U.S. patent application number 13/754037 was filed with the patent office on 2014-07-31 for gas turbine inlet silencer.
The applicant listed for this patent is Robert A. Putnam. Invention is credited to Robert A. Putnam.
Application Number | 20140212265 13/754037 |
Document ID | / |
Family ID | 50068974 |
Filed Date | 2014-07-31 |
United States Patent
Application |
20140212265 |
Kind Code |
A1 |
Putnam; Robert A. |
July 31, 2014 |
GAS TURBINE INLET SILENCER
Abstract
A system for attenuating sound emissions from an inlet to a flow
path for an air inducting machine including an inlet duct structure
having inlet and outlet passages. A planar vortex generator is
located adjacent to the outlet passage and creates vortices that
interact with a specific tonal acoustic frequency emitted from the
inlet of the machine to effect formation of a standing wave at the
vortex generator. The standing wave has an upstream propagating
component that reflects off an acoustic reflector wall to form a
reflected component that interferes with the upstream propagating
component to attenuate the specific tonal acoustic frequency from
the inlet of the machine.
Inventors: |
Putnam; Robert A.; (Winter
Springs, FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Putnam; Robert A. |
Winter Springs |
FL |
US |
|
|
Family ID: |
50068974 |
Appl. No.: |
13/754037 |
Filed: |
January 30, 2013 |
Current U.S.
Class: |
415/1 ;
415/119 |
Current CPC
Class: |
G10K 11/161 20130101;
F01D 25/00 20130101; F05D 2240/127 20130101; F02C 7/045
20130101 |
Class at
Publication: |
415/1 ;
415/119 |
International
Class: |
F01D 25/00 20060101
F01D025/00 |
Claims
1. A system for attenuating sound emissions from an inlet to a flow
path for an air inducting machine, the system comprising: an inlet
duct structure having an inlet passage and an outlet passage
downstream from the inlet passage, the outlet passage defining an
outlet plane extending span-wise generally perpendicular to flow
through the outlet passage; a vortex generator located adjacent to
or upstream of the outlet passage, the vortex generator extending
across the outlet passage and defining a plane, the vortex
generator creating vortices that interact with a specific tonal
acoustic frequency emitted from the inlet of the machine to effect
formation of a standing wave; an acoustic reflector wall located on
the inlet duct between the inlet and outlet passages, upstream of
the vortex generator and oriented generally parallel to the plane
of the vortex generator; and wherein the standing wave has an
upstream propagating component that reflects off the acoustic
reflector wall to form a reflected component that interferes with
the upstream propagating component to attenuate the specific tonal
acoustic frequency from the inlet of the machine.
2. The system of claim 1, wherein the vortex generator includes a
plurality of vortex producing rods extending in a row parallel and
in spaced relation to each other in the span-wise direction across
the outlet passage, and the rods form wake shed vortices on
downstream sides thereof in a plane generally parallel to the
outlet plane.
3. The system of claim 2, wherein the rods have a circular
cross-section defining a diameter, and at least one of the rods has
a different diameter than at least another of the rods.
4. The system of claim 3, wherein the diameters of particular rods
are selected with reference to an average velocity of air flow at
the location of each of the particular rods.
5. The system of claim 4, wherein a distance from the rods to the
acoustic reflector wall varies, depending on the diameter of the
rod.
6. The system of claim 2, including two or more rows of rods spaced
from each other in the direction of flow through the outlet passage
wherein rods in the first row of rods are aligned with rods in the
second row of rods in a direction extending perpendicular to the
outlet plane, and form either an in-line or staggered array.
7. The system of claim 1, wherein the acoustic reflector wall is
located at a junction where the flow direction changes between the
inlet and outlet passages, parallel to the plane of the vortex
generator.
8. The system of claim 1, wherein the air inducting machine is a
gas turbine engine having a compressor including rotating blade
rows, and the specific tonal acoustic frequency is a blade passing
frequency created by at least one of the rotating blade rows.
9. The system of claim 8, wherein the standing wave has a frequency
that corresponds to the blade passing frequency, and the vortex
generator is positioned relative to the acoustic reflector wall
such that a distance, d, between the standing wave and the acoustic
reflector wall is equal to n(.lamda./4), where n is an odd integer
and .lamda. is the wavelength of the blade passing frequency.
10. The system of claim 9, wherein at least one of the vortex
generator and the acoustic reflector wall is movable relative to
the other of the vortex generator and the acoustic reflector wall
to adjust the distance, d.
11. The system of claim 1, wherein interior surfaces of the inlet
duct structure, except for the acoustic reflector wall, are lined
with an acoustic absorptive structure.
12. A method of attenuating sound emissions from an inlet to a flow
path for an air inducting machine, the method comprising: providing
a flow of air through an inlet duct structure having an inlet
passage and an outlet passage, the outlet passage defining an
outlet plane extending span-wise generally perpendicular to flow
through the outlet passage; directing the flow of air over a vortex
generator located adjacent to or upstream of the outlet passage to
create vortices that interact with a specific tonal acoustic
frequency emitted from the inlet of the machine to effect formation
of a standing wave; providing an acoustic reflector wall located on
the inlet duct between the inlet and outlet passages upstream of
the vortex generator and oriented generally parallel to the plane
of the vortex generator; and wherein the standing wave has an
upstream propagating component that reflects off the acoustic
reflector wall to form a reflected component that interferes with
the upstream propagating component to attenuate the specific tonal
acoustic frequency from the inlet of the machine.
13. The method of claim 12, wherein directing the flow of air over
a vortex generator comprises providing a plurality of rods
extending in a row parallel and in spaced relation to each other,
and directing the flow of air through spaces between the rods.
14. The method of claim 13, wherein the standing wave is formed in
a plane spaced downstream from the row of rods.
15. The method of claim 14, wherein a plurality of the rods are
located at different distances from the plane of the standing
wave.
16. The method of claim 12, wherein the standing wave has a
frequency that destructively interferes with the specific tonal
acoustic frequency after the upstream propagating component
reflects off the acoustic reflector wall.
17. The method of claim 16, including moving at least one of the
vortex generator and the acoustic reflector wall relative to the
other of the vortex generator and the acoustic reflector wall to
adjust the distance between the vortex generator and the acoustic
reflector wall to tune the reflected component so as to
destructively interfere with the specific tonal acoustic
frequency.
18. The method of claim 12, wherein the air inducting machine is a
gas turbine engine having a compressor including rotating blade
rows, and the specific tonal acoustic frequency is a blade passing
frequency created by at least one of the rotating blade rows.
19. The method of claim 18, wherein the standing wave has a
frequency that corresponds to the blade passing frequency, and the
vortex generator is positioned relative to the acoustic reflector
wall such that a distance, d, between the standing wave and the
acoustic reflector wall is equal to n(.lamda./4), where n is an odd
integer and .lamda. is the wavelength of the blade passing
frequency.
20. The method of claim 19, including moving at least one of the
vortex generator and the acoustic reflector wall relative to the
other of the vortex generator and the acoustic reflector wall to
adjust the distance, d.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to gas turbine engines and,
more particularly, to an inlet silencer for attenuating sound
emissions from an inlet to the gas turbine engine.
BACKGROUND OF THE INVENTION
[0002] Gas turbine engines used for power generation include a
compressor for supplying the engine with compressed air that is
mixed with fuel and ignited to produce a hot working gas, and the
hot working gas is directed through a turbine section to produce a
work output from the engine. The compressor includes a plurality of
stages formed by pairs of rows rotating blades and stationary
vanes. The rotating blades produce an inlet noise that results from
interaction of various different phenomena related to interaction
of the blades with air being drawn into the compressor, and
produces both broadband noise and blade-passing discrete tones.
[0003] Various mechanisms have been implemented to suppress the
noise emitted from the compressor inlet. For example, acoustically
absorptive parallel inlet baffles may be installed at the inlet
duct for the compressor, between an inlet filter house and the
compressor, to absorb compressor inlet noise. However, such baffle
structure may restrict air flow and substantially increase the cost
for providing noise attenuation at the compressor inlet.
SUMMARY OF THE INVENTION
[0004] In accordance with an aspect of the invention a system is
provided for attenuating sound emissions from an inlet to a flow
path for an air inducting machine. The system comprises an inlet
duct structure having an inlet passage and an outlet passage
downstream from the inlet passage, the outlet passage defining an
outlet plane extending span-wise generally perpendicular to flow
through the outlet passage. A vortex generator is located adjacent
to or upstream of the outlet passage, the vortex generator
extending across the outlet passage and defining a plane. The
vortex generator creates vortices that interact with a specific
tonal acoustic frequency emitted from the inlet of the machine to
effect formation of a standing wave. An acoustic reflector wall is
located on the inlet duct between the inlet and outlet passages,
upstream of the vortex generator and oriented generally parallel to
the plane of the vortex generator. The standing wave has an
upstream propagating component that reflects off the acoustic
reflector wall to form a reflected component that interferes with
the upstream propagating component to attenuate the specific tonal
acoustic frequency from the inlet of the machine.
[0005] The vortex generator may include a plurality of vortex
producing rods extending in a row parallel and in spaced relation
to each other in the span-wise direction across the outlet passage,
and the rods form wake shed vortices on downstream sides thereof in
a plane generally parallel to the outlet plane.
[0006] The rods may have a circular cross-section defining a
diameter, and at least one of the rods may have a different
diameter than at least another of the rods.
[0007] The diameters of particular rods may be selected with
reference to an average velocity of air flow at the location of
each of the particular rods.
[0008] A distance from the rods to the acoustic reflector wall may
vary, depending on the diameter of the rod.
[0009] Two or more rows of the rods may be spaced from each other
in the direction of flow through the outlet passage wherein rods in
the first row of rods may be aligned with rods in the second row of
rods in a direction extending perpendicular to the outlet plane,
and may form either an in-line or staggered array.
[0010] The acoustic reflector wall may be located at a junction
where the flow direction changes between the inlet and outlet
passages, parallel to the plane of the vortex generator.
[0011] The air inducting machine may be a gas turbine engine having
a compressor including rotating blade rows, and the specific tonal
acoustic frequency may be a blade passing frequency created by at
least one of the rotating blade rows.
[0012] The standing wave may have a frequency that corresponds to
the blade passing frequency, and the vortex generator may be
positioned relative to the acoustic reflector wall such that a
distance, d, between the standing wave and the acoustic reflector
wall is equal to n(.lamda./4), where n is an odd integer and
.lamda. is the wavelength of the blade passing frequency.
[0013] At least one of the vortex generator and the acoustic
reflector wall may be movable relative to the other of the vortex
generator and the acoustic reflector wall to adjust the distance,
d.
[0014] Interior surfaces of the inlet duct structure, except for
the acoustic reflector wall, may be lined with an acoustic
absorptive structure.
[0015] In accordance with another aspect of the invention, a method
of attenuating sound emissions from an inlet to a flow path for an
air inducting machine is provided. The method comprises providing a
flow of air through an inlet duct structure having an inlet passage
and an outlet passage, the outlet passage defining an outlet plane
extending span-wise generally perpendicular to flow through the
outlet passage; directing the flow of air over a vortex generator
located adjacent to or upstream of the outlet passage to create
vortices that interact with a specific tonal acoustic frequency
emitted from the inlet of the machine to effect formation of a
standing wave; providing an acoustic reflector wall located on the
inlet duct between the inlet and outlet passages upstream of the
vortex generator and oriented generally parallel to the plane of
the vortex generator; and wherein the standing wave has an upstream
propagating component that reflects off the acoustic reflector wall
to form a reflected component that interferes with the upstream
propagating component to attenuate the specific tonal acoustic
frequency from the inlet of the machine.
[0016] The step of directing the flow of air over a vortex
generator may comprise providing a plurality of rods extending in a
row parallel and in spaced relation to each other, and directing
the flow of air through spaces between the rods. The standing wave
may be formed in a plane spaced downstream from the row of rods,
and a plurality of the rods may be located at different distances
from the plane of the standing wave.
[0017] The method may further include forming the standing wave
such that it has a frequency that destructively interferes with the
specific tonal acoustic frequency after the upstream propagating
component reflects off the acoustic reflector wall.
[0018] The method may further include moving at least one of the
vortex generator and the acoustic reflector wall relative to the
other of the vortex generator and the acoustic reflector wall to
adjust the distance between the vortex generator and the acoustic
reflector wall to tune the reflected component so as to
destructively interfere with the specific tonal acoustic
frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] While the specification concludes with claims particularly
pointing out and distinctly claiming the present invention, it is
believed that the present invention will be better understood from
the following description in conjunction with the accompanying
Drawing Figures, in which like reference numerals identify like
elements, and wherein:
[0020] FIG. 1 is a diagrammatic view of a gas turbine engine
configured to incorporate aspects of the invention;
[0021] FIG. 2 is a cross-sectional view of an inlet duct including
a sound attenuation system in accordance with aspects of the
invention;
[0022] FIG. 3 is a cross-sectional view of the inlet duct taken
along line 3-3 in FIG. 2;
[0023] FIG. 4 is an enlarged view of area A in FIG. 2,
diagrammatically illustrating a standing wave;
[0024] FIG. 5 is a cross-sectional view taken along line 5-5 in
FIG. 3;
[0025] FIG. 6 is a cross-sectional view illustrating a vortex
generator having different size rods; and
[0026] FIGS. 7A and 7B are cross-sectional views showing
alternative embodiments of vortex generators having two rows of
rods.
DETAILED DESCRIPTION OF THE INVENTION
[0027] In the following detailed description of the preferred
embodiment, reference is made to the accompanying drawings that
form a part hereof, and in which is shown by way of illustration,
and not by way of limitation, a specific preferred embodiment in
which the invention may be practiced. It is to be understood that
other embodiments may be utilized and that changes may be made
without departing from the spirit and scope of the present
invention.
[0028] Referring to FIG. 1, in accordance with an aspect of the
invention, an inlet silencer is provided at the inlet to a flow
path for an air inducting machine. In the illustrated embodiment,
the air inducting machine is a gas turbine engine 10, such as a gas
turbine engine 10 used in a power generation plant. The engine 10
includes a compressor 12 that receives air through a compressor
inlet duct structure or inlet duct 14. The illustrated inlet duct
may be located downstream from an inlet filter house (not shown).
The compressor delivers compressed air to a combustor 16 where the
air is combusted with a fuel to produce a hot working gas for a
turbine section 18 of the engine 10, producing a work output such
as to power an electric generator (not shown).
[0029] Referring to FIGS. 2 and 3, the inlet duct 14 of the
illustrated embodiment has a generally rectangular cross-section,
however, it may be understood that the aspects of the invention
described herein may be implemented in inlet ducts 14 having other
configurations, such as a circular configuration. The inlet duct 14
includes an inlet passage 28 formed by first and second side walls
20, 22, and outer and inner walls 24, 26 which connect the side
walls 20, 22. As seen in FIG. 2, the inlet passage defines a first
central axis A.sub.1.
[0030] An outlet passage 30 is located between the inlet passage 28
and a compressor inlet housing 32, and an expansion joint 34 may be
provided at the connection between the outlet passage 30 and the
compressor inlet housing 32. The outlet passage 30 includes first
and second side walls 36, 38, which may be contiguous with the
first and second side walls 20, 22 of the inlet passage 28. The
first and second walls 36, 38 are connected by a front wall 40 that
extends to the inner wall 26 of the inlet passage 28, and a rear
wall 42 that extends to the outer wall 24 of the inlet passage 28.
The outlet passage 30 defines a second central axis A.sub.2 that
may be generally perpendicular to the first central axis A.sub.1 of
the inlet passage 28. An air flow F.sub.1 entering the compressor
12 may pass into the inlet duct 14 through the inlet passage 28
generally parallel to the first central axis A.sub.1, and change
directions at a junction between the inlet and outlet passages 28,
30, i.e., at an area generally indicated by 44, and further may
pass as an air flow F.sub.2 generally parallel to the second
central axis A.sub.2 to the compressor inlet housing 32.
[0031] It should be noted that although the described inlet duct 14
is depicted as having differently oriented inlet and outlet
passages 28, 30 as a preferred embodiment, the inlet and outlet
passages 28, 30 may be at an angle other than perpendicular
relative to each other, provided only that the plane of the
acoustic reflector wall be parallel to the plane of the vortex
generator Po.
[0032] Referring to FIG. 3, the compressor 12 includes a plurality
of rotatable blades 46 arranged as a row of circumferentially
distributed blades 46, a first row of which is depicted in FIG. 3.
While operation of the turbine engine 10, and in particular the
compressor 12, produces a broad range of sound frequencies that are
emitted to the inlet duct 14, the rotation of the blades 46
produces specific tonal acoustic frequencies. In particular,
rotation of the first row of blades 46 produces a specific tonal
acoustic frequency, and at a sufficiently large amplitude, which
forms a dominant frequency that aspects of the present invention
are configured to attenuate. The dominant or specific tonal
acoustic frequency is considered a "pure tone", which is a signal
with a line spectrum consisting of a single frequency that
corresponds to a blade passing frequency of the first row of blades
46, and is referred to herein as the "dominant compressor
tone".
[0033] In accordance with an aspect of the invention, a vortex
generator 48 is provided located adjacent to, upstream from, or in
the outlet passage 30. In the embodiment illustrated in FIGS. 2 and
3, the vortex generator 48 is located within an entry to the outlet
passage 30 and includes a plurality of cylindrical pipes or rods
50. The rods 50 are arranged in spaced relation to each other in an
array or row defining a plane, P.sub.V, extending across a width,
w, of the outlet passage 30. The plane, P.sub.V, of the array of
rods 50 is oriented generally perpendicular to the second central
axis A.sub.2 of the outlet passage 30.
[0034] Referring to FIG. 4, as will be described in greater detail
below, the rods 50 have a diameter, D, selected to produce a wake
shed vortex 52 downstream from each rod 50 at a particular
frequency, or within a limited frequency range, that overlaps with
the dominant compressor tone. The illustrated wake shed vortex 52
downstream from each of the rods 50 is commonly known as a von
Karman vortex street. In addition to being provided with a
particular diameter, D, the rods 50 are separated by a
center-to-center spacing, X (see FIG. 5), to permit sufficient flow
past the sides of the rods 50 to form a von Karman vortex street,
as well as being located sufficiently close to enable the wake shed
vortex 52 of each rod 50 to interact with the wake shed vortices 52
of adjacent rods 50.
[0035] Since the engine 10 is used in a power generating plant,
which operates at a design speed for any load, the rotational speed
of the blades 46 will remain substantially constant, such that the
dominant compressor tone will not vary substantially throughout the
operation of the engine 10. Further, since gas turbine engines are
"constant volume machines," the velocity of the air flow drawn into
the inlet duct 14 and past the rods 50 will remain substantially
constant. Hence, the wake shed vortices 52 formed downstream of the
rods 50 will be in a substantially constant frequency relationship
with respect to the dominant compressor tone.
[0036] It may be noted that the frequency of the wake shed vortices
52 will normally wander or vary within a range of frequencies, such
that the frequency along the span of a rod 50 is not typically
constant. However, by superimposing an intense sound field, such as
is provided by the dominant compressor tone, E1, on the nominal
shedding frequencies of the rods 50, the shedding frequencies will
become in-phase and uniform, i.e., coupled, in a two-dimensional
sheet or plane.
[0037] In particular, the wake shed vortices 52 constructively
interact with the dominant compressor tone emitted from the
compressor 12 to create a "lock in" phenomenon which forms a plane
of coherent waves at the same frequency as the dominant compressor
tone, and depicted as a standing wave 54 in a generally planar
region 56 downstream from the rods 50, as illustrated in FIG. 4.
The region 56 of the standing wave 54 is typically located one to
five rod diameters downstream from the rods 50. The standing wave
54 is perpendicular to the plane, P.sub.V, of the array of rods.
The width, w, may be selected such that a transverse acoustic mode
of the rectangular volume defined in the outlet passage 30 is an
integer multiple of the wavelength of the standing wave 54, even
though the transverse mode is only weakly interacting with the
intense dominant compressor tone sound field, E1, and the potential
strength of the transverse mode is minimized as a result of an
acoustically absorbent wall lining on walls 40 and 42, as is
described further below.
[0038] The standing wave 54 has an upstream propagating component
58 in the form of a planar wave front that travels toward the
junction 44 with the inlet passage 28. An acoustic reflector wall
60 is located supported at the outer wall 24 adjacent to the
junction 44 and axially aligned with the array of rods 50. As seen
in FIG. 2, the second central axis A.sub.2 passes through both the
plane, P.sub.V, of the rods 50 and the reflector wall 60. The
reflector wall 60 is formed of a material, such as a hard wall
material, to efficiently acoustically reflect the upstream
propagating component 58 of the standing wave 54. The reflector
wall 60 is oriented parallel to a plane, P.sub.0 (see, FIG. 4),
defined by the planar region 56, such that a reflected wave front
62 is reflected from the reflector wall 60 parallel, and in the
opposite direction, to the upstream propagating component 58. The
plane, P.sub.0, defines an effective origin of the upstream
propagating wave 58.
[0039] A distance, d, from the plane, P.sub.0, of the standing wave
54 to the reflector wall 60 is preferably equal to a value of
n(.lamda./4), where n is an odd integer and .lamda. is the
wavelength of the standing wave 54. Hence, the reflected wave front
62 has the same wavelength but is one-half wavelength out of phase
with the wave front of the upstream propagating component 58
produced at the planar region 56, resulting in destructive
interference occurring between the reflected wave front 62 and the
upstream propagating component 58, with resulting attenuation of
the blade passing frequency.
[0040] It may be noted that the destructive interference provided
by the sound attenuation system described herein is significant in
that the dominant compressor tone within the inlet duct 14 is not
typically in the form of a uniform wave front, and therefore would
not normally be susceptible to destructive interference. The
frequency of the wake shed vortices and the dominant compressor
tone cooperate to form the "locked in" wave front that is conducive
to reflecting off the reflector wall 60 as a reflected wave front
62 out of phase with the upstream propagating wave front 58 for
destructive interference.
[0041] Other noise frequencies, such as broad band inlet noise
having an amplitude or energy that is lower than the frequency of
the dominant compressor tone, may be damped out or attenuated by
conventional inlet silencer structures. For example, the side walls
20, 22, 36, 38, a portion of the outer wall 24 not including the
reflector wall 60, the inner wall 26, and the front and rear walls
40, 42 may be provided with an acoustically absorptive liner
system, such as may be formed by a perforated plate 61 located in
front of an acoustically absorptive fiberglass pillow structure 63
(FIG. 2).
[0042] The reflector wall 60 may be supported for movement toward
and away from the plane, P.sub.0, of the standing wave 54 in order
to "tune" the acoustic attenuation system by adjusting the
distance, d, to be equal to n(.lamda./4), where n is an odd
integer. For example, the reflector wall 60 may be movable to
adjust the distance, d, about 7 to 8 centimeters in a direction
along the second central axis A.sub.2. Such tuning of the system
may be necessary, for example, to adjust for variations resulting
from changes in the ambient temperature of the air passing into the
inlet structure 14, and to fine tune the system during
installation. As is depicted diagrammatically in FIG. 2, the
reflector wall 60 may be actuated for movement relative to the
outer wall 24 by actuators 64. The actuators 64 may be any
conventional actuator such as, for example, a manually adjustable
mechanism, or a linear actuator actuated by a servomotor,
pneumatics or hydraulics. Additionally, or instead of, moving the
reflector wall 60 to tune the acoustic attenuation system, the
array of rods 50 may be actuated for movement relative to the
reflector wall 60, such as by the actuators 66 diagrammatically
depicted in FIG. 3.
[0043] It may be noted that the frequency of the wake shed vortices
52 formed downstream from the rods 50 does not necessarily have to
be exactly the same as the dominant compressor tone for the "lock
in" phenomena to occur. As long as the acoustic field associated
with the dominant compressor tone is within about .+-.10% to
.+-.20% of the wake shed vortex frequency that would be formed in
the absence of the influence of the dominant compressor tone, then
the two acoustic fields will "lock in" to form the standing wave
54.
[0044] The frequency, f, that would be formed behind the rods 50 in
the absence of the acoustic field of the dominant compressor tone
is described by the equation relating the vortex frequency in a von
Karman vortex street to the Strouhal number as follows:
f=(Su)/D
[0045] where: [0046] S is the Strouhal number, a dimensionless
number that is usually equal to about 0.2 for an isolated rod;
[0047] u is the average velocity of flow past the rod (meters/sec);
and [0048] D is the diameter of the rod, (meters)
[0049] From the above equation it can be seen that, in order to
match the frequency produced by the von Karman vortex street to the
frequency of the dominant compressor tone, the rod diameter, D, may
be selected with reference to the velocity of air flowing past the
rods 50, which velocity will be substantially determined by the
design volume flow for the engine 10.
[0050] The velocity profile across the inlet duct, normal to the
flow direction, may vary substantially and thus may produce an
associated variation in the wake shed frequency across the array of
rods 52. For example, the air flow may have a higher velocity
toward the center of the outlet passage 30 than adjacent to the
walls. In order to maintain a substantially constant frequency
across the array of rods 50, a larger diameter of rods 50 may be
provided in locations of lower flow velocity, as compared to
smaller diameter rods 50 in locations of higher flow velocity.
Additionally, since the distance from any given rod 50 to the
effective location of the associated vortice's acoustical sound
field is a function of the diameter of the rod 50, the distance of
each rod 50 to the reflector 60 should be adjusted to place the
acoustical sound field of the rods 50 at the location of the plane
P.sub.0.
[0051] An exemplary array of rods 50 having different diameters is
illustrated in FIG. 6, in which it is assumed that a lower flow
velocity may be present adjacent to the walls 40, 42 than toward
the center of the passage 30. In this illustrated embodiment the
smaller diameter rods 50 are located adjacent to the walls 40, 42,
and these smaller diameter rods 50 are located closer to the
reflector wall 60, such that the array of rods 50 is oriented
angled outwardly (i.e., angled upstream) toward the center of the
passage 30, as depicted by angles .alpha., .beta.. It should be
noted that the variation in diameter of the rods 50 illustrated in
FIG. 6 is exaggerated, and the angled array of the rods 50 is
depicted with exaggerated angles whereas in that the actual
displacement of the rods 50 relative to each other is on the order
of a few millimeters, such as a displacement along the second
central axis A.sub.2 of about one to ten millimeters.
[0052] It should also be noted that, since the velocity field
within the inlet duct 14 may also vary along the length of the rods
50, the diameter of one or more of the rods 50 may vary along the
rod length. Hence, any rod 50 having a varying diameter may also be
effectively angled along the length of the rod 50 to place the
acoustical sound field of the rod 50 at the location of the plane
P.sub.0.
[0053] The frequency of the dominant compressor tone is typically
on the order of 1000 Hz and local velocities within the inlet duct
14 may be as low as about 20 m/s. Accordingly, with a Strouhal
number of about 0.2, the required rod diameter would be about 4 mm.
In accordance with a further aspect, and referring to FIGS. 7A and
7B, an alternative configuration for the acoustic attenuation
system is illustrated, including two rows of rods 50a and 50b
located spaced along the second central axis A.sub.2. As is known
in the art, the additional set of rods increases the Strouhal
number such that a larger diameter, structurally stronger, rod may
be implemented in the vortex generator. FIG. 7A illustrates that
the rows of rods 50a, 50b may be axially aligned with each other.
FIG. 7B illustrates that the rows of rods 50a, 50b may be axially
displaced relative to each other. Further, it should be noted that
within the scope of the invention, more than two rows of rods 50
may be provided.
[0054] Additionally, it should be understood that alternative
configurations of the acoustic attenuation system relative to the
inlet duct 14 may be provided. For example, it is not necessary
that the reflector wall 60 be positioned where every change in flow
direction occurs in the inlet duct 14, and the reflector wall 60
may be positioned at any upstream location where it can be located
parallel to the plane, P.sub.0, of the standing wave 54.
[0055] While particular embodiments of the present invention have
been illustrated and described, it would be obvious to those
skilled in the art that various other changes and modifications can
be made without departing from the spirit and scope of the
invention. It is therefore intended to cover in the appended claims
all such changes and modifications that are within the scope of
this invention.
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