Abstract:
A loop heat pipe includes an evaporator and a tube hermetically connecting with the evaporator. The evaporator includes a metallic container and a wick structure disposed in an inner surface of the container. The wick structure includes a first wick portion thermally contacting the whole inner surface of the container and a second wick portion enclosed by the first wick structure and contacting with the first wick portion. A number of channels are defined between the first and second wick portions for receiving vaporized working medium. The tube communicates with the channels of the evaporator so that the vaporized working medium can flow from the channels into the tube.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to heat pipes and, more particularly, to a loop heat pipe having a good heat dissipation efficiency. 
         [0003]    2. Description of Related Art 
         [0004]    Loop heat pipes have excellent heat transfer performance due to their low thermal resistance, and are therefore an effective means for transfer or dissipation of heat form heat-generating components such as central processing units (CPUs) of computers. 
         [0005]    A conventional loop heat pipe comprises an evaporator thermally connected with a heat-generating component and disposing a wick structure therein, a condenser thermally connected with a heat sink, a vapor line and a liquid line disposed between and connecting the evaporator with the condenser. The wick structure comprises a central portion with a chamber therein and a number of extending portions radially extend outwardly from a periphery of the central portion. The outmost ends of the extending portions of the wick structure thermally contact inner surface of evaporator. A predetermined quantity of bi-phase working medium is contained in the evaporator and the liquid line. 
         [0006]    During operation of the loop heat pipe, the working medium in the extending portions of the wick structure absorbs heat from the heat-generated component and vaporizes. Thus, the vaporized working medium generates a vapor pressure which propels vaporized working medium towards the condenser via the vapor line. The vaporized working medium dissipates heat to the heat sink at the condenser and condenses to liquid thereat. The condensed working medium is then propelled through the liquid line and the evaporator in that order by the vapor pressure and by capillary action generated by the wick structure. The condensed working medium at the evaporator then evaporates and is condensed to liquid thus perpetuating the cycle. Because only the extending portions of the wick structure contact with the evaporator, an evaporation rate of the working medium is low. Thus, a heat dissipation efficiency of the loop heat pipe is low. 
         [0007]    What is needed, therefore, is a loop heat pipe having a good heat dissipation efficiency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a cross-sectional view of a loop heat pipe in accordance with a first embodiment of the present disclosure. 
           [0009]      FIG. 2  is a cross-sectional view of an evaporator of the loop heat pipe of  FIG. 1 , taken along line II-II thereof. 
           [0010]      FIG. 3  is a view similar to  FIG. 2 , showing an evaporator of a loop heat pipe in accordance with a second embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Referring to  FIGS. 1-2 , they illustrate a loop heat pipe in accordance with a first embodiment of the present disclosure. The loop heat pipe comprises an evaporator  10  and a hollow tube  20  hermetically connects with opposite ends of the evaporator  10 . A predetermined quantity of working medium (not labeled) is contained in the evaporator  10  and the tube  20 . The working medium is usually selected from a liquid which has a low boiling point such as water, methanol, or alcohol. Thus, the working medium can be easily evaporated to vapor when it absorbs heat in the evaporator  10  and condensed to liquid when it dissipates heat. 
         [0012]    The evaporator  10  comprises a container  11  and a porous elongated wick structure  13  attached on an inner surface of the container  11 . The container  11  may be constructed from any suitable metallic, such as aluminum, copper or stainless steel. In this embodiment, each of the wick structure  13  and the container  11  has a cylindrical configuration. 
         [0013]    Particularly referring to  FIG. 1 , the container  11  comprises a heat absorbing portion  112  and an enlarged extending portion  114  extending forwardly from a front end of the heat absorbing portion  112  along a central longitudinal axis of the heat absorbing portion  112 . The heat absorbing portion  112  is used to thermally contact with a heat-generating component (not shown), such as a CPU (central processing unit) of a computer. A diameter of the extending portion  114  is larger than that of the heat absorbing portion  112 . A vapor outlet  1121  is defined at a central portion of a rear end of the heat absorbing portion  112 . A liquid inlet  1141  is defined at a central portion of a front end of the extending portion  114 . 
         [0014]    The wick structure  13  consists of porous structure, such as screen mesh, or fiber inserted into the container  11  and held against the inner surface of the container  11 , or sintered powders combined to the inner surface of the container  11  using a sintering process. The wick structure  13  has a central longitudinal axis, which is coextensive with the central longitudinal axis of the heat absorbing portion  112  of the container  11 . A receiving chamber  137  extends in the wick structure  13  along the axis thereof and from an open end  134  of the wick structure  13 , which is near the liquid inlet  1141  to a closed end  132  near the vapor outlet  1121 . The receiving chamber  137  extends along a partial length of the wick structure  13 . The closed end  132  spaces a distance from an inner surface of the rear end of the absorbing portion  112  of the container  11 . The open end  134  abuts against an inner surface of the front end of the extending portion  114  of the container  11 . The receiving chamber  137  comprises a first chamber  1371  and a second chamber  1373  communicating with the first chamber  1371 . The first chamber  1371  is near to the closed end  132  of the wick structure  13 . The second chamber  1373  is near to the opening end  134  of the wick structure  13 . A diameter of the second chamber  1373  is larger than that of the first chamber  1371 . The second chamber  1373  functions as a compensation chamber for the first chamber  1371 . 
         [0015]    Particularly referring to  FIG. 2 , the wick structure  13  comprises a first wick portion  131  and a second wick portion  133 . A porosity of the first wick portion  131  is larger than that of the second wick portion  133 . In other words, a pore size of the first wick portion  131  is smaller than that of the second wick portion  133 . The first wick portion  131  is annular. An outer surface of the first wick portion  131  thermally contacts with a whole inner surface of the container  11  to improve evaporation rate of the working medium in the first wick portion  131 . The second wick portion  133  comprises a cylindrical central portion  1331  and a plurality of ribs  1333  radially extending from a periphery of the central portion  1331  to engage with an inner surface of the first wick portion  131 . The receiving chamber  137  is defined in the central portion  1331 . A cross-section of the rib  1333  is a sector. A width of the rib  1333  increases along a radially outward direction of the rib  1333 . The outmost ends of the ribs  1333  abut against the inner surface of the first wick portion  131 . The ribs  1333  and the first wick portion  131  are connected together. The ribs  1333  provide paths for the working medium to flow from the receiving chamber  137  to the first wick portion  131  to prevent the first wick portion  131  from drying. The ribs  1333  are spaced from each other. Thus, a channel  135  is defined between each two adjacent ribs  1333 . The channels  135  extend along a longitudinal direction of the second wick portion  133  from position near the  1373  to the closed end  132  of the wick structure  13  and through the closed end  132 . It is important that the channels  135  extend through the closed end  132  in order to enable vaporized working medium in the channels  135  to flow into vapor outlet  1121 . 
         [0016]    Opposite ends of the tube  20  connect with the vapor outlet  1121  and the liquid inlet  1141  of the evaporator  11 , respectively. The tube  20  is made of metallic materials compatible with the working medium, such as aluminum, copper, or stainless steel. The tube  20  can be easily bent and deformed to a desirable configuration. The tube  20  comprises a vapor line  21  and a liquid line  22  communicating with the vapor line  21 . Opposite ends of the vapor line  21  connect with the vapor outlet  1121  and the liquid line  22 , respectively. The vaporized working medium flows through the vapor line  21  to the liquid line  22 . Opposite ends of the liquid line  22  connect with the liquid inlet  1141  and the vapor line  21 , respectively. The liquid working medium flows through the liquid line  22  to the liquid inlet  1141  of the evaporator  10 . 
         [0017]    During operation of the loop heat pipe, the working medium in the first wick portion  131  absorbs heat from the heat-generating component and vaporizes. The vaporized working medium flow through the channels  135  into vapor line  21  via the vapor outlet  1121 . The vaporized working medium dissipates the heat via the tube  20  and condenses to liquid thereat. The condensed working medium is then propelled through the liquid line  22 , the second chamber  1373  and the first chamber  1371  of the receiving chamber  137  in that order by the vapor pressure and by capillary action generated by the wick structure  13 . The condensed working medium at the evaporator  10  then evaporates and is condensed to liquid thus perpetuating the cycle. A heat absorbing plate  30  thermally contacts with the vapor line  21  to absorb heat of the vaporized working medium to improve heat dissipating efficiency of the loop heat pipe. The heat absorbing plate  30  is made of a metal with a high heat conductivity, such as copper. The heat absorbing plate  30  functions as a heat sink for dissipating heat generated by the heat-generating component. 
         [0018]    Referring to  FIG. 3 , it illustrates a loop heat pipe in accordance with a second embodiment of the present disclosure. A difference between the first and second embodiments is that the first and second wick portions  131   a ,  133   a  of the evaporator  10   a  are formed by sintering a metal power. The first and second wick portions  131   a ,  133   a  have the same porosity. 
         [0019]    It is to be understood, however, that even though numerous characteristics and advantages of the disclosure have been set forth in the foregoing description, together with details of the structure and function of the disclosure, 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 disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.