ACTIVE HEAT SINK DESIGNS
A heat sink includes a surface and a first active element connected to the surface. The first active element is configured to move from a first position relative to the surface to a second position relative to the surface. The movement alters the heat transfer characteristics of the heat sink.
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The present application is related to U.S. patent application Ser. No. ______ (Docket No. Hernon 2-25-5-44-6) to Hernon, et al., entitled “Flow Diverters to Enhance Heat Sinks”, and U.S. patent application Ser. No. ______ (Docket No. Hernon 3-26-6-45-7) to Hernon, et al., entitled “Monolithic Structurally Complex Heat Sink Designs,” both of which are commonly assigned with the present application and hereby incorporated by reference as if reproduced herein in their entireties.
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 heat transfer surface area of an electronic device to reduce the operating temperature of the device. A heat sink typically consists of a base and number of parallel fins or pins. A cooling fluid, typically air, is caused to flow over the heat sink to remove heat from the fins or pins, thereby cooling the electronic device.
SUMMARY OF THE INVENTIONOne embodiment is a heat sink that has a surface. A first active element is connected to the surface and configured to move from a first position relative to the surface to a second position relative to the surface. The heat transfer characteristics of the heat sink are altered by the movement.
Another embodiment is a method that includes providing a heat sink having a surface. A first active element is formed on the surface. The active element is configured to move from a first position relative to the surface to a second position relative to the surface different from the first position. The heat transfer characteristics of the heat sink are altered by the movement.
Another embodiment is a system that includes a device configured to produce heat and a heat sink having a surface in thermal contact with the device. The heat sink includes an active element connected to the surface and configured to move from a first position relative to the surface to a second position relative to the surface. The active element is configurable to change a direction of flow of a cooling fluid in response to the heat produced by the device.
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. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
Embodiments described herein reflect the recognition that active heat sink elements may be used in heat sink designs to selectively alter flow of a coolant fluid within the heat sink to alter the heat transfer characteristics of the heat sink. The active heat sink elements may be actuated in response, e.g., to a transient heat output by an electronic device, or by a local thermal gradient within the heat sink. As discussed further below, active elements may be either commanded or uncommanded.
For brevity in the following discussion, air is discussed as the cooling fluid while explicitly recognizing that the invention can be practiced with other cooling fluids, including other gases and liquids.
In the illustrated embodiment, the active element 220 includes a first metal layer 230 and a second metal layer 240. The first metal layer 230 has a greater coefficient of thermal expansion (CTE) than that of the second metal layer 240. As illustrated in
The active element 220 is formed from two layers, each having a different CTE. While two layers are used to illustrate the embodiment, the principles discussed may be extended to three or more layers. When such a combination of layers is formed from two dissimilar metals, it is often referred to as a bimetal strip. However, such a combination of layers with different CTEs may be formed with other combinations of materials, such as, e.g., metal and polymer or polymer and polymer. Unless stated otherwise, the following discussion refers to a bilayer metallic (bimetallic) element while recognizing that other materials and more than two layers may be used.
In the context of active elements, actuation of such bimetal strips is referred to herein as uncommanded, as actuation occurs independent of any external command to the active element. In a bimetal strip, the differential CTE results in torque on the bimetal strip when the temperature is greater or less than a zero-stress temperature at which the bimetal strip assumes an a fully relaxed position. The bimetal strip will deform from its relaxed position at a temperature higher or lower than the zero-stress temperature.
In some embodiments one or more of the layers also have a thermal conductivity of about 200 W/mK or greater. Thus, when actuated the active element 220 may increase the convective heat transfer surface area of the heat sink in addition to diverting the flow of air. The layers may be metal layers bonded together by, e.g., cold rolling or electroplating. The metals may include those in the group copper, aluminum, copper, silver, and gold. In some cases, a material with a relatively poor thermal conductivity may be used, such as the metals titanium, steel or nickel, or even some polymers such as Kapton H®, manufactured by E.I. du Pont de Nemours and Co., Circleville, Ohio, USA. It may be desirable in such cases to minimize the thickness of the poorly conducting material so that the resulting active element has sufficient thermal conductivity to increase the convective surface are of the heat sink. In other cases, thermal conductivity may be less important, such as when the active elements 220 are used to obstruct air flow. In such cases, constraints on the thermal conductivity of the materials used to construct the active elements 220 may be relaxed.
In some embodiments, the active element 220 includes a copper layer and an aluminum layer. Copper has a CTE of about 17 E-6° C.−1, while aluminum has a CTE of about 24 E-6° C−1. Rolling of copper and aluminum to form a bimetal sheet is well known. The active element 220 may be cut or stamped from a formed sheet. The active element 220 may then be bonded to the fin 210 by soldering, welding, or bonding with an adhesive. When copper and aluminum are used, the active element 220 could be configured such that the first metal layer 230 is aluminum and the second metal layer 240 is copper so that the active element 220 deforms away from the surface of the fin 210 when actuated.
In some cases, the active element 220 may be configured to move into an air stream when the temperature of the bimetal strip exceeds the zero-stress temperature. Such a configuration may be desirable, e.g., to reconfigure air flow through a heat sink to provide greater air flow to another portion to accommodate a transient heat output. For example, the bimetal strip may be attached to a heat sink fin or base in such a manner that below the zero-stress temperature the bimetal strip remains flat against the surface it is attached to.
In other cases, the active element may be configured to project into an air stream when the active element is at its zero-stress temperature. The layers 230, 240 may be configured such that when the temperature of the active element 220 exceeds the zero-stress temperature, the active element 220 moves toward the surface of the fin 210. Such a configuration may be advantageous, e.g., to reduce air flow resistance in a path of the heat sink 200 as the temperature of that portion of the heat sink increases.
It should be noted that the active element 220 may be attached to a surface of a heat sink base with results that are similarly advantageous to those configurations in which the active element 220 is attached to heat sink fins. Moreover, the beneficial effects of the use of active elements may be realized when used with unconventional heat sink designs such as, e.g., those disclosed in U.S. patent application Ser. No. ______ (Hernon 3).
Movement of the active element 220 alters the heat transfer characteristics of the heat sink. For instance, diverting the flow 250 from a path parallel to the fin 210 may increase exchange of heat between the fin 210 and the air flow. Without limitation by theory, diversion of air flow may disrupt a boundary layer of air adjacent to the fin 210. When a fluid flows over a surface, a boundary layer of some thickness is formed. The boundary layer may have, depending on several factors, laminar, turbulent or transitional characteristics. In general, the laminar boundary layer is relatively less effective removing heat from the surface than other flow regimes. The thickness of the boundary layer is also thought to increase as distance of flow along the free-stream flow increases. It is therefore generally beneficial to reduce the thickness of the boundary layer to improve the rate of heat flow from the heat sink to the cooling fluid.
It is thought that the thickness of the boundary layer may be reduced by impinging a free-stream turbulent flow, e.g., the diverted flow 250, onto the laminar boundary layer. Such a reduction may be caused by generating any flow which sets up secondary flows normal to the surface that, e.g., compresses or thins the boundary layer to increase the heat transfer. Examples of these flow types include vortices and eddies, and transitional, turbulent, unstable, chaotic or resonant air flow. These aspects are discussed in greater detail in U.S. patent application Ser. No. ______ (Hernon 2).
Moving to
However, within the perimeter of the window 330, the CTE mismatch between the first outer layer 317 and the inner layer 315 causes the active element 320 to deflect when the temperature of the active element 320 changes. Again, the position of the active element 320 at a particular temperature is determined in part by the zero-stress temperature of the active element 320. When the CTE of the inner layer 315 is greater than that of the first outer layer 317, the active element 320 deflects in the direction of the first outer layer 317, as illustrated in
When the active element 320 is not coplanar with the fin 300, as illustrated in
Turning to
The active element 420 may be formed by conventional methods. In a nonlimiting example, a sacrificial layer is formed over the dielectric layer 430. A portion of the sacrificial layer is removed to expose the dielectric layer 430. A metal layer is deposited onto the sacrificial layer and the exposed dielectric layer 430. The metal layer is selectively removed to leave the active element 420. The sacrificial layer is then removed. The portion of the active element 420 deposited onto the dielectric layer 430 adheres thereto, while the portion deposited onto the sacrificial layer is free to move.
A voltage potential may be placed on the active element 420 via a control line 440 formed over the dielectric layer 430. The control line 440 may be connected to a control system that may actively control actuation of the active element 420 in response to temperature of an electronic device connected to the heat sink. An active element that is actuated in response to an external command is referred to herein as a commanded element. In another embodiment, the control system senses temperature on the electronic device or in one or more regions of the heat sink via, e.g. one or more thermocouples or thermistors, and actuates one or more active elements 420 in response to the measured temperature. In this manner, e.g., active elements 420 in the heat sink may be selectively actuated to enhance heat flow in a portion of heat sink smaller than the entire heat sink.
As is known to those skilled in the pertinent art, a MEMS device is typically formed using conventional and specialized semiconductor processing to form moving parts on a semiconductor substrate such as silicon. The moving parts may be integrated with control logic or other electronics on the substrate, and may be actuated using electrostatic fields, e.g. In some MEMS devices, such as micro-mirrors, a planar feature is attached to torsion springs so that the planar feature may be displaced from a rest position when actuated, and may then return to the rest position when not actuated. Actuation may be static, e.g., between two or more equilibrium positions, or dynamic, e.g., continuous motion between two limits at a frequency ranging from hertz to kilohertz or higher.
The active element 520 may be actuated, as illustrated in
Turning now to
Turning now to
It should be noted that in general active elements will increase back pressure through a heat sink when the active elements project into an air stream. The increased back pressure has implications in systems design issues, as greater fan capacity may be necessary, with resulting greater power consumption and system heat dissipation.
Turning now to
Now turning to
The inlet channels 1410 are configured to draw bypass air (or other cooling fluid) from an upstream location of the heat sink 1400. In many cases, the air will be cooler at the upstream portion than at a downstream location. The cooler air is directed by the inlet channels to the manifold 1120 and through the output channels 1430. The outlets 1435 are configured to output the bypass air at a location of the heat sink 1400 downstream of the inlets 1415.
The intake channels 1410 and output channels 1430 may be internal to the fins 1405 and the base 1407, may be external thereto, or may be partially internal and partially external. In an example embodiment, the intake channels 1410 and output channels 1430 are formed as integral structures of the heat sink 1400 by investment casting, as described in application Ser. No. ______ (Hernon 3). The channels 1410, 1430 may be thereby formed as passages wholly within the heat sink fins 1405 and/or the base 1407. Alternatively or in combination to internal passages, conduits along the surface, e.g., of the heat sink may be used to route air to and from the manifold 1420. When dimensions of the heat sink 1400 allow, the conduits may be formed separately of, e.g., tubing, and attached to the heat sink in the desired configuration.
In some cases, the bypass air will be cooler than the air traversing the path between the heat sink fins 1405. When the air is output at the outlets 1435, the cooler air may mix with the air stream, thereby cooling the air stream to increase heat transfer from the fins 1405 in the vicinity of the outlets 1435. Even when the air is not cooler than the air stream, the air output at the outlets 1435 may disrupt boundary layer flow between the fins 1405. Because boundary layers generally insulate the fins 1405, disruption of the boundary layers may increase heat transfer.
The active element 1440 may be any movable element configured to produce a pressure differential in the manifold 1420 that causes air to flow from the inlets 1415 to the outlets 1435. In one embodiment, the active element 1440 is a synthetic jet device. Synthetic jets are familiar to those skilled in the pertinent art, and may include, e.g., a membrane or a diaphragm configured to move air from one portion of the device to another portion of the device. The membrane or diaphragm may be driven, e.g., electromagnetically or piezoelectrically. Such a jet may be manufactured in a compact form that may be integrated within the base, e.g., of the heat sink 1400.
Each of the various embodiments presented may be used singly or in combination with other embodiments as part of a heat sink design. Thus active elements may be combined in one heat sink that have, e.g., different active element sizes, orientations, and actuation temperatures. Furthermore, some active elements may be uncommanded, and some may be commanded. Some active elements may be configured to alter air flow through the heat sink. For example, the active elements may induce unsteady, unsteady laminar, transitional, turbulent, unstable, or resonant air flow. Such air flow may, e.g., reduce a boundary layer thickness. In other cases, active elements may be configured to divert air flow from one portion of the heat sink to another.
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 surface; and
- a first active element connected to said surface and configured to move from a first position relative to said surface to a second position relative to said surface, said movement altering the heat transfer characteristics of the heat sink.
2. The heat sink as recited in claim 1, wherein said active element is configured to move in response to a signal from a control system.
3. The heat sink as recited in claim 1, wherein said active element is configured to move in response to a change of temperature of said active element.
4. The heat sink as recited in claim 1, wherein said active element comprises a first layer and a second layer, said first layer having a coefficient of thermal expansion (CTE) different than a CTE of said second layer.
5. The heat sink as recited in claim 1, wherein said active element is an active element of a micro-electrical-mechanical system (MEMS).
6. The heat sink as recited in claim 1, further comprising a second active element, wherein said first active element and second active element are configured to form a closed channel.
7. The heat sink as recited in claim 1, wherein said first active element is configurable to redirect flow of a cooling fluid from one portion of said heat sink to another portion of said heat sink.
8. The heat sink as recited in claim 1, further comprising a conduit or channel configured to transport a cooling fluid from one location of said heat sink to another location of said heat sink.
9. A method, comprising:
- providing a heat sink having a surface; and
- forming a first active element on said surface and configuring said active element to move from a first position relative to said surface to a second position relative to said surface different from said first position, said movement altering the heat transfer characteristics of the heat sink.
10. The method as recited in claim 9, wherein said active element is configured to move in response to a signal from a control system.
11. The method as recited in claim 9, further comprising configuring said active element to move in response to a change of temperature of said active element.
12. The method as recited in claim 9, wherein said active element comprises a first layer and a second layer, said first layer having a coefficient of thermal expansion (CTE) different than a CTE of said second layer.
13. The method as recited in claim 9, wherein said active element is an active element of a micro-electrical-mechanical system (MEMS).
14. The method as recited in claim 9, further comprising forming a second active element on said surface and configuring said first and second active elements to form a closed channel.
15. The method as recited in claim 9, further comprising configuring said first active element to redirect flow of a cooling fluid from one portion of said heat sink to another portion of said heat sink.
16. The method as recited in claim 9, further comprising forming a conduit or channel configured to transport a cooling fluid from one location of said heat sink to another location of said heat sink.
17. A system, comprising:
- a device configured to produce heat; and
- a first heat sink having a surface in thermal contact with said device and comprising an active element connected to said surface and configured to move from a first position relative to said surface to a second position relative to said surface, wherein said active element is configurable to change a direction of flow of a cooling fluid in response to said heat produced by said device.
18. The system as recited in claim 17, further comprising a second heat sink having an active element configured to change an air flow through said first heat sink.
19. The system as recited in claim 17, further comprising a control system configured to command movement of said active element.
20. The system as recited in claim 17, wherein said active element is a movable element of a micro-electrical-mechanical system (MEMS).
21. The system as recited in claim 17, wherein said active element is configured to draw fluid into an inlet at an upstream location of said heat sink and to output said fluid to an outlet downstream of said inlet.
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: 12/165,063
International Classification: F28F 7/00 (20060101);