Note: Descriptions are shown in the official language in which they were submitted.
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Thermal lining for jet engine combustion chamber
The invention pertains to a thermal lining for the combustion
chamber of an engine, especially a jet engine.
More precisely, the invention involves a highly ablative-resistant thermal
lining which, in a ramjet engine for instance, effectively protects the
combustion chamber case during the operating phase of the engine, and
during the acceleration phase if the accelerator is integrated to the
engine.
A ramjet engine consists of a duct with an air intake, inside which
solid or liquid fuel is burnt. Oxygen necessary for combustion is
obtained from the air inlet. The gas flow resulting from the expansion
of air and combustion gases is ejected at a speed higher than the
inlet speed.
By reaction, the engine is submitted to a thrust in a direction
opposite to the direction of the gas flow.
The combustion chamber case is therefore exposed to high temperatures
and pressures, and to erosion due to the high-speed gas flow.
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The case is generally protected by a coating of heat-resistant material
which provides a thermal lining to the combustion chamber. As this
lining is exposed to erosion by the gas flow, it should also be highly
ablative-resistant.
In addition, the engine has to be in motion to capture air. Vehicles
equipped with ramjet engines must therefore gain enough speed using,
for instance, high-thrust boosters known as accelerator propellant
charges. When the accelerator propellant charge is housed in the
engine combustion chamber, it is called an integrated charge and the
engine is an integrated-accelerator engine.
An improvement to this configuration consists in having the coating
which inhibits the combustion of the accelerator charge (propellant
grain), thermally protect the combustion chamber case not only during
the acceleration phase but also during the effective operating phase
of the engine.
The numerous types of thermal linings which have been suggested and
used so far fall into two categories:
- rigid thermal linings made of thermosetting material, such as a
phenolic resin
- "elastic" thermal linings mostly consisting of a heat- and corrosion-
resistant elastomer which can include carbon or similar fibers. The
elastomer can be a silicone resin for instance.
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The thermal linings of the first category, when used in an integrated-
accelerator engine, can be cracked by the pressure increase in the
combustion chamber. subsequent to the burning of the accelerator charge.
The cracks result from different strains in the case and the lining.
Therefore, such linings cannot protect the combustion chamber case
during the operating phase of the ramjet engine. In addition, their
thermal conductivity is generally too high to provide case protection
for long periods of engine operation.
The thermal linings of the second category can be used without major
trouble to protect combustion chambers when the engines do not operate
under excessively severe conditions. However, problems arise when
stresses become high. Especially, they cannot effectively protect the
combustion chamber case of a ramjet engine subjected to acoustic
vibrations, such as high-frequency vibrations (1000-3000 Hz) with a
peak-to-peak amplitude about 20 to 30% of the nominal pressure in
the chamber.
Under such conditions, elastomers are very soon ablated in successive
layers-Ablation results from the generation of pyrolysis gases within
the lining. The gases blow away the upper layer already pyrolyzed and
made more or less airtight.
The purpose of the invention hereunder is to remedy all these problems
by submitting a novel thermal lining which, even under severe operating
conditions, including acoustic vibrations, does not ablate rapidly
and effectively protects the combustion chamber case during ramjet
engine operation, and resists the acceleration phase without major
damage.
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To this effect, the invention proooses a thermal lining designed for the
combustion chamber of an engine, especially a jet engine, and composed of
a three-dimensional, multidirectional, self-supporting, gas-permeable,
heat-resistant fiber structure. Certain fibers in the structure are
arranged along at least one direction not contained in the plane defined
by at least two other fiber directions. The structure is impregnated
with at least one layer of elastomer material over at least one of its
sides.
A characteristic of the invention is the provision for preferred paths
within the three-dimensional structure in order to improve its permeabi-
lity to gases.
According to a first preferred manufacturing process, the three-dimensional
structure consists of a layer of fibers arranged along at least two direc-
tions. Bunches of fibers are embedded in this layer so that pins protrude
from at least one of its sides. Structure integrity is obtained by weaving
the fibers in the layer and inserting the bunches through the weft of the
resulting fabric, or by impregnating the assembly with a thermosetting
resin, such as a phenolic resin, or with a mineral binder such as silica
or carbon.
Preferred gas paths are obtained either by holes provided in the fabric
or in the fiber layer, or by loose weaving.
Also, the prefeied gas paths may be produced by low temperature
decomposing fibers which are incorporating in the three-dimensional
structure. These fibers are decomposed by melting or pyrolysis at a
lower temperature than the decomposition temperature of other materials
of the thermal lining.
Thus, according to a prefered embodiment of the invention, besides the
branches of refractory fibers, bunches of low temperature decomposing
fibers are inserted in the woven. Said decomposing fibers are made of
material decomposable at low temperature, for example at a temperature
above 200 C but low the neltinc; ~r decomposinc, ;:emperature of the
is 1 ~ 4 1559
thermosetting resin impregnating the woven or of the materials of
elastomer layer, more preferably at a temperature comprised between
200 C about and 300 C about.
These decomposing fibers may be elemental fibers which are impregnated
as the refractory fibers in order to obtain bunches, or maybe rod or bar.
Convenable fibers are, for example, polyester, polyamide fibers, teflon
rod or other materials having an appropriate decomposing temperature.
According to a second preferred manufacturing process, the three-dimensional,
multidirectional structure is composed of at least two plies of woven or
knitted fibers. The plies are superposed and loosely attached to each other
by binding wires so as to make up a mattress-like structure.
According to a third preferred manufacturing process, the three-dimensional,
multidirectional structure consists of a thick textile fabric made of at
least two layers of weft threads which are superposed and attached by
regularly undulating warp threads.
Thus, the invented thermal lining consists of a three-dimensional, multi-
directional structure and an elastic layer of elastomer material. The high-
integrity, multidirectional layer prevents excessive ablation of the elastomer
layer, which assumes the form of the c anbustion chamber case strained by
acoustic vibrations. Therefore the multidirectional structure does not
burst and provides heat insulation to the casing.
In addition, due the permeability of the multidirectional structure, gases
produced by elastomer pyrolysis can escape into the combustion chamber,
which prevents the upper pyrolyzed layer and the structure itself from being
blown away.
Another characteristic of the invention consists in installing the lining
in an engine combustion chamber in such a way that the elastomer layer
lies between the chamber case and the fiber layer of the multidirectional
structure.
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The multidirectional structure is made of suitably heat-resistant fibers,
such as carbon, silicon carbide, silica, ceramic, metal or similar fibers,
or a mixture thereof.
The multidirectional structure can also be mechanically reinforced by
nonrefractory fibers, such as polyamide, aramide or polyaramide fibers.
A further characteristic of the invention is that the elastomer layer
contains heat-resistant mineral fillers, such as fibers andJor fiber
fillers (carbon, silicon carbide, boron carbide), or granular fillers
(alumina, zirconium oxide, silicon carbide, boron carbide), or a
mixture thereof.
The thermal conductivity of the lining is improved by adding to the
elastomer some fillers able to have endothermic chemical reactions
with each other and with the binder. They can consist of a carbon-silica
combination for instance.
Another characteristic of the invention is the possibility of inserting,
between the material described above and the combustion chamber case,
an additional elastomer layer, preferably not containing any fiber
fillers or other heat-conductive fillers.
Besides, when a propellant grain has to be cast in the combustion
chamber (to serve as an accelerator charge in a ramjet engine, for
instance), the inward side of the thermal lining is coated with a
material inhibiting the combustion of the grain.
Elastomers suitable for use in this invention should offer, beside
their intrinsic properties, adequate heat and corrosion resistance.
They can be silicone polymers for instance.
Further details, characteristics and advantages of the invention
will be shown more clearly in the following description which refers
to the attached figures_ These figures are mere examples and include:
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- Fig. 1 shows the longitudinal section of an engine combustion chamber
fitted with a thermal lining in accordance with the invention
- Fig. 2 is a scaled-up view of part II in Fig. 1, which illustrates
the lining structure as per the invention
- Fig. 3 is a scaled-up schematic perspective view showing a first
manufacturing process for the three-dimensional structure of the
invented lining
- Fig. 4 is a scaled-up schematic perspective view showing a second
manufacturing process for the three-dimensional structure of the
invented lining
- Fig. 5 is a scaled-up schematic perspective view showing a third
manufacturing process for the three-dimensional structure of the
invented lining.
Referring to Fig. 1, the case 9 of the combustion chamber of a jet
engine, such as a rocket or missile ramjet engine, is protected from
heat and from etching and straining by combustion gases, by a protec-
tive layer 1, called a thermal lining. The thermal lining 1 is
bonded to the case 9 , for instance by coating the inner side of
the case 9 with an adhesive compound, such as a synthetic resin,
which adheres to the case material and the thermal lining. The nature
of this compound, which is known to the specialist, depends on the
case material, which can be metal, ceramic, composite or laminate.
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A combustion chamber means any enclosure where a combustion takes place,
liberating a large amount of gases and heat.
However, the invention can also be used to protect any enclosure
subjected to high temperatures and pressures, high-speed fluid flaw
or severe stresses.
The following refers more specifically to Fig. 2 and describes the
structure of the invented thermal lining.
According to the invention, the thermal lining 1 consists of a self-
supporting structure 2 and at least one "elastic" layer 3.
The self-supporting structure 2 is a multidirectional, three-dimensional
fiber structure, which is thick enough to provide mechanical attachment
with the elastic layer 3.
Several examples of self-supporting structures suitable for use in the
invention will be described more precisely below, with reference to Fig.
3, 4 and 5.
The "elastic" layer 3 is made of an elastomer such as room temperature
vulcanizing silicone (RTV).
The elastomer is laid on one side of the self-supporting structure 2
so that it makes up an elastic layer 3 between the combustion chamber
case 9 and the self-supporting structure 2 . Bonding between the
structure 2 and the elastic layer 3 is obtained by partial impregnation
of the structure 2 with the elastomer.
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A preferred manufacturing process for the invention consists in impregnating
the structure 2 with the elastomer throughout.
An improved elastic layer 3 is composed of an elastomer such as silicone,
containing the heat-resistant mineral fillers described above, especially
carbon or silicon carbide fibers, which increase mechanical and thermal
properties.
In order to reduce the thermal conductivity of the elastic layer (3) and
better protect the case 9 , fillers are added to the elastomer, such as
a carbon-silica mixture which produces heat-absorbing reactions, or
cooling fillers.
As shown in Fig. 2, the invention also allows the superposition of an
additional elastomer layer 7 on the elastic layer 3 . This additional
layer is preferrably made of the same elastomer material as the layer 3
However, to reduce the thermal conductivity of the lining, the material
does not contain any heat--conductive or fiber filler. But it may contain
cooling fillers or fillers causing an endothermic reaction, such as the
carbon-silica combination mentioned above.
The additional layer 7 is inserted between the thermal lining 1 and
the case 9 when the lining is installed in the combustion chamber.
Thus the elastomer material of the layer 7 fills possible gaps in the
elastic layer 3 , which provides a homogeneous protective lining without
trapped air.
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In addition, when a propellant grain (not shown here) has to be cast in
the combustion chamber, for instance to serve as the integrated accele-
rator charge in a ramjet engine, the inner side of the thermal lining 1
and more precisely of the self-supporting structure 2 , is covered with
5 a layer 8 of material, which is preferrably the same elastomer as in
the layers 3 or 7. The layer 8 is bonded to the thermal lining by
its adherence to the elastomer of the layer 3 , which impregnates the
self-supporting structure 2 .
10 The layer 8 is bonded to the propellant by means of a well-known
process, i.e., by laying a primer of organic polyisocyanate, as described
in the French patent No. 78 36 836, or directly by adding some compounds
to the elastomer, as described in the French patents No. 82 21 644,
82 21 645 and 84 02 648. The invention also covers the installation, in
the combustion chamber, of a "free", i.e., already inhibited, propellant
grain, which is then merely positioned in the chamber and not bonded to
the thermal lining.
After installing the thermal lining 1 in the combustion chamber (9),
it is preferrable to vulcanize the material(s) composing the elastic
layer 3 , and the layers 7 and 8 if applicable.
The self-supporting structure (2) is made of heat-resistant fibers
arranged in three-dimensional, multidirectional fabric, which may be
woven or not. The fabric should be permeable to gases and therefore
include preferred gas paths.
The fabric can also include nonrefractory fibers, such as polyamide
fibers, and especially the polyphenylene terephtalamide fibers marketed
by Dupont de Nemours Inc. under the trademark "Kevlar".
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Referring to Fig. 3, 4 and 5, three examples of manufacturing processes
for this fabric are given below.
The first manufacturing process for the self-supporting structure 2
shown in Fig. 3, consists in embedding fiber bunches 5 in a fiber ply 4
so that pins protrude from at least one of the ply sides, as on a wire
brush or a hedgehog's back.
The fiber ply may be woven or, as shown here, made of a first layer of
fibers 4a arranged along a given direction, and a second layer of fibers
4b arranged along a direction preferrably orthogonal to the first one.
The pins (bunches) are embedded regularly and alternately in the resulting
fabric. However, to provide paths for gases, the pins 5 are placed only
every two wefts, for instance, so as to leave holes 6 . Integrity of the
assembly is obtained by weaving the fibers 4a and 4b together, or by
impregnating the whole with a thermosetting resin, such as a phenolic
resin. The pins 5 are made of bunches of cut fibers, or of looped
fibers.
A process for generating this structure, especially by revolution, is
described in the French patents No. 2 408 676 and 2 480 261.
The elastomer of the layer 3 is then laid over the structure side
fitted with the pins 5 , so as to embed them entirely. It is also
preferrable to let the elastomer into the holes 5 , especially to
improve bonding between an adhesive layer 8 and a propellant grain.
The pin length and density (i.e., the number of pins per surface unit
of fabric) are not critical for the invention, although a minimum
density of 4 pins per sq cm is advisable, with an equal, and preferrably
greater, hole density 6
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When a layer 4 is woven, a loose weaving is preferrable to make the
fabric permeable to gases by means of paths similar to the holes 6
shown in Fig. 3.
Fig. 4 shows another manufacturing process suitable for the self-supporting
structure of the invention. This structure consists of at least two layers
of fabric 10 and 11 , superposed and loosely attached by a number of
binding threads so as to make up a kind of mattress.
The weaving of the fabric layers 10 and 11 has to be loose so that
the structure is permeable to gases and can also be infiltrated with the
elastomer.
A third type of self-supporting structure, shown in Fig. 5, consists of
a thick fabric including several laminations of weft threads 14a and
14b , attached to each other by warp threads 13 which undulate
regularly between the weft threads 14 of each lamination. The directions
of the weft threads 14 and the sine curve formed by the warp threads 13
do not have to be orthogonal. Such a structure is made permeable to gases
by loose weaving.
Other three-dimensional structures are suitable for the invention, such as
the structure described in the French patent No. 2 497 839.
The following examples of manufacturing processes for the invented thermal
lining are given for information only.
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Example 1
a) characteristics of the three-dimensional structure 2 (illustrated in fig 3)
- T 300 carboii fibers marketed by Toray
the structure was impregnated with a phenolic resin
- thickness of fabric 4 : 3 mm
- length of pins 5 7 mm
- density of pins 5 about 6 pins per sq cm of fabric
- density of holes 6 about 6 holes per sq cm of fabric
b) characteristics of the "elastic" layer 3
- elastomer: RTV 630 silicone
marketed by General Electrics
100 parts by weight
- fillers: silicon carbide powder 25 parts by weight
carbon fibers 8 parts by weight
- cross-linking agent for RTV 630 silicone 10 parts by weight
This thermal lining was tested on a bench simulating the flight of a
vehicle powered by a ramjet engine, under operating conditions
generating acoustic vibrations with a frequency of 1300 Hz at an
effective pressure greater than one bar, or a frequency of 2500 Hz
at an effective pressure near one bar.
The examination of the lining condition after a simulated flight of
10-40 seconds showed that integrity was preserved. However, elastomer
pyrolysis had occurred, to a variable extent depending on the test
duration.
Besides, a resistance test during a fast pressure increase in the
combustion chamber, designed to simulate the ignition of an integrated
accelerator charge, demonstrated the good behavior of the invented
thermal lining.
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Example 2
a) characteristics of the three-dimensional structure 2 (illustrated in fig 3)
- silicon carbide fibers marketed under the trademark "Nicalon NIN1 102"
by Nippon Carbon
- fabric thickness: 3 mm
- pin density: 6 pins per sq cm
- hole density: 6 holes per sq cm
The three-dimensional structure was densified by impregnation with a
phenolic resin.
b) characteristics of the elastic layer 3
same as Example 1
The tests described in Example 1 were conducted on this thermal lining,
with identical results concerning resistance to a steep pressure increase
and behavior during the simulated flight of a ramjet engine.
Example 3
a) characteristics of the three-dimensional structure .2
The structure has the texture shown in Fig. 5.
It is made of silicon carbide fibers marketed under the trademark
"Nicalon NLM 102" by Nippon Carbon.
- number of warp threads per cm: 146 quadruple threads, 20 2 Texes
- number of weft threads per cm: 27 quadruple threads, 20 2 Texes
- number of weft laminations: 6
- bulk factor: 31.5 % (percentage of the structure volume occupied
by fibers)
- weft porosity: 50 %
- warp distribution: 61 %
- weft distribution: 39 %
- surface density: 6200 g per sQ m
- apparent thic'kness: 8 mn
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b) characteristics of the elastic layer 3
- elastomer: RTV 141 silicone marketed by Rhone-Poulenc 100 parts
- fillers: silicone carbide powder 20 parts
- cross-linking agent for RTV 141 silicone marketed by 10 parts
5 Rhone-Poulenc by weight
Example 4
a) characteristics of the three-dimensional structure 2
10 The structure has the texture show in fig 3.
It is made of silicon carbide fibers marketed under trademark
"Nicalon NLM102" by Nippon Carbon
- fabric thickness : 3 mm
- lenght of pins 5 : 7 mm
15 - density of pins 5 : about 5,35 pins per sq.cm of fabric.
Pins are inserted in holes 6 of the fabrics. These pins are made
of polyimide aramide fibers marketed by Dupont de Nemours under
trademark Kevlar , and have the same characteristics of refractory
pins 5.
The density of kevlar fibers pins is about 5,35 pins per sp.cm of
fabric.
The three-dimensional structure 2 is impregneted with a mineral binder,
colloidal silica. The impregnating process is, for example, the process
described in French patent n 2 526 785 which consists in successive
impregnating steps in liquid phases, under vacuum and a thermal treatment
at low temperature, about 150 C.
b) characteristics of the elastic layer 3: same as example 1.
The tests described in example 1 were conducted on this thermal lining.
The obtained results show an improved bonding between the several layers
of the thermal lining, which leads to obtain very high integrity
of the structure after the tests.
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Resistance to heat, stresses and pressure is greatly improved compared
with a nonreinforced equivalent structure.
The invented thermal lining offers high ablation resistance and preserves its
integrity even under severe operating conditions, especially when
acoustic vibrations are generated. Wear resistance and preserved integrity
are obtained by means of the three-dimensional reinforcement structure of
the layer 2 which retains the upper pyrolyzed layer. Therefore this
thermal lining effectively protects the ccinbustion chamber of a ramjet
engine, during both the sustainer phase and the booster phase, especially
when an integrated accelerator is used. The lining can also be used for
protecting the combustion chamber in a vane-type engine, as described in
the French patent No. 82.02658.
Of course, any heat- and corrosion-resistant elastomer can be used in the
layers 3 and 7 .
Besides, the layer 3 can be made of an elastomer without fillers,
especially without refractory fibers.
Various compounds can also be added to the elastomer in order to improve
its bonding to the propellant, its strength and its resistance to heat.
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