Abstract:
A heat sink comprising a base, fins attached to the base and a flow diverter in contact with the base or at least one of the fins. The flow diverter has a rectangular cross-sectional profile in a plane that is coplanar with and elevated above a plane of the base and spanning the entire separation distance, and, a segment of the flow diverter is angled towards the base to direct the fluid flow towards the base.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a continuation application of U.S. patent application Ser. No. 12/165,193 to Hernon et al., entitled “FLOW DIVERTERS TO ENHANCE HEAT SINK PERFORMANCE”, filed on Jun. 30, 2008, and which is commonly assigned with the present application, and incorporated by reference herein in its entirity. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention is directed, in general, to heat sinks. 
       BACKGROUND 
       [0003]    Heat sinks are commonly used to increase the convective surface area of an electronic device to decrease the thermal resistance between the device and cooling medium, e.g., air. Such heat sinks generally employ fins or pins to exchange heat with a fluid (air or liquid) flowing thereover. Some electronic components dissipate enough power that air-cooled heat sinks are becoming inadequate to sufficiently cool these devices. Liquid cooling adds significant costs and reliability concerns to system designs, and is thus undesirable in many cases. Methods of improving the heat transfer efficiency of air-cooled heat sinks are needed to extend their use to higher power components. 
       SUMMARY 
       [0004]    One embodiment is a heat sink comprising a base, fins attached to the base and a flow diverter in contact with the base or at least one of the fins. The flow diverter has a rectangular cross-sectional profile in a plane that is coplanar with and elevated above a plane of the base and spanning the entire separation distance, and, a segment of the flow diverter is angled towards the base to direct the fluid flow towards the base. 
         [0005]    Another embodiment is a method that comprises providing a heat sink having a base and fins attached thereto. The method also comprises forming flow diverter in contact with the base or at least one of the fins. The method also comprises configuring the flow diverter to have a rectangular cross-sectional profile in a plane that is coplanar with and elevated above a plane of the base and spanning the entire separation distance, and, a segment of the flow diverter angled towards the base to direct the fluid flow towards the base. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Various embodiments are understood from the following detailed description, when read with the accompanying figures. Various features may not be drawn to scale and may be arbitrarily increased or reduced in size for clarity of discussion. Various features in figures may be described as “vertical” or “horizontal” for convenience in referring to those features. Such descriptions do not limit the orientation of such features with respect to the natural horizon or gravity. The term “surface” unless otherwise qualified applies to the combined surface of the heat sink, that is, the surface of the base, fins and any projections therefrom. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0007]      FIG. 1  illustrates a prior art heat sink; 
           [0008]      FIG. 2  illustrates air flow regions between two fins of a heat sink; 
           [0009]      FIG. 3A  illustrates a perspective view of an embodiment of a heat sink with one configuration of a flow diverter of the disclosure 
           [0010]      FIG. 3B  presents plan and sectional views of an embodiment of a heat sink similar to the heat sink presented in  FIG. 3A ; 
           [0011]      FIG. 3C  presents a sectional view of another embodiment of a heat sink of the disclosure similar to the heat sink presented in  FIG. 3A ; 
           [0012]      FIG. 3D  presents a sectional view of another embodiment of a heat sink of the disclosure similar to the heat sink presented in  FIG. 3A ; 
           [0013]      FIG. 4A  illustrates a perspective view of another embodiment of a heat sink with another configuration of the flow diverter of the disclosure; 
           [0014]      FIG. 4B  presents plan and sectional views of another embodiment of a heat sink of the disclosure similar to the heat sink presented in  FIG. 4A ; 
           [0015]      FIG. 5A  illustrates a perspective view of another embodiment of a heat sink with another configuration of the flow diverter of the disclosure; 
           [0016]      FIG. 5B  presents plan and sectional views of another embodiment of a heat sink of the disclosure similar to the heat sink presented in  FIG. 5A ; 
           [0017]      FIG. 5C  presents plan and sectional views of another embodiment of a heat sink of the disclosure similar to the heat sink presented in  FIG. 5A ; 
           [0018]      FIG. 6A  illustrates a perspective view of an embodiment of a heat sink another configuration of the flow diverter of the disclosure of the disclosure where air flow is diverted vertically with respect to a base; 
           [0019]      FIG. 6B  presents a sectional view of another embodiment of a heat sink of the disclosure similar to the heat sink presented in  FIG. 6A ; 
           [0020]      FIG. 6C  presents a three dimensional form of another embodiment of the heat sink similar to the heat sink presented in  FIG. 6A ; and 
           [0021]      FIG. 7  illustrates a perspective view of an embodiment of a heat sink with another configuration of a flow diverter of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Embodiments described herein reflect the recognition that structural features may be used in heat sinks that decrease thermal resistance between the heat sink and a fluid e.g., air. In some embodiments, these structural features may be used to produce unsteady flow of air, e.g., in selected portions of the heat sink to disturb laminar flow near surfaces of the heat sink. In other embodiments, features are formed that direct cooler, faster moving air from one region of a heat sink to a region having hotter, slower flow to increase the rate of heat transfer from the hotter regions. In some embodiments, three dimensional (3-D) rendering and investment casting may be employed to form such structural features in a cost-effective manner. 
         [0023]      FIG. 1  illustrates a prior art heat sink  100 . Features of the heat sink  100  include a base  110  and fins  120 . The fins  120  of such heat sinks are typically structurally uniform, e.g., there are no projections or depressions in the surface of the fins  120  other than surface roughness typical of the particular manufacturing method. 
         [0024]    An air stream  130  passes between the fins  120  with little obstruction. It is thought that as air enters the space between two fins, a boundary layer forms near the surfaces of the fins  120  and the base  110 . The boundary layer is a region of airflow adjacent to a surface that contains a velocity gradient. The gradient arises due to the fact that the velocity at the surface is about zero. Outside the boundary layer, in the so-called “free-stream” region, the velocity gradients are small or negligible. Therefore, the flow must go from nearly zero velocity at the wall to the free-stream velocity away from the wall within the boundary layer. The boundary layer acts as a thermal insulator. Thus, in general, the thinner the boundary layer, the lower the thermal resistance between the flowing air and a heat sink element such as a fin  120 . 
         [0025]      FIG. 2  illustrates a schematic view of a nonlimiting model of a fluid  210  flowing between two conventional fins  220  with an opening  230  between them. By convention, the direction of flow of the air stream  210  is downstream, and the opposite direction is upstream. Within a region  240 , the air stream  210  flows with free-stream characteristics. Within a region  250  the air stream  210  flows with boundary layer characteristics. A transition region  260  marks a transition from free-stream characteristics to boundary layer characteristics. The boundary layers begin at the opening  230 . The thickness of the boundary layer region  250  increases with increasing distance from the opening  230  to a point  270 . The boundary layer generally includes a laminar flow region  280  adjacent to the surface of the fin  220  that includes a region of flow parallel to the surface. The laminar flow region may include regions of non-ideal flow, e.g., not exactly parallel to the adjacent surface. Such minor departures from ideal laminar flow are considered laminar flow in the present discussion. The laminar flow region  280  and may include a region of non-parallel flow. At the point  270 , the boundary layer region  250  is fully developed, meaning that essentially all of the air flows in a region of smoothly decreasing velocity gradient with increasing distance from the fins  220 . It is thought that the resistance of heat transfer between the air stream  210  and the fins  220  decreases with increasing boundary layer thickness, and more particularly with increasing thickness of the laminar flow region  280 . At the point  270 , the heat transfer rate is thought to reach a minimum. Thus, the thermal resistance is expected to increase from the opening  230  to a maximum at about the point  270 . 
         [0026]    Embodiments described herein reflect the recognition that a laminar flow region adjacent a heat sink surface, e.g., a surface of a fin or a base, may be disturbed using structural elements, referred to herein as flow diverters. “Disturbed” as applied to a laminar flow region means that the laminar flow region has flow characteristics it would not have in the absence of the flow diverter. Examples of disturbed laminar flow region include, e.g., thinning, flow separation, and flow non-parallel to the adjacent surface. 
         [0027]    Without limitation by theory, the flow diverters are thought to produce vortexes or unsteady flow at the downstream side of the flow diverters. Unsteady flow may include, e.g., vortices and eddies, and transitional, turbulent, unstable, chaotic and resonant airflow. In some cases, a low pressure region is thought to form on the downstream side of a flow diverter. The low-pressure region is thought to cause the fluid to flow in a manner that impinges on the laminar flow region adjacent the surface, e.g., the laminar flow region  280 . Such diversion of, e.g., a fluid stream causes diverts the fluid from a greater distance above the surface to a lesser distance above the surface. Because the thermal resistance of the heat sink is in part a function of the thickness of the laminar flow region, the impinging may have the effect of increasing the rate of heat transfer between the fluid and the heat sink. The flow diverters may be configured to reduce thermal resistance of a portion of a heat sink or the entire heat sink. For example, it may be desirable to reduce thermal resistance of only a portion of a heat sink located proximate a region of an electronic device that generates more heat than other regions of the device. 
         [0028]      FIG. 3A  illustrates one embodiment of a heat sink  300  having a base  310  and a fin  320  formed thereon. Flow diverters  330  are attached to the fin  320 .  FIG. 3B  illustrates the fin  320  in plan view and sectional view. An fluid stream  340  flows past the flow diverters  330 . The fluid stream  340  may a gas or a liquid, and may be used to transfer heat to or from a heat sink, depending on the application. For simplicity of discussion a fluid stream is referred to herein after as an air stream, while recognizing that other gases or liquids may be used as a heat exchange medium. Furthermore, heat is referred to as being extracted from the heat sink, while recognizing heat could be extracted by the heat sink from the fluid stream. 
         [0029]    In this embodiment of  FIG. 3A , the flow diverters  330  are square cylindrical elements having a length equal to or less than the height of the fin  320  above the base  310 . The flow diverters  330  may have any desired cross-sectional profile, e.g., circular, square or triangular. Any shape that has the effect of causing a portion of the air stream  340  to impinge on a laminar flow region proximate the surface of the base  310  or the fin  320  is within the scope of this discussion. The flow diverters  330  are also stationary with respect to the fin  320 . In other embodiments, the flow diverters  330  may be an active element as described in U.S. patent application Ser. No. 12/165,063, incorporated herein in its entirety. There may be one or a plurality of flow diverters  330  on a fin  320 , and a particular heat sink may have flow diverters  330  formed on one or a plurality of fins  320 . Flow diverters  330  may be spaced at regular or uneven intervals on the fin  320 , and when present on adjacent fins and projecting into the same inter-fin space, may be aligned as illustrated in  FIG. 3C  or staggered as illustrated in  FIG. 3D . 
         [0030]    The flow regime of air or other cooling fluid through a heat sink may be characterized by a Reynolds number associated with the heat sink and the flowing fluid. As known by those skilled in the pertinent art, a Reynolds number describes the relationship between inertial forces and viscous forces in a fluid system. Laminar flow occurs when a fluid flows in parallel layers with little or no disruption between the layers. This flow regime is associated with a low Reynolds number. Turbulent flow is characterized by random eddies, vortices and other flow fluctuations, and is associated with a high Reynolds number. A transition regime between laminar and turbulent flow may be characterized by more predictable but non-uniform flow, such as vortices and eddies that are fairly stable over time. Thus, providing a heat sink with flow diverters may be viewed as increasing the Reynolds numbers associated with flow of the cooling fluid through the heat sink. 
         [0031]    Turbulent flow is generally associated with greater resistance to flow of fluid. In the context of heat sinks, greater flow resistance translates to a greater pressure drop across the heat sink. In some cases, a greater pressure drop is undesirable. In such cases, the flow diverters  330  may be configured to produce non-uniform flow, but not turbulent flow. In general, such a configuration must be determined experimentally for a combination of cooling fluid, velocity of the fluid, and the configuration of the heat sink. 
         [0032]      FIG. 3B  illustrates unsteady flow of an air stream  340  over the flow diverters  330 . The flow diverters  330  are thought to form a low-pressure region  350  downstream of the flow diverters  330  due to, e.g., flow separation. The low pressure region  350  may produce a standing wave or vortexes  360  at the downstream side of the flow diverters  330  depending on, e.g., the Reynolds number associated with the geometry of the heat sink  300  and the velocity of the air stream  340 . The standing wave or vortexes  360  include a flow direction component normal the surface of the fin  320 . This normal flow may have the effect of compressing the laminar flow region proximate the surface of the fin  320 , thus reducing the thermal resistance between the fin  320  and the air stream  340 . 
         [0033]      FIG. 3C  illustrates an embodiment in which the flow diverters  330  are configured to cause air flow through the heat sink  300  to be resonant. In this nonlimiting example, the flow diverters  330  cause a standing pressure wave resulting in regions of differing pressure, e.g., low pressure regions  370  and high pressure regions  380 . The formation of the standing wave is expected to occur at a range of velocity of the air stream  340  that is dependent on the geometry of the fins  320  and the flow diverters  330 . The flow diverters  330  may be configured to form the low pressure regions  370  and the high pressure regions  380  at positions that result in reduction of the thermal resistance between the fins  320  and the air stream  340  near a portion of the heat sink  300  at which lower thermal resistance between the heat sink  300  and the air stream  340  is desired. 
         [0034]      FIG. 3D  illustrates an embodiment in which the flow diverters  330  are placed on opposing faces of fins  320  in a staggered configuration. In some cases, it is thought that staggering the flow diverters  330  may aid the formation of a desired air flow characteristic, e.g., unsteady or resonant air flow, at a particular flow velocity of the air stream  340 . Configurations of the flow diverters  330  may be combined in any desired manner within a heat sink to result in the desired flow characteristics. A configuration may be determined, e.g., by wind-tunnel analysis or numerical modeling. 
         [0035]    Turning to  FIG. 4A , illustrated is an embodiment of a heat sink  400  including a base  410  and a fin  420  thereon. A number of flow diverters  430  are placed at the leading edge of the fin  420 . These flow diverters  430  present a 2-D profile to an air stream (in the plane of the fin  420 ), in contrast to the flow diverters  330 , which present a 1-D profile. In some cases, the length of the flow diverters  430  in the plane of the fin  420  is less than about the height of the fin  420 . Thus, multiple flow diverters  430  may be placed in a line with space between them, as illustrated in  FIG. 4A . In some cases, flow diverters  435  may be placed on the fin  420  downstream of the leading edge of the fin  420  instead of or in addition to the flow diverters  430 . 
         [0036]      FIG. 4B  illustrates the fin  420  in plan view and sectional view. An air stream  440  flows past the flow diverters  430 . The flow diverters  430  cause unsteady flow on the downstream side, illustrated without limitation as vortexes  450 . In this case, the vortexes  450  have a more complex motion due to the fact that the flow diverters  430  present a two-dimensional cross-section to the air stream  440 . The vortexes  450  are thought to have a direction component parallel and a direction component normal to the surface of the fin  420 . It is believed that in some flow regimes this motion is particularly effective at reducing thermal resistance between the fin  420  and the air stream  440 . 
         [0037]    As mentioned above, flow diverters  435  may be placed downstream of the leading edge of the fin  420  in addition to the flow diverters  430 . These downstream flow diverters  435  may be aligned with upstream flow diverters  430  or they may be staggered, as illustrated, causing air to take a more tortuous path between the fins  430 . 
         [0038]    Turning to  FIG. 5A , illustrated is a heat sink  500  having a base  510  and two fins  520 . A flow diverter  530  is attached to the base  510  between the fins  520 . The flow diverter  530  has, e.g., a triangular cross section in the plane of the base, but could have any other desired cross section, such as circular, elliptical, square, or a more complex cross section. The flow diverter  530  may have any height above the base  510 , though typically the height will be less than or equal to the height of the fin  520 . One flow diverter  530  is illustrated, but other embodiments include multiple flow diverters  530  between the fins  520 . Multiple flow diverters  530  may be the same or different heights, or have the same or different cross sectional profiles. 
         [0039]      FIG. 5B  illustrates plan and sectional views of the fins  520 . The embodiment  500  has a single triangular flow diverter  530  with an air stream  540  impinging thereon. Air is forced to flow between the flow diverter  530  and the fin  520 , thereby increasing its velocity. The greater air speed parallel to the fin  520  is thought to cause the laminar flow region proximate the fin  520  to thin, thus reducing the thermal resistance between the air stream  540  and the fin  520 . 
         [0040]    When a flow diverter  530  has an abrupt transition downstream of the leading edge, such as for the illustrated triangular flow diverter  530 , vortexes  550  may be formed. In some cases, such vortexes may be undesirable, such as when induced drag associated with the vortexes  550  increases the pressure drop across the heat sink. 
         [0041]    An alternate embodiment is illustrated in  FIG. 5B  in which a flow diverter  560  has an elliptical or streamlined cross section. In one embodiment, the flow diverter is configured as an elliptical airfoil. In each of these embodiments, the air stream  540  is forced to flow faster between the flow diverter and the fins  520  as before. However, the streamlined profile of the flow diverter  560  reduces the formation of vortexes at the downstream side, resulting in lower drag. This lower drag is expected to reduce the pressure drop across the heat sink  500 , improving heat transfer relative to the heat sink  500  using the triangular flow diverter  530 . 
         [0042]      FIG. 5C  illustrates an embodiment in which the flow diverter  530  is positioned at a location  580  upstream of the fins  520  and outside a volume  585  bounded by the fins  520 . The volume  585  is that volume between the fins  520  that does not extend beyond the terminus of the fins  520 . The flow diverter  530  is attached to the fins  520  by, e.g., supports  590 . The flow diverter  530  may be any shape and may be placed in any position relative to the fins  520  that disturbs laminar flow of the air stream  540  adjacent to the fins  520 . In another embodiment, not shown, the flow diverter  530  is attached to a portion of the base, e.g., the base  510 , that extends beyond the terminus of the fins  520 . 
         [0043]    In each of the embodiments illustrated in  FIG. 3 ,  FIG. 4 , and  FIG. 5 , the flow diverters may optionally be placed at a position downstream of the leading edge of the fin (e.g., fin  320 ,  420 ,  520 ) to reduce thermal resistance between the fin and the air stream in the vicinity of a hot spot. A hot spot is a region of relatively greater heat flux from an integrated circuit, e.g., relative to the surrounding areas of the circuit. By so placing the flow diverter, thermal resistance may be reduced in a portion of the heat sink where the lower resistance is particularly beneficial, while minimizing the number of flow diverters to reduce resulting pressure drop. 
         [0044]    Returning briefly to  FIG. 5B , an embodiment is illustrated in which the flow diverter  530  is placed over a hot spot  570 . It is expected that the heat flux from the hot spot  570  will be partially localized to the portion of the fins  520  immediately above the hot spot  570 . Therefore, reducing the thermal resistance between the fins  520  and the air stream  540  by decreasing the thickness of the laminar flow region in the vicinity of the hot spot  570  is expected to be particularly beneficial. 
         [0045]    In some cases, the flow diverter (e.g., flow diverter  330 ,  430  or  530 ) may be placed near the point where the boundary layers between fins become fully developed. Referring to  FIG. 2 , this point would be, e.g., about at the point  270 . Placement of the flow diverter near this point is thought to be particularly beneficial in some cases in that the number of flow diverters in an air path may be reduced. The effect of drag caused by the flow diverter may be balanced against the benefit of disrupting laminar flow regions by only placing the flow diverters at points of convergence of the boundary layers. Depending on factors such as fin spacing and the length of the path between the fins, two or more points of boundary layer convergence may possible in the path of air flow between the fins. In an embodiment, a flow diverter is placed at each convergence point in an air path. 
         [0046]    In each of the illustrated embodiments, the flow diverters may or may not be integral to the structure of the heat sink. When a flow diverter is not integral, it may be, e.g., a metal or plastic portion affixed to the remaining portion of the heat sink. The flow diverter may be affixed by adhesive, welding, or brazing, e.g., or in some cases may simply be held in place by friction. In some cases, it may be desirable to use a heat transfer agent such as thermal grease to increase thermal coupling between the flow diverter and the remaining portion of the heat sink. 
         [0047]    When the flow diverter is integral to the heat sink, the heat sink and the flow diverter may be formed as a monolithic structure, e.g., by the method of three dimensional (3-D) printing and investment casting. Such a method is disclosed in U.S. patent application Ser. No. 12/165,225, incorporated herein in it entirety. Briefly described, the method provides for using a 3-D printer to produce a sacrificial form of a heat sink. The form is used to fashion a mold, and is then melted or vaporized out of the mold. The mold is then used to form the final heat sink. This method provides the ability to form detailed 3-D patterns that might not be manufacturable by conventional methods, such as machining, die casting, folding or skiving. Moreover, the structural features are extensions of a single physical entity, e.g., a polycrystalline metallic casting. In addition to forming structural details not amendable to other methods, a monolithic structure is expected to reduce thermal resistance within the heat sink, making a greater surface area available to transfer heat to the air stream. 
         [0048]    Turning to  FIG. 6A , illustrated is an embodiment of a ducted heat sink  600  in a projection view. The heat sink  600  includes a base  610  and fins  620  thereon. Air flow is diverted by one or more ducts  630  between the fins  620 . The ducts  630  may be formed by planar segments  640 , as illustrated, or any other desired shape, such as smoothly curved surfaces. 
         [0049]    As illustrated in  FIG. 6B , in side view, the ducts  630  divert an air stream  650  from a direction generally parallel to the base  610  to a direction having a component normal to the base  610 . Thus, cooler, faster air from a portion of the heat sink  600  further from the base  610  may be diverted to a region of warmer, slower air nearer to the base  610  at a hot spot  660 . Moreover, because the air diverted by the one or more ducts  630  joins the flow of air near the base  610 , a greater volume of air per unit time may be caused to flow over the hot spot  660  than may otherwise occur absent the ducts  630 . 
         [0050]      FIG. 6C  illustrates a sacrificial form  670  of the heat sink  600  formed by 3-D printing. Ducts  680  may be seen through the semi-transparent fins  690  of the form  670 . As described previously, the form  670  may be used to render the heat sink  600  in, e.g., a metal to produce a monolithic heat sink with the ducts  680  in a practical and efficient manner. 
         [0051]      FIG. 7  illustrates an embodiment of a heat sink  700  having a base  710  and fins  720  thereon. A flow diverter  730  directs air flow from a lower level of the heat sink  700  to a higher level. The fin  720  also includes an optional opening  740  formed therein. The flow diverter  730  and the opening  740  may be positioned to allow cooler air from one portion of the heat sink  700  to flow through the fin  720  due to a pressure differential formed on the downstream side of the flow diverter  730 . The cooler air can then displace or mix with warmer air in the vicinity of a hot spot, e.g., thereby increasing the rate of heat removal from the hot spot. Optionally, another flow diverter may be positioned on the side of the fin  720  opposite the flow diverter  730  to direct air into the opening  740 . Without limitation, the investment casting method described above is well-suited to economically forming such features at the scale of heat sinks used to cool electronic components. 
         [0052]    The various embodiments described herein may be combined in any desired manner to result in a desired air flow characteristic from a heat sink. Moreover, while the embodiments are described with respect to parallel-fin heat sinks, the embodiments may be practiced with heat sinks of other geometries where thermal resistance may be reduced by disturbing laminar flow regimes near a surface of the heat sink. Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.