U.S. patent application number 13/640758 was filed with the patent office on 2013-05-30 for multiple thermal circuit heat spreader.
This patent application is currently assigned to THERMAVANT TECHNOLOGIES LLC. The applicant listed for this patent is Joseph A. Boswell, Peng Cheng, Hongbin Ma. Invention is credited to Joseph A. Boswell, Peng Cheng, Hongbin Ma.
Application Number | 20130133871 13/640758 |
Document ID | / |
Family ID | 44798998 |
Filed Date | 2013-05-30 |
United States Patent
Application |
20130133871 |
Kind Code |
A1 |
Ma; Hongbin ; et
al. |
May 30, 2013 |
Multiple Thermal Circuit Heat Spreader
Abstract
A heat spreader has more than one thermal circuit to give better
performance over a wider range of heat input regimes. Different
working fluids may be used in the different thermal circuits. The
thermal circuits may extend in three dimensions to improve the
density of the channels in limited space.
Inventors: |
Ma; Hongbin; (Columbia,
MO) ; Cheng; Peng; (Columbia, MO) ; Boswell;
Joseph A.; (San Francisco, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ma; Hongbin
Cheng; Peng
Boswell; Joseph A. |
Columbia
Columbia
San Francisco |
MO
MO
CA |
US
US
US |
|
|
Assignee: |
THERMAVANT TECHNOLOGIES LLC
Columbia
MO
THE CURATORS OF THE UNIVERSITY OF MISSOURI
Columbia
MO
|
Family ID: |
44798998 |
Appl. No.: |
13/640758 |
Filed: |
April 12, 2011 |
PCT Filed: |
April 12, 2011 |
PCT NO: |
PCT/US2011/032171 |
371 Date: |
January 9, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61323136 |
Apr 12, 2010 |
|
|
|
Current U.S.
Class: |
165/185 |
Current CPC
Class: |
H01L 23/427 20130101;
H01L 2924/0002 20130101; F28D 15/00 20130101; F28D 15/0266
20130101; H01L 2924/00 20130101; H01L 2924/0002 20130101 |
Class at
Publication: |
165/185 |
International
Class: |
F28D 15/00 20060101
F28D015/00 |
Claims
1. A heat spreading device for transferring heat from a heat source
to a heat sink, the heat spreader device comprising: a first
conduit extending in a loop and having a length between the heat
source and the heat sink; a second conduit proximate the first
conduit and in thermal communication with the first conduit over at
least some of the length of the first conduit.
2. A heat spreading device as set forth in claim 1 wherein the
second conduit is in thermal communication with the first conduit
over substantially the entire length of the first conduit.
3. A heat spreading device as set forth in claim 1 wherein the
second conduit conforms to the first conduit over the entire length
of the first conduit.
4. A heat spreading device as set forth in claim 1 wherein the
second conduit is in thermal communication with the first conduit
over a portion of the first conduit near only one of either the
heat source or the heat sink.
5. A heat spreading device as set forth in claim 1 further
comprising a working fluid in the first and second conduit.
6. A heat spreading device as set forth in claim 5 wherein at least
one of the working fluid in the first conduit and the working fluid
in the second conduit includes nanoparticles.
7. A heat spreading device as set forth in claim 5 wherein the
working fluid in the first conduit is different from the working
fluid in the second conduit.
8. A heat spreading device as set forth in claim 1 wherein the
first and second conduits are each formed at least in part out of a
single piece of material.
9. A heat spreading device as set forth in claim 1 comprising a
first plate and a second plate, portions of the first and second
conduits being defined in the material of the first plate, and
portions of the first and second conduits being defined in the
material of the second plate.
10. A heat spreading device as set forth in claim 1 wherein the
first and second conduits follow tortuous paths.
11. A heat spreading device as set forth in claim 9 wherein the
first and second conduits each include segments extending in three
different dimensions.
12. A heat spreading device as set forth in claim 1 wherein an
internal surface of at least a portion of one of the first or
second conduits is treated to optimize heat transfer.
13. A heat spreading device as set forth in claim 1 wherein the
device is adapted to receive heat from at least two spaced apart
heat sources.
14. A heat spreading device as set forth in claim 13 wherein the
device is adapted to reject heat to at least two spaced apart heat
sinks.
15. A heat spreading device as set forth in claim 1 wherein the
device is adapted to reject heat to at least two spaced apart heat
sinks.
16. A heat spreading device as set forth in claim 1 wherein the
first and second conduits are free of fluid communication with each
other.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to a heat spreader
using a plurality of thermal circuits (each thermal circuit
sometimes referred to elsewhere as an oscillating heat pipe,
loop-type heat pipe, or pulsating heat pipes) which can have many
shapes and sizes and be made of a variety of materials and working
fluids in order to handle a wide range of heat flux and total heat
load and be able to operate in different orientations all while
maintaining a minimal temperature difference between the area(s) of
the spreader that receive heat and the areas of the spreader that
reject heat.
BACKGROUND OF THE INVENTION
[0002] First proposed by Akachi (see, e.g., U.S. Pat. No.
4,921,041), an oscillating heat pipe (OHP) utilizes the oscillating
motion of liquid slugs and vapor plugs enclosed in a series of
meandering tubes to efficiently transfer heat from the heat source
at the oscillating heat pipe's evaporator section to the heat sink
at the oscillating heat pipe's condenser section. If the tubing has
a small enough diameter, the surface tension forces of the working
fluid overcome gravitational forces and distinct liquid plugs and
vapor slugs form throughout the tubing volume. When heat is added
to the evaporator section, the liquid plugs and vapor slugs inside
the tubing at the evaporator section receive the heat, causing them
to increase in temperature and pressure and expand. Portions of the
liquid slugs in the evaporator section evaporate and expand even
further. The pressure difference between the evaporator section and
condenser section drives the liquid slugs and vapor plugs toward
the condenser section where heat is transferred to the heat sink.
When the working fluid travels to the condenser section, the liquid
slugs and vapor plugs lose temperature and pressure and portions of
the vapor plugs condense. Thus, both sensible and latent heat is
transferred out of the thermal circuit to the heat sink at the
condenser section. This causes contraction of the liquid plugs and
vapor slugs inside the tubing at the condenser section. In
addition, the liquid slugs and vapor plugs previously occupying the
space in the condenser section are forced by the incoming flow to
travel toward the evaporator section, where they receive heat from
the heat source and restart the cycle. In this way, heat is
transferred from the evaporator section to the condenser section
using both convective heat transfer (liquid and vapor flows) and
phase change heat transfer (evaporation and condensation). Lastly,
because the oscillating heat pipe has no internal wick structure,
the fabrication cost can be very low and when the oscillating
motion starts, no capillary limitation exists in an oscillating
heat pipe. Experimental results show that it is not susceptible to
dry out at high thermal power densities as opposed to vapor
chambers and traditional heat pipes.
[0003] There are generally two types of oscillating heat pipes by
its tubing characteristics: tubular oscillating heat pipe and flat
plate oscillating heat pipe (FP-OHP). The latter has advantages in
the cooling of electric devices as shown by Akachi (see, e.g., U.S.
Pat. No. 5,737,840). The flat plate oscillating heat pipe has
channels engraved on a metallic plate, which design is more
suitable to its applications on electronic devices than the tubular
oscillating heat pipes. Previous research revealed that many
factors would affect the performance of flat plate oscillating heat
pipe, such as meandering turn number, working fluid, charging
ratio, and filling ratio. For example, Borgmeyer and Ma in their
article Experimental Investigations of Oscillating Motions in a
Flat Plate Pulsating Heat Pipe (J. Thermophysics and Heat Transfer,
pp. 405-409, vol. 21, no. 2 April-June 2007) describe successfully
sealing a copper flat plate oscillating heat pipe with square
internal channels of 1.59 mm hydraulic diameter. The Borgmeyer and
Ma article is incorporated herein by reference. The oscillating
motion of liquid plugs in an oscillating heat pipe has been
observed to be dependent on the working orientation. Khandekar et
al. (Thermofluid Dynamic Study of Flat-Plate Closed-Loop Pulsating
Heat Pipes, Microscale Thermophysical Eng'g 3:303-317, 2002)
conducted the experiments on aluminum 6 turn flat plate oscillating
heat pipes, all sealed with transparent glass and charged with
ethanol and water, and found that flat plate oscillating heat pipe
with larger rectangular cross section and filling ratio below 0.3
is less dependent on gravity. The lowest thermal resistance of 1
K/W was achieved with 2.2.times.2.0 mm.sup.2 rectangular channel.
The Khandekar et al. article is incorporated herein by reference.
In the research of Thompson el al. reported in Experimental
Investigation of Miniature Three-Dimensional Flat-Plate Oscillating
Heat Pipe (J. Heat Transfer, vol. 131, issue 4 043210, 9 pgs.,
April 2009), a three dimensional flat plate oscillating heat pipe
design was proposed, in which the channel density over unit heating
area is dramatically increased. The hydraulic diameter of the
channel was 0.762 mm, and charged with acetone at 0.8 volume
fraction (i.e. 80% of channel's volume filled with acetone fluid).
A much lower thermal resistance of 0.07.degree. C./W was achieved
and the heat flux was up to 20 W/cm.sup.2. The Thompson et al.
article is incorporated herein by reference. Other pertinent work
was reported by Cheng et al. in An Investigation of Flat-Plate
Oscillating Heat Pipes (J. Electron. Packaging, vol. 132, issue 4,
041009, 6 pgs., December 2010). The Cheng et al. article is
incorporated herein by reference.
[0004] Using this heat transfer mechanism, a thermal circuit
utilizes both convective as well as phase change heat transfer to
move thermal energy from the heat source at the heat spreader's
evaporator section(s) to the heat sink at condenser section(s).
Importantly, such a heat transfer mechanism requires some minimum
amount of heat load (start up power) at the evaporator section of
the heat spreader to activate the flow of liquid slugs and vapor
plugs within a thermal circuit due to inertia, gravity and
frictional forces between the working fluid and the thermal circuit
walls. At the other extreme, at some higher heat load (critical
power), the heat flux between the heat spreader's walls at the
evaporator section and the working fluid within a thermal circuit
is so great that a vapor phase remains constant in the area of the
thermal circuit nearest the evaporator section of the heat
spreader, and as a result the mass and heat transfer mechanism
described above ceases to function at such critical power. It has
been shown that thermal circuits charged with some working fluids
(e.g., acetone) have relatively low "start up power" but also have
relatively low "critical power" which makes such thermal circuit
useful in low heat load applications but not useful in higher heat
load applications. By contrast, thermal circuits charged with other
working fluids (e.g., water) have relatively high required
"start-up power" and relatively high "critical power" which make
them useful for applications requiring high heat flux heat transfer
but less useful if lesser amounts of heat need to be
dissipated.
[0005] Oscillating heat pipes have generally higher critical powers
than alternative heat spreading technologies such as heat pipes and
vapor chambers. Also, oscillating heat pipes are less affected by
gravity than traditional heat pipes and to have the ability to
transport heat greater distances with less heat transfer rate
degradation. Unfortunately, traditional single-loop oscillating
heat pipes suffer from the following undesirable attributes that
have prevented more widespread application: A) unpredictable
temperature spikes in their evaporator section(s)s; B) limited
operating power ranges (i.e. an oscillating heat pipe heat spreader
designed for minimal thermal resistance at a relatively low thermal
input power has an undesirably high thermal resistance when
relatively high thermal input power is applied; and conversely an
oscillating heat pipe heat spreader designed for minimal thermal
resistance at a relatively high thermal input power has an
undesirably high thermal resistance when relatively low thermal
input power is applied to it); and C) generally lower overall heat
transport capability (e.g. higher thermal resistance at given
thermal input powers) than is desired by end users of heat
spreading technologies. These undesirable attributes are considered
at present to be inherent to the traditional oscillating heat pipe
design and their applicability to heat spreaders.
SUMMARY
[0006] A heat spreading device for transferring heat from a heat
source to a heat sink constructed according to the principles of
the present invention generally comprises a first conduit extending
in a loop and having a length between the heat source and the heat
sink. A second conduit proximate the first conduit and in thermal
communication with the first conduit over at least some of the
length of the first conduit.
[0007] Other objects and features of the present invention will be
in part apparent and in part pointed out hereinafter.
BRIEF DESCRIPTION OF DRAWINGS
[0008] FIG. 1 is a schematic plan view of a traditional oscillating
heat pipe heat spreader;
[0009] FIG. 2 is a schematic plan view of one configuration of a
multiple thermal circuit heat spreader with two fluidly independent
thermal circuits that are thermally in communication by way of a
thermally conductive wall;
[0010] FIG. 3 is a schematic perspective of one configuration of a
three-dimensional multiple thermal circuit heat spreader;
[0011] FIG. 3A is a cross section of the schematic of FIG. 3;
[0012] FIG. 4 is a perspective of another configuration of a three
dimensional thermal circuit emplaced within a flat plate heat
spreader with its top lid removed and the thermal circuit's
channels exposed and with staggered large channels;
[0013] FIG. 5 is a fragmentary cross section of the three
dimensional heat spreader shown in FIG. 4, but showing both the top
and bottom lids attached to the flat plate;
[0014] FIG. 6 is a top plan view of thermal circuit heat spreader
shown in FIGS. 4 and 5 with the top lid removed;
[0015] FIG. 6A is an enlarged fragmentary view of an upper portion
of the heat spreader of FIG. 6, but showing a bottom side of the
flat plate with the bottom lid removed;
[0016] FIG. 6B is an enlarged fragmentary view of a lower right
side portion of the heat spreader of FIG. 6;
[0017] FIG. 7 is a schematic, fragmentary cross section of another
flat plate multiple thermal circuit heat spreader showing a stacked
arrangement of thermal circuits in the plate;
[0018] FIG. 7A is a schematic, fragmentary cross section of still
another flat plate multiple thermal circuit heat spreader showing a
stacked arrangement of thermal circuits in the plate;
[0019] FIG. 8 is a schematic, fragmentary cross section of yet
another flat plate multiple thermal circuit heat spreader showing a
stacked arrangement of thermal circuits in the plate;
[0020] FIG. 9 is a schematic perspective of another configuration
of multiple thermal circuit heat spreader; and
[0021] FIG. 10 is a graph showing thermal resistance across a range
of heat loads using multiple thermal circuits compared to a heat
spreader with only one thermal circuit and one working fluid.
[0022] Corresponding reference characters indicate corresponding
parts throughout the several views of the drawings.
DETAILED DESCRIPTION OF THE DRAWINGS
[0023] A conventional oscillating heat pipe OHP1 having a single
loop L or channel forming a single thermal circuit is schematically
illustrated in FIG. 1. The loop L has been evacuated, partially
charged with a working fluid and then sealed in order to develop
liquid slugs LS and vapor plugs VP within the loop. As drawn, the
vapor plugs VP are the spaces between the liquid slugs. When heat
(Q.sub.in) is applied to an evaporator section E of the oscillating
heat pipe OHP1 the liquid slugs LS expand and partially evaporate,
and exert a pressure on the adjacent slugs LS and plugs VP. The
pressure difference leads to a mass transfer of the vapor and
liquid in the evaporator section E to the lower pressure regions in
a condenser section C where the vapor condenses and the liquid
slugs LS contract. An adiabatic section A of the oscillating heat
pipe OHP1 is present between the evaporator section E and the
condenser section C. The heat transfer in the adiabatic section A
is essentially zero (i.e., .DELTA.Q=0). Because the thermal circuit
loop L meanders to-and-from the evaporator section E and condenser
section S, the expanding and contracting forces continuously send
slugs and plugs between the evaporator section E and condenser
section C which produces an oscillating motion of slugs and plugs.
The oscillating motion is illustrated in one of the three turnbacks
of the loop L in FIG. 1 by dashed double arrows between adjacent
liquid slugs LS. This mechanism transfers heat with both convective
heat transfer (flow of liquid slugs LS and vapor plugs VP between
sections of different temperatures) as well as phase change heat
transfer (evaporation and condensation of the working fluid within
thermal circuit).
[0024] Referring now to FIG. 2, a multiple thermal circuit spreader
constructed according to the principles of the present invention is
generally indicated at 2. The multiple thermal circuit spreader
includes in the illustrated embodiment has an inner thermal circuit
4 and an outer thermal circuit 6. Each of the thermal circuits 4, 6
forms liquid slugs 8 and vapor plugs 10 substantially as described
above for the oscillating heat pipe OHP1 of FIG. 1. Oscillation of
the liquid slugs 8 and vapor plugs 10 is shown in one of the three
turnbacks of the thermal circuits 4, 6. For clarity of
illustration, the liquid slugs 8 of the inner thermal circuit 4 are
depicted with filled-in ovals, while the liquid slugs 8 of the
outer thermal circuit 6 are depicted by unfilled ovals. The inner
and outer thermal circuits 4, 6 are each closed loops and have no
fluid communication between them. However, the inner and outer
thermal circuits are in thermal communication with each other by
way of a thermally conductive wall 12. The conductive wall is cross
hatched in FIG. 2 to facilitate understanding of the various parts
of the heat spreader 2. In this way each thermal circuit 4, 6 not
only transfers heat between an evaporator section 14 and a
condenser section 16 (like the thermal circuit loop L of FIG. 1)
but also to the adjacent thermal circuit should a temperature
difference between the inner and outer thermal circuits exist. The
heat spreader 2 further includes an adiabatic section 18 between
the evaporator section 14 and the condenser section 16 where
essentially no heat transfer occurs (i.e., .DELTA.Q=0).
[0025] It will be understood that although two thermal circuits 4,
6 are illustrated in the heat spreader of FIG. 2, a heat spreader
according to the present invention may have more thermal circuits.
The thermal circuits may not necessary be closed loops. Still
further, the thermal circuits may be in thermal communication with
each other over less than their entire lengths. For example and
without limitation, the thermal circuits may be in thermal
communication with each other on the evaporation section of the
spreader, or only in the condenser section of the spreader. There
may be more than one evaporator section and/or more than one
condenser section of the heat spreader, and they may have different
sizes and shapes. For example, perhaps on one quadrant of the plate
is an evaporator section, while the remaining area of the plate is
a condenser. In another example, there may be two or more distinct
evaporator sections and/or two or more distinct condenser sections.
Therefore a description of a spreader including one evaporator
section and one condenser section in this description will be
understood to apply to spreaders having more than one evaporator
section and/or more than one condenser section. Although the
thermal circuits 4, 6 are shown going back and forth several times
between the evaporator section 14 and the condenser section 16,
they may be constructed so as not to go back and forth.
[0026] Referring now to FIGS. 3 and 3A, a multiple thermal circuit
is diagrammatically illustrated where multiple interconnected
thermal circuits (in this case two, first thermal circuit 18 and
second thermal circuit 19) are fluidly independent but thermally in
communication but where the thermal circuits meander not only in
two dimensions (as shown in FIGS. 1 and 2) but in three dimensions
(e.g. in the z-plane) thus increasing turn number of each thermal
circuit for a given x-y surface area. The z-direction is
exaggerated somewhat in FIG. 3 to illustrate the point.
[0027] Two three-dimensional thermal circuits 20 and 22 engraved on
the both sides of two piece base 26 of a heat spreader 28 are shown
in FIGS. 4-6B. It will be understood that the base 26 may be made
of one piece of material, or more than two pieces of material
within the scope of the present invention. As illustrated, the base
26 is formed by two sheets connected together. Thermal circuits 20
and 22 may have different channel profiles. In the illustrated
embodiment, the channels of thermal circuit 20 are larger in cross
sectional area than the channels of thermal circuit 22. The cross
sectional shapes of the thermal circuit channels may be different
than shown and/or different from each other within the scope of the
present invention. The thermal circuits 20, 22 may be charged with
different working fluids, and each thermal circuit may have
different internal surface treatments. Although less preferred, the
same working fluid could be charged to both circuits. The thermal
circuits 20, 22 are arranged side-by-side on each of the opposite
faces of the solid base 26. FIG. 5 shows the heat spreader of FIG.
4, but with the top and bottom cover plates 36, 38 attached to the
top and bottom faces of the base 26. The cover plates 36, 38 are
placed on both sides of base 26 to enclose and seal thermal
circuits 20 and 22, and may be attached in a suitable manner to the
base such as by brazing, soldering, welding or other methods. The
channels of the thermal circuits 20, 22 are arranged to have the
greatest channel density in a limited space, one thermal circuit 20
has a large hydraulic diameter, and extends back and forth between
the top and bottom faces of solid base 26. Thermal circuit 22 has a
channel of much smaller hydraulic diameter, and is engraved in the
base on the face of the base opposite of thermal circuit 20. The
channels of thermal circuit 22 also extend back and forth between
the top and bottom faces of the base 26
[0028] In FIG. 6, the heat spreader in FIGS. 4 and 5 is shown in a
top plan view (x-y plane) without the cover plate 36 (FIG. 5) so
that the thermal circuits 20 and 22 in the top face of the base 26
are exposed. An upper section of the heat spreader (as oriented in
FIG. 6) represents a condenser section 42 and a lower section of
the heat spreader 28 represents an evaporator section 44. FIG. 6A
shows an enlarged, fragmentary view of the back side of the base 26
in the condenser section 42, and FIG. 6B is a further enlarged,
fragmentary view of the front side of the base 26 in the evaporator
section 44. The thermal circuits 20, 22 meander to and from
condenser section 42 and evaporator section 44. Thermal circuit 20
meanders to-and-from the condenser section 42 and the evaporator
section 44 on the front side in a series of L-shaped conduits that
at each end have a port such as 46 that transports the working
fluid in thermal circuit 20 from the front side to the back side
(see, FIG. 6B). On the back side of the base 26 the pattern is
slightly different for thermal circuit 20. For thermal circuit 22
there are only 2 ports connecting front and back side of the heat
spreader base 26--port 50 and port 52. Viewing the front side of
the heat spreader 28, thermal circuit 22 meanders to-and-from
condenser section 42 and evaporator section 44 in a lengthy pattern
that weaves in-and-around thermal circuit 20 and then connects to
the backside at one of two distant ports 50 and 52. Using these
ports, thermal circuits 20 and 22 turn in the z-plane, meaning they
travel to from the front side of the heat spreader base 26 shown in
FIG. 6 to the back side shown in FIG. 6A. A completed heat spreader
28 (FIG. 5) would have the thermal circuits 20, 22 sealed on front
and back sides (by application of the cover plates 36, 38),
evacuated, partially charged with working fluid(s), and would
transfer heat from a heat source in thermal communication with
evaporator section 44 to a heat sink in thermal communication with
condenser section 42. The completed heat spreader 28 might be
outfitted with external heat transfer enhancements (not shown) such
as fins, grooves or other heat transfer efficiency improvements
known to those practiced in heat spreading applications.
[0029] The multi-loop or multiple thermal circuit design may use
thermal circuits with different working fluids and/or different
geometries and/or channel patterns through the heat spreader.
Referring again to FIG. 5, each channel of the thermal circuit 20
having the larger cross sectional area has two small channels of
the thermal circuit 22 above or below it, alternating with each row
of channels. If the larger cross sectional area channel of the
thermal circuit 20 and corresponding two channels of thermal
circuit 22 having smaller cross section areas are considered a
"group of channels", these "group of channels" are staggered so
that the big channel is on top of the small channels in one row and
the big channel is below the small channels in adjacent rows. The
advantage of this staggered arrangement is its good thermal
coupling between the front side and back side of the heat spreader
28 as well as good thermal coupling between the two thermal
circuits 20, 22. Because the staggering is achieved by a turn in
the z-plane, turn number is increased and heat transport capability
is also raised. However, the invention's channel arrangements are
not limited to the staggered arrangement and may vary in design and
manufacturing method.
[0030] FIGS. 7, 7A and 8 are fragmentary cross sections of plates
of other heat spreaders having different arrangements of multiple
thermal circuits. FIG. 7 shows a heat spreader including thermal
circuit 60 having larger cross sectional area channels, and thermal
circuit 62 having smaller cross sectional area channels. The
thermal circuits 60, 62 are arranged one above the other and are
each confined to a single plane. FIG. 7A shows a heat spreader
having a thermal circuit 70 having larger cross sectional area
channels that are confined to a single, central plane. The heat
spreader further includes a thermal circuit 72 having smaller cross
sectional area channels that meander from one side of the heat
spreader to the other and on opposite sides of the thermal circuit
70. Finally, FIG. 8 illustrates a head spreader that includes four
different thermal circuits, each having a different cross sectional
shape. A first thermal circuit 80 has a circular cross section and
moves in the not only in the x-y direction, but also in the
z-direction through three separate levels, as may be seen in FIG.
8. By way of general illustration, in one of the middle levels, the
channels 80a, 80b are slightly vertically offset. A second of the
thermal circuits 82 has a square cross section and makes only one
pass between a middle and lower level of the heat spreader. A
larger third thermal circuit 84 has a rectangular cross section and
passes through all three levels of the heat spreader. Finally,
thermal circuit 86 has a triangular cross section and is an open,
rather than closed loop circuit. The purpose of FIG. 8 is to show
that any number of different shapes and arrangements of thermal
circuits may be used within the scope of the present invention.
[0031] The thermal circuit's shape is not limited to being square
in cross section. Circular, triangular, rectangular, T-type, and
any other cross sectional geometry may be used such as is suitable
for a given set of design parameters and issues (e.g., channel
density, heat transfer performance, heat spreader size,
manufacturability, etc.). When a thermal circuit heat spreader
includes multiple thermal circuits (or multiple loops), the channel
shape for each loop can be different from others as shown in FIG.
8. Even for one thermal circuit meandering throughout the heat
spreader's volume, the channel shape may vary from one shape such
as rectangular channel to another such as triangular one at
different regions of the heat spreader. A variety of fluids may be
used in the larger and smaller cross section area channels of the
multiple thermal circuit heat spreader of the present invention.
For example, the working fluid in a thermal circuit can be a single
fluid, a plurality of miscible fluids, a plurality of immiscible
fluids, or a combination of miscible and immiscible fluids
Furthermore, by applying low-concentration nanofluids (e.g., fluids
comprised of a base fluid that has been mixed chemically,
sonically, physically, or otherwise with nanoparticles of a
separate substance) as the working fluids in the thermal circuits
one can reduce the temperature variation at the thermal circuits
evaporator section(s) and also extend its working limits (e.g. the
lowest and highest functioning input powers). By way of general
example, the working fluids may contain suspended particles of
ultra-high conductivity material such as nanoscale particles of
diamond, gold, silver, etc. that have been suspended in the base
solution. In one more particular example, the larger cross
sectional area channel thermal circuit may be charged with a
diamond-water nanofluid to improve the effective thermal
conductivity of the big channel thermal circuit.
[0032] The internal surfaces of the thermal circuit may be smooth,
rough, treated to be hydrophobic with the working fluid, and/or
treated to be hydrophilic with the working fluid. For example,
copper surface pre-treated by oxidization has shown to enhance heat
transfer performance of certain thermal circuits charged with water
as working fluid. Further, hydrophilic surface treatments on metals
(e.g. microgrooves or chemical coatings) have proven to increase
the contact angle of working fluid and in doing so increase the
evaporative heat transfer rate between such surface and the working
fluid. Also, the inner channel surfaces may be manipulated to be
hydrophobic, hydrophilic, or hydrophobic in one area (e.g.
condenser) and hydrophilic in another area (e.g. evaporator) using
a variety of techniques including surface coatings, laser formed
micro/nano structures, and controlled chemical reactions, etc.
[0033] The shape of the multiple thermal circuit heat spreader may
be flat plate, cylindrical, or any other geometry. The evaporator
section(s) may be at the center of the heat spreader, on one or
more edges, in one or more corners, or any other location. The
orientation of the heat spreader (i.e. the location of the heat
source(s) relative to the heat sink(s) in a gravitational field)
may be with heat source(s) on top of, to the side of, below, or in
any relative position to the heat sink(s).
[0034] The multiple thermal circuit heat spreader may be made from
any shell material, including non-metals with relatively low
thermal conductivities because material conductivity is a
relatively small contributor to the heat spreader's overall heat
transport capability if the wall thickness of the material is
relatively small. For example and without limitation, at 100
microns of wall thickness the impact of the wall's thermal
conductivity will contribute little to overall thermal resistance
of the heat spreader. The multiple thermal circuit heat spreader
shell and internal tubing may be manufactured in a variety of
processes, including but not limited to: brazing, stamping,
photo-chemical etching, hot forging, cold forging, mechanical
engraving, welding, water-jet cutting, laser etching, or any other
positive or negative fabricating process of embedding and sealing
thermal circuits in shell material.
[0035] The multiple thermal circuit design may also use a
combination of loops where the combination of thermal circuits is
such that they increase or decrease the heat transfer between the
thermal circuits. For example, in FIG. 9 there are three loops or
thermal circuits comprising a top thermal circuit 90 (which is in
closest thermal communication with the heat source); a bottom
thermal circuit 92 (which is in closest thermal communication with
the heat sink); and a middle thermal circuit 94 located between top
thermal circuit 90 and bottom thermal circuit 92. Middle thermal
circuit 94 may be designed to improve or reduce the heat transfer
occurring directly through the cross-sectional area between top and
bottom thermal circuits 90, 92. It is believed that in some
instances, orthogonal heat transfer (i.e. thermal energy transfer
directly through the cross section of the heat spreader from the
heat source to the heat sink) in a single loop oscillating heat
pipe or even a multiple thermal circuit heat spreader may prevent
the lateral heat transfer needed to dissipate thermal energy in
more than one dimension. To increase that lateral heat transfer a
thermal circuit such as middle thermal circuit 94 shown in FIG. 9
may actually serve to insulate the top thermal circuit 90 from the
bottom thermal circuit 92 at one or more cross-sections of the heat
spreader. As shown in FIG. 9, the middle thermal circuit 94 is
located only at the near end. In other instances it may behoove the
heat spreader designer to use middle thermal circuit 94--as shown
in FIG. 9--to increase the orthogonal heat transfer between top
thermal circuit 90 and bottom thermal circuit 92 at one or more
cross-sections within the heat spreader. In the illustrated
embodiment of FIG. 9, the middle thermal circuit 94 has a working
fluid, but the middle thermal circuit could be embodied by, for
example, a void, a insulating material, or a high thermal
conductivity material.
[0036] The heat spreaders are comprised of more than one thermal
circuit, and each thermal circuit may meander back and forth
between the heat spreader's evaporator section(s) and condenser
section(s), or it may only traverse one Section (e.g., the middle
thermal circuit 94 of FIG. 9), or a select number of sections. A
180-degree change in direction of a thermal circuit is referred to
as a "turn", and a 90-degree turn is considered a half turn.
Because turn number is proven to be positively related to thermal
circuit heat spreader heat transfer capability, the turns in the
invented heat spreader's thermal circuits may be in two- or
three-dimensions (i.e., not only in the x-y plane but also in the
x-z or y-z plane). Three dimensional turns are beneficial when
there is a limited surface area in the x-y plane which limits
thermal circuit turn number and therefore thermal circuit heat
transfer capability; however, additional thermal circuit turns can
be achieved in the x-z or y-z planes and thus increase thermal
circuit turn number and thermal circuit heat transfer capability by
utilizing the z-plane without exceeding the x-y surface area
limitations.
[0037] Conventional oscillating heat pipes have the advantage of
being able to transport heat greater distances between their
evaporator and condenser sections than traditional heat pipes or
vapor chambers. They also have the advantage of being less affected
by gravity. However, comparing with other types of heat pipes, such
as loop heat pipe and vapor chamber, prior oscillating heat pipes
have suffered from high startup power, high thermal resistances,
and sharp temperature spikes at the evaporator section(s) which
have prevented them from finding much commercial acceptance. At
least some embodiments of the invention disclosed herein resolve
these issues by incorporating a plurality of fluidly independent
but thermally communicating thermal circuits on a single heat
spreading device. By using multiple thermal circuits on the same
heat spreader (where thermal circuits may be of the same or
different sizes, patterns, hydraulic diameters and/or working
fluids) the invented heat spreader is able to transfer heat
efficiently at both lower start-up powers and at higher critical
powers. Further, it has been empirically observed that by having
thermal circuits in thermal communication with at least one other
fluidly independent thermal circuit the likelihood of either
thermal circuit ceasing to function temporarily (which is a cause
of unpredictable temperature spikes in the evaporator section(s) of
single loop oscillating heat pipes) is exponentially reduced thus
creating an overall lower thermal resistance at any single power
input level. Finally, by utilizing three-dimensional turns the heat
spreader can transfer greater amounts of thermal energy with lower
overall thermal resistance at specified areas for receiving and
rejecting heat. FIG. 10 is a graph of a heat spreader's thermal
resistance at increasing heat loads (from 0-400 watts) with the
following thermal circuit options: a) one thermal circuit charged
with acetone; b) one thermal circuit charged with water; and c) two
thermal circuits, one charged with water and the other charged with
acetone. As can be seen the heat spreader with both a water thermal
circuit and an acetone thermal circuit has better overall
performance across a broader range of input powers. Finally, it may
be optimal to have a thermal circuit situated vis-a-vis the heat
source(s), heat sink(s), and the other thermal circuits in such a
way that increases or decreases thermal communication between the
sources, sinks and other thermal circuits in order to optimize the
spreader's overall heat transfer capacity.
[0038] Having described the invention in detail, it will be
apparent that modifications and variations are possible without
departing from the scope of the invention defined in the appended
claims.
[0039] When introducing elements of the present invention or the
preferred embodiments(s) thereof, the articles "a", "an", "the" and
"said" are intended to mean that there are one or more of the
elements. The terms "comprising", "including" and "having" are
intended to be inclusive and mean that there may be additional
elements other than the listed elements.
[0040] In view of the above, it will be seen that the several
objects of the invention are achieved and other advantageous
results attained.
[0041] As various changes could be made in the above products and
methods without departing from the scope of the invention, it is
intended that all matter contained in the above description and
shown in the accompanying drawings shall be interpreted as
illustrative and not in a limiting sense.
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