Patent Publication Number: US-8979498-B2

Title: Turbine airfoil having outboard and inboard sections

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Development of this invention was supported in part by the United States Department of Energy, Contract No. DE-FC26-05NT42644, H2 Advanced Hydrogen Turbine Development, Phase 2. Accordingly, the United States Government may have certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention is directed generally to turbine airfoils, and more particularly to hollow turbine airfoils having cooling channels for passing fluids, such as air, to cool the airfoils. 
     BACKGROUND 
     Typically, gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel and igniting the mixture, and a turbine blade assembly for producing power. Combustors often operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose turbine vane and blade assemblies to these high temperatures. As a result, turbine vanes and blades must be made of materials capable of withstanding such high temperatures. In addition, turbine vanes and blades often contain cooling systems for prolonging the life of the vanes and blades and reducing the likelihood of failure as a result of excessive temperatures. 
     Typically, turbine airfoils are formed from an elongated portion forming an airfoil having one end configured to be coupled to a disc and an opposite end configured to be a tip. As shown in  FIG. 1 , the airfoil is ordinarily composed of a leading edge, a trailing edge, a suction side, and a pressure side. The inner aspects of most turbine airfoils typically contain an intricate maze of cooling circuits forming a cooling system. The cooling circuits in the airfoils receive air from the compressor of the turbine engine and pass the air through the airfoil. At least some of the air passing through these cooling circuits is exhausted through orifices in the leading edge, trailing edge, suction side, and pressure side of the airfoil. 
     The turbine airfoil walls are load bearing in which the cumulative centrifugal loading of the airfoil is carried radially inward via the outermost wall. As such, the thickness required at the tip of the airfoil determines the thickness at the root. Typical turbine airfoils have increasing cross-sectional areas moving from the tip to the root, as shown in  FIG. 2 . The tip thickness is determined by casting tolerances that include allowances for variation in wall thickness plus the potential for internal cores to shift during the casting process. While simply designing an appropriate tip thickness and increasing the tip thickness to the root is feasible for small turbine airfoils, such is not the case for large airfoils useful in large turbine engines. In particular, when this design is scaled up to the larger engines, the root becomes larger than can be accommodated. In addition, the larger sized airfoil requires a part span snubber or tip shroud for vibration control, both of which become very difficult with the large size and temperature. Thus, an alternative configuration for a turbine airfoil is needed that is capable of being scaled up in size to without encountering the limitations of conventional cast airfoils. 
     SUMMARY OF THE INVENTION 
     This invention relates to a turbine airfoil formed from an inboard section and an outboard section attached thereto. The outboard section may be configured with a tip having an appropriate size. The remaining portions of the outboard section may be generally the same as the tip. The inboard section may be configured to support the outboard section. Forming the outboard section in this manner enables turbine airfoils to be formed in larger sizes than conventional configurations without creating centrifugal loading problems during turbine engine operation. The configuration of the outboard section enables the airfoil wall to be thinner than conventional airfoil walls and enables the airfoil wall of the outboard section to be generally constant along the length of the outboard section to the inboard section, which may begin at a point where the airfoil begins to carry the centrifugal load. 
     The turbine airfoil may be formed from a generally elongated hollow airfoil formed from an outer wall and may have a leading edge, a trailing edge, a pressure side, a suction side, a root at a first end of the airfoil and a tip at a second end opposite to the first end. The turbine airfoil may also include a cooling system positioned within interior aspects of the generally elongated hollow airfoil. The airfoil may be formed from an outboard section and an inboard section such that an inner end of the outboard section is attached to an outer end of the inboard section. The inner end of the outboard section and the outer end of the inboard section may have matching cross-sectional configurations. The inboard and outboard sections may be coupled together via one or more welds, mechanical connectors or through other appropriate ways. The outboard section may have a generally non-tapered cross-sectional area, and the inboard section may have a tapered cross-sectional area. The outboard section may have a length up to about 30 percent of a length of the outboard and inboard sections combined. The outboard and inboard sections may be formed at least in part by different materials. The outboard section may be formed at least partially from a material having a lesser density than a material used to form at least part of the inboard section. 
     An advantage of this invention is that by forming the airfoil from outboard and inboard sections, the sections may be individually cast, which allows the outboard and inboard sections to be thinner and thereby more efficient structurally than conventional airfoils. 
     Another advantage of this invention is that the outboard section may be formed from materials having a lower density than the inboard section, thereby increasing the structural efficiency of the airfoil by increasing the specific strength of the airfoil. 
     Yet another advantage of this invention is that the configuration of the outboard and inboard sections may reduce the centrifugal loads by more than 15 percent. 
     Another advantage of this invention is that the amount of stress in the outer walls forming the airfoils is reduced, thereby improving the overall structural efficiency of the airfoil. 
     Still another advantage of this invention is that the separately cast outboard section increases the structural efficiency in the critical outermost section of the airfoil and benefits the inboard section as it propagates radially inward through the airfoil. 
     These and other embodiments are described in more detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the presently disclosed invention and, together with the description, disclose the principles of the invention. 
         FIG. 1  is a perspective view of a conventional turbine airfoil. 
         FIG. 2  is a side view of the airfoil of  FIG. 1 . 
         FIG. 3  is a perspective view of a conventional turbine airfoil having features according to the instant invention. 
         FIG. 4  is a side view of the airfoil of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIGS. 3-4 , this invention is directed to a turbine airfoil  10  formed from an inboard section  12  and an outboard section  14  attached thereto. The outboard section  14  may be configured with a tip  16  having an appropriate size. The remaining portions of the outboard section  14  may be generally the same as the tip  16 . The inboard section  12  may be configured to support the outboard section  14 . Forming the outboard section  14  in this manner enables turbine airfoils  10  to be formed in larger sizes than conventional configurations without creating centrifugal loading problems during turbine engine operation. The configuration of the outboard section  14  enables the airfoil wall to be thinner than conventional airfoil walls and enables the airfoil wall of the outboard section  14  to be generally constant along the length of the outboard section  14  to the inboard section  12 , which may begin at a point where the airfoil begins to require tapering walls to carry the increasing centrifugal load. 
     The turbine airfoil  10  may be a generally elongated hollow airfoil  20  formed from an outer wall  22 . The generally elongated hollow airfoil  20  may have a leading edge  24 , a trailing edge  26 , a pressure side  28 , a suction side  30 , a root  32  at a first end  34  of the airfoil  20  and a tip  16  at a second end  38  opposite to the first end  34 . The generally elongated hollow airfoil  20  may have any appropriate configuration and may be formed from any appropriate material. The turbine airfoil  10  may include a cooling system  10  positioned within interior aspects of the generally elongated hollow airfoil. The cooling system  10  may be positioned in the generally elongated hollow airfoil  20  and may have any appropriate cross-sectional shape. 
     The outboard section  14  may include an inner end  40  that is attached to an outer end  42  of the inboard section  12 . The outboard section  14  may be attached via an appropriate manner. In at least one embodiment, the outboard section  14  may be attached to the inboard section  12  via one or more welds, or other appropriate metallurgical joining process. In other embodiments, the outboard section  14  may be attached to the inboard section  12  via one or more mechanical connectors, such as, but not limited to, one or more of the following, screw, bolt, rivot, cotter pin, and the like. 
     The outboard section  14  may have a generally non-tapered cross-sectional area, as shown in  FIGS. 3 and 4 . In particular, the outboard section  14  may be configured such that the tip  16  has an appropriate thickness because the as-cast wall thickness is more than sufficient to carry the cumulative centrifugal loading below stress limits. The thickness of the outboard section  14  may remain constant moving radially inward toward the root  32 . The thickness of the outboard section  14  may remain generally constant until the airfoil  20  begins to require tapering to support centrifugal loads. The outboard section  14  may terminate generally at the location at which the airfoil  20  begins to taper to support centrifugal loads. In at least one embodiment, the outboard section  14  may have a length up to about 30 percent of a length of the outboard and inboard sections  14 ,  12  combined. 
     The inboard section  12  may have a tapered cross-sectional area that increases in size moving radially inward. In particular, the outer end  42  of the inboard section  12  and the inner end  40  of the outboard section  14  may have matching cross-sectional configurations such that the sections  12 ,  14  may be coupled together. The inboard and outboard sections  12  and  14  may be formed from different materials so that the airfoil  20  may be optimized. For example, the outboard section  14  may be formed at least partially from a material having a lesser density than a material used to form at least part of the inboard section  12 . Because the outboard section  14  does not support as much centrifugal loads as does the inboard section  12 , the outboard section  14  may be formed from materials that may have less strength than the one or more materials forming the inboard section  12 , and thus may weigh less per unit area than the materials forming the inboard section  12 . The different materials may have the same general chemistry but may differ in specific composition. 
     The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of this invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of this invention.