Porous composite structure

A porous composite structure for controlling of gas flow therethrough. The porous composite structure includes a first porous element formed from a sheet having a plurality of perforations with a first total predetermined cross-sectional area. A second porous element formed from a perforated sheet having a second total predetermined perforation cross-sectional area is disposed parallel to the first porous element and spaced a predetermined distance therefrom. At least one intermediate porous element is interposed the first porous element and the second porous element. The intermediate porous element has a total predetermined cross-sectional pore area which is less than the first and second total predetermined cross-sectional areas. Each of the porous elements are bonded to adjacent porous elements to form an integral structure, whereby the intermediate porous element regulates the flow of gas through the porous composite structure while the first and second porous elements mechanically strengthen the porous composite structure. A method of producing the porous element is also disclosed.

BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
The present invention relates to a porous composite structure having 
enhanced structural strength. In particular, the porous composite 
structure of the present invention is suitable for fluidization 
application and for the control of aerodynamic noise. 
2. DESCRIPTION OF THE PRIOR ART 
There are a variety of porous composite structure which are designed to 
solve a number of individual problems. For example, in U.S. Pat. No. 
3,260,370, issued to Schwartzwalder on July 12, 1966, relates to a 
composite filter element for use in filtering and conditioning dry 
cleaning solvents. In another example, U.S. Pat. No. 3,679,062, issued to 
Burkhart on July 25, 1972, the composite filter element is designed to 
resist outward distortion to the filter sheets during the backwashing and 
cleaning of the filter sheets. Thus, while each of the many prior art 
composite porous structures solve a unique specific problem, most are 
unsuitable for a variety of applications such as influidizing media and 
the control of acoustic noise. 
Fluidization occurs when either a liquid or a gas, most commonly ambient 
air, is moved or blown through a dry powder to separate the particles and 
permit them to behave as a fluid. More exactly, when a fluid is passd 
upwardly through a bed of closely sized granular particles, a pressure 
gradient is required to overcome friction. In order to increase the rate 
of flow, a greater pressure gradient is required. When the pressure drop 
approaches the weight per unit surface area of the particles, they begin 
to move and exhibit fluid properties. There are several major devices 
advantageously using the fluidization of powder material, including 
gravity conveyors, powder dryers and coolers, batch powder mixers, bulk 
storage silos, separators and heat transfer beds. 
In the case of gravity conveyors, the fluidization process is used to 
facilitate transporting a dry powdered material a predetermined distance. 
A porous sheet having a longitudinally formed trough and having a small 
angle of incline is used as the conveyor. Thus, while the material is 
fluidized, it flows somewhat like water along the trough. The conveyor may 
have a very small angle of inclination which is considerably less than the 
angle of repose of the powdered material because the material is 
fluidized. 
With respect to powder dryers and coolers, the powder material is fluidized 
in order to evaporate moisture from the powder or to exchange heat through 
the powder. Batch powder mixers use fluidization to accomplish the mixing 
of different powders within a batch. The powders become agitated because 
of the low resistance to fluid flow while fluidized and thus the powders 
become throughly mixed. 
Silos for holding cement powder and for other bulk storage are additional 
examples of fluidizing applications. To enable easy removal of the powder 
from the silo, a fluidized bed at the bottom of the silo blows air through 
the powder to impart to the powder a fluidized state. Once fluidized, the 
powder can be removed from the silo by simply blowing the powder out of 
the silo, since the powder acts very much like a liquid. 
An example of an application for an illutration separator is separation of 
grain from impurities such as weed seeds and small stones. In this 
example, the grain mixture is passed over and moved across a fluidization 
bed at a very small angle relative to the horizontal. The fluidized 
mixture is then moved outside of the fluidization bed and is separated by 
gravity, since the trajectory of the particles will vary. The stones, 
being heavy, will fall within a comparatively short distance. The grain 
seeds will fall within a moderately longer distance than the stones while 
the weed seeds, which are generally much lighter particles than the grain 
seeds, travel still further. 
The use of fluidization in the heat transfer beds consists of placing a 
part in a fluidization bed of silica or aluminum oxide utilizing hot air 
as the fluidizing media. The part is heated very rapidly due to the 
convection effect caused by the motion of the small particles of silica 
sand or aluminum oxide and the impingement of these particles on the part. 
In all of the above mentioned fluidizing applications, porous elements are 
useful. When choosing a porous element, its strength, its resistance to 
abrasion and puncture, its ease of cleaning either chemically or by steam, 
its flow characteristics, its cost, and its ability to operate over a wide 
range of temperatures are all important considerations. 
In addition to the above mentioned considerations, fluidizing applications 
require that the element have a a strong construction yet have appropriate 
perforation to provide a high pressure drop uniformly over the element as 
compared to the pressure drop through the solid particles. This is 
necessary because, if the pressure drop across the media is similar to the 
pressure drop across the element, the operation will become unstable 
which, of course, is undesirable. 
One prior art design of a porous element for fluidizing applications 
utilizes a single plate with a plurality of drilled holes therethrough. It 
has been found that this design was not satisfactory since the holes 
weakened the element. Furthermore, it was prohibitively expensive and did 
not provide the pressure drop characteristics required for fluidizing 
applications. 
In another prior art design of a porous element for fluidizing 
applications, a layer of wire cloth which has been roll calendered and 
fusion bonded between two layers of plain mesh weave wire has been found 
to be moderately successful in some fluidizing media applications. 
However, an element of this type of construction is also expensive and is 
difficult to fabricate. 
Thus, none of the known art porous element designs provides an inexpensive 
and easy to fabricate porous media suitable for fluidizing applications. 
The control of aerodynamic noise created by gas flow through restrictions 
and piping systems has also become increasingly important as noise levels 
in airplanes and in industrial facilities have been subjected to close 
governmental regulation. A major source of noise in such situations is an 
aerodynamic phenomenon associated with high velocity flow rate. The high 
velocity flow rate is created by a rapid expansion of a gas after passing 
through a flow restriction, thereby creating a localized high velocity 
flow condition. 
To prevent such localized high velocity flow conditions, sophisticated flow 
path elements have been used to gradually decrease the pressure of the gas 
so that the velocity remains substantially constant and at a relatively 
low rate. Such flow path elements may accomplish the desired control of 
gas velocity, but at a relatively high cost. Furthermore, they limit the 
flexibility of design because of the varying flow conditions under which 
these devices must operate. This has led to the consideration of using 
porous materials. Unfortunately, it is difficult to accurately control 
pore size in porous materials to prevent localized conditions of high 
velocity flow created by the currents of relatively small openings. 
One such material in which relatively precise control of pore size is 
obtained is described in U.S. Pat. Nos. 2,457,657 and 3,123,466 and U.S. 
Application No. 945,261, filed Sept. 22, 1978. This material is formed by 
a precision winding operation in which wire ribbon material is wound on a 
mandrel with successive windings being crossed relative to each other to 
create porous layers having openings of precisely controlled size. The 
layers of the windings are subsequently diffusion bonded to provide a 
unitary structure. This approach, however, provides a material which is 
expensive to make and is susceptible to damage and to plugging of the 
exposed wire cloth layers. 
Accordingly, it is an object of the present invention to provide a 
controlled porousity composite structure which is adaptable for use in 
both fluidizing bed and acoustical applications and which is inexpensive 
and simple to fabricate. The porous composite structure is formed of 
several porous elements by bonding one layer of weave cloth between two 
layers of perforated sheets wherein the airflow resistance is controlled 
by transverse flow paths between the sheets and the cloth with the open 
mesh transferring stress to the strong perforated sheets. 
SUMMARY OF THE INVENTION 
The present invention is directed to a composite porous structure useful 
for both acoustical and fluidizing media applications, which is strong and 
permits the accurate control of the gas flow characteristics therethrough. 
The present invention provides a porous composite structure for control of 
gas flow expansion therethrough. The composite structure includes a first 
porous element and a second porous element disposed parallel to the first 
porous element and spaced a predetermined distance therefrom. The first 
and second porous elements preferably are perforated sheets having, 
respectively, a first and second total predetermined cross-sectional 
perforation area. At least one intermediate porous element is interposed 
the first and second porous elements. The intermediate porous element has 
a third total predetermined cross-sectional pore area which is smaller 
than both the first and second predetermined cross-sectional areas. Each 
of the porous elements is bonded to adjacent porous elements so that 
together these porous elements form a single integral composite porous 
structure. The third total predetermined cross-sectional area is selected 
to regulate the flow of gas through the composite structure. The first and 
second porous elements provide mechanical strength to the composite pore 
structure. 
The invention also provides a method of controlling the flow rate of gas 
flowing from a region of relatively high pressure to a region of 
relatively low pressure. The method includes the step of disposing a first 
porous element in juxtaposition to the high pressure region to cause the 
gas to flow through the first porous element. The first porous element is 
composed of a perforated sheet having a first total predetermined 
perforation cross-sectional area. Next, an intermediate porous element is 
placed contiguous to the first porous element. The intermediate porous 
element has a third total predetermined pore cross-sectional area which is 
less than the first predetermined cross-sectional area and is selected to 
regulate the gas flow velocity flowing through the first porous element 
and the intermediate porous element. Next, a second porous element similar 
to the first porous element is placed contiguous to the intermediate 
porous element in juxtaposition to the low pressure region to cause the 
gas to flow through the second porous element. The second porous element 
is a perforated sheet having a second predetermined perforation 
cross-sectional area which is greater than the third predetermined 
cross-sectional area. Finally, the first porous element and the second 
porous element are each bonded to the intermediate porous element to 
provide a unitary composite porous structure having a mechanical strength 
greater than the intermediate porous element but having the flow 
resistance characteristics of the intermediate porous element. 
The primary object of the present invention is to provide an inexpensive 
composite porous structure having desired flow characteristics yet 
providing great structural strength. 
Another object of the present invention is to provide a composite flow 
structure having the flow characteristics of wire mesh material yet having 
greater structural strength. 
Still another object of the present invention is to provide an inexpensive 
means for manufacturing composite porous structures for acoustical 
applications. 
Still another object of the present invention is to provide an method for 
inexpensively manufacturing composite porous structures useful for 
fluidizing media applications. 
These and many other objects, features, and advantages of the present 
invention will become apparent to those skilled in the art when the 
following detailed description is read in conjunction with the drawings 
appended hereto.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the following description and in the appended drawings, a particular 
embodiment is described utilizing specific terminology for the sake of 
clarity. The embodiment described and illustrated is the best mode 
contemplated by the inventor at the time of filing the present application 
for carrying out the invention. However, it is understood that this 
description is not intended to be limiting and should not be so construed 
inasmuch as the invention may take many forms and variations within the 
spirit and scope of the appended claims. 
Referring now to the drawings, and more particularly to FIG. 1 thereof, a 
porous composite structure according to the present invention is generally 
designated by the numeral 10. The porous composite structure includes a 
first outer porous element 12, four intermediate porous elements 14 
through 20 and a second outer porous element 22. Those skilled in the art 
will recognize that any number of intermediate elements may be utilized in 
practicing the invention and that the invention is not limited to any 
specific number of intermediate elements between the first outer porous 
element 12 and the second outer porous element 22. Each of the elements 12 
through 22 are bonded to adjacent elements to form a unitary composite 
element 10. 
The first and second outer porous elements 12 and 22, respectively, are 
preferably flat, thin metallic sheets having a plurality of apertures 24 
and 26, respectively. 
The apertures 24 in the first outer porous element 12, in the preferred 
embodiment are arranged in a rectangular array as illustrated in FIGS. 1 
and 2. The apertures 24 extend along a first transverse axis 30 and along 
a second transverse axis 32 which is perpendicular to the first transverse 
axis 30. Each of the apertures 24 along the first transverse axis 30 are 
separated by a first distance "a" and, similarly, each of the apertures 24 
along the second transverse axis 32 are separated by a second distance 
"b". Each of the apertures 24 has a diameter measurement "c". The 
apertures 24 in the first outer porous element 12, therefore, have a first 
predetermined total aperture cross-sectional area which may be determined 
by the following formula, where A is the total cross-sectional area of the 
first outer porous element 12 and P is the total cross-sectional aperture 
area of the first outer porous element 12: 
EQU p=(Pi.c.sup.2.A)/(4.a.b) 
In one example of structure, the first outer porous element 12 is formed 
from a perforated sheet which has a plurality of apertures having a 
diameter of three hundredths of an inch (0.030") arranged in a rectangular 
array having holes spaced twenty-five hundredths of an inch (0.25") apart. 
This produces a porous element having an open area equal to one percent 
(1%) of the total area of the sheet. 
The second outer porous element 22 may be similar to the first outer porous 
element 12. In the example illustrated in the drawing, the apertures 26 in 
the second outer porous element are arranged in a similar rectangular 
array having longitudinal axes parallel to the longitudinal axes 30 and 32 
of the first outer porous element 12 but spaced, respectively, a distance 
d and e therefrom so as to be somewhat offset from the apertures in the 
first outer porous element 12. 
In the preferred embodiment, each of the first and second outer porous 
elements 12 and 22, respectively are made from 304 stainless steel having 
a thickness "t" of fifteen thousandths of an inch (0.015"). Each of the 
outer porous elements 12 and 22, respectively, have an outer surface 34 
exposed to the working fluid and an inner surface 36 for bonding to the 
intermediate porous elements 14 through 20. 
Each of the intermediate porous elements 14 through 20 have a plurality of 
apertures 38 through 44, respectively, formed therein. The apertures 38 
through 44 are formed in arrays each having a third total predetermined 
aperture cross-sectional area. This third predetermined cross-sectional 
area is less than both the first predetermined cross-sectional area and 
the second cross-sectional area of, respectively, the first and second 
outer porous elements 12 and 22. Therefore, the third predetermined 
cross-sectional area determines the flow characteristics of the working 
fluid therethrough. 
Preferably, the intermediate porous elements 14 through 20 are formed from 
a woven wire cloth. In the preferred embodiment, four intermediate 
elements are provided, each consisting of a 30.times.30.times.0.012 sheet 
of wire mesh material. That is, the wire mesh material is formed of woven 
wire having a diameter of twelve thousandths of an inch (0.012") which is 
woven to form cloth having thirty (30) wires per inch in each transverse 
direction. Preferably, the wire mesh material has been calendered to a 
thickness of sixteen thousandths of an inch, of an inch to eighteen 
thousandths (0.016" -0.018") to produce a comparatively flat intermediate 
porous element 14 through 20. The flattening of the wire mesh material 
decreases the pore size in the wire mesh material and, further, increases 
the contact area between the adjacent intermediate porous elements 14 
through 20. Furthermore, this process increases the contact area between 
the outermost intermediate porous elements 14 and 20 and the inner 
surfaces 36 of the first and second outer porous elements 12 and 22, 
respectively so as to produce a better bond therebetween. 
Each of the porous elements 12 through 22 may be bonded to adjacent porous 
elements by diffusion bonding procedures, well known in the art, to 
produce a single unitary composite porous structure. For example, the 
several porous elements 12 through 22 maybe secured together by such 
temporary fastening means as tape and, subsequently, fusion bonded by 
heating in a vacuum or hydrogen atmosphere. 
Those skilled in the art will recognize that for acoustic applications the 
porous composite structure made according to the present invention must be 
less than thirty percent (30%) transparent. Thus, for this purpose, the 
composite element is preferably constructed as described above with the 
first outer porous element 12 offset from the second outer porous element 
22 by one hundred and twenty-five thousandths of an inch (0.125") or, half 
of the distance between adjacent apertures in both transverse directions 
30 and 32. Furthermore, for this purpose, the intermediate porous elements 
16 and 20 are preferably offset from the adjacent intermediate porous 
elements 14 and 18 by forty-five degrees (45.degree.) so as to provide a 
tortuous path for the working fluid through the composite structure. 
In an alternate embodiment for fluidizing applications, the intermediate 
porous elements 14 through 20 used are four layers of zero to forty-five 
degrees (0.degree.-45.degree.) cross axis oriented one hundred and 
forty-five (145) mesh plain square weave bolting cloth. These layers of 
cloth are laminated between two outer porous elements 12 and 22 consisting 
of layers of perforated plastic sheet with between thirty and fifty 
percent (30-50%) open area. The outer porous elements 12 and 22 are 
oriented to provide an offset hole pattern as previously described. The 
resultant structure is cemented together with reticulating epoxy to form 
the above described porous composite structure 10 having the advantages 
and features referred to above. 
It will be appreciated by one skilled in the art that the structure formed 
thereby will have great structural strength in comparison to the 
structural strength of the wire mesh intermediate porous elements 14 
through 20 alone. Nonetheless, the flow characteristics through the porous 
composite structure 10 will be determined by the flow characteristics 
through the intermediate porous elements 14 through 20. This is true since 
the total effective predetermined cross-sectional area of the composite 
structure will be determined by the pore size of the intermediate porous 
elements 14 through 20. Those skilled in the art will recognize that the 
number of intermediate porous elements used in constructing the porous 
composite structure 10 and the flatness of each intermediate porous 
element will affect the permeability of the composite structure to fluid 
flow. Furthermore, the permeability of the porous composite structure 10 
can also be affected by a partial angular offset of successive 
intermediate elements, yfor example, by providing a crossing angle varying 
from zero to forty-five degrees (0.degree. to 45.degree.) between the 
adjacent intermediate porous elements 14 through 20. As the crossing angle 
is decreased, the actual effective pore size of the composite structure 
decreases. 
The stresses normally found in the intermediate porous elements 14 through 
20 are transferred to the first and second outer porous elements 12 and 22 
in a manner similar to the load carrying characteristics of a conventional 
"I" beam construction, in common use in civil engineering applications. 
Thus, the weaker portion of the construction controls the flow 
characteristics of the gas whereas the stronger portion of the 
construction controls the strength of the composite structure. This 
composite construction further provides a porous element which is 
comparatively light and abrasion resistant in comparison to structures 
having similar strength and porosity characteristics. Finally, the 
composite structure 10 provides a porous element which is comparatively 
lightweight and abrasion resistant in comparison to other porous elements 
constructed by prior art methods having similar flow rate and weight 
characteristics. 
While the present invention has been described in connection with preferred 
embodiments and methods, it will be understood that it is not intended to 
limit the present invention to those embodiments and methods. On the 
contrary, it is intended to cover all alternatives, modifications and 
equivalents as may be included within the spirit and scope of the present 
invention as defined by the appended claims.