Abstract:
The invention relates to an airfoil for a gas turbine, which comprises a compound fillet disposed between the airfoil and a platform. The compound fillet consists of a first arc and a second arc. A first end of the first arc tangentially adjoins an outer surface of the airfoil. A second end of the first arc tangentially adjoins a first end of the second arc. A second end of the second arc tangentially adjoins a surface of the platform, wherein the following equation is satisfied: 0.15≦R1/s≦0.45, and 0.09≦a/s≦0.27.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European application 13170564.2 filed Jun. 5, 2013, the contents of which are hereby incorporated in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to an airfoil for a gas turbine, in particular, to a compound fillet between an airfoil and a platform. The present invention also relates to a blade and a vane for a gas turbine. 
     BACKGROUND 
     A gas turbine typically includes at least one rotor assembly in which a plurality of blades/vanes, comprising airfoils radially extending from platforms, are circumferentially fitted and distributed around a rotor disk. During operation, centrifugal forces generate circumferential rim stress in the rotating blades. As for vanes, gas pressure and vibration may also generate stress. These stresses can concentrate at the transition between the platform and the airfoil. This stress concentration can be minimized by fillets at the platform/airfoil connection portion. Adequate stress relief can however only be achieved with an adequately sized and shaped fillet. 
     Generally, it is desirable to reduce the size of the platform, or alternatively avoid reworking of a blade/vane when it is discovered in final design stages that there is insufficient space between the airfoil and platform edge to enable tangentially joining the fillet to the platform surface. It is therefore desirable to provide a compound fillet consisting of multiple sections of curves that requires less platform surface space without compromising mechanical integrity. 
     A compound fillet for a turbine blade is disclosed in EP2184442A1, which covers an airfoil to platform join and is configured to comprise a first arc and a second arc. The first arc has a first end tangential to the airfoil surface. The second arc having a first end tangentially adjoins the second end of the first arc and a second end adjoins the plat form surface. The radius of the first arc is larger than the radius of the second arc. Furthermore, another compound fillet also is disclosed in this reference, which comprises a first arc and a second arc wherein the second arc adjoins non-tangentially the platform surface. 
     A kind of transition between a surface of a blade/vane airfoil and a platform at an end of the airfoil is disclosed in GB2353826A, which comprises at least two curves of different radii, the radius of the curve nearest the surface of the airfoil being larger than the radius of the curve nearest to the platform. The transition may comprise two curves of different radii separated by a straight line section, or it may form a section of an ellipse. 
     A cooled moving blade for a gas turbine is disclosed in U.S. Pat. No. 6,190,128 B1, which has a base portion of a profile formed by an elliptically curved surface and a rectilinear surface portion, wherein the rectilinear surface portion is provided at a hub portion of the blade where thermal stress is large. 
     Even though different compound fillets are proposed in the above mentioned publications, there exists much more space to develop in respect of optimizing the parameters of compound fillet in order to improve the stress relief capacity of the blade/vane, hence improve the working life of the blade or vane. 
     SUMMARY 
     It is an object of the present invention is to provide an airfoil for a blade and/or a vane, which could optimize the structure for stress relief, in order to prolong the working lives thereof. 
     In one aspect of the present invention, an airfoil for a gas turbine is proposed, which comprises a compound fillet disposed between the airfoil and a platform, wherein the compound fillet consists of a first arc and a second arc, a first end of the first arc tangentially adjoining an outer surface of the airfoil, a second end of the first arc tangentially adjoining a first end of the second arc, and a second end of the second arc tangentially adjoining a surface of the platform, wherein the following equation is satisfied:
 
0.15≦ R 1 /s≦ 0.45, and
 
0.09≦ a/s ≦0.27,
 
where R 1  represents the radius of the first arc, s represent the chord length of the airfoil, and a represents the distance between the point where the first end of the first arc adjoins the outer surface of the airfoil and the top surface of the platform in the direction along the extension of the outer surface of the airfoil.
 
     According to one possible embodiment of the present invention, the following equation is further satisfied:
 
0.024≦ R 2 /s≦ 0.072,
 
where R 2  represents the radius of the second arc.
 
     In another aspect of the present invention, a blade for a gas turbine is proposed, which comprises the airfoil according to the present invention. 
     In another aspect of the present invention, a vane for a gas turbine is proposed, which comprises the airfoil according to the present invention. With the present invention, the structure of the airfoil, the blade comprising the same and/or the vane comprising the same, are improved in stress relief capacity, and prevented from pre-mature cracks during operation of the blade and/or vane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, advantages and other features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments thereof, given for the purpose of exemplification only, with reference to the accompany drawing, in which: 
         FIG. 1  shows a schematic cross section view of an airfoil for a blade of a gas turbine according to an example embodiment of the present invention; 
         FIG. 2  shows a schematic cross-section view of the airfoil for the blade of the gas turbine according to an example embodiment of the present invention; and 
         FIG. 3  shows a schematic cross section view of an airfoil for a vane of a gas turbine according to an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic cut-away view of an airfoil  110  for a blade  100  of a gas turbine according to example embodiments of the present invention. The blade  100  comprises the airfoil  110  with an outer surface  112 , and a platform  120  with a top surface  122 . A compound fillet  130  is disposed between the airfoil  110  and the platform  120 . Generally, the profile of the blade  100  represents a symmetrical structure. Thus, as shown in  FIG. 1 , one side of the airfoil  110  is numerated and described for purpose of simplicity and clarity. The compound fillet  130  comprises a first arc  132  with a radius R 1  and a center O 1 , and a second arc  134  with a radius R 2  and a center O 2 , where the first arc  132  tangentially adjoins at its first end  133  the outer surface  112  of the airfoil  110  at the point A, and the second arc  134  tangentially adjoins at its second end  137  the top surface  122  of the platform  120  at the point B, and the second end  135  of the first arc  132  and the first end  136  of the second arc  134  tangentially adjoin with each other. As shown in  FIG. 1 , it is defined that a length a indicates the distance between the point A and the platform  120  in the direction of the extension of the outer surface  112  of the airfoil, i.e. a represents the distance between the point A where the first end  133  of the first arc  132  adjoins the outer surface  112  of the airfoil  110  and the top surface  122  of the platform  120  in the direction along the extension of the outer surface  112  of the airfoil  110 . As shown in  FIG. 1 , it is shown the extension of the outer surface  112  of the airfoil  110 , represented by broken line, and the extension of the top surface  122  of the platform  120 , represented also by broken line, intersect at point C. In this case, the length a represents the length AC. 
       FIG. 2  shows the section view of the airfoil  110  of the blade  100  or vane  200  (shown in  FIG. 3 ), where s represents a chord of the blade  100  or vane  200 . Those skilled in the art should understand the term of “chord” as the common meaning as that mentioned in the art. That is, the chord of a blade/vane refers to the length of the perpendicular projection of the blade/vane profile onto the chord line, where the chord line refers to, if a two dimensional blade/vane section were laid convex side up on a flat surface, the line between the points where the front and rear of the blade/vane section would touch the surface. 
     According to one possible embodiment, the airfoil  110  is structured to satisfy the following equation:
 
0.15≦ R 1 /s≦ 0.45, and   (1)
 
0.09 ≦a/s≦ 0.27.   (2)
 
     According to another possible embodiment, other than the equation (1) and (2), the airfoil is further structured to satisfy the following equation:
 
0.024 ≦R 2 /s≦ 0.072.   (3)
 
     The airfoil  110  that satisfy the equations (1), (2) and (3) according to embodiments of the present invention, may provide a blade that optimizes stress relief capacity as the blade is operated with high speed under high temperature and pressure. Thus, the working life of the blade is substantially prolonged. 
       FIG. 3  shows a schematic cut-away view of an airfoil  210  for a vane  200  of a gas turbine according to example embodiments of the present invention. The vane  200  comprises an airfoil  210  with an outer surface  212 , and a platform  220  with a top surface  222 . A compound fillet  230  is disposed between the airfoil  210  and the platform  220 . The compound fillet  230  comprises a first arc  232  with a radius R 1  and a center O 1 , and a second arc  234  with a radius R 2  and a center O 2 , where the first arc  232  tangentially adjoins at its first end  233  the outer surface  212  of the airfoil  210  at the point A, and the second arc  234  tangentially adjoins at its second end  237  the top surface  222  of the platform  220  at the point B, and the second end  235  of the first arc  232  and the first end  236  of the second arc  234  tangentially adjoin with each other. As shown in  FIG. 3 , it is defined that a length a indicates the distance between the point A and the platform  220  in the direction of the extension of the outer surface  212  of the airfoil, i.e. a represents the distance between the point A where the first end  233  of the first arc  232  adjoins the outer surface  212  of the airfoil  210  and the top surface  222  of the platform  220  in the direction along the extension of the outer surface  212  of the airfoil  210 . As shown in  FIG. 3 , it is shown the extension of the outer surface  212  of the airfoil, represented by broken line, and the extension of the top surface  222  of the platform  220 , represented also by broken line, intersect at point C. In this case, the length a represents the length AC. 
     As shown in  FIG. 3 , and as is known by those skilled in the art, the axis of the vane  200  is generally angled with respect the platform by certain angles. Thus, the compound fillets  230  on the left and right side of the airfoil  210  differ in shape from each other. According to embodiments of the present invention, it is designed that R 1 , R 2 , a and s are adopted on both sides, except that the positions of O 1  and O 2  are different. 
     As described herein, the present invention may extensively apply to both blades and vanes of a gas turbine. Those skills in the art should understand that, the general concept of the present invention intends to cover both blade and vane utilized in a gas turbine. The objective is to optimize the structure of the blade and/or the vane, in order to prolong their working life and preventing pre-mature cracking due to stress generated by high speed rotation, high temperature and/or high pressure. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.