Patent Publication Number: US-11021988-B2

Title: Turbine ring assembly

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a turbine ring assembly comprising a plurality of ring sectors made of ceramic-matrix composite material as well as a ring support structure. 
     The field of application of the invention is in particular that of the aeronautical gas turbine engines. The invention is however applicable to other turbomachines, for example industrial turbines. 
     In the case of entirely metallic turbine ring assemblies, it is necessary to cool all the elements of the assembly and particularly the turbine ring which is subjected to the hottest flows. This cooling has a significant impact on the engine performance since the cooling flow used is taken from the main flow of the engine. In addition, the use of metal for the turbine ring limits the possibilities to increase the temperature at the turbine, which would however allow improving the performance of the aeronautical engines. 
     In order to solve these problems, it has been envisaged to produce turbine ring sectors made of ceramic-matrix composite material (CMC) in order to overcome the implementation of a metal material. 
     CMC materials have good mechanical properties making them capable of forming structural elements and advantageously preserve these properties at high temperatures. The implementation of CMC materials has advantageously allowed reducing the cooling flow to be imposed during the operation and therefore increasing the performance of the turbomachines. In addition, the implementation of CMC materials advantageously allows decreasing the weight of the turbomachines and reducing the effect of hot expansion encountered with the metal parts. 
     However, the existing solutions proposed can implement an assembling of a CMC ring sector with metal attachment portions of a ring support structure, these attachment portions being subjected to the hot flow. Consequently, these metal attachment portions undergo hot expansions, which can lead to mechanical stressing of the CMC ring sectors and to an embrittlement thereof. 
     Furthermore, documents FR 2 540 939, GB 2 480 766, EP 1 350 927, US 2014/0271145, US 2012/082540 and FR 2 955 898 which disclose turbine ring assemblies, are known. 
     There is a need to improve existing turbine ring assemblies and their mounting, and in particular the existing turbine ring assemblies implementing a CMC material in order to reduce the intensity of the mechanical stresses to which the CMC ring sectors are subjected during the operation of the turbine. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The invention aims at proposing a turbine ring assembly allowing to maintain each ring sector in a deterministic manner, that is to say, so as to control its position and prevent it from vibrating, on the one hand, while allowing the ring sector, and by extension the ring, to deform under the effects of temperature rises and pressure variations, and this in particular independently of the interface metal parts and, on the other hand, while improving the sealing between the off-flowpath sector and the flowpath sector and while simplifying the manipulations and reducing their number for the mounting of the ring assembly. 
     An object of the invention proposes a turbine ring assembly comprising a plurality of ring sectors forming a turbine ring and a ring support structure, each ring sector having, according to a section plane defined by an axial direction and a radial direction of the turbine ring, a portion forming an annular base with, in the radial direction of the turbine ring, an inner face defining the inner face of the turbine ring and an outer face from which a first and a second attachment tabs protrude, the ring support structure including a central shroud from which a first and a second radial clamps protrude between which the first and second attachment tabs of each ring sector are maintained. 
     According to a general characteristic of the object, the turbine ring assembly comprises a first annular flange and a second annular flange disposed upstream of the first annular flange with respect to the direction of an air flow intended to pass through the turbine ring assembly, the first and second annular flanges having respectively a first free end and a second end opposite to the first end, the first end of the first flange bearing against the first attachment tab, the first end of the second annular flange being spaced apart from the first end of the first annular flange in the axial direction, and the second end of the second annular flange comprising an upstream bearing shroud protruding upstream in the axial direction, the upstream bearing shroud having a radial bearing in contact with the central shroud of the ring support structure. 
     In a particular embodiment, the ring sectors may be made of ceramic-matrix composite material (CMC). 
     The second annular flange separated from the first annular flange at its free end allows providing the turbine ring assembly with an upstream flange dedicated to take up the force of the high-pressure distributor (DHP). The second annular flange upstream of the turbine ring and free from any contact with the ring is configured to transit the maximum axial force induced by the DHP directly into the ring support structure without passing through the ring which, when it is made of CMC, has a low mechanical permissible element. 
     Indeed, leaving a space between the first ends of the first and second annular flanges allows deflecting the force received by the second flange, upstream of the first annular flange which is in contact with the turbine ring, and transiting it directly toward the central shroud of the ring support structure via the second end of the second annular flange, without impacting the first annular flange and therefore without impacting the turbine ring. The first end of the first flange do not undergo a force, the turbine ring is thus preserved from this axial force. 
     The transit of the DHP force via the second annular flange can induce its tilting. This tilting can cause an uncontrolled contact between the low portions, that is to say the first ends, of the second annular flange and the first annular flange in contact with the turbine ring, which would have the consequence of transmitting directly the DHP force to the ring. 
     The upstream bearing shroud ensures higher resistance to the tilting induced by the DHP force. The bearing shroud takes up the significant tangential stresses caused by the DHP force and thereby limits the tilting of the second annular flange. 
     In addition, the removable nature of the annular flanges makes it possible to have axial access to the cavity of the turbine ring. This allows assembling the ring sectors together outside the ring support structure and then axially sliding the assembly thus assembled into the cavity of the ring support structure until bearing against the second radial clamp, before fastening the annular flanges on the central shroud of the ring support structure. 
     During the operation of fastening the turbine ring on the support structure of the ring, it is possible to use a tool including a cylinder or a ring on which the ring sectors are pressed or sucked during their crown assembling. 
     The fact of having two annular flanges each in one piece, that is to say describing the entirety of a ring over 360°, allows, compared to sectored annular flanges, limiting the passage of the air flow between the off-flowpath sector and the flowpath sector, in so far as all the inter-sector leaks are eliminated, and therefore controlling the sealing. 
     The solution defined above for the ring assembly thus makes it possible to maintain each ring sector in a deterministic manner, that is to say to control its position and prevent it from starting to vibrate, while improving the sealing between the off-flowpath sector and the flowpath sector, while simplifying the manipulations and while reducing their number for the mounting of the ring assembly, and while allowing the ring to deform under the effect of temperature and pressure in particular independently of the interface metal parts. 
     According to a first aspect of the turbine ring assembly, the second annular flange may comprise a contact abutment extending in the axial direction of the turbine ring and separating the second end of the second annular flange from the second end of the first annular flange. 
     The contact abutment provided between the second ends of the first and second annular flanges allows further reducing the contact between the low portion of the second annular flange, disposed upstream of the first flange, and that of the first annular flange, following this tilting. The direct transit of the DHP force toward the ring is therefore avoided. 
     According to a second aspect of the turbine ring assembly, the assembly may further comprise an omega seal mounted between the first end of the second annular flange and the first end of the first flange, the second annular flange being fastened to the ring support structure on a portion upstream of the radial bearing. 
     The omega seal allows ensuring the sealing between the flowpath cavity and the off-flowpath cavity upstream of the ring. 
     According to a third aspect of the turbine ring assembly, the ring sector may have an inverted Greek letter section pi (π) along the section plane defined by the axial direction and the radial direction, and the assembly may comprise, for each ring sector, at least three pins to radially hold the ring sector in position, the first and second attachment tabs of each ring sector each comprising a first end secured to the outer face of the annular base, a second free end, at least three lugs for receiving said at least three pins, at least two lugs protruding from the second end of one of the first or second attachment tabs in the radial direction of the turbine ring and at least one lug protruding from the second end of the other attachment tab in the radial direction of the turbine ring, each receiving lug including an orifice for receiving one of the pins. 
     According to a fourth aspect of the turbine ring assembly, the ring sector may have a section with an elongated K-shape along the section plane defined by the axial direction and the radial direction, the first and a second attachment tabs having an S-shape. 
     According to a fifth aspect of the turbine ring assembly, the ring sector may have, on at least one radial range of the ring sector, an O-shaped section along the section plane defined by the axial direction and the radial direction, the first and second attachment tabs each having a first end secured to the outer face and a second free end, and each ring sector comprising a third and a fourth attachment tabs each extending, in the axial direction of the turbine ring, between a second end of the first attachment tab and a second end of the second attachment tab, each ring sector being fastened to the ring support structure by a fastening screw including a screw head bearing against the ring support structure and a thread cooperating with a tapping formed in a fastening plate, the fastening plate cooperating with the third and fourth attachment tabs. 
     Another object of the invention proposes a turbomachine comprising a turbine ring assembly as defined above. 
    
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood upon reading the following, by way of indication but without limitation, with reference to the appended drawings in which: 
         FIG. 1  is a schematic perspective view of a first embodiment of a turbine ring assembly according to the invention; 
         FIG. 2  is an exploded schematic perspective view of the turbine ring assembly of  FIG. 1 ; 
         FIG. 3  is a schematic sectional view of the turbine ring assembly of  FIG. 1 ; 
         FIG. 4  is a schematic sectional view of a second embodiment of the turbine ring assembly; 
         FIG. 5  is a schematic sectional view of a third embodiment of the turbine ring assembly; 
         FIG. 6  is a schematic sectional view of a fourth embodiment of the turbine ring assembly. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a high-pressure turbine ring assembly comprising a turbine ring  1  made of ceramic-matrix composite material (CMC) and a metal ring support structure  3 . The turbine ring  1  surrounds an assembly of rotary blades (not represented). The turbine ring  1  is formed of a plurality of ring sectors  10 ,  FIG. 1  being a radial sectional view. The arrow D A  indicates the axial direction of the turbine ring  1  while the arrow D R  indicates the radial direction of the turbine ring  1 . For reasons of simplification of presentation,  FIG. 1  is a partial view of the turbine ring  1  which is actually a complete ring. 
     As illustrated in  FIGS. 2 and 3 , which respectively have an exploded schematic perspective view and a sectional view of the turbine ring assembly of  FIG. 1 , the sectional view being along a section plane comprising the radial direction D R  and the axial direction D A , each ring sector  10  has, along a plane defined by the axial D A  and radial D R  directions, a section with substantially the shape of the inverted Greek letter (π). The section comprises indeed an annular base  12  and upstream and downstream radial attachment tabs, respectively  14  and  16 . The terms “upstream” and “downstream” are used here with reference to the flowing direction of the gas flow in the turbine represented by the arrow F in  FIG. 1 . The tabs of the ring sector  10  could have another shape, the section of the ring sector having a shape other than π, such as a K- or an O-shape. 
     The annular base  12  includes, along the radial direction D R  of the ring  1 , an inner face  12   a  and an outer face  12   b  opposite to each other. The inner face  12   a  of the annular base  12  is coated with a layer  13  of abradable material forming a thermal and environmental barrier and defines a flow path of gas flow in the turbine. The terms “inner” and “outer” are used herein with reference to the radial direction D R  in the turbine. 
     The upstream and downstream radial attachment tabs  14  and  16  protrude, along the direction D R , from the outer face  12   b  of the annular base  12  away from the upstream and downstream ends  121  and  122  of the annular base  12 . The upstream and downstream radial attachment tabs  14  and  16  extend over the entire width of the ring sector  10 , that is to say, over the entire arc of circle described by the ring sector  10 , or over the entire circumferential length of the ring sector  10 . 
     As illustrated in  FIGS. 1 to 3 , the ring support structure  3  which is secured to a turbine casing comprises a central shroud  31 , extending in the axial direction D A , and having an axis of revolution coincident with the axis of revolution of the turbine ring  1  when they are fastened together, as well as a first radial annular clamp  32  and a second radial annular clamp  36 , the first radial annular clamp  32  being positioned upstream of the second radial annular clamp  36  which is therefore downstream of the first radial annular clamp  32 . 
     The second radial annular clamp  36  extends in the circumferential direction of the ring  1  and, along the radial direction D R , from the central shroud  31  towards the center of the ring  1 . It comprises a first free end  361  and a second end  362  secured to the central shroud  31 . The second radial annular clamp  36  includes a first portion  363 , a second portion  364 , and a third portion  365  comprised between the first portion  363  and the second portion  364 . The first portion  363  extends between the first end  361  and the third portion  365 , and the second portion  364  extends between the third portion  365  and the second end  362 . The first portion  363  of the second radial annular clamp  36  is in contact with the downstream radial attachment clamp  16 . The second portion  364  is thinned relative to the first portion  363  and the third portion  365  so as to give some flexibility to the second radial annular clamp  36  and thus not to stress too much the CMC turbine ring  1 . 
     The first radial annular clamp  32  extends in the circumferential direction of the ring  1  and, along the radial direction D R , from the central shroud  31  to the center of the ring  1 . It comprises a first free end  321  and a second end  322  secured to the central shroud  31 . 
     As illustrated in  FIGS. 1 to 3 , the turbine ring assembly  1  comprises a first annular flange  33  and a second annular flange  34 , the two annular flanges  33  and  34  being removably fastened to the first radial annular clamp  32 . The first and second annular flanges  33  and  34  are disposed upstream of the turbine ring  1  with respect to the flowing direction F of the gas flow in the turbine. 
     The first annular flange  33  is disposed downstream of the second annular flange  34 . The first annular flange  33  has a first free end  331  and a second end  332  removably fastened to the ring support structure  3 , and more particularly to the first radial annular clamp  32 . 
     In addition, the first annular flange  33  has a first portion  333  extending from the first end  331  and a second portion  334  extending between the first portion  333  and the second end  332 . When the ring assembly  1  is mounted, the first portion  333  of the first annular flange  33  bears against the upstream radial attachment tab  14  of each of the ring sectors  10  forming the turbine ring  1 , and the second portion  334  of the first annular flange  34  bears against at least part of the first radial annular clamp  32 . 
     The radial holding of the ring  1  is ensured by the first annular flange  33  which is pressed on the first radial annular clamp  32  of the ring support structure  3  and on the upstream radial attachment tab  14 . The first annular flange  33  ensures the sealing between the flowpath cavity and the off-flowpath cavity of the ring. 
     The second annular flange  34  has a first free end  341  and a second end  342  removably fastened to the ring support structure  3 . 
     The second annular flange  34  is dedicated to take up the force of the high-pressure distributor (DHP) on the ring assembly  1 , on the one hand, by deforming and, on the other hand, by transiting this force towards the casing line which is more mechanically robust, that is to say toward the line of the ring support structure  3  as illustrated by the force arrow E represented in  FIG. 3 . 
     In the first embodiment illustrated in  FIGS. 1 to 3 , the first annular flange  33  and the second annular flange  34  are in contact at their second end respectively  332  and  342 . The second end  342  of the second annular flange  34  comprises a contact abutment  340  protruding in the axial direction D A  between the second annular flange  34  and the first annular flange  33 . The contact abutment  340  allows maintaining a distance between the first end  331  of the first annular flange  33  and the first end  341  of the second annular flange  34  during the tilting of the second annular flange  34  induced by the DHP force. The second end  342  of the second annular flange  34  is fastened to the first radial annular clamp  32  via the abutment and the first annular flange  33 . 
     In addition, the second end  342  of the second annular flange  34  comprises a bearing shroud  346  protruding upstream in the axial direction D A . 
     In other words, the second annular flange  34  has an upstream face  34   a  receiving the gas flow F and a downstream face  34   b  facing the first annular flange  33 , and the second end  342  of the second annular flange  34  comprises a contact abutment  340  extending in the axial direction D A  from the downstream face  34   b  downstream, that is to say towards the first annular flange  33 , and a bearing shroud  346  extending in the axial direction D A  from the upstream face  34   a  of the second annular flange  34 . 
     The bearing shroud  346  has an inner face  346   a  and an outer face  346   b , a first free end  3461 , and a second end  3462  secured to the upstream face  34   a  of the second annular flange  34 , the first end  3461  being upstream of the second end  3462  when the turbine ring assembly is mounted. The bearing shroud  346  comprises, on its first end  3461 , a radial bearing  348  protruding from the outer face  346   b  of the bearing shroud  346 . The radial bearing  348  is in contact with the central shroud  31  of the ring support structure  3 . 
     The bearing shroud  346  ensures a higher resistance to the tilting induced by the DHP force. The bearing shroud  346  takes up the significant tangential stresses caused by the DHP force and thereby limits the tilting of the second annular flange  34 . 
     The second annular flange  34  ensures the connection between the downstream portion of the DHP, the ring support structure  3 , or casing, by radial surface contact, and the first annular flange  33  by axial surface contact. 
     The first and second annular flanges  33  and  34  are fastened, by shrink-fitting, to the ring support structure  3 . 
     The second annular flange  34  is shrink-fitted onto the central shroud  31  of the ring support structure  3 , the shrink-fitting being carried out, on the one hand, between the central shroud  31  and a portion  345  protruding from the contact abutment  340 , in the radial direction D R  away from the axis of revolution of the ring that is to say towards the central shroud  31  and, on the other hand, between the central shroud  31  and the radial bearing  348 . 
     The first annular flange  33  is shrink-fitted onto the first radial annular clamp  32  of the ring support structure  3 . More precisely, the shrink-fitting is carried out between a radial surface  335  approximately in the middle, in the radial direction D R , of the first annular flange  33  and a radial surface  325  at mid-height of the first radial annular clamp  32 , the two radial surfaces  335  and  325  facing each other, and even in contact with each other in the radial direction D R . The radial surface  335  of the first annular flange  33  extends over the entire circumference of the first annular flange  33 , and on the face of the first annular flange  33  facing the first annular clamp  32  and the first radial fastening tab  14 . More specifically, the radial surface  335  of the first annular flange  33  may be formed anywhere on the portion of the first annular flange  33  intended to be in contact with the first radial annular clamp  32 , the radial surface  325  of the first radial annular clamp  32  being formed at a corresponding height on the face of the first radial annular clamp  32  facing the first annular flange  33 . 
     The ring support structure  3  further comprises screws  38  which allow pressing the ring in a low radial position that is to say towards the flowpath, in a deterministic manner. There is indeed a clearance between the axial pins and the bores on the ring to compensate for the hot-operating differential expansion between the metal and the CMC elements. 
       FIG. 4  represents a schematic sectional view of a second embodiment of the turbine ring assembly. 
     The second embodiment of the invention illustrated in  FIG. 4  differs from the first embodiment illustrated in  FIGS. 1 to 3  mainly in that the second annular flange  34  is not in direct contact with the first annular flange  33 . 
     The first annular flange  33  and the second annular flange  34  are connected by an omega seal  40  allowing to ensure the sealing between the flowpath cavity and the off-flowpath cavity upstream of the ring  1 . 
     In the second embodiment, the second annular flange  34  does not comprise a contact abutment  340  unlike the first embodiment illustrated in  FIGS. 1 to 3 . 
     The bearing shroud  346  of the second annular flange  34  also comprises a radial bearing  348  protruding from the outer face  346   b  of the bearing shroud  346 . In  FIG. 4 , the radial bearing  348  is disposed on an upstream portion of the bearing shroud  346  without being directly on the first end  3461 , the radial bearing  348  may be disposed over the entire length of the outer face  34   b  in the axial direction D A , the most upstream position allowing an increased resistance. 
     In the second embodiment illustrated in  FIG. 4 , the first annular flange  33  is fastened to the first annular clamp  32  of the ring support structure  3  using screws  60  and fastening nuts  61 , the screws  60  passing through the second portion  334  of the first annular flange  33  as well as the upstream radial annular clamp  32 . 
     The radial bearing  348 , protruding in the radial direction D R  in a direction away from the axis of revolution of the ring  1 , comprises a first face  348   a  extending in the radial direction D R  and receiving the flow F and a second face  348   b  extending in the radial direction D R  and opposite to the first face  348   a , the second face  348   b  forming an axial shoulder bearing on a radial rib  314  of the central shroud  31 . The radial rib  314  protrudes in the radial direction D R  from the central shroud  31  in a direction towards the axis of revolution of the ring  1 . The radial rib  314  comprises a first face  314   a  extending in the radial direction D R  facing the flow F and in contact with the second face  348   b  of the radial bearing  348 , and a second face  314   b  extending in the radial direction D R  and opposite to the first face  314   a.    
     The axial shoulder formed by the second face  348   b  of the radial bearing  348  of the second annular flange  34  is pressed against the radial rib  314  of the central shroud  31  of the ring support structure  3 . A DHP casing, not represented in  FIG. 4 , located upstream of the second annular flange  34  ensures a blocking in the axial direction D A  of the second annular flange  34  on the other side of the radial rib  314 . The second annular flange  34  is thus held axially in position between the radial rib  314  and the DBH casing upstream of the second annular flange  34 . 
     At the radial level, there is a functional clearance between the radial bearing  348  of the second annular flange  34  and the central shroud  31  of the ring support structure  3 . This clearance has no influence on the behavior of the mounting, in particular in dynamics since the second annular flange  34  remains static during the operation of the engine. In addition, its radial positioning has no influence on the radial positioning of the other parts. 
       FIG. 5  represents a schematic sectional view of a third embodiment of the turbine ring assembly. 
     The third embodiment illustrated in  FIG. 5  differs from the first embodiment illustrated in  FIGS. 1 to 3  in that the ring sector  10  has, in the plane defined by the axial D A  and radial D R  directions, a K-shaped section instead of an inverted π-shaped section. 
       FIG. 6  represents a sectional view of a fourth embodiment of the turbine ring assembly. 
     The fourth embodiment illustrated in  FIG. 6  differs from the first embodiment illustrated in  FIGS. 1 to 3  in that the ring sector  10  has, in the plane defined by the axial D A  and radial R D  directions, on a portion of the ring sector  10 , an O-shaped section instead of an inverted π-shaped section, the ring section  10  being fastened to the ring support structure  3  by means of a screw  19  and a fastener  20 , the screws  38  being removed. 
     In each of the embodiments of the invention illustrated in  FIGS. 1 to 6 , in the axial direction D A , the second radial annular clamp  36  of the ring support structure  3  is separated from the first annular flange  33  by a distance corresponding to the spacing of the upstream and downstream radial attachment tabs  14  and  16  so as to maintain these between the first annular flange  33  and the second radial annular clamp  36 . 
     In the first and second embodiments illustrated in  FIGS. 1 to 4 , in order to hold the ring sectors  10 , and therefore the turbine ring  1 , in position, with the ring support structure  3 , the ring assembly comprises two first pins  119  cooperating with the upstream attachment tab  14  and the first annular flange  33 , and two second pins  120  cooperating with the downstream attachment tab  16  and the second radial annular clamp  36 . 
     In these two embodiments illustrated respectively in  FIGS. 1 to 3  and in  FIG. 4 , for each corresponding ring sector  10 , the second portion  334  of the first annular flange  33  comprises two orifices  3340  for receiving the two first pins  119 , and the third portion  365  of the radial annular clamp  36  comprises two orifices  3650  configured to receive the two second pins  120 . 
     For each ring sector  10 , each of the upstream and downstream radial attachment tabs  14  and  16  comprises a first end  141  and  161  secured to the outer face  12   b  of the annular base  12  and a second free end  142  and  162 . The second end  142  of the upstream radial attachment tab  14  comprises two first lugs  17  each including an orifice  170  configured to receive a first pin  119 . Similarly, the second end  162  of the downstream radial attachment tab  16  comprises two second lugs  18  each including an orifice  180  configured to receive a second pin  120 . The first and second lugs  17  and  18  protrude in the radial direction D R  of the turbine ring  1  respectively from the second end  142  of the upstream radial attachment tab  14  and from the second end  162  of the downstream radial attachment tab  16 . 
     The orifices  170  and  180  may be circular or oblong. Preferably, all the orifices  170  and  180  comprise a portion of circular orifices and a portion of oblong orifices. The circular orifices make it possible to tangentially index the rings and to prevent them from moving tangentially (in particular in the event of contact by the vane). The oblong orifices allow accommodating the differential expansions between the CMC and the metal. The CMC has a coefficient of expansion much lower than that of the metal. At high temperature, the lengths in the tangential direction of the ring sector and of the casing portion vis-à-vis each other will be different. If there were only circular orifices, the metal casing would impose its displacements to the CMC ring, which would be a source of very high mechanical stresses in the ring sector. Having oblong holes in the ring assembly allows the pin to slide into this hole and to avoid the overstress phenomenon mentioned above. Therefore, two drilling patterns can be imagined: a first drilling pattern, for a case with three lugs, would comprise a radial circular orifice on a radial attachment clamp and two tangential oblong orifices on the other radial attachment clamp, and a second drilling pattern, for a case with at least four lugs, would comprise a circular orifice and an oblong orifice by radial attachment clamp vis-à-vis each other each time. Other appended cases may be considered as well. 
     For each ring sector  10 , the two first lugs  17  are positioned at two different angular positions with respect to the axis of revolution of the turbine ring  1 . Likewise, for each ring sector  10 , the two seconds lugs  18  are positioned at two different angular positions with respect to the axis of revolution of the turbine ring  1 . 
     As illustrated in  FIG. 5 , in the third embodiment, each ring sector  10  has, along a plane defined by the axial D A  and radial D R  directions, a substantially K-shaped section comprising an annular base  12  with, along the radial direction D R  of the ring, an inner face  12   a  coated with a layer  13  of abradable material forming a thermal and environmental barrier and which defines the flow path of gas flow in the turbine. Substantially S-shaped upstream and downstream radial attachment tabs  140 ,  160  extend, along the radial direction D R , from the outer face  12   b  of the annular base  12  over the entire width thereof and above the upstream and downstream circumferential end portions  121  and  122  of the annular base  12 . 
     The radial attachment tabs  140  and  160  have a first end, referenced respectively  1410  and  1610 , secured to the annular base  12  and a second free end, referenced respectively  1420  and  1620 . The free ends  1420  and  1620  of the upstream and downstream radial attachment tabs  140  and  160  extend either parallel to the plane in which the annular base  12  extends, that is to say along a circular plane, or rectilinearly while the attachment tabs  140  and  160  extend annularly. In this second configuration where the ends are rectilinear and the annular attachment tabs, in the case of a possible swing of the ring during the operation, the surface bearings then become linear bearings thereby providing a greater sealing than in the case of ad hoc bearings. The second end  1620  of the downstream radial attachment tab  160  is held between a portion  3610  of the second radial annular clamp  36  protruding in the axial direction D A  from the first end  361  of the second radial annular clamp  36  in the opposite direction to the flow F direction and the free end of the associated screw  38 , that is to say the screw opposite to the screw head. The second end  1410  of the upstream radial attachment tab  140  is held between a portion  3310  of the first annular flange  33  protruding in the axial direction D A  from the first end  331  of the first annular flange  33  in the flow F direction and the free end of the associated screw  38 . 
     In the fourth embodiment illustrated in  FIG. 6 , the ring sector  10  comprises an axial attachment tab  17 ′ extending between the upstream and downstream radial attachment tabs  14  and  16 . The axial attachment tab  17 ′ extends more precisely, in the axial direction D A , between the second end  142  of the upstream radial attachment tab  14  and the second end  162  of the downstream radial attachment tab  16 . 
     The axial attachment tab  17 ′ comprises an upstream end  171 ′ and a downstream end  172 ′ separated by a central portion  170 ′. The upstream and downstream ends  171 ′ and  172 ′ of the axial attachment tab  17 ′ protrude, in the radial direction D R , from the second end  142 ,  162  of the radial attachment tab  14 ,  16  to which they are coupled, so as to have a central portion  170 ′ of axial attachment tab  17 ′ raised relative to the second ends  142  and  162  of the upstream and downstream radial attachment tabs  14  and  16 . 
     For each ring sector  10 , the turbine ring assembly comprises a screw  19  and a fastener  20 . The fastener  20  is fastened on the axial attachment tab  17 ′. 
     The fastener  20  further comprises an orifice  21  equipped with a tapping cooperating with a thread of the screw  19  to fasten the fastener  20  to the screw  19 . The screw  19  comprises a screw head  190  whose diameter is greater than the diameter of an orifice  39  made in the central shroud  31  of the support structure of the ring  3  through which the screw  19  is inserted before being screwed to the fastener  20 . 
     The radial securing of the ring sector  10  with the ring support structure  3  is carried out using the screw  19 , whose head  190  bears on the central crown  31  of the ring support structure  3 , and the fastener  20  screwed to the screw  19  and fastened to the axial attachment tab  17 ′ of the ring sector  10 , the screw head  190  and the fastener  20  exerting forces of opposite directions in order to hold together the ring  1  and the ring support structure  3 . 
     In a variant, the radial holding of the ring downwards can be ensured using four radial pins plated on the axial attachment tab  17 ′, and the radial holding of the ring upwards can be ensured by a pickaxe head, secured to the screw  19 , placed under the ring in the cavity between the axial attachment tab  17 ′ and the outer face  12   b  of the annular base. 
     In each of the embodiments of the invention illustrated in  FIGS. 1 to 6 , each ring sector  10  further comprises rectilinear bearing surfaces  110  mounted on the faces of the upstream and downstream radial attachment tabs  14  and  16  in contact respectively with the first annular flange  33  and the second radial annular clamp  36 , that is to say on the upstream face  14   a  of the upstream radial attachment tab  14  and on the downstream face  16   b  of the downstream radial attachment tab  16 . In a variant, the rectilinear bearings could be mounted on the first annular flange  33  and on the second downstream radial annular clamp  36 . 
     The rectilinear bearings  110  allow having controlled sealing areas. Indeed, the bearing surfaces  110  between the upstream radial attachment tab  14  and the first annular flange  33  on the one hand, and between the downstream radial attachment tab  16  and the second radial annular clamp  36  on the other hand, are comprised in the same rectilinear plane. 
     More precisely, having bearings on radial planes allows overcoming the effects of de-cambering in the turbine ring  1 . 
     A method for producing a turbine ring assembly corresponding to that represented in  FIG. 1 , that is to say according to the first embodiment illustrated in  FIGS. 1 to 3 , is now described. 
     Each ring sector  10  described above is made of ceramic-matrix composite material (CMC) by formation of a fibrous preform having a shape close to that of the ring sector and densification of the ring sector by a ceramic matrix. 
     For the production of the fibrous preform, it is possible to use ceramic fiber yarns, for example SiC fiber yarns, such as those marketed by the Japanese company Nippon Carbon under the name “Hi-NicalonS”, or carbon fiber yarns. 
     The fibrous preform is advantageously made by three-dimensional weaving, or multilayer weaving with arrangement of debonding areas allowing the portions of preforms corresponding to the attachment tabs  14  and  16  of the sectors  10  to be spaced apart. 
     The weaving can be of the interlock type, as illustrated. Other weaves of three-dimensional or multilayer weaving can be used such as for example multi-plain or multi-satin weaves. Reference can be made to document WO 2006/136755. 
     After weaving, the blank can be shaped to obtain a ring sector preform which is consolidated and densified by a ceramic matrix, the densification can be achieved in particular by gas-phase chemical infiltration (CVI) which is well known per se. In a variant, the textile preform can be a little cured by CVI so that it is rigid enough to be manipulated, before raising liquid silicon by capillarity in the textile for carrying out the densification (“Melt Infiltration”). 
     A detailed example of manufacture of CMC ring sectors is in particular described in document US 2012/0027572. 
     The ring support structure  3  is for its part made of a metal material such as a Waspaloy® or inconel 718® or C263® alloy. 
     The production of the turbine ring assembly is continued by the mounting of the ring sectors  10  on the ring support structure  3 . 
     For this, the ring sectors  10  are assembled together on an annular tool of the “spider” type including, for example, suckers configured to each hold a ring sector  10 . 
     Then, the two second pins  120  are inserted into the two orifices  3650  provided in the third portion  365  of the second radial annular clamp  36  of the ring support structure  3 . 
     The ring  1  is then mounted on the ring support structure  3  by inserting each second pin  120  into each of the orifices  180  of the second lugs  18  of the downstream radial attachment clamps  16  of each ring sector  10  forming the ring  1 . 
     All the first pins  119  are then placed in the orifices  170  provided in the first lugs  17  of the radial attachment tab  14  of the ring  1 . 
     Then, the first annular flange  33  and the second annular flange  34  are fastened to the ring support structure  3  and to the ring  1 . The first and second annular flanges  33  and  34  are fastened by shrink-fitting to the ring support structure  3 . The DHP force exerted in the direction of the flow F reinforces this fastening during the operation of the engine. 
     It should be noted that in the case of a method for producing a turbine ring assembly corresponding to that represented in  FIG. 4 , the mounting is carried out by fastening the first flange  33  to the ring support structure  3  by bolted connection, then by putting the omega seal  40  in place in the groove provided for this purpose in the first flange  33  before assembling the second flange  34  to the ring support structure  3 . 
     In order to radially hold the ring  1  in position, the first annular flange  33  is fastened to the ring by inserting each first pin  119  into each of the orifices  170  of the first lugs  17  of the upstream radial attachment tabs  14  of each ring sector  10  forming the ring  1 . 
     The ring  1  is thus axially held in position using the first annular flange  33  and the second radial annular clamp  36  bearing respectively upstream and downstream on the rectilinear bearing surfaces  110  of the respectively upstream  14  and downstream  16  radial attachment tabs. During the installation of the first annular flange  33 , an axial pre-stressing may be applied to the first annular flange  33  and to the upstream radial attachment tab  14  to overcome the effect of differential expansion between the CMC material of the ring  1  and the metal of the ring support structure  3 . The first annular flange  33  is maintained in axial stress by mechanical elements placed upstream as illustrated in dashed lines in  FIG. 3 . 
     The ring  1  is radially held in position using the first and second pins  119  and  120  cooperating with the first and second lugs  17  and  18  and the orifices  3340  and  3650  of the first annular flange  33  and the radial annular clamp  36 . 
     The invention thus provides a turbine ring assembly allowing to maintain each ring sector in a deterministic manner while allowing, on the one hand, the ring sector, and by extension the ring, to deform under the effects of temperature rises and pressure variations, and in particular independently of the interface metal parts and, on the other hand, while improving the sealing between the off-flowpath sector and the flowpath sector and while simplifying manipulations and reducing their number for the mounting of the ring assembly. 
     In addition, the invention provides a turbine ring assembly comprising an upstream annular flange dedicated to take up the DHP force and thus to induce low levels of forces in the CMC ring, a contact abutment between the annular flange dedicated to take up the DHP force and the annular flange used to maintain the ring, the abutment allowing to ensure the non-contact of the low portions of the two flanges upon tilting of the upstream flange. The turbine ring assembly according to the invention also allows controlling the rigidity at the upstream and downstream axial contacts between the CMC ring and the metal casing. As a result, the sealing is ensured in all circumstances without inducing too high axial forces on the ring.