Abstract:
A turbine blade for use in a gas turbine engine, where the turbine blade is made from a spar and shell construction in which a thin walled shell is formed from carbon or molybdenum nanotubes arranged in a direction such that the nanotubes are under tension when the blade is rotating in the engine. The carbon nanotubes are allotropes of carbon in which the length to diameter ratio exceeds 1,000,000 in order to produce very high tensile strength, unique electrical properties, and a very efficient conductor of heat. The nanotube shell includes a metal insert having a tear drop shape and the lower end of the shell wraps around the metal insert to form a wedge in which the shell is held in place against radial displacement between the platform and the attachment portion of the blade.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit to an earlier filed and co-pending U.S. patent application Ser. No. 12/176,930 filed on Jul. 21, 2008 and entitled TURBINE BLADE WITH CARBON NANOTUBE SHELL. 
    
    
     FEDERAL RESEARCH STATEMENT 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a gas turbine engine, and more specifically to a turbine blade formed from a spar and shell. 
     2. Description of the Related Art Including Information Disclosed under 37CFR 1.97 and 1.98 
     In a gas turbine engine, a compressed air from a compressor is burned with a fuel in a combustor to produce a hot gas flow. The hot gas flow is passed through a multiple stage turbine to convert most of the energy from the gal flow into mechanical work to drive the compressor, and in the case of an aero engine to drive a fan, and in the case of an industrial gas turbine (IGT) engine to drive an electric generator to produce electrical power. 
     The efficiency of the engine can be increased by passing a higher temperature gas into the turbine, or a higher turbine inlet temperature. However, the maximum turbine inlet temperature will depend upon the material properties of the first stage turbine stator vanes and rotor blades, since these airfoils are exposed to the highest gas flow temperature. Modern engine has a turbine inlet temperature around 2,400 degrees F., which is much higher than the melting point of a typical modern vane or blade. These airfoils can be used under these high temperature conditions due to airfoil cooling using a mixture of convection cooling along with impingement cooling and film cooling of the internal and the external surfaces of these airfoils. 
     A few very high temperature materials exist that have melting points well above modern engine turbine inlet temperatures. Columbian has a melt temperature of up to 4,440 F; TZM Moly up to 4,750 F; hot pressed silicon nitride up to 3,500 F; Tantalum up to 5,400 F; and Tungsten up to 6,150 F these materials would allow for higher turbine inlet temperatures. However, these materials cannot be cast or machined to form turbine airfoils. 
     On prior art method of forming a turbine airfoil from one of these exotic high temperature materials is disclosed in U.S. Pat. No. 7,080,971 B2 issued to Wilson et al on Jul. 25, 2006 and entitled COOLED TURBINE SPAR SHELL BLADE 
     CONSTRUCTION, the entire disclosure being incorporated herein by reference. The shell is formed from a wire EDM process to form a thin walled airfoil shell, and the shell is held in compression between a spar tip and the blade platform or root section. The shell can take the higher gas flow temperatures, and the spar provides internal cooling for the airfoil walls. 
     It is well recognized that forming a steel wire with a long length/diameter ratio will improve its tensile strength compared to a standard forging. Carbon nanotubes (CNTs) are allotropes of carbon. This results in a nanostructure where the length/diameter ratio exceeds 1,000,000. Such cylindrical carbon molecules have novel properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science. CNTs exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized. 
     All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as ballistic conduction, but good insulators laterally to the tube axis. It is predicted that carbon nanotubes will be able to transmit up to 6,000 watts per meter per degree Kelvin at room temperature compared to copper—a metal well known for its good thermal conductivity—which only transmits 385 W/(m*K). The temperature stability of carbon nanotubes is estimated to be up to 2,800 degrees Celsius in vacuum and about 750 degrees Celsius in air. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide for a turbine airfoil with a very long life prediction for the shell. 
     It is another object of the present invention to provide for a shell formed from a nanotube material. 
     It is another object of the present invention to provide for a spar and shell turbine blade in which the shell made from a nanotubes material that is supported in tension to increase the life of the blade. 
     It is another object of the present invention to provide for a spar and shell turbine blade with a thermally free platform to relieve thermal fight. 
     It is another object of the present invention to provide for a spar and shell turbine blade which eliminates bonds, welds and brazes. 
     It is another object of the present invention to provide for a spar and shell turbine blade with a much lighter weight. 
     A turbine blade made from a spar and shell construction in which the spar is connected to the attachment section of the blade by only a mechanical fastener without bonds, welds or brazing. The shell is formed from Carbon or Molybdenum nanotubes extending in the spanwise direction of the shell and held in tension during rotation of the blade to provide for an infinite life. The shell is constructed of a single piece with the nanotubes running in the airfoil spanwise direction. The lower end of the shell includes a tear drop shaped metal insert in order to wedge the shell in place between a separate platform and the root or attachment portion of the blade. The lower end of the nanotube shell wraps around the tear drop shaped metal insert. The platform includes an inner top surface that slants inward toward the attachment portion in order to pinch the shell between the platform and the attachment and thus hold the shell against radial outward displacement during rotation of the blade. 
     The shell is held in place on the lower end and is free to slide radially at the tip end. The platform is a separate piece from the attachment portion in order to provide for a thermally free platform to relieve the thermal fight between the platform and the airfoil portion. The shell can be held in tension so that an infinite life for the blade can be obtained. A tie bolt is used to fasten the spar to the attachment or root portion of the blade, and the attachment includes a cavity and an opening on the bottom in which a hex nut and be inserted onto the tie bolt and a tool inserted to tighten the tie bolt and secure the spar to the attachment or root portion of the blade. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a pressure side view of a cross section of the turbine blade of the present invention. 
         FIG. 2  is a detailed view of the tip of the blade in  FIG. 1 . 
         FIG. 3  shows a top view of a cross section of the spar and shell and the platform of the blade of  FIG. 1 . 
         FIG. 4  shows a detailed side view of the spar to root attachment connection of the blade of  FIG. 1 . 
         FIG. 5  shows a detailed front view of the lower part of the attachment connection of  FIG. 4 . 
         FIG. 6  is a detailed view of the connection between the lower end of the shell and the platform and attachment interface. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a turbine blade with a spar and shell construction that reduces or eliminates the problems discussed above in the background. The blade  10  is shown in  FIG. 1  and includes a spar  11  with a tip end  12  and a platform end  13 , a shell  21  made from high temperature resistant Carbon or Molybdenum nanotubes extending along the spanwise direction of the shell and under tension, and an attachment or root  31  section in which the spar  11  fastened. The attachment  31  can be a single piece or made from several pieces secured together to form the root of the blade with a rotor disk attachment such as a fir tree configuration, and a platform to form a seal with adjacent airfoils in the turbine. A platform section  61  with fingers  62  that form part of a labyrinth seal is mounted onto the attachment  31  and forms the platform for the blade. In the  FIG. 1  embodiment, the tip  12  and the spar  11  are a single piece. However, in another embodiment the tip that forms the blade tip can be formed as a separate piece from the spar and secured to the spar by any well known means such as bonding, welding or brazing or a mechanical fastener. In another embodiment, the attachment  31  and the platform  61  can be formed as a single piece instead of separate pieces as shown in  FIG. 1 . Also, the spar  11  and the attachment  31  can be formed as a single piece since the shell is not held in compression by the blade tip portion of the spar in the present invention. The spar can be cast for ease of manufacture since the shell  21  is separate from the spar  11 . 
     The shell  21  is made from a carbon or molybdenum nanotube material in which the nanotubes extend substantially along the spanwise direction of the airfoil such that the nanotubes are under tension during rotation of the blade. The shell  21  is a thin walled surface that forms the airfoil portion of the blade and includes the leading edge and the trailing edge, and the pressure side and the suction side walls. The shell  21  thickness about 0.060 inches. The nanotube shell has an 80% taper. The shell  21  is held in tension during engine operation by a dear drop shaped metal insert  71  as seen in  FIG. 6 . The nanotube shell wraps around the metal insert  71  on the lower end of the shell. The platform  61  includes a slanted inner surface on the upper end and the attachment or root portion  31  includes a slanted outward surface on the outer end adjacent to the platform slanted surface so that the metal insert and the shell can be wedged between the two slanted surfaces when the shell is in place between the platform  61  and the attachment  31 . 
     The spar  11  includes a tip section  12  as seen in more detail in  FIG. 2 . The tip  12  includes a squealer tip  14  formed by the tip walls around the airfoil surfaces, cooling holes  15  on the tip and the side of the spar  11  to provide cooling for the squealer tip and the backside surface of the shell  21 . The outside edges of the tip  12  also includes a seal groove with an upper groove surface slanted so that a wire seal  56  placed within the groove will be forced upward and thus outward and against the inner surface of the shell to provide for a tight fitting seal when the blade is rotating. The lower end of the spar  13  includes a threaded hole about in the center in which a tie bolt screws into in order to pull the spar  11  against the upper surface of the platform  31  and secure the shell  21  in-between the spar tip  12  and the platform  31 . The spar and the platform can be cast or machined, and can be made from different materials. The shell and the spar are placed in tension when the blade is rotating. The shell can extend outward beyond the end of the tip or can be flush with the tip. 
       FIG. 3  shows a top view of a cross section through the blade which shows the platform outer surface and the cooling passages formed within the spar and the shell assembly. The platform  31  is standard in shape. The spar  11  includes a leading edge with a row of metering and impingement holes  17  and two rows of impingement holes  15  one on the pressure side and the second on the suction side. The spar  11  forms a cooling air supply cavity  23  and has a row of exit cooling holes  16  on the trailing edge side of the spar  11 . The shell  21  includes a leading edge with a showerhead arrangement of film holes  22 . A leading edge impingement cavity  24  is formed between the spar and the shell. The trailing edge region of the shell includes a trailing edge cavity  25  with a plurality of trip strips  26  spaced along the side walls in an alternating arrangement to act as turbulent promoters for the cooling air. A row of trailing edge exit holes  27  is formed along the trailing edge of the shell  21 . The spar and shell form a pressure side impingement cavity and a suction side impingement cavity between the metering hole  17  and the exit hole  16 . Impingement holes  15  formed on the spar  11  force pressurized cooling air from the cavity  23  to impinge against the inner side walls of the shell to provide impingement cooling. Cooling air from the cavity  23  also flows through the exit holes  16 , then through the trailing edge cavity  25  and out the exit holes  27  to provide cooling for the trailing edge region. 
     Ribs can also be used to prevent bulging of the airfoil wall. The ribs can formed on the inner surface of the shell and extend inward to abut the spar, or the ribs can be formed on the spar and extend outward and abut against the shell. In one embodiment, one rib formed on the shell extends inward and abuts against the spar at about a midpoint within the suction side impingement cavity as seen in  FIG. 3 . One or more ribs can be included on the pressure side of the airfoil to provide support for the shell  21  against the spar  11 . 
       FIG. 4  shows a detailed view of the tie bolt and spar to attachment connection. The spar  11  includes a threaded hole on the bottom end  13  in which the tie bolt  51  screws into. The attachment  31  includes an inner cavity  35  and a top surface with a hole for insertion of the tie bolt  51 . The lower end of the tie bolt  51  also includes threads on the outer surface in which an Allen nut  52  screws onto. The Allen nut  52  includes a hex shaped opening on the bottom in which a wrench or other tool is inserted into and screw the Allen nut onto the threaded bottom portion of the tie bold  51 . A self locking nut  64  is threaded onto the tie bolt to lock the Allen nut  52  in place. The attachment  31  includes a slot  34  on the bottom and an opening  33  on the top surface of the slot  34  for insertion of the Allen nut, the self locking nut  64  and the wrench to remove or secure the Allen nut  52  and the self locking nut  64  to the tie bolt  51 .  FIG. 5  shows a front view of the tie bolt and spar and shell interface when assembled. The tie bolt  51  is made from MP159 for resistance to the high temperature environment of the platform and the attachment, has a diameter of 0.750 inches, and includes 16 threads. However, other diameters and thread numbers are possible in order to retain the spar  11  to the attachment  31 . The tie bolt  51  must be capable of withstanding very high stress levels in order to secure the shell  11  between the spar tip and the platform  61  during engine operation. One or more tie bolts  51  can be used to secure the spar to the shell. 
     The shell  21  is secured to the spar  11  and attachment  31  in a thermally free manner by allowing for a space to exist between the bottom of the shell  21  and the top surface of the attachment  31 . As seen in  FIG. 1 , a lower wire seal  55  is held within an inward facing groove formed in the platform  61  with a slanted upper surface. The wire seal  55  is forced upward from the centrifugal force developed during rotation of the blade. This forces the wire seal  55  up against the attachment  31  surface and the upper groove surface to form a tight fitting seal. A second or upper wire seal  56  is placed within an outward facing groove formed on the blade tip  12  as seen in  FIGS. 1 and 2 . The top of the groove is also slanted upward so that the upper wire seal  56  is forced upward and against the inner surface of the shell  21  to produce a tight fitting seal under rotation of the blade. 
     Because the shell  21  is held under tension during engine operation and because the nanotubes are much stronger under tension than under compression, an infinite life for the shell is predicted. A life of from 5 to 25 times longer than the prior art blades is predicted. Thus, the turbine blade with the spar and shell construction of the present invention can be used in an engine, such as an industrial gas turbine engine, for long periods without repair or replacement. 
     Another benefit from the turbine blade with the spar and shell construction of the present invention is the weight savings over the prior art blade. A large IGT engine used for power production includes 72 blades in the first stage of the turbine, and each blade weighs 14.7 pounds including the TBC. The blade of the present invention weighs 10.9 pounds which is almost 4 pounds less than the prior art. A lighter blade will produce a lower stresses on the rotor disk due to the lower centrifugal forces developed than in the prior art blade. Lower stress on the rotor disk will allow for smaller and lower weight rotor disks, or improved disk LCF life at the life limiting location. 
     The process for assembling the turbine blade is described next. The nanotube shell with the metal insert is assembled. The tip is secured to the spar if separate pieces are used. The spar with the tip secured on the end is fastened to the attachment  31 . The spar and attachment  31  assembly is then placed inside the shell from the lower end of the shell so that the lower end of the shell will abut against the slanted outward surfaces of the attachment  31 . The platform  61  is then placed over the shell and secured to the attachment such that the slanted inward facing surfaces of the platform wedge the metal insert in place between the two slanted surfaces. The tie bolt is tightened and the self locking nut is secured to prevent the tie bolt from loosening. 
     Another feature of the spar and shell turbine blade of the present invention is the reduction in the casting technology used to form the blade. A lower level of casting technology allows for alternative casting vendors to be used to manufacture the blade. The present invention provides approximately 30% reduction is size of casting footprint. Casting costs are a function of parts per mold and casting yield. Removing the platform would allow more parts per mold for airfoil spar and increased yield. Separate platform would permit (if cast) cored platforms and other high technology features to be used.