Abstract:
A method of forming a curved cooling channel into a gas turbine component such as a turbine blade uses an electrode in the form of a helix. The electrode is driven to rotate around the central rotational axis of the helix and axially along the central rotational axis. A turbine blade for a gas turbine component is provided with at least one helical cooling channel.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for providing a curved cooling channel in a gas turbine component. The invention furthermore relates to a coolable blade for a gas turbine component having at least one curved cooling channel. 
     BACKGROUND OF THE INVENTION 
     In general, cooling channels in gas turbine components are designed in the form of open passageways that extend along straight line axes. In a number of applications, such as, for example, gas turbine blades subject to high thermal loads and having complex geometries, it is difficult to form cooling channels in a suitable manner for especially highly loaded sections. Points on the blades that are hard to access, such as, for example, in the transition area between the blade hub and the platform or in wall sections with high thermal loads, may require cooling channels having axes that do not extend linearly. A cooling channel that curves three-dimensionally in space would have to be provided in such locations. 
     European publication EP 0 659 978 A1 discloses a coolable turbine blade that is constructed in a known manner to include a blade hub, a blade root, and a platform. The blade hub includes a suction-side wall and a pressure-side wall that are connected with each other along a leading edge and a trailing edge, with a cavity formed in between the suction-side wall and the pressure-side wall. Curved channels are provided in the region of the blade tip. 
     With respect to providing the curved channels, general reference is made to electrochemical processes and also to laser beam drilling. However, European publication EP 0 659 978 A1 does not provide further details related to this. The curved channels shown in this publication have also been positioned in the region of the blade tip. Starting from the pressure side of the blade, the curved channels extend to the blade tip. Accordingly, the area where the curved channels are formed is easily accessible, and the curved channels can be formed without great difficulty. Difficulties are encountered, however, when attempting to form curved channels in areas of a gas turbine blade that are hard to access. 
     SUMMARY OF THE INVENTION 
     In view of the above-discussed difficulties, the invention is directed to a method for forming curved cooling channels, even in regions of the blades that are hard to access. The invention also is directed to a coolable blade for a gas turbine component, in particular a turbine blade, that is provided with a curved cooling channel that enables the required heat removal even at positions on the blade that are hard to access. 
     According to an embodiment of the invention, a method for forming the curved cooling channels in regions of the blade that are hard to access utilizes an electrode in the form of a helix. The electrode is driven during the formation of the cooling channel so as to rotate around its central rotational axis, resulting in a curved channel in a helical shape. This makes it possible to provide in a simple manner cooling channels in areas subjected to high thermal loads, especially at the transition areas between the blade platform and the blade hub, or in the wall of the blade hub. 
     The electrode is positioned axially movable in relation to the rotational axis so as to permit the corresponding advance of the electrode according to the pitch of the helix. The electrode is preferably driven in a forcibly coupled manner, moving axially and rotating relative to the rotational axis. This ensures that the electrode is guided optimally in the respective channel section that was just created. 
     The method according to the invention allows for the selection of a variety of cross-sectional shapes for the electrode wire to form cooling channels having any desired cross section. Examples of cooling channel cross-sections can include rectangular cross-sections, circular cross-sections and ellipsoid cross-sections, with the cross-section chosen to ensure optimum flow conditions within the cooling channel. 
     Even though the method described above can be used in practically all gas turbine components, it is used preferably in coolable blades. 
     A coolable blade constructed according to an embodiment of the invention preferably includes a cooling channel with at least one section having a helical shape. Such cooling channels enable an extremely efficient cooling of the blade, especially in regions that are subject to especially strong thermal stresses. The cooling channels according to the invention can be provided in regions of the blade that are hard to access, such as the transition area from blade hub to platform or in wall areas of the blade hub that are subject to especially high loads due to hot gas, and allow for optimal cooling of these regions. 
     In a preferred embodiment of the invention, a blade is provided with the at least partially helical cooling channels in the wall of the blade hub. The cooling channel can extend substantially continuously over the entire height of the blade hub, thereby allowing for even cooling of the blade hub in the direction of the blade height. Such a cooling channel also can be produced economically using the method of the invention since it can be drilled in a single working step. 
     Depending on the thermal load introduced by hot gas flow, it may be advantageous to provide several cooling channels that are separate from each other. For example, several laterally juxtaposed cooling channels can be provided. In the area of the leading edge, three to five of these cooling channels can be provided with their axes arranged substantially parallel to each other in order to always ensure a safe cooling of the corresponding wall section in case of a potential shifting of the stagnation point. 
     In another embodiment, the cooling channels can be arranged below each other. In this case, the cooling channels do not extend continuously over the entire height of the blade hub, but only over a specific partial section. This makes it possible to account for the thermal load that varies over the blade height, and to provide cooling channels at the places where the thermal load is highest. This configuration also makes it possible to increase the cooling power since the cooling medium is added and removed at several places along the blade height. 
     In another embodiment several cooling channels can be stacked inside each other. The radial and/or axial offset of the individual cooling channels is selected so that they all extend separately from each other. This allows a strong cooling effect at places with especially high thermal loads without weakening the cross-section of the wall too much. 
     According to another aspect of the invention, ejection openings can be provided for forming a cooling film. The ejection openings are designed as so-called film cooling holes that start from the cooling channel and end at the surface of the blade hub. A suitable design of the helical extension of the cooling channel makes it possible to achieve an optimum cooling film. The optimum cooling film is also supported by the swirl of the cooling air flow created by the helical shape. 
     Another important application of the invention relates to providing such a cooling channel in the transition area from the blade hub to the platform. This transition area usually has a transition radius that is subject to very large thermal and mechanical loads. This area therefore must be cooled in a targeted manner in order to not exceed the maximum permissible load values. 
     It is preferred that the cooling channel is provided with several supply and outlet channels so that the coolant is not excessively heated when flowing through the cooling channel. Such a cooling channel can be created in a simple manner since during the forming of the cooling channel, the helical electrode is positioned so that only one angle sector of one turn extends inside the blade, and the remaining sector is located in the area of a cavity through which the coolant flows. This creates several cooling channel sections located on top of each other, which can be supplied optimally by the coolant flowing in the cavity. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings show exemplary embodiments of the invention in schematic form. 
     FIG. 1 shows a perspective view, partially in phantom, of a blade section with an electrode forming a cooling channel according to an embodiment of the invention. 
     FIG. 2 shows a perspective view of a blade section with an electrode forming a cooling channel. 
     FIG. 3 shows a perspective view of a blade section with cooling channels formed by an electrode according to the invention. 
     FIG. 4 shows a cross-sectional view of a blade section with cooling channels according to a first embodiment of the invention. 
     FIG. 5 shows a cross-sectional view of a blade section with cooling channels according to a second embodiment of the invention. 
     FIG. 6 shows a cross-sectional view of a blade section with cooling channels according to a third embodiment of the invention. 
     FIG. 7 shows a cross-sectional view of a wall section of a blade with cooling channels according to a first embodiment of the invention. 
     FIG. 8 shows a cross-sectional view of a wall section of a blade with cooling channels according to a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The method according to the invention is explained with reference to the exemplary embodiment shown in FIGS. 1 to  3 , illustrating the section of a blade  10  in the transition area from a blade hub  12  to a platform  16 . Below the platform  16 , a cavity  28  is formed, which is limited on one side by the platform  16 , and on the other side by a blade root  14 , and whose function is explained in more detail below. 
     According to the invention, an electrode in the shape of a helix  2  is used and is driven so as to rotate around its rotational axis R. Forcibly coupled with the rotational movement, the electrode  2  is also driven in an axial direction, causing the electrode  2  to advance into the material in the area of the platform  16  and of the blade root  14 . A spark erosion method, electrical discharge machining, electrochemical drilling, or other known processes can be used with the electrode and processes according to the invention. 
     The coupled rotational and axial movement causes the electrode  2  to be driven forward along a helical path. This creates a helical channel such as cooling channel  20  that is formed in the section processed by the electrode  2 . If the electrode is positioned completely within a portion of the blade, such as the platform  16 , a continuous cooling channel  20  is created. 
     In the exemplary embodiment shown in FIGS. 1-3, the electrode  2  is positioned in relation to the platform  16  and the blade root  14  in such a way that, when seen from the top, an angle sector is located in the area of the cavity  28 . The electrode  2  therefore exits the blade root  14  in each case, and enters the area of the platform  16  following another rotation of about 90°. This does not create a single, continuous cooling channel  20 , but a plurality of parallel extending channel sections with inlet openings  22  and outlet openings  24 . 
     The geometric design of the helix  2  can be varied depending on the desired results. The pitch of the helix  2  determines the amount of axial movement of the electrode relative to the rotation of the electrode. Forcible coupling of the rotation of the electrode with its axial movement is generally only needed at the beginning of the drilling process as long as the electrode has sufficient mechanical stability since the helix  2  is guided in the already drilled section. 
     The embodiments shown in FIGS. 4 to  6  show different possibilities for using such cooling channels in order to realize different cooling concepts in the transition area between the blade hub and platform. Because of the small radius at this point, in connection with the high thermal loads, this transition area is especially at risk and therefore must be cooled optimally. 
     FIG. 4 shows a blade  10  with a blade hub  12 , a blade root  14 , and a platform  16 . The blade hub  12  is constructed hollow, i.e., a cavity  19  exists between the walls  18 , whereby a coolant K can flow through said cavity (not shown here). 
     In the transition area between blade hub  12  or blade root  14  and platform  16 , a helically extending cooling channel  20  is illustrated. An inlet opening  22  is provided in the area of the blade root  14 , and an outlet opening  24  is provided in the area of the platform  16 . Below the platform  16 , a baffle plate  30  is positioned so that a cavity  28  is created between the platform  16  and the baffle plate  30 . 
     The baffle plate  30  has baffle holes  32 , through which the coolant K first enters the cavity  28 , and from there leaves the blade  10  in the area of the platform  16  through ejection openings in the form of film holes  26 . 
     A portion of the cooling air flow K furthermore flows through the inlet opening  22  located in the area of the blade root  14  below the baffle plate  30 . As a result of the pressure differential between this area and the cavity  28 , this portion of the cooling air flow K flows through the cooling channel  20 , and exits from the cooling channel through the outlet opening  24  in the area of the cavity  28 . There, it is mixed with the remaining coolant K and exits the blade  10  through the film holes  26 . 
     As illustrated in particular in FIG. 3, a plurality of inlet openings  22  and outlet openings  24  are provided, enabling an even cooling along a plane perpendicular to the drawing plane of FIG.  4 . 
     FIGS. 5 and 6 show variations of the cooling channels  20  in a blade similar to that shown in FIG.  4 . 
     The embodiment according to FIG. 5 has a first group of cooling channels  20  in the transition area between the wall  18  and the platform  16 , as well as a second group of cooling channels  20  in the end portion of the platform  16 . Both groups of cooling channels  20  are supplied exclusively through the cavity  28 . 
     The embodiment shown in FIG. 6 shows a helical cooling channel  20  that is formed continuously within the wall  18  at the transition area between the wall  18  and the platform  16  in a direction perpendicular to the drawing plane of FIG.  6 . The cooling channel  20  is supplied by a feeding channel  23  and has a outlet channel  25  that ends in the area of the hollow space  19 . It is also possible to provide a plurality of feeding channels  23  and outlet channels  25  in order to even the cooling effect. 
     The embodiments shown in FIGS. 7 and 8 show cooling concepts that can be realized within a wall  18  (for example in the stagnation point area of the blade  10 ). Five cooling channels  20  are shown that extend substantially over the entire height of the blade hub  12  (not shown,) i.e., perpendicular to the illustrated section plane. The arrangement provides each of the cooling channels  20  extending separately from the others and at least partially overlapping with an adjacent cooling channel. The laterally and vertically stacked arrangement achieves optimal cooling of the blade. 
     The embodiment shown in FIG. 8 differs from the one in FIG. 7 in that outlet channels  25  have been provided. These enable the formation of a cooling film (not shown in detail). 
     In another variation of the embodiments shown in FIGS. 7 and 8, the individual cooling channels  20  are provided not continuously over the entire height of the blade hub. Instead, individual helical cooling channels, each having a few turns, are provided so as to be positioned on top of each other, but separate from each other. These can be arranged, for example, specifically more densely in the middle of the blade height in order to account for the local heat introduction through the hot gas flow. 
     The concept according to the invention makes it possible to economically realize optimum cooling concepts, and in particular to provide cooling channels even in regions that are hard to access.