Abstract:
A cooling system for a turbine airfoil of a turbine engine having a trailing edge cooling channel with bifurcated exhaust channels formed by suction and pressure side trailing edge cooling channels in fluid communication with a central trailing edge cooling channel. The suction and pressure side trailing edge cooling channels may be separated with a trailing edge rib. The suction and pressure side trailing edge cooling channels may be recessed from the airfoil external surface to control the flow of cooling fluids from the cooling system such that the exhaust flow minimizes shear mixing and thus lowers the aerodynamic loss yet maintains high film cooling effectiveness for the airfoil trailing edge.

Description:
FIELD OF THE INVENTION 
     This invention is directed generally to turbine airfoils, and more particularly to cooling systems in hollow turbine airfoils. 
     BACKGROUND 
     Typically, gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel and igniting the mixture, and a turbine blade assembly for producing power. Combustors often operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose turbine blade assemblies to these high temperatures. As a result, turbine blades must be made of materials capable of withstanding such high temperatures. In addition, turbine blades often contain cooling systems for prolonging the life of the blades and reducing the likelihood of failure as a result of excessive temperatures. 
     Typically, turbine blades are formed from a root portion having a platform at one end and an elongated portion forming a blade that extends outwardly from the platform coupled to the root portion. The blade is ordinarily composed of a tip opposite the root section, a leading edge, and a trailing edge. The inner aspects of most turbine blades typically contain an intricate maze of cooling channels forming a cooling system. The cooling channels in a blade receive air from the compressor of the turbine engine and pass the air through the blade. The cooling channels often include multiple flow paths that are designed to maintain all aspects of the turbine blade at a relatively uniform temperature. However, centrifugal forces and air flow at boundary layers often prevent some areas of the turbine blade from being adequately cooled, which results in the formation of localized hot spots. Localized hot spots, depending on their location, can reduce the useful life of a turbine blade and can damage a turbine blade to an extent necessitating replacement of the blade. 
     Typically, the trailing edge of turbine airfoils develop hot spots. Trailing edges are thus often designed to be thin and include cooling channels that exhaust cooling cooling fluids from the pressure side of the trailing edge. This design minimizes the trailing edge thickness but creates shear mixing between the cooling air and the mainstream flow as the cooling air exits from the pressure side. The shear mixing of the cooling fluids with the mainstream flow reduces the cooling effectiveness of the trailing edge overhang and thus, induces over temperature at the airfoil trailing edge suction side location. Frequently, the hot spot developed in the trailing edge becomes the life limiting location for the entire airfoil. Thus, a need exists for a cooling system capable of providing sufficient cooling to trailing edge of turbine airfoils. 
     SUMMARY OF THE INVENTION 
     This invention relates to a turbine airfoil cooling system for a turbine airfoil used in turbine engines. In particular, the turbine airfoil cooling system may include one or more internal cavities positioned between outer walls of a generally elongated, hollow airfoil of the turbine airfoil. The cooling system may include one or more trailing edge cooling channels positioned within the generally elongated, hollow airfoil and proximate to a trailing edge and may be bifurcated and recessed from the airfoil external surface to minimize shear mixing at the trailing edge, thereby reducing aerodynamic loss while maintaining high film cooling effectiveness for the trailing edge. In at least one embodiment, the trailing edge cooling channel may include a central trailing edge cooling channel, a suction side trailing edge cooling channel extending from the central trailing edge cooling channel through the trailing edge, and a pressure side trailing edge cooling channel extending from the central trailing edge cooling channel through the trailing edge. The suction side trailing edge cooling chamber and the pressure side trailing edge cooling channel may be separated by a trailing edge rib forming the trailing edge and positioned in a general spanwise direction. 
     The turbine airfoil may be formed from a generally elongated, hollow airfoil formed by an outer wall and having a leading edge, a trailing edge, a tip section at a first end, a root coupled to the airfoil at an end generally opposite the first end for supporting the airfoil and for coupling the airfoil to a disc, and a cooling system formed from at least one cavity in the elongated, hollow airfoil positioned in internal aspects of the generally elongated, hollow airfoil. The suction side trailing edge cooling chamber and the pressure side trailing edge cooling channel may each include support ribs. The support ribs in the at least one pressure side trailing edge cooling channel may be aligned in a spanwise direction with the plurality of suction side chordwise support ribs in the suction side trailing edge cooling channel. In another embodiment, the plurality of pressure side chordwise support ribs in the pressure side trailing edge cooling channel may be offset in a spanwise direction from the plurality of suction side chordwise support ribs in the suction side trailing edge cooling channel. 
     A plurality of pin fins may be included in the central trailing edge cooling channel to increase the turbulence and cooling effectiveness of the central trailing edge cooling channel. The pin fins may extend from an inner surface of the outer wall forming the suction side to an inner surface of the outer wall forming the pressure side. The plurality of pin fins in the central trailing edge cooling channel may be aligned into rows extending in a spanwise direction. 
     The cavity in the elongated, hollow airfoil of the cooling system may include a serpentine cooling channel having an opening for receiving cooling fluids from a fluid supply source and includes at least one exhaust orifice in an internal rib for exhausting cooling fluids into the at least one trailing edge cooling channel. A plurality of trip strips may extend inwardly from inner surfaces of the outer wall forming the serpentine cooling channel. A leading edge cooling channel may be positioned proximate to the leading edge, extending generally spanwise to the leading edge, and in fluid communication with the at least one cavity forming the cooling system. 
     During use, cooling fluids may flow into the cooling system from a cooling fluid supply source. A portion of the cooling fluids may flow into the leading edge supply channel, through the supply orifices and into the leading edge cooling channel. The cooling fluids may then flow from the leading edge supply channel through film cooling holes forming a showerhead in the leading edge. The remaining portion of cooling fluids may flow from the cooling fluid supply source into the serpentine cooling channel. The cooling fluids may flow back and forth spanwise between the root to the tip section in the serpentine cooling channel. A portion of the cooling fluids in the serpentine cooling channel may be exhausted through the film cooling holes. The remaining portion of the cooling fluids may be passed through the one or more inlets into the central trailing edge cooling channel. The cooling fluids may then flow past the pin fins and around the trailing edge rib through either the suction or pressure side trailing edge cooling chambers. The cooling fluids may then be exhausted from the trailing edge of the elongated airfoil. 
     An advantage of this invention is that bifurcated trailing edge cooling channels exhaust cooling fluids from the trailing edge forming a concurrent cooling fluid flow that minimizes shear mixing between the cooling fluid and the mainstream flow, thereby enhancing the effectiveness of the airfoil trailing edge. 
     Another advantage of this invention is that bifurcated and recessed trailing edge cooling channels reduce the airfoil trailing edge thickness, thereby lowering the airfoil aerodynamic blockage and increase turbine stage performance and efficiency. 
     These and other embodiments are described in more detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the presently disclosed invention and, together with the description, disclose the principles of the invention. 
         FIG. 1  is a perspective view of a turbine airfoil having features according to the instant invention. 
         FIG. 2  is a cross-sectional view of the turbine airfoil shown in  FIG. 1  taken along line  2 - 2 . 
         FIG. 3  is a detailed cross-sectional view of the trailing edge cooling chamber shown in  FIG. 2  along line  3 - 3 . 
         FIG. 4  is a partial front view of the trailing edge looking chordwise taken at line  4 - 4  in  FIG. 1 . 
         FIG. 5  is a partial front view of an alternative embodiment of the trailing edge looking chordwise taken at line  4 - 4  in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIGS. 1-5 , this invention is directed to a turbine airfoil cooling system  10  for a turbine airfoil  12  used in turbine engines. In particular, the turbine airfoil cooling system  10  may include one or more internal cavities  14 , as shown in  FIG. 2 , positioned between outer walls  16  of a generally elongated, hollow airfoil  20  of the turbine airfoil  12 . The cooling system  10  may include one or more trailing edge cooling channels  18  positioned within the generally elongated, hollow airfoil  20 . The trailing edge cooling channels  18  may be positioned proximate to a trailing edge  22  and may be bifurcated to minimize shear mixing at the trailing edge  22 , thereby reducing aerodynamic loss while maintaining high film cooling effectiveness for the trailing edge  22 . In at least one embodiment, the trailing edge cooling channel  18  may include a central trailing edge cooling channel  24 , a suction side trailing edge cooling channel  26  extending from the central trailing edge cooling channel  24  through the trailing edge  22 , and a pressure side trailing edge cooling channel  28  extending from the central trailing edge cooling channel  24  through the trailing edge  22 . The suction side trailing edge cooling chamber  26  and the pressure side trailing edge cooling channel  28  may be separated by a trailing edge rib  42  forming the trailing edge and positioned in a general spanwise direction. The suction side trailing edge cooling chamber  26  and the pressure side trailing edge cooling channel  28  may be recessed from an outer surface  21  forming the trailing edge  22  to create space for the exhaust cooling fluids to collect. The trailing edge rib  42  forms the effective thickness of the trailing edge  22 , as shown with arrows  76 . 
     The turbine airfoil  12  may be formed from a generally elongated, hollow airfoil  20  coupled to a root  30  at a platform  32 . The turbine airfoil  12  may be formed from conventional metals or other acceptable materials. The generally elongated airfoil  20  may extend from the root  30  to a tip section  34  and include a leading edge  36  and the trailing edge  22 . Airfoil  20  may have an outer wall  16  adapted for use, for example, in a first stage of an axial flow turbine engine. Outer wall  16  may form a generally concave shaped portion forming a pressure side  38  and may form a generally convex shaped portion forming the suction side  40 . The cavity  14 , as shown in  FIG. 2 , may be positioned in inner aspects of the airfoil  20  for directing one or more gases, which may include air received from a compressor (not shown), through the airfoil  20  to reduce the temperature of the airfoil  20 . The cavity  14  may be arranged in various configurations and is not limited to a particular flow path. 
     The cooling system  10 , as shown in  FIGS. 2-3 , may include the trailing edge cooling channel  18  positioned within the generally elongated, hollow airfoil  20  and proximate to the trailing edge  22 . The at least one trailing edge cooling channel  18  may include the central trailing edge cooling channel  24 , one or more suction side trailing edge cooling channels  26  extending from the central trailing edge cooling channel  24  through the trailing edge  22 , and one or more pressure side trailing edge cooling channels  28  extending from the central trailing edge cooling channel  24  through the trailing edge  22 . The suction side trailing edge cooling chamber  26  and the pressure side trailing edge cooling channel  28  may be recessed from an outer surface  21  forming the trailing edge  22  to create space for the exhaust cooling fluids to collect. By recessing the suction and pressure side trailing edge cooling channels  26 ,  28  into the airfoil  20 , additional space may be created for the cooling fluids being exhausted from the airfoil  20  to reduce turbulence in the film cooling. The suction side trailing edge cooling channel  26  and the pressure side trailing edge cooling channel  28  may be separated by a trailing edge rib  42  forming the trailing edge  22  and positioned in a general spanwise direction. 
     The cooling system  10  may also include one or more chordwise support ribs  44  extending chordwise from the outer wall  16  into contact with the trailing edge rib  42 . In at least one embodiment, the cooling system  10  may include a plurality of chordwise support ribs  44 . The plurality of chordwise support ribs  44  may include one or more suction side chordwise support ribs  46  positioned in the suction side trailing edge cooling channel  26 . Similarly, the plurality of chordwise support ribs  44  may include one or more pressure side chordwise support ribs  48  positioned in the pressure side trailing edge cooling channel  28 . As shown in  FIG. 4 , the pressure side chordwise support ribs  48  in the pressure side trailing edge cooling channel  28  may be aligned in a spanwise direction with the plurality of suction side chordwise support ribs  46  in the suction side trailing edge cooling channel  26 . In another embodiment, as shown in  FIG. 5 , the pressure side chordwise support ribs  48  in the pressure side trailing edge cooling channel  28  may be offset in a spanwise direction from the suction side chordwise support ribs  46  in the suction side trailing edge cooling channel  26 . 
     The cooling system  10  may also include a plurality of pin fins  50  in the central trailing edge cooling channel  24 . The pin fins  50  may extend from an inner surface  52  of the outer wall  16  forming the suction side  40  to an inner surface  52  of the outer wall  16  forming the pressure side  38 . The pin fins  50  in the central trailing edge cooling channel  24  may be aligned into rows extending in a spanwise direction. The pin fins  50  within the rows may be aligned or offset in the spanwise direction from each other. 
     The cooling system  10  may also include a serpentine cooling channel  54  positioned within central aspects of the elongated airfoil  20 . The serpentine cooling channel  54  may include an opening  56  for receiving cooling fluids from a fluid supply source and may include an exhaust orifice  58  in an internal rib  60  for exhausting cooling fluids into the trailing edge cooling channel  18 . The serpentine cooling channel  54  may be formed from three legs, as shown in  FIG. 2 , or in other number of legs. The serpentine cooling channel  54  may also include one or more trip strips  62  extending inwardly from inner surfaces  52  of the outer wall  16  forming the serpentine cooling channel  54 . The trip strips  62  may be orthogonal to the flow of cooling fluids through the channels or may be positioned at other angles. 
     The cooling system  10  may include one or more leading edge cooling channels  64  positioned proximate to the leading edge  36 . The leading edge cooling chamber  64  may extend generally spanwise and along the leading edge  36 . The leading edge cooling chamber may be in fluid communication with the cavity  14  forming the cooling system  10  and in particular, may be in contact with a leading edge supply channel  66  through one or more supply orifices  68 . 
     During use, cooling fluids may flow into the cooling system  10  from a cooling fluid supply source. A portion of the cooling fluids may flow into the leading edge supply channel  66 , through the supply orifices  68  and into the leading edge cooling channel  64 . The cooling fluids may then flow from the leading edge supply channel  66  through film cooling holes  70  forming a showerhead in the leading edge  36 . The remaining portion of cooling fluids may flow from the cooling fluid supply source into the serpentine cooling channel  54 . The cooling fluids may flow back and forth spanwise between the root  30  to the tip section  34  in the serpentine cooling channel  54 . A portion of the cooling fluids in the serpentine cooling channel  54  may be exhausted through the film cooling holes  70 . The remaining portion of the cooling fluids may be passed through the one or more exhaust orifices  58  into the central trailing edge cooling channel  24 . The cooling fluids may then flow past the pin fins  50  and around the trailing edge rib  42  through either the suction or pressure side trailing edge cooling chambers  26 ,  28 . The cooling fluids may then be exhausted from the trailing edge  22  of the elongated airfoil  20 . 
     The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of this invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of this invention.