Abstract:
A portion of the stator of a gas turbine known as turbine shroud, which isituated radially opposite the mobile vanes of a turbine stage includes an &#34;abradable&#34; member supported by a first shell braced by a second shell and connected with turbine casing 2 by a thin, i.e. flexible, annular wall member embedded in a casing. Cooling air inflates the second shell so as to brace the exterior shell and circulates between the two shells so as to cool them and also cools the &#34;abradable&#34; member by impingement. The present invention is applicable to aviation turbojets.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to gas turbines and concerns, more precisely, the portion of a turbine stator known as the turbine shroud, which is located so as to face the mobile vanes of a turbine stage. This invention is applicable particularly to aviation turbojets. 
     2. Description of the Prior Art 
     It is important to achieve as small a radial tolerance as possible between the turbine shroud and the tips of the mobile vanes, any excessive play being prejudicial to the efficiency of the turbine. One known technique used to insure very small tolerances is to form the inside surface of the turbine shroud as one piece, called an &#34;abradable&#34; member capable of wearing down under the friction caused by the vanes without any risk of deteriorating the latter. However, if such wear occurs at a certain turbine load, it will increase the play under a different load. Thus, it is necessary to avoid wear as much as possible and, to that effect, prevent relative deformation of the turbine shroud in relation to the turbine shroud, whatever the load conditions in the turbine. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to permit the production of a shroud supporting the &#34;abradable&#34; member whose thermal expansions will be as identical as possible with those of the mobile blading, and which will maintain its circular shape as it expands and contracts under thermal variations. 
     The turbine shroud in accordance with this invention comprises, in combination, a braced and cooled annular shell supporting the &#34;abradable&#34; member connected to the turbine casing by means of a thin annular wall whose one extremity is rigidly anchored to said shell and whose other extremity is attached to said casing. 
     Owing to its structure as a braced shell, the shroud supporting the &#34;abradable&#34; member presents considerable mechanical inertia, so that it does not run the risk of deformation, such as ovaling. Thanks to cooling, this shroud can conform to the thermal expansions and contractions of the turbine rotor, the differential expansions and contractions of this shroud in relation to the turbine casing being absorbed by the bending of the thin annular wall. 
     In one embodiment, bracing is formed by a second shell disposed inside the first one, means being provided to feed compressed air into the second shell, thus giving support to said first shell, and to circulate cooling air between the two shells, thereupon leading this air to the &#34;abradable&#34; member so as to cool the latter by impingement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is an axial, partial, half-sectional view of one stage of an aviation turbojet, presenting a turbine shroud in accordance with the invention: and 
     FIG. 2 is a view similar to that of FIG. 1, showing a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1 can be seen one stage of gas turbine that is part of an aviation turbojet which is not shown in its entirety and which comprises, in the familiar manner, a compressor forcing compressed air into an annular casing containing a combustion chamber in which fuel is burned to produce hot gases which perform work in the turbine before being expelled through a nozzle to produce a propulsive jet. The turbine stage shown comprises a distributor blading 1 connected to turbine casing 2 by a flange 1a, and a mobile blading 3 rotating in a turbine shroud 4. 
     Turbine shroud 4 comprises an annular part 5, called an &#34;abradable&#34;  member made of a material that is capable of being worn down by friction as the vanes of mobile blading 3 bear down upon it, without any risk of deteriorating said vanes, this &#34;abradable&#34; member being anchored to inside wall 7 of annular shell 6 whose outside wall 8 extends toward the upstream end of the turbine by means of a wall portion 8a which is welded at 9 to another wall 10 that is integral with flange 10a. Wall 8, 8a is of an essentially cylindrical shape and carries at its downstream extremity an annular portion 8b that is thickened toward the outside and whose external surface 8c forms a cylindrical bearing surface. 
     Internal wall 7 is integral with upstream wall 7a and downstream wall 7b which run radially toward the exterior and which are thicker than wall 7 itself. 
     Downstream wall 7a is in turn integral with cylindrical wall 7c running upstream and welded at 11 to another wall 12 whose upstream extremity rests on cylindrical bearing surface 1b of the annular support of distributor 1. Downstream wall 7b is integral with cylindrical appendage 7d which fits onto cylindrical bearing surface 8c of the thick portion of wall 8b and which is immobilized axially in relation to the latter by a plurality of smooth pins 13 which are force-fitted in a crown-like configuration. 
     The inside face of interior wall 7 comprises several circular ribs 14 to which is attached &#34;abradable&#34; member 5. This &#34;abradable&#34; member 5 is made of porous material similar to the one described in French Patent application Ser. No. 77 26638 of Aug. 26, 1977 and, like the latter, features walls 5a produced by electron bombardment in order to prevent the cooling fluid, which is described hereinbelow, from circulating axially in the &#34;abradable&#34; member. Since the &#34;abradable&#34; member is not part of this invention, it is not deemed necessary to describe it in greater detail. Further details regarding the &#34;abradable&#34; member and its mounting on the &#34;abradable carrier&#34; can be obtained from the above-mentioned French Patent application. Between ribs 14, wall 7 is traversed by oblique passages 14a. 
     Inside annular shell 6 is mounted a second annular shell 15 made of sheet metal. This shell 15 is composed of two elements 15a and 15b, joined side-by-side by their radial walls which are perforated by a plurality of apertures 16 in crown-like disposition. The upstream wall of upstream element 15a is also perforated at its junction with the external cylindrical wall of this element 15a by a plurality of apertures 17 in a crown-like configuration. Shell 15 thus forms a hollow shroud, and onto its four faces (i.e. the two cylindrical faces--interior and exterior, the upstream face and the downstream face) are welded ball bearings 18 which, at rest, bear on the interior surfaces opposite shell 6. 
     The portion shown of turbine casing 2 comprises an upstream sleeve 19 provided at its downstream extremity with a flange 19a, and a downstream sleeve 20 provided at its upstream extremity with an outside flange 20a. Flanges 1a and 10a are pressed between these flanges 19a and 20a by the bolts 21. Downstream sleeve 20 is internally equipped opposite downstream wall 7b of sleeve 6, with an annular flange 20b that has an L-shaped cross-section, to which is attached annular support 22 of the distributor of the following stage of the turbine, which is partially represented in 23. 
     Between upstream sleeve 19 of the turbine casing and distributor 1, there is an annular conduit 24 connected with the casing of the combustion chamber (not shown) which is fed compressed air at 25 bars. This annular conduit 24 is connected, by apertures 1c in flange 1a, with an annular conduit 25 provided between cylindrical walls 8a, 10 and 7c, 12 and which opens up into annular shell 6. Apertures 26 in flanges 1a and 10a through which pass bolts 21 have a slightly larger diameter than these bolts and thus form passages around them which connect with conduit 24 and annular space 27 formed between sleeve 2 and walls 8, 8a and, 10 by means of canals 19b and 20c machined radially into the downstream face of flange 19a and into the upstream face of flange 20a, respectively. 
     Space 27 connects, by means of an annular passage 28 between L-shaped flange 20b and cylindrical appendage 7d, with space 29 formed between wall 7b and support 22, and space 29 is separated, by an annular omega-shaped seal 30 perforated by small apertures, from the internal flow of hot gases or &#34;jet&#34; 31 of the turbine. The static pressure of the jet at the exit from blading 3 being on the order of 5 bars, the apertures perforated in seal 30 are calibrated so as to create the drop in pressure required to avoid any perturbation of the jet at the downstream end of the turbine and, simultaneously, to maintain in space 27 a level of pressure sufficient to keep the shroud from becoming core-shaped under the pressure of the jet. 
     During operation, cooling air taken from the combustion chamber casing (not shown) flows through annular conduit 24 and, at the downstream extremity of the latter, separates into two streams; a first stream which flows through canals 19b, passages 26 surrounding bolts 21 and canals 20c into space 27, from where it flows into jet 31 through annular passage 28, space 29 and the apertures of seal 30 and a second stream, with a clearly higher rate of flow than the first one, which flows through apertures 1c into annular conduit 25 and, at the downstream extremity of the latter, separates into several currents some of which flow between wall 8 and exterior wall of shell 15, then between the wall of shell 15B and the downstream wall of this shell 15, ending up in space 32 formed between wall 7 and the interior wall 7 of shell 6, the other currents end up in this same space 32 by flowing between wall 7a and the upstream wall of shell 15. The cooling air thus entering space 32 escapes from the latter through oblique passages 14a so as to cool by impingement the &#34;abradable&#34; member 5 and flows through the pores of the latter into jet 31. The air flowing through the downstream extremity of annular passage 24 also enters shell 15 through apertures 17, keeping said shell under pressure and &#34;inflating&#34; it in such a manner that ball bearings 18 are tightly pressed against the walls of shell 15, which has the effect of maintaining at a high level the mechanical inertia of the latter and, consequently, of preventing any deformation, such as ovaling, under the action of the thermal stresses applied to it. 
     The cooling air taken from the combustion chamber casing is at a fairly elevated temperature, so that shell 6 and the elements that are integral with it (i.e. portions of walls 8a and 7c) must be made of a material characterized by a low coefficient of expansion in order to avoid differential thermal expansions between shell 6 and rotor blading 3. In the described embodiment, shell 6 and walls 8a and 7c are made of NCK 20 D alloy. Elements 10 and 12, on the other hand, are made of the same material as turbine casing 2. 
     It will be noted that, thanks to the flexibility of wall 8a, 10 which connects it to casing 2, the &#34;abradable carrier&#34; of high mechanical inertia formed by the combination of shells 6 and 15 can expand and contract freely to conform to the thermal expansions and contractions of rotor blading 3. These expansions and contractions cause only wall 8a, 10--and, secondarily, wall 7a, 12--to bend. In order to minimize the bending stress applied to welds 9 and 11, the latter are placed midway between wall 7a of shell 6 and framing flange 10a, i.e. at a point where the bending stresses are almost zero. 
     The shell shape of &#34;abradable carrier&#34; shroud 6 offers the advantage of giving it significant inertia so as to avoid any buckling under the pressure of the cooling air. To this effect, as can be seen in the drawing, radial walls 7a and 7b are made comparatively thick. Inside, shell 15 contributes to the efficient cooling of these thick walls and insures even cooling of the latter. An additional advantage of this shell 15 is that it serves as a dust trap since the particles carried by the air stream which flows through conduit 25 have an inertia that is too high for them to traverse the corner between outside shell 6 and inside shell 15 and they thus penetrate through apertures 17 onto the latter, where they are slowed down and trapped. 
     FIG. 2, in which elements serving the same function as in FIG. 1 are designated by the same reference numbers augmented by a 100 series, shows an embodiment in which outside shell 106 is composed on two annular parts, to wit a first part 33, which forms walls 107, 107a and 107b and whose 107c portion extends to a flange 107d which is attached to flanges 119a and 120a of the casing, and a part 34 which completes the shell between walls 107a and 107b. This annular part 34 is perforated by apertures 35 of a crown-like configuration, each facing an aperture of casing 102. Each aperture 35 is connected to aperture 36 facing it by a length of tube 37 mounted with swivel joints at both ends. Internal sheet metal shell 115 is perforated opposite each aperture 35 by an aperture 38 which is thus connected through tube 37 with the secondary air flow (indicated, schematically, by arrow 39) around casing 102. Internal shell 115 is thus &#34;inflated&#34; by secondary air flow 39. 
     This secondary air flow escapes from inside shell 115 through two series of apertures 40, 41, circulates between the two shells and enters space 132, which is here limited internally by a cylindrical sheet metal wall 43. After flowing through this perforated wall, the air flows through oblique passages 114a of wall 107 so as to cool &#34;abradable&#34; member 105 by impingement and crosses the latter to end up in jet 131. 
     The relatively warm air taken from the combustion chamber casing by means of annular conduit 124, flows at reference numeral 43 through flanges 101a and 107d, enters spaces 127, then 129 and joins up with jet 131. Flange 107d being thus ventilated by relatively warm air, it is necessary--in order to insure the mechanical strength of wall 107c--to provide for as linear as possible a temperature gradient between this wall 107c and shroud 104. To this effect, a reflux heat exchanger is created by means of a compound sleeve of sheet metal between wall 107c and distributor 101. Part of the fresh air circulating between the two shells flows through apertures 45 of wall 107a into annular space 46 formed between wall 107c and sleeve 44, through apertures 47 close to the upstream extremity of this sleeve 44 to enter annular space 48 formed between the latter and distributor 101, and escapes into jet 131 through the space formed between this distributor 101 and wall 107a. Thus, the internal face of flexible wall 107c is bathed by secondary air that is relatively cool, while its external face is exposed to the relatively warm air of the combustion chamber casing. 
     One advantage of the embodiment shown in FIG. 2 is that the cooling air taken through tubes 37 may have its temperature and rate of flow controlled by a secondary control device. Therefore, annular shell 106 may constitute a material of the same nature as that of casing 102. Another advantage is to permit, through appropriate metering of the warm air taken in the combustion chamber casing and cold air taken from the secondary air flow, control of the radial tolerance between &#34;abradable&#34; member 105 and mobile blading 103. 
     Having generally described this invention, a further understanding can be obtained by reference to certain specific embodiments which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.