Abstract:
A turbine vane for a stationary gas turbine is provided. The turbine vane includes a hollow vane blade, wherein a rib is provided inside the vane blade to allow a pressure side wall and a suction side wall to support one another. The rib has an opening penetrating the rib near a wall at a height of an external fillet between a side wall, and a platform surface for extending a life of the turbine vane. By the opening, material accumulations in a transition region are avoided or the accumulation is reduced, whereby increases in stiffness and higher temperature gradients associated therewith are avoided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Stage of International Application No. PCT/EP2009/051909 filed Feb. 18, 2009, and claims the benefit thereof. The International Application claims the benefits of European Application No. 08003728.6 EP filed Feb. 28, 2008. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention refers to a turbine blade for a stationary gas turbine, with at least one platform section which comprises a platform with a platform surface on which is arranged a blade airfoil which is profiled in cross section, having a pressure-side wall and a suction-side wall, wherein the surfaces of the pressure-side wall and of the suction-side wall which are exposable to a hot gas merge in each case into the platform surface via an outer rounding, and with at least one cavity which is arranged in the blade airfoil, extends into the platform section, and in which provision is made for at least one rib which connects the pressure-side wall to the suction-side wall and, extending along a longitudinal direction of the blade airfoil, sub-divides the cavity. 
       BACKGROUND OF INVENTION 
       [0003]    Aforesaid turbine blades have been known for a long time from the prior art. As a rule, they have a blade airfoil which is traversed by cavities which are separated from each other by means of ribs. The ribs extend from the suction-side wall to the pressure-side wall and along the longitudinal direction of the blade airfoil, i.e. from the platform to the blade tip. Cast turbine blades in this case have a transition region between blade airfoil and platform surface which, by means of a fillet-like rounding, thickens the blade sidewalls, i.e. the suction-side wall and the pressure-side wall, in this region. In the transition region, therefore, there is a material accumulation which also brings with it a sudden change of rigidity of the blade airfoil. The blade airfoil is therefore stiffer on the platform side than in its middle region or on the blade-tip side. On account of this sudden change of rigidity, larger temperature gradients can occur during In order to avoid secondary damage in the gas turbine, in which such a turbine blade is installed and used during operation, as a result of fragments which become detached from the turbine blade, the turbine blade is exchanged after achieving a predetermined maximum service life. 
         [0004]    From the prior art it is known, moreover, to at least partially extend the service life by a thicker thermal barrier coating being applied in the regions of sudden changes of rigidity than in the regions without such material accumulations. As a result, the temperature gradients can be reduced. 
         [0005]    In addition, a gas turbine blade, the leading edge of which, exposable to inflow by a hot gas, is impingement cooled, is known from EP 1 420 142 A1. The impingement cooling openings which are required for this purpose are arranged in a rib which supports the blade airfoil between suction-side and pressure-side walls. In this case, the impingement cooling openings for the most part are distributed uniformly over the height of the blade airfoil and invariably arranged in the middle between pressure side and suction side in order to ensure uniform cooling of the leading edge. 
       SUMMARY OF INVENTION 
       [0006]    It is therefore an object of the present invention to provide a turbine blade for a stationary gas turbine, which blade has an extended service life. 
         [0007]    The object is achieved by means of a turbine blade according to the independent claims. 
         [0008]    The invention provides that in a turbine blade which is referred to in the introduction at least one of the ribs which are arranged in the blade airfoil at the level of the outer rounding has an opening, off-center and close to the wall, which penetrates the rib. 
         [0009]    The opening is provided at the level of the outer rounding inside the turbine blade in the rib which is arranged there. Close to the wall in this case can mean that its position is off-center between the inner sides of pressure-side wall and suction-side wall. As a result of this, the material accumulation at the level of the outer rounding can be reduced. This simple constructional means leads to balancing of the sudden change of rigidity and to the reduction of the temperature gradient in the then-reduced material accumulation. If necessary, the effects which are induced by the opening upon the cooling air system of the turbine blade, and also upon the stress increase around the opening, are additionally to be taken into consideration. The same applies to the so-called creep life as a result of the reduced supporting cross section of the rib and to the possibly altered natural frequencies on account of the mass which is then absent. Consequently, it may be wise to provide an oval opening with a suitable orientation. Extending the rib into the platform section and furthermore onto the root section or fastening section of the turbine blade may also be wise. 
         [0010]    According to a further advantageous development, accommodating a further rounding which is arranged between rib and sidewall may also be wise. As a result of this, mechanical loads are reduced. Naturally, the proposed measures can also be combined in order to compensate the changes which occur as a result of using the opening close to the wall in order to achieve overall an extended service life of the turbine blade. In all, with the invention the load on the material accumulation can be reduced and therefore the service life can be increased. 
         [0011]    The measure according to the invention to make provision in the rib, at the level of the outer rounding, for an opening which is close to the wall and penetrates the rib, can be simply realized and can also be subsequently retrofitted in operationally stressed turbine blades as long as accessibility to the rib through the blade root is ensured. On the other hand, the opening can be achieved in a simple way during the production of new parts if the blade airfoil and the platform are cast in one piece and the casting core, which is used for producing the cavities in the casting device, for the subsequent opening close to the wall which is provided in the rib, is realized by means of a hole which is provided in the core. This is particularly advantageous since the hole can also be used for stabilizing the casting core, and other so-called crossover holes, which are provided neither close to the wall nor at the level of the outer rounding in a rib which is arranged between the suction-side wall and the pressure-side wall, can be dispensed with. 
         [0012]    Advantageous developments and further developments are disclosed in the dependent claims. 
         [0013]    An opening which penetrates the rib is not only close to the wall when it is arranged off-center between suction-side wall and pressure-side wall but also when it is tangent to, or intersects, the sidewall plane which is spanned by the inner side of the suction-side wall and/or pressure-side wall. 
         [0014]    The opening is expediently round or oval. These openings can be produced in a particularly simple manner, especially if the turbine blade is cast essentially in one piece. A casting core then only needs to have a corresponding hole at the corresponding place. 
         [0015]    According to an alternative solution, the service life of a turbine blade can also be extended by the platform-side rib end extending by a longer or shorter distance on the inner side of the pressure-side wall than on the inner side of the suction-side wall. 
         [0016]    A recess instead of the opening close to the wall and penetrating the rib is understood by this, i.e. the opening is not fully encompassed by rib material. Also, with a turbine blade which is equipped in such a way the mass accumulation in the transition region can be locally reduced. The turbine blade according to the second configuration, in an advantageous development, can have a platform surface which is part of an imaginary platform plane which extends through the cavity, wherein the platform-side end of the rib lies on the pressure side on one side of the platform plane and lies on the suction side on the other side of the platform plane. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Further features, characteristics and advantages of the present invention are described in more detail in the following text based on exemplary embodiments with reference to the attached figures. The described features can be advantageous both individually and in combination with each other. 
           [0018]      FIG. 1  shows a perspective view of a turbine blade according to the invention with a schematically represented blade airfoil, 
           [0019]      FIG. 2  shows the detail Z as a detail from the turbine blade according to the invention according to  FIG. 1  in a perspective view, and 
           [0020]      FIG. 3  shows the detail Z with an alternative solution. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0021]      FIG. 1  shows in a perspective view a turbine blade  10  for a stationary gas turbine. The turbine blade  10  according to  FIG. 1  is formed as a rotor blade. The invention, however, can also be used in stator blades of a stationary gas turbine. The cast, one-piece turbine blade  10  comprises a blade root  14  along a longitudinal direction  12 , to which is connected a platform section  16 . The platform section  16  essentially comprises a platform  18  with a platform surface  20 . The platform surface  20  is essentially planar and is therefore part of an imaginary platform plane  22 . A blade airfoil  24 , which is profiled in cross section, is arranged on the platform surface  20 . The blade airfoil  24  is formed by a pressure-side wall  26  and a suction-side wall  28  which extend from a common leading edge  30  to a common trailing edge  32  and merge into each other both at the leading edge  30  and at the trailing edge  32  in the process. The surfaces of the suction-side wall  28  and pressure-side wall  26 , and also the platform surface  20 , in this case are passed by a hot gas of the gas turbine. Both the pressure-side wall  26  and the suction-side wall  28  merge into the platform  18  via a fillet-like, encompassing rounding  34 . The rounding  34  or the transition section is also known as a fillet. 
         [0022]    The cavity which is enclosed by the sidewalls  26 ,  28  is sub-divided into sub-cavities by means of a plurality of ribs  36 . Each rib  36 , at least inside the blade airfoil  24 , extends along its longitudinal direction  12 . In  FIG. 1 , only a stub of the blade airfoil  24  is shown. The complete blade airfoil up to the blade tip is only indicated by means of a broken line. 
         [0023]      FIG. 2  shows the detail Z of the turbine blade  10  according to  FIG. 1  in a perspective view, wherein for reasons of clarity irrelevant elements in the direction towards the leading edge  30  and trailing edge  32  are blanked out.  FIG. 2  shows in detail the features which are already described in relation to  FIG. 1 , these being the platform surface  20 , the pressure-side wall  26 , the suction-side wall  28 , the platform  18 , the rib  36  and the rounding  34 . 
         [0024]    According to the invention, provision is made in the rib  36 , as viewed along the longitudinal direction  12  of the blade airfoil  24 , for an opening  40  close to the wall, which penetrates the rib  36  and is at the level of the outer rounding  34 . The opening  40  close to the wall is round in construction in the configuration which is shown. An oval opening  40  is also possible. The opening  40 , with regard to an inner side  42  of the pressure-side wall  26 , is arranged in such a way that the sidewall plane  44  which is spanned by it is intersected by the opening  40 . Hereby, a material reduction, which is shown by hatching and provided with the designation  46 , results in the region of the outer rounding  34 . On account of the material reduction in the region of the outer rounding  34  inside the turbine blade  10 , a sudden change of rigidity can be avoided since the mass increase in the region of the outer rounding  34  is compensated at least partially on account of the recess which exists as a result of the opening  40 . On account of the opening  40  which exists in the rib  36 , a bridge  50  remains with regard to the rib end  48  and connects the suction-side wall  28  to the pressure-side wall  26 . 
         [0025]    The effect according to the invention can also be achieved with a turbine blade  10  in which there is no bridge  50 . This results in an alternative turbine blade  10 , the detail Z of which is shown in  FIG. 3 . The detail Z which is shown in  FIG. 3  essentially corresponds essentially to the detail which is referred to in  FIG. 2  and is therefore not described in more detail in this case. Identical features are provided with identical designations in  FIG. 3 . In contrast to  FIG. 2 , no provision is made in the rib  36  for an opening  40  which is entirely enclosed by material. Instead, on the platform side the rib  36  ends at a non-consistent height with regard to the longitudinal extent of the turbine blade  10 . Therefore, instead of the opening  40  provision is made for a recess. That part of the rib  36  which is arranged directly on the inner side  43  of the suction-side wall  28  ends at a different point, as seen in the longitudinal direction of the blade axis  12 , to that part of the rib  36  which is arranged directly on the inner side  42  of the pressure-side wall  26 . In other words, the platform-side rib end extends far less on the inner side  42  of the pressure-side wall  26  than the rib end which is arranged on the inner side  43  of the suction-side wall  28 . An unnecessary mass accumulation, which leads to an unnecessary sudden change of rigidity, can therefore be avoided at least for the pressure-side section of the outer rounding  34 . 
         [0026]    The platform surface  20  is part of an imaginary platform plane  22  which extends through the cavity. Preferably, the platform-side end of the rib  36  is arranged on the pressure side on one side, i.e. above (on the blade tip side) the platform plane  22 , and is arranged on the pressure side on the other side, i.e. below (blade root side) the platform plane  22 . Also, a reverse arrangement of the rib ends is possible, in which on the platform side the pressure-side end of the rib  36  ends beneath the platform plane  22  and the suction-side end of the rib  36  ends above the platform plane  22 . The manner of the course of the platform-side rib end from the pressure side  26  to the suction side  28  can be optionally formed in this case. The course can be for example in a straight line or, like the configuration shown in  FIG. 3 , can be convex/concave. In order to take into consideration the effects upon the cooling air system and also upon the stress situation in the opening  40  which arise as a result of using the opening  40  or recess according to the invention, an additional rounding  41 , which is provided in the transition from rib  36  to inner wall  42 ,  43  of the pressure-side wall  26  and/or suction-side wall  28 , can preferably also be accommodated. The accommodation leads to different radii R 1 , R 2  for the additional rounding  41  at different positions along the longitudinal extent  12  of the blade airfoil  24 . The radius R 1  of the additional rounding  41  can be greater at the level of the outer rounding  34  than the radius R 2  of the additional rounding  41  at mid-height of the blade airfoil  24 . 
         [0027]    As long as the ribs  36  are arranged in the middle region between leading edge  30  and trailing edge  32  of the blade airfoil  24 , the opening  40  or recess is provided on the pressure-side wall. If, however the rib  36  is located comparatively close to the leading edge  30  or comparatively close to the trailing edge  32 , then the opening  40  or the recess according to the invention can be arranged on the suction-side wall since higher hot gas temperatures and material temperatures occur in the corresponding regions. 
         [0028]    The recess at the level of the outer rounding  34 , which is brought about by the opening  40  in the inner side  42  of the pressure-side wall  26  or in the inner side  43  of the suction-side wall  28 , can extend further along the inner side  42 ,  43  even beyond the region of the rib  36  so that the recess on the inner side is also arranged in the section of the transition region where no rib  36  supports the sidewalls  26 ,  28 . The recess deepens the associated spanned plane of the sidewalls  26 ,  28  in the manner of a fillet in each case, as a result of which a mass reduction can also be achieved in the section of the outer rounding  34  in which there is no arrangement for a rib  36 . This recess can also be used in the case of a turbine blade which is formed according to  FIG. 3 . In this case also, stress reductions according to the invention can therefore be achieved, which allows the occurrence of crack development and possibly crack propagation in this section of the transition region to be further delayed. 
         [0029]    In all, the invention refers to a turbine blade  10  for a stationary gas turbine which has a hollow blade airfoil  24  in which there is at least one rib  36  inside, mutually supporting the pressure-side wall  26  and the suction-side wall  28 , in which rib provision is made at the level of the outer rounding  34  between sidewall  26 ,  28  and platform surface  20  for an opening  40  close to the wall, penetrating the rib  36 , for extending the service life of the turbine blade  10 . By means of the opening  40 , material accumulations in the transition region are avoided or the accumulation is reduced in comparison to when there is no opening  40 , as a result of which sudden changes of rigidity and the larger temperature gradients which are associated therewith can be avoided.