Turbomachine stator having a double skin casing including means for preventing gas flow longitudinally therethrough

A turboshaft engine in which the stator comprises an outer casing surrounding a ring formed by a number of ring elements each having an apertured rib which spaces the element from the casing, the ribs dividing the space between the casing and the ring into separate compartments, and each rib having a sealing element disposed alongside it to cover the apertures and seal one compartment from the next. The casing and the ring are fixed together by virtue of bolts which have their axes inclined to the radial direction so that tightening the bolts causes the casing to press radially on the sealing elements at the same time as pressing them axially against the respective ribs.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The invention relates to a turboshaft engine having a stator equipped with 
means for preventing the longitudinal or axial flow of gas around the 
guide vane stages of the stator. 
2. Summary of the prior art 
The stator of many turboshaft engines comprises a double envelope formed by 
an outer casing and a ring which is surrounded by the casing and which is 
constituted by a plurality of interconnected elements to which the vanes 
of the guide stages are secured. The ring elements are screwed to the 
casing and are provided with radially-extending circumferential ribs which 
engage the casing to maintain the spacing between the ring and the casing. 
However, as these ribs are the cause of a substantial outward heat loss by 
virtue of their good thermal conductivity, they are provided with 
apertures so that they only touch the casing at circumferentially spaced 
positions. The drawback with this arrangement is that the volume between 
the ring and the outer casing becomes continuous, with the result that it 
becomes impossible to achieve fine adjustment of the clearance between the 
rotor blades and each ring element by blowing heating or cooling gas at 
different temperatures or rates into different sections of the volume 
between the casing and the ring in order to vary the thermal expansion of 
each ring element independently. Only an overall setting is possible, and 
this fails by far to provide the same degree of precision in reducing 
clearances to a minimum, so that the output of the engine is affected. 
Re-establishment of a seal at each rib is therefore desirable in order to 
prevent the axial flow of gas between the casing and the ring elements 
through the apertures of the ribs. Several solutions have been proposed in 
French Patents Nos. 2468738, 2482661 and 2575221, and in U.S. Pat. No. 
4,314,793, but are generally fairly complicated. The aim of the invention 
is to provide a particularly simple solution to this problem. 
SUMMARY OF THE INVENTION 
According to the invention, there is provided a turboshaft engine including 
a stator comprising a casing, a plurality of ring elements surrounded by 
said casing, stages of guide vanes fixed to said ring elements, 
screw-threaded members fixing said ring elements to said casing, said ring 
elements being provided with ribs extending outwardly therefrom into 
contact with said casing to space said ring elements from said casing, 
said ribs each having apertures, such that only circumferentially spaced 
portions of the rib touch said casing, and a substantially radial bearing 
surface disposed inwardly of said apertures, and annular sealing elements 
for preventing the axial flow of gas between said ring elements and said 
casing through said apertures of said ribs, said sealing elements each 
comprising a circular soleplate and a web extending substantially radially 
inwards from said soleplate to engage frictionally said bearing surface of 
a respective one of said ribs, said web covering said apertures of said 
respective rib, and said casing being formed with a plurality of steps, 
one step for each of said sealing elements, each of said steps defining a 
first compression surface acting radially on said soleplate of the 
respective sealing element and a second compression surface acting axially 
on said soleplate to press said sealing element against the respective 
rib, said screw-threaded members being inclined with respect to the radial 
direction so that tightening said screw-threaded members causes said first 
and second compression surfaces of said casing steps to press on said 
sealing elements simultaneously. A preferred embodiment of the invention 
will now be described, by way of example, with reference to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The stator of the compressor illustrated in FIG. 1 comprises a slightly 
frusto-conical outer casing 1, which may be formed by a number of 
interconnected parts, and a Plurality of frusto-conical ring elements 2 
which are connected together end to end by interlocking joints 3 to form a 
single continuous ring surrounded by, and substantially parallel to the 
casing 1. Each ring element 2 carries a stage of fixed stator or guide 
vanes 4, and a stage of movable rotor blades 5 is disposed between 
successive stages of the fixed vanes 4. 
Each ring element 2 is provided with a circumferentially extending rib 6 
which projects radially outwards so that its outer edge 7 contacts the 
casing 1. The ribs 6 divide the volume between the casing 1 and the ring 
elements 2 into compartments, into each of which opens a device which 
supplies gas for regulating the clearance between the respective ring 
element 2 and the movable blade stage 5 surrounded thereby. These devices 
do not form part of the invention, and only the holes 13 which open 
through the casing 1 into the different compartments are shown. 
Each rib 6 is cut away over large parts of its circumference to form 
apertures 14 (best seen in FIG. 3) which extend over most of the radial 
height of the ribs 6 up to the casing 1, so that the ribs touch the casing 
1 only through the outer edges 7 of the relatively small portions of the 
ribs between the apertures, thereby extensively limiting the heat 
transmitted to the casing 1 by the ribs 6. Associated with each rib 6 is a 
sealing element 8 (described below in more detail) which covers the 
apertures 14 and serves to maintain the compartments divided by the rib 
separate from each other. 
At its outer edge 7 each rib portion between the apertures 14 is provided 
with an axially projecting flange 9 having a block 10 formed between it 
and the rib 6 at a position opposite another block 11 formed by an outward 
thickening of the casing 1. The blocks 10 and 11 have aligned holes 
extending through them for accommodating bolts 12 which serve to connect 
the casing 1 and the ring elements 2 rigidly together. 
As can be seen in FIG. 2, each rib 6 has a circumferentially extending 
bearing surface 15 situated in a radial plane and disposed radially 
inwards from the apertures 14, and each of the flanges 9 has a further 
bearing surface 16 lying in the same radial plane, the bearing surfaces 
15,16 facing towards the associated sealing element 8. 
Each sealing element 8 is a continuous ring comprising an outer circular 
soleplate 18 which abuts the bearing surfaces 16 of the associated rib 
flanges 9, and a web 19 which extends inwardly from the soleplate 18. The 
web 19 has an outer portion 21 which is inclined towards the associated 
rib 6, and a radial inner end portion 22 having a bearing surface 23 which 
abuts the bearing surface 15 of the rib 6. The web 19 is slightly 
resilient and when the sealing element 8 is unstressed the inner end 
portion 22 protrudes axially beyond the plane of one end of the soleplate 
18 as shown by the dashed lines in FIG. 2. 
In the region of each rib 6 and associated sealing element 8 the interior 
of the casing 1 is formed with a step defining an axially extending 
circular surface 17 which acts radially on the outer surfaces of the 
flanges 9 and the soleplate 18, and a radially extending surface 20 which 
acts axially on the soleplate 18 to press it towards the rib 6 and against 
the bearing surfaces 16. 
When the stator is assembled, the tightening of the bolts 12 attaching a 
particular ring element 2 to the casing 1 brings about both an axial 
displacement of the associated sealing element 8 towards the rib 6 as a 
result of the step surface 20 acting axially on the soleplate 18, causing 
the web 19 to yield resiliently when the surfaces 15 and 23 come into 
contact with each other, and a radial displacement as a result of the 
compression exerted radially on the soleplate 18 by the step surface 17. A 
situation is thus reached where the sealing element 8 bears firmly against 
the surfaces 15 and 16 of the rib 6 and against the surfaces 17 and 20 of 
the casing 1. The surfaces 15,16,17 and 20, as well as the surfaces of the 
sealing element 8 which bear upon them, are slightly roughened, for 
example by a turning operation, to prevent any significant flow of gas 
between the compartments. 
The axis X of the bolts 12 may be inclined in the longitudinal or axial 
direction at an angle of about 40.degree. or 50.degree. depending on 
whether axial or radial compression is to be favoured, which is a matter 
particular to each embodiment. It is of course possible to employ other 
angles of inclination. 
The fitting of the sealing elements 8 raises no particular difficulties, as 
they may be introduced through the wider mouth of the frusto-conical 
casing 1 prior to mounting and bolting the ring elements 2 in position.