Abstract:
There is described a turbine blade, with a platform and with an aerodynamically profiled blade leaf which extends transversely thereto and which comprises a suction-side blade leaf wall and a pressure-side blade leaf wall which extend from an inflow-side leading edge to an outflow-side trailing edge, with respect to a hot gas flowable along the platform or the blade leaf walls during operation, the platform and/or one of the two blade leaf walls having at least two adjacent regions. To prolong the service life of the turbine blade by equalizing the thermal stresses arising there during operation, the two regions have different heat transfer coefficients on the hot-gas side.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of European application No. 06010252.2 EP filed May 18, 2006, which is incorporated by reference herein in its entirety. 
       FIELD OF INVENTION 
       [0002]    The invention relates to a turbine blade for a gas turbine, with a platform and with an aerodynamically profiled blade leaf which extends transversely thereto and which comprises a suction-side blade leaf wall and a pressure-side blade leaf wall which extend from an inflow-side leading edge to an outflow-side trailing edge, with respect to a hot gas flowable along the blade leaf walls during operation, the platform having at least two adjacent regions. The invention relates, furthermore, to a gas turbine having a turbine blade of this type. 
       BACKGROUND OF INVENTION 
       [0003]    EP 1 469 163 A2 discloses, in this context, a cooled moving blade of a gas turbine which inside it has cooling ducts running in a meander-shaped manner. Provided on the inner walls of the blade leaf which delimit the cavities are turbulators which improve the heat transfer from the blade material into the coolant flowing through the cavity. Owing to the increased heat transfer, the turbine blade can permanently withstand higher operating temperatures. 
         [0004]    The disadvantage, in this case, is that cracks may occur, due to inadmissibly high temperature gradients, in the region of the flute-like transition from platform to blade leaf, which is also designated as a fillet, and/or in the platform. If the cracks which have occurred overshoot a critical crack length, then a reliable operation of the gas turbine equipped with such a turbine blade is not ensured. 
         [0005]    Moreover, it is known from U.S. Pat. No. 4,023,350 to provide between the supports of an exhaust gas duct of a turbomachine a multiplicity of swirl generators in order to reduce the pressure loss which occurs and consequently to increase the efficiency of the turbomachine. 
       SUMMARY OF INVENTION 
       [0006]    A particularly long service life of the turbine blade is therefore a design aim by which the availability of a gas turbine equipped with it can be further increased. An object of the invention is, accordingly, to provide a turbine blade for a gas turbine, the service life of which is prolonged. 
         [0007]    The object directed at the turbine blade is achieved by means of a generic turbine blade which is designed according to the features of the independent claims. The invention is based on the knowledge that the wear and the occurrence of cracks and also the subsequent crack growth are thermally induced. The material of the turbine blade is exposed to thermal stresses, since substantially different material temperatures arise in at least two regions of the turbine blade which are adjacent to one another. Conventionally, these regions are influenced by suitable measures, for example by cooling taking place inside the turbine blade, in such a way that these regions withstand the temperatures. However, since the cooling of the regions often takes place over a large area and therefore cannot be adjusted to local requirements, for example on account of convective cooling, the regions subjected to a different thermal load are cooled uniformly, thus giving rise to particularly high temperature gradients in the blade material. However, these high temperature gradients lead to the occurrence of cracks and crack growth which shorten the service life. It is proposed by the invention that the two material regions of the turbine blade which are adjacent to one another have different heat transfer coefficients on the hot-gas side (that is to say, occurring between the blade material and the hot gas), in order to equalize the thermal stresses occurring during operation in the regions. Since the cold-side heat transfer coefficient (that is to say, occurring between the blade material and the coolant) can sometimes be set only with difficulty, the heat transfer coefficient on the hot-gas side is set for the first time in order to equalize the temperatures in the blade material. The local thermal stresses occurring between the two regions can thereby be reduced substantially. In order to achieve this, in a generic turbine blade, there is provision for a turbulence element arranged on the hot-gas side at the edge of the platform to be arranged in one of the regions in order to set the heat transfer coefficient on the hot-gas side there. Owing to the equalized thermal stress between the two adjacent regions, cracks therefore occur more rarely than hitherto. And even if cracks should occur, their growth will take place only more slowly, as compared with a turbine blade known from the prior art. Correspondingly, by virtue of the invention, a particularly long-life turbine blade can be specified, by means of which the period of availability of a gas turbine equipped with it is also increased further. In particular, by virtue of the proposed measure, the fatigue lifetime (low cycle fatigue=LCF) for the platform and its transition into the blade leaf, that is to say in the fillet, is prolonged. 
         [0008]    Contrary to the common view that the introduction of heat from the hot gas into the blade material should always be kept as low as possible, the measure proposed here leads to an increased introduction of heat on account of an increased turbulence in the flow, with the result that the difference of the material temperatures of the higher-loaded region and the lower-loaded region is reduced. Due to the reduced difference in the material temperatures, a lower thermal stress occurs between the two regions, thus achieving an equalization of the material temperature which has the effect on the turbine blade of a prolonged service life. 
         [0009]    Moreover, since cracks also occur at the edge of the platform, the turbulence elements are also provided at this location. 
         [0010]    Advantageous refinements are specified in the dependent claims. 
         [0011]    It has proved advantageous that that region of the platform which is adjacent to the suction-side blade leaf wall has the turbulence elements. Preferably, the turbulence elements are provided in the middle region of the platform which lies between the leading edge and trailing edge, as seen in the flow direction of the hot gas. Particularly in this suction-side region of the platform, comparatively high temperature gradients along the platform occur, due to the fillet enriched with more mass and to the usually convectively cooled platform or blade leaf, and are conducive to defects, such as the occurrence of cracks and crack growth. The provision of the means according to the invention in this region is accordingly of particular advantage. 
         [0012]    Turbulators, dimples, ribs or pins are employed as turbulence elements. These known designs serve for inciting the turbulence of the hot gas flowing past, in order thereby to increase the heat transfer coefficient on the hot-gas side. 
         [0013]    Instead of turbulence elements, the platform may also have a wavy surface as means for setting different heat transfer coefficients, the wave front of the wave form being oriented transversely, preferably perpendicularly, to the flow direction of the hot gas. The wave crests of the wave form serve in this case for the slight increase in turbulence in the hot gas flow, with the result that the heat transfer coefficient on the surface on the hot-gas side rises slightly. A slightly slowed flow occurs in the wave troughs of the wave form, with the result that the heat transfer coefficient falls slightly at this location. The wave troughs and the wave crests are arranged in regions in which different thermal stresses have hitherto occurred. Consequently, even with a preferably slightly wave-like surface, an equalization of the thermal stresses arising during operation can be achieved. 
         [0014]    Preferably, the platform has a first transverse edge onto which the hot gas is capable of flowing and a second transverse edge lying opposite the first transverse edge, the platform having only two concave wave troughs along the longitudinal extent of the platform between the first and second transverse edge. 
         [0015]    The proposed measures have proved to be particularly efficient when the turbine blade has been produced by a casting method, that is to say is cast, and when the blade leaf and/or the platform can be cooled, preferably can be cooled convectively. In particular, convectively cooled turbine blades experience comparatively uniform cooling along the cooling duct in which the coolant, mostly cooling air, flows. Accordingly, the adaption of the heat transfer coefficient on the hot-gas side is particularly appropriate here. Expediently, the turbine blade is also free of any heat insulation layer and is therefore designed to withstand material temperatures of 850° C. to 1000° C. These temperatures normally arise in the second or third turbine stage of a stationary gas turbine used for current generation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Further advantages and features may be gathered from the following description of exemplary embodiments. Elements remaining essentially the same are designated by the same reference symbols. Furthermore, as regards identical features and functions, reference is made to the description of the exemplary embodiment. In the drawing: 
           [0017]      FIG. 1  shows a perspective illustration of a turbine moving blade, 
           [0018]      FIG. 2  shows a sectional top view of a turbine moving blade, 
           [0019]      FIG. 3  shows the temperature profile along the platform of the turbine blade, 
           [0020]      FIG. 4  shows the adapted heat transfer coefficient α along the platform of the turbine blade, and 
           [0021]      FIG. 5  shows the wavy platform in cross section according to the sectional line V-V. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0022]      FIG. 1  shows a perspective illustration of a turbine blade  10 , designed as a moving blade, which has a fastening foot  12  of hammer-shaped cross section for reception in a groove, not illustrated, of the rotor disk of the rotor of a gas turbine. The fastening foot  12  has adjoining it a platform  14  which delimits the flow duct of the turbine radially, that is to say transversely to the direction of the Z-axis. Provided on the surface  16  of the platform  14  is a blade leaf  18  which extends transversely to the platform  14  and which comprises a suction-side blade leaf wall  20  and a pressure-side blade leaf wall  22  which extend from an inflow-side leading edge  24  to an outflow-side trailing edge  26 . Inflow side and outflow side in this context relate in each case to the hot gas  28  which flows through the turbine during operation and which flows around the blade leaf  18  essentially in the axial direction X. The turbine blade  10  is uncoated, that is to say it has no heat insulation layer, and is intended for use in the second or third turbine stage of the stationary gas turbine. 
         [0023]    The blade leaf  18  of the turbine blade  10  is designed to be partially hollow and has two cavities  32  which are separated by a supporting rib  30  and through which a coolant, preferably cooling air  36 , supplied on the fastening side can flow in parallel in the radial direction Z. 
         [0024]    While the preferably stationary gas turbine is in operation, the blade leaf  18  of a generic turbine blade heats up to approximately 850° C. to 1000° C., and it is cooler, in particular, at its trailing edge  26  near the platform  14  in a pressure-side region  38  than in the opposite suction-side region  40 . In the turbine blade known from the prior art, the region  38  is, as a rule, more than 130° C. cooler than the region  40 . 
         [0025]    Since the cast turbine blade thus has two regions  38 ,  40  which are adjacent to one another and are loaded differently during operation, according to the invention, to equalize the thermal stress on the suction-side region  38  at the trailing edge  26 , turbulators  44  are provided which increase the heat transfer coefficient α on the hot-gas side with respect to the other of the two regions  40 , as a result of which the introduction of heat from the hot gas  28  into the blade material is also increased, contrary to the otherwise customary efforts. On account of the higher introduction of heat, in this region  38 , the blade material is hotter than without the arrangement of turbulators  44 . However, the admissible material temperature is in this case not overshot. Since the opposite region  40  on the suction-side blade leaf wall  20  is in any case subject to substantially higher load during operation, that is to say, as a rule, is more than 130° C. hotter, without the presence of the turbulators  44  too great a temperature difference would occur between the two regions  38 ,  40  in the blade material, which would keep the thermal stresses at these locations at an inadmissibly high level, insofar as this is not effectively counteracted by the measure according to the invention. The turbulators  44  provided on the pressure side in the near-platform region near the trailing edge  26  improve the heat transfer from the hot gas  28  into the blade material, so that, in the turbine blade  10  according to the invention, the difference between the temperature in the suction-side blade material and in the pressure-side blade material is equalized in such a way that a difference of less than 100° C. can be achieved. On account of the reduced temperature gradients, correspondingly lower thermal stresses arise, so that the two regions  38 ,  40  remain permanently free of critical and lifetime-shortening fatigue phenomena, such as cracks. 
         [0026]    The turbulence elements  42  may be designed as turbulators  44 , dimples, ribs or pins and have been co-manufactured directly during the casting of the turbine blade  10 . Turbulators  44  may be designed both as rib-shaped ribs, that is to say ribs running straight essentially along their longitudinal extent, or as sickle-shaped ribs. 
         [0027]      FIG. 2  shows a cross section through the blade leaf  18  of the turbine blade  10  as a top view, the blade leaf  18  in this case having four cavities  32  through which cooling air  36  can flow sequentially. Thermal stresses arise in the material of the platform  14  on account of the hot gas  28  flowing along it and are dependent on the suction-side width, as seen in the circumferential direction Y, of the platform  14  between the edge  50  of the platform and the suction-side blade leaf wall  20 . In a first region A, the suction-side width of the platform  14  is greater than it is in a second region B. In a third region C, the suction-side width between the platform edge  50  and the blade leaf  18  increases again. In these regions, because of the cooling of the blade leaf  18 , different thermal gradients arise which could hitherto have led to defects. In particular, the region B has hitherto been affected by crack growth. In order to reduce the temperature gradients in the blade material, in particular in the region B, turbulence elements  42  in the form of turbulators  44  are provided locally in the surface  16  of the platform  14  and increase the introduction of heat from the hot gas  28  flowing past them into the turbine blade material. In this case, the turbulence elements  42  are arranged one behind the other with respect to the flow direction of the hot gas, in order to adapt thermally a particularly large area. The temperature difference between the first region A or the third region C, which have hitherto in any case been subjected to higher thermal load, and the region B, which has hitherto been subjected to lower thermal load, can be reduced significantly, with the result that, overall, the thermal stress between the regions A, B, C is equalized. The occurrence of cracks and crack growth can be effectively avoided, thus resulting in a prolonged service life for the turbine blade  10 . 
         [0028]      FIG. 3  shows the temperature profile T in the platform material  14  along the axial direction X. The temperature T A  of the region A of the inflow-side transverse edge  52  is comparatively high, for example 850° C., and decreases in the direction of the hot gas  28  flowing along to a temperature minimum T B  which is to be found in the region B. From there on, the material temperature rises again to a mean temperature value T C  which occurs during operation in the region of the outflow-side transverse edge  54  of the platform  14 . The temperatures may be measured by means of a suitable measurement method or be determined simulatively with the aid of a finite element computing program. The temperature difference between the region A and the region B has hitherto been of an order of magnitude higher than 130° C. This results in a temperature gradient over a distance of approximately 10 cm which leads to thermal stresses in the blade material and may be conducive to crack growth. By turbulence elements  42  being arranged in the region B, the difference of the temperature T A  and T B  was reduced to a value of below 100° C., so that the thermally induced stresses could be reduced to an extent such that the occurrence of cracks and crack growth arise only with a delay or not at all. 
         [0029]      FIG. 4  shows the heat transfer coefficient α as a function of the X-axis. The heat transfer coefficient α on the hot-gas side is higher in the region B than in the regions A and C which are to be found at the inlet-side end  52  of the platform  14  and at the outlet-side end  54  of the platform  14 . This wave-like characteristic curve of the heat transfer coefficient α along the platform  14  is attributable to the turbulence elements  42  which are provided in the region B for equalizing the material temperatures of the platform  14 . 
         [0030]      FIG. 5  shows, in an alternative embodiment, the surface  16  of the platform  14  along the sectional line V-V of  FIG. 2 . Instead of turbulence elements  42 , the surface  16  has a wavy design, so that, as seen in the radial direction Z, the height of the platform is increased in the region B, as compared with the regions A and C. Thus, between the wave troughs A and C, a maximum B is provided which likewise leads to a heat transfer coefficient α adapted as required. The wave front of the wave-shaped surface  16  of the platform  14  may run transversely to the flow direction of the hot gas  28  or else perpendicularly to the platform edge  50 . 
         [0031]    Although the turbulence elements  42  cause slight aerodynamic flow losses in the hot gas  28 , the service life of the turbine blade  10  according to the invention can be prolonged significantly, as compared with a generic turbine blade, since adaption of the heat transfer coefficient α on the hot-gas side for equalizing the thermal stresses and material temperatures is provided in one of at least two hitherto differently loaded thermal regions.