Note: Descriptions are shown in the official language in which they were submitted.
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Field of the Invention
:- Tllis invention concerns control of aerodynamic noise
created by gaseous flow through a flow restriction, and more
particularly, tortuous path type elements designed to minimize
such aerodynamic noise by gradually dissipating the pressure
energy of the gas and limiting the velocity of the gas flow below
that at which noise levels become troublesome.
Background of the Invention
Control of noise created by gas flow through restric-
tions in piping systems has become increasingly important as
noise levels in manufacturing and other industrial facilities
have been subjected to close governmental regulation. Irl addi-
tion, in certain defense applica.ions, i.e. submarines, noise
control is a critical design objective. A major source of noise
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such situations have been identified as beiny caused by an aero-
dynamic pllenomenon associated with hiyh velocity flow levels
created by a rapid expansion of the gas after passing through a
flow restriction, creating localized high velocity flow conditions.
In order to prevent such excessive velocities, tortuous
flow ~ath elements havebeen used in conjunction with valves, etc.,
to gradually decrease the pressure of the gas so that its velo- --
city remains su~s~antially constant and at a relatively low level.
Such tortuous pa~h devices have in the past been ,urovided by rela-
tively expensive machined parts or stacked discs in which laby-
rinth passages are formed.
While such devices may accomplisn tne desired control
of gas velocities, the cost penalty is relatively high and the
flexibility of design is limited in adapting to varying flow con-
ditions which has led to the consideration of porous materials,
but in most such materials it is difficult to accurately control
pore size to prevent localized conditions of high velocity flow
created by the occurrence of relatively sr,lall openings.
One such material in which relatively precise control
over the pore size mai7 be had is a precision wound material o~ -
the sort described in U.S. Patent Nos. 2,857,657 and 3,123,446.
Thls material is formed by a precision winding operation in which
wire ribbon material is wound on a mandrel with successive wind-
ings being crossed with respect to each other to create porous
~ layers having openings of precisely controlled size. The layers
'~j; of windings are subsequently diffusion bonded to provide a unitary
;~ structure. This approach provides a material in which porosity
may be relatively easily controlled by varying the pitch, the
crossover angle of the windings, wire, size, etc.
~hile these features would seem to lend this construction
, to the above described tortuous path flow element, an additional
requirement of such a flow element is that the openings should be
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progressively larger to compensate for expansion of the gaS as
~ the pressure declines in passing through the tortuous path.
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In the above described design, a slight taper in the
opening size occurs as windings of the same pitch are wound on an
increasingly greater diameter created by the previous layer, but
such taper is not nearly great enough to compensate for the change
in density of the gas. If the pitch of successive windings is
increased to provide such increased area openings, periodic
interference patterns between windings are created which form
small area openings and blockages, causing localized high flow
velocities, at least partially defeating the objectives describ~d.
In addition, minimum pore sizes must sometimes be he~d to
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prevent blockages by particles suspended in the gas.
Accordingly, the present invention provides a controlled
porosity material which is formed by successive windings of
,i wire and in which the winding pitch is varied in successive
'i windings, while the interference patterns described are avoided
,,, to provide a porous acoustic element.
According to the present invention there is provided
a porous acoustic gas expansion control element comprising:
a first porous layer comprised of a plurality of layers of
windings of wire, each of said windings being spaced apart at
~ the same pitch with successive layers being wound to cross each
;~!, other to create a multiplicity of first spaces having a total
cross-sectional area; a second porous layer comprised of a
plurality of layers of windings of wire, each of said windings
~ being spaced apart at the same pitch, said pitch being greater
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than the pitch of said first porous layer windings, with
successive layers being wound to cross each other to create a
multiplicity of second spaces, said multiplicity of second
spaces having a total cross-sectional area larger than the total
cross-sectional area of said multiplicity of first spaces;
'~! a transition layer interposed between said first and second
r porous layers, said transition layer being formed with a multi-
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plicity of plenum openings, said muitiplicity of openings having
a cross-sectional area at ieast as great as the cross-sectional
area of said multiplicity of second spaces, said transition layer
being configured and oriented to allow gas expansion from said
first porous layer to said second porous layers, whereby the
interference pattern of the overlay of said transition layer with
each of said first and second porous layers does not produce
blockages or openings of smaller area than the spaces of either
of said first or second porous layers; and said first and second
porous layers and said transition layer being bonded together
to form an integral structure.
The present invention also provides a method of reducing
aerodynamic noise in a gas flowing from a region of relatively
high pressure to a region of relatively lower pressure by limiting
the gas flow velocity comprising: disposing a first porous layer,
in juxtaposition to said high pressure region to cause said gas
to flow through said first porous layer, said first porous layer
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: being comnrisedof a plurality of layers of windings ofwire, each
of said windings being spaced apart at the same pitch with successive
layers being wound to cross each other to create a multiplicity
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of first spaces, said first spaces having a total predetermined
cross-sectional area; disposing a transition layer adjacent
said first porous layer remote from said high pressure region,
said transition layer being formed with a multiplicity of plenum
openings of total cross-sectional area at least as great as the
total predetermined cross-sectional area of said multiplicity
of first spaces in said first porous layer; disposing a second
porous layer, said second porous layer being comprised of a plur-
ality of layers of windings of wire in juxtaposition to said
transition layer, each of said windings being spaced apart at
the same pitch, said pitch being greater than the pitch of said
first porous layer windings, with successive layers being wound .
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to cross each other to create a multiplicity of second spaces,
said second spaces having a total ~redetermined cross-sectional
area greater than said tstal predetermined cross-sectional area
of said first spaces, such that the difference in total cross-
sectional area corresponds to the increase in the volume of said
gas as said pressure decreases in flowing through said first
porous layer and said transition layer; said transition layer
further being oriented to said first porous layer so as to pre-
clude an interference pattern producing blockages or openings
of sma~ler area than either of said first or second porous
layers; bonding together said first and second porous layers and
said transition layer; and then passing said gas through a
sufficient number of alternating porous and transition layers
from said high pressure region to said lower pressure region
to decrease said gas pressure to that existing in said low
pressure region without exceeding a predetermined gas flow
velocity.
Thus, according to the present invention the porous
acoustic gasexpansion control element is formed by successively
winding layers of increasing pitch but which layers are not
directly wound on each other but rather on transition layers
having openings therein acting as plenum spaces, such transition
layers in the preferred embodiment being provided by a layer of
mesh material having openings of an area on the order of the
lar~est ContiauouswOund layer. The particular configuration
of the transition layer, i.e., the difference in the angle of the
mesh material and the size Ofthe oPenings precludes formation of
an interference pattern to allow the mesh layer openings to act
as plenum spaces and provide a transition space for flow
therethrough into the next wound layer.
The present invention will be further illustrated by
way of the accompanying drawings in which:
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Figure 1 is a fragmentary view of a section of a porous
acoustic element according to one embodiment of the present
invention.
Figure 2 is a view of the section 2-2 taken in Figure 1.
Figures 3- 7 are diagrammatic representations of steps
in the manufacture of the porous acoustic element according to
the present invention.
Referring to the drawings and particularly Figures 1
and 2, the porous acoustic element 10 consists of a plurality of
porous layers 12 each created by windings of flattened wires 14
of a construction as taught by U.S. Pa~ent 2,8~7,657. Successive
windings of the ribbon wire 14 cross each other and are of
constant spaced apart pitch so as to create a controlled porosity
layer.
Interposed between the porous layers 12 are transition
or plenum layers 16 in the preferred embodiment provided by layers
of square weave mesh 16 placed over the first porous layer 12 with
the square weave pattern extending transversely to the windings
in the porous iayers.
According to the concept of the present invention, each
of the cross-winding layers 12 is of a progressively more open
construction in the direction toward which flow is to occur, i.e.
from the region of high pressure towards the region of relatively
low pressure, as indicated in Figure 2. The relationship between
the openings of successive winding layers as well as the number
of successive layers should be designed to correspond to the
pressure gradient conditions existing in the particular
application.
Each of the transition or plenum layers 16 have openings
sized to be larger than either cross-winding layer 12 contiguous
therewith, and of such a
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configuration that the overlay of the transition layer 16 and
either contiguous cross-winding layer 12 does not create the inter-
ference pattern described above, thereby preventing the occurrence
of undersized openings or blockages. This configuration is pro~
vided in this embodiment by the square weave pattern extending
transversely to t]le wire windings. Thus, flow may occur from one
cross-winding layer 12 through the transition layer 16 to a
successive cross-winding layer of substantially different pitch
without creating the overlay interference problem.
The entire assemblage is diffusion bonded to create a
unitary or monolithic structure for incorporation into a valve
housing or other application device.
A specific example, for illustrative purposes only,
will be given in conjunction with a brief description of a typical
fabrication process, depicted diagrammatically in Figure 3-7.
In the first step, a first porous layer 12 ~as produced
by winding on a mandrel 18 rotate~ in a winding machine 20, a
.008 inch thick b~ 0.125 inch wide stainless steel ribbon, with
a 9 helix angle and a 0.185 inch center-to-center spacing between
winds and 0.060 d,stance between wires. This produces a porous
layer having a 10.5% open area.
Next, a layer of wire mesh was calendered to reduce its
thickness at wire cross-over points to improve contact area.
This woven wire mesh was of .016 inch diameter stainless steel
wire having 16 wires per inch to produce a spacing of 0.0465
with an open area greater than 50%.
The layer of wire mesh 16 was placed over the existing
cross-wound layer 12 as shown in Figure 4 and held in place by
some temporary fastener such as tape, etc., until the first few
windings were made of the next cross-wound layer 12.
A second cross-wound layer 12 is wound over the transi-
tion layer 16 as shown in Figure 5. This layer was of 0.010 inch
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thick stainless s-teel wire ribbon 0.063 inches wide, center-to-
center spacing of 0.104 inches and a distance between wires of
0.041 inches and 9 helix angle, producing an open area of 15.5%
Additional transition and cross-wound layers were
added alternately as described to produce a desired porosity
characteristic.
Next, the assemblage is sintered or diffusion bonded by
heating in a vacuum or hydrogen atmosphere as depicted in Figure
6 to bond the contacting intersections of the wires and produce a
unitary porous tube.
The layers of wound wires are then removed from the
mandrel 18 for further fabrication steps, such as sectioning
or forming sheets of the material depending on the requirements
of the particular application.
While a square-weave mesh material has been found to ~;~
provide spaces which will act as plenums, and not create restric-
tions when overlaying the cross-wound layers, other materials
could also be utilized to accomplish this same end. For example,
photoetched or punched layers of thin metal are disclosed in ~.S.
Patent 3,900,629 having appropriately sized parallel slots which
could be utilized.
Another alternative would be to wind a single layer of
flattened wire at a very low helix angle (i.e. 1 or less) at
spacing equal or greater than the desired minimum pore size.
Accordingly, it can be seen that a relatively easily
fabricated porous acoustic element which is readily adapted to
almost any situatio~ has been provided by the present invention,
inasmuch as the number of layers and their characteristics may
be varied to tailor the overall porosity of the material. This
element is used to great advantage in the disclosed method of
controlling aerodynamic noise.
