U.S. patent application number 11/041767 was filed with the patent office on 2005-08-25 for high efficiency flat panel microchannel heat exchanger.
This patent application is currently assigned to International Mezzo Technologies, Inc.. Invention is credited to Kelly, Kevin W., McCandless, Andrew.
Application Number | 20050183851 11/041767 |
Document ID | / |
Family ID | 34859783 |
Filed Date | 2005-08-25 |
United States Patent
Application |
20050183851 |
Kind Code |
A1 |
Kelly, Kevin W. ; et
al. |
August 25, 2005 |
High efficiency flat panel microchannel heat exchanger
Abstract
An apparatus providing high efficiency heat exchange between two
fluids is disclosed. The apparatus most commonly comprises a flat
panel with microchannels directing the flow of the two fluids,
specifically: with a small hydraulic diameter in order to increase
the heat transfer effect; while, at the same time, the flow length
and cross-section of the microchannels are controlled to reduce the
pressure losses normally associated with such small hydraulic
diameters.
Inventors: |
Kelly, Kevin W.; (Baton
Rouge, LA) ; McCandless, Andrew; (Baton Rouge,
LA) |
Correspondence
Address: |
Anthony P. Iannitelli
LBTC
D-108
South Stadium Drive
Baton Rouge
LA
70803
US
|
Assignee: |
International Mezzo Technologies,
Inc.
|
Family ID: |
34859783 |
Appl. No.: |
11/041767 |
Filed: |
January 24, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11041767 |
Jan 24, 2005 |
|
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10003882 |
Oct 25, 2001 |
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6892802 |
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Current U.S.
Class: |
165/148 ;
165/165 |
Current CPC
Class: |
F28F 2260/02 20130101;
F28F 7/02 20130101 |
Class at
Publication: |
165/148 ;
165/165 |
International
Class: |
F28D 001/00 |
Goverment Interests
[0002] The work leading to the invention described herein was made
with Government support under Grant No. DABT63-95-C-0020 awarded by
the Defense Advanced Projects Research Agency. The Government has
certain rights in this invention.
Claims
We claim:
1. A heat exchanger for transferring heat between a first fluid and
a second fluid, comprising: a. A first set of fluid channels
through which the first fluid may flow, in which each channel is
substantially identical to the others in cross sectional dimensions
and length; and b. A second set of fluid channels through which the
second fluid may flow; each channel having a flow length less than
8.0 mm; and having a value of said channel flow length divided by
the square of the channel hydraulic diameter between 5 mm.sup.-1
and 30 mm.sup.-.
2. The heat exchanger as recited in claim 1, in which each channel
of the second set of fluid channels has a length less than 4.0
mm.
3. The heat exchanger as recited in claim 1, in which the second
set of fluid channels directs the second fluid flow in paths which
are not parallel to the first set of fluid channels.
4. The heat exchanger as recited in claim 1, in which the second
set of fluid channels is comprised of individual channels, each
with a hydraulic diameter between 0.250 mm and 2.0 mm.
5. The heat exchanger as recited in claim 3, in which the second
set of fluid channels is comprised of individual channels, each
with a hydraulic diameter between 0.250 mm and 2.0 mm.
6. The heat exchanger as recited in claim 1, in which the second
set of fluid channels is comprised of individual channels with
substantially rectangular cross section.
7. The heat exchanger as recited in claim 3, in which the second
set of fluid channels is comprised of individual channels with
substantially rectangular cross section.
8. The heat exchanger as recited in claim 7, in which the
individual channels of the second set of fluid channels each have a
hydraulic diameter between 0.250 mm and 2.0 mm.
9. The heat exchanger as recited in claim 7, in which the cross
section of each channel of the second set of fluid channels is
oriented so that its longest side is substantially parallel to the
direction of fluid flow through the first set of fluid
channels.
10. The heat exchanger as recited in claim 9, in which the cross
section of each channel of the second set of fluid channels has a
smallest dimension measuring between 0.125 mm and 1.0 mm.
11. The heat exchanger as recited in claim 1, in which the first
set of fluid channels is comprised of individual channels with
substantially rectangular cross section.
12. The heat exchanger as recited in claim 11, in which the cross
section of each channel of the first set of fluid channels has a
longest side substantially parallel to the direction of the fluid
flow through the second set of fluid channels.
13. The heat exchanger as recited in claim 1, in which the first
set of fluid channels is arranged substantially in a single plane,
defined by the general direction of fluid flow through said first
set of fluid channels.
14. The heat exchanger as recited in claim 13, in which the second
set of fluid channels is interspersed throughout the first set of
fluid channels, providing flow through the second set of fluid
channels that intersects the plane of the first set of fluid
channels.
15. The heat exchanger as recited in claim 13, in which the second
set of fluid channels provides fluid flow substantially parallel to
the normal of the single plane of the first set of fluid
channels.
16. The heat exchanger as recited in claim 15, in which the first
set of fluid channels is comprised of individual channels with
substantially rectangular cross section.
17. The heat exchanger as recited in claim 16, in which the second
set of fluid channels is comprised of individual channels with
substantially rectangular cross section.
18. The heat exchanger as recited in claim 17, in which the
individual channels of both the first and second set of fluid
channels have a smallest cross sectional dimension less than 1.0
mm.
19. The heat exchanger as recited in claim 17, in which the
individual channels of both the first and second set of fluid
channels have a smallest cross sectional dimension less than 0.50
mm.
20. The heat exchanger as recited in claim 1, in which the first
set of fluid channels and the second set of fluid channels are
contained in a unitary body panel which separates the sets of fluid
channels with walls.
21. The heat exchanger as recited in claim 20, in which the wall
thickness between the first set of fluid channels and the second
set of fluid channels is less than 0.250 mm.
22. The heat exchanger as recited in claim 20, in which the wall
thickness between the first set of fluid channels and the second
set of fluid channels is less than 0.100 mm.
23. The heat exchanger as recited in claim 18, in which the first
set of fluid channels and the second set of fluid channels are
contained in a unitary body panel which separates the sets of fluid
channels with walls.
24. The heat exchanger as recited in claim 23, in which the wall
thickness between the first set of fluid channels and the second
set of fluid channels is less than 0.250 mm.
25. The heat exchanger as recited in claim 23, in which the wall
thickness between the first set of fluid channels and the second
set of fluid channels is less than 0.100 mm.
26. A heat exchanger that transfers heat between two fluids,
comprising: a. A first set of fluid channels, arrayed substantially
in a plane, which directs the first fluid flow; b. A second set of
fluid channels, interspersed throughout the plane of the first set
of fluid channels, which directs the second fluid flow to intersect
the plane of the first fluid channels, and in which the second set
of fluid channels is made up of individual channels with hydraulic
diameter of less than 1.0 mm, and in which the individual channels
of the second set of fluid channels each have a ratio of flow
length to hydraulic diameter less than one half of the same ratio
in the individual channels of the first set of fluid channels.
27. The heat exchanger as recited in claim 26, in which each
channel of the second set of fluid channels has a flow length less
than 8.0 mm.
28. The heat exchanger as recited in claim 26, in which each
channel of the second set of fluid channels has a flow length less
than 4.0 mm.
29. The heat exchanger as recited in claim 26, in which each
channel of the second set of fluid channels has a hydraulic
diameter between 0.250 mm and 2.0 mm.
30. The heat exchanger as recited in claim 27, in which each
channel of the second set of fluid channels has a hydraulic
diameter less than 2.0 mm.
31. The heat exchanger as recited in claim 26, in which the second
set of fluid channels in comprised of individual channels with
substantially rectangular cross section.
32. The heat exchanger as recited in claim 30, in which the second
set of fluid channels is comprised of individual channels with
substantially rectangular cross section.
33. The heat exchanger as recited in claim 31, in which the
cross-sectional area of each channel of the second set of fluid
channels has a longest dimension substantially parallel to the
direction of the first fluid flow.
34. The heat exchanger as recited in claim 31, in which the
rectangular cross section of each channel of the second set of
fluid channels has a smallest dimension between 0.125 mm and 1.0
mm.
35. The heat exchanger as recited in claim 35, in which the
cross-sectional area of each channel of the second set of fluid
channels has a longest dimension substantially parallel to the
direction of the first fluid flow.
36. The heat exchanger as recited in claim 35, in which the
rectangular cross section of each channel of the second set of
fluid channels has a smallest dimension less than 1.0 mm.
37. The heat exchanger as recited in claim 26, in which the first
set of fluid channels consists of individual channels with
substantially rectangular cross section, intermittently connected
to adjacent channels.
38. The heat exchanger as recited in claim 37, in which the
cross-sectional area of each channel of the first set of fluid
channels has a longest dimension substantially parallel to the
direction of the second fluid flow.
39. The heat exchanger as recited in claim 38, in which each
channel of the second set of fluid channels has a flow length less
than 8.0 mm.
40. The heat exchanger as recited in claim 39, in which each
channel of the second set of fluid channels has a hydraulic
diameter between 0.250 mm and 2.0 mm.
41. The heat exchanger as recited in claim 40, in which the second
set of fluid channels is comprised of individual channels with
substantially rectangular cross section.
42. The heat exchanger as recited in claim 26, in which the first
set of fluid channels and the second set of fluid channels are
comprised by a unitary body flat panel, which separates the sets of
fluid channels with walls.
43. The heat exchanger as recited in claim 42, in which the walls
are no thicker than 0.250 mm.
44. The heat exchanger as recited in claim 26, in which the
individual channels of the second set of fluid channels each have a
ratio of flow length to hydraulic diameter less than one fifth of
the same ratio in the individual channels of the first set of fluid
channels.
45. The heat exchanger as recited in claim 44, in which the first
set of fluid channels and the second set of fluid channels are
comprised by a unitary body flat panel, which separates the sets of
fluid channels with walls.
46. The heat exchanger as recited in claim 45, in which the walls
are no thicker than 0.250 mm.
47. The heat exchanger of claim 26, in which the individual
channels of the second set of fluid channels each have a ratio of
flow length to hydraulic diameter less than one tenth of the same
ratio in the individual channels of the first set of fluid
channels.
48. The heat exchanger as recited in claim 47, in which the first
set of fluid channels and the second set of fluid channels are
comprised by a unitary body flat panel, which separates the sets of
fluid channels with walls.
49. The heat exchanger as recited in claim 48, in which the walls
are no thicker than 0.250 mm.
50. A primary surface heat exchanger that transfers heat between
two fluids, comprising: a. A first set of fluid channels, arrayed
substantially in a plane, which directs a first fluid flow and in
which the hydraulic diameter of the individual first fluid channels
is less than 1.0 mm; b. A second set of fluid channels which
directs a second fluid flow so that it intersects the plane of the
first fluid channels and in which the hydraulic diameter of the
individual second fluid channels is less than 1.0 mm; and which
provides an open area to the second fluid of more than 25%.
51. The heat exchanger as recited in claim 50, in which the first
set of fluid channels is comprised of individual channels with
substantially rectangular cross section.
52. The heat exchanger as recited in claim 50, in which each
channel in the first set of fluid channels has a hydraulic diameter
between 0.250 mm and 1.0 mm.
53. The heat exchanger as recited in claim 50, in which the second
set of fluid channels is comprised of individual channels with
substantially rectangular cross section.
54. The heat exchanger as recited in claim 50, in which each
channel in the second set of fluid channels has a hydraulic
diameter between 0.250 and 1.0 mm.
55. The heat exchanger as recited in claim 50, in which the first
set of fluid channels is comprised of individual channels with
substantially rectangular cross section, and in which the second
set of fluid channels is comprised of individual channels with
substantially rectangular cross section.
56. The heat exchanger as recited in claim 55, in which the
individual channels in the first set of fluid channels each have a
hydraulic diameter between 0.250 and 1.0 mm, and in which the
individual channels in the second set of fluid channels each have a
hydraulic diameter between 0.250 and 1.0 mm.
57. The heat exchanger as recited in claim 50, in which the second
set of fluid channels is comprised of individual channels with a
flow length less than 8.0 mm.
58. The heat exchanger as recited in claim 50, in which the second
set of fluid channels is comprised of individual channels with a
flow length less than 4.0 mm.
59. The heat exchanger as recited in claim 56, in which the
individual channels in the second set of fluid channels each have a
flow length less than 8.0 mm.
60. The heat exchanger as recited in claim 56, in which the
individual channels in the second set of fluid channels each have a
flow length less than 4.0 mm.
61. The heat exchanger as recited in claim 50, wherein the second
set of fluid channels provides an open area to the second fluid
between 25% and 50%.
62. The heat exchanger as recited in claim 50, wherein the second
set of fluid channels provides an open area to the second fluid
between 30% and 45%.
63. The heat exchanger as recited in claim 59, wherein the second
set of fluid channels provides an open area to the second fluid
between 25% and 50%.
64. The heat exchanger as recited in claim 59, wherein the second
set of fluid channels provides an open area to the second fluid
between 30% and 45%.
65. A primary surface heat exchanger for transferring heat between
a first fluid and a second fluid, comprising: a. A first set of
fluid channels, arrayed substantially in a plane, which directs the
first fluid flow; and b. A second set of fluid channels,
interspersed throughout the plane of the first set of fluid
channels, which directs the second fluid flow to intersect the
plane of the first fluid channels; and in which each channel of the
second set of fluid channels has a value of length to hydraulic
diameter which is less than the ratio of length to hydraulic
diameter of each channel of the first set; and in which each
channel of the second set of fluid channels has both a flow length
of less than 8.0 mm, and a value of said flow length divided by the
square of the channel's hydraulic diameter between 5 mm.sup.-1 and
30 mm.sup.-1.
66. The heat exchanger as recited in claim 65, in which each
channel of the second set of fluid channels has a flow length less
than 4.0 mm.
67. The heat exchanger as recited in claim 65, in which each
channel in the first set of fluid channels has a hydraulic diameter
between 0.250 mm and 2.0 mm.
68. The heat exchanger as recited in claim 65, in which each
channel in the second set of fluid channels has a hydraulic
diameter between 0.250 mm and 2.0 mm.
69. The heat exchanger as recited in claim 65, in which the second
set of fluid channels comprises individual channels with
substantially rectangular cross section.
70. The heat exchanger as recited in claim 69, in which each
channel in the second set of fluid channels has a cross sectional
dimension substantially parallel to the direction of fluid flow in
the first set of channels.
71. The heat exchanger as recited in claim 69, in which each
channel in the second set of fluid channels has a smallest cross
sectional dimension measuring between 0.125 mm and 1.0 mm.
72. The heat exchanger as recited in claim 68, in which the second
set of fluid channels consists of individual channels with
substantially rectangular cross section.
73. The heat exchanger as recited in claim 72, in which each
channel in the second set of fluid channels has a smallest cross
sectional dimension measuring between 0.250 mm and 0.50 mm.
74. The heat exchanger as recited in claim 65, in which the first
set of fluid channels consists of individual channels with
substantially rectangular cross section.
75. The heat exchanger as recited in claim 74, in which each
channel in the first set of fluid channels has a cross sectional
dimension substantially parallel to the direction of fluid flow
through the second set of fluid channels.
76. The heat exchanger as recited in claim 73, in which the first
set of fluid channels consists of individual channels with
substantially rectangular cross section.
77. The heat exchanger as recited in claim 76, in which the cross
section of each channel in the first set of fluid channels has a
longest side substantially parallel to the direction of fluid flow
through the second set of fluid channels.
78. The heat exchanger as recited in claim 77, in which each
channel in the first set of fluid channels has a smallest cross
sectional dimension measuring between 0.250 mm and 0.50 mm.
79. The heat exchanger as recited in claim 65, in which the first
set of fluid channels and the second set of fluid channels are
comprised by a unitary body flat panel and separated from one
another by walls.
80. The heat exchanger as recited in claim 79, in which the walls
have a thickness less than 0.250 mm.
81. The heat exchanger as recited in claim 78, in which the first
set of fluid channels and the second set of fluid channels are
contained in a unitary body flat panel and separated from one
another by walls.
82. The heat exchangers as recited in claim 81, in which the walls
have a thickness less than 0.250 mm.
83. The heat exchanger as recited in claim 65, in which each
channel in the second set of fluid channels has a value of flow
length divided by hydraulic diameter less than one half of the
ratio of flow length divided by hydraulic diameter provided by each
channel in the first set of fluid channels.
84. The heat exchanger as recited in claim 65, in which each
channel in the second set of fluid channels has a value of flow
length divided by hydraulic diameter less than one fifth the ratio
of flow length divided by hydraulic diameter provided by each
channel in the first set of fluid channels.
85. The heat exchanger as recited in claim 82, in which each
channel in the second set of fluid channels has a value of flow
length divided by hydraulic diameter less than one half of the
ratio of flow length divided by hydraulic diameter provided by each
channel in the first set of fluid channels.
86. The heat exchanger as recited in claim 82, in which each
channel in the second set of fluid channels has a value of length
divided by hydraulic diameter less than one fifth of the ratio of
flow length divided by hydraulic diameter provided by the first set
of fluid channels.
87. A primary surface heat exchanger for transferring heat between
a first fluid and a second fluid, comprising: a. A network of fluid
channels consisting of substantially parallel channels defining a
primary direction of flow for the first fluid, in which said
channels are intermittently connected to one another by channels
allowing secondary flow substantially perpendicular to the primary
direction of the first fluid; and b. A set of fluid channels,
arrayed throughout the network of fluid channels, which directs the
second fluid flow to intersect with the plane defined by the
primary and secondary flow through the network; and in which each
channel of the second set of fluid channels has both a flow length
of less than 8.0 mm and a nominal hydraulic diameter less than 2.0
mm. c. A plurality of walls separating the set of fluid channels
from the network, each wall providing a potential conduit of heat
transfer between the first fluid and the second fluid.
88. The heat exchanger as recited in claim 87, wherein the set of
fluid channels provides an open area to the second fluid between
25% and 50%.
89. The heat exchanger as recited in claim 87, wherein the set of
fluid channels provides an open area to the second fluid between
30% and 45%.
90. The heat exchanger as recited in claim 87, wherein the set of
fluid channels is comprised of individual channels with a
substantially rectangular cross section.
91. The heat exchanger as recited in claim 88, wherein the set of
fluid channels is comprised of individual channels with a
substantially rectangular cross section.
92. The heat exchanger as recited in claim 87, in which the set of
fluid channels directs the second fluid flow to be substantially
parallel to the normal of the first fluid flow through the network
of fluid channels.
93. The heat exchanger as recited in claim 92, in which the set of
fluid channels provides an open area to the second fluid between
25% and 50%.
94. The heat exchanger as recited in claim 92, in which the set of
fluid channels provides an open area to the second fluid between
30% and 45%.
95. The heat exchanger as recited in claim 94, in which the set of
fluid channels is comprised of individual channels with a
substantially rectangular cross section.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This Application is a continuation-in-part of co-pending
application Ser. No. 10/003,882, filed Oct. 25, 2001 and published
as U.S. patent application 2002/0125001 A1, the disclosure of which
is incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to heat exchangers,
particularly very high efficiency flat panel microchannel heat
exchangers.
SUMMARY OF THE INVENTION
[0004] Heat exchangers are used in a great many mechanical and
electrical systems. Some of the most commonly known applications
include the condenser of a refrigerator or the radiator of an
automobile. Increasingly, customers have demanded high efficiency
heat exchangers for cooling computer chips and other electronic
components. This demand, among others, produces a continuing need
for increased efficiency in a smaller volume.
[0005] Compact heat exchangers are used in applications which
demand low pressure drops in the working fluids and low overall
mass or weight. Until recently, such a heat exchanger was incapable
of transferring the amounts of heat necessary for certain
applications, such as an automobile. It is desirable to reduce the
size of the necessary heat exchanger while maintaining the rate of
heat exchange.
[0006] The rate of heat transfer for a given heat exchanger is
related to the surface area-to-volume ratio of the fluid channels.
Advances in microfabrication allow the design of a heat exchanger
to increase the amount of surface area in relation to the overall
volume. As is well understood in the field, the overall heat
transfer of a given fluid channel increases when the hydraulic
diameter of that channel decreases. So-called microchannels provide
the reduction in hydraulic diameter necessary to produce the
required heat transfer performance.
[0007] In prior heat exchangers, the hydraulic diameter of at least
one set of fluid channels is reduced. In exchange for this increase
in heat transfer performance, however, prior heat exchangers impose
a steep penalty in lost pressure (pump losses) in the working
fluids. That pressure loss has been the limit on reduction in
size.
[0008] Typically, the prior art defines a microchannel as any fluid
flow channel with a smallest dimension less than 2.0 mm. See, e.g.,
WO 2004/017008 A1.
[0009] In further illustration, U.S. patent application
2002/0125001 A1 discusses, in detail, the history of so-called
micro heat exchangers and highlights the high pressure drops
created in those designs. The heat exchanger disclosed in that
application consists of two sets of microchannels. The length of
the first fluid channels, claimed for the first time, is less than
6.0 mm and preferably less than 1.0 mm. At the same time, that heat
exchanger shows that the first fluid channels number at least 50
per square centimeter, preferably as much as 1000 per square
centimeter.
[0010] We have discovered a better design which captures the
benefits of microchannels through increased heat transfer
efficiency and limits the pressure drop to a range which makes the
heat exchangers preferable in terms of performance, weight, size
and cost. Instead of maximizing the number of fluid channels, we
have optimized the cross-section of the microchannels to maintain
the increased heat transfer performance without suffering pumping
losses to the extent the previous design requires.
[0011] The current invention uses microchannels for the first fluid
flow (typically a liquid such as water).
[0012] The current invention also uses microchannels for the second
fluid flow (typically a gas such as air), substantially reducing
the required thermal diffusion lengths for the first fluid. The
reduced fluid flow length provides a much greater value for heat
transfer per unit volume or per unit mass than has been achieved
with traditional heat exchanger design. At the same time, the
reduction in thermal diffusion length of the microchannels offsets
the pressure loss caused by the reduction in hydraulic
diameter.
[0013] The fluid channels in the first set may have a greater flow
length than that provided by the first fluid channels, but tend to
have a smaller nominal cross-sectional area.
[0014] With both fluids contained in microchannels, the current
invention does not require secondary surfaces such as fins or pins
to achieve high thermal efficiency. This allows for simpler designs
and ease of manufacturing.
[0015] As discussed in U.S. patent application 2002/0125001 A1, the
innovation which provides intersecting flows between the first
fluid channels and the second fluid channels serves to allow
relatively short flow lengths in the gas-side channels. It is
well-understood in the art that increased flow length in a
microchannel provides very little benefit in heat transfer but
produces a greater pressure drop.
[0016] In its preferred embodiment, the current invention is a
unitary body, flat panel heat exchanger. The fluid channels
described are contained in the panel. This embodiment allows heat
transfer through primary surfaces between two fluids flowing in
substantially perpendicular (or at least non-parallel) channels.
One fluid, typically water or a water/ethylene glycol mixture,
flows in the plane of the flat panel heat exchanger. The other
fluid, typically air or some other gas, flows in channels
perpendicular to the plane of the coolant flow--typically across
the shortest dimension of the flat panel.
[0017] In its preferred embodiment, the shortest dimension of the
flat panel, and the fluid flow length for the gas, is less than 8.0
mm. Other than the heat exchanger described in U.S. patent
application 2002/0125001 A1, it is believed no prior gas-fluid
cross-flow heat exchangers have been thinner than about 2.0 cm in
this dimension.
[0018] In the majority of its current applications, the current
invention provides heat transfer from a gas to a liquid. The fluid
channels for the gas usually provide very short thermal diffusion
lengths. The current invention uses microchannels with a typical
width from 0.200 mm to 1.0 mm. As is well understood in the art,
this reduction in size causes increased pressure gradients along
the length of the channels. To combat that increase, the current
invention uses a large number of these microchannels, providing
parallel flow paths for the gas which intersect the plane of the
coolant flow. As opposed to the competing design, the current
invention does not maximize the number of these cross flow
channels.
[0019] Instead, the current invention provides the cross flow
channels in the shape of substantially flat slots. Flat slots
provide a small hydraulic diameter but allow for greater flow area
to the fluid, resulting in reduced pressure drop without a loss of
heat transfer efficiency. At the same time, flat slots are easier
to manufacture than numerous smaller features.
[0020] For the same reasons, the preferred embodiment of the
current invention provides substantially flat slots for the coolant
flow.
[0021] It has been discovered that the use of flat slots allows the
current invention to provide a highly increased open area to the
gas flow. That is, the area of open channels divided by the total
frontal area seen by the gas side flow is more than 25%, and
potentially as high as 50%.
[0022] Finally, the flat slot geometry used allows for much reduced
wall thicknesses in both sets of microchannels. Prototypes of the
current invention result in wall thicknesses as low as 75 .mu.m
(0.075 mm). As is well understood in the art, the thermal
resistance of a solid is proportional to the length of the
conduction path. With conduction paths as short as 75 .mu.m, even
materials with poor thermal conductivity are effective in the
construction of the current invention. This allows the use of
polymers, ceramics, and other materials previously thought
inappropriate for use in heat exchangers.
[0023] A further advantage of the current invention is that it is a
primary surface heat exchanger. This means that the surfaces are
all transferring heat from one fluid to the other. The use of
features such as pins, fins and the like require material which has
a low thermal resistance. Just as with the reduced wall thickness
created by our design, materials with poor thermal conductivity are
appropriate if indicated by other concerns, such as strength,
corrosive materials or weight.
[0024] As an added bonus, once the flat panel cross flow micro heat
exchanger is created, a person having ordinary skill in the art
would be able to design heat exchanger systems which incorporate
multiple panels in various alignments, such as stacking panels. One
example of a heat exchanger system incorporates several flat panels
in a corrugated pattern to increase thermal transfer as a factor of
frontal area.
[0025] These and other embodiments and features of the present
invention will become even more apparent from the following
detailed description of preferred embodiments, the accompanying
figures, and the appended claims. As used in this description, the
term "microchannel" is used to mean a channel with at least one
dimension on the scale of 2.0 mm or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 illustrates schematically a cross section of an
embodiment of a flat panel cross flow micro heat exchanger in
accordance with the present invention.
[0027] FIG. 2 depicts a Scanning Electron Microscope view of a high
efficiency flat panel micro-channel heat exchanger.
[0028] FIG. 3 is a picture showing a complete view of the high
efficiency flat panel micro-channel heat exchanger panel, including
tubes which provide the coolant-side flow to the panel.
[0029] FIG. 4 is a photograph of an heat exchanger system employing
dual high efficiency flat panel microchannel heat exchangers to
increase heat transfer as a function of frontal area.
[0030] FIG. 5 is a representation of a corrugated multi-panel
microchannel heat exchanger, one of the applications of the current
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The schematic illustration of a cross section of an
embodiment of a flat panel cross flow micro heat exchanger is shown
in FIG. 1 (not drawn to scale). In FIG. 1, the cross-hatched
regions denote solid structures through which fluid may not flow.
The dotted regions denote channels through which the coolant fluid
may flow in the plane of the figure, and the open squares denote
cross-sections of the channels through which air, or some other
fluid, may flow perpendicular to the plane of the figure.
[0032] The current invention employs microchannels as fluid flow
channels for both fluids. Typical dimensions range from 200 .mu.m
to 1.0 mm. FIG. 2 is a SEM view of one embodiment of the high
efficiency flat panel microchannel heat exchanger.
[0033] In exchange for the benefits of reduced thermal resistance,
microchannels result in an increase in pressure losses compared to
macro-scale channels. The current invention alleviates that concern
by keeping the total flow length of the microchannels small. In
addition, the advanced manufacturing technique provides thinner
walls, as well as maximizing the open area available to the
fluid.
[0034] Open Area is defined as the sum of the cross-sectional area
of all the channels for a given fluid divided by the total frontal
area for a given flow. Prior inventions are believed to provide an
open area to the gas-side flow of less than 25%. The current
innovation allows open area to the gas-side flow above 25% and,
perhaps, as high as 50%. As previously mentioned, pressure losses
are reduced as open area is increased.
FABRICATION OF THE INVENTION
[0035] The invention may be manufactured by any of several methods.
An early prototype was manufactured in two halves and bonded
together. Each side was made using the LIGA process.
[0036] The LIGA process produces microstructures and is well-known
in the art. See, e.g., A. Maner et al., "Mass Production of
Microdevices with Extreme Aspect Ratios by Electroforming," Plating
and Surface Finishing, pp. 60-65 (March 1988); W. Bacher, "The LIGA
Technique and Its Potential for Microsystems--A Survey," IEEE
Trans. Indust. Electr., vol. 42, pp. 431-441 (1995); and E. Becker
et al., "Production of Separation-Nozzle Systems for Uranium
Enrichment by a Combination of X-Ray Lithography and
Galvanoplastics," Naturwissenschaften, vol. 69, pp. 520-523
(1982).
[0037] LIGA can be used to create an array of high aspect ratio
microstructures on a conductive substrate. Electroplating (often
nickel, in the most common embodiments) is performed to fill the
"open" volumes in the LIGA array with metal. The array is dissolved
through chemical processes known in the art. This leaves a metal
mold insert with micro-scale features (here, channels).
[0038] This mold insert is used to mold or emboss a polymer. The
resulting polymer negative of the mold insert is covered with a
layer of conductive metal, such as gold, through well known
processes, such as sputtering. The metal-coated polymer can be
electroplated, creating a metal shell around the polymer core.
Finally, the polymer can be dissolved by chemical processes known
in the art. The remaining metal shell provides a hollow shell with
flow paths for two intersecting streams.
Application of Flat Panel Heat Exchangers
[0039] Once in possession of the high efficiency flat panel
microchannel heat exchanger, a person having ordinary skill in the
art will be able to optimize heat exchanger systems designs using
the panel. For instance, the size of the microchannels may be
changed, the exterior dimensions may all be changed, and the panels
may be mounted at some angle other than perpendicular to the gas
side flow.
[0040] FIG. 3 is a picture of a high efficiency flat panel
microchannel heat exchanger made part of a heat exchanger system
which delivers coolant to the panel through tubes. Each tube has a
slot in which the panel may be mounted. The panel is brazed or
otherwise fixedly connected to the tubes.
[0041] FIG. 4 includes several pictures of a more complex heat
exchanger system. In this system, four (4) large high efficiency
flat panel microchannel heat exchangers are mounted in a
v-configuration. This system allows greater thermal efficiency in a
small frontal area than a single panel would allow.
[0042] Finally, FIG. 5 is a conceptual design using multiple high
efficiency flat panel microchannel heat exchangers in a corrugated
arrangement. The dimensions shown are illustrative only. Persons
having ordinary skill in the art will be able to use these high
efficiency flat panel microchannel heat exchangers in a variety of
configurations which might take advantage of the unique qualities
of the panel.
Theoretical and Experimental Performance of High Efficiency Flat
Panel Microchannel Heat Exchangers
[0043] Experimental and theoretical models provide results which
highlight the significant impact that the current invention
provides. These results demonstrate the potential of high
efficiency flat panel microchannel heat exchangers representing
substantial improvement over existing systems. In addition to
performance factors, high efficiency flat panel microchannel heat
exchangers made in accordance with this invention provide
advantages in terms of ease of use and ease of manufacturing.
[0044] Theoretical discussion of flow in channels reveals that a
fully developed flow occurs after a region of entrance effects. In
a fully developed flow, the Nusselt number becomes constant,
according to Equation 1: 1 Nu = hD h k = const . ( 1 )
[0045] Where:
[0046] Nu=the Nusselt number of the fully developed flow
[0047] h=the convective heat transfer coefficient
[0048] D.sub.h=the hydraulic diameter of the channel
[0049] k=the thermal conductivity of the fluid
[0050] Since the thermal conductivity (k) of a given fluid is
constant, the heat transfer coefficient (h) must increase when the
hydraulic diameter (D.sub.h) of the channel is reduced. These
increases are highly valuable for gas streams where the low thermal
conductivity of the gases (relative to liquids) leads to low
gas-side heat transfer coefficients.
[0051] The increase in heat transfer coefficient in microchannels
allows for a significant reduction of the heat exchanger size with
negligible increase in the pressure drop for that stream. Equation
2 approximates the heat transfer to or from a fluid, air for
instance, passing through a cross-flow heat exchanger. Equation 3
shows the pressure drop across the ends of the heat exchanger (not
including the inlet and exit contraction/expansion losses),
assuming fully developed laminar flow, a standard assumption for
the analysis of microchannel flows. 2 T coolant - T air - exit T
coolant - T air - inlet = exp [ - k air air C p - air V L D h 2 ] (
2 ) P = 32 V air L D h 2 ( 3 )
[0052] Nomenclature:
[0053] T.sub.coolant=Heat exchanger average wall (also coolant)
temperature
[0054] T.sub.air-inlet=Temperature of the air entering the heat
exchanger
[0055] T.sub.air-exit=Temperature of the air exiting the heat
exchanger
[0056] .DELTA.P=Air pressure drop across the heat exchanger
thickness
[0057] .beta.=Constant depending upon the Reynolds number of the
air flow
[0058] k.sub.air=Air thermal conductivity
[0059] L=Length of the air microchannels (also, heat exchanger
thickness)
[0060] .rho..sub.air=Air density
[0061] C.sub.p-air=Air constant pressure specific heat
[0062] V=Air velocity through (or across) the heat exchanger
[0063] D.sub.h=Hydraulic diameter of the micro (air) channels
[0064] .mu..sub.air=Air dynamic viscosity
[0065] It can be readily observed that the geometric parameter that
controls both the heat transfer and pressure drop of the gas stream
is L/D.sub.h.sup.2. Keeping L/D.sub.h.sup.2 constant and reducing
the hydraulic diameter D.sub.h, therefore, allows the reduction in
heat exchanger thickness while maintaining the same heat transfer
and pressure drop characteristics. That is, there is no pressure
drop penalty (to the first order) associated with making a heat
exchanger for a given thermal load small by using micro-scaled flow
passages.
[0066] Nonetheless, there do not appear to be any competing heat
exchangers which are designed to take advantage of this scaling
opportunity. Although some heat exchangers operate in the range of
L/D.sub.h.sup.2 between 5 mm.sup.-1 and 30 mm.sup.-1, they do not
also use an overall length less than 8.0 mm, as in the current
invention.
[0067] Within the parameters discussed above, using long slots as
the cross-sectional profile for the channels provides ease of
manufacturing compared to smaller features. In addition, long slots
provide small hydraulic diameter while leaving a large total area
for fluid flow. This selection improves the pressure drop
performance without degrading the thermal performance.
[0068] Each and every patent, patent application and printed
publication referred to above is incorporated herein by reference
in toto to the fullest extent permitted as a matter of law.
[0069] This invention is susceptible to considerable variation in
its practice. The forgoing description, therefore, is not intended
to limit, and should not be construed as limiting, the invention to
the particular embodiments presented hereinabove. Rather, what is
intended to be covered is as set forth in the ensuing claims and
the equivalents thereof permitted as a matter of law.
* * * * *