Patent Publication Number: US-7713029-B1

Title: Turbine blade with spar and shell construction

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is related to U.S. patent application Ser. No. 11/243,308 filed on Oct. 4, 2005 by Wilson et al and entitled TURBINE VANE WITH SPAR AND SHELL CONSTRUCTION. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to fluid reaction surfaces, and more specifically to a turbine blade with a spar and shell construction. 
   2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
   In a gas turbine engine, such as an aero engine used to power an aircraft or an industrial gas turbine engine used to produce electrical power, a turbine section includes a plurality of stages of rotor blades and stator vanes to extract the energy from the hot gas flow passing through. The engine efficiency can be improved by increasing the temperature of the hot gas flow entering the turbine. However, the inlet temperature is limited to the material properties of the first stage vanes and rotor blades. To improve the efficiency, a complex internal cooling circuits have also been proposed to provide impingement and film cooling to these airfoils in order to allow for a higher gas flow temperature. 
   A recent improvement in the high temperature resistant airfoils is the use of a spar and shell construction in which a shell having the shape of the airfoil is secured to a spar for support. The shell is typically made from a material that cannot be cast or forged like the nickel based super-alloys used to make turbine blades and vanes. The shell is fabricated from exotic high temperature materials such as Niobium or Molybdenum or their alloys in which the airfoil shape is formed by a well known electric discharge machining process (EDM) or a wire EDM process that can make a thin wall shell suitable for near wall impingement cooling in an airfoil. Because the turbine blade would be under high centrifugal forces during operation, the shell could even be made from a ceramic material because the spar would support the load, allowing for the shell to be exposed to the high temperature gas flow. 
   Turbine rotor disks also include blade attachment slots in which a root of the turbine blade having a fir-tree configuration is inserted to secure the blade to the rotor disk in the radial direction. The single piece cast nickel super-alloy turbine blade includes the root portion with the fir-tree configuration to fit within the disk slot. There is a need in the prior art for a spar and shell constructed blade to be capable of replacing the nickel super-alloy blade by using the attachment slot within the rotor disk to insert the spar and shell constructed blade. 
   The Prior Art U.S. Pat. No. 4,790,721 issued to Morris et al on Dec. 13, 1988 and entitled BLADE ASSEMBLY discloses a turbine blade with a metal core having a cap or blade tip, a metal liner functioning as a coolant containing surface, and a ceramic blade jacket secured between the blade tip of the metal core and the platform of the base having the fir-tree configuration. The metal core that holds the ceramic blade jacket (the shell) is secured to the fir-tree base by bonding. This construction is considered to be very weak in holding the blade together during operating speeds producing high centrifugal forces that tend to pull the spar away from the fir-tree root or base. Also, this construction does not permit removal and replacement of the shell component, which is known to be the life limiting part of the spar and shell constructed blade. 
   It is an object of the present invention to provide for a turbine blade of the spar and shell construction that can be inserted into the dove-tail slot of the rotor disk. 
   It is another object of the present invention to provide for a turbine blade having a spar and shell construction that can withstand the high centrifugal forces during operation of the engine. 
   It is another object of the present invention to provide for a turbine blade of the spar and shell construction with a fir-tree root in which the shell can be easily replaced. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is a turbine blade having a spar and shell construction, in which the spar includes a dove-tail that fits within a two piece platform and root assembly having a fir-tree configuration that slides into a dove-tail slot of the rotor disk. The two piece platform assembly includes the blade platforms to form the gas flow path, and is secured together by a clamping screw or other fastener to facilitate installation of the blade assembly into the rotor disk. The spar and the platform halves each include cooling air passages to supply cooling air to the blade. The spar includes a blade tip, and the shell is compressed between the blade tip of the spar and the platform halves to form the turbine blade. Because of the spar and shell construction, the blade can be made from an exotic high temperature resistant material that cannot be cast or forged into the airfoil shape in order that a higher gas flow temperature can be used in the engine. Also, the blade with the spar and shell construction can be inserted into a dove-tail slot in a rotor disk that is typically is used for a single piece nickel super-alloy turbine blade. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  shows a schematic view of the turbine blade having the spar and shell construction of the present invention. 
       FIG. 2  shows a cross section of a front view of the turbine blade of  FIG. 1 . 
       FIG. 3  shows a side view of a cross section of the turbine blade of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is shown schematically in  FIG. 1  and is a turbine blade made from a spar and shell construction having a fir-tree root and platform that will slide into an attachment slot of a rotor disk. The turbine blade includes a spar  11  with a blade tip  12  and a dove-tail  13  having a tear-drop shape on the opposite end from the tip. The spar  11  includes one or more cooling air passages  14  to channel cooling air from the root to portions of the blade for cooling. The fir-tree root and platform of the blade assembly includes two platform halves  21  and  22  that, when combined, form a fir-tree configuration that can slide within the attachment slot formed in the rotor disk. Each platform halve  21  and  22  includes a slot  26  that, when joined together, form a slot that can secure the dove-tail  13  of the spar  11  to the platform halves. The spar dove-tail and the platform halves slots  26  are of such size and shape to secure the spar  11  to the dove-tail configuration of the platform halves against radial displacement from high rotational speeds of the rotor disk. The platform halves  21  and  22  pinch the dove-tail  13  of the spar  11  to secure the spar  11  against radial displacement during engine operation. A hole  24  is formed in the two platform halves  21  and  22  to receive a screw  25  or rivet (or other well known fastener) in order to secure the two platform halves together. The platform halves  21  and  22  also include one or more cooling air passages to connect the cooling air source to the cooling air passages  14  within the spar  11 . 
   Although not shown in the Figures, the spar can have radial or serpentine flow cooling channels connected to impingement cooling holes to direct jets of impingement cooling air onto the backside surface of the shell  31  in order to provide impingement cooling for the shell and spar. Also, cooling holes could be used on the blade tip  12  of the spar  11  to provide cooling for the blade tip  12 . 
   The shell  31  is a thin wall airfoil surface that can be made from a high temperature resistant refractory or exotic material such as Niobium or Molybdenum that cannot be cast or forged, but must be formed from one of the well known processes such as electric discharge machining (EDM) or wire EDM that can form the thin walled shell without having to cast or forge the shell. The spar and shell construction of the present invention can also be used with shells made from machined or cast pieces as well.  FIG. 2  shows a front view of a cross section of the blade with the spar and shell construction of  FIG. 1 .  FIG. 3  shows a side view of a cross section of the blade of  FIG. 1 . The shell  31  is secured between the blade tip  12  of the spar  11  and the platforms  23  of the two platform halves  21  and  22 . Because the spar  11  includes the dove-tail  13  that is pinched between the platform halves  21  and  22 , the blade assembly can operate at very high rotational speeds without the spar  11  breaking away from the rotor disk and the shell coming off. Also, the turbine blade of the present invention can be used to replace a prior art nickel super-alloy single piece turbine blade in which the root with the fir-tree attachment is formed as a single piece with the airfoil portion. When the spar is inserted into the shell and the two platform halves are fastened together, the platform halves will not quite come together in their operational position because the shell must be compressed between the tip and the platforms. Because of the dove-tail in the spar and the slot in the platform halves having the configuration and shape as shown in the Figures, when the fastener is tightened and the two platform halves  21  and  22  come together, a compression of the shell between the spar tip and the platform surfaces is produced. 
   To cool the blade, the spar blade tip  12  includes cooling holes  12  to provide cooling for the tip  12 . the cooling air passing through the impingement cooling holes in the main body of the spar  11  will pass in the space between the spar and the shell and then up through the tip cooling holes  12  as seen by the arrows in  FIG. 2 . Two fastener screws are shown to secure the platform halves together. However, the number of fasteners and the kind used can vary depending upon the size of the blade or other factors such as reliability concerns. The platform and the blade tip can include a groove to fit the end of the shell within when the blade assembly is fastened together. Ribs extending from the spar can also be used to secure the shell against deflections during operation and add rigidity to the shell.