U.S. patent application number 12/165225 was filed with the patent office on 2009-12-31 for monolithic structurally complex heat sink designs.
This patent application is currently assigned to Alcatel-Lucent Technologies Inc.. Invention is credited to Domhnaill Hernon, Marc Hodes, Shankar Krishnan, Alan Lyons, Alan O'Loughlin.
Application Number | 20090321045 12/165225 |
Document ID | / |
Family ID | 41446002 |
Filed Date | 2009-12-31 |
United States Patent
Application |
20090321045 |
Kind Code |
A1 |
Hernon; Domhnaill ; et
al. |
December 31, 2009 |
MONOLITHIC STRUCTURALLY COMPLEX HEAT SINK DESIGNS
Abstract
A heat sink includes a base and a heat exchange element
monolithically connected to the base. The heat exchange element has
a surface that at least partially bounds first and second paths
through the heat exchange element. The surface forms an upper
boundary of the first and second paths and includes an opening
therethrough connecting the first and second paths.
Inventors: |
Hernon; Domhnaill; (Meath,
IE) ; Hodes; Marc; (Dublin, IE) ; Lyons;
Alan; (New Providence, NJ) ; O'Loughlin; Alan;
(Dubllin, IE) ; Krishnan; Shankar; (Richland,
WA) |
Correspondence
Address: |
HITT GAINES, PC;ALCATEL-LUCENT
PO BOX 832570
RICHARDSON
TX
75083
US
|
Assignee: |
Alcatel-Lucent Technologies
Inc.
Murray Hill
NJ
|
Family ID: |
41446002 |
Appl. No.: |
12/165225 |
Filed: |
June 30, 2008 |
Current U.S.
Class: |
165/80.2 |
Current CPC
Class: |
B33Y 80/00 20141201;
F28F 13/003 20130101; Y10T 29/4935 20150115; H01L 23/467 20130101;
H01L 23/3672 20130101; H01L 2924/0002 20130101; H01L 23/367
20130101; F28F 2255/00 20130101; H01L 2924/0002 20130101; B22D
25/02 20130101; H01L 2924/00 20130101; H05K 7/20009 20130101 |
Class at
Publication: |
165/80.2 |
International
Class: |
F28F 7/00 20060101
F28F007/00 |
Claims
1. A heat sink, comprising: a base; and a heat exchange element
monolithically connected to said base and having a surface that at
least partially bounds first and second paths through said heat
exchange element, wherein said surface forms an upper boundary of
said first and second paths and includes an opening therethrough
connecting said first and second paths.
2. The heat sink as recited in claim 1, wherein said first and
second paths are unobstructed paths about parallel to said
base.
3. The heat sink as recited in claim 1, wherein said heat exchange
element is a portion of a foam structure.
4. The heat sink as recited in claim 1, wherein said heat exchange
element defines a closed channel that is about parallel to said
base and has a closed circular or polygonal cross-section.
5. The heat sink as recited in claim 4, wherein said polygonal
cross-section is a hexagon.
6. The heat sink as recited in claim 1, wherein said heat exchange
element divides space into two congruent labyrinths.
7. The heat sink as recited in claim 6, wherein said heat exchange
element forms a minimum area surface.
8. The heat sink as recited in claim 1, wherein said heat exchange
element includes a re-entrant void.
9. The heat sink as recited in claim 1, wherein a width of said
path varies along said path.
10. The heat sink as recited in claim 1 formed by a process
comprising the steps of: forming a sacrificial pattern of said heat
sink using stereolithography, and providing said pattern to an
investment casting process to form a heat sink.
11. A method comprising: providing a sacrificial heat sink pattern
comprising: a base form; and a heat exchange element form connected
to said base form and having a surface that at least partially
bounds first and second paths through said heat sink pattern,
wherein said surface forms an upper boundary of said first and
second paths and includes an opening therethrough connecting said
first and said second paths; and providing said pattern to an
investment casting process to form a monolithic heat sink.
12. The method as recited in claim 11, wherein said first and
second paths are unobstructed paths about parallel to said base
form.
13. The method as recited in claim 11, wherein said heat exchange
element form is a portion of a form of a foam structure.
14. The method as recited in claim 11, wherein said heat exchange
element form defines a closed channel that is about parallel to
said base form and has a closed circular or polygonal
cross-section.
15. The method as recited in claim 14, wherein said polygonal
cross-section is a hexagon.
16. The method as recited in claim 11, wherein said heat exchange
element form divides space into two congruent labyrinths.
17. The method as recited in claim 16, wherein said heat exchange
element form is a minimum area surface.
18. The method as recited in claim 11, wherein said heat exchange
element form includes a re-entrant void.
19. The method as recited in claim 11, wherein a width of said path
varies along said path.
20. The method as recited in claim 11, further comprising the step
of forming said heat sink using stereolithography.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] 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 "Flow Diverters to Enhance Heat Sink Performance,"
both of which are hereby incorporated by reference as if reproduced
herein in their entirety.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention is directed, in general, to heat
sinks.
BACKGROUND OF THE INVENTION
[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 a cooling medium, e.g., air.
Various manufacturing methods are used, including extrusion,
machining and die-casting. These methods are suitable for
relatively simple heat sinks. But more complex structures are
needed to improve the performance of heat sinks. Traditional
methods of manufacturing heat sinks are not suited to making such
complex structures.
SUMMARY OF THE INVENTION
[0004] One embodiment is a heat sink that includes a base and a
heat exchange element monolithically connected to the base. The
heat exchange element has a surface that at least partially bounds
first and second paths through the heat exchange element. The
surface forms an upper boundary of the first and second paths and
includes an opening therethrough connecting the first and second
paths.
[0005] Another embodiment is a method that includes providing a
sacrificial heat sink pattern comprising a base form and a heat
exchange element form connected to the base form. The heat exchange
element form has a surface that at least partially bounds first and
second paths through the heat sink pattern. The surface forms an
upper boundary of the first and second paths and includes an
opening therethrough connecting the first and the second paths. The
pattern is provided to an investment casting process to form a
monolithic heat sink.
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. 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 elements of heat sinks in accordance with
the invention;
[0009] FIG. 3 illustrates a method;
[0010] FIG. 4 illustrates a periodic fin-foam heat sink;
[0011] FIG. 5A illustrates a minimum-surface structure heat
sink;
[0012] FIG. 5B illustrates a path with varying cross-sectional
area;
[0013] FIG. 6 illustrates a slotted honeycomb heat sink;
[0014] FIGS. 7A, 7B and 7C respectively illustrate elements of the
embodiments of FIGS. 4, 5A and 6; and
[0015] FIG. 8 illustrates performance characteristics of heat
sinks.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0016] Embodiments described herein reflect the recognition that
three dimensional (3-D) rendering and investment casting may be
employed to manufacture monolithic heat sinks with structural
complexity unattainable by prior art methods. Such complexity in a
monolithic heat sink design provides a means to form heat sinks
with novel structural features to improve the performance of such
heat sinks over prior art heat sinks. The described embodiments
make structural elements available to heat sink designers hitherto
unattainable. The availability of these elements provides the
designer with the ability to take greater advantage of flow
mechanics and heat dissipation physics than with "simple" heat
sinks, defined below. Embodiments are described herein that result
in a significant improvement of heat transfer characteristics of a
structurally complex heat sink relative to simple heat sinks.
[0017] The present discussion introduces the concept of using 3-D
printing of a sacrificial pattern and subsequent investment casting
to form a heat sink in which heat exchange elements can be
monolithically attached to a base of the heat sink. As used herein,
monolithic is defined with respect to an element of a heat sink to
mean that the element and base are a single, continuous entity. In
other words, the element and the base are portions of a single,
cast unit, and are not fastened to the remaining portion by
adhesive, screws, welds, crimps, or any similar chemical or
mechanical means. However, a heat exchange element and base are
still monolithically connected if they are polycrystalline, if any
of these fastening means are used to attach another element to the
monolithic portion or to attach the heat sink to a circuit or
assembly.
[0018] A typical 3-D printer uses a laser and a liquid photopolymer
to produce a 3-D form by a succession of solid layers. An example
is a stereolithography rapid prototyping system. Those skilled in
the pertinent art are familiar with such systems and the
photopolymers used in them. For example, one type of printer uses
the laser to produce a solid pattern in a thin layer of liquid
photopolymer on a translatable stage. The stage is advanced and
another layer is formed on the first layer. By a succession of
layers, a 3-D form of an object of almost arbitrary complexity may
be formed with potential resolution of features on the order of 100
.mu.m. In some systems, a wax or soluble photopolymer is also used
to mechanically support fragile portions of the 3-D form. The 3-D
form may be used directly as a pattern in a conventional investment
casting process described further below.
[0019] Heat sinks formed using patterns generated by 3-D printing
are referred to herein as "structurally complex" heat sinks to
reflect the potential for structural complexity. It is understood,
however, that the presence of specific physical features is not a
prerequisite to including a heat sink in the class of complex heat
sinks defined here.
[0020] 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 are
structurally uniform, e.g., there are no projections from or
depressions in the surface of the fins 120 other than surface
roughness typical of the particular manufacturing method. The heat
sink 100 is representative of the class of heat sinks formed by
conventional methods including extrusion, sand-casting,
die-casting, bonding, folding, forging, skiving and machining of
metal blocks or sacrificial forms. Machining is defined as the
removal of material from a block by mechanical means. The maximum
aspect ratio of the fins, i.e., the ratio of the fin height H to
fin thickness T, is typically limited to a range of about 8:1 to
about 20:1, depending on the manufacturing method. Heat sinks in
this class are defined herein as "simple" heat sinks, and are
expressly disclaimed.
[0021] FIG. 2 illustrates various structural features of a
structurally complex heat sink 200 that may be formed using 3-D
printing and casting. Coordinate axes are shown for reference in
the following discussion. A base 205 provides a foundation for
various illustrated heat exchange elements. The base is shown as
planar, but may be any desired shape. For example, a base may be
formed in a shape that conforms to underlying topography of a
circuit board or electronic device. Several examples of heat
exchange elements are illustrated in FIG. 2. It is noted that these
examples are not exclusive, and that the heat sink 200 may include
each type of element alone or in combination with other
elements.
[0022] A fin 210 is a rectangular solid element projecting from the
base 205. The fin may have a conventional aspect ratio, (the ratio
of the height to the thickness) less than about 20:1, or may have a
greater aspect ratio. The fin 210 may include a coolant channel 215
through which a coolant such as, e.g., water or air, may be
circulated to augment heat transfer from the fin to, e.g., an air
stream adjacent to the fin 210. The coolant channel may be routed
in a manner not achievable by prior art methods of forming heat
sinks, e.g., in an arbitrary path in the X-Z plane. Such channels
may also be provided in the base 205 if desired. The aspect ratio
of the fin 210 may be limited by such factors as, e.g., material
strength, ability to fill high aspect ratio voids during casting,
and mechanical strength required of the fins to withstand loads
during service. It is conservatively estimated that fins may be
constructed with an aspect ratio exceeding 100:1.
[0023] A fin 230 includes bends 235 formed in the Y-Z plane. Such
bends may be desirable to, e.g., increase fin surface area without
increasing fin height above the base 205. Depending on complexity,
the bends 235 may be difficult to manufacture by the aforementioned
methods, especially if combined with other features illustrated in
FIG. 2. For example, bends may be formed in both the Y-Z and the
X-Y planes. The conventional manufacturing methods are not amenable
to such structurally complex features.
[0024] In another embodiment, a fin 240 includes an extension 245.
The extension 245 may be thin in the X-direction, in which case the
minimum thickness will depend on factors including the material
used for the heat sink. The thickness in the X-direction may range
from this minimum to greater than the full length of the fin 240 in
the X-direction. The thickness in the X-direction may exceed the
length of the fin 240 when, e.g., the extension 245 forms a portion
of a vortex generator placed upwind of the heat sink 200. See,
e.g., U.S. patent application Ser. No. ______ . (Hernon 2) The
height of the extension 245 in the Z-direction may range from a
minimum formable thickness to greater than the height of the fin
240. In some embodiments, the extension forms a flat plate, e.g., a
thin planar feature projecting from the fin 240 into an air stream
flowing past the fin 240. The extension 245 configured in this way
may be, e.g., a flow diverter as described in the ______
application (Hernon 2). In other embodiments, the extension forms a
bump, which may be circular, elliptical, or pyramidal, e.g.
[0025] A fin 250 includes a depression 255. The depression 255 may
be, e.g., a dimple having circular or elliptical cross-section in
the X-Z plane. The profile of the depression 255 in the Y-Z plane
may be any desired profile, such as, e.g., circular (as
illustrated), triangular, square, or even a re-entrant cavity. As
was described for the extension 245, the depression 255 may also
extend in the X-direction the entire length of the fin 250, or in
the Z-direction for the entire height of the fin 250.
[0026] A fin 260 includes an opening 265. The opening 265
intersects both opposing surfaces of the fin 260. The opening 265
may be any desired shape, e.g., circular, triangular, square or
hexagonal, and the fin 260 may include any desired number of
openings 265. Of course, the configuration of openings 265 may be
constrained by the mechanical strength of the material used, the
fin thickness, and the service environment to preserve the physical
integrity of the fin 265.
[0027] Fins 270 include bridging elements 272, 274, 276. Such
bridging elements may be oriented such that a major surface is
oriented, e.g., in the Y-Z plane, such as bridging element 272, or
in the X-Y plane, such as bridging element 274. Bridging features
may also include openings, such as bridging element 276. Bridging
elements may also be used to form ducts to direct air from one
portion of the heat sink to another. See, e.g., U.S. patent
application Ser. No. ______ (Hernon 3).
[0028] Fin 280 includes re-entrant voids 285. The voids 285 have a
concave volume accessible only through an opening that is smaller
than the largest cross-sectional area of the void. Such features
provide a means to significantly increase the surface area of the
fin 280 to reduce thermal resistance between the fin 280 and the
ambient. Novel heat sink structures such as a minimum area surface
may also be produced, as described below.
[0029] In some cases, fins are not even used. Honeycomb channels
290 are one such heat exchange element. In this embodiment,
channels 295 formed by the honeycomb run parallel to each other and
to the base 205. The channels 295 are closed channels, meaning the
cross-section of each channel is a closed polygon at some point
along the channel. The walls of the channels 295 may include other
features already described, including, e.g., openings 297,
extensions and depressions. As the term "closed channel" is used
herein, a channel may include openings such as the openings 297 in
the channel walls and still be considered closed.
[0030] The foregoing physical features are not exhaustive of the
possible features that may be formed by the described method.
Moreover, the elements described may be combined in innovative ways
to achieve heat transfer characteristics hitherto unobtainable. The
advantages provided by the possible combinations of elements are
extended by the fact that these elements are integral to the
monolithic heat sink 200. Thus the elements are not partially
insulated from the heat sink by thermal grease or an adhesive
material, and thermal conductivity throughout the heat sink is
improved. Moreover, the homogeneous thermal conductivity of the
heat sink may provide a more consistent environment for modeling of
the thermal performance of the heat sink, easing the design burden.
The advantages of forming an element and a base as a monolithic
structure are not lost if additional structural elements are
attached to the heat sink in a non-monolithic manner.
[0031] Heat sinks formed by the described embodiments are intended
for applications in which machining of features of a complex heat
sink are impractical, uneconomical or impossible. As such, the
target applications are limited to those in which physical
dimensions of features of the heat sink are below a size for which
machining may be economically and practically used. Certainly,
machining of features on surfaces of a heat sink separated by 1 mm
or less is considered impractical, uneconomical or impossible. Such
machining when surfaces are separated by 5 mm would still be
considered at least impractical or uneconomical, and may be
infeasible. Above 1 cm, machining might be feasible, even if at
great expense, in the most demanding applications. Accordingly,
heat sinks are expressly disclaimed that have opposing surfaces
separated by more than about 1 cm.
[0032] FIG. 3 illustrates a method 300 for forming a structurally
complex heat sink. In a step 310, a designer reduces a concept to a
design. The heat sink may be designed in any manner amenable to
later transfer of design data to a 3-D rendering system. One
particularly useful technique includes the use of a 3-D
computer-aided design and manufacturing (CAD/CAM) system to define
the structure of the structurally complex heat sink. Data provided
by the CAD/CAM system may be provided directly to a 3-D rendering
system in a step 320. The data may also be advantageously provided
to a thermal modeling system to predict and optimize the
performance of the heat sink design under various conditions such
as air speed, thermal load and maximum heat flux. While thermal
modeling may be advantageous during the design phase of the heat
sink, it should be understood that the method 300 does not require
such modeling.
[0033] In the step 320, the design resulting from the step 310 is
rendered as a heat sink form in a sacrificial material. The
material may be, e.g., a photopolymer used in a stereolithography
rapid prototyping system. A base form and a heat exchange form may
be produced as a monolithic pattern. The resulting pattern may be
of almost arbitrary complexity. In those cases which a single
pattern cannot capture a desired design two or more forms may be
joined to produce the final desired pattern.
[0034] In a step 330, the heat sink is rendered in a desired metal
using the pattern produced in the step 320 as a sacrificial form in
an investment casting process. Those skilled in the art of
investment casting are familiar with various methods of investment
casting. In a preferred embodiment, a phosphoric acid bonded
plaster casting method is used.
[0035] In a step 340, the heat sink is integrated into a system,
such as an electronic assembly. In some cases, the heat sink is
joined to an electronic component, e.g., an integrated circuit such
as a microprocessor or power amplifier, an optical amplifier, or
similar heat-dissipating device. In some cases, the heat sink could
be attached to the cold side of a thermo-electric device when the
warm side is used to heat a device. Thermal grease or a heat
conducting pad may be used to improve thermal conduction between
the device package and the heat sink. In other cases, cooling lines
may be attached to the heat sink when liquid coolant channels such
as the coolant channel 245 are provided in the heat sink.
[0036] The following embodiments are non-limiting applications of
the described method of forming a monolithic heat sink. These
applications illustrate the use of various structural features
previously described and illustrated in FIG. 2. It is understood,
however, that any heat sink design not otherwise disclaimed and
including structural features such as those illustrated in FIG. 2
and formed by the described method are within the scope of this
disclosure.
[0037] Turning to FIG. 4, illustrated is an embodiment of a
fin-foam heat sink 400. The fin-foam heat sink 400 includes
vertical fins 410 and a foam structure 420 on a base 430. The foam
structure 420 is a structurally complex assemblage of heat transfer
elements having a porous structure that fills space in a heat sink.
When a foam structure is combined with heat sink fins, the combined
structure is referred to as a fin-foam.
[0038] In some cases, the foam structures are unstructured
(pseudo-random). In other cases, the foam structures have one or
more heat transfer elements configured in unit cells with two- or
three-dimensional periodicity. In FIG. 4, e.g., X-Y elements 440
have a major surface that is about parallel to the X-Y plane as
denoted by the XYZ coordinate reference, and Y-Z elements 450 have
a major surface that is about parallel to the Y-Z plane. A unit
cell 460 in this nonlimiting example includes one Y-Z element and
two X-Z elements.
[0039] The heat transfer elements are configured to provide a path
470 for air flow through the heat sink 400. In some cases, the path
470 is an unobstructed path, meaning that the path 470 provides a
straight-line route for air flow through the heat sink 400 that may
additionally be parallel to the base 430. In other cases, the path
470 is a tortuous path, meaning that a route of air flow through
the heat sink 400 includes bends. The mean path of the tortuous
path is about parallel to the base 430. A particular heat sink
design, such as the illustrated fin-foam design, may include a
combination of unobstructed and tortuous paths.
[0040] In the fin-foam heat sink 400 the distance between the
vertical fins 410 is equal to the unit cell width, but in other
embodiments, the unit cell width may be smaller than this distance.
For example, the space between the fins 410 may include two or more
unit cells. In some embodiments the fins 410 are omitted
completely, so the heat sink consists only of the foam structure
420 on the base 430. When a periodic foam structure is desired, the
foam structures may be generated with body-centered cubic (BCC),
face-centered cubic (FCC), A15 lattice arrangements, e.g., or any
other desired lattice arrangement. The foam may include fractal
geometries, or plates or spikes projecting from a horizontal or
vertical plate to increase surface area for heat exchange.
[0041] The foam structures may also be designed to produce
beneficial flow characteristics downstream of the foam voids within
the fin passages. Such structures may be configured to produce,
e.g., flow instabilities, unsteady laminar, transitional,
turbulent, chaotic and resonant flows that increase heat transfer
between the fin-foam heat sink 400 and the ambient. See, e.g., U.S.
patent application Ser. No. ______ . (Hernon 2)
[0042] The fins 410 and the foam structure 420 may be formed as a
single, monolithic cast structure by the casting process described
above. Such a design provides a significant advantage over a heat
sink assembled from separate subassemblies in that there are no
thermal resistance penalties associated with having extra thermal
barriers due to adhesives, e.g. The fin-foam embodiment results in
a significant increase of the surface area available for heat
transfer to or from the fin-foam heat sink 400 compared to a simple
heat sink design. For example, the surface area available for heat
transfer on the fin-foam heat sink 400 is approximately 15% greater
than the surface area of a parallel fin heat sink with identical
length, height and width dimensions.
[0043] Turning now to FIG. 5, illustrated is an embodiment of a
heat sink element 500 having only one interior surface 510 and one
exterior surface 520. The illustrated embodiment is referred to as
a Schwarz' P surface, and is characterized by smoothly varying
curvature of the surfaces. Formally, the Schwarz' P structure is
characterized by having zero mean curvature, and is sometimes
referred to as a "minimum-surface" structure. Of course, other
structures besides a Schwarz' P structure may be used, need not be
area-minimizing, and may include flat or angular features.
[0044] The element 500 may comprise any shape or size of unit cell
that includes an interior and an exterior volume separated by a
continuously-connected surface, e.g., the Schwarz' P structure. The
element 500 divides space into two congruent labyrinths. The
element 500 also provides an unobstructed path 530. In some
embodiments, the internal flow within the element 500 is disrupted
by general instability through separation effects or simple
acceleration and deceleration effects due to changes of
cross-sectional area within the internal flow passages. Also, the
unit cell need not be symmetric, but may be an arbitrary array of
structures that may, e.g., sustain self-oscillations of flow.
[0045] The interior surface 510 defines an interior region and the
exterior surface 520 defines an exterior region. The element 500
may be used in forced-air applications, in which case air flows
over both the interior and exterior surfaces for cooling. In other
cases, the element 500 may be used in liquid-cooled applications,
in which a liquid coolant is caused to flow through the inner
region. If desired, one or more caps 540 may be used to direct or
limit fluid flow. The cap 540 may be, e.g., an active element as
disclosed in U.S. patent application Ser. No. ______ (Hernon 1). In
one embodiment, more air or cooling fluid may be directed to a
portion of the element near an area of an electronic device
dissipating greater power than other areas of the device. Varying
the minimum or maximum diameter of passages through the element 500
may also be employed to preferentially direct the flow of air or a
liquid.
[0046] Turning to FIG. 5B, a path 550 of a cooling fluid such as
air through a channel cross-section 560 is illustrated. The
nonlimiting case of the Schwartz' P structure is illustrated as an
example. One aspect of such structures is that the width of a
channel through which a fluid moves through the heat sink varies
along the path of flow. In some embodiments, the structure is
configured to be conducive to self-sustaining flow oscillations in
the laminar flow regime. Such oscillations may be used to enhance
heat transfer without large increases in flow resistance. Such
structures may also trigger instabilities such as
Tollmien-Schlichting waves or Kelvin-Helmholtz instabilities, or
may trigger transition to turbulence.
[0047] Turning now to FIG. 6, illustrated is an embodiment of a
monolithic heat sink element 600. The element 600 may be used,
e.g., as a finless heat sink, or as a heat transfer element between
fins (not shown). The element 600 includes a base 610, parallel
channels 620 and openings 630. The channels 620 have a hexagonal
cross-section, and collectively form a honeycomb-like pattern.
Other shapes that form a closed polygonal cross-section may also be
used, e.g., square, triangular or circular channels. The parallel
channels 620 provide unobstructed paths through the heat sink
element 600.
[0048] The openings 630 may be, e.g., offset (staggered)
rectangular or circular, or they may be otherwise positioned along
the length of the channels 620 in a manner beneficial to the heat
transfer and pressure characteristics of the element 600. It is
thought that in some cases the openings 630 may improve convection
or air flow away from the base 610. In some cases, the openings 630
may reduce thermal resistance between the heat sink and a cooling
fluid by restarting a boundary layer region adjacent to the walls
of the channels 620. The boundary layer is a region of relatively
static air adjacent the channel wall that acts as a thermal
insulator. Restarting the boundary layer may cause free-stream air
to flow closer to the channel wall thereby increasing heat
transfer. Complex geometries such as those shown in FIG. 6 that
result in such flow effects are not achievable at the scale of
component heat sinks using the conventional processes described
previously.
[0049] FIG. 7 illustrates a geometrical features shared by the
described embodiments. FIG. 7A shows a detail 710 of the foam
structure 420. FIG. 7B shows a detail 735 of the Schwartz' P
structure of the heat sink element 500. FIG. 7C shows a detail 735
of the channel 620 of the heat sink element 600. Each detail 710,
735, 755 has a surface that at least partially bounds adjacent
paths through the respective heat exchange element. A surface of a
heat sink includes all surface area thereof, whether contiguous or
non-contiguous.
[0050] Focusing first on the detail 710, the underside of a foam
element 715 is a surface that partially bounds and forms an upper
boundary of a path 720 through the foam structure 420. The
underside of a foam element 725 is a surface that partially bounds
and forms an upper boundary of a path 730 through the foam
structure 420 that is adjacent to the path 720. An opening, hidden
from view, connects the path 720 and the path 730. With respect to
the detail 735, the underside of a portion 740 of the heat sink
element 500 is a surface that partially bounds and forms an upper
boundary of a path 745 and a path 750 through the heat sink element
500. A neck region 752 forms an opening between the path 745 and
the path 750. With respect to the detail 755, the underside of a
portion 760 of the heat sink element 600 is a surface that
partially bounds and forms an upper boundary of a path 765. The
underside of a portion 770 of the heat sink element 600 is a
surface that partially bounds and forms an upper boundary of a path
775. An opening 780 connects the path 760 and the path 765.
[0051] Turning to FIG. 8, illustrated is a graph comparing the
experimental performance of a honeycomb heat sink, such as the heat
sink 600, and a fin-foam heat sink, such as the heat sink 400 with
a standard finned heat sink such as the heat sink 100. The
performance curves show thermal resistance of the three cases as a
function of air velocity directly upstream of the heat sinks. The
heat sinks are controlled for heat sink width, height, length and
heat sink base. All designs are placed in fully ducted flow, so
that velocity through each heat sink is constant.
[0052] For the configurations tested, both the fin-foam and the
honeycomb heat sinks outperform the finned heat sink, and the
fin-foam heat sink outperforms the honeycomb design. While specific
heat sink performance will depend on many factors, the performance
characteristics clearly illustrate the potential benefit of the
fin-foam design and the slotted honeycomb design over the
traditional finned heat sink. This improvement over simple heat
sinks is unexpectedly large. The magnitude of the improvement makes
it possible to extend the use of air-cooled heat sinks to high
power-dissipating electronic components that would otherwise
require more expensive means of cooling, such as liquid
cooling.
[0053] 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.
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