Abstract:
The subject invention provides a heat sink for a liquid cooled cooling assembly for removing heat generated by an electronic device. The heat sink includes a flow diverter having a hyperbolic cross section disposed on a base for absorbing a significant portion of the heat from the electronic device. An inlet tube directs an impinging flow of cooling fluid directly onto the flow diverter to remove the heat stored within the flow diverter. A spiral wall extends in an increasing spiral from the flow diverter to define a spiral channel for discharging the flow of cooling fluid. The spiral wall includes a plurality of louvers for creating turbulence in the flow of cooling fluid for maintaining a high heat transfer coefficient between the spiral wall and the cooling fluid.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The subject invention relates to a cooling assembly for cooling an electronic device such as a microprocessor or a computer chip. 
   2. Description of the Prior Art 
   These electronic devices generate a high concentration of heat, typically a power density in the range of 5 to 35 W/cm 2 . Accordingly, research activities have focused on developing more efficient cooling assemblies capable of efficiently dissipating the heat generated from such electronic devices, while occupying a minimum of space. 
   A forced air cooling assembly typically includes a heat exchanger and a heat sink, and cools the electronic device by natural or forced convection cooling methods. The electronic device is attached to the heat sink and transfers heat thereto. The heat exchanger typically uses air to directly remove the heat from the heat sink. However, air has a relatively low heat capacity. Such forced air cooling assemblies are suitable for removing heat from relatively low power heat sources with a power density in the range of 5 to 15 W/cm 2 . However, the increased computing speeds have resulted in a corresponding increase in the power density of the electronic devices in the order of 20 to 35 W/cm 2 , thus requiring more effective cooling assemblies. 
   In response to the increased heat produced by the electronic devices, liquid-cooled cooling assemblies, commonly referred to as liquid cooled units (“LCUs”) were developed. The LCUs employ a heat sink in conjunction with a high heat capacity cooling fluid, like water or water-glycol solutions, to remove heat from these types of higher power density heat sources. One type of LCU circulates the cooling fluid through the heat sink to remove the heat absorbed from the heat source affixed thereto. The cooling fluid is then transferred to a remote location where the heat is easily dissipated into a flowing air stream with the use of a liquid-to-air heat exchanger and an air moving device such as a fan or a blower. These types of LCUs are characterized as indirect cooling units since they remove heat form the heat source indirectly by a secondary working fluid. Generally, a single-phase liquid first removes heat from the heat sink and then dissipates it into the air stream flowing through the remotely located liquid-to-air heat exchanger. Such LCUs are satisfactory for a moderate heat flux less than 35 to 45 W/cm 2 . 
   The U.S. Pat. No. 5,304,846, issued to Azer et. al., and the U.S. Pat. No. 6,422,307, issued to Bhatti et. al., each disclose a typical heat sink assembly used in a LCU. The heat sink assemblies include a base plate with a plurality of fins having smooth sidewalls extending upwardly from the base plate. In operation, the base plate absorbs the heat from the electronic device and transfers the heat to the fins. A cooling fluid flows past the smooth walled fins, drawing the heat from the fins, thereby removing the heat from the heat sink. The flow of cooling fluid may be directed parallel to the fins or impinged thereon. 
   The U.S. Pat. No. 5,019,880, issued to Higgins, discloses a heat sink that includes a circular base with a central flow diverter having a conical shape extending upwardly from the base. A plurality of planar fins is disposed radially about the circumference of the flow diverter and extend upwardly from the base to a lid. An inlet is disposed above the lid for directing a flow of cooling fluid perpendicularly onto the flow diverter. The flow of cooling fluid then circulates radially outward to the outer periphery of the base through a plurality of flow channels defined between the planar fins. 
   The amount of heat transferred between the fins and the cooling fluid is dependent on a heat transfer coefficient therebetween. The heat transfer coefficient is dependent on a thermal boundary layer, which is a layer of stagnant cooling fluid adjacent each of the fins. The thermal boundary layer acts as an insulator, limiting the heat transfer coefficient. As the cooling fluid flows past the fins uninterrupted, the thermal boundary layer becomes thicker, decreasing the heat transfer coefficient and thereby decreasing the effectiveness of the heat sink assembly. Additionally, the amount of heat stored in each of the fins varies according to the distance between each of the fins and the heat source, with the highest concentration of heat occurring directly above the heat source, with the fins disposed farther from the heat source absorbing less heat. Therefore, the heat transfer to the cooling fluid at the outer periphery of the heat sink is less efficient than the heat transfer to the cooling fluid directly above the heat source. 
   SUMMARY OF THE INVENTION AND ADVANTAGES 
   The subject invention provides a heat sink assembly for removing heat from an electronic device. The heat sink assembly includes a base having a top surface and a lid having a bottom surface in spaced relationship with and parallel to the top surface of the base. A flow diverter extends upwardly from the top surface of the base toward the bottom surface of the lid. The lid defines an inlet aligned with the flow diverter for impinging the flow of cooling fluid on the flow diverter. A spiral wall extends between the top surface of the base and the bottom surface of the lid and is disposed in an increasing spiral from the flow diverter to define a spiral channel having an outlet for directing the flow of cooling fluid radially relative to the flow diverter. 
   Accordingly, the subject invention provides a heat sink with a flow diverter for absorbing heat generated from the electronic device. The flow of cooling fluid removes the heat stored in the flow diverter, with the spiral wall directing the flow of cooling fluid away from the flow diverter. The flow diverter provides a large mass directly above the heat source for absorbing a significant amount of the heat generated thereby. The spiral wall absorbs additional heat from the heat source, which is transmitted radially outward from the heat source through the base plate. The direct impingement of the flow of cooling fluid on the flow diverter removes the majority of heat stored therein, with the remaining heat stored in the spiral wall and the base removed by the flow of cooling fluid as the cooling fluid circulates through the spiral channel. The subject invention, therefore, provides a more efficient heat sink for a cooling assembly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
       FIG. 1  is a schematic view of a liquid cooling system. 
       FIG. 2  is a perspective view of the heat sink of the subject invention; 
       FIG. 3  is a perspective view of the heat sink without the lid; 
       FIG. 4  is an exploded perspective view of the heat sink of  FIG. 2 ; 
       FIG. 5  is a top view of the heat sink as shown in  FIG. 3 ; 
       FIG. 6  is a cross sectional view of the heat sink along cut line  6 — 6  of  FIG. 5 ; 
       FIG. 7  is an enlarged fragmentary side view of the spiral wall of the heat sink; 
       FIG. 8  is a fragmentary top view of the spiral wall of  FIG. 7 ; 
       FIG. 9  is an enlarged side view of the flow diverter of the heat sink; and 
       FIG. 10  is an enlarged side view of an alternative embodiment of the flow diverter. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the Figures, wherein like numerals indicates like or corresponding parts throughout the several views, a heat sink assembly is generally shown at  10 . 
   Referring to  FIG. 1 , the operation of the heat sink  10  is incorporated into a liquid cooling assembly, generally shown at  44  in  FIG. 1 . A working fluid mover, such as a pump P, moves the flow of cooling fluid, usually a liquid, through a cooling fluid storage tank T, that stores excess cooling fluid. The pump P moves the cooling fluid through a heat extractor assembly to dissipate heat from the cooling fluid. The heat extractor includes a fan F and a radiator R. The radiator R can be of the well known type including tubes with cooling tins between the tubes to exchange heat between the cooling fluid passing through the tubes and air forced through the radiator R by the fan F. 
   Referring to  FIGS. 2 through 6 , the heat sink  10  includes a base  12  having a top surface  14  and a lid  16  having a bottom surface  18 , best shown in  FIGS. 4 and 6 , in spaced relationship with and parallel to the top surface  14  of the base  12 . A flow diverter  20  extends upwardly from the top surface  14  of the base  12  toward the bottom surface  18  of the lid  16 . The lid  16  includes a top surface  22  and an inlet  24  including an inlet tube  26  extending upwardly form the top surface  22  of the lid  16 . The inlet  24  is aligned with the flow diverter  20  for directing an impinging flow of cooling fluid on the flow diverter  20 . A spiral wall  28  extends between the top surface  14  of the base  12  and the bottom surface  18  of the lid  16  and is disposed in an increasing spiral from the flow diverter  20 . The spiral wall  28  defines a spiral channel  30  having an outlet  32  for directing the flow of cooling fluid radially in a spiral relative to the flow diverter  20 . Preferably, the base  12 , the lid  16 , the spiral wall  28  and the flow diverter  20  are manufactured from a thermally conductive material. 
   The base  12  and the lid  16  are generally circular with the outlet  32  extending tangentially therefrom. An outer wall  34  extends between the base  12  and the lid  16  about an outer periphery of the base  12  and the lid  16  for encapsulating the heat sink  10 , so that the flow of cooling fluid enters through the inlet tube  26  and is discharged through the outlet  32 . 
   The spiral wall  28  includes a plurality of louvers  38  for creating turbulence in the flow of cooling fluid. Preferably, as best shown in  FIGS. 7 and 8 , the louvers  38  are perpendicular to the base  12  and parallel to each other. If the flow of cooling fluid is allowed to flow past the surface for the spiral wall  28  uninterrupted, the thermal boundary layer between the spiral wall  28  and the flow of cooling fluid increases in thickness, thereby decreasing the heat transfer coefficient therebetween. The louvers  38  in the spiral wall  28  interrupt the flow of cooling fluid adjacent the spiral wall  28 , keeping the thermal boundary layer to a minimum and maintaining a high heat transfer coefficient. Referring to  FIGS. 7 and 8 , the louvers  38  are preferably created in the spiral wall  28  by forming two parallel cuts  40  in the spiral wall  28  and twisting the portion of the spiral wall  28  therebetween to form the louvers  38 . It should be appreciated, however, that any obstruction in the spiral wall  28  that interrupts the flow of cooling fluid and augments the thermal boundary layer is contemplated by the subject invention to maintain a high heat transfer coefficient. 
   Referring to  FIG. 4 , the top surface  14  of the base  12  defines a spiral groove  42 , and the bottom surface  18  of the lid  16  defines a corresponding spiral groove  42 . The spiral wall  28  is disposed in the grooves  42  and sandwiched between the base  12  and the lid  16 . The top surface  14  of the base  12  and the bottom surface  18  of the lid  16  include a braze coating (not shown). During the manufacturing process, the heat sink  10  is assembled and sent through a brazing furnace where the braze coating melts, thereby attaching the outer wall  34 , the spiral wall  28 , and the flow diverter  20  to the base  12 ; and the outer wall  34  and the spiral wall  28  to the lid  16  by metallurgical bonding. 
   Referring to  FIGS. 1 and 6 , an electronic device  46  generates an amount of heat to be dissipated, the heat being transferred from the electronic device  46  to a bottom surface  48  of the base  12  of the heat sink  10 . Referring back to  FIGS. 2 through 6 , the heat is then conducted from the base  12  to the flow diverter  20  and the spiral wall  28 , where the impinging flow of cooling fluid removes the heat therefrom as it circulates through the heat sink  10 . 
   Referring to  FIG. 9 , the flow diverter is generally shown at  20 , and includes a circular base  50  having a circular cross section. The flow diverter  20  preferably includes a cross section defined by two hyperbolic arcs  52  meeting at an apex  54 . However, referring to  FIG. 10  in which an alternative embodiment of a flow diverter is generally shown at  120 , the flow diverter  120  includes a conical shape. A distinct advantage of the flow diverter  20  having a hyperbolic cross section, as shown in  FIG. 9 , is that its mass is 40% less than that of the conical flow diverter  120 , as shown in  FIG. 10 , each having the same base area and the same height. 
   Referring to  FIGS. 9 and 10 , the flow diverter  20  includes a height (c) defined by the equation: 
                   c   a     =         0.1     Bi   d       -     1   2                 (   1   )               
where “a” is the radius of the circular base  50  and “Bi d ” is a dimensionless quantity called the Biot number for the flow diverter  20  defined by the equation:
 
                   Bi   d     =     ha     k   d               (   2   )               
where “k d ” is the thermal conductivity of the flow diverter material and “h” is the heat transfer coefficient of the cooling fluid surrounding the flow diverter  20 .
 
   The desired values of the radius (a) of the circular base  50  are in the range 0.2 in≦a≦0.4 in (5.1 mm≦a≦10.2 mm) and those of the Biot number (Bi d ) in the range 0.0005≦Bi d ≦0.05 corresponding to the preferred values of c/a, which are in the range of 1.22≦c/a≦14.12. 
   The efficiency (φ) of the flow diverter  20  is defined as the ratio of the heat flux ({dot over (q)} d ″) on the surface of the flow diverter  20  to the heat flux ({dot over (q)} b ″) at the circular base  50  of the flow diverter  20 . Since heat flux is the heat dissipation rate per unit area, the efficiency is defined by the equation: 
                 ϕ   =           q   .     d   ″         q   .     b   ″       =           q   .     d     /     S   d             q   .     b     /     S   b                   (   3   )               
where “{dot over (q)} d ” is the heat dissipation rate from the surface of the flow diverter  20 , “{dot over (q)} b ” is the heat dissipation rate from the circular base  50  of the flow diverter  20 , “S d ” is the surface area of the flow diverter  20 , and “S b ” is the area of the circular base  50 .
 
   When the flow diverter  20  having a hyperbolic cross section and the flow diverter  120  having a conical cross section are compared, wherein each of the flow diverters  20 ,  120  is intended to dissipate the same amount of heat and include the same circular base area but define a different surface area, the efficiency ratio between the two flow diverters  20 ,  120  is defined by the equation: 
                     ϕ   hyperbolic       ϕ   conical       =       S   conical       S   hyperbolic               (   4   )               
where “φ hyperbolic ” is the efficiency of the hyperbolic-shaped flow diverter  20 , “φ conical ” is the efficiency of the conical-shaped flow diverter  120 , “S hyperbolic ” is the surface area of the hyperbolic-shaped flow diverter  20 , and “S conical ” is the surface area of the conical-shaped flow diverter  120 .
 
   Since the surface area (S hyperbolic ) of the hyperbolic-shaped flow diverter  20  is less than the surface area (S conical ) of the conical-shaped flow diverter  120 , each having the same base area, it follows from Equation (4) that the efficiency (φ hyperbolic ) of the hyperbolic-shaped flow diverter  20  is greater than the efficiency (φ conical ) of the conical-shaped flow diverter  120 . Presented in Table 1 are the numerical values of the surface area (S hyperbolic ) of the hyperbolic-shaped flow diverter  20  and the surface area (S conical ) of the conical-shaped flow diverter  120  normalized by the area of the circular base  50  over a range of the ratio of the height (c) of the flow diverters  20 ,  120  to the radius (a) of the circular base  50 . Also included in Table 1 are the numerical values of the flow diverter efficiency ratios suggested by Equation (4). The tabular results show that for the entire range of the c/a ratio, the efficiency of the hyperbolic-shaped flow diverter  20  is 17% to 49% higher than that of the conical shaped flow diverter  120 . 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               c/a 
               S hyperbolic /πa 2   
               S conical /πa 2   
               S conical /S hyperbolic  = φ huperbolic /φ conical   
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               1 
               1.2127 
               1.4142 
               1.1662 
             
             
               2 
               1.6806 
               2.2361 
               1.3305 
             
             
               3 
               2.2485 
               3.1623 
               1.4064 
             
             
               4 
               2.8585 
               4.1231 
               1.4424 
             
             
               5 
               3.4890 
               5.0990 
               1.4615 
             
             
               6 
               4.1308 
               6.0828 
               1.4725 
             
             
               7 
               4.7793 
               7.0711 
               1.4795 
             
             
               8 
               5.4322 
               8.0623 
               1.4842 
             
             
               9 
               6.0881 
               9.0554 
               1.4874 
             
             
               10 
               6.7461 
               10.0499 
               1.4897 
             
             
                 
             
           
        
       
     
   
   Referring to  FIGS. 5 and 6 , the spiral wall  28  includes a spiral angle (θ) defined by the equation: 
                 θ   =       [         (     r   a     )     2     -   1     ]       2   /   3               (   3   )               
where “r” is the local radius of the spiral and “θ” its angle of rotation in radians from the beginning of the spiral wall  28  at the flow diverter  20 .
 
   The spiral channel  30  includes a width (s θ ) defined by the equation: 
                     s   θ     a     =         1   +       (       2   ⁢           ⁢   π     +   θ     )       3   /   2           -       1   +     θ     3   /   2                     (   5   )               
where “a” is the radius of the circular base  50  of the flow diverter  20 . As the spiral angle (θ) increases, the width (s θ ) of the spiral channel  30  becomes constant. The preferred values of s θ /a are in the range 2≦s θ /a≦3.
 
   The spiral wall  28  includes a height (b) defined by the equation: 
                   b   t     =     0.6498       Bi   w                 (   6   )               
where “t” is the thickness of the spiral wall  28  and “Bi w ” is a dimensionless quantity called the Biot number for the spiral wall  28  defined by the equation:
 
   
     
       
         
           
             
               
                 
                   Bi 
                   w 
                 
                 = 
                 
                   ht 
                   
                     k 
                     w 
                   
                 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
         
       
     
   
   where “k w ” is the thermal conductivity of the spiral wall material and “h” is the heat transfer coefficient of the cooling fluid surrounding the spiral wall  28 . The preferred values of the thickness (t) of the spiral wall  28  lie in the range of 0.001 in≦t≦0.006 in (0.025 mm≦t≦0.152 mm) and those of the Biot number (Bi w ) lie in the range of 0.000005≦Bi w ≦0.00005 corresponding to the preferred values of b/t, which are in the range of 29≦b/t≦290. 
   As the spiral angle (θ) increases, the spiral width (s θ ) of the spiral channel  30  becomes constant to ensure that a flow velocity (ū θ ) of the cooling fluid is maintained as the cooling fluid flows in a spiral fashion around the flow diverter  20  and through the spiral channel  30 . The cooling fluid impinges onto the flow diverter  20  filling the inner spiral wall  28  spanning between 0 rad.≦θ≦2π rad. Once the inner spiral wall  28  is filled, the flow of cooling fluid is constrained to flow primarily in a spiral fashion around the flow diverter  20  within the spiral channel  30   
   The mean flow velocity (ū θ ) of the flow of cooling fluid through the spiral channel  30  is defined by the equation: 
                     u   _     θ     =       m   .       ρ   ⁢           ⁢     s   θ     ⁢   b               (   8   )               
where “{dot over (m)}” is the mass flow rate of the cooling fluid impinging on the flow diverter  20 , “ρ” is the fluid density of the cooling fluid, “s θ ” is the width of the spiral channel  30 , and “b” is the height of the spiral wall  28 .
 
   Although the cooling fluid flows primarily in the θ-direction through the spiral channel  30 , there is intrusion and extrusion of the fluid into the spiral channel  30  at each of the louvers  38  in the spiral wall  28  through the gaps created by the twisting of the louvers  38 . The movement of the fluid at the louvers  38  serves to destabilize the thermal boundary layer on the spiral wall  28 , thereby augmenting the heat transfer coefficient. 
   The heat transfer coefficient (h) between the spiral wall  28  and the flow of cooling fluid in the presence of the louvers  38  is defined by the equation: 
                   h     h   o       =           na     ⁢       (     1   +     θ     3   /   2         )       1   /   4         +   α       1   +   α               (   9   )               
where “h o ” is the heat transfer coefficient in the absence of louvers  38 , “n” is the linear density of the louvers  38  in the spiral wall  28 , and “α” is the aspect ratio of the spiral channel  30 . The aspect ratio (α) is defined by the equation:
 
   
     
       
         
           
             
               
                 α 
                 = 
                 
                   b 
                   
                     s 
                     θ 
                   
                 
               
             
             
               
                 ( 
                 10 
                 ) 
               
             
           
         
       
     
   
   The heat transfer coefficient (h o ) in the absence of louvers  38  is defined for uniform wall temperature (UWT) boundary conditions by the equation: 
                       h   o     ⁢   b     k     =     3.770   ⁢     (     1   +   α     )     ⁢     (     1   -     2.610   ⁢           ⁢   α     +     4.970   ⁢           ⁢     α   2       -     5.119   ⁢           ⁢     α   3       +     2.702   ⁢           ⁢     α   4       -     0.548   ⁢           ⁢     α   5         )               (   11   )               
and for uniform wall heat flux (UHF) boundary conditions by the equation:
 
                       h   o     ⁢   b     k     =     4.118   ⁢     (     1   +   α     )     ⁢     (     1   -     2.042   ⁢           ⁢   α     +     3.085   ⁢           ⁢     α   2       -     2.477   ⁢           ⁢     α   3       +     1.058   ⁢           ⁢     α   4       -     0.186   ⁢           ⁢     α   5         )               (   12   )               
where “b” is the height of the spiral wall  28 , “k” is the thermal conductivity of the cooling fluid flowing through the spiral channel  30 , and “α” is the aspect ratio of the spiral channel  30  defined by equation 10.
 
   The preferred values of the louver density (n) are in the range of 15≦n≦35 louvers per inch along the spiral wall  28  (6≦n≦14 louvers per cm). 
   The foregoing invention has been described in accordance with the relevant legal standards; thus, the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.