CA1087406A - Sound suppressing structure with thermal relief - Google Patents
Sound suppressing structure with thermal reliefInfo
- Publication number
- CA1087406A CA1087406A CA285,937A CA285937A CA1087406A CA 1087406 A CA1087406 A CA 1087406A CA 285937 A CA285937 A CA 285937A CA 1087406 A CA1087406 A CA 1087406A
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- Canada
- Prior art keywords
- panel
- high frequency
- frequency panel
- tubes
- recited
- Prior art date
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Abstract
SOUND-SUPPRESSING STRUCTURE
WITH THERMAL RELIEF
Abstract Sound-suppressing structure comprising stacked acoustic panels wherein the inner high frequency panel is mounted for thermal expansion with respect to the outer low frequency panel. Slip joints eliminate the potential for thermal stresses, and a thermal expansion gap between the panels provides for additional relative thermal growth while reducing heat convection into the low frequency panel.
WITH THERMAL RELIEF
Abstract Sound-suppressing structure comprising stacked acoustic panels wherein the inner high frequency panel is mounted for thermal expansion with respect to the outer low frequency panel. Slip joints eliminate the potential for thermal stresses, and a thermal expansion gap between the panels provides for additional relative thermal growth while reducing heat convection into the low frequency panel.
Description
79~6 In recent years, increasing attention has been directed at the noise characteristically emitted by aircraft gas turbine engines, and Federal regulations now limit the permissible noise levels. Accordingly, more effective noise suppression techniques are continually being sought by the gas turbine engine design community. One technique which has found wide-spread acceptance in reducing the noise propagating from engine inlet and exhaust ducts is to line the duct walls with a sound-suppression, or sound-absorbent, material. In one form, the material comprises a sand-wich of two thin metal facing sheets or skins separated by a core material, generally of the cellular honeycomb variety. This -honeycomb sandwich material has its inner skin perforated so that all the cells are vented to the duct flow path. As is well known, the cells function as Helmholtz resonators to tune out noise within a frequency band which is related to the cell size. In order to broaden the band of frequencies suppressed without increasing treatment length, a stacked configuration may be employed wherein a plurality of cellular cavities having a variety of cavity volumes are spaced from the duct by a variety of distances, with a plurality of neck passages provided for communicating between the various cavities and the duct. U.S.
Patent 3,819,009, Motzinger, entitled "Duct Wall Acoustic Treatment," which is assigned to the assignee of the present invention, is representative of such a structure.
When such a stacked sandwich material is employed for noise suppression in a hot gas environment, typified by bas turbine engine exhaust nozæles and ducts, a potential differential thermal expansion problem exists. This is due to the large temperature gradient which exists between the hot ::
.. . .
~ ' ~' :.
., ~ . . : , -~74~6 flow path defining honeycomb facing sheet and the relatively cooler opposite (backside) tacing sheet. As the engine is cycl~d Illr(mgh()ut its opcrating range, cyclic thermal stresses are impose-l on the~ sound-suppression material. These -thermal stresses and the resulting distortion and fatigue may reduce the structural life and, thus, effectively increase the cost of the engine over its life cycle.
Therefore, a means is needed for making use of the inherent acoustic advantages of honey;comb sandwich material in a hot gas environ-ment without subjecting it to high levels of thermal stress. In short, the lQ problem is to use the honeycomb structure so as to take advantage of its acoustic properties without inCUrring structural liabilities in a hot gas environment.
SUMMARY OF THE INVENTION
Acoordingly, it is the primary object of the present invention to provide a sound-suppressing structure of the honeycomb variety in which the potential for thermal stresses is minimized.
It is a further object of the present invention to minimize the - potential for thermal stresses in stacked cellular acoustic suppression material for disposition in a gas turbine engine exhaust duct. -These and other objects and advantages will be more clearly understood from the following detailed description, drawings and spccific examples, all of which are intended to be typical of rather than in any way limiting to the scope of the present invention.
Briefly stated, the above objectives are accomplished in a structure comprising a duct wall and coannular inner, high frequency, and outer, low frequency, panels. The high frequency panels include an inner . :
' ' :,' ' .
perforated shect defining a hot gas flow path ancl an outer nonperrorated sheet sandwiching a cellular honeycomb core. Thc low frequency panel includes annular resonating chambers external to the high frequency panel with integral hollow tubes which pass through aper-Lures in the high frequency panel to vent the chambers to the hot gas flow path. The chambers vary i size and the tubes vary in length to provide wide-band sound suppression.
Since the high frequency panel shields the low frequency panel from the hot gas environment, a considerable temperature differential exists between the two panels. `To accommodate relative thermal expansion, the low frequency panel is recessed into and rigidly attached to the duct wall so as to form a pocket therein. The high frequency panel is slidingly receivcd wlthin the pocket and connected to the duct wall by at least one slip joint to permit relative thermal expansion. The tubes are integral with the~ low frequency panel and, to ensure against interference betwcen thcse tu~)es ~ncl the high frequency panel due to thermal expansion of the latter, predeter-mined clearance is established therebetween which results in the apertures being concentric about the tubes at a predetermined temperature differential.
Expansion slots are pro~rided in the nonperforated outer sheet of the\ high frequency panel to accomrnodate the thermal expansion effects within that panel itself. Since the slots reduce the hoop stiffness of the panel, ring stiffeners are attached to the inner perforated skin on the surface thereof adjacent the honeycomb core. A thermal expansion gap between the two panels provides for additional relative growth and reduces heat convection into the low fre~uency panel.
~.
DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims particularly ~ , , :
.. -. . . . .
8~4~
pointing out and distinctly claiming the subject matter which is regarded as part of the present invention, it is believed that the invention will be more fully under6tood froln the following dcscription of the preferred (~-mho~;rn- n1whi-:h is giverl by way of cxamplc with the ~ccompanying drawings, in which:
Figure 1 is a partial cross-sectional view of a portion of a gas turbine engine exhaust duct which is acoustically treated in accordance with the present invention;
Figure 2 is an enlarged view of the exhaust duct of the engine of Figure 1 depicting the sound suppressing structure of the present invention in greater detail;
l~'igure :3 ix a partial s(?ctional vicw takc n along lin(~ ol l~`.i.~urc 2; ;nl(l ~igurc 4 is a further enlarged view showing fabrication details of the sound-suppressing structure of Figure 2.
DESCRIPTION OF THE PREFERRE'D EMBODIMENT
Referring to the drawings wherein like numèrals correspond to like elements throughout, attention is first directed to Figure l wherein a gas turbine engine nozzle 10 embodying the present invention is diagrammati-cally shown. Hot gases of combustion ;lre cxpanded through a turbine tnot shown)in a manner well known in the art and exit through exhallst noz.~.le 1() in the direction indicated by arrow 12 to generate a propulsive thrust in thc opposite direction. The nozzle is shown to include a rigid centerbody 14 symmetrical about axis 16 and a generally coannular duct wall 18 defining a hot gas passage 20 therebehYeen. Both the centerbody and duct wall are sho~n to be provided with acoustical treatment 22 and 24, respectively, of a type now to be described.
,: .
7~
~eferring now to Figures 2 and 4 wherein the duct acoustic treatment 24 is shown in greater detail, there is provided a high frequency acoustic panel 26 consisting of an inner sheet 28, perforated with a plurality of small diameter holes 30, and an outer nonperforated sheet 32 sandwiching therebetween a cellular core 34 of the honeycomb variety. This pair of sheets is bonded to the honeycomb core by any o~ several methods such as brazing or diffusion bonding. Each cell 36 is aligned with one of the holes 30 to provide communication between the cell interior and flow passage 20, the cells lunctioning as Helmholtz resonators to tune out noise within a frequency band which is related to the cell size. This high frequency acoustic panel is mounted concentrically within a low frequency acoustic panel indicated generally at 38 and comprising a plurality of coannular resonating chambers 40. As used herein, the terms "low fre~uency panel" and "high frequency panel" are relative terms, it being well understood that larger chambers are required to suppress lower noise frequencies. These chambers may be formed in a variety of ways but are here forrned between a cylindrical inner `~i facing sheet 44 and an outer annular stepped facing sheet 46 separated radially from sheet 44 by a plurality of upstanding partitions 48. The facingS
sheets and partitions may be attached as by welding or brazing to form a rigid structure. In fact, in the embodiment shown, stepped facing sheet 46 comprises a plurality of rings S0 between adjacent pairs of partitions 48.
Each chamber 40 is provided with a hollow tube 42 which is integral with facing sheet 44 and which penetrates into the chamber. The high frequency panel 26 has a plurality of apertures 52 extending completely ;~
therethrough, the apertures being in general alignment with, and of a larger diameter than, the tubes. Accordingly, the tubes pass through, but are not -5- ,, .
.. ..
-.
7~
connected lo, the high frequency panel and thus provide communication between lh(~ hot gas r~assage 20 wherein the lloise lo 1)(~ su~)pressed is localed and the resonating chambers 40. The resonating chambcrs vary in size (volume) and the tubes vary in length to provide for wide-band noise suppression as is taught and described in U. S. Patent 3, 819, 009, previously noted herein.
As discussed earlier, differential thermal expansion exists relative to the low and high frequency panels. This condition is due to the direct exposure oE the high frequency panel, particularly facing sheet 28, to I() the e~haust gas flow through T)assage ~(). Thc low rre(luerlcy pancl :38, on the other hand, is shieldeà from the hot gases by the prescnce of thc high --~
frequency treatment and, thus, remains substantially cooler. This tempera-ture differential is most pronounced during transient operation such as during engine start-up.
Ir) To providc for this relative therrnal cxpansion, slip joints 54 `, are incorporated between duct walls 18 and panel 26 to permit the high frequency panel to expand as a whole relative to the cooler outer structure.
As shown in Figurc 2, facing sheet 44 is recessed from duct 18 to for m a pocket 55 and the high frequency panel is nested within this low frequency panel pocket utilizing the slip joints to permit both axial and radial thermal growth. The tubes 42 are attached to the cooler low frequency panel rather than the high frequency panel since it is desirable from an acoustic point of view to have chambers 40 totally sealed with the only access to the interior thereof provided through the hollow tubes. If the tubes were attached to panel 26, a slip fit would be necessary between the tubes and facing sheet 44 to accommodate thermal expansion, thus presenting the possibility of an acoustic "leak. "
, .
7~
To ensure against interference betwecrl thc tubes and the higsh frequency panel due to thermal expansion of the latter, a clearance 56 is established in the apertures encircling the tubes. Preferably, the apertures are not placed concentrically about the tubes during fabrication when the entire structure is cold. Instead, since high frequency panel 26 will expand generally uniformly from its center outwardly when heated, each such clear-ance 56 can be preset such that each tube will be concentric within its aperture when the panel is heated and expanded, Thi~ ensures against interference during ~n~ine operation when vibrati~n~ and aerodynamic loading could tend to distort the nozzle. Note also that an annular gap 58 : ....,:
has been provided between the low and high frqquency panels, not only to provide for relative thermal expansion, but also for the purpose of reducing conductivity of heat to the low frequency panel which clearly assists the duct wall 1~ in Eunctioning as the load-bearing structure in this portion of the noz~le. It is clear, therefore, that the thermal growth problem between the low and high frequency panels has been overcome.
The remaining concern is with respect to intrapanel thermal stresses, particularly in the high frequency panel. ,~ince the inner-facing sheet 28 will tend to expand at a much greater rate than outer-facing sheet 32, stresses will be imposed on the honeycomb core sandwiched and brazed therebetween which will tend to lean, and to possibly buckle, the honeycomb core. The eEfect will be most pronounced on the panel ends since the panel can be expected to expand rather symmetrically from the center outwardly.
Accordingly, and as shown in Figure 3, facing sheet :~2 has been scored hy as axial and circumferential ~lot$ 60 and 62, respcctively, into a plurality of rectangular mosaic-like pieces 64. Preferably, the pieces should be made " ~ .
... . ~ - .. , , . ~
37~
as small as possible to minimize the end effects without excessively sacriEicing the structural integrity of the panel or its acoustic properties.
~Note that the scoring "unseals" some of the high frequency chambers. ) ThusJ the pieces have relative freedom of expansion in all directions. For a typical gas turbine engine nozzle, slots having a width of about . 01 - . 02 inch will significantly rcduce the thermal stresses without seriously affecting the noise suppression effectivencss.
Although the sandwich-like structure is not relied upon primarily for its structural strength and stiffness but rather for its acoustic properitcs, a certain degree of structural rigidity is required. Because the axial slot 60 may r ccluce the hoop stiffness to the point that it could not withstand anticipated vibrational and other loadings, it may become necessary to attach a plurality of thin ring stiffeners 66 to the backside of the inner-facing sheet 2~ adjacent the honeycomb core. Such a stiffener does not suffer thermal stress problems since its excellent heat conduction capability limits the temperature gradient in the stiffener.
It will be obvious to one skilled in the art that certain changes can be made to the above-described invention without departing from the broad inventive concept~ thereof. For example, while the present invention has been directed to annular panels for use in gas turbine engine exhaust nozzles, it is clear that the invention is equally applicable to any acoustically treated hot gas flow path regardless of its shape, and the use of the word "panel" is not meant to be limited to any particular shape, Similarly, as noted earlierJ the terms "high" and "low" frequency are mercly relative terms as used herein and do not limit the scope of the invention to any particular band or bands of frequency.
:
.` ~ ,'; . .. , , - . ~ . , .' ,. ~ , :
. . , - . .
Patent 3,819,009, Motzinger, entitled "Duct Wall Acoustic Treatment," which is assigned to the assignee of the present invention, is representative of such a structure.
When such a stacked sandwich material is employed for noise suppression in a hot gas environment, typified by bas turbine engine exhaust nozæles and ducts, a potential differential thermal expansion problem exists. This is due to the large temperature gradient which exists between the hot ::
.. . .
~ ' ~' :.
., ~ . . : , -~74~6 flow path defining honeycomb facing sheet and the relatively cooler opposite (backside) tacing sheet. As the engine is cycl~d Illr(mgh()ut its opcrating range, cyclic thermal stresses are impose-l on the~ sound-suppression material. These -thermal stresses and the resulting distortion and fatigue may reduce the structural life and, thus, effectively increase the cost of the engine over its life cycle.
Therefore, a means is needed for making use of the inherent acoustic advantages of honey;comb sandwich material in a hot gas environ-ment without subjecting it to high levels of thermal stress. In short, the lQ problem is to use the honeycomb structure so as to take advantage of its acoustic properties without inCUrring structural liabilities in a hot gas environment.
SUMMARY OF THE INVENTION
Acoordingly, it is the primary object of the present invention to provide a sound-suppressing structure of the honeycomb variety in which the potential for thermal stresses is minimized.
It is a further object of the present invention to minimize the - potential for thermal stresses in stacked cellular acoustic suppression material for disposition in a gas turbine engine exhaust duct. -These and other objects and advantages will be more clearly understood from the following detailed description, drawings and spccific examples, all of which are intended to be typical of rather than in any way limiting to the scope of the present invention.
Briefly stated, the above objectives are accomplished in a structure comprising a duct wall and coannular inner, high frequency, and outer, low frequency, panels. The high frequency panels include an inner . :
' ' :,' ' .
perforated shect defining a hot gas flow path ancl an outer nonperrorated sheet sandwiching a cellular honeycomb core. Thc low frequency panel includes annular resonating chambers external to the high frequency panel with integral hollow tubes which pass through aper-Lures in the high frequency panel to vent the chambers to the hot gas flow path. The chambers vary i size and the tubes vary in length to provide wide-band sound suppression.
Since the high frequency panel shields the low frequency panel from the hot gas environment, a considerable temperature differential exists between the two panels. `To accommodate relative thermal expansion, the low frequency panel is recessed into and rigidly attached to the duct wall so as to form a pocket therein. The high frequency panel is slidingly receivcd wlthin the pocket and connected to the duct wall by at least one slip joint to permit relative thermal expansion. The tubes are integral with the~ low frequency panel and, to ensure against interference betwcen thcse tu~)es ~ncl the high frequency panel due to thermal expansion of the latter, predeter-mined clearance is established therebetween which results in the apertures being concentric about the tubes at a predetermined temperature differential.
Expansion slots are pro~rided in the nonperforated outer sheet of the\ high frequency panel to accomrnodate the thermal expansion effects within that panel itself. Since the slots reduce the hoop stiffness of the panel, ring stiffeners are attached to the inner perforated skin on the surface thereof adjacent the honeycomb core. A thermal expansion gap between the two panels provides for additional relative growth and reduces heat convection into the low fre~uency panel.
~.
DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims particularly ~ , , :
.. -. . . . .
8~4~
pointing out and distinctly claiming the subject matter which is regarded as part of the present invention, it is believed that the invention will be more fully under6tood froln the following dcscription of the preferred (~-mho~;rn- n1whi-:h is giverl by way of cxamplc with the ~ccompanying drawings, in which:
Figure 1 is a partial cross-sectional view of a portion of a gas turbine engine exhaust duct which is acoustically treated in accordance with the present invention;
Figure 2 is an enlarged view of the exhaust duct of the engine of Figure 1 depicting the sound suppressing structure of the present invention in greater detail;
l~'igure :3 ix a partial s(?ctional vicw takc n along lin(~ ol l~`.i.~urc 2; ;nl(l ~igurc 4 is a further enlarged view showing fabrication details of the sound-suppressing structure of Figure 2.
DESCRIPTION OF THE PREFERRE'D EMBODIMENT
Referring to the drawings wherein like numèrals correspond to like elements throughout, attention is first directed to Figure l wherein a gas turbine engine nozzle 10 embodying the present invention is diagrammati-cally shown. Hot gases of combustion ;lre cxpanded through a turbine tnot shown)in a manner well known in the art and exit through exhallst noz.~.le 1() in the direction indicated by arrow 12 to generate a propulsive thrust in thc opposite direction. The nozzle is shown to include a rigid centerbody 14 symmetrical about axis 16 and a generally coannular duct wall 18 defining a hot gas passage 20 therebehYeen. Both the centerbody and duct wall are sho~n to be provided with acoustical treatment 22 and 24, respectively, of a type now to be described.
,: .
7~
~eferring now to Figures 2 and 4 wherein the duct acoustic treatment 24 is shown in greater detail, there is provided a high frequency acoustic panel 26 consisting of an inner sheet 28, perforated with a plurality of small diameter holes 30, and an outer nonperforated sheet 32 sandwiching therebetween a cellular core 34 of the honeycomb variety. This pair of sheets is bonded to the honeycomb core by any o~ several methods such as brazing or diffusion bonding. Each cell 36 is aligned with one of the holes 30 to provide communication between the cell interior and flow passage 20, the cells lunctioning as Helmholtz resonators to tune out noise within a frequency band which is related to the cell size. This high frequency acoustic panel is mounted concentrically within a low frequency acoustic panel indicated generally at 38 and comprising a plurality of coannular resonating chambers 40. As used herein, the terms "low fre~uency panel" and "high frequency panel" are relative terms, it being well understood that larger chambers are required to suppress lower noise frequencies. These chambers may be formed in a variety of ways but are here forrned between a cylindrical inner `~i facing sheet 44 and an outer annular stepped facing sheet 46 separated radially from sheet 44 by a plurality of upstanding partitions 48. The facingS
sheets and partitions may be attached as by welding or brazing to form a rigid structure. In fact, in the embodiment shown, stepped facing sheet 46 comprises a plurality of rings S0 between adjacent pairs of partitions 48.
Each chamber 40 is provided with a hollow tube 42 which is integral with facing sheet 44 and which penetrates into the chamber. The high frequency panel 26 has a plurality of apertures 52 extending completely ;~
therethrough, the apertures being in general alignment with, and of a larger diameter than, the tubes. Accordingly, the tubes pass through, but are not -5- ,, .
.. ..
-.
7~
connected lo, the high frequency panel and thus provide communication between lh(~ hot gas r~assage 20 wherein the lloise lo 1)(~ su~)pressed is localed and the resonating chambers 40. The resonating chambcrs vary in size (volume) and the tubes vary in length to provide for wide-band noise suppression as is taught and described in U. S. Patent 3, 819, 009, previously noted herein.
As discussed earlier, differential thermal expansion exists relative to the low and high frequency panels. This condition is due to the direct exposure oE the high frequency panel, particularly facing sheet 28, to I() the e~haust gas flow through T)assage ~(). Thc low rre(luerlcy pancl :38, on the other hand, is shieldeà from the hot gases by the prescnce of thc high --~
frequency treatment and, thus, remains substantially cooler. This tempera-ture differential is most pronounced during transient operation such as during engine start-up.
Ir) To providc for this relative therrnal cxpansion, slip joints 54 `, are incorporated between duct walls 18 and panel 26 to permit the high frequency panel to expand as a whole relative to the cooler outer structure.
As shown in Figurc 2, facing sheet 44 is recessed from duct 18 to for m a pocket 55 and the high frequency panel is nested within this low frequency panel pocket utilizing the slip joints to permit both axial and radial thermal growth. The tubes 42 are attached to the cooler low frequency panel rather than the high frequency panel since it is desirable from an acoustic point of view to have chambers 40 totally sealed with the only access to the interior thereof provided through the hollow tubes. If the tubes were attached to panel 26, a slip fit would be necessary between the tubes and facing sheet 44 to accommodate thermal expansion, thus presenting the possibility of an acoustic "leak. "
, .
7~
To ensure against interference betwecrl thc tubes and the higsh frequency panel due to thermal expansion of the latter, a clearance 56 is established in the apertures encircling the tubes. Preferably, the apertures are not placed concentrically about the tubes during fabrication when the entire structure is cold. Instead, since high frequency panel 26 will expand generally uniformly from its center outwardly when heated, each such clear-ance 56 can be preset such that each tube will be concentric within its aperture when the panel is heated and expanded, Thi~ ensures against interference during ~n~ine operation when vibrati~n~ and aerodynamic loading could tend to distort the nozzle. Note also that an annular gap 58 : ....,:
has been provided between the low and high frqquency panels, not only to provide for relative thermal expansion, but also for the purpose of reducing conductivity of heat to the low frequency panel which clearly assists the duct wall 1~ in Eunctioning as the load-bearing structure in this portion of the noz~le. It is clear, therefore, that the thermal growth problem between the low and high frequency panels has been overcome.
The remaining concern is with respect to intrapanel thermal stresses, particularly in the high frequency panel. ,~ince the inner-facing sheet 28 will tend to expand at a much greater rate than outer-facing sheet 32, stresses will be imposed on the honeycomb core sandwiched and brazed therebetween which will tend to lean, and to possibly buckle, the honeycomb core. The eEfect will be most pronounced on the panel ends since the panel can be expected to expand rather symmetrically from the center outwardly.
Accordingly, and as shown in Figure 3, facing sheet :~2 has been scored hy as axial and circumferential ~lot$ 60 and 62, respcctively, into a plurality of rectangular mosaic-like pieces 64. Preferably, the pieces should be made " ~ .
... . ~ - .. , , . ~
37~
as small as possible to minimize the end effects without excessively sacriEicing the structural integrity of the panel or its acoustic properties.
~Note that the scoring "unseals" some of the high frequency chambers. ) ThusJ the pieces have relative freedom of expansion in all directions. For a typical gas turbine engine nozzle, slots having a width of about . 01 - . 02 inch will significantly rcduce the thermal stresses without seriously affecting the noise suppression effectivencss.
Although the sandwich-like structure is not relied upon primarily for its structural strength and stiffness but rather for its acoustic properitcs, a certain degree of structural rigidity is required. Because the axial slot 60 may r ccluce the hoop stiffness to the point that it could not withstand anticipated vibrational and other loadings, it may become necessary to attach a plurality of thin ring stiffeners 66 to the backside of the inner-facing sheet 2~ adjacent the honeycomb core. Such a stiffener does not suffer thermal stress problems since its excellent heat conduction capability limits the temperature gradient in the stiffener.
It will be obvious to one skilled in the art that certain changes can be made to the above-described invention without departing from the broad inventive concept~ thereof. For example, while the present invention has been directed to annular panels for use in gas turbine engine exhaust nozzles, it is clear that the invention is equally applicable to any acoustically treated hot gas flow path regardless of its shape, and the use of the word "panel" is not meant to be limited to any particular shape, Similarly, as noted earlierJ the terms "high" and "low" frequency are mercly relative terms as used herein and do not limit the scope of the invention to any particular band or bands of frequency.
:
.` ~ ,'; . .. , , - . ~ . , .' ,. ~ , :
. . , - . .
Claims (12)
1. Sound-suppressing structure comprising:
a duct wall partially defining a fluid flow path and having a hollow cavity therein;
a low frequency acoustic panel within said cavity and having a first cellular core sandwiched between a pair of first facing sheets, one of said first sheets penetrated by a plurality of hollow tubes rigidly attached thereto, one end of each tube extending into a core cell and providing the only access to the interior thereof; and a high frequency acoustic panel within said cavity and covering said low frequency panel to further define the fluid flow path, said high frequency panel having a second cellular core sandwiched between a pair of second facing sheets, one of said second sheets perforated with a plurality of small diameter holes communicating with the interiors of the second core cells, wherein said high frequency panel includes a plurality of apertures extending completely therethrough in general alignment with the hollow tubes and freely receiving the ends thereof; and wherein said high frequency panel is slidingly connected to said duct wall by at least one slip joint to permit relative thermal growth of one of said panels without inducing stresses in the other of said panels.
a duct wall partially defining a fluid flow path and having a hollow cavity therein;
a low frequency acoustic panel within said cavity and having a first cellular core sandwiched between a pair of first facing sheets, one of said first sheets penetrated by a plurality of hollow tubes rigidly attached thereto, one end of each tube extending into a core cell and providing the only access to the interior thereof; and a high frequency acoustic panel within said cavity and covering said low frequency panel to further define the fluid flow path, said high frequency panel having a second cellular core sandwiched between a pair of second facing sheets, one of said second sheets perforated with a plurality of small diameter holes communicating with the interiors of the second core cells, wherein said high frequency panel includes a plurality of apertures extending completely therethrough in general alignment with the hollow tubes and freely receiving the ends thereof; and wherein said high frequency panel is slidingly connected to said duct wall by at least one slip joint to permit relative thermal growth of one of said panels without inducing stresses in the other of said panels.
2. The structure as recited in claim 1 wherein the high frequency panel is spacially separated from the low frequency panel to form a thermal expansion gap therebetween.
3. The structure as recited in claim 1 wherein said duct wall and said perforated facing sheet define a high temperature fluid flow path.
4. The structure as recited in claim 2 wherein the apertures are of a larger cross section than the tubes received thereby, and wherein the tubes are initially located within the apertures such that, when the high frequency panel is subjected to a predetermined temperature differential with respect to the low frequency panel, the relative thermal growth of the high frequency panel causes the apertures to be substantially concentric about the tubes.
5. The structure as recited in claim 2 wherein the apertures are of larger cross section than the tubes received thereby, and wherein the apertures are initially off-centered with respect to the tubes by an amount substantially equal to the thermal growth of the high frequency panel with respect to each tube when the high frequency panel is subjected to a predetermined temperature differential with respect to the low frequency panel.
6. The structure as recited in claim 2 wherein the high frequency panel core is of the honeycomb variety and is attached to its associated facing sheets.
7. The structure as recited in claim 6 wherein the high frequency panel facing sheet bounding the thermal expansion gap is scored through to the core in the form of a mosaic pattern for thermal expansion relief.
8. The structure as recited in claim 6 wherein the high frequency panel facing sheet bounding the thermal expansion gap comprises a plurality of small sheet pieces arranged mosaic-like and separated laterally from each other to provide for relative thermal growth.
9. The structure as recited in claim 8 wherein the high frequency panel is provided with a pluralityof stiffening ribs attached to the perforated facing sheet on the side thereof adjacent the honeycomb core.
10. The structure as recited in claim 2 wherein said low frequency panel and said high frequency panel are generally coannular, with the high frequency panel being the innermost of the two panels to partially define the fluid flow path.
11. The structure as recited in claim 2 wherein the tubes generally extend from the perforated facing sheet and partially into the low frequency core cells, the length of the tubes and the size of the cells being such as to form Helmholtz resonators tuned to suppress noise of a predetermined frequency band.
12. A sound-suppressing structure comprising:
a low frequency acoustic panel having a first cellular core and a multiplicity of rigidly attached hollow tubes penetrat-ing said core cells;
a high frequency acoustic panel covering said low frequency panel having a second cellular core sandwiched between a pair of facing sheets, one of which partially defines a fluid flow path and which is perforated with a plurality of small holes providing communication between the flow path and the second cellular core, and a plurality of larger holes extending completely through the high frequency panel for the receipt of an end of the hollow tubes; and expansion means between said low and high frequency panels for permitting relative thermal growth thereof.
a low frequency acoustic panel having a first cellular core and a multiplicity of rigidly attached hollow tubes penetrat-ing said core cells;
a high frequency acoustic panel covering said low frequency panel having a second cellular core sandwiched between a pair of facing sheets, one of which partially defines a fluid flow path and which is perforated with a plurality of small holes providing communication between the flow path and the second cellular core, and a plurality of larger holes extending completely through the high frequency panel for the receipt of an end of the hollow tubes; and expansion means between said low and high frequency panels for permitting relative thermal growth thereof.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA285,937A CA1087406A (en) | 1977-09-01 | 1977-09-01 | Sound suppressing structure with thermal relief |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA285,937A CA1087406A (en) | 1977-09-01 | 1977-09-01 | Sound suppressing structure with thermal relief |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1087406A true CA1087406A (en) | 1980-10-14 |
Family
ID=4109449
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA285,937A Expired CA1087406A (en) | 1977-09-01 | 1977-09-01 | Sound suppressing structure with thermal relief |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1087406A (en) |
-
1977
- 1977-09-01 CA CA285,937A patent/CA1087406A/en not_active Expired
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