Abstract:
A hybrid vane ( 50 ) for a gas turbine engine having a ceramic matrix composite (CMC) airfoil member ( 52 ) bonded to a substantially solid core member ( 54 ). The airfoil member and core member are cooled by a cooling fluid ( 58 ) passing through cooling passages ( 56 ) formed in the core member. The airfoil member is cooled by conductive heat transfer through the bond (( 70 ) between the core member and the airfoil member and by convective heat transfer at the surface directly exposed to the cooling fluid. A layer of insulation ( 72 ) bonded to the external surface of the airfoil member provides both the desired outer aerodynamic contour and reduces the amount of cooling fluid required to maintain the structural integrity of the airfoil member. Each member of the hybrid vane is formulated to have a coefficient of thermal expansion and elastic modulus that will minimize thermal stress during fabrication and during turbine engine operation.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of gas turbine engines, and more particularly to an internally cooled hybrid ceramic matrix composite vane. 
     BACKGROUND OF THE INVENTION 
     Gas turbine engines are known to include a compressor section for supplying a flow of compressed combustion air, a combustor section for burning a fuel in the compressed combustion air, and a turbine section for extracting thermal energy from the combustion air and converting that energy into mechanical energy in the form of a shaft rotation. Many parts of the combustor section and turbine section are exposed directly to the hot combustion gasses, for example the combustor, the transition duct between the combustor and the turbine section, and the turbine stationary vanes, rotating blades and surrounding ring segments. 
     It is also known that increasing the firing temperature of the combustion gas may increase the power and efficiency of a combustion turbine. Modern, high efficiency combustion turbines have firing temperatures in excess of 1,600° C., which is well in excess of the safe operating temperature of the structural materials used to fabricate the hot gas flow path components. Accordingly, several methods have been developed to permit operation of these materials in this environment. These include film cooling, backside cooling and thermal barrier coatings. 
     Film cooling involves the delivery of a film of cooling fluid, such as compressed air extracted from the compressor section, between the structural component and the flow of hot combustion gasses. The film of cooling fluid may be provided from a bleed flow from the compressor through holes formed in the surface of the component to be cooled. Film cooling systems are generally very effective in cooling a component, however they may significantly reduce the efficiency of the machine. Energy is needed to compress the cooling fluid, a decrease in combustion gas temperature is induced by the addition of the relatively cold fluid, and disturbance may be created in the smooth flow of air over an airfoil component such as a blade or vane. 
     Backside cooling generally involves the passage of a cooling fluid over a backside of a component that has a front side exposed to the hot combustion gasses. The cooling fluid in backside cooling schemes may be compressed air that has been extracted from the compressor or steam that is available from other fluid loops in a combustion turbine power plant. Backside cooling does not affect the exhaust gas composition or the flow of air over an airfoil component, it does not dilute the hot combustion air with colder fluid, and it can generally be supplied at a lower pressure than would be needed for film cooling. However, backside cooling creates a temperature gradient across the thickness of the cooled wall, and thus becomes decreasingly effective as the thickness of the component wall increases and as the thermal conductivity of the material decreases. 
     Insulation materials such as ceramic thermal barrier coatings (TBC&#39;s) have been developed for protecting temperature-limited components. While TBC&#39;s are generally effective in affording protection for the current generation of combustion turbine machines, they may be limited in their ability to protect underlying metal components as the required firing temperatures for next-generation turbines continue to rise. 
     Ceramic matrix composite (CMC) materials offer the potential for higher operating temperatures than do metal alloy materials due to the inherent nature of ceramic materials. This capability may be translated into a reduced cooling requirement that, in turn, may result in higher power, greater efficiency, and/or reduced emissions from the machine. However, CMC materials generally are not as strong as metal, and therefore the required cross-section for a particular application may be relatively thick. Due to the low coefficient of thermal conductivity of CMC materials and the relatively thick cross-section necessary for many applications, backside closed-loop cooling is generally ineffective as a cooling technique for protecting these materials in combustion turbine applications. Accordingly, high temperature insulation for ceramic matrix composites has been described in U.S. Pat. No. 6,197,424 B1, which issued on Mar. 6, 2001, and is commonly assigned with the present invention. That patent describes an oxide-based insulation system for a ceramic matrix composite substrate that is dimensionally and chemically stable at a temperature of approximately 1600° C. That patent also describes a stationary vane for a gas turbine engine formed from such an insulated CMC material. A similar gas turbine vane  10  is illustrated in FIG. 1 as including an inner wall  12  and stiffening ribs  14  formed of CMC material covered by an overlying layer of insulation  16 . Backside cooling of the inner wall  12  is achieved by convection cooling, e.g. via direct impingement through supply baffles (not shown) situated in the interior chambers  18  using air directed from the compressor section of the engine. 
     If baffles or other means are used to direct a flow of cooling fluid throughout the airfoil member for backside cooling and/or film cooling, the cooling fluid is typically maintained at a pressure that is in excess of the pressure of the combustion gasses on the outside of the airfoil so that any failure of the pressure boundary will not result in the leakage of the hot combustion gas into the vane. Such cooling passages must generally have a complex geometry in order to provide a precise amount of cooling in particular locations to ensure an adequate degree of cooling without over-cooling of the component. It is generally very difficult to form such complex cooling passages in a ceramic matrix composite component. Alternatively, large central chambers  18  as illustrated in FIG. 1 may be used with appropriate baffling to create impingement of the cooling fluid onto the backside of the surface to be cooled. Such large chambers create an internal pressure force that can result in the undesirable ballooning of the airfoil structure due to the internal pressure of the cooling fluid applied to the large internal surface area of the passage  18 . Furthermore, the geometry of FIG. 1 is also limited by stress concentrations at the intersection of the stiffening ribs  14  and the inner wall  12 . 
     Even higher operating temperatures are envisioned for future generations of combustion turbine machines. Accordingly, further improvements in the design of ceramic matrix composite airfoils and the cooling of such airfoils are needed. 
     SUMMARY OF THE INVENTION 
     Accordingly, a hybrid turbine component is described herein as including a CMC airfoil member defining a core region and a core member bonded to the airfoil member within the core region. The core member includes cooling channels for the passage of a cooling fluid for removing heat from the CMC material through the bond. The cooling passage may be formed as a groove on an outside surface of the core member, thereby providing both convective and conductive cooling of the CMC member. By bonding the core member to at least 30% of the inside area of the CMC airfoil member, the internal stress caused by the cooling fluid pressure is reduced. An insulating material may be deposited over the CMC airfoil member to reduce the cooling flow requirements. The materials properties of the various components are selected to minimize the stresses in the system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other advantages of the invention will be more apparent from the following description in view of the drawings that show: 
     FIG. 1 is a cross-sectional view of a prior art gas turbine vane made from a ceramic matrix composite material covered with a layer of ceramic thermal insulation. 
     FIG. 2 is a cross-section view of a solid-core ceramic matrix composite gas turbine vane. 
     FIG. 3 is a cross-section of the vane of FIG. 2 as viewed along Section  3 — 3 . 
     FIG. 4 is a perspective view of the vane of FIG. 2 with the core member partially inserted prior to being bonded to the CMC airfoil member. 
     FIG. 5 is a cross-sectional view of a solid-core ceramic matrix composite gas turbine vane having a layer of thermal insulation deposited over the CMC material. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 2-4 illustrate an improved stationary vane  20  for a gas turbine engine. The vane  20  includes an airfoil member  22  formed from a ceramic matrix composite material having an outer surface  24  defining an airfoil and an inner surface  26  defining a core region. The term ceramic matrix composite is used herein to include any fiber-reinforced ceramic matrix material as may be known or may be developed in the art of structural ceramic materials. The fibers and the matrix material surrounding the fibers may be oxide ceramics or non-oxide ceramics or any combination thereof. A wide range of ceramic matrix composites (CMCs) have been developed that combine a matrix material with a reinforcing phase of a different composition (such as mulite/silica) or of the same composition (alumina/alumina or silicon carbide/silicon carbide). The fibers may be continuous or long discontinuous fibers. The matrix may further contain whiskers, platelets or particulates. Reinforcing fibers may be disposed in the matrix material in layers, with the plies of adjacent layers being directionally oriented to achieve a desired mechanical strength. 
     A core member  28  is disposed within the core region of airfoil member  22 . The core member  28  is preferably formed from a different material than the airfoil member  22 , for reasons that will be explained in more detail below. One or more cooling passages  30  are formed in the core member  28  for passing a cooling fluid  32  to remove heat from the vane  20 . In this embodiment, the cooling passages  30  are partially defined by grooves formed into an outer surface  34  of the core member  28 . Alternatively, the cooling passages may be holes formed below the outer surface  34  of the core member  28 , preferably proximate the outer surface  34  to promote heat transfer between the outer surface  34  and the cooling fluid  32 . A plenum  36  is formed in the core member  28  for the introduction of the cooling fluid  32  at one end of the vane  20 . Openings  38  connect the plenum  36  and respective ones of the cooling passages  30  for the passage of cooling fluid  32 . The size of the plenum is selected to maintain the pressure of the cooling fluid  32  within a predetermined range at each of the plurality of openings  38  along the length of the vane  20 . The cooling fluid  32  passes along the cooling passages  30  and eventually exits the vane  20  along its trailing edge  39 . 
     The outer surface  34  of the core member  28  is attached to the inner surface  26  of airfoil member  22  by a bond  40 , as may be best seen in FIG.  3 . The bond  40  may be a layer of adhesive, or it may be a sintered bond joint created by curing the adjoining core member  28  and airfoil member  22  materials together. The bond  40  provides a heat removal pathway for conductive transfer of heat energy away from the airfoil member  22  into the core member  28 , and in turn into the cooling fluid  32 . In the embodiment illustrated, there will be some direct heat transfer from the airfoil member  22  to the cooling fluid  32 , since the inner surface  26  of the airfoil member  22  forms part of the pressure boundary for the cooling passage  30 . Such direct heat transfer between the airfoil member  22  and the cooling fluid  32  will not occur in embodiments where the cooling passage  30  is formed as a subsurface hole in the core member  28 . To ensure an adequate heat transfer between the core member  28  and the airfoil member  22 , the outer surface  34  of the core member  28  may be bonded to at least 30% of the area of the inner surface  26  of the airfoil member  22 . In other embodiments, the core member  28  may be bonded to at least 50% or at least 75% or at least 80% of the inner surface  26  of the airfoil member  22 . In one embodiment, the width of the grooves forming the cooling passages  30  is 3 mm and the distance between adjacent cooling passages is 12 mm, providing a bond of approximately 80% of the inner surface  26  of the airfoil member  22  to the outer surface  34  of the core  28 . 
     The ceramic matrix composite material of the airfoil member  22  provides mechanical strength necessary to withstand the thermal and mechanical stresses imposed on the vane  20 . The core member  28  substantially fills the hollow center of the airfoil member  22  and limits the area of the inner surface  26  that is exposed to the internal pressure loads created by the high pressure cooling fluid  32  and eliminating the ballooning effect experienced with the prior art design of FIG.  1 . The reduction of such internal pressure loads is especially beneficial near the trailing edge  39  where the thickness of the CMC material of the airfoil member  22  may be reduced. The core member  28  also provides a damping effect on the dynamics of the vane  20 , increasing the rigidity and stiffness of the vane  20  and providing a robust product that is more impact resistant and that may produce a reduced level of acoustic noise than prior art designs. Moreover, the core member  28  provides a much simpler mechanism for defining cooling channels  30  than the prior art techniques of forming passages within the CMC laminate or forming passages by using an internal metal sheath or baffle structure. The cooling passages  30  may be formed on the outer surface  34  of core member  28  by casting the part to include the passages or by machining the passages  30  into the surface  34 . Airfoil member  22  may be formed first and used as a mold for the casting of the core member  28 . For such a process, a fugitive material may be used to define the space for the passages  30  during the casting of the core member  28 . The fugitive material is then removed by heating during a subsequent process step to create the cooling passages  30 . Typical fugitive materials include wax, plastic, polystyrene, etc. 
     The material of construction of the core member  28  affects the performance of vane  20  in the environment of a gas turbine engine. Material properties of particular importance are discussed below. 
     Because the airfoil member  22  is exposed to higher temperatures than the core member  28 , the relative thermal expansion of these two materials may cause tensile stresses throughout the airfoil member  22  and bond  40 . Accordingly, it may be desired that the coefficient of thermal expansion (CTE) of the core member  28  be greater than the CTE of the airfoil member  22 , in one embodiment at least 10% greater. Other embodiments may have a CTE of the core member  28  that is about 7% greater than the CTE of the airfoil member  22 , or in the range of &gt;0 to 14% greater. For other applications, it may be acceptable to have the CTE of the core member  28  in the range of 94% to 120% of the CTE of the airfoil member  22 . This difference in thermal expansion coefficients will at least partially compensate for the difference in temperature ranges experienced by the respective materials, thereby more closely matching the physical growth of the materials and minimizing the amount of thermal stress induced in the vane  20 . 
     In order to safeguard the integrity of the heat removal pathway through the bond  40 , it may be desired that the mechanical strength of the core member  28  be less than the mechanical strength of the bond  40  between the airfoil member  22  and the core member  28 . Cracking of the airfoil member  22  material could result in the undesirable leakage of the cooling fluid  32  into the combustion gas flow. Delamination of the bond  40  could result in the loss of cooling of the airfoil member  22 . Accordingly, it may be desired to maintain the core member  28  as the mechanical weak link in the structure by using a material that has a tensile strength that is less than the tensile strength of the bond  40  and the airfoil member  22 . 
     The core member  28  may be designed to be strain tolerant in order to relax the loads imposed by thermal stresses. A ceramic material such as AN-191 may be used and will exhibit creep when exposed to tensile loads as a result of micro cracking of the material. Furthermore, it may be desired to maintain the elastic modulus of the core member material to be less than one-half that of the CMC airfoil member material, or in other embodiments to be less than one-third or less than one-tenth that of the CMC airfoil member material. The CMC airfoil member is the desired structural material and it is intended that the airfoil member bear the majority of the loads. If the modulus of the core is too high, the core, not the CMC airfoil member, takes the loads. 
     The required thermal conductivity of the core member material will depend upon the overall heat load, the number and location of the cooling channels  30 , and the thermal conductivity of the CMC airfoil member material. Generally, oxide CMC materials have lower thermal conductivity than do non-oxide based CMC materials, and thus a higher thermal conductivity core member material will be desirable for the oxide CMC materials. 
     FIG. 5 illustrates another embodiment of an improved gas turbine vane  50 . Vane  50  includes a ceramic matrix composite airfoil member  52  having a center that is substantially filled with a core member  54 . A first plurality of cooling passages  56  (shown in phantom) are formed through the core member  54 , with the cooling fluid  58  passing from an inlet plenum  60  formed along a length of the core member  54  into each respective cooling passage  56 . The cooling passages are openings for cooling fluid  58  that are formed proximate the outside surface of the core member  54 , and they may take the form of subsurface holes (as illustrated in FIG. 5) or grooves formed in the surface of the core member (as illustrated in FIGS.  2 - 4 ). The cooling passages  56  terminate in an outlet plenum  62  formed along the length of the vane  50  proximate a trailing edge portion  64 . The outlet plenum functions to redistribute the cooling fluid  58  into the respective inlets of a second plurality of cooling passages  66  formed proximate the trailing edge  64  for eventual discharge into the hot combustion gas passing over the vane  50 . One may appreciate that as the cross-sectional thickness of the vane  50  decreases and the temperature of the cooling fluid  58  increases toward the trailing edge portion  64 , the cooling requirements for the vane  50  change. Thus, the outlet plenum  62  provides a transition location for establishing a different cooling passage geometry for the trailing edge portion  64 . In one embodiment, the number of trailing edge cooling passages  66  is twice the number of cooling passages  56  upstream of the outlet plenum  62 , and the size of each respective trailing edge cooling passage  66  is reduced accordingly. Trailing edge cooling passages  66  may be formed as subsurface holes or grooves in a surface of a trailing edge portion  68  of core member  54 . In this embodiment, the bond  70  between the airfoil member  52  and the core member  54  encompasses 100% of the surface area of the inside surface of the airfoil member  52  local to the trailing edge region. The bond  70  provides a heat transfer path from the airfoil member  52  to the cooling fluid  58 . 
     The trailing edge portion  68  of the core member  54  of FIG. 5 is fabricated to be a separate piece of material from the remainder of the core member  54 . One may appreciate that in other embodiments the core member may be formed of one, two or more separate sections of material. The interface between such sections may be spaced apart as shown in FIG. 5 to form a plenum, or they may be in close contact. A multiple section core may be beneficial for limiting the stresses generated within the core. The joint between such sections may be located along any axis of the airfoil; for example, extending from the leading edge to the trailing edge along the length of the vane to join a top section and a bottom section, or extending from the low pressure side of the airfoil to the high pressure side of the airfoil along the length of the vane to join a leading edge section and a trailing edge section, or extending from the leading edge to the trailing edge along the cord of the airfoil to join an inner section and an outer section. The various sections may be formed of the same material or a different material. In one embodiment, the trailing edge portion  68  of FIG. 5 may be formed of a CMC material while the remainder of the core  54  may be formed of AN-191 material. 
     Vane  50  of FIG. 5 also includes a layer of insulating material  72  disposed over the airfoil member  52 . Insulating material  72  may be of the composition described in U.S. Pat. No. 6,197,424 or other appropriate insulating material, and may be formed (cast) or machined to provide a desired airfoil shape. The airfoil member  52  may be commercially available A-N720 CMC material (from COI Ceramics, San Diego, Calif.) having mullite-alumina Nextell 720 reinforcing fibers in an alumina matrix, or it may be any other appropriate oxide or non-oxide CMC material. The core member material may be commercially available AN-191 (Saint-Gobain, Worcester, Mass.), for example, or it may be a material having properties specially selected as described above for a particular application. 
     Thus, the hybrid vane of the present invention utilizes an efficient internal cooling scheme and a thermal barrier coating layer to permit the use of an 1,100° C. oxide CMC structural vane in a 1,600° C. combustion environment. The solid core with integral cooling channels and supply plenum is constructed of a material selected to improve the heat transfer and to minimize the stress at the bond interface. In addition, the solid core reduces the stress in the CMC layer caused by the internal cooling fluid pressure. 
     While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.