Abstract:
A turbine blade for a gas turbine is provided. The quantity of coolant flowing off the rear edge thereof is set relatively simply and exactly directly upon casting the turbine blade, without reworking the cast turbine blade with respect to the setting of coolant consumption being necessary. Raised areas are situated on the inner surfaces of the intake side wall or pressure side wall, between which a throttle element is present, by means of which the quantity of coolant flowing out is set. This arrangement allows a core tool to be produced simply, by means of which the casting cores required for casting the turbine blade are produced having the desired precision in great quantities.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2011/064811, filed Aug. 29, 2011 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 10175235.0 EP filed Sep. 3, 2010. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a turbine blade comprising a main blade part, around which a hot gas can flow and which comprises a suction-side wall and a pressure-side wall which extend in the direction of flow of the hot gas from a common leading edge to a trailing edge, wherein at least one opening for blowing out a coolant which cools the main blade part beforehand is arranged on the trailing edge, which at least one opening is fluidically connected to a cavity arranged in the main blade part by means of a channel, wherein the channel is also delimited by an inwardly facing face of the suction-side wall and by an inwardly facing face of the pressure-side wall and a throttling element is provided for setting the quantity of coolant emerging from the opening. 
       BACKGROUND OF INVENTION 
       [0003]    A turbine blade of the type mentioned in the introduction and a casting core for producing such a turbine blade are known, for example, from WO 2003/042503 A1. The known turbine blade has a cooled trailing edge, on which a plurality of openings for blowing out the cooling air are separated from one another by interposed webs (also known as “tear drops”). A common cavity is arranged upstream of the openings arranged on the trailing edge, in which cavity there are three rows of pillar-like pedestals (also known as “pin fins”), which are provided for increasing the transfer of heat of the cooling air which brushes past them and for increasing the pressure loss there. 
         [0004]      FIG. 7  of WO 2003/042503 A1 shows a perspective illustration of the casting core required for producing such a turbine blade. The space occupied by the casting core remains, after the cast turbine blade has been produced, as a cavity in the turbine blade, with openings arranged in the casting core being filled with casting material. In this respect, the casting core represents the negative reflection of the interior of the turbine blade. 
         [0005]    The pin fins known from WO 2003/042503 A1 have a cylindrical shape and connect the inner faces of the suction-side wall and pressure-side wall, which are located opposite one another, of the main blade part of the turbine blade. 
         [0006]    In this context, it is known to set the quantity of cooling air emerging at the trailing edge of the turbine blade by a suitable selection of the maximum pressure loss and/or the smallest cross-sectional area close to the trailing edge through which the cooling air is to flow. However, this procedure can lead to casting cores in which the openings provided on the casting core trailing edge become so large that only still relatively thin separating webs remain between them. During handling of the casting core, however, the casting core can fracture precisely at this point, and therefore it then becomes unusable. 
         [0007]    Furthermore, WO 2003/042503 A1 discloses C-shaped guide elements for cooling air, which are arranged in turning regions of cooling channels and which are intended to bring about low-loss deflection and guidance of the cooling air in downstream zones. 
         [0008]    Furthermore, EP 1 091 092 A2 discloses an air-cooled turbine blade. In order to achieve particularly efficient cooling of a hollow-walled suction or pressure side of the main blade part, pins are arranged in grid form in the cavity of the double wall. In principle, the pins are diamond-shaped, with the corners thereof being rounded off and the edges thereof being curved concavely inward. Between the pins, a network of passages therefore arises for cooling air, these passages each having a narrowed inlet opening and a narrowed outlet opening, between which there is a diffuser and nozzle portion. 
         [0009]    The portions are intended to be used to decelerate and accelerate the cooling air in order to achieve the efficient cooling. 
         [0010]    Furthermore, U.S. Pat. No. 5,752,801 discloses an internally cooled turbine blade, the cooling channels of which on the trailing edge side are configured with a zigzag shape by cast-in c-shaped fins. A better cooling action can thereby be achieved. In addition, the casting cores required for the production can thereby be stiffened. 
       SUMMARY OF INVENTION 
       [0011]    It is therefore an object of the invention to provide a turbine blade of the type mentioned in the introduction for a gas turbine, which can be cooled efficiently and sufficiently using the smallest quantity of coolant possible. 
         [0012]    The object relating to the turbine blade is achieved by a turbine blade according to the features of the claims, with advantageous solutions being presented in the claims. 
         [0013]    The turbine blade for a gas turbine comprises a main blade part, around which a hot gas can flow and which comprises a suction-side wall and a pressure-side wall which extend in the direction of flow of the hot gas from a common leading edge to a trailing edge, wherein at least one opening for blowing out a coolant which cools the main blade part beforehand is arranged on or in the trailing edge, which at least one opening is fluidically connected to a cavity arranged in the main blade part by means of a channel, wherein the channel is also delimited by an inwardly facing face of the suction-side wall and by an inwardly facing face of the pressure-side wall and a throttling element is provided for setting the quantity of cooling air emerging from the opening, wherein, according to the invention, the throttling element is arranged upstream—in relation to the throughflow direction of the channel—of the opening in question and comprises two elevations which are each arranged on one of the two inwardly facing faces. 
         [0014]    In other words: the throttling element comprises elevations which are arranged on the inwardly facing faces and which extend transversely to the throughflow direction of the channel, and between which there is arranged the minimum throughflow cross section of the channel. To determine the minimum throughflow cross section, it is necessary to detect the minimum perpendicular distance between respective fibers of the neutral fibers of the coolant flow and one of the two side faces in the cooling channel. 
         [0015]    The invention is based on the recognition that the coolant consumption can be set in a particularly simple and exact manner using the proposed design by arranging the throttling element upstream of the trailing edge opening in the interior of the blade. In this case, the throttling element is to be formed by two elevations placed in relation to one another, of which one is arranged on the inwardly facing face of the suction-side wall and one on the inwardly facing face of the pressure-side wall. Neither of the elevations connects the suction-side wall to the pressure-side wall. This embodiment of the throttling element is particularly advantageous for turbine blades produced by a casting process. It is known that turbine blades are mostly produced by casting processes, in which so-called lost casting cores are used to produce the inner cooling system. These casting cores are produced mostly with the aid of a core die. The core die comprises two slider elements, which can be moved toward one another and away from one another. When pushed together, these slider elements surround a cavity, which has the same contour as the cavity of the turbine blade to be cast. To produce the casting core, the casting core material is introduced into the cavity of the slider elements. After the casting core material has dried, the casting core is available for producing the turbine blade. 
         [0016]    According to the invention, the slider elements are designed, for producing a first prototype of the turbine blade series to be produced, in such a way that, in the turbine blade prototype to be produced, the throttling, minimum distance between the elevations is in any case smaller than that required in theory. The first turbine blade prototype thus produced is then subjected to a coolant flow rate measurement. As desired, on account of the distance between the elevations being initially too small, the throttling action is too great, which for the time being leads to an excessively small flow rate. Depending on the result of the flow rate measurement, the slider elements are then modified. The elevations thereof are modified slightly, as a result of which the minimum distance therebetween increases when pushed together. Then, a further casting core is produced therewith. This is used to produce a further turbine blade prototype, the flow rate of which is then determined again and compared with the desired rate. If the flow rate determined corresponds to the desired flow rate, the process for producing the slider elements is concluded. The slider elements are then formed in such a way that casting cores with which appropriate turbine blades can be produced in series are always produced with them. If the most recently determined flow rate does not correspond to the desired flow rate, all steps are carried out again for producing a further turbine blade prototype with a minimum distance which is increased somewhat compared to the preceding prototype. 
         [0017]    The particular advantage of the proposed solution is that each of the two sliders can be machined on their own—for instance by grinding the elevation arranged thereon—without fundamentally changing the structure of the turbine blade and the cooling system thereof. It is possible in this respect for only one of the slider elements or else both slider elements to be machined during one iteration step. 
         [0018]    This method is also suitable particularly in the case of modifications to already existing blades in the case where more cooling air is needed for sufficient cooling. In this case, only extremely small modifications are needed to the blade design. An additional qualification owing to an otherwise required change in casting is therefore not necessary. 
         [0019]    In this case, the two elevations are arranged offset in relation to one another—as seen in the throughflow direction of the cooling channel. The offset arrangement makes it possible for the perpendicular distance between the inner face of the pressure-side wall and the inner face of the suction-side wall to be reduced further, which leads to particularly narrow trailing edge regions of main blade parts. This reduces aerodynamic losses in the hot gas flowing around the main blade part. 
         [0020]    As a whole, the invention leads to a reduction in the reject rate during the production of turbine blades, which significantly improves the production costs and the production time for turbine blades. 
         [0021]    It is advantageous that that elevation which is arranged on the inwardly facing face of the pressure-side wall is arranged downstream of that elevation which is arranged on the inwardly facing face of the suction-side wall. This design enforces a flow of coolant in the channel which flows in an intensified manner past the inwardly facing face of the suction-side wall. This makes it possible, particularly in the case of the so-called cut-back trailing edges, to achieve a lengthened film cooling action of the unprotected end of the suction-side trailing edge, which reduces wear phenomena there and lengthens the service life of the turbine blade. 
         [0022]    It is preferable that a plurality of openings are arranged on the trailing edge, the cooling channel collectively connecting a plurality of openings to the cavity. If the elevations are in the form of fins, it is also possible for turbulences to be generated in the coolant during operation with the aid of this angular contour of the inwardly facing faces of the side walls of the main blade part. These turbulences can contribute firstly to the throttling action and secondly to an increase in the transfer of heat on account of a more turbulent coolant flow. 
         [0023]    The interior of the turbine blade as proposed by the invention can be employed both for turbine blades having a common (for the side walls) trailing edge and for turbine blades having a so-called cut-back trailing edge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    A further advantageous embodiment of the invention will be explained in more detail with reference to the drawing, in which: 
           [0025]      FIG. 1  shows a perspective illustration of a turbine rotor blade, 
           [0026]      FIG. 2  shows a longitudinal section through the region of the trailing edge of the turbine rotor blade known from the prior art, 
           [0027]      FIG. 3  shows a cross section through the trailing edge region of a turbine blade according to the invention according to a first configuration, and 
           [0028]      FIG. 4  shows a cross section through the trailing edge region of a turbine blade according to the invention according to a second configuration. 
       
    
    
       [0029]    The same features are provided with identical reference signs in all the figures. 
       DETAILED DESCRIPTION OF INVENTION 
       [0030]      FIG. 1  is a perspective illustration of a gas turbine blade  10  relating to the invention. According to  FIG. 1 , the gas turbine blade  10  is in the form of a rotor blade. The invention can also be used in a guide vane (not shown) of a gas turbine. The turbine blade  10  comprises a blade root  12 , with a fir tree-like cross section, and also a platform  14  arranged thereon. An aerodynamically curved main blade part  16  adjoins the platform  14  and comprises a leading edge  18  and also a trailing edge  20 . Cooling openings arranged as a so-called “shower head” are provided on the leading edge  18 , from which cooling openings an internally flowing coolant, preferably cooling air, can emerge. The main blade part  16  comprises a—with respect to FIG.  1 —rear-side suction-side wall  22  and also a front-side pressure-side wall  24 . A multiplicity of openings  28  separated from one another by interposed webs  30  are provided along the trailing edge  20 . In this case, the trailing edge  20  is in the form of a so-called cut-back trailing edge, and therefore the openings  28  lie more on the pressure side than in the center of the trailing edge  20 . 
         [0031]      FIG. 2  shows the interior of a turbine blade known from the prior art in a longitudinal section along a plane, spanned by a center line, which extends from the leading edge  18  to the trailing edge  20  of the main blade part  16 , and by the longitudinal direction of the blade, which extends from the blade root  12  toward the blade tip. 
         [0032]    In  FIG. 2 , the trailing edge openings  28 , between which the webs  30  are arranged, are shown arranged further to the right. The webs  30  extend substantially parallel to a flow of hot gas which, during operation, flows around the main blade part  16  from the leading edge  18  to the trailing edge  20 . Shown on the left in  FIG. 2 , a multiplicity of pillars or pedestals  32  arranged in a grid are provided. In this case, both the pedestals  32  and the webs  30  extend from an inner face  34  of the suction-side wall  22  to an inner face (not shown in  FIG. 2 ) of the pressure-side wall  24 . Consequently, the pedestals  32  are arranged in a cavity  38  of the turbine blade  10 , which is laterally delimited by the suction-side wall  22  and the pressure-side wall  24 . 
         [0033]    If the turbine blade  10  is used in a gas turbine, a coolant, for example cooling air  40  or cooling steam, flows through the cavity  38  during operation. The part of the turbine blade  10  which is not shown in  FIG. 2  is generally internally designed such that the field of pedestals  32  is subjected to a substantially uniform incident flow of cooling air  40 . The uniform incident flow onto the pedestals  32  arranged in the grid is shown by the arrows marked with  40 . The cooling air  40  impinges on individual pedestals  32  and, in the process, is deflected by these, with the main direction of flow of said cooling air remaining substantially unchanged. Turbulences are thereby produced in the cooling air  40 . The heat introduced by the hot gas into the blade walls  22 ,  24  is thereby conducted further into the pedestals  32 , where the cooling air  40  impinging on the pedestals  32  absorbs the heat and carries it away. Once the cooling air  40  has flowed through the field of pedestals, it enters passages  41  which connect the cavity  38  to the openings  28 . Once it has flowed through the passages  41 , the cooling air  40  passes out of the turbine blade  10  through the openings  28  and blends with the hot gas flowing around the main blade part  16 . 
         [0034]    In order here to set the quantity of coolant leaving the openings  28 , elevations  42 ,  44  ( FIG. 3 ,  FIG. 4 ) are provided on the inner faces  34 ,  36  of the suction-side wall  22  and pressure-side wall  24 . One ( 42 ) of the two elevations  42 ,  44  is arranged on the inner face  34  or part thereof, and the other ( 44 ) of the two elevations  42 ,  44  is situated on the inner face  36  or part thereof. The inner faces  34 ,  36  delimit a cavity  38  and also a cooling channel  46 , which connects the cavity  38  to the openings  28 . In this respect, it is possible for the cavity  38  and channel  46  to merge into one another. According to the invention, the minimum distance between the inner face  34  and the inner face  36  is then provided in the region of the two elevations  42 ,  44 . In this respect, what is shown is the neutral fiber  47 —in  FIG. 3  in relation to the cross section shown therein through the trailing edge  20  of the turbine blade  10  of the cooling channel  46  which is always at the same perpendicular distance from the inner face  34  and the inner face  36 . The minimum distance A forming the throttling element is situated here between the two elevations  42 ,  44 , as a result of which the latter are in relation to one another. 
         [0035]    The elevations  42 ,  44  replace neither the pedestals  32  nor the webs  30 . 
         [0036]    As shown in  FIG. 3 , the elevations  42 ,  44  extend along the longitudinal direction of the blade (perpendicular to the plane of the sheet) over the entire height of the cooling channel  46 . The contours of the elevations  42 ,  44  are configured, as in the cross section shown in  FIG. 3 , such that they make a continuous and edge-free profile of the cooling channel possible in the direction of flow of the coolant toward the trailing edge opening  28 . Here, the cooling channel  46  converges. Alternatively, it may be provided that the elevations are also in the form of fins, as shown in  FIG. 4 . 
         [0037]    As shown in  FIG. 4 , the elevations  42 ,  44  have a fin-like contour with a height H 1  and H 2 , respectively. 
         [0038]    During the production of a first prototype of the turbine blade according to the invention, the heights H 1  and H 2  are relatively large, and therefore it is possible to determine a coolant consumption which lies below the desired or predefined consumption. By modifying the core die, i.e. the corresponding slider elements, it is possible to successively produce further prototypes which, on account of reduced fin heights H 1 , H 2 , always consume slightly more coolant than the prototype produced before. Each iteration in this case includes the production of a turbine blade having a defined fin height H 1  and H 2  and the determination of the coolant consumption of the corresponding turbine blade prototype. As soon as a coolant consumption corresponding to the desired or predefined quantity is established, the production of the slider elements is ended, and therefore the core die which is then available can be used to produce casting cores and therefore turbine blades with the desired coolant consumption to an increased extent, which significantly reduces the reject rate. 
         [0039]    De facto, the proposed configuration provides a turbine blade  10  which, during the phase of die production, makes a simple and cost-effective test phase possible, in order to provide a core die produced exactly for a series of turbine blades  10  after the conclusion of the iterations. 
         [0040]    Furthermore, it is even possible that the casting cores required to cast the turbine blade  10  according to the invention fracture less frequently upon handling than the casting cores known from the prior art. 
         [0041]    It is of course also possible for the throttling element to comprise only a single elevation  44  (or  42 ) instead of two elevations  42 ,  44 , such that the minimum distance which determines the flow rate is situated between a single elevation  44  (or  42 ) and the then inwardly directed face  34  (or  36 ) of the suction-side wall  22  (or of the pressure-side wall  36 ) which lies opposite it. In this case, the opposing face  34  or  36  can then also have a planar configuration in the region of the minimum distance. 
         [0042]    Overall, the invention specifies a turbine blade  10 , the quantity of coolant  40  of which flowing out from the trailing edge  20  is set relatively simply and exactly immediately upon casting of the turbine blade  10 , without it being necessary to rework the cast turbine blade  10  in terms of setting the coolant consumption. In order to achieve this, it is proposed that elevations  42 ,  44  are situated on the inner faces  34 ,  36  of the suction-side wall  22  and pressure-side wall  24 , between which elevations the throttling element used to set the quantity of coolant flowing out is located. This arrangement makes it possible to simply produce a core die with which the casting cores required for casting the turbine blade  10  can always be produced in large quantities with the desired accuracy.