Abstract:
A turbine ring assembly for a gas turbine includes a one-piece split ring ( 10 ) of ceramic matrix composite (CMC) material, a CMC wedge-shaped part ( 20 ) having flanks in contact with the ends of the ring, on either side of the split, so as to close the ring, and an annular metal support structure ( 40 ) surrounding the CMC ring and in contact therewith over the major fraction of its outline, the CMC ring being mounted with prestress in the metal structure, at least one element ( 26 ) exerting a resilient return force on the wedge-shaped part to keep it in contact with the ends of the CMC ring when the split opens under the effect of differential expansion between the annular metal structure and the CMC ring, and at least one element for preventing the CMC ring from turning about its axis.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to turbine rings for gas turbines, whether industrial gas turbines or gas turbines forming aeroengines. 
     In a gas turbine, a turbine ring defines a flow section at a rotary wheel of the turbine for a stream of hot gas passing therethrough. In order to ensure the best possible efficiency, it is important to avoid gas passing directly between the tips of the blades of the turbine wheel and the inside surface of the ring. Thus, in the usual way, a turbine ring is provided on its inside face with a layer of abradable material with which the turbine tips can come into contact without significant damage under the effect of dimensional variations of thermal origin or as the result of the centrifugal force that is applied to the blades. 
     Turbine rings are usually made as a plurality of adjacent sectors of metal material. 
     For example, document U.S. Pat. No. 6,758,653 proposes replacing the metal material of the turbine ring sectors by a thermostructural composite material, and more particularly by a ceramic matrix composite (CMC) material. Such a material presents mechanical properties that make it suitable for constituting structural elements and also has the ability to conserve these properties at high temperature, while presenting density that is much lower than that of the metal materials commonly used in such an application. 
     It is therefore attractive to replace the metal material of the turbine ring sectors with a CMC material. Nevertheless, it is necessary to design an assembly for the ring sectors that is rather complex in order to accommodate the difference between the coefficients of expansion of a CMC material and of the material of a metal casing in which the ring sectors are assembled, and while minimizing leaks against adjacent sectors. 
     OBJECT AND SUMMARY OF THE INVENTION 
     An object of the invention is to provide a simplified CMC turbine ring assembly that also serves to minimize leaks of gas between a support structure of the ring and the flow section through a turbine wheel inside the ring. 
     This object is achieved by a turbine ring assembly for a gas turbine, the assembly comprising:
         a one-piece split ring of ceramic matrix composite (CMC) material;   a wedge-shaped CMC part having flanks in contact with the ends of the ring on either side of the split, so as to close the ring;   an annular metal support structure surrounding the CMC ring, in contact therewith over a major fraction of its outline, the CMC ring being mounted with prestress inside the metal structure;   at least one element exerting a resilient return force on the wedge-shaped part to keep it in contact with the ends of the CMC ring when the split opens under the effect of differential expansion between the annular metal structure and the CMC ring; and   at least one element for preventing the CMC ring from turning about its axis.       

     Thus, with a one-piece ring, the structure of the turbine ring assembly is simplified. In addition, using a CMC material makes it possible to reduce cooling requirements, thereby reducing the need for a stream of cooling air. 
     In a first embodiment the metal structure comprises two annular metal supports with the CMC ring being mounted between them. 
     The element exerting a resilient return force may be a prestressed elastically-deformable blade bearing firstly against the annular metal supports and secondly against the wedge-shaped part. 
     In a second embodiment, the metal structure comprises a metal hoop surrounding the outer peripheral surface of the CMC ring. 
     The element exerting a resilient return force may then be a prestressed elastically-deformable tongue integral with the metal hoop and bearing against the wedge-shaped part. 
     The metal structure may further comprise two annular metal supports with the CMC ring and the metal hoop being mounted between them, enabling differential expansion to take place at least in a radial direction between the metal hoop and the annular metal supports. 
     Advantageously, centering means are provided for centering the metal hoop and the CMC ring. 
     The metal hoop may be mounted between the annular metal supports by means of elastically-deformable blades. 
     Advantageously, a sealing gasket is interposed between at least one of the lateral faces of the CMC ring and a facing face of one of said annular metal supports. 
     In both embodiments, and preferably, the wedge-shaped part presents an inner end face that lies substantially in continuity with the inside peripheral surface of the CMC ring at the temperature to which the turbine ring assembly is normally exposed in operation. 
     The CMC ring may be provided with a layer of abradable material on its inside peripheral surface. 
     Advantageously, the material of the CMC ring is a self-healing ceramic matrix composite material. 
     The material of the CMC ring may be provided with a coating forming an environmental barrier for protection against corrosion. 
     Advantageously, the CMC ring includes fiber reinforcement made by three-dimensional weaving. 
     Also advantageously, the CMC ring and the wedge-shaped part are made out of the same material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood on reading the following description made by way of non-limiting indication with reference to the accompanying drawings, in which: 
         FIG. 1  is a highly diagrammatic fragmentary axial half-section view showing a first embodiment of a turbine ring assembly of the invention incorporated in its environment in a gas turbine; 
         FIG. 2  is a fragmentary section view on plane II-II of  FIG. 1 ; 
         FIGS. 3A and 3B  are fragmentary radial section views on a larger scale showing the turbine ring assembly on planes IIIA-IIIA and IIIB-IIIB of  FIG. 2 ; 
         FIG. 4  is a fragmentary perspective view on a larger scale showing a detail of the turbine ring assembly of  FIGS. 1 and 2 ; 
         FIGS. 5A and 5B  show a detail in section of  FIG. 2 , on a larger scale, respectively when the turbine ring assembly is cold, and when it is at high temperature under conditions of use; 
         FIG. 6  is a fragmentary view on a larger scale showing a detail of the turbine ring assembly of  FIGS. 1 and 2 ; 
         FIG. 7  is a highly diagrammatic fragmentary view in axial half-section showing how a second embodiment of a turbine ring assembly of the invention is incorporated in its environment in a gas turbine; 
         FIG. 8  is a lateral elevation view of a CMC ring and a metal hoop in the second embodiment of the invention; 
         FIG. 9  is a section view on plane IX-IX of  FIG. 8 ; 
         FIG. 10  is a fragmentary perspective view on a larger scale showing the CMC ring and the metal hoop surrounding the ring in the second embodiment of  FIGS. 7 to 9 ; 
         FIG. 11  shows a detail on a larger scale of the section view of  FIG. 9 ; 
         FIGS. 12 and 13  are fragmentary radial section views on a larger scale showing details of the turbine ring assembly of  FIGS. 8 and 9 ; and 
         FIG. 14  shows an example of an interlock type weave for three-dimensionally weaving fiber reinforcement for a CMC ring for a turbine ring assembly of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A first embodiment of the invention is described with reference to  FIGS. 1 to 6 . 
       FIG. 1  shows, in highly diagrammatic form, and on going from upstream to downstream in the flow direction of the gas stream through a gas turbine: a combustion chamber  1 ; a turbine nozzle  2  placed at the outlet from the combustion chamber; a high pressure (HP) turbine  3 ; a flow straightener  4 ; and a first stage of a low pressure (LP) turbine  5 . 
     The HP turbine  3  comprises a turbine ring assembly and a wheel  6  movable in rotation and carrying blades  7 . 
     The turbine ring assembly comprises a single-piece split turbine ring  10  made of CMC material. The CMC ring  10  is supported by a metal structure comprising upstream and downstream metal annular supports  30  and  40  with the ring  10  being placed between them. 
     The metal supports  30  and  40  are connected to a turbine casing  8 . An annular space  9  is formed outside the ring  10 , between the supports  30  and  40  and defined by a bottom wall  9   a . Cooling air is fed to the annular space  9  in well-known manner. 
     As shown in greater detail in  FIGS. 3A and 3B , lateral portions of the ring  10  adjacent to its opposite lateral faces  12   a  and  12   b  engage in cylindrical housings formed by steps in the inner faces  32  and  42  of the supports  30  and  40  that are situated facing the ring  10 . 
     In its lateral portions, the ring  10  has its lateral faces  12   a  and  12   b  adjacent to the end walls  32   a  and  42   a  of said housings, and it presses via its outer peripheral surface against the peripheral cylindrical walls  32   b  and  42   b  of the same housings ( FIG. 3A ). The split ring  10  is inserted with circumferential prestress in the housings of the supports  30  and  40  within the limit of its capacity for elastic deformation so that, as explained below, contact with pressure between the ring  10  and the cylindrical walls  32   b  and  42   b  continues to be maintained in the event of the supports  30  and  40  expanding for thermal reasons, given that the metal material from which they are made has a coefficient of expansion that is greater than that of the CMC material of the ring  10 . 
     When cold, the ring  10  is almost closed, with the gap between its ends being small. The ring  10  has its end portions  10   a  and  10   b  chamfered, and a wedge-shaped closure part  20  is pressed against the chamfered surfaces  15   a  and  15   b  of the end portions of the ring  10  ( FIGS. 5A ,  5 B). The section of the part  20  is substantially trapezoidal, with a rear face  22   a  and a shorter front face  22   b  that is connected to the rear face  22   a  via inclined side faces  24   a  and  24   b  that press against the chamfered surfaces  15   a  and  15   b  and that have substantially the same angles of inclination as they do. The part  20  is made of CMC material, preferably out of the same material as the ring  10 , and is of a width that is substantially equal to the width of the ring  10 . 
     The part  20  is pressed against the chamfered end portions of the ring  10  with a resilient bearing force that is exerted by an elastically-deformable blade  26  that is received in a prestressed state between the rear face  22   a  of the part  20  and end walls  36   a  and  46   a  of housings  36  and  46  formed over a sector of each of the annular supports  30  and  40 , starting from their inner faces  32  and  42  ( FIGS. 3B and 4 ). In the example shown, the blade  26  has a curved shape with its central portion pressing against the part  20  and with its end portions pressing against the walls  36   a  and  46   a . The front face  22   b  of the part  20  has a circular profile of radius substantially equal to that of the inside peripheral surface  16  of the ring  10 . 
       FIGS. 5A and 5B  show the relative positions of the ring  10  and of the part  20  respectively when cold and when hot, i.e. once the operating temperature has been reached while the gas turbine in which the turbine ring assembly is mounted is operating under normal conditions. When cold, the part  20  is set back from the inside surface  16  of the ring  10 . When hot, the split in the ring  10  enlarges as a result of the ring  10  that was assembled with pre-stress “follows” the expansion of the metal annular supports  30  and  40 . The dimensions of the part  20  are selected as a function of the differential expansion between the ring  10  and the supports  30  and  40 , so that when hot the front face  22   b  of the part  20  is situated substantially in continuity with the inside surface  16  of the ring  10 , the flexible blade  26  continuing to apply a pressure force on the part  20 . 
     The continuous pressing with prestress between the outside surface of the ring  10  and the surfaces  32   b  and  42   b  ensures that the ring  10  is centered. This continuous pressing also serves to limit leaks between the outside of the ring  10  and the hot gas flow section inside the ring  10 . When the turbine ring assembly is cooled in operation by feeding air to the outside of the turbine ring, leaks of cooling air into the hot gas flow section could be minimized even further, if so desired, by placing an annular sealing gasket between the ring  10  and the downstream annular support  40 . 
     On its inside peripheral surface, the ring  10  is provided with a layer  11  of abradable material with which the tips of the blades of a rotary wheel surrounded by the ring  10  can come into contact without significant damage. In the example shown, the layer  11  is placed in an annular setback  18  formed in the inside surface  16  over a major fraction of the width of the ring  10  in the axial direction. As a result, the exposed face of the layer  11 , the portions of the inside surface  16  situated on either side thereof, and the inside peripheral surfaces  38  and  48  of the annular metal supports  30  and  40  define a continuous surface for the hot gas flow section, which surface does not present any sudden variation in diameter. It is nevertheless possible to envisage forming the layer of abradable material  11  as an extra thickness on the inside surface  16  of the ring  10 . A layer  21  of abradable material is advantageously formed on the front face  22   b  of the part  20  so that, when hot, it comes into continuity with the layer  11 . 
     The ring  10  is prevented from turning relative to the annular supports  30  and  40 , or at least relative to one of them. This ensures that contact between a blade tip of the rotary wheel and the abradable coating  11  does not cause the ring to turn. By way of example, turning may be prevented by means of teeth  35  and  45  ( FIG. 6 ) projecting from the walls  32  and  42  and engaging in notches formed in the lateral faces  12   a  and  12   b  of the ring  10 . 
     A second embodiment of the turbine ring assembly of the invention is described below with reference to  FIGS. 7 to 14 . 
       FIG. 7  is a fragmentary diagrammatic view in axial half-section of a gas turbine that differs from  FIG. 1  essentially by the way in which the CMC split turbine ring  110  is mounted between the annular metal supports  130  and  140 , the other elements of the gas turbine being similar to those of  FIG. 1  and having the same references. 
     The split ring  110  is mounted with circumferential prestress inside a metal hoop  150 , within the limit of its capacity for elastic deformation. Mounting is performed so that contact with pressure between the outside peripheral surface of the ring  110  and the inside surface of the hoop  150  continues to be maintained in the event of differential expansion of thermal origin at the temperatures encountered in operation by the turbine ring assembly, the metal material of the hoop  150  having a coefficient of expansion that is greater than that of the CMC material of the ring  110 . The hoop  150  is of a width that is slightly less than the width of the ring  110 , with its lateral edges being set back from the lateral faces  112   a  and  112   b  of the ring  110  ( FIGS. 8 and 12 ). 
     When cold, the ring  110  is almost closed, and the gap between its ends is small. The ring  110  has its end portions  110   a  and  110   b  chamfered and a closure part  120  in the form of a wedge is pressed against the chamfered surfaces  115   a  and  115   b  of the end portions of the ring ( FIGS. 9 ,  10 , and  11 ). The part  120  is similar to part  20  of the above-described embodiment. It presents a rear face  122   a , a front face  122   b  of curvature substantially equal to that of the inside peripheral surface  116  of the ring  110 , and lateral faces  124   a  and  124   b  that press against the chamfered surfaces  115   a  and  115   b . The part  120  is made of CMC material, preferably of the same material as the ring  110  and is of a width that is substantially equal to the width of the ring  110 . 
     The part  120  is pressed against the chamfered end portions of the ring  110  with a resilient pressure force exerted by an elastically-deformable tongue  156 . As shown in  FIGS. 10 and 11 , the hoop  150  is spaced apart from the ring over a larger-diameter portion  158  that is connected to the remainder of the hoop, thereby leaving a gap for the part  120  in the vicinity of the end portions of the ring  110 , which ring remains pressed against the inside surface of the hoop  150  over the major fraction of its outline. In the example shown, the tongue  156  is cut in the circumferential direction in the middle zone of the portion  158  of the hoop  150  and remains connected thereto at one end. After being cut, the tongue is deformed so as to be curved and pressed against the part  120 , exerting a resilient force thereagainst, including when the hot part  120  closes the split in the ring  110  with its front face  122   b  being substantially in continuity with the inside surface  116  of the ring  110 . Naturally, it is possible to use other shapes for the elastically-deformable parts that exert a pressure force on the part  120 , e.g. a blade similar to that in the above-described embodiment. 
     The ring  110  together with the hoop  150  is placed between the metal annular supports  130  and  140 , the lateral faces  112   a  and  112   b  of the ring being adjacent to the inner lateral faces  132  and  142  of the supports  130  and  140  that are situated facing the ring  110 . 
     The ring  110  provided with the hoop  150  is held between the supports  130  and  140  by means of elastically-deformable metal blades  160  ( FIGS. 8 ,  10 , and  12 ) that extend radially. A plurality of blades  160  are provided that are preferably distributed regularly around the axis of the ring  110 , there being at least three such blades. Each blade  160  has a central portion  162  connected to the outside surface  152  of the hoop  150  that is connected to curved lateral portions  162   a  and  162   b  extending away from the surface  152 . The ends of the lateral portions  162   a  and  162   b  are engaged in corresponding recesses  134  and  144  formed in the faces  132  and  142  of the supports  130  and  140 . The blades  160  are connected to the outside surface  152  of the hoop  150 , e.g. by clip-fastening, welding, or riveting. In the example shown, the blades  160  are provided with hooks  164   a  and  164   b  on either side of their central portions  162 , which hooks are engaged with elastic deformation in housings formed in the outside surface  152  of the hoop  150  ( FIG. 10 ). 
     The blades  160  enable the desired centering of the ring  110  to be conserved inside the hoop  150  while still allowing differential expansion to take place in a radial direction between the hoop  150  and the annular supports  130  and  140 . As mentioned above, for the above-described embodiment, it can be desirable to minimize or control leaks between the outside of the ring  110  and the hot gas flow section inside the ring  110 , at least beside the downstream annular support  140 . For this purpose, a resilient washer  166  is placed in a groove  143  formed in the inner surface  142  of the annular support  140  ( FIGS. 12 and 13 ). The resilient washer is prestressed and has its circumferentially outer end pressing against the bottom of the groove  143  and its circumferentially-inner end pressing against a thrust washer  168  pressed against the lateral face  112   b  of the ring  110 . In a variant, a sealing gasket with an ω-shaped profile could be used. 
     Holes may be formed through the elastic washer  166  in order to balance pressure in the groove  143  on either side of the washer  166  and allow a flow of cooling air to pass for the annular support  140 . 
     On its inside periphery, the ring  110  is provided with a layer  111  of abradable material. As in the above-described embodiment, the layer  111  is received in an annular setback  118  formed in the inside surface  116  of the ring  110  so as to co-operate with the inside peripheral surfaces  138  and  148  of the annular metal supports  130  and  140  to form a continuous surface for the hot gas flow section without any sudden change in diameter. A similar abradable coating  121  is formed on the front face  122   b  of the part  120 . Naturally, it is possible to envisage forming a coating  111  that projects from the inside surface  116  of the ring  110 . 
     As in the above-described embodiment, the ring  110  with the hoop  150  is prevented from turning relative to the supports  130  and  140 , or relative to at least one of them. This is achieved, for example, by means of pegs  159  ( FIG. 13 ) that are secured to the hoop  150  and that engage in blind holes formed in the outside face of the ring  110 . The hoop  150  is prevented from turning relative to the supports  130  and  140  by means of the blades  160  having their ends engaged in the recesses  134  and  144  in the faces  132  and  142 . In a variant, the recesses  134  and  144  could be in the form of continuous grooves, in which case the hoop  150  would be prevented from turning relative to the supports  130  and  140  by other means, e.g. by forming one or more teeth on at least one of the faces  132  and  142 , each of said teeth engaging in a notch formed in the hoop  150  and optionally in the ring  110 . 
     The CMC material of the ring  10  or  110  and of the part  20  or  120  may be of known type obtained by densifying a fiber preform with a ceramic matrix, the fiber preform providing the fiber reinforcement of the material. The fibers of the preform are refractory fibers such as carbon fibers or ceramic fibers, e.g. fibers of silicon carbide (SiC). It should be observed that the term “ceramic” also covers compounds of the refractory oxide type. 
     A first step may consist in making a fiber preform that serves, after densification by the ceramic matrix, to obtain a part from which the ring  10  or  110  or the wedge-shaped part  20  or  120  can be machined or cut out. It should be observed that machining is preferably performed at an intermediate stage of densification so as to ensure that after a subsequent final stage of densification the fibers of the fiber preform are well protected by a layer of matrix. 
     One way of making a fiber preform for the ring  10  or  110  consists in making a strip of desired thickness by three-dimensional weaving.  FIG. 14  is a diagram showing an interlock type weave suitable for three-dimensional weaving (the weft yarns being shown in section). The end portions of the woven strip may be made with decreasing thickness corresponding to the chamfered ends of the ring. The decreasing thickness may be obtained during weaving by progressively reducing the number of layers of warp and weft yarns. Three-dimensional weaves other than interlock weaves can be used, such as multilayer weaves, e.g. of the multi-plain or multi-satin type, as described in document WO 2006/136755. 
     Other ways of making a fiber preform for the ring  10  or  110  can be envisaged. For example, it is possible to form a strip of desired thickness by superposing a plurality of fiber plies, e.g. strips of woven fabric with the strips being bonded together, e.g. by needling. 
     The ceramic matrix may be a refractory ceramic matrix such as SiC, or advantageously it may be a “self-healing” ceramic matrix. A “self-healing” ceramic matrix is obtained by making at least one of the component phases of the matrix out of a material that, by passing to the viscous state in a certain temperature range, is capable of filling in or “healing” cracks that form in the matrix, in particular under the effect of thermal cycling. Compositions having “self-healing” properties are in particular vitreous compositions, e.g. of the aluminosilicate type, or compositions that, under the effect of oxidation, are capable of forming vitreous compositions. Matrix phases of boron carbide B 4 C or of an Si—B—C ternary system are precursors of vitreous compositions. The matrix may be formed by chemical vapor infiltration (CVI) and an interphase coating, e.g. of pyrolytic carbon (PyC) or of boron nitride BN may previously be formed on the fibers of the preform. The fiber preform may be kept in the desired shape during an initial stage of densification until the preform is consolidated, i.e. until it has been partially densified to an extent that is sufficient to enable the preform thereafter to conserve its shape without the help of tooling. Methods of making a composite material with a self-healing ceramic matrix are described in particular in documents U.S. Pat. Nos. 5,965,266, 6,291,058, and 6,068,930. 
     After densification and machining, the CMC material may be protected against corrosion by an environmental protection barrier, in known manner. Such a barrier may for example comprise an outer layer of yttrium-stabilized zirconia and a bonding underlayer of mullite. It is also known to provide corrosion resistance by means of a layer made of a compound of the type comprising an aluminosilicate of an alkaline earth metal, such as the compound BaO 0.75 SrO 0.25 Al 2 O 3 (SiO 2 ) 2  commonly known by the abbreviation BSAS. With a CMC material that contains silicon, a chemical barrier layer may then be interposed, e.g. a layer of mullite or comprising a mixture of mullite plus BSAS, while a bonding or keying underlayer of Si may be provided. A thermal barrier layer of yttrium-stabilized zirconia may be formed on the BSAS layer. Environmental barriers that are particularly suitable for CMC materials having a matrix containing silicon are described in particular in the following documents: U.S. Pat. No. 6,866,897, EP 1 416 066, U.S. Pat. No. 6,759,151, FR 06/51880, and FR 06/55578. The various layers of the environmental barrier may be deposited by physical vapor deposition, e.g. by plasma or thermal plasma sputtering, or by chemical vapor deposition (CVD), possibly with the assistance of a plasma. 
     The layer of abradable material is for example made of a refractory oxide such as zirconia or alumina. It may be formed by physical vapor deposition, e.g. by plasma or thermal plasma sputtering. The abradable material is preferably porous. In known manner, its porosity may be controlled by depositing the material of the abradable layer together with a powder of material that can be eliminated at high temperature, e.g. polyethylene powder. 
     The layer of abradable material may be formed on the environmental protection barrier. 
     The wedge-shaped part  20  or  120  with the layer of abradable material  21  or  121  can be obtained in a manner similar to that described for the ring  10  or  110 . To make its fiber preform, it is nevertheless possible to form a strip of fiber texture of desired thickness by three-dimensional weaving or by superposing and bonding together fiber plies, and then cutting said strip up into preforms having shapes that correspond to the shapes desired for the parts  20  or  120 . 
     The turbine ring assembly of the invention is particularly suitable for an HP turbine of a gas turbine. Nevertheless, it can also be used for an LP turbine, or indeed for an intermediate turbine if a gas turbine has more than two stages.