FLOW DIVERTERS TO ENHANCE HEAT SINK PERFORMANCE
A heat sink includes a base and fins attached to the base. A flow diverter is in contact with the base or at least one of the fins and is configured to disturb a laminar flow region of a fluid flowing adjacent to at least one of the fins or the base.
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The present application is related to U.S. patent application Ser. No. ______ to Hernon, et al., entitled “Active Heat Sink Designs”, and which is commonly assigned with the present application, and U.S. patent application Ser. No. ______ to Hernon, et al., entitled “Monolithic Structurally Complex Heat Sink Designs,” both of which are hereby incorporated by reference as if reproduced herein in their entirety.
TECHNICAL FIELD OF THE INVENTIONThe present invention is directed, in general, to heat sinks.
BACKGROUND OF THE INVENTIONHeat 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 OF THE INVENTIONOne embodiment is a heat sink that includes a base and fins attached to the base. A flow diverter is in contact with the base or at least one fin and is configured to disturb a laminar flow region of a fluid flowing adjacent to at least one of the fins or the base.
Another embodiment is a method that includes providing a heat sink having a base and fins attached thereto. A flow diverter is placed in contact with said fin or said base. The flow diverter is configured to disturb a laminar flow region of a fluid flowing adjacent to at least one of said fins or said base.
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:
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.
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.
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.
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.
In this embodiment of
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.
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.
Turning to
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.
Turning to
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.
An alternate embodiment is illustrated in
In each of the embodiments illustrated in
Returning briefly to
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
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.
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. ______ (Hernon 3). 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.
Turning to
As illustrated in
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.
Claims
1. A heat sink comprising:
- a base;
- fins attached to said base; and
- a flow diverter in contact with said base or at least one of said fins and configured to disturb a laminar flow region of a fluid flowing adjacent to at least one of said fins or said base.
2. The heat sink as recited in claim 1, wherein said flow diverter is configured to divert said fluid from a greater distance above a surface of said heat sink to a lesser distance above said surface.
3. The heat sink as recited in claim 1, wherein said heat sink is in thermal contact with a device configured to dissipate heat, and said flow diverter is configured to direct said fluid from a region of relatively lower power dissipation by said device to a region of relatively greater power dissipation by said device.
4. The heat sink as recited in claim 1, wherein said flow diverter comprises a duct between two of said fins.
5. The heat sink as recited in claim 1, wherein said flow diverter is configured to produce unsteady flow in said fluid.
6. The heat sink as recited in claim 1, wherein said flow diverter is located on said base between two of said fins.
7. The heat sink as recited in claim 6, wherein a height of said flow diverter is less than a height of said fins.
8. The heat sink as recited in claim 6, wherein said flow diverter has a streamlined profile.
9. The heat sink as recited in claim 1, wherein said flow diverter is attached to said base or said fin and positioned upstream of said fin and outside a space bounded by said fins.
10. The heat sink as recited in claim 1, wherein said flow diverter is a monolithic feature of said heat sink.
11. The heat sink as recited in claim 1, further comprising an opening in said surface, wherein said flow diverter is configured to direct said fluid through said opening.
12. The heat sink as recited in claim 1, wherein said flow diverter is an active element.
13. A method, comprising:
- providing a heat sink having a base and fins attached thereto;
- placing a flow diverter in contact with said fin or said base; and
- configuring said flow diverter to disturb a laminar flow region of a fluid flowing adjacent to at least one of said fins or said base.
14. The method as recited in claim 13, wherein said flow diverter is configured to divert said fluid from a greater distance above said base to a lesser distance above said base.
15. The method as recited in claim 13, further comprising placing said heat sink in thermal contact with a device configured to dissipate heat, wherein said flow diverter is configured to divert said fluid from a region of relatively lower power dissipation by said device to a region of relatively greater power dissipation by said device
16. The method as recited in claim 13, further comprising configuring a duct between two of said fins to divert said fluid.
17. The method as recited in claim 13, further comprising configuring said flow diverter to produce unsteady flow in said fluid.
18. The method as recited in claim 13, further comprising locating said flow diverter on said base between said fins and configuring said flow diverter to have a height less than a height of fins.
19. The method as recited in claim 13, further comprising forming said flow diverter, said base and said fins as a monolithic structure.
20. The method as recited in claim 13, wherein said flow diverter is an active element.
21. The method as recited in claim 13, wherein said fluid is air.
Type: Application
Filed: Jun 30, 2008
Publication Date: Dec 31, 2009
Applicant: Alcatel-Lucent Technologies Inc. (Murray Hill, NJ)
Inventors: Domhnaill Hernon (Meath), Marc Hodes (Dublin), Alan Lyons (New Providence, NJ), Alan O'Loughlin (Dubllin), Shankar Krishnan (Richland, WA)
Application Number: 12/165,193
International Classification: F28F 7/00 (20060101);