U.S. patent application number 13/312249 was filed with the patent office on 2012-06-21 for acoustic liner.
This patent application is currently assigned to ROLLS-ROYCE PLC. Invention is credited to Richard J. ASTLEY, Andrew J. KEMPTON, Paul B. MURRAY, Rie SUGIMOTO.
Application Number | 20120156006 13/312249 |
Document ID | / |
Family ID | 43567235 |
Filed Date | 2012-06-21 |
United States Patent
Application |
20120156006 |
Kind Code |
A1 |
MURRAY; Paul B. ; et
al. |
June 21, 2012 |
ACOUSTIC LINER
Abstract
An acoustic liner for a gas turbine engine comprises a folded
cavity with an opening and a resistive facing sheet covering the
opening, characterised in that the folded cavity further comprises
a resistive septum that increases attenuation at high frequencies.
The presence of the septum permits the attenuation of noise over a
much wider range of frequencies than in known liners. By suitable
choice of the resistance and reactance of the septum and of its
position in the cavity, the liner can provide low frequency
attenuation comparable with known liners while also providing
improved attenuation at higher frequencies.
Inventors: |
MURRAY; Paul B.; (Horsham,
GB) ; ASTLEY; Richard J.; (Southampton, GB) ;
SUGIMOTO; Rie; (Southampton, GB) ; KEMPTON; Andrew
J.; (Derby, GB) |
Assignee: |
ROLLS-ROYCE PLC
London
GB
|
Family ID: |
43567235 |
Appl. No.: |
13/312249 |
Filed: |
December 6, 2011 |
Current U.S.
Class: |
415/119 ;
181/290 |
Current CPC
Class: |
F02C 7/045 20130101;
G01K 17/04 20130101; G01N 25/4833 20130101; F05D 2250/38 20130101;
G01N 25/02 20130101; F05D 2250/31 20130101 |
Class at
Publication: |
415/119 ;
181/290 |
International
Class: |
F04D 29/66 20060101
F04D029/66; E04B 1/82 20060101 E04B001/82 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 15, 2010 |
GB |
1021265.2 |
Claims
1. An acoustic liner for a gas turbine engine, comprising a folded
cavity with an opening and a resistive facing sheet coveting the
opening, characterised in that the folded cavity further comprises
a resistive septum that increases attenuation at high
frequencies.
2. An acoustic liner as claimed in claim 1, in which the folded
cavity has a single bend.
3. An acoustic liner as claimed in claim 2, in which the bend turns
the cavity through about 90 degrees.
4. An acoustic liner as claimed in claim 2, in which the septum is
located immediately before the bend.
5. An acoustic liner as claimed in claim 1, in which the facing
sheet has a resistance of between 0.1 and 4 .rho.c.
6. An acoustic liner as claimed in claim 1, in which the facing
sheet has a resistance of between 0.2 and 1 .rho.c.
7. An acoustic liner as claimed in claim 1, in which the septum has
a resistance of between 0.05 and 3 .rho.c.
8. An acoustic liner as claimed in claim 1, in which the septum has
a resistance of between 1 and 2 .rho.c.
9. An acoustic liner as claimed in claim 1, and further comprising
a honeycomb layer in the opening.
10. A casing arrangement for a gas turbine engine, comprising a
plurality of acoustic liners as claimed in claim 1.
11. A gas turbine engine comprising a casing arrangement as claimed
in claim 10.
Description
[0001] This invention relates to gas turbine engines. In
particular, it relates to acoustic liners, such as those used in
fan casings of gas turbine engines to reduce noise and
vibration.
[0002] Many current gas turbine engines, particularly for aerospace
use, comprise an engine core and a ducted fan which is driven by a
turbine of the engine core. The ducted fan comprises a fan rotor
having an array of fan blades which rotate within a duct
surrounding the fan rotor, to provide a substantial part of the
thrust generated by the engine.
[0003] It is known to provide measures in the fan casing to reduce
the noise generated by the fan in operation. Aircraft engine fan
noise may occur at frequencies as low as 50 Hz or as high as 10
kHz. It is known to provide acoustic liners in the duct to
attenuate the fan noise, comprising resistive layers (otherwise
known as septums) above cavities. Typical configurations consist of
one or more resistive layers, backed respectively by one or more
honeycomb cavities. Those with one resistive layer and one
honeycomb layer are known as single layer liners, while those with
two resistive layers (one facing sheet and one septum) are known as
double layer liners. Further resistive layers and honeycomb layers
may also be added, These liners are typically sized to attenuate
sound from approximately 500 Hz to 10 kHz (i.e. those frequencies
which contribute most to community noise), and have typical overall
depths (in the radial direction when installed) between 5 mm and 50
mm. To target frequencies below about 500 Hz, the liner depth must
increase, but deeper liners can be difficult or impossible to
accommodate within the nacelle cowling. A schematic illustration of
such a liner is shown in FIG. 1. Fan blades 12 (only one shown) of
a gas turbine engine rotate in a gas flow 14 in a duct 16. The
rotational axis of the blades 12 is not shown, but lies below FIG.
1 and parallel with arrow 14.
[0004] The duct 16 is defined by an annular fan casing 18, which
has an inner wall 20 which is washed by the gas flow 14 and an
outer wall 22 which is a structural casing. A plurality of acoustic
liners 24 (only one shown) are secured circumferentially around the
casing 18. The illustrated liner 24 comprises a radially-extending
cavity 26 and a perforate facing sheet 28. The cavity 26 is
therefore in fluid communication with the duct 16. The liner
parameters are selected to provide an optimised impedance match to
that for maximum duct attenuation at the principal target
frequencies and duct flow and sound pressure spectral conditions.
Liner parameters are designed using a series of semi-empirical
formulae which relate the liner construction to the liner surface
impedance. One such formula, for example, is that the liner depth
should be of the order of a quarter of the wavelength of the target
frequency.
[0005] For lower target frequencies, it is known to provide
acoustic liners in which the cavities are L-shaped rather than
straight, and such liners are commonly referred to as "folded"
liners. The advantage of a folded liner is that it requires less
radial depth in the engine nacelle, and therefore aids installation
in what is typically a restricted space. FIG. 2 is a schematic
illustration of such a folded liner. A disadvantage of such liners
is that because they are optimised for a low target frequency their
attenuation at higher frequencies is inferior.
[0006] Typically, if dedicated low frequency liner configurations
are installed in aircraft engine nacelle ducts, they occupy space
previously assigned to community noise liners, and therefore their
presence degrades the attainable level of community noise
attenuation. Dedicated low frequency liners are also typically
heavier and more expensive than community noise liners. The present
invention therefore seeks to provide a novel low frequency acoustic
liner that provides significant attenuation also at high
frequencies, thereby increasing the overall attenuation efficiency
of the lined duct.
[0007] Poor mid- to high-frequency performance is a characteristic
of both straight and folded cavities. This is a result of the
presence of multiple frequencies in the spectral range of interest,
where attenuation is at a minimum. Folded cavities provide more
scope for improved high frequency performance given the tendency
for an additional tuned direct reflection, at the fold, from the
more directional high-frequency sound.
[0008] The invention provides additional mid- to high-frequency
attenuation from folded cavities, realised via the addition of a
resistive septum inside the cavity. The septum resistance and
position is chosen to attain the optimum impedance match over the
complete low and high frequency target frequency range and spectral
content.
[0009] According to the invention, there is provided an acoustic
liner for a gas turbine engine as set out in the claims.
[0010] The invention will now be described, by way of example only,
with reference to the accompanying drawings in which:
[0011] FIG. 1 shows a schematic circumferential view of a
conventional deep low frequency acoustic liner;
[0012] FIG. 2 shows a schematic circumferential view of a
conventional folded cavity low frequency acoustic liner;
[0013] FIG. 3 shows a schematic circumferential view of an acoustic
liner according to the invention; and
[0014] FIG. 4 shows a schematic circumferential view of a specific
embodiment of an acoustic liner according to the invention.
[0015] Referring first to FIG. 3, a folded cavity liner 124 is
installed in a casing 18 of a gas turbine engine. The liner 124
comprises a cavity 126 and a resistive facing sheet (perforate or
wire-mesh-on-perforate) 128 covering an opening to the cavity
126.
[0016] The liner further comprises a perforate septum 132, in this
embodiment located immediately before the bend in the liner. The
impedance of the septum 132 may be different from the impedance of
the facing sheet 128.
[0017] The inventors have discovered that the presence of the
septum 132 in the liner 124 permits the attenuation of noise over a
much wider range of frequencies than in known liners. By suitable
choice of the impedance (resistance and reactance) of the septum
132 and of its position in the cavity 126, the liner can provide
low frequency attenuation comparable with known liners while also
providing improved attenuation at higher frequencies, when compared
with the performance of conventional low frequency deep straight
cavity or folded cavity liners.
[0018] In particular preferred embodiments of the invention, the
resistance of the facing sheet is in the range 0.2-1 .rho.c and the
resistance of the septum is in the range 1-2 .rho.c. It is
envisaged that the benefits of the invention can substantially be
achieved with facing sheet resistances in the range 0.1-4 .rho.c
and with septum resistances in the range 0.05-3 .rho.c. The
expression pc represents the characteristic impedance of the air,
where .rho. is the density of the air and c is the speed of sound
in it. For the purposes of defining liner properties, pc may be
considered to be constant, and is generally calculated at ICAO
standard atmosphere conditions.
[0019] FIG. 4 shows a specific embodiment of an acoustic liner
according to the invention. Some features are the same as
corresponding features in FIG. 3, and these have been given the
same reference numbers.
[0020] As in FIG. 3, a folded cavity liner 124 is installed in a
casing 18 of a gas turbine engine. The liner 124 comprises a cavity
126 and a resistive facing sheet (perforate or
wire-mesh-on-perforate) 128.
[0021] The liner further comprises a perforate septum 132, in this
embodiment located immediately before the bend in the liner. By
"before" is meant that the septum is located on the "duct" side of
the bend, rather than on the "cavity" side of the bend. The septum
comprises a mesh layer 146, which for convenience of manufacture
extends along the whole length of the cavity 126.
[0022] In this embodiment, the wall 20 of the duct 18 comprises a
single layer liner 142 of conventional design, comprising a
honeycomb layer with a resistive facing sheet 128. (In other
embodiments this liner may be absent.) To provide fluid
communication between the duct 18 and the cavity 126, an opening is
provided over the region 154. This may (as shown in FIG. 4) have an
open honeycomb layer 144, or may be covered only by the facing
sheet 128. In this embodiment, the dimension 154 is 5 cm and the
dimension 152 is 10 cm. The thickness 156 of the single layer liner
is 1.5 cm. The radial depth 158 of the cavity 126 is 5 cm, and its
length (160+162) is 10 cm. The dimensions 160 and 162 are each 5
cm. The region 166 is dead space, and is not in fluid communication
with any other part of the assembly.
[0023] In practice, a number of folded cavity liners 124 would be
arranged circumferentially around the annulus of the fan casing 18.
In the embodiment shown in FIG. 4, each liner 124 has a
circumferential width (in the direction into and out of the paper
in FIG. 4) of 5 cm.
[0024] Although the invention has been described by reference to
certain specific embodiments, it will be understood that variations
and alternatives may be employed in other embodiments without
departing from the scope of the invention.
[0025] The impedance and location of the septum 132 may be varied
to target specific frequency ranges.
[0026] The septum 132 may be made of a perforate, or of a metal or
non-metal mesh.
[0027] One or more additional septums may be added in different
positions in the cavity, to provide more degrees of freedom to the
optimisation of the overall design.
[0028] A highly open mesh layer 146, or other perforate support
sheet, may be combined with a resistive septum 132, so that the
support sheet will provide structural continuity and the overall
septum impedance will control the acoustic behaviour.
* * * * *