Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MIXED CARBON FOAM/METALLIC HEAT EXCHANGER
TECHNICAL FIELD
[001] The disclosure relates to ram air heat exchangers
for aircraft. More particularly, the disclosure relates to
a mixed carbon foam/metallic heat exchanger having
thermally conductive carbon foam layers which alternate
with metal foam layers to allow for the fabrication of heat
exchanger cores using materials having vastly different
coefficients of thermal expansion (CTE).
BACKGROUND
[002] In the manufacture of ram air heat exchangers
using thermally conductive carbon foam as a thermal
management material, metallic and carbon elements may be
used in fabrication of the heat exchanger core. The
metallic and carbon elements used in fabrication of the
heat exchanger core may have different coefficients of
thermal expansion (CTE). Therefore, during fabrication,
high-temperature vacuum brazing processes may generate
thermal stresses within the heat exchanger core during the
heat-up and cool-down phases of the brazing process.
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(003] Therefore, fabrication processes that address
thermal stresses caused by mismatched coefficients of
thermal expansion (CTE) in a mixed carbon foam/metallic
heat exchanger may be desirable.
SUMMARY
[004] The disclosure is generally directed to a heat
exchanger. An illustrative embodiment of the heat
exchanger includes a thermally-conductive fluid barrier
having first and second surfaces, at least one first type
of foam element placed in thermally-conductive contact with
the first surface of the thermally-conductive fluid barrier
and having a first coefficient of thermal expansion and at
least one second type of foam element placed in thermally-
conductive contact with the second surface of the
thermally-conductive fluid barrier and having a second
coefficient of thermal expansion. The first coefficient of
thermal expansion of the first type of foam element and the
second coefficient of thermal expansion of the second type
of foam element are different by at least a factor of
three.
[005] The disclosure is further generally directed to a
mixed carbon foam/metallic foam heat exchanger method. An
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illustrative embodiment of the method includes providing a
reticulated metal foam layer, providing a thermally
conductive carbon foam layer in thermally-conductive
contact with the reticulated metal foam layer, distributing
a first fluid through the reticulated metal foam layer and
distributing a second fluid through the carbon foam layer.
BRIEF DESCRIPTION OF THE ILLUSTRATIONS
[006] FIG. 1 is a perspective view of an illustrative
embodiment of the heat exchanger.
[007] FIG. 2 is an enlarged sectional view, taken along
section line 2 in FIG. 1, of a reticulated metal foam layer
of the heat exchanger.
[008] FIG. 2A is an enlarged sectional view, taken
along section line 2A in FIG. 1, of a thermally conductive
carbon foam layer of the heat exchanger.
[009] FIG. 2B is a sectional view illustrating a
reticulated metal foam layer and a thermally conductive
carbon foam layer attached to opposite sides of a
thermally-conductive fluid barrier.
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[0010] FIG. 3 is an enlarged sectional view illustrating
staggered fluid flow channels in the reticulated metal foam
layers of the heat exchanger.
[0011] FIG. 4 is an end view of the heat exchanger shown
in FIG. 1.
[0012] FIG. 5 is a flow diagram which illustrates an
illustrative embodiment of a mixed carbon foam/metallic
foam heat exchanger method.
[0013] FIG. 6 is a flow diagram of an aircraft
production and service methodology.
[0014] FIG. 7 is a block diagram of an aircraft.
DETAILED DESCRIPTION
[0015] Referring initially to FIGS. 1-4, an illustrative
embodiment of the mixed carbon foam/metallic foam heat
exchanger, hereinafter heat exchanger, is generally
indicated by reference numeral 1 in FIG. 1. The heat
exchanger 1 may include a heat exchanger frame 2 which may
be aluminum, for example and without limitation, and may
include an upper end plate 3; a lower end plate 4 placed in
an opposed relationship with respect to the upper end plate
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3; and spaced apart end plates 5 at respective ends of the
upper end plate 3 and the lower end plate 4. At each end
of the heat exchanger frame 2, carbon foam layers 14 may be
exposed through plate slots 6 which separate adjacent side
bar members 5 from each other.
[0016] At least one ductile thermal management material
layer 10 may be provided in the heat exchanger frame 2. As
shown in FIG. 2, the ductile thermal management material
layer 10 may be reticulated metal foam such as reticulated
aluminum foam, for example and without limitation. In
certain applications the fluids in the heat exchanger may
require the use of other ductile materials such as copper,
copper alloys, stainless steels, nickel alloys, etc. At
least one thermally conductive carbon foam layer 14 may be
provided in the heat exchanger frame 2 in thermally-
conductive contact with at least one ductile thermal
management material layer 10. The carbon foam may, in
certain applications be replaced by other foams, such as a
ceramic. The ductile thermal management material layer 10
and the thermally conductive carbon foam layer 14 may have
different coefficients of thermal expansion (CTEs), for
example the CTEs of the two materials may differ by a
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factor of three or more. As shown in FIG. 2B, in some
embodiments each ductile thermal management material layer
may be separated from each carbon foam layer 14 by a
thermally-conductive fluid barrier 18. Accordingly, the
ductile thermal management material layer 10 may be
attached to a first surface 18a and the carbon foam layer
14 may be attached to a second surface 18b of the
thermally-conductive fluid barrier 18 according to the
knowledge of those skilled in the art. The thermally-
conductive fluid barrier 18 may be a metal braze foil
layer, for example and without limitation.
[0017] As shown in FIGS. 1 and 3, multiple stress relief
blind slots 11 may extend into each ductile thermal
management material layer 10. The stress relief blind
slots 11 may be placed in generally parallel, staggered
relationship with respect to each other and may be oriented
in generally perpendicular relationship with respect to a
longitudinal axis of the ductile thermal management layer
10. Each stress relief slot 11 may or may not extend
across the entire thickness of the ductile thermal
management material layer 10. As shown in FIGS. 1 and 4,
stress relief blind slots 15 may also be provided in the
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thermally conductive carbon foam layer 14 and each may or
may not extend across the entire thickness of the carbon
foam layer 14. The stress relief blind slots 11 and stress
relief blind slots 15 may provide stress relief for the
heat exchanger 1 during manufacturing and in operation.
Furthermore, the stress relief blind slots 11 and stress
relief blind slots 15 may provide control of fluid flow
losses through the ductile thermal management material
layer 10 and the thermally conductive carbon foam layer 14
,respectively, in operation of the heat exchanger 1.
[0018] As shown in FIGS. 1 and 4, the ductile thermal
management material layers 10 and the thermally conductive
carbon foam layers 14 may be arranged in the heat exchanger
frame 2 in alternating relationship with respect to each
other, with each carbon foam layer 14 sandwiched between a
pair of ductile thermal management material layers 10. The
heat exchanger frame 2 may include multiple side bar
members 7 each of which may extend into a plate slot 6
between the end plates 5 at respective ends of the heat
exchanger frame 2. Each side bar member 7 may be generally
placed between ductile thermal management material layers
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and generally adjacent to a thermally conductive carbon
foam layer 14.
[0019] In some applications of the heat exchanger 1, CTE
induced thermal stresses may be a function of length scale.
Therefore, as shown in FIGS. 1 and 4, the thermally
conductive carbon foam layers 14 may be segmented in
multiple sections and tolerance-fitted together in the heat
exchanger frame 2. Segmentation of the carbon foam layers
14 may reduce the total length scale between each element
of the carbon foam layers 14 and the metallic portions of
the heat exchanger 1 such as the various elements of the
heat exchanger frame 2, for example and without limitation,
to reduce CTE induced thermal stresses between the carbon
foam layers 14 and those metallic portions of the heat
exchanger 1 during operation of the heat exchanger 1.
[0020] During fabrication of the heat exchanger 1, a
vacuum brazing process may be used as is known to those
skilled in the art. Accordingly, the ductile thermal
management material layers 10 and the thermally conductive
carbon foam layers 14, separated by thermally-conductive
fluid barriers 18, may be stacked and brazed together
during fabrication. It will be appreciated by those
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skilled in the art that during the vacuum brazing process,
the high thermal stresses resulting from thermal expansion
and contraction induced in the heat exchanger frame 2 of
the heat exchanger 1 may be absorbed by the ductile thermal
management material layers 10. The thermal management
material layers 10 may not transfer the thermal stresses
from the heat exchanger frame 2 to the thermally conductive
carbon foam layers 14. This may prevent the application of
excessive thermally induced stress on the carbon foam
layers 14.
[0021] In application of the heat exchanger 1, a first
slot (not shown) may be placed in fluid communication with
the ductile thermal management material layers 10 and a
second slot (not shown) may be placed in fluid
communication with the thermally conductive carbon foam
layers 14. A first fluid (not shown) may be distributed
from the first slot through the thermal management material
layers 10, and a second fluid (not shown) may be
distributed from the second slot through the carbon foam
layers 14. Accordingly, heat may be transferred by
convection and conduction from the hotter to the cooler of
the first fluid and the second fluid through the thermally-
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conductive fluid barrier 18 (FIG. 2B). The high thermal
stresses resulting from thermal expansion induced in the
heat exchanger 1 by the hotter of the first fluid and the
second fluid may be absorbed by the ductile thermal
management material 10. This may prevent the application
of excessive thermally induced stress on the carbon foam
layers 14. The upper end plate 3, lower end plate 4 and
side bar members 5 of the heat exchanger frame 2 may
prevent loss of fluid from the heat exchanger 1.
[0022] Referring next to FIG. 5, a flow diagram 500
which illustrates an illustrative embodiment of a mixed
carbon foam/metallic foam heat exchanger method is shown.
In block 502, a reticulated metal foam layer is provided.
In block 503, a thermally-conductive fluid barrier is
provided in thermally conductive contact with the
reticulated metal foam layer. In block 504, a thermally
conductive carbon foam layer is provided in thermally-
conductive contact with the thermally-conductive fluid
barrier. In block 506, a first fluid is distributed
through the reticulated metal foam layer. In block 508, a
second fluid is distributed through the carbon foam layer.
Heat is transferred from the hotter to the cooler of the
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first fluid and the second fluid. The reticulated metal
foam layer may absorb stresses which are induced by thermal
expansion during transfer of the heat between the fluids,
minimizing or eliminating thermal stresses exerted on the
carbon foam layer.
[0023] Referring next to FIGS. 6 and 7, embodiments of
the disclosure may be used in the context of an aircraft
manufacturing and service method 78 as shown in FIG. 6 and
an aircraft 94 as shown in FIG. 7. During pre-production,
exemplary method 78 may include specification and design 80
of the aircraft 94 and material procurement 82. During
production, component and subassembly manufacturing 84 and
system integration 86 of the aircraft 94 takes place.
Thereafter, the aircraft 94 may go through certification
and delivery 88 in order to be placed in service 90. While
in service by a customer, the aircraft 94 may be scheduled
for routine maintenance and service 92 (which may also
include modification, reconfiguration, refurbishment, and
so on).
[0024] Each of the processes of method 78 may be
performed or carried out by a system integrator, a third
party, and/or an operator (e.g., a customer). For the
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purposes of this description, a system integrator may
include without limitation any number of aircraft
manufacturers and major-system subcontractors; a third
party may include without limitation any number of vendors,
subcontractors, and suppliers; and an operator may be an
airline, leasing company, military entity, service
organization, and so on.
[0025] As shown in FIG. 7, the aircraft 94 produced by
exemplary method 78 may include an airframe 98 with a
plurality of systems 96 and an interior 100. Examples of
high-level systems 96 include one or more of a propulsion
system 102, an electrical system 104, a hydraulic system
106, and an environmental system 108. Any number of other
systems may be included. Although an aerospace example is
shown, the principles of the invention may be applied to
other industries, such as the automotive industry.
[0026] The apparatus embodied herein may be employed
during any one or more of the stages of the production and
service method 78. For example, components or
subassemblies corresponding to production process 84 may be
fabricated or manufactured in a manner similar to
components or subassemblies produced while the aircraft 94
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is in service. Also, one or more apparatus embodiments may
be utilized during the production stages 84 and 86, for
example, by substantially expediting assembly of or
reducing the cost of an aircraft 94. Similarly, one or
more apparatus embodiments may be utilized while the
aircraft 94 is in service, for example and without
limitation, to maintenance and service 92.
[0027] Although the embodiments of this disclosure have
been described with respect to certain exemplary
embodiments, it is to be understood that the specific
embodiments are for purposes of illustration and not
limitation, as other variations will occur to those of
skill in the art.
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