Abstract:
A turbine rotor blade with a spar and shell construction, the spar including an internal cooling supply channel extending from an inlet end on a root section and ending near the tip end, and a plurality of external cooling channels formed on both side of the spar, where a middle external cooling channel is connected to the internal cooling supply channels through a row of holes located at a middle section of the channels. The spar and the shell are held together by hooks that define serpentine flow passages for the cooling air and include an upper serpentine flow circuit and a lower serpentine flow circuit. the serpentine flow circuits all discharge into a leading edge passage or a trailing edge passage.

Description:
GOVERNMENT LICENSE RIGHTS 
     This invention was made with Government support under contract number DE-FG02-07ER84668 awarded by Department of Energy. The Government has certain rights in the invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a gas turbine engine, and more specifically to an air cooled turbine rotor blade with a spar and shell construction. 
     2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
     A gas turbine engine, such as an industrial gas turbine (IGT) engine, compresses air that is then burned with a fuel to produce a high temperature gas flow, which is then passed through a turbine having multiple rows or stages or stator vanes and rotor blades to power and aircraft or, in the case of the IGT, drive an electric generator. It is well known in the art of gas turbine engine design that the efficiency of the engine can be increased by passing a higher gas flow temperature through the turbine. However, the turbine inlet temperature is limited by the material properties of the turbine, especially for the first stage airfoils since these are exposed to the highest temperature gas flow. As the gas flow passes through the various stages of the turbine, the temperature decreases as the energy is extracted by the rotor blades. 
     Another method of increases the turbine inlet temperature is to provide more effective cooling of the airfoils. Complex internal and external cooling circuits or designs have been proposed using a combination of internal convection and impingement cooling along with external film cooling to transfer heat away from the metal and form a layer of protective air to limit thermal heat transfer to the metal airfoil surface. However, since the pressurized air used for the airfoil cooling is bled off from the compressor, this bleed off air decreases the efficiency of the engine because the work required to compress the air is not used for power production. It is therefore wasted energy as far as producing useful work in the turbine. 
     Recently, airfoil designers have proposed a new air cooled turbine rotor blade or stator vane design that is referred to as a spar and shell airfoil. U.S. Pat. No. 7,080,971 issued to Wilson et al. on Jul. 25, 2006 and entitled COOLED TURBINE SPAR SHELL BLADE CONSTRUCTION discloses one of these latest airfoils, the entire disclosure being incorporated herein by reference. The spar and shell construction allows for the use of a shell that can be made from an exotic high temperature alloy or material such as tungsten, molybdenum or columbium that could not be used in the prior art investment casting blades or vanes. Airfoils made from the investment casting technique are formed from nickel super-alloys and as a single piece with the internal cooling circuitry cast into the airfoil. Film cooling holes are then drilled after the airfoil has been cast. Without much improvement in the cooling circuitry of these investment cast nickel super-alloy airfoils, the operating temperature is about at its upper limit. 
     Thus, these new spar and shell airfoils will allow for the shell to be formed from the exotic high temperature materials because the shell can be formed using a wire EDM process to form a thin wall shell, and then the shell is supported by a spar to form the blade or vane. The exotic high temperature metals such as tungsten, molybdenum or columbium cannot be cast using the investment casting process because of there very high melting temperatures. However, thin walled shells can be formed using the wire EDM process. With a spar and shell airfoil having a shell made from one of these materials, the operating temperature can be increased way beyond the maximum temperature for an investment cast airfoil. Thus, the engine turbine inlet temperature can be increased and the engine efficiency increased. 
     One major problem with these new spar and shell rotor blades is securing the shell to the blade assembly without inducing too high of a stress level on the blade spar or tip section. Since the rotor blade rotates in the engine, high stress levels are formed on the blade parts that form the blade assembly. In some designs, the blade tip is formed as part of the spar to maintain low stress levels. In some designs, the blade tip is a separate piece from the spar and thus must be attached to the spar while securing the shell to the blade assembly. Because the blade assembly must be supplied with cooling air to provide cooling for the shell, the spar must not be solid but include at least one central passage for supplying the cooling air to the blade assembly. This hollow spar can result in less metal material in the tip region for the tip cap to be secured to the spar. High stress levels have been observed in computer modeling of various designs for the tip cap and spar connection. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide for a turbine rotor blade of the spar and shell construction with a tip cap as a separate piece from the main spar. 
     It is another object of the present invention to provide for a turbine rotor blade of the spar and shell construction with hooks that secure the shell to the spar and form cooling passages for the blade. 
     It is another object of the present invention to provide for a turbine rotor blade of the spar and shell construction with an upper serpentine flow cooling circuit and a lower serpentine flow cooling circuit that will minimize an effect of cooling flow leakage across openings in slots used to secure hooks from the shell. 
     It is another object of the present invention to provide for a turbine rotor blade of the spar and shell construction with a relatively low stress level in the blade tip cap to spar connection of less than 50 ksi. 
     It is another object of the present invention to provide for a turbine rotor blade of the spar and shell construction with cooling channels for the shell. 
     These objectives and more can be achieved by the turbine rotor blade with the spar and shell construction in which the spar includes a number of channels formed on the pressure and the suction sides of the spar that form cooling air passages for the blade assembly when the shell is secured onto the spar. Because of these external formed channels on the spar, the tip section of the spar can be large enough to support a tip cap that is secured to the spar through a tongue and groove connection. The tip cap also functions to retain the shell against radial displacement during rotation of the blade assembly. 
     A main cooling air supply passage extends almost the entire length of the spar and supplies cooling air to the middle cooling channels on the spar through a series of holes located in the spar at a point about midway between the blade tip and the platform. The cooling air flows through the holes and into the two upper serpentine flow passages and two lower serpentine flow passages to provide cooling for the shell on both the pressure side and the suction side of the blade. The cooling air then flows into the leading edge and the trailing edge channels. The spent cooling air is then discharged through rows of film holes on the leading edge and rows of exit holes on the trailing edge of the blade. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows a schematic exploded view of a first embodiment of the spar and shell blade of the present invention. 
         FIG. 2  shows a cross section view of the spar with the cooling supply channel of the present invention for a second embodiment. 
         FIG. 3  shows a cross section view of the blade along a line parallel to the spanwise direction of the blade with the shell hooks engaging the slots formed on the spar outer surface for the second embodiment of the present invention. 
         FIG. 4  shows a side view of the spar of the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is an air cooled turbine rotor blade that has a spar and shell construction The shell is a thin walled shell to provide for relatively low metal temperature due to backside convection and impingement cooling, the shell being secured to the spar by a number of hooks extending from the shell that prevent bulging of the shell due to high cooling air pressure in channels formed between the spar and the shell, to produce a seal between adjacent cooling channels formed between the shell and the spar, and to allow for a relatively large metal surface in the tip region for attaching a separate tip cap to the spar while maintaining low stress levels at the tip section during rotor blade rotation. 
       FIG. 1  shows the turbine rotor blade of the first embodiment of the present invention in an exploded view with a root section  11  that also includes the blade platform, the root section  11  also includes two fir tree legs  12  on the two sides that form a means to secure the blade assembly to a slot in a rotor disk (not shown), a spar  13  that has a fir tree configuration on the bottom end of the same cross section shape as the fir tree legs  12  of the root section  11 , a shell  14 , and a tip cap  15 . The spar  13  also includes a cooling air supply channel  16  that extends from an opening in the root section  13  and ends near to the tip section  17  of the spar  13 . The tip section  17  includes a dovetail slot  18  that extends along a chordwise direction on the tip end for insertion of a dovetail projection formed on a bottom side of the tip cap  15 . The spar  13  also includes a row of holes  19  that connect the cooling supply channel  16  to both the pressure and suction sides of the spar  13 . In the first embodiment, the cooling holes are located at the bottom of the middle cooling passages formed on the outer surface of the spar  13  on both the pressure side and the suction side. In a second embodiment as shown in  FIGS. 2 and 3 , the cooling supply holes are located at a mid-point between the middle cooling passage formed on the external surface of the spar  13  as seen in  FIGS. 2 and 4 . The spar  13  can be formed by the well known investment casting process and form conventional materials such as nickel super alloys, while the shell can be formed from an exotic high temperature material such as molybdenum or columbium that could not be cast but must be formed by an EDM process (electro discharge machining). 
       FIG. 3  shows the shell  14  in place on the spar  13  in a cross section top view for the second embodiment of the spar and shell rotor blade. the spar  13  and the shell  14  both includes hooks  21  and  22  that engage with each other to secure the shell to the spar and prevent the shell from bulging outward due to the high cooling air pressure. Also, the hooks  21  and  22 —when engaged together—form the separation walls between the adjacent radial extending cooling air passages that are formed on the outer surface of the spar  13  and between the shell  14 . The cooling air flowing through the radial passages will also cool the hooks  21  and  22  because the cooling air will also contact the hooks  21  and  22 . 
     The spar  13  includes a central cooling supply passage  16  to deliver cooling air from a source external to the blade. A middle cooling air passage is formed on both sides of the spar  13  with a pressure side middle passage  31  on the pressure side and a suction side middle passage  41  formed on the suction side wall of the spar  13 . Both middle passages  31  and  41  extend the length of the shell as seen in  FIG. 4 . A row of three pressure side cooling holes  29  and a row of suction side cooling holes  29  connect the central cooling supply passage  16  to the respective middle passages  31  and  41 . 
     The spar  13  also forms cooling air passages on both sides to channel the cooling air from the middle passages  31  and  41  to both of the leading edge passage  24  and the trailing edge passage  25  in a serpentine flow path. The pressure side of the spar  13  includes an upper leading edge passage  33  and an upper trailing edge passage  32  located on the sides of the pressure side middle passage  31 . The pressure side of the spar  13  also includes a lower leading edge passage and a lower trailing edge passage also located on the sides of the pressure side middle passage  31 . 
     The spar  13  includes a suction side with similar cooling passages that lead into the leading edge and trailing edge passages  24  and  25 . The middle passage  41  is connected to a suction side leading edge passage  43  and a suction side trailing edge passage  42 . Like on the pressure side of the spar  13 , the suction side also includes an upper serpentine flow passage and a lower serpentine flow passage. 
     The shell  14  includes a row of film cooling holes  51  connected to the leading edge passage  24  that open onto the suction side surface of the leading edge region. The shell  14  also includes a row of trailing edge exit holes  52  to discharge cooling air from the trailing edge passage  25  and cool the trailing edge region of the shell  14 . 
     The blade assembly is assembled by inserting the spar  13  up through an opening formed in the root section  11  from the bottom end. The root section  11  and the spar  13  are formed so that the spar  13  can be inserted further up through the opening in the root section than required in the final assembly arrangement so that the tip cap can be secured to the dovetail groove  18 . With the spar inserted into the opening of the root section  11 , the shell  14  is placed over the spar  13  and the spar  13  inserted far enough into the root section opening so that the dovetail slot  18  extends beyond the top edge of the shell so that the dovetail projection on the tip cap can be inserted into the dovetail slot  18 . With the tip cap  15  in place on the spar  13 , the spar is then backed out of the root section opening  11  so that the fir tree sections on the spar  13  and the root legs  12  are aligned. At this position, the shell is adequately secured between the platform and the tip cap  15 . A shallow groove is formed on the platform surface so that the bottom end of the shell can be inserted into. The platform grooves will allow for thermal expansion of the shell within the blade assembly without inducing stress into the tip cap  15  and the spar  13  so that the shell can be thermally uncoupled from the spar. 
     The slots formed on the spar that receive the hooks on the shell extend below the platform section so that the tip cap can be inserted into the spar with the shell in place. Because the spar  13  will extend further into the opening of the platform so that the spar tip end will extend beyond the top of the shell in order to insert the tip cap into the spar tip groove  18 , the slots are required to extend further toward the lower end of the spar. This section of the slots is left open when the shell is in place and forms a leakage flow path for the cooling air. Because of the lower serpentine flow path formed in the spar  13 , the leakage path formed by the open slots will be minimized because any leakage flow will be part of the serpentine flow passage in the cooling channels. This leakage path thus becomes part of the normal cooling air flow path for the blade. 
     The blade assembly is cooled by passing pressurized cooling air through the cooling supply channel  16  of the spar  13 . The cooling air then flows through the rows of holes  29  and into the middle channels  31  and  41  on the pressure side and the suction side of the spar  13 . as seen in  FIG. 4 , cooling air that flows through the holes  29  and into the middle channels on both sides of the spar  13  will flow into the upper serpentine flow path and the lower serpentine flow path. The cooling air from the serpentine flow paths on the pressure side and the suction side of the spar  13  will then merge into the leading edge passage  24  or the trailing edge passage  25 . The leading edge passage  24  and the trailing edge passage  25  both form a common collection passage for the cooling air from the serpentine flow passages. The cooling air in the leading edge passage  24  will then flow out through the film cooling holes  51  and the cooling air in the trailing edge passage  25  will flow out through the row of exit holes  52 . 
     One of the features of the present invention is that the hooks on the spar and the shell form the cooling air passages. Because of this feature, the cooling air flowing through the passages also acts to cool the hooks. The hooks  21  extending from the shell  14  are hotter than the hooks from the spar because the shell  14  is exposed to the higher temperature. 
     Because of the structure of the spar with the cooling channels formed on the outer surfaces, the tip region of the spar  13  can be large enough with enough metal material to form the dovetail slot and projection arrangement in order to secure the tip cap to the spar  13  while keeping the stress level low enough in a range of less than 50 ksi but preferably below 40 ksi. 
     The tip cap  15  will remain secured into position on the spar  13  when the blade assembly is secured into the slot of the rotor disk. Because of the fir tree arrangement on the root section and the spar bottom end, with the fir trees aligned together and inserted into the disk slot, the tip cap will not be capable of sliding out from the tip groove  18  because of the presence of the shell  14  secured between the tip cap  15  and the platform.