U.S. patent application number 13/674523 was filed with the patent office on 2014-05-15 for acoustic structure with internal thermal regulators.
This patent application is currently assigned to HEXCEL CORPORATION. The applicant listed for this patent is HEXCEL CORPORATION. Invention is credited to Earl Ayle.
Application Number | 20140133964 13/674523 |
Document ID | / |
Family ID | 50681849 |
Filed Date | 2014-05-15 |
United States Patent
Application |
20140133964 |
Kind Code |
A1 |
Ayle; Earl |
May 15, 2014 |
ACOUSTIC STRUCTURE WITH INTERNAL THERMAL REGULATORS
Abstract
Thermally insulating septums are located internally within the
cells of an acoustic honeycomb to regulate heat flow into the
acoustic structure. The internally located insulating septums
protect the honeycomb and acoustic septums located within the
honeycomb cells from heat damage that might otherwise be caused by
a heat source, such as the hot section of a jet engine. The
internal thermal regulators are useful in combination with heat
blankets or other thermal insulating structures to provide a
reduction in size and/or weight of the insulating structure while
still providing the same overall degree of thermal insulation for
the acoustic honeycomb.
Inventors: |
Ayle; Earl; (Chandler,
AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HEXCEL CORPORATION |
Dublin |
CA |
US |
|
|
Assignee: |
HEXCEL CORPORATION
Dublin
CA
|
Family ID: |
50681849 |
Appl. No.: |
13/674523 |
Filed: |
November 12, 2012 |
Current U.S.
Class: |
415/119 ;
181/292; 29/896.2 |
Current CPC
Class: |
F05D 2250/283 20130101;
F02C 7/24 20130101; Y10T 29/4957 20150115; Y02T 50/60 20130101;
E04B 1/8209 20130101; F02K 1/822 20130101; Y02T 50/672 20130101;
F05D 2260/963 20130101; Y02T 50/675 20130101; F02K 1/827 20130101;
G10K 11/172 20130101; F05D 2260/231 20130101 |
Class at
Publication: |
415/119 ;
181/292; 29/896.2 |
International
Class: |
E04B 1/82 20060101
E04B001/82; F02C 7/24 20060101 F02C007/24 |
Claims
1. An acoustic structure having an internal thermal regulator, said
acoustic structure comprising: a honeycomb comprising a first edge
that is to be located closest to a high temperature area and a
second edge, said honeycomb comprising a cell defined by a
plurality of walls that extend between said first and second edges;
a thermally insulating septum located within said cell adjacent to
the first edge of said honeycomb to provide said internal thermal
regulator, said thermally insulating septum comprising sides which
are attached to the walls of said cell; an acoustic damping
material located within said cell between said thermally insulating
septum and said second edge; and a perforated acoustic panel
attached to the second edge of said honeycomb wherein sound waves
can enter said cell through said perforated acoustic panel.
2. An acoustic structure according to claim 1 which further
comprises a solid protective sheet which is attached to the first
edge of said honeycomb.
3. (canceled)
4. An acoustic structure according to claim 1 wherein said acoustic
damping material is an acoustic septum.
5. An acoustic structure according to claim 1 wherein said acoustic
damping material is difunctional filler material that provides both
thermal insulation and sound attenuation within said cell.
6. An acoustic structure according to claim 1 which further
includes a thermal insulation structure located between the first
edge of said honeycomb and said high temperature area.
7. An acoustic structure according to claim 6 wherein an air gap is
located between said thermal insulation structure and the first
edge of said honeycomb.
8. An acoustic structure according to claim 1 wherein said
honeycomb walls comprise fibers and a cured resin.
9. A jet engine that comprises an acoustic structure according to
claim 1.
10. A jet engine that comprises an acoustic structure according to
claim 6 and wherein said jet engine comprises a high temperature
area having a temperature of between 750.degree. F. and 900.degree.
F.
11. A method for making an acoustic structure having an internal
thermal regulator, said method comprising the steps of: providing a
honeycomb comprising a first edge that is to be located closest to
a high temperature area and a second edge, said honeycomb
comprising a cell defined by a plurality of walls that extend
between said first and second edges; locating a thermally
insulating septum within said cell adjacent to the first edge of
said honeycomb to provide said internal thermal regulator, said
thermally insulating septum comprising sides which are attached to
the walls of said cell; locating, an acoustic damping material
within said cell between said thermally insulating septum and said
second edge; and attaching a perforated acoustic panel to the
second edge of said honeycomb wherein sound waves can enter said
cell through said peforated acoustic panel.
12. A method for making an acoustic structure according to claim 11
which includes the additional step of attaching a solid protective
sheet to the first edge of said honeycomb.
13. (canceled)
14. A method for making an acoustic structure according to claim 11
wherein said acoustic damping material is an acoustic septum.
15. A method for making an acoustic structure according to claim 11
wherein said acoustic damping material is a difunctional filler
material that provides both thermal insulation and sound
attenuation within said cell.
16. A method for making an acoustic structure according to claim 11
which further includes the additional step of locating a thermal
insulation structure adjacent to the first edge of said
honeycomb.
17. A method for making an acoustic structure according to claim 16
wherein said thermal insulation structure is located adjacent to
the first edge of said honeycomb such that an air gap is located
between said thermal insulation structure and the first edge of
said honeycomb.
18. A method for making an acoustic structure according to claim 11
wherein said honeycomb walls comprise fibers and a cured resin.
19. A method for providing thermal insulating and sound dampening
for a jet engine that comprises a high temperature area, said
method comprising the step of locating an acoustic structure having
an internal thermal regulator according to claim 1 adjacent to said
high temperature area.
20. A method for providing thermal insulating and sound dampening
for a jet engine that comprises a high temperature area, said
method comprising the step of locating an acoustic structure having
an internal thermal regulator according to claim 6 adjacent to said
high temperature area.
21. An acoustic structure according to claim 1 wherein said
thermally insulating septum comprises hollow microspheres and a
matrix of resin.
22. An acoustic structure according to claim 21 wherein said matrix
of resin comprises polyimide resin.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to acoustic
structures that are used to attenuate or dampen noise that emanates
from a particular source. More particularly, the present invention
is directed to acoustic structures that are exposed to relatively
high temperatures and the systems that are used to protect such
acoustic structures from damage that might be caused by such heat
exposure.
[0003] 2. Description of Related Art
[0004] It is widely recognized that the best way of dealing with
noise generated by a specific source is to treat the noise at the
source. This is typically accomplished by adding acoustic damping
structures to the structure of the noise source. One particularly
problematic noise source is the jet engine used on most passenger
aircraft. Acoustic structures are typically incorporated in the
engine inlet, nacelle and combustion/exhaust structures. These
acoustic structures include acoustic resonators that contain
relatively thin acoustic materials or grids that have millions of
holes that create acoustic impedance to the sound energy generated
by the engine.
[0005] Honeycomb has been a popular material for use in aircraft
and aerospace vehicles because it is relatively strong and
lightweight. For acoustic applications, acoustic materials are
added to the honeycomb structure so that the honeycomb cells are
acoustically closed at the end located away from the noise being
dampened and covered with a porous covering at the end located
closest to the noise. The closing of the honeycomb cells with
acoustic material in this manner creates an acoustic resonator that
provides attenuation, damping or suppression of the noise. Acoustic
septums are also usually located within the interior of the
honeycomb cells in order to provide the resonator with additional
noise attenuation properties.
[0006] Large jet engines include a combustion or hot section that
is located centrally within the engine. The hot section produces
large amounts of hot combustion gases. The hot section is
surrounded by an annular passageway through which air flows at much
colder temperatures. The hot sections of present day jet engines
typically operate at temperatures on the order of 500.degree. F. to
750.degree. F. The next generation of jet engines is being designed
to have hot sections that operate at higher temperatures which are
expected to be as high as 900.degree. F. The higher hot section
operating temperature is necessary in order to produce lower
emissions and to achieve greater fuel economy.
[0007] Acoustic structures that are located near the hot sections
must be protected against the relatively high temperatures in order
to avoid damage to the honeycomb and/or acoustic septums. This is a
particular problem for acoustic honeycomb made from composite
materials which utilize matrix resins that have maximum operating
temperatures on the order of 350.degree. F. to 500.degree. F.
depending upon the type of resin. The material used to make the
acoustic septum may also be damaged when exposed directly to the
heat generated by the hot section.
[0008] One current approach that is used to protect acoustic
structures from heat generated by the hot section is to place an
insulating structure, such as a heat blanket between the hot
section and the acoustic structure being protected. The heat
blanket reduces the flow of heat into the acoustic structure to
provide the required thermal protection. Although heat blankets
provide adequate thermal insulation, they also take up valuable
space and add weight to the engine. In addition, the service life
of a typical heat blanket is limited so that it must be replaced at
specified time intervals. The thermal blanket must also be removed
to allow inspection of underlying structures. This removal and
reinstallation process is time consuming and many times results in
the heat blanket being damaged. Repairing and/or replacing a
damaged heat blanket can involve significant added time and
costs.
[0009] Another approach used to thermally protect acoustic
structures is to coat the high temperature side of the acoustic
structure with high temperature silicone. Such high temperature
silicone coatings provide adequate thermal protection. However, the
insulating coatings must be scrapped and peeled off in order to
inspect the underlying acoustic structure. This is a time consuming
process that also destroys the coating. A new coating must be
applied to the acoustic structure once the inspection has been
completed. Application of a new silicone coating is a time
consuming process that includes the additional cost of the new high
temperature silicone coating material.
[0010] There presently is a need to design thermal protection
systems for acoustic structures that are more efficient, smaller
than and not as heavy as existing thermal protection systems. The
need is especially great for acoustic structures that will be used
in the next generation of large jet engines where even higher hot
section operating temperatures are expected.
SUMMARY OF THE INVENTION
[0011] In accordance with the present invention, it was discovered
that thermally insulating septums can be located internally with
the acoustic honeycomb to regulate heat flow into the acoustic
structure and provide an effective thermal insulation system that
protects the honeycomb and acoustic septums from heat damage that
might otherwise be caused by a heat source, such as the hot section
of a jet engine. The internal thermal regulators may be used alone
or in combination with heat blankets or other external thermal
insulating structures depending upon the temperatures to which the
acoustic honeycomb is exposed.
[0012] The present invention is directed to acoustic structures, in
general, and to acoustic honeycomb located near the hot section of
a jet engine, in particular. The acoustic structures in accordance
with the present invention include a honeycomb that has a first
edge located closest to a high temperature area and a second edge
located away from the high temperature area. The honeycomb includes
a plurality of cells that are defined by walls that extend between
the first and second edges of the honeycomb.
[0013] As a feature of the present invention, thermally insulating
septums are located internally within the cells. The thermally
insulating septums are located adjacent to the first edge of the
honeycomb to function as an internal thermal regulator to control
or prevent the flow of heat into the body of the honeycomb.
[0014] As a further feature of the present invention, acoustic
damping material is located internally within the cells between the
thermally insulating septums and the second edge of the honeycomb
to provide attenuation of noise. The acoustic damping material is
in the form of acoustic septums and/or a difunctional filler
material. The difunctional filler material provides both sound
damping and thermal insulation internally within the honeycomb
cells.
[0015] The use of internally located insulating septums in
accordance with the present invention provides a thermal regulator
in the honeycomb which allows one to reduce or even eliminate the
need for a separate heat blanket or other external thermal barrier
depending upon the operating temperature of the hot section, the
temperature limits of the honeycomb material and the amount of
difunctional material located in the honeycomb cells.
[0016] The internalization of a portion of the heat protection
system by using internal insulating septums provides a heat
regulation system that has a number of design variables which may
be used to achieve thermal regulation efficiencies that are not
possible when using only an external heat blanket. As a result, the
overall size and weight of the thermal protection system can be
reduced while still maintaining the necessary degree of thermal
protection for the acoustic honeycomb. This feature is particularly
useful for thermally protecting acoustic structures in the next
generation of jet engines where acoustic structures need to be
thermally protected against higher operating temperatures while at
the same time minimizing the weight and size of the thermal
protection system as much as possible.
[0017] The above described and many other features and attendant
advantages of the present invention will become better understood
by reference to the following detailed description when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 shows a simplified partial cross-sectional view of a
jet engine in which an acoustic structure that includes internal
thermal regulators in accordance with the present invention.
[0019] FIG. 2 is a simplified depiction of a portion of an acoustic
structure that includes internal thermal regulators (insulating
septums) in accordance with the present invention.
[0020] FIG. 3 is an exploded view showing the exemplary acoustic
honeycomb, solid protective sheet and a perforated acoustic panel
prior to their being assembled to form the exemplary acoustic
structure.
[0021] FIG. 4 is a simplified end view of FIG. 1 showing the
acoustic structure ocated adjacent to the hot section of the jet
engine.
[0022] FIG. 5 is a simplified depiction of a portion of an
exemplary preferred acoustic structure that includes internal
thermal regulators in accordance with the present invention and
which additionally includes an external thermal blanket. This
exemplary preferred acoustic structure is intended for use in next
generation large jet engines that have hot sections that operate at
temperatures up to 900.degree. F. and higher.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The acoustic structure of the present invention may be used
for damping noise from a wide variety of noise sources where the
acoustic structure is exposed on one side to elevated temperatures.
The acoustic structure is well-suited for use in damping noise
generated by aircraft engines and particularly the large jet
engines used for commercial aircraft. The acoustic structure
includes internal thermal regulators so that it may be employed,
without a heat blanket or other external thermally insulating
structure, at locations within current engine designs which operate
at maximum temperatures on the order of the 600.degree. F. to
750.degree. F. A preferred acoustic structure in accordance with
the present invention includes a heat blanket or other external
thermally insulating structure in order to meet the increased
thermal load produced by the next generation large jet engines. The
next generation of large jet engines will operate in some hot
sections at temperatures up to 900.degree. F. and higher.
[0024] The following detailed description is limited to exemplary
embodiments of acoustic structures located within a jet engine. The
embodiments include acoustic structures both with and without
external thermally insulating structures, such as heat blankets. It
will be understood that the acoustic structures of the present
invention may also be used in any situation where damping of noise
from a noise source is desired and wherein the acoustic structure
is exposed on one side to high temperatures.
[0025] An exemplary jet engine is shown at 10 in the FIG. 1. The
jet engine 10 includes a combustion core or hot section 12 which
generates a primary hot air flow as represented by arrow 14. The
hot air flow within the hot section or high temperature area 12 can
be at temperatures ranging from 600.degree. F. to 900.degree. F.
and higher depending upon the jet engine type and design. A nacelle
structure 16 is located around the hot section 12 to provide an
annular duct 18 through which cold secondary air flows as
represented by arrow 20. The cold air flow enters the jet engine at
a temperature equal to the outside air temperature and is heated as
it passes through the annular duct 18 to temperatures that are
equal to or slightly less than the temperature of the hot section
12.
[0026] An exemplary acoustic structure in accordance with the
present invention is located in the outer portion of the hot
section 12 as shown at 22. The acoustic structure 22 includes a
first side 24 that is located closest or adjacent to the hot
section or high temperature area 12 of the jet engine. The acoustic
structure 22 also includes a second side 26 that is located closest
or adjacent to the cool air duct or low temperature area 18 of the
jet engine. The acoustic structure 22 is shown in FIGS. 1 and 2
without a heat blanket. A heat blanket may be added, if necessary,
to provide additional thermal protection. An exemplary thermal
insulation system in accordance with the present invention that
includes a heat blanket is described below and shown in FIG. 5.
[0027] A detailed simplified cross-sectional view of the acoustic
structure 22 is shown in FIG. 2. The acoustic structure 22 includes
a honeycomb 28 that includes the walls 30a-30e which define
honeycomb cells 32a-32d that extend from a first edge 34 of the
honeycomb to a second edge 36 of the honeycomb. As a feature of the
present invention, thermally insulating septums 38a-38d are located
within the honeycomb cells adjacent to the first edge 34 of the
honeycomb to provide an internal thermal regulator within each
cell. Acoustic septums 40a to 40c are also located within the
honeycomb cells to provide desired acoustic dampening. If desired,
more than one acoustic septum may be located in an individual
honeycomb shell as shown in cell 32a wherein two septums 40a are
located therein. A solid protective sheet 42 is attached to the
first edge of the honeycomb and a perforated acoustic panel 44 is
attached to the second edge of the honeycomb. If desired,
difunctional material that is both insulating and sound damping may
be placed within the honeycomb as shown at 48a, 48b and 48c.
[0028] In FIG. 3, the acoustic structure 22 is shown in prior to
the solid protective sheet 42 and perforated acoustic panel 44
being attached to the edges of honeycomb 28. The thermally
insulating septums are referenced as a group 38 and the acoustic
septums are referenced as a group 40. The acoustic structure is
shown as a planar structure in FIG. 3. The actual final structure
will typically be curved, as shown in FIG. 4 to provide an annular
structure that surrounds the hot section of the jet engine
[0029] In FIG. 4, a simplified end view of a section of FIG. 1 is
shown where arrows 50 depict the heat radiated from the hot section
12 which is regulated by the thermally insulating septums 38. The
identifying numbers in FIG. 4 correspond to the identifying numbers
used in FIGS. 1-3. As will be discussed below, a thermal blanket or
other external heat insulating structure will optionally be located
between the acoustic structure 22 and the hot section 12 to provide
additional thermal insulation in those situations where the
insulating septums alone cannot adequately protect a given
honeycomb material from the heat generated by the hot section.
[0030] The materials used to make the honeycomb 28 can be any of
those typically used in acoustic structures including metals,
ceramics and composite materials. Exemplary metals include
stainless steel, titanium and aluminum alloys. The present
invention is particularly useful for honeycomb made from composite
materials which tend to have maximum operating temperatures that
are much lower than metals and ceramics. Exemplary composite
materials include fiberglass, Nomex and various combinations of
graphite or ceramic fibers with suitable matrix resins. Matrix
resins that can withstand relatively high temperatures (450.degree.
F. to 650.degree. F.) are preferred. For example, when the matrix
resin is polyimide, the maximum operating temperature for the
honeycomb is on the order of 500.degree. F. to 650.degree. F.
Composite honeycomb in which the matrix resin is a high-performance
epoxy typically have a much lower maximum operating temperature on
the order of 350.degree. F. to 400.degree. F. It is preferred that
the heat transfer into the honeycomb be regulated such that the
temperature of the honeycomb remains at a level that is equal to or
less than the maximum operating temperature of the matrix
resin.
[0031] The desired reduction in temperature between the high
temperature area and the second edge of the honeycomb will vary
depending upon the highest operating temperature of the hot section
and the maximum operating temperature of the honeycomb resin. The
larger the difference between the two temperatures, the greater the
amount of thermal regulation that must be designed into the
insulating septums and heat blanket, if required. In general, the
type of material used to make the insulating septums as well as the
thickness and location of the septums should provide a steady-state
reduction in temperature of at least 225.degree. F. Steady-state
temperature reductions of at least 375.degree. F. are typically
needed for hot sections operating in the higher temperature ranges
of 750.degree. F. to 900.degree. F.
[0032] As an example, if the operating temperature of the high
temperature area is 700.degree. F. and the maximum operating
temperature of the honeycomb matrix resin is 450.degree. F., then
the insulating septums are chosen such that a steady-state
temperature of the honeycomb is obtained that is at least
250.degree. F. below the operating temperature of the hot section
or high temperature area. In some instances it may be desired to
achieve the required 250.degree. F. drop in temperature using only
insulating septums. Optionally, a thermal blanket or other external
thermal insulator may be used to provide a portion of the required
heat regulation.
[0033] A preferred exemplary embodiment of a thermally regulated
acoustic structure in accordance with the present invention is
shown at 60 in FIG. 5. The acoustic structure 60 includes an
acoustic honeycomb 62 which includes internally located thermally
insulating septums 63. The acoustic honeycomb 62 is used in
combination with an external thermal insulator, such as thermal or
heat blanket 64. The thermal blanket is spaced from the edge of the
honeycomb 62 using spacers 66 in order to form a thermally
insulating chamber 68. The thermally insulating chamber 68 may be
formed using spacers 66 to keep the heat blanket 64 spaced away
from the acoustic honeycomb 62 or any other type of connection
structure may be used provided that the heat blanket 64 is securely
attached to the acoustic honeycomb 62 in such a way that a space or
chamber is formed between the heat blanket 64 and the acoustic
honeycomb 62.
[0034] The acoustic structure 60 has a honeycomb matrix resin that
has an exemplary maximum operating temperature of 450.degree. F.
The acoustic structure is designed to be used near exemplary hot
sections that operate at temperatures as high as 900.degree. F. As
shown in FIG. 5, the heat blanket 64 has a thickness and weight
that are designed to regulate heat flow such that the temperature
on the low temperature side (inside) of the heat blanket is
200.degree. F. below the hot section side (outside) of the heat
blanket. The combination of the air gap or thermally insulating
chamber 68 and the thermally insulating septums 63 provides further
heat regulation such that the temperature on the low temperature
side of the insulating septums is 250.degree. F. below the
temperature on the inside of the heat blanket.
[0035] In a conventional thermally protected acoustic system, the
acoustic structure would be protected only by a heat blanket, as
shown at 70. The heat blanket 70, by itself, would have to be
sufficiently thick and heavy to provide the desired heat regulation
from 900.degree. F. down to 450.degree. F. Such a structure
(acoustic honeycomb+heat blanket) would have a thickness
represented by "t". As shown in FIG. 5, the present invention uses
thermally insulating septums to provide a design variable where the
thickness and weight of the heat blanket is substantially reduced
while maintaining the same thickness (t) of the overall structure.
This design variable allows one to replace a portion of the heat
blanket with a thermally insulating chamber 68 that is much lighter
than the heat blanket. Although the air gap or thermally insulating
chamber is not as thermally insulating as the portion of the heat
blanket that it replaces, the combination of the air gap and
internally located insulating septums provide the same degree of
heat regulation at a much lighter weight.
[0036] The acoustic structure 60 shown in FIG. 5, where the heat
blanket 64 is separated from the acoustic honeycomb 62 by an air
gap 68, is only exemplary. If desired, the acoustic structure 62
may be placed directly in contact with the thermal blanket 64. This
may be desirable in those situations where the thickness (t) of the
thermally protected acoustic structure 60 is to be kept at a
minimum to meet design requirements.
[0037] The thickness of the insulating septums 38a-38d and the
material used to form the insulating septums can be varied in order
to provide desired levels of insulation and heat regulation so that
the temperature of the honeycomb remains below the maximum
operating temperature of the honeycomb, as described above. It is
not necessary that the insulating septums function as a heat
blanket or other thermal blocking structure that completely
insulates the honeycomb from heat. Instead, the insulating septums
are intended to regulate the amount of heat transferred into the
honeycomb cells so that the temperature within the honeycomb
remains below levels that could be potentially destructive to the
honeycomb.
[0038] The thermally insulating septums 38a-38d can be made from
any suitable insulating material that provides the necessary heat
regulation or insulation between the hot side 24 of the acoustic
structure and the cool side 26 of the acoustic structure. As a
feature of the invention, the insulating septums are located within
the honeycomb cells to provide an "in-core" heat regulation system
as opposed to an exterior system, such as an insulating blanket or
sheet. The thermally insulating septums are preferably made from
hollow ceramic or glass high temperature insulating microspheres
that held together by a matrix of high temperature resin. They may
also be made from combinations of high temperature insulating
fibers in a matrix of high temperature resin or a low conductivity
ceramic material in a foamed resin matrix.
[0039] The hollow ceramic microspheres are typically made from
glass, alumina, titanium dioxide, iron oxide and fly ash. The
hollow microspheres can have diameters that range in size from 50
microns to 250 microns. Exemplary hollow microspheres are described
in published United States Patent Appl. No. US 2010/0107611 A1, the
contents of which are hereby incorporated by reference. The
microspheres are preferably combined with uncured high temperature
resin to form a viscous material that is formed into a layer into
which the first edge of the honeycomb is dipped. The thickness of
the viscous layer determines the thickness of the thermally
insulating septums that are formed when the matrix resin is
subsequently cured. Alternatively, a layer of insulating material
may be formed and then "cookie-cut" into the honeycomb cells using
the edge of the core to cut through the insulating material. In
addition, the insulating septums may be pre-formed and then
inserted into the honeycomb cells where they are friction fit
and/or glued in place.
[0040] The amount of high temperature matrix resin is chosen so
that only the minimum amount of matrix resin is present to provide
adequate agglomeration of the microspheres and adherence to the
honeycomb walls. Exemplary high temperature matrix resins for the
hollow ceramic microspheres include polyimide resins, such as
Skybond 700 and 705, which are available from Industrial Summit
Technology Corporation (Parkin, N.J.) or Unitech RP46 and RP50,
which are available from Unitech Corporation (Arlington, Va.).
Typically, the hollow ceramic microsphere will make up from 85 to
95 weight percent of the viscous material used to form the
insulating septums with the remainder of the material being the
matrix resin. After the honeycomb has been inserted into the
viscous layer of insulating material, the resin matrix is cured
according to standard procedures for the particular matrix resin to
form the insulating septum. The insulating septum is held in place
by adhesion between the matrix resin and the honeycomb walls. The
insulating septum is essentially a disk of closely packed hollow
ceramic microspheres that are held together and held in place
within the honeycomb by the high temperature resin matrix.
[0041] The insulating septums may be formed such that all of the
honeycomb cells contain insulating septums made from the same layer
of microsphere insulating material. Alternatively, one or more
cells may be selectively plugged with foam wax or other removable
material. After formation of the first set of insulating septums,
the first set of septums is covered and additional insulating
septums can be formed in the previously plugged cells. This type of
selective plugging and/or protection of the honeycomb cells allows
one to make an acoustic structure that contains insulating septums
that are made from different insulating materials and which have
different thicknesses.
[0042] The acoustic septums 40a-40c can be made from any of the
standard acoustic materials used it to provide noise attenuation
including woven fibers and perforated sheets. The use of the woven
fiber acoustic septums is preferred. These acoustic materials are
typically provided as relatively thin sheets of an open mesh fabric
that are specifically designed to provide noise attenuation. It is
preferred that the acoustic material be an open mesh fabric that is
woven from monofilament fibers. The fibers may be composed of
glass, carbon, ceramic or polymers. Monofilament polymer fibers
made from polyamide, polyester, polyethylene
chlorotrifluoroethylene (ECTFE), ethylene tetrafluoroethylene
(ETFE), polytetrafluoroethyloene (PTFE), polyphenylene sulfide
(PPS), polyfluoroethylene propylene (FEP), polyether ether ketone
(PEEK), polyamide 6 (Nylon 6, PA6) and polyamide 12 (Nylon 12,
PA12) are just a few examples. Open mesh fabric made from PEEK is
preferred for high temperature applications, such as nacelles for
jet engines. Exemplary septums are described in U.S. Pat. Nos,
7,434,659; 7,510,052 and 7,854,298, the contents of which are
hereby incorporated by reference. Septums made by laser drilling
plastic sheets or films may also be used.
[0043] The solid protective sheet 42 is preferably a high
temperature nonmetallic skin that is able to withstand relatively
high temperatures on the order of 600.degree. F. to 900.degree. F.
The material is preferably, but not necessarily, thermally
insulating. The protective sheet is intended to protect the
honeycomb structure from direct contact with the hot gases formed
in the hot section 12. The protective sheet may be eliminated, as
shown in FIG. 5, when an external insulator, such as a heat blanket
or insulator is used. Any of the materials normally used to protect
underlying structures from hot gases may be used to form the solid
protective sheet.
[0044] It is preferred that a heat blanket be used in combination
with the thermally regulated acoustic honeycomb. The hot engine
temperatures are blocked first by the insulation or heat blanket
which also provides physical protection of the acoustic honeycomb
from potentially abrasive gases. In addition to reducing the
temperature and protecting underlying structures, the insulation
blanket can also be removed to check the acoustic bypass duct. The
acoustic bypass duct structure is inspected to confirm it has not
seen excess temperatures which would affect structural integrity.
The insulation blankets are also inspected and/or replaced during
this periodic inspection.
[0045] The material used to make the perforated acoustic panel 44
can be any of the materials commonly used for such porous acoustic
structures provided that the pores or perforations in the structure
are sufficient to allow the sound waves from the jet engine or
other source to enter into the acoustic cells or resonators.
[0046] In general, the honeycomb cells will typically have a
cross-sectional area ranging from 0.05 square inch to 1 square inch
or more. The depth of the cells (honeycomb thickness or core
thickness "T" in FIG. 2) will generally range from 0.25 to 3 inches
or more. For honeycomb used in acoustic structures 22 that are
located adjacent to the hot section 12 of a jet engine, the
honeycomb cells will typically have a cross-sectional area of
between about 0.1 to 0.5 square inch and a thickness (T) of between
about 1.0 and 2.0 inches.
[0047] As mentioned above, additional difunctional material 48 can
be added to the honeycomb cells either alone, as shown at 48c in
FIG. 2 or between the acoustic septum and the thermally insulating
septum, as shown at 48a and 48b. The additional insulating material
is preferably a difunctional material. This means that the material
not only provides additional thermal insulation, but also provides
some degree of sound attenuation. Exemplary difunctional materials
include spun fibers, such as glass fibers or high temperature
foams. The type, amount and location of the additional difunctional
material may be varied widely within the honeycomb cells to achieve
an equally wide variety of thermal regulation and noise attenuation
objectives.
[0048] The acoustic structure of the present invention provides a
number of advantages which include a substantial reduction in the
amount of heat flow from the high temperature side of the acoustic
structure to the low temperature side. This can reduce or eliminate
the need for a separate external heat shield. In addition, one can
form different sizes and types of insulating septums inside of the
honeycomb cells in order to fine tune and carefully regulate the
amount of heat that flows through various parts of the
honeycomb.
[0049] The heat flow control or regulation feature provided by the
internal insulating septums works well in combination with the
acoustic septums that are also located in the honeycomb cells. The
insulating septums provide thermal protection for the acoustic
septums, which like the honeycomb tend to fail at temperatures well
below the hot section operating temperatures. The present invention
provides a number of advantages, as described above, that can only
be obtained by the unique combination of septums described herein
where internally located septums provide both thermal protection
and sound attenuation.
[0050] It should be noted that the operating temperatures for the
hot sections and the maximum operating temperatures for the
honeycomb matrix resin are exemplary only. The present invention
may be applied to a wide range of noise damping situations where it
is necessary to achieve effective heat protection using a minimum
amount of weight and space. The present invention moves at least a
portion of the heat protection system into the honeycomb by
providing internal insulating septums. The use of internal
insulating septums alone or in combination with an external heat
protective structure provides an effective way to minimize the
weight and size of the overall heat-protected acoustic structure,
especially with respect to next generation jet engines that operate
at relatively high temperatures.
[0051] Having thus described exemplary embodiments of the present
invention, it should be noted by those skilled in the art that the
within disclosures are exemplary only and that various other
alternatives, adaptations and modifications may be made within the
scope of the present invention. Accordingly, the present invention
is not limited by the above-described embodiments, but is only
limited by the following claims.
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