Note: Descriptions are shown in the official language in which they were submitted.
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.
-4-
,: .
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.
--8--
:
.` ~ ,'; . .. , , - . ~ . , .' ,. ~ , :
. . , - . .
