, and a central section  520  located between the evaporating section  420  and the condensing section  620 . First, second and third capillary wicks  242 ,  252  and  262  are formed at the evaporating, central and condensing sections  420 ,  520  and  620  respectively. Main differences between the second and third embodiments are that the thickness of the first capillary wick  242  at the evaporating section  420  and the third capillary wick  262  at the condensing section  620  are uniform. Each of the first and second capillary wicks  242  and  262  has a difference in thickness compared to the second capillary wick  252  formed at the central section  520 . 
       FIG. 4  illustrates a heat pipe in accordance with a fourth embodiment of the present invention. A thin tube  300  is disposed in the central section  510  of the heat pipe on the base of the second embodiment of the present invention to separate the evaporated working fluid from the liquid working fluid. An entrainment limit caused by contra-flow between the different ends of the heat pipe can therefore be avoided. Heat transfer performance of the heat pipe is improved. The tube  300  is attached on an inner surface of the second capillary wick  251  at the central section  510 . The tube  300  is of a thin film, meshed, metallic or nonmetallic material. The tube  300  can extend towards the evaporating and condensing sections  410 ,  610  in a proper range. A shape of a section of the tube  300  can be round, ellipsoid or polygonal when a section of a casing (not labeled) of the heat pipe is round, ellipsoid or polygonal. 
     It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.