Ceramic lining

An arrangement for insulating a wall of a metallic combustion chamber is disclosed. A layer of insulating material is applied to the metallic wall and a plurality of ceramic wall panels are applied over the ceramic insulation material. The wall panels are retained by fastening elements that pass through an opening in the wall panels. The opening in the wall panels has a frusto-conical surface and the engagement surface of the fastening element is at least partially spherical to avoid mechanical and thermal stresses.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to ceramic linings a insulating of metallic walls of 
combustion chambers. 
2. Discussion of Background 
A ceramic lining of the type mentioned at the beginning is known from DE 
195 02 730 A1. The lining there has the purpose of an uncooled detachable 
lining of a combustion space with ceramic elements which withstand the 
high mechanical and thermal stresses in a commercial heavy-duty combustion 
chamber. 
For this purpose, the lining comprises at least one wall panel, made of 
refractory structural ceramic and having at least one through-opening, and 
a fastening element per opening. The fastening element is fastened by its 
foot in a metallic holding device fastened on the metallic supporting 
wall. The head of the fastening element rests in the opening in the wall 
panel. Arranged between the metallic wall and the ceramic wall panel is an 
insulating layer. The fastening element consists of refractory structural 
ceramic and is resiliently coupled to the holding device. Regarded as the 
advantages are that the lining can withstand very high mechanical and 
thermal stresses on account of its homogeneity and the material used and 
that the lining can be disassembled without being destroyed and can 
therefore be used repeatedly. Furthermore, the resilient coupling of the 
ceramic structure to the metallic holding construction allows the thermal 
expansions between metallic and ceramic components or deformations of the 
insulating layer due to mechanical stresses to be absorbed. It is 
considered particularly expedient for the fastening element to have a 
thermically optimized form, preferably a concavity in the center of the 
head, a rounded-off head and rounded-off cross-sectional transitions with 
large radii from the head to the shank and from the shank to the foot. 
This achieves the effect that the mechanically and thermally induced loads 
cause only minor stresses. 
In the case of this known lining, a surface contact was chosen between the 
component to be fastened and the component via which the pressing force is 
introduced. For this purpose, the seat of the bolt on the tile is designed 
as a ball/ball seat. One of the reasons for this is to ensure a pendulum 
motion of the bolt free from any bending moments, since said bolt can get 
into a skewed position as a result of production and assembly inaccuracies 
and/or also due to operationally caused displacements of the components. 
In the ideal case, when the ball diameter of the bolt corresponds precisely 
to that of the tile, there is negligible Hertzian stress at the contact 
surface. If, however, the two ball diameters involved deviate from each 
other, which may be the consequence of production tolerances and/or 
thermal expansions, there is immediately just linear contact of the two 
components at one edge of the tile-ball seat. The ball seat terminates via 
radii at its ends. As a result, with the differences in diameter mentioned 
there are immediately two convex surfaces facing each other. This leads to 
very high undesired Hertzian stresses. 
SUMMARY OF THE INVENTION 
Accordingly, on the basis of the finding that a surface contact abruptly 
changes into an uncontrolled linear contact or even just point contact if 
differences occur in the geometry of the elements involved, one object of 
the invention is to provide for a fastening of brittle components on which 
the contact pressure can change its direction of application, a novel seat 
geometry which ensures controllable linear contact throughout all 
operating states. 
This is achieved according to the invention by the fastening element being 
provided with a cross-sectionally rounded-off head, which is pressed with 
the rounded portion on a cross-sectionally straight bearing surface of the 
wall panel opening. 
The advantage of the invention is to be seen in particular in the 
simplicity of the measure. While retaining the angular mobility of the 
bolt, the solution is distinguished by low-cost production.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views and only the 
elements essential for understanding the invention are shown FIG. 1 shows 
there is a partial longitudinal section of the lining according to the 
invention for a gas-turbine combustion chamber. Applied on top of the 
metallic supporting wall 1 of the combustion chamber is an insulating 
layer 2. This preferably consists of ceramic fibrous material. Arranged in 
turn on the insulating layer are ceramic wall panels 3, which consist of 
refractory structural ceramic, for example SiC or Si.sub.3 N.sub.4. The 
wall panels 3 and the insulating layer 2 are fastened on the metallic 
supporting wall 1 with the aid of fastening elements 4, which are in each 
case arranged in a metallic holding device 5, which serves as a holding 
means for resiliently urging the head portion 20 against the wall panel 
bearing surface 22, and is described in detail later. Like the wall 
elements 3, these fastening elements 4 likewise consist of refractory 
structural ceramic. 
The outer form and dimensions of the wall panels 3 can be adapted 
unproblematically to the geometry of the space to be lined and are not 
predetermined in any way. 
FIG. 2 shows a possible form of the wall panels 3. In this design variant, 
they have a hexagonal outer contour. For reasons of simple manufacture and 
uniform stress distribution under thermal and mechanical stress, 
symmetrical forms are to be preferred. The thickness d of the wall panel 3 
is governed on the one hand by the required mechanical stability and on 
the other hand by a minimization of the thermal stresses on account of 
temperature gradients in the component. In the simplest case, a square 
contour may also be used, in order to line planar or only slightly curved 
combustion spaces. Similarly, wall panels 3 with a rectangular outer 
contour can also be used. 
In the center of the wall panel 3 there is arranged a through-opening 6 for 
receiving a fastening element 4, which in this case is a bolt which 
comprises a head, shank and foot. It goes without saying that, in other 
exemplary embodiments not shown here, there may also be a plurality of 
openings 6 in each wall panel 3. 
As revealed by FIG. 3, which shows an enlarged section of the wall panel 3 
according to FIG. 2 in the region of the opening 6 along the line IV--IV, 
the opening 6 is drawn in in the direction of the metallic supporting wall 
1. As a result, on the one hand the contact surface between the fastening 
element 4 and the wall panel 3 is enlarged, on the other hand the heat 
flows in cases of stress gradients of a steady state and non-steady state 
are influenced in such a way that only minimal thermal stresses occur. The 
geometrical shaping of this zone results from a tradeoff between the 
heat-accumulating and heat-conducting properties of the materials used. A 
ratio of the thickness d of the wall panel 3 to the depth t of the 
drawn-in part of the wall panel 3 in the region of the opening 6 of about 
5 to 3 has proven advantageous. 
The contact surface between the head 20 of the fastening element 4, 
arranged in the opening 6, and the wall panel 3 is configured according to 
the invention in an optimum way, in order to ensure definite contact even 
in the case of slight angular positions of the bolt. For this purpose, the 
head 20 is provided with a spherical bearing surface, which bears with its 
rounded portion 21 on a cross-sectionally straight bearing surface 22. 
This straight portion is the wall of the wall panel 3 bounding the opening 
6. With this measure, the region in which linear contact occurs can be 
easily determined, since the tolerances to be expected are generally 
known. The Hertzian stress occurring in the case of the surfaces pressed 
against one another - here convex surface against planar surface - is much 
less than in the case of the known ball/ball seats and can easily be 
determined analytically. Nevertheless, a good sealing effect is also 
achieved here, if desired in the first place. 
In the case of the example, the cavity between the shank of the fastening 
element 4 and the insulating layer 2 is filled by a divided sleeve 15 of 
strengthened, preformed insulating material. 
There is provided an expansion-tolerant flexible restraint of the ceramic 
fastening element 4 on the outside of the metallic supporting wall 1. 
According to FIG. 1, the metallic holding device 5 comprises a 
longitudinally divided threaded sleeve 7, which encloses the foot of the 
fastening element. Arranged on the external thread of the threaded sleeve 
7 is a threaded nut 9, by means of which the restraining force can be set, 
as explained further below. At the same time, the nut 9 holds the two 
halves of the threaded sleeve 7 together. The mutual positioning of the 
two halves of the threaded sleeve can be secured by additional 
constructional elements, for example the bolts. A square 10 serves the 
purpose of holding the divided sleeve during the tightening of the 
threaded nut 9. Items 7 and 10 are part of the divided sleeve. 
Furthermore, the metallic holding device 5 comprises a guide ring 11, which 
is recessed into the metallic supporting wall 1, a one-part guide sleeve 
12 for the fastening element 4 and spring elements 13 arranged between the 
guide sleeve 12 and the guide ring 11. The spring 13 is, for example, as 
represented in FIG. 1, a cup spring. The resilient coupling of the ceramic 
structure to the metallic holding device achieves the effect that relative 
thermal expansions between the metallic and ceramic components or 
deformations of the insulating layer 2 ("settling") are absorbed by 
mechanical stresses, for example pulsations in the combustion space, 
without inadmissibly high stresses in the ceramic component being induced 
at the contact surfaces. Virtually constant restraining forces are ensured 
by means of a specific resilient excursion of the restraint (which can be 
set by means of the threaded nut 9 screwed on to the external thread of 
the sleeve 7). 
It goes without saying that the invention is not restricted to the 
embodiment described. As a departure from the spherical form of the head 
and the conical form of the bearing surface, in the case of linearly 
expanded seats (perpendicularly with respect to the plane of the drawing), 
the head could also be of a circular design and the bearing surface could 
be of a trapezoidal design. Moreover, the proposed introduction of force 
may advantageously be considered for fastening all possible brittle 
components, provided that the introduction of force by means of a 
ball/cone seat is possible. 
It should also be mentioned that a kinematic reversal of the principle does 
not achieve the object. This is because a conical head in interaction with 
a then toroidal bearing surface does not ensure that linear contact is 
maintained if there is an angular deflection of the bolt. 
Obviously, numerous modifications and variations of the present invention 
are possible in the light of the above teachings. It is therefore to be 
understood that, within the scope of the appended claims, the invention 
may be practiced otherwise than as specifically described herein.