Patent Publication Number: US-10329930-B2

Title: Turbine ring assembly with sealing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to French Patent Application No. 1552815, filed Apr. 1, 2015, the entire contents of this application is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a turbine ring assembly for a turbine engine, which assembly comprises a ring support structure and a plurality of single-piece ring sectors made of ceramic matrix composite material. 
     The field of application of the invention is in particular that of gas turbine aeroengines. Nevertheless, the invention is applicable to other turbine engines, e.g. industrial gas turbines. 
     Ceramic matrix composite materials (CMCs) are known for conserving their mechanical properties at high temperatures, thereby making them suitable for constituting hot structural elements. 
     In gas turbine aeroengines, improving efficiency and reducing certain polluting emissions has led to seeking operation at ever-higher temperatures. When turbine ring assemblies are made entirely out of metal, it is necessary to cool all of the elements of the assembly, and in particular the turbine ring, which is subjected to very hot streams, typically above the temperature that can be withstood by the metal material. Such cooling has a significant impact on the performance of the engine since the cooling stream that is used is taken from the main stream through the engine. In addition, the use of metal for the turbine ring puts a limit on potential increases of temperature in the turbine, even though such increases would nevertheless make it possible to improve the performance of aeroengines. 
     That is why it has already been envisaged to use CMCs for various hot portions of engines, particularly since CMCs present the additional advantage of density that is lower than that of the refractory metals that have traditionally been used. 
     Thus, making turbine ring sectors as single pieces of CMC is described in particular in Document US 2012/0027572. Each ring sector comprises an annular base having an inside face that defines the inside face of the turbine ring and an outside face from which there extend two tab-forming portions with ends that are engaged in housings of a metal ring support structure. 
     The use of CMC ring sectors makes it possible to reduce significantly the ventilation needed for cooling the turbine ring. Nevertheless, holding the ring sectors in position remains a problem, in particular in the face of differential expansion that can occur between the metal support structure and the CMC ring sectors. In addition, another problem lies in the stresses generated by the imposed movements. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The invention seeks to avoid such drawbacks, and for this purpose it proposes a turbine ring assembly comprising a ring support structure and a plurality of ring sectors made of ceramic matrix composite material making up a turbine ring, each ring sector comprising an annular base with, in a radial direction of the turbine ring, an inside face defining the inside face of the turbine ring and an outside face facing the inside face of the ring support structure, each said annular base including at each circumferential end a circumferential edge that is held facing a circumferential edge of the circumferential end of the annular base of a ring sector that is adjacent in the turbine ring, the assembly being characterized in that said assembly comprises resilient holder devices, each resilient holder device exerting a force on the circumferential ends of two adjacent ring sectors, holding said adjacent sectors in position, and in that each resilient holder device comprises a spring element present beside the outside face of the ring support structure. 
     By using resilient holder devices, it is ensured that the ring sectors are held in position in the event of differential expansion between the sectors and the support structure, with such movements being compensated by the resilient holding. In addition, by placing the ring element of each resilient holder device beside the outside face of the ring support structure, the spring element is spaced away from the hot stream flowing in the passage and is exposed only to temperatures that are compatible with the material of the spring. There is therefore no need to cool the spring elements, and it is then possible to use more ordinary materials for fabricating them, such as metal materials. 
     According to a characteristic of the turbine ring assembly of the invention, it includes a plurality of gaskets, each gasket extending axially over the circumferential ends of two adjacent sectors at the facing edges of said ends. The resilient holder devices exert force that holds the gaskets in contact with the circumferential ends of two adjacent ring sectors. 
     By placing and holding a gasket over the zone where the axial edges of the sectors face one another in the ring, leaks of the gas stream flowing inside the passage formed by the inside face of the ring sectors are limited. In addition, since the gaskets are held by resilient holder devices, the gaskets are held in position and consequently the passage is sealed. 
     According to an embodiment of the turbine ring assembly of the invention, the ring support structure has an upstream annular radial flange and a downstream annular radial flange with the ring sectors being held between them without being attached to said flanges, each gasket having an upstream end passing through a slot formed in the upstream radial flange and a downstream end passing through a slot formed in the downstream radial flange. Each ring sector presents a K-shape in a plane defined by the radial direction and the circumferential direction of the turbine ring, with tabs extending from the outside face of the annular base over the circumferential ends of said annular base, circumferential edges of the tabs and the circumferential edges of the circumferential ends of each ring sector being held respectively facing the circumferential edges of tabs and the circumferential edges of ring sectors that are adjacent in the ring. Each resilient holder device exerts a force on the facing tabs of two adjacent ring sectors that is directed radially towards the outside of the turbine ring, holding the corresponding gasket in contact with the circumferential ends of two adjacent ring sectors. Since the ring sectors are not fastened directly to the support structure, the imposed movements are significantly reduced, and consequently the stresses on the ring sectors are significantly reduced. The ring sectors can thus be positioned more easily relative to one another in order to define a more coherent shape for the turbine ring. 
     In an aspect of the embodiment of the turbine ring assembly of the invention, the gaskets are constituted by strips of ceramic matrix composite material. 
     In another aspect of the embodiment of the turbine ring assembly of the invention, each resilient holder device comprises a bolt and a spring, the bolt having a head present between the outside face of a gasket and the tabs of two adjacent sectors, the spring being mounted in a prestressed state between a shroud of the ring support structure and a nut fastened to the end of the bolt remote from its end having the head. 
     In another embodiment of the turbine ring assembly of the invention, the ring support structure includes tabs that extend from the inside face of a shroud, each tab including a folded-over portion. Each ring sector includes tabs extending from the outside face of its annular base, each tab having a folded-over portion engaged with the folded-over portion of a tab of the ring structure. Each resilient holder device exerts a pressing force on each gasket holding the gasket in contact with the circumferential ends of two adjacent ring sectors. 
     In another aspect of the embodiment of the turbine ring assembly of the invention, each resilient holder device comprises a shoe in contact with the top surface of the gasket, a rod extending vertically from the outside surface of the shoe, and a spring exerting a pressing force on the free end of the rod. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood on reading the following description given by way of non-limiting indication and with reference to the accompanying drawings, in which: 
         FIGS. 1 and 2  are perspective views showing a portion of a turbine ring assembly in accordance with an embodiment of the invention; 
         FIG. 3  is a radial section view of the turbine ring assembly of  FIGS. 1 and 2 ; and 
         FIG. 4  is a perspective view showing a portion of a turbine ring assembly in accordance with another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIGS. 1 and 2  show a high-pressure turbine ring assembly  10  in an embodiment of the invention. The assembly  10  comprises a CMC turbine ring  11  and a metal ring support structure  12 . The turbine ring  11  surrounds a set of rotary blades  20 . The turbine ring  11  is made up of a plurality of ring sectors  110 , with  FIGS. 1 and 2  being perspective views showing a portion of the high-pressure turbine ring assembly  10  with an axial section showing the edges of a ring sector  110 . Arrow D A  points in the axial direction of the turbine ring  11  and arrow D R  points in the radial direction of the turbine ring  11 . 
     As shown in  FIG. 3 , each ring sector  110  is K-shaped in a plane defined by the radial direction D R  and by the circumferential direction of the turbine ring  11 , the sector having an annular base  111  with its inside face in the radial direction D R  coated in a layer  112  of abradable material, this inside face defining the flow passage for the gas stream through the turbine. Substantially S-shaped tabs  113 ,  114  extend from the outside face of the annular base  111  in the radial direction D R , over its entire width, and above circumferential ends  1110  and  1111  of the annular base  111 . Each annular sector  110  thus has two circumferential edges  1110   a  &amp;  113   a  and  1111   b  &amp;  114   b  at each of its ends in the circumferential direction of the ring  11 . The edges  1110   a  and  113   a  situated on a first end of a sector  110  are for being held facing respective edges  1111   b  and  114   b  of the ring sector that is adjacent in the turbine ring. 
     The ring support structure  12  is secured to a turbine casing  13 . The structure  12  has an upstream annular radial flange  121  and a downstream annular radial flange  122  that extend from a shroud  123  of the turbine casing. The terms “upstream” and “downstream” are used herein with reference to the flow direction of the gas stream through the turbine (arrow F in  FIGS. 1 and 2 ). The flanges  121  and  122  present respective bottom edges  121   a  and  122   a.    
     The ring sectors  110  are arranged in annular manner between the flanges  121  and  122  of the metal ring support structure  12 , the inside face of the ring having the layer  112  of abradable material extending beyond the bottom edges  121   a  and  122   a  of the flanges  121  and  122 . 
     In order to provide good sealing between the flow passage for the gas stream through the turbine and the outside of the turbine ring, gaskets  130  are placed between adjacent ring sectors at their facing edges. More precisely, the gaskets  130  are dimensioned and placed in such a manner as to cover the end portions  1111  and  1110  of the annular bases  111  of two adjacent ring sectors  110 . The gaskets  130  are placed in respective housings  115 , each having its bottom formed by the circumferential ends  1111  and  1110  of two adjacent sectors in combination, the top portion of each housing  115  being formed by the tabs  114  and  113  of two adjacent sectors in combination. In this example, the gaskets  130  are made of CMC. The upstream ends  131  and the downstream ends  132  of the gaskets  130  pass through respective slots  1210  and  1220  formed respectively in the upstream and downstream flanges  121  and  122  ( FIGS. 1 and 2 ). 
     The two adjacent ring sectors  110  and the gasket  130  present between the two adjacent sectors are held by a traction device  140  constituted by a bolt  141  and a spring  142 . The bolt  141  has a head  1410  that is placed between the outer face  130   a  of the corresponding gasket  130  and the tabs  114  and  113  of two adjacent sectors. Notches  1140  and  1130  are formed respectively in the tabs  114  and  113  so as to pass the shank  1411  of the bolt  141 . Likewise, orifices  1310  are formed in the shroud  123  of the turbine casing  13  so as to pass the shank  1411  of the bolt  141 . 
     The spring  142  is a compression spring mounted in a prestressed state between the shroud  123  and a nut  1412  engaged on the end of the bolt  141  remote from its end having the head  1410 . Thus, the spring  142  exerts a force on the nut  1412  that is directed radially towards the outside of the ring  11  in a direction D shown in  FIGS. 1 and 3  and transmitted to the head  1410  of the bolt  141  via the shank  1411  of the bolt. The head  1410  then exerts a force that is directed in the direction D on the tabs  113  and  114  of two adjacent sectors  110 . This force is also transmitted to the circumferential ends  1110  and  1111  of two adjacent sectors  110  that in turn exert a force FT that is directed radially towards the outside of the ring  11  against the gasket  130  interposed between the circumferential ends  1110  and  1111  of the tabs  113  and  114  of two adjacent sectors  110 . Under the effect of this force, the gaskets  130  are held in abutment against the top portions of the slots  1210  and  1220  formed respectively in the flanges  121  and  122 . Sealing between adjacent sectors, i.e. sealing between the gas flow passage on the inside of the ring sectors and on the outside of the ring sectors, is thus provided by the gaskets  130 . In addition, since both the ring sectors  110  and the gaskets  130  are held in position by resilient means (springs  142 ), mechanical connection and sealing between the ring sectors is ensured even when movements are imposed by differential thermal expansion. 
     Since each spring  142  is placed beside the outside face of the ring support structure (outside face of the shroud  123 ), it is spaced away from the hot stream flowing in the passage and is exposed only to temperatures that are compatible with the material of the spring. There is therefore no need to cool the springs, and it is possible to use materials such as metal materials for fabricating them. 
       FIG. 4  shows a high-pressure turbine ring assembly  30  in accordance with another embodiment of the invention. The assembly  30  comprises a CMC turbine ring  31  and a metal ring support structure  32 . The turbine ring  31  surrounds a set of rotary blades  40 . The turbine ring  31  is made up of a plurality of ring sectors  310 , with  FIG. 4  being a perspective view showing a portion of the high-pressure turbine ring assembly  30  with an axial section showing the edge of a ring sector. 
     As shown in  FIG. 4 , each ring sector  310  comprises an annular base  311  having its inside face coated in a layer  312  of abradable material defining the flow passage for the gas stream through the turbine. Tabs  313 , each having a folded-over portion  3130 , extend from the outside face of the annular base  311  at circumferential ends  3110  and  3111  of the annular base  311 . In the presently-described example, each sector  310  has four tabs  313  arranged in pairs on each annular edge of the sector, the folded-over portion  3130  of each tab  313  extending axially over the annular base  311 . Each annular sector  310  has a first circumferential edge  3110   a  and a second circumferential edge  3110   b . The edge  3110   a  situated at a first end of a sector  310  is for being held facing the edge  3110   b  of the ring sector that is adjacent in the turbine ring. 
     The ring support structure  32  is secured to a turbine casing (not shown in  FIG. 4 ). The structure  32  has tabs  320  that extend from the inside face of a shroud  50  that may form part of the turbine casing (not shown in  FIG. 4 ). Each tab  320  has a folded-over portion  3200  that is to co-operate with tabs  313  of the ring sectors  310 . The ring sectors  310  are arranged in annular manner beneath the shroud  50 , with the folded-over portions  3130  of the tabs  313  being engaged with the folded-over portions  3200  of the tabs  320 . 
     In order to provide good sealing between the flow passage for the gas stream through the turbine and the outside of the turbine ring, gaskets  330  are placed between adjacent ring sectors at their facing edges. More precisely, the gaskets  330  are dimensioned and placed in such a manner as to cover the circumferential ends  3111  and  3110  of two adjacent ring sectors  310 . By way of example, the gaskets  330  are made of a thermally insulating material, such as a felt of oxide (alumina) fibers, or each of them made be made of an elastically deformable insulating material such as a fiber structure or an insulating foam, that is held inside a braid made with fibers that withstand high temperatures, such as ceramic fibers. 
     The ring sectors  310  and the gaskets  330  are held by a pressing device  340  constituted by a shoe  341  in contact with the top surface of the gasket  330 , a rod  343  extending vertically from the outside surface of the shoe, and a spring  342  exerting a pressing force on the free end  3430  of the rod  343 . Orifices  51  are formed through the shroud  50  to allow the rod  343  to pass. 
     The spring  342  is a compression spring mounted in the prestressed state between the free end  3430  of the rod  343  and an annular tab  52  extending from the outside surface  50   a  of the shroud  50 . The tab  52  defines a housing between the inside surface  52   b  of said tab and the outside surface  50   a , the height H 2  of the housing being defined as a function of the height H 1  of the rods  343  so that H 1 &lt;H 2 , thereby enabling the springs  342  to be mounted in a prestressed state. 
     Thus, the spring  342  exerts a force on the rod  343  that is directed radially towards the inside of the ring  31  in a direction DI shown in  FIG. 4  and that is transmitted to the shoe  341  via the rod  343 . The shoe  341  then exerts a pressing force F A  that is directed radially towards the inside of the ring  31  against the gasket  330  and against the circumferential ends  3110  and  3111  of two adjacent sectors  310  via said gaskets  330 . Sealing between adjacent sectors, i.e. sealing between the gas flow passage on the inside of the ring sectors and on the outside of the ring sectors, is thus provided by the gaskets  330 . In addition, since both the gaskets  330  and the ring sectors  310  are held in position by corresponding resilient means (springs  342 ), mechanical connection and sealing between the ring sectors is ensured even in the event of movements imposed by differential thermal expansion. 
     Since each spring  342  is placed beside the outside face of the ring support structure (outside face  50   a  of the shroud  50 ), it is spaced apart from the hot stream flowing in the passage and it is exposed only to temperatures that are compatible with the material of the spring. There is thus no need to cool the springs and materials such as metal materials can be used for fabricating them. 
     Each above-described ring sector is made of CMC by forming a fiber preform of shape close to the shape of the ring sector and by densifying the ring sector with a ceramic matrix. 
     In order to make the fiber preform, it is possible to use yarns made of ceramic fibers, e.g. SiC fiber yarns such as those sold by the Japanese supplier Nippon Carbon under the name “Nicalon”, or carbon fiber yarns. 
     The fiber preform is advantageously made by three-dimensional weaving, or by multilayer weaving with zones of non-interlinking being provided to make it possible to space preform portions corresponding to the tabs  113  and  114  apart from the sectors  110  or corresponding to tabs  313  apart from the sectors  310 . 
     The weaving may be of the interlock type, as shown. Other three-dimensional or multilayer weaves can be used, such as for example multi-plain or multi-satin weaves. Reference may be made to Document WO 2006/136755. 
     After weaving, the blank may be shaped in order to obtain a ring sector preform that is consolidated and densified with a ceramic matrix, it being possible for densification to be performed in particular by chemical vapor infiltration (CVI) as is well known. 
     A detailed example of fabricating CMC ring sectors is described in particular in Document US 2012/0027572.