Patent Publication Number: US-2005126757-A1

Title: Internally enhanced tube with smaller groove top

Description:
FIELD OF INVENTION  
      The present invention relates to internally enhanced tubes for improved heat transfer and specifically to a heat transfer tube with inner grooves having a groove top opening that is smaller than the largest opening in the groove.  
     BACKGROUND OF THE INVENTION  
      Heat pipes are typically used in heat exchangers for air conditioning and refrigeration and for thermal management of electronics devices such as computer CPU&#39;s. A heat pipe is a tube which is sealed at both ends and provided with a limited quantity of refrigerant. One end of the tube is exposed to a heat source, where the liquid inside the tube is heated so that the liquid is evaporated. The vapor flows to the opposite end of the tube which is exposed to a heat sink. The vapor releases its heat to the heat sink and condenses back to liquid form. The liquid will then flow back to the end where the heat source is located to be evaporated. These evaporation and condensation processes continue such that heat is transferred from the heat source to the heat sink in a continuous manner. The heat pipe described above has a much higher heat transfer rate than solid heat conductors made of highly conductive materials such as copper.  
      In order to draw liquid from the heat sink end back to the heat source end, a wick structure is required, which has a capillary effect. The capillary effect functions as a pump to move liquid from the heat sink end to the heat source end. In current heat pipes, the inner groove structure has been used as the wick of a heat pipe. However, the current inner groove structures expose the liquid flow to the vapor flow in the center of the heat pipe and in the opposite flow direction to the liquid flow. The vapor flow entrains liquid droplets and carries these droplets away from the liquid stream. This entrainment of the liquid droplets into the vapor flow has a detrimental effect on the performance of the heat pipe.  
      The current designs have an inner groove wick structure with a trapezoidal groove shape with the groove top being larger than the groove bottom. This structure enhances the entrainment effect discussed above so that the resulting heat pipe is less efficient with regard to heat transfer. Accordingly, there is a need for a heat pipe design that provides increased heat transfer performance by reducing the entrainment effect described above.  
     SUMMARY OF THE INVENTION  
      The present invention meets the above-described need by providing an internally enhanced tube with a groove opening size that is smaller than the size of the largest opening in the groove.  
      The present invention reduces the entrainment effect described above by shielding the liquid flow from the vapor flow. Due to the narrower groove opening at the top, the vapor flow in the center of the tube is partially separated from the liquid flow inside the groove. Accordingly, the liquid droplets are more difficult to be carried away by the vapor flow traveling in the opposite direction. Due to this shielding effect, the entrainment effect is reduced so that more liquid can reach the heat source end of the heat pipe and therefore the total heat transfer can be increased.  
      In a first embodiment, the groove geometry is defined by a plurality of trapezoidal-shaped fins.  
      In a second embodiment, the groove geometry is defined by a plurality of T-shaped fins.  
      In a third embodiment, the groove geometry is defined by a plurality of mushroom-shaped fins.  
      Common characteristics of the embodiments include, but are not limited to, the following aspects. The groove opening is smaller than the groove bottom. The groove cross-sectional area is equal to or larger than the cross-sectional area of the fins that form the grooves. And the height of the grooves are equal or larger than the width of the grooves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention is illustrated in the drawings in which like reference characters designate the same or similar parts throughout the figures of which:  
       FIG. 1  is a side elevation view of a first embodiment of the present invention;  
       FIG. 2  is a side elevation view of a second embodiment of the present invention;  
       FIG. 3  is a side elevation view of a third embodiment of the present invention;  
       FIG. 4  is a side elevational view of a fourth embodiment of the present invention;  
       FIG. 5  is a side elevational view of a fifth embodiment of the present invention; and,  
       FIG. 6  is a side elevational view of a sixth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      In  FIG. 1 , a section  10  of heat pipe  13  is shown. The pipe  13  may be constructed of copper, copper alloy, or other heat conductive materials. The pipe  13  is shown in a partial view that does not show the overall profile of the pipe. As will be evident to those of ordinary skill in the art, the enhancement of the present invention may be provided for pipe having many cross-sectional shapes including, but not limited to, round, oval, square, rectangular, etc. The longitudinal axis of the pipe  13  is oriented normal to the page. The heat pipe  13  has an outer wall  16  and an internally enhanced inner wall  19 . The heat pipe  13  has a wall thickness  22  measured from the bottom surface  24  of the groove  25  to the outer wall  16 . The groove  25  has an opening  29  at the top with respect to the orientation of  FIG. 1 . As shown, grooves  25  are formed by trapezoidal shaped fins  26  that result in grooves  25  having bottom surface  24  and opposed angled walls  31  and  34 . The walls  31  and  34  angle inward toward each other. As a result, the width  39  at the bottom of the groove  25  is larger than the width  42  of opening  29  at the top of groove  25 . By reversing the groove opening size to be smaller than the groove bottom, the liquid flow from the heat sink end to the heat source end is better shielded from the vapor flow. The cross-sectional area of the groove  25  is equal to or larger than the cross-sectional area of the fins  26  that form the grooves  25 . Also, the height  70  of the grooves  25  is equal to or larger than the width  39  of the grooves  25 . As a result of the shielding effect of the groove shape, the entrainment of liquid into the vapor stream is reduced so that more liquid can reach the heat source end of the heat pipe, and the total heat transferred can be increased.  
      The heat pipe  13  of the present invention also has the following properties. The groove height  70  is between 0.05 mm to 5 mm. The groove opening  29  is 0.05 mm to 5 mm in length, and the groove pitch is 0.10 to 5 mm. The ratio of groove cross-sectional area to groove height is 0.02 mm to 1 mm. The ratio of groove cross-sectional area to groove wall length is 0.01 mm to 1 mm. And the ratio of groove opening to the largest width of the groove is 0.01 to 0.99.  
      Turning to  FIG. 2 , an alternate embodiment for a heat pipe  99  of the present invention includes a set of grooves  100  formed between T-shaped fins  103 . The heat pipe  99  has an outer surface  106 . The longitudinal axis of the pipe is oriented normal to the page with respect to  FIG. 2 . The pipe  99  has a wall thickness  109  measured between the bottom surface  112  of the groove  100  and the outer surface  106 . The groove  100  is formed in part by opposed walls  115 ,  118 . The outer end  113  of the T-shaped fins  103  defines an opening  121 . The opening  121  has a width  124 . The width  124  is smaller than the width  127  along the bottom surface  112 . The cross-sectional area of the groove  100  is equal to or larger than the cross-sectional area of the fins  103 . Also, the height  150  of the groove  100  is equal to or larger than the width  127 .  
      In  FIG. 3 , another alternate embodiment of the present invention is shown. Heat pipe  200  has a plurality of fins  203  having a mushroom-shaped profile. A plurality of grooves  201  are formed between the fins  203 . Heat pipe  200  has an outer surface  206 . The longitudinal axis of the pipe is oriented normal to the page with respect to  FIG. 2 . The pipe  200  has a wall thickness  209  measured between the bottom surface  212  of the groove  201  and the outer surface  206 . The groove  201  is formed in part by opposed walls  215 ,  218 . The ends  213  of adjacent fins  203  define an opening  221 . The opening  221  has a width  224  that is smaller than the width  227  along bottom surface  212 . The cross-sectional area of the groove  201  is equal to or larger than the cross-sectional area of the fins  203 . Also, the height  250  of the groove  201  is equal to or larger than the width  127 .  
      Turning to  FIG. 4 , another embodiment of the present invention is shown. Heat pipe  300  has a plurality of fins  303  forming grooves  306  between adjacent fins  303 . As shown, the bottom of groove  306  is round. Other shapes for the bottom wall may also be suitable including flat and other non-round shapes.  
      The longitudinal axis of the pipe is oriented perpendicular to the page. The opening  309  at the top of the groove  306  is smaller than the largest width  312  of the groove  306 . The largest width  312  is located in a midportion of groove  306 .  
      In  FIG. 5 , angled fins  400  provide triangular shaped grooves  403 . The top of the grooves  403  have an opening  406  with a width  409  that is smaller than the largest width of the grooves  403 . The largest width for the groove  403  is located at the bottom wall  412 .  
      In  FIG. 6 , Y-shaped fins  500  provide grooves  503  located therebetween. The width of the opening  506  at the top of the groove  503  is smaller than the widest opening  512  of the groove  503 .  
      While the invention has been described in connection with certain embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.