Wire cloth as porous material for transpiration-cooled walls

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Material Information

Title:
Wire cloth as porous material for transpiration-cooled walls
Series Title:
NACA RM
Physical Description:
38 p. : ill. ; 28 cm.
Language:
English
Creator:
Eckert, E. R. G ( Ernst Rudolf Georg ), 1904-
Lewis Research Center
United States -- National Advisory Committee for Aeronautics
Publisher:
NACA
Place of Publication:
Washington, D.C
Publication Date:

Subjects

Subjects / Keywords:
Wire screens   ( lcsh )
Airplanes -- Motors -- Cooling   ( lcsh )
Aerodynamics -- Research   ( lcsh )
Genre:
federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Abstract:
Abstract: The permeability characteristics and tensile strength of a porous material developed from stainless-steel corduroy wire cloth for use in transpiration-cooled walls where the primary stresses are in one direction were investigated. The results of this investigation are presented and compared with similar results obtained with porous sintered metal compacts. A much wider range of permeabilities is obtainable with the wire cloth than with the porous metal compacts considered and the ultimate tensile strength in the direction of the primary stresses for porous materials produced from three mesh sizes of wire cloth are from two to three times the ultimate tensile strengths of the porous metal compacts.
Bibliography:
Includes bibliographic references (p. 16).
Statement of Responsibility:
by E.R.G. Eckert ... et al.
General Note:
"Report date November 13, 1951."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 003810642
oclc - 135003787
sobekcm - AA00006217_00001
System ID:
AA00006217:00001

Full Text
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RM E51H23


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RESEARCH MEMORANDUM
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.E WIRE CLOTH AS POROUS M
:I; TRANSPIRATION-COOLE

By E. R. G. Eckert, Martin
and Reeves P. Coc
i Lewis Flight Propulsion I
Cleveland, Ohio


A .UNIVERSITY OF FLORF
DOCUMENTS DEPART
.'120 MARSTON SCIEN(
~R. P.O. BOX 117011
GAINESVLLE, FL 3261



NATIONAL ADVISORY
FOR AERONAL
WASHINGTON
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MATERIAL FOR
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NACA RM E51E23


NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

RESEARCH MEMORANDUM

WIRE CLOTH AS POROUS MATERIAL FOR TRANSPIRATION-COOLED WALLS

By E. R. G. Eckert, Martin R. Kinsler, and Reeves P. Cochran


SUMMARY

An investigation was made to determine the properties of wire cloth
as a porous material for transpiration-cooled walls where a coolant is
forced through the porous material to form an insulating layer of fluid
on the heated surface of the wall. Materials presently available for
transpiration cooling, such as sintered porous metals, do not have suf-
ficient strength for applications in which the operating stresses are
high. For applications where the stresses act primarily in one direc-
tion, a porous material with high strength in that direction is desirable.
An example of such an application is in turbine-rotor blades where the
centrifugal stress predominates. The suitability of a corduroy-type wire
cloth manufactured from AISI type 304 stainless steel was investigated
for this purpose.

The cloth was woven with considerably more wires in one direction
than in the other. As woven, the cloth was too permeable for most
transpiration-cooling applications, but by cold-rolling a porous material
may be obtained with a wide range of permeabilities, which should cover
most requirements for transpiration-cooled walls. The stiffness of the
wire cloth could be increased by a brazing process that bonded the wires
together. In order to provide an adequate basis for comparison of
various porous materials, a reduced tensile strength was introduced for
aircraft applications where strength-density ratio is important. The
reduced tensile strength of the cloth after brazing and rolling, was as
high as 130,000 pounds per square inch, which is 2 to 3 times the ulti-
mate comparable strength of compacted sintered metals. Spot-welding was
found to be a satisfactory method of attaching wire cloth to solid struc-
tures and seam brazing afforded a means of attaching layers of cloth to
each other.


INTRODUCTION

Transpiration cooling has been shown to be an effective means for
cooling structures in high-temperature high-velocity gas streams (refer-
ence 1). In the transpiration-cooling process, the walls of the struc-
ture are made of a porous material and a coolant is forced through the
porous wall to form an insulating layer of fluid between the wall and








NACA RM E5H123


the hot gas stream. This method holds particular promise for air-cooled
gas-turbine rotor blades where conventional convection-cooling methods
become inadequate at high gas temperatures. The transpiration-cooling
method also offers attractive possibilities for the cooling of other
structural elements in the aircraft propulsion system, such as com-
bustion chambers, transition ducts, turbine stators and casings, and
jet-nozzle components. This method may also be used to protect the skin
of missiles from aerodynamic heating effects. Additional applications
for porous walls may be found in deicing where warm air is forced through
the walls, and in boundary-layer control devices where suction is applied
to improve flow characteristics. cQ

A porous material for transpiration cooling and the other applica-
tions mentioned must fulfill certain basic requirements. These require-
ments are:

(a) Controllable permeability which is locally uniform or changes
locally in a prescribed manner to meet a required coolant-flow distri-
bution

(b) Sufficiently wide range of permeabilities to accommodate large
variations in coolant flow requirements

(c) Adequate strength for a proposed application

(d) Availability in quantity in pieces of sufficient size and thick-
ness to meet a specific application

(e) Adaptability to conventional fabrication methods

Usually a porous material produced by sintering of powdered metals
is considered for transpiration-cooling applications. However, sintered
metals do not as yet satisfactorily fulfill all the previously-mentioned
requirements. The applicability of other materials for the
transpiration-cooling process was therefore studied. In many
transpiration-cooling applications, the primary stresses are in one
direction in the plane of the porous wall, as, for example, in gas-
turbine rotor blades where the primary stresses are caused by centri-
fugal forces and act in the direction of those forces. In such a case,
it is advantageous to have a porous material that is composed of fibers
or wires which run parallel to the direction of these primary stresses.
In the process of investigating materials which meet this idea, a type
of wire cloth having the greater portion of the wires woven in one direc-
tion was selected for consideration. As woven, the wire cloth has a
permeability which is too large for most transpiration-cooling applica-
tions; however, its permeability can be reduced to any desired degree by
cold-rollin'. The stiffness was increased by a brazing process which
bonded the wires together.









NACA RM E51H23


The results are presented herein of an investigation conducted at
the NACA Lewis laboratory of the permeability and strength of wire cloth
both in its normal state and as modified by the previously-mentioned
brazing and rolling processes for transpiration cooling.

Samples of wire cloth for these investigations were obtained from
The W. S. Tyler Company of Cleveland. The assistance of this company
and particularly Mr. Hugh Brown in the development of wire cloth as a
material for transpiration cooling is gratefully acknowledged.


PREPARATION OF WIRE CLOTH

A wire cloth suitable for the outlined purpose should be such that
the greater portion of the wires run in the direction of the primary
stresses. A special weave called corduroy cloth fulfills this require-
ment. The mesh of wire cloth is designated by the number of openings
per inch between wires in the warp and shoot, respectively. (See
fig. 1.) In this report the wires in the shoot of the cloth are referred
to as lengthwise wires, and the wires in the warp as crosswise wires.
Data for the three different meshes of the wire cloth, which were inves-
tigated in this report, are contained in table I. The number of wires
in the second and fourth columns was determined by actual count on the
samples used. For the 20X350 wire cloth, this number deviated consider-
ably from the nominal number.

The material in the cloth tested in this investigation is AISI
type 304 stainless steel. However, the wire cloth can be manufactured
from other materials as well. The wires from which the cloth is woven
are in an annealed state. The weaving process itself causes some work
hardening. Front and side views of such a wire cloth are shown in fig-
ure 2. This figure depicts the special type of weave which is used in
the manufacturing process.

As woven the wire cloth has a permeability that is too high for
most of the applications considered. However, the permeabilities may be
decreased to any desired degree by a cold-rolling process. The strength
of the wires is increased by this rolling process as long as the reduc-
tion in thickness stays below a certain limit, approximately half the
original thickness. Front and side views of the wire cloth after it was
rolled to half its original thickness are presented in figure 3. This
photograph may be somewhat misleading as it gives the impression that
the spaces between the wires are closed up except for the rows of dark
holes. In reality, the slots that run obliquely to the surface of the
cloth are still present between the wires. In addition to decreasing
the permeability, the rolling process has another beneficial effect.
For many applications, the surface of the transpiration-cooled wall has
to be smooth. It can be seen from a comparison of figures 2 and 3 that
the surface roughness is decreased considerably by the rolling process.








NACA RM E5LH23


In order to be effective, the rolling has to be applied in the
lengthwise direction, which is the axial direction of the larger number
of wires. In some cases, porous material prepared in this way is
undesirable because it is considerably less rigid against bending forces
than a piece of solid sheet metal. The rigidity can be increased by
interconnecting the wires at all places where they touch each other.
Such interconnection was obtained in the following way: The wire cloth was
sprayed with a low-temperature silver brazing alloy by the use of a metal
spray gun; the brazing material applied this way did not close up the
pores but covered the wires only on the surface facing the spray gun. A
view of the surface of the sprayed cloth is shown in figure 4(a). A sil-
ver brazing alloy with a melting point of 12600 F was used in this inves-
tigation. The sprayed cloth was dipped into a salt bath at about 14000 F
temperature until the sprayed alloy was brazed to the surface of the
wires (figs. 4(b) and 4(c)). It can be seen that the brazing material
gave a very good interconnection of the wires without closing up the
spaces needed for the flow of the coolant.

The material containing no brazing material is referred to herein
as "unbrazed wire cloth", and the sprayed and heated material as "brazed
wire cloth". The specific application determines whether preference
should be given to the brazed or the unbrazed wire cloth. An advantage
of the brazed cloth is its greater stiffness, but, it is to be expected
that unbrazed cloth has good vibration damping characteristics, which
are desirable in many cases. Also, the permissible temperature will be
higher for the unbrazed cloth. For these reasons, the investigations
were conducted on both types of cloth. Air-flow tests showed that the
permeability was not decreased seriously by the brazing process. The
permeability of the brazed material was reduced to the desired degree by
rolling after the brazing process was finished. Photographs of this
material after its thickness has been reduced 15 and 37 percent of the
original value are presented in figures 5(a) and 5(b), respectively.
The material prepared in this way has a smooth surface and a stiffness
comparable to solid sheet metal of the same thickness.

Methods were developed by The W. S. Tyler Company for joining pieces
of wire cloth by a brazing process and also for interconnecting several
layers of wire cloth by local application of heat in a manner correspond-
ing to spot welding or seam welding. Of the different procedures tested
at the Lewis laboratory for connecting wire cloth to a solid metal struc-
ture, spot welding was found to be most satisfactory.


PERMEABILITY OF WIRE CLOTH

The amount of coolant that can be forced through a porous material
with a given pressure drop must be known for the designer to select the
material and operating pressure for a specific application of transpira-
tion cooling.









NACA RM E51H23


In order to determine the amount of air that will flow through
rolled wire cloth of various permeabilities in both the brazed and the
unbrazed state, tests were performed during which the weight rate of
flow and the pressure drop across the cloth were measured.


Apparatus and Procedure

A schematic diagram of the test equipment used for determining the
permeability of wire cloth is shown in figure 6. Air at room tempera-
ture and at a gage pressure of 120 pounds per square inch is filtered
and passed through a pressure regulator. The air flow is controlled by
a hand valve and measured by a rotameter. It then passes through a
specimen of wire cloth held between two copper gaskets in a pipe-to-tube
connector coupling. This arrangement is shown in detail in figure 6.
The temperature and the pressure of the air passing through the rotameter
are also measured.

Pieces of unbrazed wire cloth selected from sheets supplied by the
manufacturer were rolled various amounts to maximum reduction of 50 per-
cent of their original thickness. Disks, 1 inches in diameter, were
cut from the selected pieces and their average thicknesses were deter-
mined. These disks were then clamped tightly between the copper gaskets
in the connector coupling. Air was permitted to flow through the test
apparatus. The pressures on both sides of the wire-cloth specimen as
well as the weight flow through it were determined. For the most part,
measurements were made on three layers of the cloth stacked one on top
of the other. However, in order to determine whether or not there is a
difference in permeability when different numbers of layers are used,
tests were also performed on one and five layers of the cloth. Similar
permeability tests were made on brazed- and rolled-wire cloth.


Results of Permeability Tests

The weight rate of flow of gases through a plane wall of porous
material at large pressure differences and under isothermal conditions
depends on the pressure-square difference (reference 2). Therefore, the
results of the permeability tests are plotted in figures 7 and 8 as the
difference in the squares of the pressures on both sides of the cloth
per unit of thickness of cloth against the weight rate of flow of gases
through the cloth. Two mesh sizes of the unbrazed cloth (fig. 7), and
three mesh sizes of brazed cloth (fig. 8) were investigated .

Each of the figures contains results with different numbers of
layers at the same percentage of thickness reduction by rolling. An
examination of the corresponding points reveals that the pressure-square
difference per unit thickness of the cloth does not depend systematically








NACA RM E51H23


on the number of layers. Differences that occur in some cases are
attributed to inaccuracies in the preparation of the porous cloth or the
thickness measurements. Especially at large values of thickness reduc-
tion, the thickness measurement has to be extremely accurate in order to
obtain reproducible results. This point is discussed in the following
section. The fact that the pressure-square difference per unit thick-
ness does not depend on the number of layers indicates that the pressure
drop through a specified number of layers is the same regardless of
whether the layers are placed some distance apart or stacked closely
together. The two unconnected points in figure 8(b) are for data
obtained in an attempt to investigate a 20X350 specimen of very low per-
meability. There was insufficient pressure drop available to test at
higher weight-flow rates.

A comparison of unbrazed and brazed cloth of a specific mesh (figs.
7(a) and 8(a) or figs. 7(b) and 8(c), respectively) indicates that the
pressure-square difference of the brazed cloth is about 20-percent
larger than the pressure-square difference of unbrazed cloth with the
same thickness. The brazed cloth must be rolled in order to obtain con-
siderable reduction in the permeability. However, the difference in the
pressure-square difference values of unbrazed and brazed cloth become
greater at large values of thickness reduction (figs. 7 and 8). At
high values, it is therefore unnecessary to roll brazed cloth as much as
unbrazed cloth in order to obtain the same permeability. This fact
becomes important when embrittlement is a consideration in cloths rolled
to very low permeability; it is discussed in the section entitled
"TENSILE STRENGTH OF WIRE CLOTH."

No consistent differences can be found between the values of the
pressure-square difference per unit thickness for cloth of different
meshes in either the brazed or the unbrazed condition.


Comparison With Compacted Sintered Porous Metals

A comparison of the relation between porosity and permeability of
rolled wire cloth with that of some compacted sintered porous metals is
of interest because such a comparison should give some indication of
the nature of the flow as influenced by the geometry of the channels in
both materials.

For this purpose, the porosity of the wire cloth will be calculated.
The porosity is the ratio of the volume of voids Vv to the total
volume Vt (all symbols are defined in the appendix):


f V
Vt(1)
fvt








NACA RM E51H23 7


or if the volume of metal Vm is used

Vm
f = ---
Vt

However, the volume of the metal includes the volume of the steel Vs
and the volume of the brazing alloy Vb, therefore


V = V + V

When the corresponding weight W and specific weight y are used,
this equation may be written

Ws Wb
*s rb

Then


1 W s + \
f = 1 +'- -+-
Vt Ts ITb

and because Vt = AT where A is the surface area and T is the
thickness of the cloth


1 1 s b
f = 1 --+- I (2)
T Ts A ybA


By this formula the porosity can be calculated because the specific
weight of the steel and of the brazing material is known. The weight
per unit area of the unbrazed material is Ws/A and the difference in
weight of the brazed and the unbrazed material is Wb/A.

In figure 9, the porosity of rolled-wire cloth, as determined by
equation (2), is plotted against the percentage reduction in original
thickness. The value of the porosity of unbrazed wire cloth at a given
value of thickness reduction increases with an increase in the ratio of
the diameter of the crosswise wires to the diameter of the lengthwise
wires (see table I). A large diameter ratio causes more bending of the
wires in weaving and consequently more void space. The amount of braze
material added to the cloth was largest for the 28X500 mesh size. This
fact explains why the reduction in porosity by the brazing process was
largest for this type of cloth.








NACA RM E51H23


The permeability coefficient K is defined by Darcy's law (refer-
ence 3) as follows:


pl2 p22 1
(2RBT)G (3)


where ZT is the total thickness of a number of layers of cloth in
series. A linear relation between the pressure-square difference and
the weight rate of flow is assumed in this law. Actually, the relation
is not quite linear (figs. 7 and 8). This relation was expressed in
reference 2 by the equation


pl2 p22 2RT 2
a= (2RTp)G + G (4)
ET g

By equating equations (3) and (4) and solving for K, it is found that


K = (5)
1 + G


This equation shows that the permeability coefficient actually varies
somewhat with the weight rate of flow and the temperature (viscosity) of
the fluid passing through the porous material as well as the configura-
tion of the passages in the material. A permeability coefficient based
on Darcy's law is used herein in order to compare the wire cloth with
compacted sintered metals which were evaluated on this same basis in
reference 4. For small values of weight flow, this method gives a good
approximation of the permeability coefficient K in equation (5)

because the second term G in the denominator becomes small as com-
MaMg
pared with the first term. For large values of weight flow, the effects
of this second term become appreciable and cannot be neglected.

The permeability coefficient K or 1/a is plotted against the
percentage reduction in original thickness in figure 10. The test points
appear to be grouped around two curves, one for brazed cloth and one for
unbrazed cloth.

Finally, the porosity was plotted against the permeability coeffi-
cient in figure 11 along with test results on sintered metal compacts
obtained from reference 4. From figure 11 it is seen that although the
same range of porosities is considered the range of permeability coeffi-
cients for the wire cloth is much greater than that of the sintered








NACA RM E51H23


compacts. For a small change in porosity of the wire cloth, it is pos-
sible to obtain a large change in the permeability. On the other hand,
because the porosity is mainly controlled by the rolling process, a
small error in obtaining a required thickness of the cloth means a rela-
tively large error in the permeability. As an example, it is found from
figures 9 and 11 that for brazed 20X250 mesh cloth having a porosity in
the region of 14 percent an error of 0.0001 inch causes a _13 percent
variation in the permeability coefficient. The rolling process must
therefore be controlled with extreme care to obtain reproducible results.
This fact also explains the scatter in figures 9 and 10. For equal per-
meability values, the required porosity of sintered material is generally
greater than that of the wire cloth probably because of the smaller
cross section and the more tortuous course of the passages.


TENSILE STRENGTH OF WIRE CLOTH

Experimental Procedure and Results

Sufficient strength is an important consideration in many applica-
tions of porous materials to transpiration cooling as was mentioned in
the INTRODUCTION. The three meshes of wire cloth tested were woven from
AISI type 304 stainless steel, which has a tensile strength in the
annealed state of 87,000 pounds per square inch. This material will
elongate about 65 percent in a 2-inch gage length before rupturing
(reference 5). The manufacturer of the cloth estimates that a 20-percent
elongation occurs during the weaving process, and that this cold-working
of the material raises the tensile strength of the cloth to about
100,000 pounds per square inch. The effect of cold work on the tensile
strength and percent elongation of AISI type 302 stainless steel (refer-
ence 5) is shown in figure 12. The behavior of AISI type 304 stainless
steel is similar except for a slightly higher rate of work hardening
because of the lower carbon content. It can readily be seen from these
curves that in addition to bringing the permeability of the wire cloth
into a desirable range, the rolling will increase the tensile strength
and reduce the percentage elongation.

Tensile tests have been made at room temperature on rolled and
unrolled wire cloth, both brazed and unbrazed. These tests were made on
strips of mesh approximately 0.8 inch wide and 10 inches long. In order
to obtain better gripping in the jaws of the testing machine, the ends
of the test strips were coated with soft solder. Particularly with the
unbrazed cloth this coating insured that all wires would be stressed
equally. During the tests, the load was increased progressively in
increments of 100 pounds and the elongation of the test specimen was
measured at each loading up to and including the breaking load. The
elongation of rolled specimens of brazed and unbrazed cloth is shown in
figures 13 and 14 as percentage elongation in 4 inches. The percentage








NACA RM E51H23


elongation due to tensile stress decreases with increased amount of roll-
ing performed on the cloth. This decrease is partly due to compacting
of the woven structure, and partly to the increase in the hardness of
the material. The marked effect of cold-working on the elongation of
AISI type 302 stainless steel is apparent in figure 12. Cold-working
of AISI type 304 stainless steel will produce similar effects. If the
allowable stresses in a structure are determined by the elongation, then
the stresses must be kept below 0.5 to 0.7 of the tensile strength
depending on the amount of rolling (figs. 13 and 14).

The effects of rolling on the tensile strength of the wire cloth
are shown in figures 15 and 16. In this report, the term tensile
strength refers to the ultimate tensile strength unless otherwise indi-
cated. The left sides of both figures were computed by dividing the
breaking load by the sum of the cross-sectional areas of the lengthwise
wires as determined from the diameters before rolling and the actual
number of wires per inch given in table I. For structural elements like
turbine blades which have to carry their own weight in a centrifugal
field, the ratio of the tensile strength a to the specific weight r
is a value more suitable for comparing different materials than the ten-
sile strength a alone. Also, for nonrotating parts such as combustion-
chamber liners, the weight is the main factor limiting the thickness of
the material. In general, the ratio a/y is therefore the best basis
for a comparison of different materials for aircraft structural compon-
ents. All the porous materials which will be compared herein are manu-
factured from 18-8 stainless steel. A reduced tensile strength a' was
therefore determined by multiplying the strength specific-weight ratio
by the specific weight ys of the stainless steel:


S= rs (6)


This value can be compared with the familiar strength values of solid
stainless steel for which y = ys.


For the wire cloth, the reduced tensile strength was determined in
the following way: The tensile strength a is defined by the equation


F = (7)
a

The specific weight of a specimen of the cloth of length L, cross-
sectional area a, and weight W is


W
aL








NACA RM E51H23


The reduced tensile strength for the cloth is therefore

F
T T s (8)

This strength can be calculated from the measured values of the breaking
force and weight per unit length of the specimen used for the rupture
test. The right sides of figures 15 and 16 show these reduced tensile
strengths. The value of the reduced tensile strength a' is always
lower than the value of the tensile strength a because in computing
the reduced strength the lengthwise wires are considered to carry in
addition to their own weight the weight of the crosswise wires. The
values of the reduced strength a' increase at a faster rate with
reduction of thickness by rolling than the values of the tensile
strength a. This more rapid increase is explained by the fact that the
length of the wires increased and the corresponding cross-sectional area
decreased in the rolling process; a fact which is accounted for in the
determination of the reduced strength a', whereas the strength a is
based on the nominal cross-sectional area before rolling. The differ-
ence between the original and the reduced strength is larger for the
brazed 28X500 mesh cloth than for the two other types because the weight
of the 28X500 mesh cloth was increased 30 percent by the brazing process
as compared with 16 percent for the 20X250 type and 17 percent for the
20X350 type. It is probably possible to reduce the amount of weight
addition from the brazing of the 28X500 mesh cloth by maintaining a
closer control on the spraying process.

As shown in figure 15, the tensile strength of the unbrazed cloth
reaches a maximum when the cloth has been reduced 40 percent in original
thickness by rolling. Although the tensile strength of the brazed cloth
increases even after a 45-percent reduction in original thickness with
the exception of the 20x350 type (fig. 16), a practical limit is reached
at about 40 or 45 percent because the material becomes too brittle to
bend beyond this point.

Two specimens of 20X250 brazed wire cloth reduced 40 percent in
original thickness were tested for tensile strength in the direction of
the crosswise wires with an average result of 150,000 pounds per square
inch. This tensile strength is based on the nominal cross-sectional
areas of the crosswires as listed in table I. This value is higher than
that for the tensile strength a of the lengthwise wires, as the cross-
wise wires are less deformed by the rolling process than are the length-
wise wires. However, because the number of crosswise wire-s per inch is
much less than the number of lengthwise wires, the strength of the cloth
in the crosswise direction will be only a fraction of the strength in
the lengthwise direction.








NACA RM E51H23


Comparison With Compacted Sintered Porous Metals

Usually sintered porous metals are considered as material for
transpiration-cooled walls. The strength of compacted sintered porous
metals as described in reference 4 will therefore be compared with the
strength of wire cloth. Two of the porous metals (designated in
figs. 11, 17, and 18 as the Unexcelled Powder and the Hardy compacts)
were produced from AISI type 302 stainless-steel powder. The third
type of porous metal compact (designated in figs. 17 and 18 as VA com-
pacts) was made from atomized powder of AISI type 301 stainless steel.
The comparison will be made for the reduced tensile strength a'. From
the values of tensile strength a given in reference 4, a reduced ten-
sile strength a' was determined as follows: The ratio of the specific
weight of the porous metal to the specific weight of stainless steel can
be expressed in terms of porosity f of the porous metals.


-L = 1 f
Ts

The reduced tensile strength is therefore


a' = a (9)
1-f

This evaluation credits the porous material with its lighter weight which
is advantageous for aircraft applications. The reduced tensile strengths
of the porous compacts and the wire cloth are compared for various values
of porosity in figure 17, where the curves for the wire cloth were
obtained by cross-plotting figures 9, 15, and 16. The tensile strengths
of the 20X250 mesh and 20X350 mesh cloth, both brazed and unbrazed, are
from 2 to 3 times that of the sintered compacts for a range of porosities
between 15 and 20 percent. The reduced tensile strength of the brazed
28X500 mesh cloth is about 1 to 2 times that of the sintered compacts
for this same range of porosities.

A further strength comparison between the wire cloth and the porous
metals was made on the basis of the permeability coefficient K defined
in the previous section. The results of this comparison are given in
figure 18. Permeability data for the porous metals were obtained from
reference 4, and the strength figures for the wire cloth are determined
from cross plots of figures 10, 15, and 16. The strengths of the wire
cloth are as much as 4 times higher in the range of permeabilities con-
sidered than the strengths of the compacted sintered materials (fig. 18).
These comparisons are on the basis of the ultimate strengths of the
materials, but even if the working strengths of the wire cloth, which are
shown in figures 13 and 14 to be from 60,000 to 80,000 pounds per square
inch, are compared with the ultimate strengths of the porous compacts,
the wire cloth is still about twice as strong.








NACA RM E51H23


Strength of Connections Between Wire Cloth and Solid Metals

Various methods of attaching the wire cloth to a supporting struc-
ture have been considered. Tests were made on the use of spot-welding
to determine the strength of such an attachment and the amount of sur-
face interruption caused. Specimens of brazed 20X250 mesh cloth reduced
40 percent in original thickness were spot-welded to the edge of
0.040-inch-thick fins using a 1/32-inch-diameter electrode. The shear
strength per spot weld for such an attachment was found to be upwards of
150 pounds. This strength is comparable to that obtained on a 0.018-inch-
thick strip of sheet metal in the same test. No measurement of tensile
strength of the spot welds was made, but from tests to determine the
deflection of the cloth under differential air pressure, it may be con-
cluded that any spacing of spot welds which will keep this deflection in
a range comparable to casting tolerances for turbine blade profiles
(about 0.005 in. deviation in 0.25 in. linear distance) will have suffi-
cient strength to withstand any differential pressure likely to be used
in transpiration cooling.


SUMMARY OF RESULTS

An experimental investigation was conducted to determine the per-
meability and strength characteristics of wire cloth for use as the
porous material for transpiration-cooled walls and the following results
were obtained:

1. By cold-rolling of corduroy wire cloth, a porous material may be
obtained with a wide range of permeabilities, which should cover most
requirements for transpiration-cooled walls.

2. In' the range of low porosities, small changes in the reduction
of thickness by rolling caused very large changes in the permeability.
The rolling had to be very carefully controlled in order to obtain repro-
ducible results.

3. The tensile strength at room temperatures increased with
moderate amounts of rolling but started to decrease for the unbrazed
material when the thickness had been reduced to approximately 60 percent
of the original value.

4. The stiffness of the cloth against bending forces was increased
considerably by a brazing process that interconnected the wires where
they touched each other without closing up the channels necessary for
the coolant flow.

5. The tensile strength of the wire cloth at room temperature based
on the sum of the wire cross sections in the stress direction was
increased by the brazing process.








NACA bM E5123


6. The tensile strengths of the wire cloth were in the range of
values between 100,000 and 130,000 pounds per square inch. The reduced
tensile strength, which was introduced in order to evaluate the material
properly for aircraft application, comprised values between 80,000 and
120,000 pounds per square inch with the exception of brazed 28X500 mesh
cloth, which had values around 70,000 pounds per square inch.

7. As compared with sintered porous material made from stainless-
steel powder, the reduced tensile strength of 20X250 and 20X350 mesh
wire cloth in the direction of the larger number of wires was 2 to
3 times as large, and the reduced tensile strength of brazed 28X500 wire
cloth from 11 to 2 times as large.


8. Interconnection between layers of wire cloth was accomplished by
a seam brazing process, whereas for a connection with solid metal parts,
spot-welding proved satisfactory.


Lewis Flight Propulsion Laboratory
National Advisory Committee for Aeronautics
Cleveland, Ohio








NACA RM E51H23


APPENDIX SYMBOLS

The following symbols are used in this report:

A surface area, sq in.

a cross-sectional area, sq in.

F force, lb

f porosity, dimensionless

G weight rate of flow, lb/(sec)(sq in.)

g gravitational constant, in./(sec)2

K permeability coefficient, sq in.

L length, in.

p static pressure, Ib/sq in.

R gas constant, in./OR

T static temperature, OR

V volume, cu in.

W weight, Ib

a constant, in.-2

0 constant, in.-l

r specific weight, lb/cu in.

Mi absolute viscosity, (lb)(sec)/sq in.

E summation, dimensionless

a tensile strength, lb/sq in.

a' reduced tensile strength, Ib/sq in.

T thickness of one layer of porous material, in.








NACA RM E51H23


Subscripts:

1 side of porous material at high pressure

2 side of porous material at low pressure

b braze

m metal

s steel

t total

v voids


REFERENCES

1. Eckert, E. R. G., and Esgar, Jack B.: Survey of Advantages and
Problems Associated with Transpiration Cooling and Film Cooling
of Gas-Turbine Blades. NACA RM E50K15, 1950.

2. Green, Leon, Jr.: Fluid Flow Through Porous Metals. Prog. Rep.
No. 4-111, Jet Prop. Lab., C.I.T., Aug. 19, 1949. (Contract No.
W-04-200-ORD-455, Ordnance Dept.)

3. Duwez, Pol, and Martens, Howard E.: The Powder Metallurgy of Porous
Metals and Alloys Having a Controlled Porosity. Trans. A.I.M.M.E.,
Metals Div., vol. 175, 1948, pp. 848-874.

4. Hill, M., Reen, 0. W., Vermilyea, D. A., and Lenel, F. V.: Produc-
tion of Porous Metal Compacts. Bi-Monthly Prog. Rep. No. 3, Pow-
der Metallurgy Lab., Rensselaer Polytechnic Institute, Oct. 6,
1950. (Navy Res. Contract NOa(s) 11022.)

5. Thum, Ernest E.: The Book of Stainless Steels. Am. Soc. Metals,
2d ed., 1935, p. 371.








NACA RM E51H23


TABLE I SPECIFICATIONS FOR THREE MESHES OF WIRE CLOTH


Mesh Lengthwise wires Crosswise wires Original thick-
Number Diameter Number Diameter ness of wire
per (in.) per (in.) cloth as woven
in. in. (in.)

20X250 250 0.008 20 0.010 0.0269
20X350 315 .0065 20 .010 .0238
29X500 496 .004 28 .008 .0169











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Figure 6. Schematic diagram of test equipment for measuring air flow
through specimens of wire cloth.


regulator























































































Weight-flow rate, G, lb/(sec)(sq in.)

(a) Mesh, 20X250.

Figure 7. Correlation of air-flow data for unbrazed and rolled staintriej-teel
corduroy wire cloth.


NACA RM E51H23


RI

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NACA RM E51H23


.0004 .0006 .0008 .001 .002
Weight-flow rate, G, lb/(sec)(sq in.)

(b) Mesh, 28X500.


Figure 7. Concluded.


Correlation of air-flow data for unbrazed and rolled stainless-
steel corduroy wire cloth.


.0i002






































































Weight-flow rate, G, lb/(sec)(sq in.)

(a) Mesh, 20X250.

Figure 8. Correlation of air-flow data for brazed and rolled stainless-steel corduroy
wire cloth.


NACA RM E51H23


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NACA RM E51H23


Figure 8. Continued.


.0006 .0008 .001 .002 .004 .006
Weight-flow rate, G, lb/(sec)(sq in.)

(b) Mesh, 20X350.

Correlation of air-flow data for brazed and rolled stainless-steel
corduroy wire cloth.











NACA RM E51H23


.wUU .uJUO .U J .UU,
Weight-flow rate, G, Ib/(sec)(sq in.)

(c) Mesh, 28)600.


Figure 8. Concluded. Correlation at air-flow data for brazed and rolled stainless-steel
corduroy wire cloth.


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NACA RM E51H23


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20x350
28X500
20X250
20x350
28x500


0 0


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Brazed
Brazed
Unbrazed
Unbrazed
Unbrazed


Reduction in original thickness, percent


Figure 9. Effect of rolling on porosity of brazed and unbrazed wire cloth
in three mesh sizes.


t0o


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NACA RM E51H23


0 10 20 30 40 50 61
Reduction in original thickness, percent

Figure 10. Effect of rolling on permeability coefficient of brazed and
unbrazed wire cloth in three mesh sizes.












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Wire Cloth
Mesh
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20X350 brazed
28X500 brazed
20X250 unbraz
20X350 unbraz
28X500 unbraz


ed
ed
ed


Compacted Sintered Metals (reference 4)

7 Unexcelled powder, AISI type 302
8 -200+ 325 mesh, VA, AISI type 501
9 -200 +325 mesh, Hardy, AISI type 302
10 -100+200 mesh, VA, AISI type 301
11 -100 +200 mesh, Hardy, AISI type 302


Porosity, f, percent


Figure 17. Comparison of reduced tensile strength and porosity for brazed
and unbrazed rolled stainless-steel corduroy wire cloth and some compacted
sintered metals.


- 1
2
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--
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