BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention related to two-phase heat dissipation devices, particularly to the transport line of loop heat pipes, capillary pump loops, spray cooling devices or others. The two-phase heat dissipation device is sued for heat removal of heat generating devices, such as the central processing unit (CPU) or other integrated circuit (IC) chips.  
         [0003]     2. Brief Description of Related Art  
         [0004]     As electronic technology advances, more electronic function is performed in a smaller area on a semiconductor chip. More electronic function is invariably accompanied with temperature increase which may damage the chip. To maintain a safe temperature, it is necessary to remove the heat generated in the chip at the chip mount.  
         [0005]     A widely used method for cooling the chip mount is to utilize the two-phase heat transfer during phase transition between liquid phase and vapor phase of a coolant. In this method, a vaporization section vaporizes the coolant and carries away a large amount of heat energy, and the vapor fills the originally evacuated space. In the condensation section, the vapor condenses into liquid for recycling and releases a large amount of heat energy. The heat pipe is a commonly applied heat dissipation device in this category. However in the traditional heat pipe, the vapor and the recycling liquid move in opposite directions. This impedes the recycling capillary pumping action and tends to limit the maximum heat dissipation capability of the heat pipe.  
         [0006]      FIG. 1  shows a prior art to remedy this problem, using a loop heat pipe (LHP). The LHP comprises sequentially an evaporator  11 , a wick  12 , a vapor conduit  13 , a condensation section  14 , a liquid line  15 , and a compensation chamber  16 . The principle of operation is similar to that of a traditional heat pipe except that the vapor and the liquid flow in single way along a guiding loop. The operation still depends on the capillary action. The liquid compensation chamber is used to compensate the dispersion of the liquid in the loop to avoid the evaporator drying out.  
         [0007]      FIG. 2  shows another prior art disclosed in U.S. Pat. No. 6,381,135. The LHP  200  has an evaporator  212  placed on a heat-generating device  204  on top of a substrate  202 . A thermal interface material  206  is placed between the device  204  and the evaporator  212 . The evaporator  212  comprises a wick  213 , one end of which is connected to a liquid line  214 , which also has a wick (not shown). The second wick need not be the same as the first wick and may be of sintered metal powder, metallic wire mesh, or packed spherical particles. The vapor line  216  is simply a hollow tube. The condenser  218  is a hollow block of metal, which can be mounted with fins to dissipate heat. The condenser  218  may have a third wick  219 , which need not be the same as the first two wicks. The drawback of this structure is that the wicks in the condenser  218  and the liquid line  214  increase the friction to the liquid flow and hence retard coolant recycling. Therefore, the evaporator tends to dry out at high heat dissipation and limit the heat cooling capability. For the reason that the volume flow rate of the vapor far exceeds that of the liquid, the liquid line  214  requires much smaller cross-sectional area than the vapor line  216 . When the liquid section and the vapor section use the same large tubing, the liquid line cannot provide additional capillary force. If both sections use the same small tubing, the vapor speed and the friction in the vapor line becomes excessive.  
       SUMMARY OF THE INVENTION  
       [0008]     The object of this invention is to provide efficient flow of a coolant in a transport line for loop heat pipes, capillary pump loops or spray cooling devices. Another object of this invention is to provide additional pumping force for the liquid flow without significantly increasing the friction force.  
         [0009]     These objectives are achieved by using grooved microchannels in the liquid line. Grooved microchannels can be optionally made in the inner surface of the condensation section. Further, a plug can be inserted in the liquid line to reduce its effective cross-sectional area to enhance the pumping force. Another way is to shrink the liquid line section to reduce its effective cross-sectional area to enhance the pumping force. The grooved microchannels in the liquid line provide additional pumping force for coolant recycling with limited friction force.  
         [0010]     The grooved microchannels can be constructed on the inner surface of the tube by means of extrusion molding of the tube, or by lining a groove-corrugated wire mesh along the inner wall of the transport line. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows a prior art loop heat pipe.  
         [0012]      FIG. 2  shows a second prior loop heat pipe disclosed in U.S. Pat. No. 6,381,135  
         [0013]      FIG. 3  shows a first embodiment of the present invention with grooved microchannels along the inner wall of the condensation section and liquid line, and a plug inserted in the liquid line.  
         [0014]      FIG. 4  shows a second embodiment of the present invention with grooved microchannels covered with a layer of wire mesh along the inner wall of the condensation section and liquid line, and a plug inserted in the liquid line.  
         [0015]      FIG. 5  shows a third embodiment of the present invention with a groove-corrugated wire mesh lining along the inner wall of the condensation section and the liquid line to form microchannels, and a plug inserted in the liquid line.  
         [0016]      FIG. 6  shows a fourth embodiment of the present invention with grooved microchannels along the inner wall of the condensation section and liquid line, and without a plug in the liquid line.  
         [0017]      FIG. 7  shows a fifth embodiment of the present invention with grooved microchannels covered with a layer of wire mesh along the inner wall of the condensation section and liquid line, and without a plug in the liquid line.  
         [0018]      FIG. 8  shows a sixth embodiment of the present invention with a groove-corrugated wire mesh lining along the inner wall of the condensation section and the liquid line, and without a plug in the liquid line.  
         [0019]      FIG. 9  shows a seventh embodiment of the present invention with shrunk cross-section of the liquid line. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]      FIG. 3  shows the first embodiment of the present invention. The flow path of the transport line  100  has sequential three sections: a vapor line  103 , a condensation section  104 , and a liquid line  105 . The inner wall of the tubing  101  of the condensation section and liquid line is made with grooved microchannels  102 , which can be made by extrusion molding during the fabrication of the tubing  101 . The grooved microchannel has a hydraulic diameter smaller than the 500 μm. The cross-section of the groove can be triangular, rectangular, trapezoidal, wavy, or others. A plug  106  is inserted into the core of the liquid line  105  to reduce its effective cross-sectional area to only the groove microchannels, to enhance the pumping force. The plug  106  can be fabricated with metal, plastic or other heat resistant materials. The bottom corners of the grooved microchannels as shown in A-A section view help to collect the condensed liquid and convey it to the liquid line section. With the grooved microchannels closed by a plug  106  in the liquid line  105 , the liquid can be effectively pumped with capillary action back to the wick (not shown) in the main body of the two-phase heat dissipation device. At the end of the liquid section  105 , a capillary material  107  is optionally inserted. The capillary material  107  provides smoother connection with the wick (not shown) in the main body of the two-phase heat dissipation device. The vapor line section  103  can be a tube only or inserted with a grooved microchannels. The condensation section  104  is inserted with a grooved microchannels, which has a cross-section shown in cross-section A-A; the upper part of the liquid line  105  has a cross-section B-B; and, if the capillary material  107  is inserted, the lower part of the liquid line  105  has a cross-section C-C.  
         [0021]     The grooved microchannels can also be fabricated on the surface of the plug  106 .  
         [0022]     With a reduced effective cross-sectional area in the liquid section  105  by inserting a plug  106  and leaving the grooved microchannels only as a passage for the liquid, additional pumping force is provided for coolant recycling without significantly increasing friction in the liquid flow.  
         [0023]      FIG. 4  shows a second embodiment of the present invention. This embodiment differs from the first embodiment in that a layer of wire mesh  109  is added to cover the grooved microchannels  102  for at least the condensation section  104 , as shown in cross-section A-A, to improve the pumping ability. The material of the wire mesh can be metals or nonmetals.  
         [0024]      FIG. 5  shows a third embodiment of the present invention. The grooved microchannels are fabricated with a layer of a corrugated wire mesh to line in along the inner wall of the tubing  101  of the transport line. The vapor section  103  can be a tube only or inserted with a corrugated wire mesh as shown in the cross-section A-A, The condensation section  104  has a cross-section A-A with corrugated wire mesh lining  108 . The liquid line section has a cross-section B-B with corrugated wire mesh enclosing a plug  106  which reduces the effective cross-sectional area for the liquid to flow. The wire mesh is corrugated with a cross-section shape either of triangular, rectangular, trapezoidal, wavy, or other groove shape with equivalent function. The corrugated wire mesh is basically inserted into the condensation section  104  and liquid line  105 . A plug  106  is optionally inserted as a core in the liquid line  105  to reduce its effective cross-sectional area. An additional layer of wire mesh  109  can be optionally placed against the corrugated wire mesh  108  to form closed grooved microchannels in the condensation section  104 , as shown in cross-section A′-A′, to improve the pumping ability. The material of the wire mesh can be metals or nonmetals.  
         [0025]      FIG. 6  shows a fourth embodiment of the invention. The structure is similar to  FIG. 3  with the same reference numerals denoting the corresponding parts. The only difference is that the plug  106  in the liquid line  105  in  FIG. 3  is removed. The optional capillary material  107  provides smoother connection with the wick (not shown) in the main body of the two-phase heat dissipation device.  
         [0026]      FIG. 7  shows a fifth embodiment of the invention. The structure is similar to  FIG. 4  with the same reference numerals denoting the corresponding parts. The only difference is that the plug  106  in the liquid line  105  in  FIG. 4  is removed. The optional capillary material  107  provides smoother connection with the wick (not shown) in the main body of the two-phase heat dissipation device. In this embodiment, the layer of wire mesh  109  covers at least the condensation section  104  and the liquid line  105  to improve the pumping ability.  
         [0027]      FIG. 8  shows a sixth embodiment of the invention. The structure is similar to  FIG. 5  with the same reference numerals denoting the corresponding parts. The only difference is that the plug  106  in the liquid line  105  in  FIG. 5  is removed. The optional capillary material  107  provides smoother connection with the wick (not shown) in the main body of the two-phase heat dissipation device. Again, an additional layer of wire mesh  109  can be placed inside the corrugated wire mesh to form closed microchannels in the condensation section  104  and the liquid line  105  (as shown in section A′-A′ of  FIG. 5 ) to improve the pumping ability.  
         [0028]      FIG. 9  shows a seventh embodiment of the present invention. The inner wall of the tubing  101  of the condensation section  104 , and the liquid line  105  has grooved microchannels as shown in cross-section A-A, and the liquid section  105  has a cross-section B-B which is made smaller than that of the condensation section  104  as shown in the cross-section A-A. Also, a layer of wire mesh  109  can be added to cover the grooved microchannels  102  for at least the condensation section  104  (not shown). The shrunk liquid line  105  enhances capillary action for the coolant recycling. In addition, a plug can be inserted in the shrunk liquid line  105  (not shown) to further reduce its effective cross-section area An optional capillary material  107  can be added (as shown in  FIG. 8 ) in the end of the liquid line  105 .  
         [0029]     Other embodiments having a smaller effective cross0section area of the liquid line  105  can be made without grooved mnicrochannels on the inner wall of the transport line (not shown). This can be achieved by simply inserting a plug  106  having a size slightly smaller than of the transport line into the liquid line  105 . The small gap between the non-grooved inner wall surface of the evaporator. Alternatively, this can be achieved by shrinking the liquid line  105 . In addition, a plug can be inserted into the shrunk liquid line to further reduce its effective cross-sectional area. Again, a layer of wire mesh  109  can be added to cover the inner surface of at least the condensation section  104 . An optional capillary material  107  can be added in the end of the liquid line  105 .  
         [0030]     While the preferred embodiments of the invention have been described, it will be apparent to those skilled in the art, the various modifications may be made in the embodiments without departing from the spirit of the present invention Such modifications are all within the scope of the present invention.
Grooved microchannels are used to enhance the capillary action in the transport line of two-phase heat dissipation devices, such as loop heat pipes, capillary pump loops, or spray cooling devices, or others. Efficient heat dissipations achieved by enhancing the capillary pumping force for the liquid flow without significantly increasing the friction force. The effective cross-sectional area of the liquid line is made smaller than that of the condensation section, either by inserting a plug or shrinking the liquid line, to provide additional pumping force for the coolant recycling.