Abstract:
An air cooled turbine airfoil, such as a rotor blade, with a sequential impingement cooling circuit that provides a high level of cooling with a low amount of cooling air. The airfoil is formed of a spar and shell construction in which the spar is formed from a series of alternating layers that form the cooling circuit with the shell forming the gap for the impingement cooling channels. Two different layers form the impingement cooling channels with a third layer being a separation layer. The impingement cooling circuit includes a first impingement channel to cooling a forward half of the pressure side wall, a second impingement cooling channel to cool the aft half of the pressure side wall, a third impingement cooling channel to cool a forward section of the suction side wall, and a fourth impingement cooling channel to cool the aft section of the suction side wall.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a CONTINUATION-IN-PART of U.S. patent application Ser. No. 12/351,624 filed on Jan. 9, 2009 and entitled AIR COOLED TURBINE AIRFOIL WITH SEQUENTIAL COOLING. 
    
    
     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 an air cooled turbine airfoil. 
     2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
     In a gas turbine engine, air is first compressed to a high pressure in a compressor. The high pressure air is then mixed with fuel and burned at nearly constant pressure in the combustor. The high temperature gas exhausted from the combustor is then expanded through a turbine which then drives the compressor. If executed correctly, the exhaust stream from the turbine maintains sufficient energy to provide useful work by forming a jet, such as in aircraft jet propulsion or through expansion in another turbine which may then be used to drive a generator like those used in electrical power generation. The efficiency and power output from these machines will depend on many factors including the size, pressure and temperature levels achieved and an agglomeration of the efficiency levels achieved by each of the individual components. 
     Current turbine components are cooled by circulating relatively cool air (relative to the hot gas stream passing through the turbine), which is extracted from the compressor, within passages located inside the component to provide a convective cooling effect. In many recent arrangements, the spent cooling flow is discharged onto the surfaces of the component to provide an additional film cooling effect. 
     The challenge to cool first stage turbine vanes (these are exposed to the highest temperature gas flow), in particular, is complicated by the fact that the pressure differential between the vane cooling air and the hot gas which flows around the airfoil must necessarily be small to achieve high efficiency. Specifically, coolant for the first stage turbine vane is derived from the compressor discharge, while the hot gas is derived from the combustor exit flow stream. The pressure differential available for cooling is then defined by the extremely small pressure drop which occurs in the combustor. This is because the pressure of the coolant supplied to the vane is only marginally higher than the pressure of the hot gas flowing around the airfoil as defined by the combustor pressure loss, which is desirably small. This pressure drop is commonly on the order of only a few percentage points. Further, it is desirable to maintain coolant pressure inside the vane higher than the pressure in the hot gas flow path to insure coolant will always flow out of the vane and not into the vane. Conversely, in the event hot gas is permitted to flow into the vane, serious material damage can result as the materials are heated beyond their capabilities and progression to failure will be swift. As a consequence, current first stage turbine vanes are typically cooled using a combination of internal convection heat transfer using single impingement at very low pressure ratio, while spent coolant is ejected onto the airfoil surface to provide film cooling. 
     The efficiency of the convective cooling system is measured by the amount of coolant heat-up divided by the theoretical heat-up possible. In the limits, little coolant heat-up reflects low cooling efficiency while heating the coolant to the temperature of the surface to be cooled (a theoretical maximum) yields 100% cooling efficiency. In the previous methods using single impingement, the flow could only be used once to impinge on the surface to be cooled. This restriction precludes the ability to heat the coolant substantially, thereby limiting the cooling efficiency. 
     Designers and operators of gas turbine engines are always looking for ways to improve the performance of the engines. For example, an industrial gas turbine engine is operated by a power company to produce electricity. To improve the performance of the first stage turbine stator vane, additional cooling air can be provided to the vane to produce better cooling, or additional film cooling holes can be drilled into the vane to provide more film cooling for specific surfaces of the vane. This requires more cooling air from the compressor and therefore does not provide much of an improvement. 
     Another way to improve the performance of a turbine first stage stator vane is to redesign the internal cooling circuit and then cast a new stator vane with the redesigned cooling circuit. The new vane is then placed in the engine and tested during normal operation. This is a very expensive way to improve the performance of the first stage vane because of the high costs to cast and machine the new vane. If the new vane has little or no improvement in the engine, then the cost of the redesign and casting is a loss. 
     BRIEF SUMMARY OF THE INVENTION 
     A turbine vane for a first or second stage industrial gas turbine engine, where the vane includes an inner spar or insert that forms a series of impingement cooling passages to cool the airfoil walls. The insert is formed from a stack of layers that are bonded together to form the cooling air passages. A first layer provides impingement cooling to a forward section on the pressure wall side followed by impingement cooling of the aft section of the pressure wall side of the airfoil. The spent cooling air from the aft section of the pressure wall side is then split up into two flows with one flow producing impingement cooling to the aft section of the suction wall side which is then discharged through the trailing edge region of the airfoil and a second flow that produces impingement cooling on the forward section of the suction wall side which is then discharged through film cooling holes on the leading edge region of the suction side of the airfoil. 
     The stacked layers that form the serial impingement cooling insert is formed from a metal printing process that can produce very small cooling air features that cannot be formed using the lost wax or investment casting process. 
     The turbine stator vane can be a spar and shell construction with separate end walls from the shell and the stacked insert secured within the shell and end wall assembly to form the vane having the serial impingement cooling circuit that the stacked inserts provides for. 
     In another embodiment of the present invention, an old turbine stator vane can be improved by hollowing out the internal cooling air features to leave a shell of the airfoil with the inner and outer end walls intact, and the serial impingement insert of the present invention can be secured within the hollowed out shell to produce a new and improved stator vane using the old structure without having to produce an entirely new stator vane. The vane is hollowed out and hooks are cut or later welded to the inner surface of the airfoil so that the stacked insert can be secured in place within the airfoil to produce the serial impingement cooling. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows an isometric view of a shell with hooks of the present invention. 
         FIG. 2  shows a close-up view of a section of the shell in  FIG. 1  with one of the hooks. 
         FIG. 3  shows an isometric view of a top layer on an impingement layer of the stacked insert of the present invention. 
         FIG. 4  shows an isometric view of an impingement layer of the stacked insert of the present invention. 
         FIG. 5  shows an isometric view of a collection layer of the stacked insert of the present invention. 
         FIG. 6  shows an isometric view of a top section of the stacked inserts of the present invention in assembled formation. 
         FIG. 7  shows a top view of the pressure side cooling layer of the stacked insert of the present invention secured within an airfoil wall. 
         FIG. 8  shows a top view of the suction side cooling layer of the stacked insert of the present invention secured within an airfoil wall. 
         FIG. 9  shows the outer and inner end walls used to form the vane of the present invention. 
         FIG. 10  shows a cross section side view of a stator vane with a stacked insert for serial impingement cooling of the present invention. 
         FIGS. 11 and 12  shows cross section views of the upper section and the lower section of the stacked ins 
         FIG. 13  shows a cross section view of a spar and shell vane with separate end walls having the stacked insert of the present invention. 
         FIG. 14  shows a cross section view of one method of securing the stacked insert within an airfoil of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A turbine stator vane, especially for a vane used in an industrial gas turbine engine, includes an insert secured within the airfoil section to provide serial impingement cooling for the backside walls of the airfoil. The impingement cooling insert is formed from an alternating stack of plates that form the cooling circuit for the vane airfoil section. The impingement cooling insert can be secured with a shell formed separately from inner and outer endwalls or in a hollowed out airfoil with the endwalls formed as a single piece with the airfoil section. 
       FIG. 1  shows a shell  11  that forms the spar and shell type of vane or blade. The shell includes hooks  12  that secure the shell walls to the insert as described below. 
       FIGS. 3 through 5  shows the three plates used to form the insert that produces the impingement cooling for the backside walls of the shell or airfoil section of the vane. The insert is formed with a top layer or plate  21  followed by an alternating arrangement of plate A  31  and plate B  32 .  FIG. 3  shows a top layer  21  with a plate A below it. The top layer  21  includes a cooling air supply cavity  22 , receivers  23  for the hooks from the airfoil, and two holes  24  and  25  that can be used for bolts that secure the stacked plates  21 ,  31  and  41  together to form the insert. In  FIGS. 3 through 5 , the thickness or length in the airfoil spanwise direction is exaggerated in order to show the details of the film cooling holes and other features. The actual plates that are used to form the insert would be very thin in order to provide very small spacing between film cooling holes in the spanwise direction. The plates are so thin that the features formed within the plates could not be cast using the investment casting process because of the small size required. Ceramic pieces of this size would be broken during the liquid metal pouring process. 
       FIG. 4  shows the plate A  31  with a cooling air supply cavity  22  aligned with the cooling air supply cavity in the top plate  21 , hook receivers aligned with the hook receivers of the top plate  21 , a row of impingement cooling holes  32  located in an forward section of the pressure side wall of the airfoil, a collection and supply channel  33  for an aft section of the suction side wall of the airfoil, a collection and supply channel  34  for a forward section of the suction side wall of the airfoil, a row of impingement cooling holes for the aft section of the suction side wall of the airfoil, and a row of impingement cooling holes for a forward section of the suction side wall of the airfoil. 
       FIG. 5  shows the plate B  41  with a cooling air supply cavity  22  aligned with the cooling air supply cavities in the top plate  21  and the plate A, hook receivers aligned with the hook receivers of the top plate  21 , a pressure side wall spent impingement cooling air collection channel  42 , a supply channel  43  for an aft section of the pressure side wall, and a row of impingement holes  44  for the aft section of the pressure side wall. 
       FIG. 6  shows an upper section of the insert with the top plate  21  on top and the alternating arrangement of the plates A and B ( 31  and  41 ) stacked below the top plate  21 . The collection chambers  33  and  34  for the aft section of the pressure side wall impingement chamber is shown and the collection chamber  42  for the forward section of the pressure side impingement chamber is shown in  FIG. 6  formed between the stack of plates  31  and  41 . When the plates  31  and  41  are stacked together, a gap is formed between the plates ( 31  and  41 ) and the inner side of the airfoil or shell wall for the cooling air to flow toward the wall or away and into the next channel and row of impingement cooling holes. The gaps extend along the spanwise direction of the airfoil and are only separated by the hooks in the chordwise direction. The plates  21 ,  31  and  41  can be bonded together such as with a TLP (transient liquid phase) bonding process or with bolts that pass through the bolt holes  24  and  25 . 
       FIGS. 7 and 8  shows the two plates A and B ( 31  and  41 ) that form the serial impingement cooling circuit for the airfoil with the plates secured to the hooks that extend from the inner side of the airfoil or shell that forms the vane. In this embodiment of the present invention, three hooks are used that separate the cooling channels for the airfoil into four sections which are the forward pressure side impingement channel or chamber  51  (see  FIG. 7 ), an aft pressure side impingement chamber  52  (see  FIG. 8 ), an aft suction side impingement chamber  53  ( FIG. 7 ), and a forward suction side impingement chamber  54  ( FIG. 7 ). Each of the four impingement chambers  51 - 54  have about the same chordwise length. However, the four chambers  51 - 54  can have different chordwise lengths depending upon the cooling requirements of the shell. In this particular embodiment, the insert forms a cooling circuit in which the forward pressure side chamber  51  is cooled first, the spent impingement air collected and passed into the aft pressure side chamber  52  in series with the forward pressure side chamber  51 , where the spent impingement cooling air from the aft pressure side chamber  52  is collected and then split up into two channels  33  and  34 . Channels  33  and  34  then flow the cooling air to the aft suction side impingement chamber  53  and the forward suction side impingement chamber  54  in parallel to provide impingement cooling to the suction side wall of the airfoil or shell. The spent impingement cooling air from the aft suction side impingement chamber is channeled out through exit holes formed along the trailing edge of the shell or airfoil. The spent impingement cooling air from the forward suction side impingement chamber  54  is channeled out through film cooling holes formed in a leading edge region of the shell or airfoil on the suction side. 
     When all of the plates are stacked together to form the insert, a series of impingement cooling circuits are formed that extend along the spanwise or radial direction of the insert. Each of the impingement cooling circuits that form the series includes the four impingement cooling chambers  51 - 54  as described in  FIGS. 7 and 8 . The three hooks separate the four impingement chamber  51 - 54  in the chordwise direction. The plates  21 ,  31 , and  41  are all recessed within the shell so that a gap is formed between the outer side of the plates and the inner surface of the shell. In the forward pressure side impingement chamber  51 , each of the impingement cooling holes  32  in the plate  31  and the return chamber  42  in the plate  41  are connected to the gap formed between the plates and the inner side of the shell formed between the two hooks and extend from the platform to the blade tip. All of the impingement cooling air discharged into this gap can flow back into any one of the return chambers  42  formed in any one of the plates  41  because the gap is open to all of the impingement holes  32  and return chambers  42 . The same open gap is formed with the other three impingement chambers  52 - 54 . 
     As seen in  FIG. 4 , the cooling air flows through the impingement holes  32  to produce impingement cooling for the forward section of the pressure side wall. The spent impingement cooling air is then collected in the chamber  42  formed by the plate  41  shown in  FIG. 5 . The spent impingement cooling air is then channeled through channel  43  to the impingement holes  44  to produce impingement cooling for the aft section of the pressure side wall. The spent impingement cooling air from the impingement holes  44  is then channeled through channels  33  and  34  toward the two impingement chambers on the suction side wall. The spent impingement cooling air from the impingement holes  35  is channeled in the chamber formed between the airfoil inner surface and the insert to the trailing edge exit holes. The spent impingement cooling air from the impingement holes  36  is channeled in the chamber formed between the airfoil inner surface and the insert to the leading edge film cooling holes. 
     The separate plates are bonded together using a process such as Transient Liquid Phase (TLP) bonding, or by mechanical fasteners. The impingement cooling holes and the cooling circuit formed on the plates are of very small size that cannot be formed using the lost wax or investment casting process. Thus, the plates must be formed using a process such as that developed by Mikro Systems, Inc. of Charlottesville, Va. that can print metallic and ceramic parts as a single piece with very fine details that cannot be produced using the prior art investment casting process. Each of the plates  21 ,  31  and  41  can be printed as a separate piece and then bonded together using the TLP bonding process. If the impingement cooling holes and other cooling air passages or features of the plates can be formed by investment casting process, then the entire insert can be formed as a single piece without using the stacked plates that are bonded together. The reason for using the stacked plates that are bonded together is because the fine cooling air features used in the one embodiment of the present invention that cannot be cast using the investment casting process because of the casting limitations such as with the ceramic core. 
       FIG. 9  shows an outer end wall  61  and an inner end wall  62  used to form the stator vane. A shell forming the airfoil section is secured between the two end walls  61  and  62  with the insert formed from the stacked plates secured within the shell and between the endwalls.  FIG. 10  shows the shell secured between the two endwalls  61  and  62  in a thermally free state and with rope seals (preferably made from Nextel) providing a seal between the end wall and the shell.  FIGS. 11 and 12  shows the outer end wall  61  and the inner end wall  62  with rope seals  63  to secure the gap formed between the shell and the end wall. The insert is secured within the shell and the endwalls.  FIG. 13  shows outer and inner endwalls  61  and  62  with the shell  1  secured between them in a thermally free state and the insert formed from the stacked plates secured within the shell and between the endwalls. 
     In another embodiment of the invention, an old vane can be improved by hollowing out the interior cooling air passages and securing the insert of the present invention formed from the stacked plates to provide for backside wall impingement cooling of the airfoil surface of the old vane. The old vane with its two endwalls can be machined to leave a hollow space within the airfoil walls in which the insert formed from the stacked plates can be then secured.  FIG. 14  shows an embodiment in which a vane having an outer end wall  61  and an inner end wall  62  and an airfoil surface  65  is hollowed out and the insert formed from the stacked plates  66  is secured by welding a plate  67  that covers over the top ends of the outer end wall  61  and the insert  66 . Seals are placed around the lower end between the insert and the hollowed out interior of the airfoil  65 . With this embodiment, the old vane can be tested with various different cooling circuits by using different inserts formed from stacked plates until a desired amount of cooling capability is found without having to recast a new vane having the newly designed cooling circuit.