U.S. patent application number 09/725928 was filed with the patent office on 2002-05-30 for bypass duct fan noise reduction assembly.
Invention is credited to Tse, Man-Chun.
Application Number | 20020064453 09/725928 |
Document ID | / |
Family ID | 24916501 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020064453 |
Kind Code |
A1 |
Tse, Man-Chun |
May 30, 2002 |
BYPASS DUCT FAN NOISE REDUCTION ASSEMBLY
Abstract
A gas turbine engine fan assembly includes fan blades spaced
axially from stator vanes inside an annular bypass duct. A
plurality of perforated baffle plates are installed in the annular
bypass duct downstream of the stator vanes. The perforated baffle
plates extend in a generally axial direction and are unevenly,
circumferentially spaced apart from one another to divide a major
section of the annular bypass duct into a plurality of axial
flow-path segments in an asymmetrical pattern to disrupt
continuity, destroy a symmetrical pattern and absorb sound energy
of a spinning mode of sound pressures imbedded in the air flow
downstream of the stator vanes, without substantially affecting a
thrust provided by the air flow when discharged from the bypass
duct. The invention provides a solution for suppressing rearward
noise propagation of a fan BPF tone and higher harmonics thereof
through the annular bypass duct of a gas turbine engine, which
contributes to the reduction of aircraft noises, particularly
during take-off.
Inventors: |
Tse, Man-Chun; (Brossard,
CA) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C.
ATTN: GREGORY LAPOINTE
900 CHAPEL STREET
SUITE 1201
NEW HAVEN
CT
06510-2802
US
|
Family ID: |
24916501 |
Appl. No.: |
09/725928 |
Filed: |
November 30, 2000 |
Current U.S.
Class: |
415/119 ;
415/208.3 |
Current CPC
Class: |
B64D 2033/0206 20130101;
F05D 2260/96 20130101; F02K 1/386 20130101; F02C 7/045 20130101;
F02C 7/24 20130101 |
Class at
Publication: |
415/119 ;
415/208.3 |
International
Class: |
F01D 001/00 |
Claims
I/we claim:
1. An apparatus for suppressing rearward noise propagation of a fan
BPF tone and higher harmonics thereof through an annular bypass
duct of a gas turbine engine comprising at least one perforated
baffle plate extending generally in an axial direction with respect
to the gas turbine engine and adapted to divide a major section of
the annular bypass duct downstream of a plurality of stator vanes
into a plurality of axial flow-path segments, the axial flow-path
segments being in fluid communication with one another through the
perforations in the at least one baffle plate.
2. An apparatus as claimed in claim 1 comprising a plurality of
perforated baffle plates extending generally in an axial direction
with respect to the gas turbine engine, each adapted to span across
an annulus defined between inner and outer walls of the annular
bypass duct, the axial flow-path segments being in fluid
communication with adjacent ones through the perforations in the
respective baffle plates.
3. An apparatus as claimed in claim 2 wherein the plurality of
perforated baffle plates are positioned downstream of, and axially
aligned with, a plurality of struts spanning across the annulus of
the annular bypass duct.
4. An apparatus as claimed in claim 1 wherein the perforated baffle
plates are unevenly circumferentially spaced apart from one another
so that the axial flow-path segments are formed in an asymmetrical
pattern.
5. An apparatus as claimed in claim 1 wherein the perforations in
the respective baffle plates have a percentage of opening area POA
ranging from 20% to 50%.
6. An apparatus as claimed in claim 1 wherein the perforations in
the respective baffle plates are in a staggered hole pattern.
7. An apparatus as claimed in claim 1 wherein the perforations in
the respective baffle plates include holes having uniform diameters
and have a ratio of individual hole diameter to plate thickness
between 0.5 and 2.0.
8. A gas turbine engine fan assembly comprising: a plurality of
circumferentially spaced rotor blades; a plurality of
circumferentially spaced stator vanes axially spaced apart from the
rotor blades; an annular duct surrounding the rotor blades and
stator vanes, and having an inlet for receiving an air flow and an
outlet for discharging at least a portion of the air flow
compressed by the rotor blades and passed over the stator vanes;
and means installed in the annular duct downstream of the stator
vanes for disrupting continuity, destroying a symmetrical pattern
and absorbing sound energy of a spinning mode of sound pressures
imbedded in the air flow downstream of the stator vanes, without
substantially affecting a thrust provided by the air flow when
discharged from the annular bypass duct.
9. A fan assembly as claimed in claim 8 wherein the means comprises
at least one perforated baffle plate extending generally in an
axial direction with respect to the fan assembly, dividing a major
section of the annular duct downstream of the stator vanes into at
least two axial flow-path segments, the two axial flow-path
segments being in fluid communication with each other through the
perforations in the at least one baffle plate.
10. A fan assembly as claimed in claim 8 wherein the means
comprises a plurality of perforated baffle plates extending
generally in an axial direction with respect to the fan assembly,
each spanning across an annulus defined between inner and outer
walls of the annular bypass duct, whereby a major section of the
annular duct downstream of the stator vanes is divided into a
plurality of axial flow-path segments, the axial flow-path segments
being in fluid communication with adjacent ones through the
perforations in the respective baffle plates.
11. A fan assembly as claimed in claim 10 wherein the perforated
baffle plates are located downstream of, and aligned with, a
plurality of struts spanning across the annulus of the annular
bypass duct.
12. A fan assembly as claimed in claim 10 wherein the perforated
baffle plates are unevenly circumferentially spaced apart from one
another so that the axial flow-path segments are formed in an
asymmetrical pattern.
13. A fan assembly as claimed in claim 10 wherein the perforated
baffle plates are separated axially from the stator vanes by a
space substantially small with respect to a length of the
perforated baffle plates.
14. A fan assembly as claimed in claim 10 wherein each of the
perforated baffle plates is slightly curved so that a downstream
section of the plate is gently and gradually deviated from an axial
orientation in a circumferential direction opposite to a rotation
of the rotor blades, the circumferential deviation being
substantially small relative to a length of the perforated baffle
plate.
15. A fan assembly as claimed in claim 10 wherein the perforations
in the respective baffle plates have a percentage of opening area
POA ranging from 20% to 50%.
16. A fan assembly as claimed in claim 10 wherein the perforations
in the respective baffle plates are in a staggered hole
pattern.
17. A fan assembly as claimed in claim 10 wherein the perforations
in the respective baffle plates include holes having uniform
diameters, and have a ratio of hole diameter to plate thickness
between 0.5 and 2.0.
18. A method for suppressing rearward noise propagation of a fan
BPF tone and higher harmonics thereof through an annular bypass
duct of a gas turbine engine comprising: in the annular bypass duct
downstream of a plurality of stator vanes, disrupting continuity,
destroying a symmetrical pattern and absorbing sound energy of a
spinning mode of sound pressures imbedded in an air flow downstream
of the stator vanes, without substantially affecting a thrust
provided by the air flow when discharged from the annular bypass
duct.
19. A method as claimed in claim 18 wherein the disrupting
continuity, destroying the symmetrical pattern and absorbing sound
energy of the spinning mode of sound pressures is conducted by
using at least one perforated baffle plate extending generally in
an axial direction with respect to the gas turbine engine and
adapted to divide a major section of the annular bypass duct
downstream of a plurality of stator vanes into at least two axial
flow-path segments, the two axial flow-path segments being in fluid
communication with each other through the perforations in the at
least one baffle plate.
20. A method as claimed in claim 18 wherein the disrupting
continuity, destroying the symmetrical pattern and absorbing sound
energy of the spinning mode of sound pressures is conducted by
using a plurality of perforated baffle plates extending generally
in an axial direction with respect to the gas turbine engine, each
adapted to span across an annulus defined between inner and outer
walls of the annular bypass duct, whereby a major section of the
annular bypass duct downstream of a plurality of stator vanes is
divided into a plurality of axial flow-path segments, the axial
flow-path segments being in fluid communication with adjacent ones
through the perforations in the respective baffle plates.
21. A method as claimed in claim 20 wherein the perforated baffle
plates are unevenly circumferentially spaced apart from one another
so that the axial flow-path segments are formed in an asymmetrical
pattern.
Description
FIELD OF THE INVENTION
[0001] This invention relates to suppression of noise sound waves
emitted from a jet engine, more particularly to the suppression of
noise propagation from a gas turbine engine fan assembly downstream
through the bypass duct thereof.
BACKGROUND OF THE INVENTION
[0002] Noise has been a significant negative factor associated with
the commercial airline industry since the introduction of the
aircraft gas turbine engine. Considerable effort has been directed
toward quieting aircraft engines.
[0003] Gas turbine engine noise is generated by two primary
sources. First, there is noise associated with viscous shearing of
rapidly moving gases exhausted into the relatively quiescent
surrounding atmosphere. In turbo fan aircraft engines, such gases
are emitted from the fan and core nozzles at the rear of the
engine. Various approaches have been utilized to reduce this
"shear" noise, most approaches incorporating mixers to co-mingle
fan and exhaust gases with each other and with the surrounding
environment.
[0004] The second source of noise is the rotating turbo machinery
itself, as the result of rapidly rotating blade rows disposed
within the gas stream. Fans and compressors include at least one
row of a plurality of circumferentially spaced apart rotor blades
for compressing air channeled therethrough, and a row of
circumferentially spaced apart stator vanes axially spaced apart
from the rotor blades. The rotor blades rotate about a longitudinal
centerline axis of the engine at a rotational speed and effect a
tonal noise at a blade passing frequency (BPF). The aerodynamic
interaction of rotor blade-wakes and stationary vanes adds a
significant contribution to the noise produced by the jet gas
turbine engine. Interaction tones are generated in the region
between the rotor blades and the stator vanes, within the annular
duct surrounding the blades and the vanes, conventionally known as
spinning mode tones or noises.
[0005] The blade rotation-wakes of the air compressed by the rotor
blades form rotational pressure fields and impinge on the stator
vanes, thereby creating the spinning mode tones. The spinning mode
tones occur at discrete frequencies including the fundamental blade
passing frequency BPF, alternatively referred to herein as the
first harmonic, and higher order frequencies including the second,
third and higher harmonics. When this spinning mode speed is fast
enough to reach a local Mach number to be greater than about 1.1,
the spinning mode tones will propagate outside, both upstream
through the duct inlet and downstream through the bypass duct,
enhancing BPF tone levels generated directly by the rotor
blades.
[0006] In order to reduce the spinning mode tone noises, it is well
known in the art to direct the spinning mode tones to impinge on
the walls of the interior of the engine, including the bypass duct,
which is lined with a sound absorbent material. This technique
causes the spinning mode tones to decay before exiting the engine.
Normally, the bypass duct liners are tuned for the BPF tone and the
higher harmonics cannot be efficiently suppressed. In addition,
because of the limited duct wall area within most conventional jet
engines, such acoustical wall treatment has only made small
reductions in fan noise levels, and this is compounded by engine
nacelle length-to-radius ratios becoming smaller.
[0007] Efforts have been made to seek alternative solutions to
reduce engine fan noise levels. Reduction of the spinning mode
noise can be achieved by reduction of the production processes at
the source of the noise which reduces the incident aerodynamic
unsteadiness or the mode generation from fan-stator interactions.
It is conventionally known in the art to select the number of vanes
and the number of blades to create a spinning mode propagation
cut-off phenomenon, as described, for example, by Gliebe et al. in
their U.S. Pat. No. 5,169,288 issued on Dec. 8, 1992. In practice,
for the spinning mode propagation cut-off a number V of vanes and
number B of blades are selected to achieve V>2.3B. In some
designs, however, particularly in high bypass turbine fan engines
requiring a relatively large number of rotor blades, a cut-on
fan-stator V<2B may be selected in order to find a compromise
with other design criteria. In such cases, the spinning mode is
always cut-on, resulting in increasing the fan BPF tone noise and
its higher harmonics.
[0008] In U.S. Pat. No. 4,300,656 issued to Burcham on Nov. 17,
1981, Burcham describes an acoustic noise elimination assembly
having the capability of disrupting the continuity of rotating
fields of sound pressures forwardly projected from fans or rotors
of a type commonly found in the front or compressor first stage of
air-breathing engines, when operating at tip speeds in the
supersonic range. The assembly includes a tubular cowl defining a
duct for delivering an air stream axially into the intake of a jet
engine and sound barrier, defined by a plurality of intersecting
flat plates or struts having a line of intersection coincident with
a longitudinal axis of the tubular cowl, which serves to disrupt
the continuity of rotating fields of multiple tonal components of a
noise.
[0009] Nevertheless, in addition to the conventional bypass duct
acoustic liner, few attempts have been made to reduce the rearward
propagation of a fan BPF tone and its harmonics which increase a
total noise level emitted from the rear of an engine, thereby more
severely affecting the environment especially in a take-off
condition. Therefore, it is desirable to develop new methods and
apparatus to attenuate the fan BPF tone and its harmonics within
the bypass duct of gas turbine engines.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to provide a method
and an apparatus to suppress the fan BPF tone noise and higher
harmonics thereof within bypass ducts of gas turbine engines.
[0011] It is another object of the present invention to provide an
apparatus to disrupt continuity, destroy a symmetrical pattern and
absorb sound energy of a spinning mode of sound pressures in a
bypass duct of a gas turbine engine.
[0012] It is a further object of the present invention to provide a
gas turbine engine fan assembly with a low level fan BPF tone noise
and higher harmonics thereof propagating rearwardly out of the
engine.
[0013] It is a still further object of the present invention to
provide a method and an apparatus for suppressing rearward noise
propagation of a fan BPF tone and higher harmonics thereof through
a bypass duct of a gas turbine engine without substantially
affecting the thrusts provided by the air flow when discharged from
the bypass duct.
[0014] In general terms according to the present invention,
rearward noise propagation of fan BPF tone and higher harmonics
thereof through an annular bypass duct of a gas turbine engine is
suppressed in the annular bypass duct downstream of a plurality of
stator vanes by disrupting continuity, destroying a symmetrical
pattern and absorbing sound energy of a spinning mode of sound
pressures imbedded in an air flow downstream of the stator vanes
without substantially affecting a thrust provided by the air flow
when discharged from the annular bypass duct.
[0015] In accordance with one aspect of the present invention an
apparatus for suppressing rearward noise propagation of a fan BPF
tone and higher harmonics thereof through an annular bypass duct of
a gas turbine engine comprises at least one perforated baffle
plate, preferably a plurality of perforated baffle plates,
extending generally in an axial direction with respect to the gas
turbine engine and adapted to divide a major section of the annular
bypass duct downstream of a plurality of stator vanes, into a
plurality of axial flow-path segments. The axial flow-path segments
are in fluid communication with one another through the
perforations in the at least one baffle plate. When the plurality
of the perforated baffle plates are provided, the axial flow-path
segments are in fluid communication with adjacent ones through the
perforations in the respective baffle plates.
[0016] It is desirable that the perforated baffle plates are
unevenly circumferentially spaced apart from one another so that
the axial flow-path segments are formed in an asymmetrical
pattern.
[0017] In one embodiment of the present invention the perforations
in the respective baffle plates have a percentage of opening area
(POA) ranging from 20% to 50%, and are in a staggered hole pattern.
It is also preferable that the perforations in the respective
baffle plates include holes having uniform diameters and a ratio of
individual hole diameter to plate thickness between 0.5 and
2.0.
[0018] In accordance with another aspect of the present invention,
there is a gas turbine engine fan assembly which comprises a
plurality of circumferentially spaced rotor blades; a plurality of
circumferentially spaced rotor vanes axially spaced apart from the
rotor blades; an annular duct surrounding the rotor blades and
stator vanes, and having an inlet for receiving an air flow and an
outlet for discharging at least a portion of the air flow
compressed by the rotor blades and passed over stator vanes; and
means installed in the annular duct downstream of the stator vanes
for disrupting continuity, destroying a symmetrical pattern and
absorbing sound energy of a spinning mode of sound pressures
imbedded in the air flow, downstream of the stator vanes, without
substantially affecting a thrust provided by the air flow when
discharged. The means according to one embodiment of the invention
comprises at least one perforated baffle plate extending generally
in an axial direction with respect to the fan assembly, dividing a
major section of the annular duct downstream of the stator vanes
into at least two axial flow-path segments. The two axial flow-path
segments are in fluid communication with each other through the
perforations in at least one baffle plate.
[0019] According to another embodiment of the present invention,
the means comprises a plurality of perforated baffle plates
extending generally in an axial direction with respect to the fan
assembly, each adapted to span across an annulus defined between
inner and outer walls of the annular bypass duct. Thus, a major
section of the annular duct downstream of the stator vanes is
divided into a plurality of axial flow-path segments. The axial
flow-path segments are in fluid communication with adjacent ones
through the perforations in the respective baffle plates. The
perforated baffle plates are preferably separated axially from the
stator vanes by a space relatively small with respect to a length
of the perforated baffle plates. Each of the perforated baffle
plates may be slightly curved so that a downstream section of the
plate is gently and gradually deviated from an axial orientation in
a circumferential direction opposite to a rotation of the rotor
blades. The circumferential deviation is preferably small relative
to the length of the perforated baffle plate. It is also preferably
to have the perforated baffle plates unevenly circumferentially
spaced apart from one another so that the axial flow-path segments
are formed in a asymmetrical pattern.
[0020] The present invention provides a solution effective for
suppressing rearward noise propagation of fan noise in a bypass
duct of a gas turbine engine without substantially affecting the
thrust provided by the air flow when discharged from the bypass
duct. Other advantages and features will be better understood with
reference to preferred embodiments to be described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Having thus generally described the nature of the present
invention, reference will now be made to the accompanying drawings,
showing by way of illustration the preferred embodiments thereof,
in which:
[0022] FIG. 1 is a longitudinal cross-sectional schematic view of a
gas turbine engine having a short cowl nacelle incorporating one
embodiment of the present invention;
[0023] FIG. 2a is a radial view of the bypass duct of the gas
turbine engine illustrated in FIG. 1 taken along line 2-2, showing
a flat baffle plate axially spaced apart from a stator vane;
[0024] FIG. 2b is a view similar to FIG. 2a, showing a flat baffle
plate integrated with a stator vane according to an alternative
embodiment;
[0025] FIG. 2c is a view similar to FIG. 2a showing a curved baffle
plate integrated with a stator vane and slightly deviated from its
axial orientation; and
[0026] FIG. 3 is a cross-sectional view taken along line 3-3 in
FIG. 1 showing an asymmetrical pattern of the flow-path segments
divided by the baffle plates in the bypass duct.
[0027] FIG. 4 is a longitudinal cross-sectional schematic view of a
gas turbine engine having a long cowl nacelle, incorporating an
embodiment of the present invention; and
[0028] FIG. 5 is a longitudinal cross-sectional schematic view of
the engine shown in FIG. 4, incorporating another embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0029] Referring to the drawings, particularly FIG. 1, an exemplary
gas turbine engine 10 includes in serial flow communication about a
longitudinal center axis 12, a fan having a plurality of
circumferentially spaced apart fan, or rotor blades 14, a
conventional low pressure compressor 16, a conventional high
pressure compressor 18, a conventional annular combustor 20, a
conventional high pressure turbine 22 and a conventional low
pressure turbine 24. The low pressure turbine 24 is securely
connected to both the low pressure compressor 16 and the fan blades
14 by a first rotor shaft 26, and the high pressure turbine 22 is
securely connected to the high pressure compressor 18 by a second
rotor shaft 28. Conventional fuel injecting means 30 are provided
for selectively injecting fuel into the combustor 20 for powering
the engine 10.
[0030] A conventional annular casing 32 surrounds the engine 10
from the low pressure compressor 16 to the low pressure turbine 24,
and defines, with the low pressure compressor 16, a low pressure
compressor inlet 34 for receiving a portion of ambient air 36
thereof. The downstream end of the casing 32 defines with a
conventional annular exhaust plug 40 an annular exhaust outlet 42.
A portion of the air 36 compressed by the fan blades 14 adjacent to
the blade roots 38 are further compressed by the low pressure
compressor 16 and the high pressure compressor 18 and forced into
the combustor 20. The mixture of the compressed air 36 and the fuel
injected by the fuel injecting means 30 generate combustion gases
52. The combustion gases 52 cause the high pressure turbine 22 and
the low pressure turbine 24 to rotate respectively for powering the
high pressure compressor 18, low pressure compressor 16 and the fan
blades 14. Surrounding the blades 14 and the upstream portion of
the casing 32 is a short cowl nacelle 44 which is spaced radially
outwardly from the casing 32 to define with the casing 32 an
annular duct 55 for permitting the radially outer portion of the
air 36 compressed by the fan blades 14 to bypass the engine. A
plurality of circumferentially spaced stator vanes 46 extend
radially between the casing 32 and the nacelle 44, and are spaced
apart axially downstream of the fan blades 14. The nacelle 44
includes an inlet 48 at its upstream end for receiving the ambient
air 36 and an outlet 50 for discharging the portion of the air 36
which is compressed by the fan blades 14 and passed over the stator
vanes 46 for providing a portion of a thrust. The air flow
discharged from the outlet 50 of the bypass duct 55 mixes with the
combustion gases 52 discharged from the exhaust outlet 42 of the
engine to form jet exhaust, thereby creating jet exhaust noise in
the surrounding air.
[0031] In addition to the jet exhaust noise, there is fan noise.
During operation of the engine 10, air 36 enters the inlet 48 and
passes the fan blades 14, due to both the aircraft movement and the
suction generated by the rotating fan blades 14. The air 36 passing
the rotating fan blades 14 effects a blade passing frequency BPF
noise which is a product of the rotational speed N.sub.b and the
number B of the fan blades 14. The portion of the air 36 compressed
by the fan blades 14 and passing the stator vanes 46, under the
fan-stator interaction, generates spinning mode tones, resulting in
increasing the BPF tone level and higher harmonics thereof. The BPF
tone noise and higher harmonics thereof propagate both forwardly
and rearwardly out of the gas turbine engine 10 through the inlet
48 and the outlet 50. In order to significantly reduce rearward fan
noise propagation, at least one perforated baffle plate, but
preferably a plurality of perforated baffle plates 54 are installed
in a section of the annular bypass duct 55 downstream of the stator
vanes 46. The perforated baffle plates 54 in this particular
embodiment of the present invention, are flat, which is more
clearly shown in FIG. 2a, and axially spaced apart from the stator
vanes 46 and span across an annulus defined between inner and outer
walls of the bypass duct 55, to divide a major section of the
annular bypass duct 55 downstream of the stator vanes 46 into a
plurality of axial flow-path segments 55a as shown in FIG. 3.
[0032] The axial space between the perforated baffle plates 54 and
the stator vanes 46, indicated by letter S as shown in FIG. 2a, is
substantially small with respect to the length L of the perforated
baffle plates 54. The baffle plates 54 are unevenly,
circumferentially spaced apart from one another so that the axial
flow-path segments 55a of the annular bypass duct 55 are formed in
an asymmetrical pattern, as illustrated in FIG. 3.
[0033] Each perforated baffle plate 54 includes holes 56 in a
staggered pattern and the holes 56 have a percentage of opening
area POA ranging from 20% to 50%. The holes 56 preferably have
uniform diameters and a ratio of individual hole diameter to plate
thickness between 0.5 and 2.0. The holes 56 in the baffle plates 54
communicate the adjacent axial flow-path segments 55a of the
annular bypass duct 55.
[0034] The principle of the present invention is explained with
details below. Downstream of the fan blades 14, the air 36
compressed by the fan blades 14 is swirling because of the rotating
fan blades 14. This swirl will cause loss of momentum before a
portion of the air 36 compressed by the fan blades 14 exists the
outlet 50 of the annular bypass duct 55, and therefore it is
straightened out with the stator vanes 46. These stator vanes 46
are a source of fan noise as the symmetrical rotation-wakes of the
air 36 compressed by the fan blades 14 impinge on the stator vanes
46, thereby creating a spinning mode tone noise including the
fundamental BPF tone and higher harmonics. When this spinning mode
reaches the local Mach number M greater than about 1.1, spinning
mode tone noise will propagate both forwardly and rearwardly
outside of the annular bypass duct 55. The local Mach number M is
described by the following equation:
M=V.sub.m/a
[0035] wherein:
[0036] a is a sound speed in the local sound propagation medium,
and
[0037] V.sub.m is the maximum tangential speed of the rotational
spinning mode and is proportional to the spinning mode rotational
speed N.sub.m.
[0038] The spinning mode rotational speed N.sub.m is also
proportional to the rotational speed N.sub.b of the fan blades 14,
which may be represented by the following equation:
N=(nB/m)N.sub.b
[0039] wherein:
[0040] B is the number of the fan blades 14,
[0041] n is the blade passing frequency harmonic integer number,
and
[0042] m is the spinning mode number determined by an equation as
follows:
m=nB+kV
[0043] wherein:
[0044] V is the number of the stator vanes 46, and
[0045] k is the index number that may take on all positive and
negative integers, including zero as an integer.
[0046] From the above equations it is apparent that when the engine
operation condition is certain, i.e. N.sub.b unchanged a proper
selection of the number B and V will affect the spinning mode speed
N.sub.m, resulting in a change of the local Mach number M. Based on
this principle, the prior art spinning mode propagation cut-off
technology has been developed. However, as a compromise to other
design criteria, the selection of the number B and V cannot always
satisfy the propagation cut-off requirements. Therefore the
spinning mode tone noise will be present in the annular bypass duct
55 and subsequently discharged at the outlet 50 of the annular
bypass duct 55. The principle of the present invention is that
reduction of rearward noise propagation in the bypass duct is
conducted through direct attenuation or flow modulation to disrupt
continuity, destroy the symmetrical pattern and absorb the sound
energy of the spinning mode of the sound pressures. This is
implemented with the perforated baffle plates 54 which absorb and
reflect the rotational components of the incident wave,
particularly through the action of flow through the baffle holes
56. As the spinning mode is imbedded in the flow, a modulation of
the bypass flow by the perforated baffle plates 54 will attenuate
the spinning mode tones. The perforated baffle plates 54 extend in
a generally axial direction so that the perforated baffle plates 54
interfere only with the rotational components of the spinning mode
of sound pressures imbedded in the air flow, but as straighteners
they do not affect the thrust provided by the air flow when
discharged from the annular bypass duct 55.
[0047] FIGS. 2b and 2c illustrate alternative embodiments. The
space S between the perforated baffle plates 56 and the stator
vanes 46 can be reduced to zero. In one alternative embodiment of
the present invention illustrated in FIG. 2b, a perforated baffle
plate 54a is integrated with a stator vane 46 that is axially
aligned with the perforated baffle plate 54a. However, not all
stator vanes 46 can be integrated with the baffle plates because
the number of stator vanes 46 is usually much greater than the
number of perforated baffle plates 54a and only a few stator vanes
are aligned with respective baffle plates 54a. In another
alternative of the present invention illustrated in FIG. 2c, a
perforated baffle plate 54b is integrated with one aligned stator
vane 46 and gently and gradually curved to deviate slightly from
the axial orientation in a circumferential direction opposite to
the rotational speed N.sub.b of the fan blades 14. The angled
position of the baffle plate 54b will more effectively interfere
with the rotational components of the air flow and the spinning
mode of the sound pressures. Nevertheless, the deviation indicated
by W must be small relative to the length L1 of the perforated
baffle plate 54b. The perforations can spread over the area of the
baffle plate 54b as shown in FIG. 2c or they may just cover a major
section thereof, downstream of the stator vane position as shown in
FIG. 2b.
[0048] The present invention is also applicable to a long cowl
nacelle of a gas turbine engine which is illustrated schematically,
in a plan view, partly in section, in FIG. 4. An exemplary gas
turbine engine 60 is a long cowl mixed flow exhaust fan engine
including a centrally disposed core engine 62. The core engine 62
is coupled to drive a plurality of fan blades 64 disposed upstream
of the core engine 62. The fan blades 64 and the core engine 62 are
disposed inside a nacelle structure 66 which together with the core
engine 62 forms an annular bypass duct 68 for directing a
predetermined portion of the air flow 36 from the fan blades 64
over a plurality of stator vanes 69 and a mixer device 70 toward
the exhaust nozzle 72 for producing the thrust in a manner well
known in the art.
[0049] According to one embodiment of the present invention a
plurality of perforated baffle plates 74 are provided in the
annular bypass duct 68 to divide a major section of the annular
bypass duct 68 between the stator vanes 69 and the mixer device 70,
into a plurality of axial flow-path segments in an asymmetrical
pattern. The structural details of the baffle plates 74 and the
options for alternative embodiments as well as the asymmetrical
pattern of the flow-path segments are similar to those illustrated
in FIGS. 1-3, and will not be redundantly described.
[0050] According to another embodiment of the present invention, as
illustrated in FIG. 5, a plurality of perforated baffle plates 74
are provided in the annular bypass duct 68 downstream of struts 73.
The perforated baffle plates 74 are aligned with the respective
struts 73, either slightly spaced apart therefrom, or integrated
therewith, thereby dividing the section of the annular bypass duct
68 between the struts 73 and the mixer device 70, into a plurality
of axial flow-path segments.
[0051] Modifications and improvements to the above-described
embodiments of the present invention may become apparent to those
skilled in the art. The forgoing description is intended to be
exemplary rather than limiting. The scope of the present invention
is therefore intended to be limited solely by the scope of the
appended claims.
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