Abstract:
A gas turbine engine blade has a relatively large fillet to improve the characteristics of the air flow thereover. The fillet has a thin wall which, together with an impingement rib, defines a fillet cavity therebetween, and cooling air is provided to flow through impingement holes in the impingement rib and impinge on the rear surface of the fillet. The impingement holes are elongated in cross sectional shape with their elongations being orient in a direction generally transverse to a radial direction.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to turbine blades, and more particularly, to turbine blades with a large fillet and associated cooling features.  
         [0002]     Present turbine blade design configurations include little or no leading edge fillets at the transition between the blade and the associated platform. As a result, several gas path vortices are developed in this region so as to cause hot gases to be trapped in certain areas of the airfoil, thereby resulting in severe distress to those regions.  
         [0003]     One way to alleviate the problem is to introduce large fillets that have a substantial radius such that the gas path vortices are substantially eliminated. A large fillet on the other hand, will tend to add metal and therefore mass to the blade. Such an increase in thermal mass in a fluid area would have negative effects in terms of centrifugal loading and thermal stress fatigue and creep. It is therefore desirable to not only substantially increase the fillet radius but also to reduce the mass that is associated with a larger fillet, and to also provide proper cooling for this area.  
       SUMMARY OF THE INVENTION  
       [0004]     Briefly, in accordance with one aspect of the invention, the thickness of the relatively large fillet is minimized to reduce its mass the impingement cavity behind the leading edge is extend radially inwardly and curve forwardly behind an substantial conformity with the curve of the fillet.  
         [0005]     In accordance with another aspect of the invention, the impingement cavity flattens and widens as it extends towards its radially inner end to thereby provide improved cooling to the fillet.  
         [0006]     In accordance with another aspect of the invention, the impingement cavity is defined on its one side by an impingement rib having impingement holes that are elongated in cross sectional form.  
         [0007]     In accordance with another aspect of the invention, the impingement holes near the blade leading edge are orientated with their elongations radially aligned, and those impingement holes adjacent the fillet are aligned with their elongations in the transverse direction.  
         [0008]     In the drawings as hereinafter described, preferred and alternate embodiments are depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIGS. 1A and 1B  are schematic illustrations of vortex flow models for turbine blades in accordance with the prior art.  
         [0010]      FIG. 2  is a top view of a turbine blade showing the streamlines flowing therearound in accordance with the prior art.  
         [0011]      FIG. 3A  shows comparisons of gas temperature reductions between large and small fillet blades.  
         [0012]      FIG. 3B  shows comparisons of adiabatic wall temperatures between large and small fillet blades.  
         [0013]      FIGS. 4A and 4B  are cutaway views of a large fillet blade in accordance with the present invention.  
         [0014]      FIG. 4C  is a sectional view as seen along lines CC of  FIG. 4B . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     Referring now to  FIGS. 1A and 1B , there is shown an artist&#39;s conception of a vortex structure that results from the flow of hot gases over a turbine blade having no fillet (i.e. with the blade portion intersecting with the platform section at substantially an orthogonal angle). Here, it will be seen, that because of laminar separation that occurs, secondary flow vortices are formed such that hot gases can be trapped on the suction side of the airfoils as shown, and these can then result in severe distress in these regions.  
         [0016]     In  FIG. 2 , there is shown a computational fluid dynamics simulation of the streamlines of gases passing around an airfoil having little or no fillet as discussed hereinabove. Here again, there is evidence of secondary flow vortices that tend to affect the thermal load to the airfoil.  
         [0017]     In an effort to address the problems discussed hereinabove, the airfoil was modified to include a leading edge fillet with a substantial radius. For example, present blade design configurations use leading edge fillets to the blade platforms with a radius, or offset, in the range of 0.080 inches or less. In accordance with the present design of increased fillet size, a fillet is provided having a radius that may be as high as a quarter of the size of the entire radial span or about ⅜ inches or higher. This modification has been found to improve the flow characteristics of the airfoil and to thereby substantially reduce the temperatures in the fillet region. For example, in  FIG. 3A , there is shown a color coded indication of temperatures in three gradations, A, B and C for both an airfoil with no fillet (at the bottom) and one with a large fillet (at the top). In each of these, the cooler range of temperatures is shown by the darker colors A at the bottom and the hotter temperature ranges are shown by the lighter colors C at the top. As will be recognized, the gas temperatures flowing over the modified airfoil (i.e. with a fillet) has a substantially greater portion in the cooler zone A than the airfoil without the fillet. This is the result of the fillet tending to suppress the end wall vortices.  
         [0018]     Similarly, in  FIG. 3B , wherein there is shown a comparison of adiabatic wall temperatures between an airfoil having no fillet (as shown at the left) and one with the fillet (as shown at the right). In each case, the darker portion D is indication of cooler temperature range and the lighter portion E is indicative of a higher temperature range. Again, it will be seen that the adiabatic wall temperatures of the airfoil having a fillet are substantially reduced from those of the airfoil having no fillet.  
         [0019]     Although the use of larger fillets successfully addresses the problem of the secondary flow vortices as discussed hereinabove, the use of such large fillets can also introduce other problems associated with the design and use of an airfoil. Generally, it will be understood that the introduction of a larger fillet will also increase the amount of metal that is in the airfoil. This substantial increase in the mass in the area of the fillet could have a negative effect in terms of centrifugal loading and thermal stress, fatigue and creep. The present invention therefore addresses this problem by reducing the mass of the larger fillet blade and providing for various cooling features that have been found effective in cooling the large fillet leading edges.  
         [0020]     Referring now to  FIGS. 4A and 4B , wherein a turbine blade  11  is shown in a front view and a side view, respectively, the turbine blade  11  has a fir tree  12  for attaching the blade  11  to a rotating member such as a disk, an airfoil portion  13  and a platform  14  having a leading edge  15  and a trailing edge  20  that define a plane x-x. The airfoil portion  13  has a pressure side (i.e. concave side) and a suction side (i.e. convex side), a leading edge  16  that defines a plane Y 1 -Y 1  that is substantially orthogonal to plane x-x and a trailing edge  17 . At the point where the leading edge  16  transitions into and is attached to the platform  14 , there is a relatively large-radius fillet  18  that extends from a point  25  on the platform  14  to a point  30  on the leading edge  16  as shown. The distance D defines the offset between the plane Y 1 -Y 1  and a plane Y 2 -Y 2  that is parallel to plane Y 1 -Y 1  and passes through point  25 . A fillet line F-F extending between points  25  and  30  and forming a fillet angle of θ defines the extent of the fillet  18 . In accordance with the present invention the large fillet  18  is defined by the parameters D and θ with the offset D being in the range of 0.080″ to 0.375″ and the fillet angle θ being in the range of 10° to 60°. It is this large radius fillet that overcomes the problems of end wall vortices as discussed hereinabove.  
         [0021]     As is conventional in these types of blades, there is provided behind the leading edge wall a leading edge cavity  19 , and parallel to that is a coolant supply cavity  21 . The coolant supply cavity  21  is supplied with a source of cooling air that flows up through a pair of radial passages  22 A and  22 B which pass through the fir tree  12 . The coolant supply cavity  21  is fluidly connected to the leading edge cavity  19  by a plurality of impingement cooling passages  23 . These impingement cooling passages  23  are formed in what eventually becomes an impingement rib  35  during the casting process by the insertion of small ceramic core rods which are subsequently removed to leave the impingement cooling passages  23 . Thus, the cooling air passes through the radial passages  22 A and  22 B and into the coolant supply cavity  21 . It then passes through the impingement cooling passages  23  and into the leading edge cavity  19  where it impinges on the inner surface of the leading edge before being discharged to the outside of the blade by way of film holes. In accordance with one aspect of the present invention, the leading edge cavity  19  extends downwardly toward the platform  14  into an expanded fillet cavity  24  directly behind the fillet  18 . The coolant supply cavity  21  is fluidly connected to the fillet cavity  24  by impingement holes  26  formed in the lower portion of the impingement rib  35 .  
         [0022]     In operation, cooling air is introduced into the radial passages  22 A and  22 B, passes into the supply cavity  21  on the back side of the impingement rib  35  and then a portion of the cooling air passes through the impingement cooling passages  23  to cool the leading edge  16  of the blade and a portion thereof passes through the impingement holes  26  to impinge on the inner surface  27  of the fillet  18  and then flow through film cooking holes formed in the fillet  18 .  
         [0023]     Considering now some of the features of the present invention, it will be recognized that the radial passage  22 A is radially aligned with the impingement holes  26  at the lower portion of the impingement rib  35  such that the cooling air flowing through the radial passage  22 A impinges directly on the impingement holes  26  leading to the impingement cavity  24 , where it impinges on the fillet inner surface  27 , such that effective cooling of the inner wall  27  of the fillet  18  can be accomplished.  
         [0024]     Another feature that tends to enhance the cooling function is that of the fillet cavity  24  being wider toward its radially inner end  28  as shown in  FIG. 4A , and also flattened towards its radially inner end as shown in  FIG. 4B . That is, as the fillet cavity  24  approaches its inner end  28 , the distance between the impingement rib  35  and the fillet inner wall  27  decreases so as to place the impingement holes  26  closer to the inner wall  27 . By making the fillet cavity  24  as wide as possible, a wider area of the large fillet  18  is cooled by impingement and more metal is removed from the large fillet  18 , thereby resulting in less mass, stress and creep damage in the blade and attachment.  
         [0025]     Another feature of the present invention is shown in  FIG. 4C  wherein the impingement cooling passages  23  in the radially outer portion of the impingement rib  35 , are elongated in form, with the elongations aligned substantially radially as shown. In the radially inner portion of the impingement ribs  35 , however, the impingement holes  26  are elongated in the lateral direction as shown to thereby more effectively cool the full width of the large fillet  18 .  
         [0026]     The shape of the elongated impingement cooling passages  23  and the impingement holes  26  can be of any generally oval shape such as elliptical or racetrack in form. The limiting factor for how thin and wide the fillet cavity  24  can be made is the geometric constraints of the casting process for the core. A minimum corner radius and draft angle is required for the core features which will dictate a minimum thickness for a given width of the fillet cavity  24 .  
         [0027]     While the present invention has been particularly shown and described with reference to preferred and alternate embodiments as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the true spirit and scope of the invention as defined by the claims.