Turbine blade with carbon nanotube shell

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.

FEDERAL RESEARCH STATEMENT

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.

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 blade10is shown inFIG. 1and includes a spar11with a tip end12and a platform end13, a shell21made from high temperature resistant Carbon or Molybdenum nanotubes extending along the spanwise direction of the shell and under tension, and an attachment or root31section in which the spar11fastened. The attachment31can 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 section61with fingers62that form part of a labyrinth seal is mounted onto the attachment31and forms the platform for the blade. In theFIG. 1embodiment, the tip12and the spar11are 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 attachment31and the platform61can be formed as a single piece instead of separate pieces as shown inFIG. 1. Also, the spar11and the attachment31can 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 shell21is separate from the spar11.

The shell21is 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 shell21is 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 shell21thickness about 0.060 inches. The nanotube shell has an 80% taper. The shell21is held in tension during engine operation by a dear drop shaped metal insert71as seen inFIG. 6. The nanotube shell wraps around the metal insert71on the lower end of the shell. The platform61includes a slanted inner surface on the upper end and the attachment or root portion31includes 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 platform61and the attachment31.

The spar11includes a tip section12as seen in more detail inFIG. 2. The tip12includes a squealer tip14formed by the tip walls around the airfoil surfaces, cooling holes15on the tip and the side of the spar11to provide cooling for the squealer tip and the backside surface of the shell21. The outside edges of the tip12also includes a seal groove with an upper groove surface slanted so that a wire seal56placed 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 spar13includes a threaded hole about in the center in which a tie bolt screws into in order to pull the spar11against the upper surface of the platform31and secure the shell21in-between the spar tip12and the platform31. 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. 3shows 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 platform31is standard in shape. The spar11includes a leading edge with a row of metering and impingement holes17and two rows of impingement holes15one on the pressure side and the second on the suction side. The spar11forms a cooling air supply cavity23and has a row of exit cooling holes16on the trailing edge side of the spar11. The shell21includes a leading edge with a showerhead arrangement of film holes22. A leading edge impingement cavity24is formed between the spar and the shell. The trailing edge region of the shell includes a trailing edge cavity25with a plurality of trip strips26spaced along the side walls in an alternating arrangement to act as turbulent promoters for the cooling air. A row of trailing edge exit holes27is formed along the trailing edge of the shell21. The spar and shell form a pressure side impingement cavity and a suction side impingement cavity between the metering hole17and the exit hole16. Impingement holes15formed on the spar11force pressurized cooling air from the cavity23to impinge against the inner side walls of the shell to provide impingement cooling. Cooling air from the cavity23also flows through the exit holes16, then through the trailing edge cavity25and out the exit holes27to 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 inFIG. 3. One or more ribs can be included on the pressure side of the airfoil to provide support for the shell21against the spar11.

FIG. 4shows a detailed view of the tie bolt and spar to attachment connection. The spar11includes a threaded hole on the bottom end13in which the tie bolt51screws into. The attachment31includes an inner cavity35and a top surface with a hole for insertion of the tie bolt51. The lower end of the tie bolt51also includes threads on the outer surface in which an Allen nut52screws onto. The Allen nut52includes 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 bold51. A self locking nut64is threaded onto the tie bolt to lock the Allen nut52in place. The attachment31includes a slot34on the bottom and an opening33on the top surface of the slot34for insertion of the Allen nut, the self locking nut64and the wrench to remove or secure the Allen nut52and the self locking nut64to the tie bolt51.FIG. 5shows a front view of the tie bolt and spar and shell interface when assembled. The tie bolt51is 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 spar11to the attachment31. The tie bolt51must be capable of withstanding very high stress levels in order to secure the shell11between the spar tip and the platform61during engine operation. One or more tie bolts51can be used to secure the spar to the shell.

The shell21is secured to the spar11and attachment31in a thermally free manner by allowing for a space to exist between the bottom of the shell21and the top surface of the attachment31. As seen inFIG. 1, a lower wire seal55is held within an inward facing groove formed in the platform61with a slanted upper surface. The wire seal55is forced upward from the centrifugal force developed during rotation of the blade. This forces the wire seal55up against the attachment31surface and the upper groove surface to form a tight fitting seal. A second or upper wire seal56is placed within an outward facing groove formed on the blade tip12as seen inFIGS. 1 and 2. The top of the groove is also slanted upward so that the upper wire seal56is forced upward and against the inner surface of the shell21to produce a tight fitting seal under rotation of the blade.

Because the shell21is 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 attachment31. The spar and attachment31assembly 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 attachment31. The platform61is 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.