Abstract:
A stator vane assembly for a gas turbine includes a ceramic matrix composite airfoil held between radially inner and outer metal platforms wherein an interface between the airfoil and at least one of the radially inner and outer platforms is shaped to create a circuitous leakage path for gas from the gas turbine hot gas path.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to turbine nozzle assemblies and specifically, to platform interface configurations for stage 2 CMC nozzle vanes. 
   Sealing between high temperature components such as ceramic matrix composite (CMC) nozzle vanes and radially inner and outer metallic attachments or platforms creates problems relating to steep thermal gradients with associated high thermal stresses and reduced component life; internal pressure due to cooling air resulting in air flow wall distortion; and time varying performance erosion due to historical seal degradation. Eliminating the seal between a CMC vane and metal inner and outer platforms, however, results in an open channel for hot gas ingestion. Accordingly, there remains a need for a new geometry at the interface of the CMC vane and either one or both of the radially inner and outer metallic platforms that accommodates the inherent difficulties in the matching of ceramic and metal components, and that also eliminates the need for separate and discrete sealing elements. Seal-less design is also synonymous with unpressurized vane design. 
   BRIEF DESCRIPTION OF THE INVENTION 
   Controlled leakage is the key to the success of a seal-less design. Controlled leakage can be accommodated by creative interface configurations on the platform interface surface, the vane interface surface, or both. In the exemplary embodiments of this invention, creative interface configurations are provided that establish a circuitous gas leak path for increased flow resistance, resulting in the desired controlled leakage. 
   In the various embodiments described herein, a CMC stator vane (also referred to herein as an airfoil shell or, simply airfoil) is assembled between a pair of radially inner and outer metal platforms that may be radially interconnected by a pair of spars extending through the airfoil shell. Each of the platforms is formed on its interior face with an airfoil-shaped recess adapted to receive the CMC airfoil shell. The seal-less configurations described herein are located on the airfoil shell and/or on adjacent interior peripheral surfaces of the airfoil-shaped recesses on the inner and/or outer platforms. 
   In one exemplary embodiment, mating step joints are formed on the peripheral surface of each platform recess and the respective adjacent airfoil shell surfaces. 
   In a second exemplary embodiment, the interface configuration is in the form of a scarf joint, i.e., with mating angled surfaces extending about the adjacent peripheries of each platform recess and respective airfoil shell surface. 
   In a third exemplary embodiment, the platform airfoil surfaces are formed with a plurality of laterally projecting, abradable knife edges that interface with adjacent smooth surfaces on the airfoil shell. 
   In a fourth exemplary embodiment, a compliant or spring interface is provided on the peripheral surface of each platform recess for engagement with a respective smooth surface on the adjacent airfoil shell. It will be appreciated that the free end or edge surface of the compliant interface may also be formed with a step joint or scarf joint as described above, to interface with the adjacent mating surface on the respective airfoil shell to provide the desired circuitous or tortuous path. 
   Accordingly, in one aspect, the present invention relates to a stator vane assembly for a gas turbine comprising a ceramic matrix composite airfoil held between radially inner and outer metal platforms wherein an interface between the airfoil and at least one of the radially inner and outer platforms is shaped to create a circuitous leakage path for gas from the gas turbine hot gas path. 
   In another aspect, the invention relates to a stator vane assembly for a gas turbine comprising a ceramic matrix composite airfoil held between radially inner and outer metal platforms wherein each of the platforms is formed with a recess adapted to receive the inner and outer platforms, each recess including a peripheral edge, the peripheral edge shaped to create the circuitous leakage path in cooperation with an adjacent surface on the airfoil. 
   In still another aspect, the invention relates to a stator vane assembly for a gas turbine comprising a ceramic matrix composite vane held between radially inner and outer metal platforms wherein an interface between the vane and at least one of the radially inner and outer platforms is shaped to provide a compliant face for engagement with a smooth surface on the vane. 
   The invention will now be described in detail in connection with the drawings identified below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective exploded view of a CMC airfoil shell and associated inner and outer metal platforms connected by radial spars; 
       FIG. 2  is a schematic of a baseline or reference configuration at the interface of a CMC airfoil shell and a radially inner metal platform; 
       FIG. 3  is a schematic of a step joint interface between a CMC airfoil shell and an inner metal platform in accordance with a first exemplary embodiment of the invention; 
       FIG. 4  is a schematic of a scarf joint interface between a CMC airfoil shell and an inner metal platform in accordance with a second exemplary embodiment of the invention; 
       FIG. 5  is a schematic of an abradable knife edge interface between a CMC airfoil shell and an inner metal platform in accordance with a third exemplary embodiment of the invention; 
       FIG. 6  is a schematic illustrating a compliant interface between a CMC airfoil shell and an inner metal platform in accordance with a fourth exemplary embodiment of the invention; 
       FIG. 7  is a schematic of a combined compliant/step joint interface between a CMC airfoil shell and an inner metal platform in accordance with a fourth exemplary embodiment of the invention; and 
       FIG. 8  is a schematic of a combined compliant scarf joint interface between a CMC airfoil shell and an inner metal platform in accordance with a fifth exemplary embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , a CMC airfoil shell and metal platform assembly  10  is shown in exploded form. More specifically, a pair of radially inner and outer metal platforms  12 ,  14  are interconnected by a pair of radial spars  16 ,  18 . A pair of airfoil-shaped recesses  20 ,  22  are formed in the metal platform surfaces  24 ,  26 , respectively, with the open sides of the recesses facing each other. In the exemplary embodiment, the larger spar  18  is in the shape of a hollow channel that supplies cooling air to the airfoil shell  28 . In this regard, the airfoil shell  28  is a hollow member that can be slidably received over the spars during assembly, with opposite ends of the airfoil shell received in the recesses  20 ,  22 . Note that in the exemplary embodiment, the platforms  12 ,  14  are each formed with two recesses such that a pair of adjacent airfoil shells may be supported between the inner and outer platforms. 
   In an alternative arrangement, the spars  16 ,  18  could be combined into a single airfoil-shaped channel, sized to also receive the external airfoil shell  28  in telescoping relationship, with appropriate dimensional tolerances. 
   As illustrated in  FIG. 1 , the recesses  20 ,  22  are shaped in a manner complementary to the airfoil shell  28 . It will be appreciated that the tolerances between the airfoil shell and the platform recesses must be controlled to avoid harmful excessive vibration, but at the same time, avoid problems associated with thermal mismatch between the components. 
   Turning to  FIG. 2 , the airfoil shell  28  is schematically represented as seated in the airfoil-shaped recess  20  of the inner metal platform  24 . The recess  20  is defined by the closed peripheral edge  30  that interfaces with surfaces  32 ,  34  on the pressure and suction sides of the airfoil shell  28 . This illustration provides a baseline reference for the interface configurations described below. In this regard, the unique interface configurations described herein are formed at the interface between recess surface  30  and opposed surfaces  32 ,  34  of the airfoil shell at the radially inner platform  24 , and/or substantially identically at the radially outer platform  14 . For convenience, only the interfaces at the radially inner platforms are shown. 
   With reference now to  FIG. 3 , the CMC airfoil shell  36  is shown in assembled relationship with an inner metal platform  38 . In this example, the interface configuration (or simply interface) is in the form of a step joint, with at least two, laterally oriented steps  40 ,  42 , oriented perpendicular to a radial centerline through the vane, formed in the peripheral edge  43  of the lower platform recess  44  engaged with lateral shoulders  46 ,  48  formed at the lower end of the airfoil shell  36 . Note that this arrangement allows the insertion of the airfoil shell from below the lower platform  38 . The step joint at the opposite end of the airfoil shell would be reversed, however, to permit one-way installation of the shell  36  between both the inner and outer platforms. With appropriate tolerances between the interfacing surfaces, it will be appreciated that any gas leaking out of the hot gas path of the turbine, will necessarily be forced to follow a circuitous route through the interface, establishing the desirable controlled leakage, and without having to use discrete sealing elements. 
   With reference now to  FIG. 4 , another interface is illustrated that is of simpler design than the configuration in  FIG. 3 . Specifically, a CMC airfoil shell  50  is shown in assembled relationship with respect to an inner metal platform  52 . In this embodiment, the radially inner platform recess  54  is formed with a peripheral edge surface  56  that is slanted at about a 45° angle to a radial centerline through the airfoil shell  50 . At the same time, the lower surface  58  of the airfoil shell  50  is formed at a similar angle, thus forming a scarf joint between the airfoil shell and the inner platform  52 . Here again, for purposes of facilitating one-way installation, the interface at the upper end of the airfoil shell would be reversed. 
   In  FIG. 5 , yet another embodiment is shown where a CMC airfoil shell  58  is seated within the recess  62  in the inner metal platform  60 . In this embodiment, the recess  62  in the platform  60  is formed with a peripheral edge  63  made up of a plurality (for example, at least four) of inwardly projecting abradable knife edges  64  (four shown), spaced from each other in the radial direction. The edges  64  interface with an adjacent smooth surface  66  on the airfoil shell  58 , with appropriate tolerance between the two. Here again, it will be appreciated that resistance to leakage gas is increased by reason of the circuitous path through the platform. 
   In  FIG. 6 , a compliant interface is provided between a CMC airfoil shell  68  and an inner metal platform  70 . In this embodiment, the recess  72  in the inner platform is formed with a peripheral edge having oppositely directed cutouts or slots  74 ,  76  extending in a radial direction that, in effect, permit the edge  80  of the recess  72  to act in the nature of a spring, in compliant or resilient “engagement” (i.e., with minimal clearance) with an adjacent smooth surface  78  of the airfoil shell. In order to incorporate the circuitous leakage gas feature of the earlier-described embodiments, it will be appreciated that the edge  80  of the platform recess  72  may be configured to incorporate a step joint as illustrated in  FIG. 3  or a scarf joint as illustrated in  FIG. 4 . These alternative interface configurations are shown schematically in  FIGS. 7  and  8 , respectively. Specifically,  FIG. 7  shows a compliant step joint where the CMC airfoil shell  82  is seated within the recess  84  in an inner metal platform  86 , with the edge  88  of the compliant recess (formed by slots  90 ) formed with a step joint  92  that interfaces with a complementary step joint  94  on the airfoil shell. 
   In  FIG. 8 , the CMC airfoil shell  96  is seated within the recess  98  in an inner metal platform  100 , with the edge  102  of the recess  84  (formed by slots  104 ) formed with an angled surface  106  that interfaces with a complementary angled peripheral surface  108  on the airfoil shell  96 , thus forming a compliant scarf joint at the interface. 
   By providing increased flow resistance resulting in controlled leakage, it is possible to eliminate the steep thermal gradients and associated reduction in thermal stresses and increased component life; thinner wall sections of the CMC vane to the elimination of internal pressure due to cooling air; and robust consistent performance by eliminating seal degradation. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.