Patent 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 partially defines a fillet cavity therebehind, and cooling air is provided to the fillet cavity and is then routed to the outer surface by way film cooling holes. Various design features are provided to enhance the effectiveness of the cooling air being provided to both the fillet cavity and other cavities within the blade.

Full Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to turbine blades, and more particularly, to turbine blades with a large fillet and associated cooling features. 
   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. 
   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 
   Briefly, in accordance with one aspect of the invention, the thickness of the relatively large fillet is minimized to reduce its mass while a dedicated radial passage is introduced to pass cooling air over the back side of the fillet and leading edge before venting through a series of film holes. 
   In accordance with another aspect of the invention, the dedicated radial passage introduces the flow of coolant air so as to impinge at the base of the fillet area and flow upwardly over a series of cooling features such as hemispherical dimples, before exiting from leading edge film holes. 
   In accordance with another aspect of the invention, the ceramic core which ties the supply and leading edges cores and when removed forms impingement cooling passages between the internal cavities of the blade, are replaced with a refractory metal core which involves a very small core height with features such as pedestals that can be lasered in the core to enhance heat transfer. 
   In accordance with another aspect of the invention, the cross-over holes between the internal cavities is modified from a circular shape to a race-track shape for better target wall coverage. 
   In accordance with another aspect of the invention, the placement of the leading edge impingement cross-over holes are off-set from the mid plane toward the pressure side of the blade. 
   By yet another aspect of the invention, trip strips are included in the impingement feed cavity, and the impingement cross-over holes are located substantially between adjacent trip strips so as to avoid interference between the structures. 
   In accordance with another aspect of the invention, the entrance to the leading edge fed passage is bell-mouthed in shape in order to enhance the flow characteristics of the cooling air. 
   In accordance with another aspect of the invention, the radial gap between the leading edge showerhead holes and the fillet showerhead holes is reduced to enhance the cooling effect thereof. 
   By yet another aspect of the invention, the discrete laser holes are replaced with forward-diffused shaped holes to increase the film cooling coverage and reduce the potential for plugged holes with adverse impacts on local metal temperatures. 
   By yet another aspect of the invention, the feed holes are metered so as to provide for desirable flow control. 
   By yet another aspect of the invention, a trench is provided on the inner surface of the leading edge so as to take better advantage of the cooler portion of the air stream. 
   By another aspect of the invention, micro-circuit internal features are used to uniformly distribute and reduce cooling flow, and micro-circuit pedestals are used to serve as conduction paths and flow turbulence promoters while offering structural integrity to the micro-circuit inside the large fillet. 
   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 
       FIGS. 1A and 1B  are schematic illustrations of vortex flow models for turbine blades in accordance with the prior art. 
       FIG. 2  is a top view of a turbine blade showing the streamlines flowing therearound in accordance with the prior art. 
       FIG. 3A  shows comparisons of gas temperature reductions between large and small fillet blades. 
       FIG. 3B  shows comparisons of adiabatic wall temperatures between large and small fillet blades. 
       FIGS. 4A and 4B  are cut away views of a large fillet blade in accordance with the present invention. 
       FIGS. 5A and 5B  are illustrations of an alternate embodiment thereof. 
       FIGS. 6A and 6B  show features of the cross-over holes in accordance with the present invention. 
       FIG. 7  shows the placement and use of dimples in accordance with an embodiment of the present invention. 
       FIGS. 8A and 8B  are illustrations of another alternative embodiment of a large fillet blade in accordance with the present invention. 
       FIGS. 9A–9C  show the use of micro-circuit cores in the blade leading edge fillet area in accordance with the present invention. 
       FIG. 10  shows the location of the cross-over holes in accordance with an embodiment of the present invention. 
       FIGS. 11A and 11B  show another embodiment of the cross-over hole location in accordance with the present invention. 
       FIG. 12  shows the entrance at the bottom of the leading edge feed passage in accordance with one embodiment of the invention. 
       FIG. 13  shows the relationship between the leading edge showerhead holes and the fillet showerhead holes in accordance with one embodiment of the invention. 
       FIGS. 14A–14D  show the shaped holes and an associated trench in accordance with an embodiment of the present invention. 
       FIG. 15  shows the use of metering holes at the feeds for flow control. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIGS. 1A and 1B , there is shown an artists 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. 
   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. 
   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 code 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. 
   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. 
   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. 
   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  111  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 a 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. 
   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 the radial passage  22  which passes 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 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 passage  22  and into the coolant supply cavity  21 . It than 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 . There is further provided a dedicated fillet feed passage  26  that extends radially up through the fir tree  12  as shown. The fillet feed passage  26  is fluidly connected to the fillet cavity  24  by a cross-over openings  27 . 
   In operation, cooling air is introduced into the fillet feed passage  26 , passes through the cross-over openings  27  and into the fillet cavity  24  to cool the fillet  18  prior to being discharged through film holes (not shown). 
   Heretofore, the impingement cooling passages  23  have been circular in cross sectional form. We have found that if these passages are elongated in the radial direction to a racetrack form as shown in  FIG. 6B , better target wall coverage will be obtained as the cooling air passes through these passages to flow into the leading edge cavity  19 . 
   Referring now to  FIGS. 5A and 5B , an alternate embodiment is shown to again include a dedicated fillet feed passage  26  extending radially up through the fir tree  12  and through a cross-over opening  27 . As in the  FIGS. 4A and 4B  embodiments, the cross-over opening  27  interconnects with a fillet cavity  24 . However, the coolant flow is directed to impinge at the base of the fillet area and flow upwards over a series of cooling features, such as hemispherical dimples before exiting by way of leading edge film holes. Such a design is shown in  FIG. 7  wherein a plurality of dimples  29  are formed on the inner surface  31  of the airfoil leading edge  16  as shown. These dimples provide for an enhanced cooling effect of the leading edge in the fillet region. 
   An alternative embodiment of the present invention is shown in  FIGS. 8A and 8B  wherein, rather than the ceramic core which ties the supply and leading edge cores as discussed hereinabove with respect to the  FIGS. 4A and 4B  embodiment, the supply and leading edge cores are connected with a refractory metal core (RMC)  32 . These features are more clearly shown in  FIGS. 9A–9C . The RMC  32  allows for very small core height with features, such as pedestals, lasered in the core to enhance heat transfer. The advantage of this configuration is that of increased heat transfer which is due to enhanced impingement at the fillet cavity  24 . 
   Another feature to enhance cooling characteristics is shown in  FIG. 10 . Here it will be recognized that the common approach for the placement of the impingement cooling passages is mid-way, or on the mid-plane  33 , between the suction side  34  and the pressure side  36  of the blade  11 . In the present design, however, the impingement cooling passages  28  are off-set towards the pressure side  36  as shown. This results in improved cooling by taking advantage of the Coriolis forces that result from rotation of the blade. 
   The use of trip strips in a flow passage is a common way to enhance the flow and cooling characteristics in an airfoil. A pair of such trip strips  37  are shown in  FIGS. 11A and 11B  as applied to the fillet feed passage  26 . We have recognized that the placement of the cross-over opening  27  can be critical in preventing the interference that the trip strips may have on the flow to the cross-over opening  27 . Accordingly, the cross-over opening  27  is preferably placed in a position substantially intermediate between a pair of adjacent trip strips  37  as shown. This same concept is equally applicable to the placement of the impingement cooling passages  28  with respect to trip strips that may be placed in the coolant supply cavity  21 . 
   Referring now to  FIG. 12 , another feature to enhance cooling characteristics is shown. Here, both the radial feed passage  22  and the fillet feed passage  26  has a bell shaped inlet as shown at  38  and  39 , respectively. These bell shaped inlet openings have been found to decrease the resistance and the pressure losses of the airflow into the passages and thereby increase the amount of cooling effect that can be obtained. 
   The function of the film holes that conduct the cooling air from leading edge cavity  19  and the fillet cavity  24  to the leading edge  16  of the blade has been discussed hereinabove. The radial spacing of these film holes has generally been uniform along the leading edge  16  of the blade. In  FIG. 13 , these film holes as shown at  41  are not parallel as is generally the case for those connecting the leading edge cavity  19  to the leading edge of the blade  16 . Instead, they are canted to accommodate their individual positions along the curve of the fillet  18  as shown. In addition to this canting of the film holes  41 , we have recognized that, unlike the cooling holes in the principal portion of the blade, the film holes  41  are preferably placed closer together so as to increase the number of film holes  41  for a given length along the fillet  18 . For example, the typical spacing between film holes (i.e. the pitch between the center of adjacent holes) on the principal portion of the blade is in the area of two times the diameter of the film holes, whereas the spacing of the film holes  41  along the fillet are preferably in the range of one-and-one half times the diameter of the film holes. 
   Shown in  FIGS. 14A–14D , is an alternative embodiment of the film cooling holes at the leading edge of the blade and of the fillets. Here, a trench  42  is formed in the leading edge  16  and extends down to and transitions into the fillet  18  as shown. A plurality of film holes  43  then interconnects the inner surface  31  of the leading edge  16  to the trench  42  as shown. Preferably, the film holes  43  are formed with a cross sectional shape that is a racetrack shape rather than a round shape as discussed hereinabove. The affect of the trench is to allow the cooling air to pass through the film holes and fill the trench before spilling over onto the surface of the leading edge  16 . 
   Referring now to  FIG. 15 , a further modification of the film holes can be made such that their shape, when extending from the inner surface  31  to the leading edge  16 , includes a metering portion  44  and a diffusion portion  46 . The metering potion  44  is preferably cylindrical or racetrack in cross-sectional form, and the diffusion portion  46  is conically shaped as shown to enhance the cooling effect of the cooling air flowing therethrough. The diffusion portion  46  will then discharge its cooling air to the trench  42  as described hereinabove. 
   The angles of these portions may, of course, be varied to meet the requirement of the particular application. Typical values may be, for example, an angle α of 20° and an angle β of 14°. 
   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.

Technology Classification (CPC): 5