Abstract:
An airfoil for a gas turbine engine has opposed pressure and suction sidewalls extending between a leading edge and a trailing edge. The airfoil includes an array of radially-spaced apart longitudinally-extending lands which define a plurality of trailing edge slots therebetween. Each of the trailing edge slots has an inlet in fluid communication with an interior of the airfoil and an exit in fluid communication with the trailing edge. At least one of the lands is tapered such that a width of the land measured in a radial direction decreases from the suction sidewall to the pressure sidewall.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to gas turbine components, and more particularly to turbine airfoils. 
   A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and ignited for generating hot combustion gases. These gases flow downstream to one or more turbines that extract energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight. In a turbofan engine, which typically includes a fan placed at the front of the core engine, a high pressure turbine powers the compressor of the core engine. A low pressure turbine is disposed downstream from the high pressure turbine for powering the fan. Each turbine stage commonly includes a stationary turbine nozzle followed in turn by a turbine rotor, each of which includes a plurality of hollow airfoils which are cooled by a combination of internal convective cooling and gas side film cooling. 
   For reasons of aerodynamic efficiency, the high pressure turbine blades or nozzles typically have thin trailing edges and therefore can not accommodate convection cooling openings (i.e. holes or slots) extending all the way to the end of the trailing edge. These openings typically break out from the pressure side surface upstream of the trailing edge. The cooling air leaving the openings mixes with some of the external hot gases and becomes film cooling to protect the downstream trailing edge surface. Slot film cooling typically is the only cooling mechanism for the very aft end of the trailing edge. Trailing edge slots are typically discrete and separated with partition walls. The partition walls are extended outside the slots to form so-called lands. The lands have a tapered height in an axial direction and provide structural support for the trailing edge. These lands are exposed to the hot gas on pressure side and receive a very limited amount of film cooling spilled over from the slots. Therefore, the lands are typically much hotter than the suction side wall on the slot floor between lands. Accordingly, there is a need for providing cooling for these lands. 
   BRIEF SUMMARY OF THE INVENTION 
   The above-mentioned need is met by the present invention, which according to one aspect provides an airfoil for a gas turbine engine, having opposed pressure and suction sidewalls extending between a leading edge and a trailing edge. The airfoil includes an array of radially-spaced apart longitudinally-extending lands defining a plurality of trailing edge slots therebetween. Each of the trailing edge slots has an inlet in fluid communication with an interior of the airfoil and an exit in fluid communication with the trailing edge. At least one of the lands is tapered such that a width of the land measured in a radial direction decreases from the suction sidewall to the pressure sidewall. 
   According to another aspect of the invention, an airfoil for a gas turbine engine has opposed pressure and suction sidewalls extending between a leading edge and a trailing edge. The airfoil includes an array of radially-spaced apart longitudinally-extending lands defining a plurality of trailing edge slots therebetween. Each of the trailing edge slots has an inlet in fluid communication with an interior of the airfoil and an exit in fluid communication with the trailing edge. At least one of the lands is tapered such that a width of the land measured in a radial direction decreases from the suction sidewall to the pressure sidewall; a width of the land measured in a radial direction decreases from the exit to the trailing edge; and a thickness of the land measured in a circumferential direction decreases from the exit to the trailing edge. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
       FIG. 1  is a perspective view of a prior art turbine blade; 
       FIG. 2  is a side view of a portion of the turbine blade of  FIG. 1 ; 
       FIG. 3  is a view taken along lines  3 - 3  of  FIG. 2 ; 
       FIG. 4  is a perspective view of a turbine blade constructed in accordance with the present invention; and 
       FIG. 5  is a side view of a portion of the turbine blade of  FIG. 4 ; 
       FIG. 6  is a view taken along lines  6 - 6  of  FIG. 5 ; 
       FIG. 7  is a view taken along lines  7 - 7  of  FIG. 5 , showing a cross-sectional shape of a trailing edge land; 
       FIG. 8  is a cross-sectional view of an alternative trailing edge land; 
       FIG. 9  is a cross-sectional view of another alternative trailing edge land; and 
       FIG. 10  is a rear view of a turbine airfoil showing a variable-radius slot fillet. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates a prior art turbine blade  10 . The turbine blade  10  includes a conventional dovetail  12  for radially retaining the blade  10  to the disk as it rotates during operation. A blade shank  14  extends radially upwardly from the dovetail  12  and terminates in a platform  16  that projects laterally outwardly from and surrounds the shank  14 . The platform defines a portion of the combustion gases past the turbine blade  10 . A hollow airfoil  18  extends radially outwardly from the platform  16  and into the hot gas stream. The airfoil  18  has a concave pressure sidewall  20  and a convex suction sidewall  22  joined together at a leading edge  24  and at a trailing edge  26 . The blade incorporates a number of trailing edge slots  28  on the pressure side  20  of the airfoil. The trailing edge slots  28  are separated by a number of longitudinally extending lands  30 , which extend from exits  34  of the trailing edge slots  28  to the trailing edge  26 . 
     FIGS. 2 and 3  illustrate the trailing edge portion of the turbine blade  10  in more detail. The width “w” of each land  30  measured in a radial direction decreases from the trailing edge slot exit  34  to the trailing edge  26 . The thickness “t” of each land  30  measured in a circumferential direction (i.e. from the pressure sidewall  20  of the airfoil  18  to the suction sidewall  22  of the airfoil) decreases from the trailing edge slot exit  34  to the trailing edge  26 . 
   In operation, cooling air is supplied to the interior of the airfoil  18 . After optionally being used for other cooling purposes, the cooling air transitions through the trailing edge slots  28  to cool the trailing edge  26 . The lands  30  are exposed to the hot gas at the pressure sidewall  20  and receive a very limited amount of film cooling spilled over from the slots  28 . Therefore, the lands  30  are typically much hotter than the suction sidewall  22  on the slot floor between lands  30 . 
   An exemplary turbine blade  110  constructed according to the present invention is shown in  FIG. 4 . It is noted that the present invention is equally applicable to other types of hollow cooled airfoils, for example stationary turbine nozzles. The turbine blade  110  includes a dovetail  112 , a blade shank  114 , and a platform  116 . A hollow airfoil  118  extends radially outwardly from the platform  116  and into the hot gas stream. The airfoil  118  has a concave pressure sidewall  120  and a convex suction sidewall  122  joined together at a leading edge  124  and at a trailing edge  126 . The blade  110  is similar in overall construction to the prior art blade  10  except for the trailing edge portion, which is shown in more detail in  FIGS. 5 ,  6 , and  7 . The blade  110  incorporates a number of trailing edge slots  128  on the pressure sidewall  120  of the airfoil  118 . The trailing edge slots  128  are separated by a number of longitudinally extending lands  130 . 
   Each trailing edge slot  128  has an inlet (not shown) in fluid communication with the interior of the airfoil  118  and a downstream exit  134  in which exhausts through the pressure sidewall  120  of the blade  110  upstream of the trailing edge  126 . Each land  130  has a forward end  138  at the trailing slot exit  134  and an aft end  140  at the trailing edge  126  of the airfoil  118 . As shown in  FIG. 7 , each land  130  also has a base  142  adjacent the suction sidewall  122 , and a top surface  144  flush with the pressure sidewall  120 . A pair of side faces  146  and  148  extend between the forward end  138  and aft end  140  of each land  130 . 
   The lands  130  are tapered to reduce the amount of surface area at the hottest locations and to improve cooling film coverage. In the example shown in  FIGS. 5 ,  6 , and  7 , the lands  130  are tapered in three directions. The width “W” of each land  130  measured in a radial direction decreases from the trailing edge slot exit  134  to the trailing edge  126 . The thickness “T” of each land  130  measured in a circumferential direction (i.e. from the pressure sidewall  120  of the airfoil  118  to the suction sidewall  122  of the airfoil) decreases from the trailing edge slot exit  134  to the trailing edge  126 . Finally, the width “W” of each land  130  measured in a radial direction decreases from the base  142  of the land  130  (i.e. adjacent the suction sidewall  122 ) to the top surface  144  of the land  130 . 
   The taper of the width “W” from the base  142  to the top surface  144  may be implemented in various ways. for example, as shown in  FIG. 7 , the side faces  146  and  148  of the land  130  are generally planar, and the top surface  144  is a curved surface with a small circular radius.  FIG. 8  depicts another land  130 ′ in which the top surface  144 ′ is substantially planar and has a width greater than that of the top surface  144 . Such a design may be easier to produce than the radiused top surface  144 .  FIG. 9  shows yet another alternative land  130 ″ in which the side faces  146 ″ and  148 ″ have a concave curvature, and the top surface  144 ″ is substantially planar. This may help diffusion of the cooling flow exiting the trailing edge slot  128  and promote film coverage of the land  130 ″. 
   A concave fillet  150  is disposed between the side faces  146  and  148  and the suction sidewall  122 , at the base  142  of the land  130 . The radius “R” of the fillet  150  may be varied from the slot exit  134  to the trailing edge  126  to improve cooling film attachment. For example, as shown in  FIG. 10 , the fillet  150  may have a relatively small first radius R 1  at the slot exit  134 , increasing to a larger second radius R 2  at a position axially aft of the slot exit  134 , and then decreasing to an intermediate third radius R 3  larger than the first radius R 1  but smaller than the second radius R 2 , further downstream near the trailing edge  126 . The fillet  150 , the shape of the top surface  144  and the shape of the side faces  146  and  148  as described above may be selected to suit a particular application. For example, a particular land may include the curved top surface  144 ′ depicted in  FIG. 7  along with the concave side faces  146 ″ and  148 ″ shown in  FIG. 9 . 
   In operation, cooling air provided to the airfoil  110  flows through the interior thereof, where a portion of the flow may be used for cooling purposes such as convection, impingement, leading edge film cooling, etc., in a known manner. Cooling air then flows through the trailing edge slots  128  and out their exits  134 , as shown by the arrows “A”, to provide film cooling for the downstream suction sidewall  122 . As the cooling air flows out the trailing edge slots  128 , the tapered lands  130  encourage diffusion of the flow and promote attachment of a cooling film. The tapered lands  130  have a reduced hot land surface area compared to prior art trailing edge lands, further encouraging the exit film to spread wider and improve the film coverage. 
   The foregoing has described a cooled airfoil for a gas turbine engine. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.