Abstract:
A heat pipe includes a hollow elongated casing, which defines therein an enclosed chamber, a working fluid filled in the enclosed chamber; and a wick, which is formed on the inside wall of the casing around the enclosed fluid chamber and has two parts that have different porosities.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to heat transferring devices and more particularly, to a high-performance heat pipe of which the wick has different porosities at different parts for different functions.  
         [0003]     2. Description of the Related Art  
         [0004]     A conventional heat pipe generally comprises a hollow metal tube, which has the both ends closed, a working fluid, for example, pure water filled in the metal tube, and a wick formed of copper powder on the inside wall of the metal tube by sintering. The wick has pores for absorbing the working fluid.  
         [0005]     When one end of the heat pipe touched a heat source, the pure water at the heating end is vaporized, and produced steam is quickly dissipated to the other end, namely, the cold end where steam is condensed into water to release latent heat. At this time, condensed water flows back to the heating end through the pores in the wick, completing one thermal cycle. By means of the alternation of the working fluid between the liquid phase and the gas phase, the heat pipe transfers a big amount of heat energy.  
         [0006]     To facilitate fabrication, the wick of a heat pipe is made having a uniform porosity. However, the porosity has a great concern with the performance of the heat pipe. A wick having a relatively lower porosity provides a relatively better capillary effect, however its flow path function for carrying the working fluid is relatively poorer. On the contrary, a wick having a relatively higher porosity provides a relatively better flow patch function for carrying the working fluid, however its capillary effect is relatively weaker. Therefore, it is an important work how to maintain the capillary effect of the wick while improving the flow path function for carrying the working fluid.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention has been accomplished under the circumstances in view. It is the main object of the present invention to a high-performance heat pipe, which uses a wick that provides a satisfactory capillary effect and flow path function for carrying the working fluid.  
         [0008]     To achieve this and other objects of the present invention, the wick of the heat pipe has at least two different porosities to provide a satisfactory capillary effect and flow path function for carrying the working fluid, thereby improving the thermal transfer performance of the heat pipe.  
         [0009]     In one embodiment of the present invention, the wick has axially divided multiple parts that have different porosities. In another embodiment of the present invention, the wick has radially divided multiple parts that have different porosities. Preferably, the wick has two parts, namely, the first part that has a porosity within about 55%-60%, and the second part that has a porosity within about 65%-80%. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     FIG. I is a schematic sectional view of a heat pipe according to the present invention.  
         [0011]      FIG. 2  is a schematic sectional view of an alternate form of the heat pipe according to the present invention.  
         [0012]      FIG. 3  is a schematic sectional view of another alternate form of the heat pipe according to the present invention.  
         [0013]      FIG. 4  is a sectional view taken along line  4 - 4  of  FIG. 3 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     Referring to  FIG. 1 , a heat pipe in accordance with the present invention is shown comprising a casing  10 , a working fluid  20 , and a wick  30 .  
         [0015]     The casing  10  is a metal tube having the both ends closed and defining therein an enclosed chamber  11 . The working fluid  20  is disposed in the enclosed chamber  11 . According to this embodiment, the working fluid  20  is pure water. Other fluids may be selectively used as a substitute. Further, the casing  10  has a heating end  12  and a cooling end  14 .  
         [0016]     The wick  20  is formed of copper powder on the inside wall of the casing  10  by sintering, having axially divided three parts, namely, the first part  32 , the second part  34 , and the third part  36 . These three parts  32 ,  34  and  36  have different porosities. The porosity of the first part  32  of the wick is 55%. The porosity of the second part  34  of the wick is 80%. The porosity of the third part  36  of the wick is 60%.  
         [0017]     When the heating end  12  of the casing  10  touched a heat source, the working fluid  20  is vaporized, and produced steam flows along the chamber  11  to the cooling end  14  where steam is condensed into liquid. At this time, the third part  36  provides a better capillary effect to absorb the working fluid  20  around the cooling end. When the working fluid  20  is returning to the heating end  12 , and the second part  34  provides a better flow path function to reduce the resistance to the reverse flowing of the working fluid  20 . Further, the first part  36  also provides a better capillary effect to absorb the working fluid  20  from the second part  34  to the heating end  12 .  
         [0018]     Because the first, second and third parts  32 ,  34  and  36  of the wick  30  have different porosities, the wick  30  provides a better flowing path function and a satisfactory capillary effect, achieving a high performance in heat transfer.  
         [0019]     Further, alloy powder of copper and silver or other suitable materials may be selectively used for sintering into the desired wick  30  instead of copper powder. A porosity ranging from 55%-60% provides a better capillary effect. A porosity ranging from 65%-80% provides a better flow path function. In actual fabrication, the porosity of each part of the wick  30  may be changed subject to actual requirements, and each part may be made having different porosities gradually increased from one end to the other.  
         [0020]      FIG. 2  shows an alternate form of the heat pipe according to the present invention. According to this embodiment, the heat pipe comprises a casing  10 , a working fluid  20 , and a wick  30 . The wick  30  is axially divided into a first part  32  and a second part  34 . The porosity of the first part  32  is 75%. The porosity of the second part  34  is 55%. By means of this design, the second part  34  provides a better capillary effect, and the first part  32  provides a better flow path function.  
         [0021]      FIGS. 3 and 4  show another alternate form of the heat pipe according to the present invention. According to this embodiment, the heat pipe comprises a casing  10 , a working fluid  20 , and a wick  30 . The wick  30  is radially divided into a first part  32  and a second part  34 . The first part  32  is disposed at an outer side and bonded to the inside wall of the casing  10 . The porosity of the first part  32  is 70%. The second part  34  is disposed at an inner side and bonded to the first part  32 . The porosity of the second part  34  is 58%. The second part  34  provides a better capillary effect to absorb the working fluid  20 . The first part  32  provides a better flow path function to reduce the resistance to the reverse flowing of the working fluid  20 . Therefore, the heat pipe provides a better flowing path function and a satisfactory capillary effect, achieving a high performance in heat transfer.  
         [0022]     Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.