Abstract:
A device and a method are for producing a blade including two outer walls and at least one cavity between the outer walls, for a turbine. An outer mould and several cores are used in forming the outer walls and the at least one cavity of the blade. At least one of the cavities is divided into two channels by a middle segment. One channel is located between the first outer wall and the middle segment, while the other channel is located between the middle segment and the second outer wall. Two cores which are separate from each other are used accordingly. This provides a simple and economical means of reducing the thickness of the outer wall.

Description:
This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/EP01/10600 which has an International filing date of Sep. 13, 2001, which designated the United States of America and which claims priority on European Patent Application number EP 00120035.1 filed Sep. 14, 2000, the entire contents of which are hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to a device for producing a blade having two outer walls and at least one cavity, arranged between the outer walls, for a turbine, comprising an outer mold and a plurality of cores for forming the outer walls and the at least one cavity. 
     The invention also generally relates to a method of producing a blade having two outer walls and at least one cavity, arranged between the outer walls, for a turbine, an outer mold and a plurality of cores being provided for forming the outer walls and the at least one cavity. 
     Further general subject matter of the invention includes a blade for a turbine, in particular a gas turbine, having two outer walls and at least one cavity arranged between the outer walls. 
     BACKGROUND OF THE INVENTION 
     Blades, in particular blades for gas turbines, must be cooled from inside on account of the high operating temperatures. For this purpose, the blades have one or more cavities. In the hitherto known blades, these cavities extend from the one outer wall of the blade up to the other outer wall. A section of a core is provided for forming each cavity. The individual sections are connected to one another. The core is accommodated in a suitable receptacle of an outer mold for producing the blade by a casting process. In this case, the length of the core can assume comparatively high values. 
     In blades cooled from inside, the wall thickness of the outer walls is to be selected to be as small as possible. A substantial improvement in the cooling can be achieved by a small wall thickness. The minimum wall thickness provided must always be greater than the tolerance of the wall thickness. Otherwise, there is the risk of the core being displaced and/or deformed during the casting in such a way that it comes into contact with the outer mold and the blade produced has a hole. In practice, therefore, a comparatively large wall thickness must be selected. 
     A further disadvantage of the known methods is that shifting of the core during the casting has consequences for both outer walls of the blade. The reason for this is that the core extends from the one outer wall up to the other outer wall. Therefore, the core has to be produced with high precision in these known methods. Tolerances which occur during the production of the core must likewise be taken into account. 
     To improve the cooling, blades having cavities are known. Such a blade and also a method and a device for producing it have been disclosed by WO 99/59748 originating from the same applicant. This publication proposes a multiplicity of cores which are connected to one another and the outer mold via connecting elements. The production of this blade is complicated and costly. 
     SUMMARY OF THE INVENTION 
     An object of a preferred embodiment of the present invention is therefore to provide a simple and cost-effective device and a cost-effective method for producing a blade with small wall thicknesses. A further object of an embodiment of the invention is to provide a blade for a turbine, this blade having outer walls with a substantially smaller wall thickness. 
     The device according to an embodiment of the invention provides for each of the cores to have at least one section which extends from an associated outer wall up to a center web of the blade without being involved in the formation of the other outer wall. 
     The method according to an embodiment of the invention is characterized in that at least one section of each core is supported in such a way that the distance between the outside of the section of the one core and the inside of the outer mold is independent of the distance between the outside of the section of the other core and the inside of the outer mold, so that the wall thicknesses of the two outer walls, at least in the region of the sections, are formed independently of one another. 
     According to an embodiment of the invention, in a blade of the type mentioned at the beginning, this object is achieved in that at least one cavity is divided into two passages by a center web, the one passage being arranged between the one outer wall and the center web and the other passage being arranged between the center web and the other outer wall. 
     One basic idea of an embodiment of the invention is that the two outer walls of the blade are produced independently of one another at least in sections. At least one cavity of the blade is divided into two passages by a center web. The one passage extends from the first outer wall up to the center web and the other passage extends from the center web up to the second outer wall. A plurality of cores are provided. A first core has one or more sections for forming the passages between the first outer wall and the center web. The further passages are formed by sections of a second core which is provided separately from the first core. Displacements and deformations of the first core which bring about a change in the wall thickness of the one outer wall are not transmitted to the second core. The wall thickness of the two outer walls, at least in regions, are therefore formed independently of one another. 
     The method according to an embodiment of the invention provides for those sections of each core which serve to form the passages to be supported in such a way that a minimum wall thickness is ensured. Projections which are supported on the inside of the outer mold are advantageously used for this purpose. 
     During the production of the cores, only the outside, facing the inside of the outer mold, of the sections is critical for the wall thickness of the outer walls. In particular, comparatively coarse tolerances may be applied to the side of the sections which is assigned to the center web. As a result, the production accuracy of the outside of the cores can be substantially improved, the outside of the cores being critical for the wall thickness of the outer walls. All the tolerances are shifted into the region of the center web. This does not result in disadvantages for the cooling effect, since the hot fluid flowing through the turbine is not admitted directly to the center web. Furthermore, the center web is cooled on both sides by the passages. The center web also provides the requisite strength for the blade when the outer walls have small wall thicknesses. 
     According to an advantageous development of an embodiment of the invention, the cores are provided with projections for supporting on the outer mold. They are then advantageously supported on one another during the casting and pressed against the inside of the outer mold. The support may be effected by rigid, in particular wedge-shaped, or elastic spacers. 
     With this procedure, a minimum wall thickness for the outer walls is reliably maintained. Displacements of the cores toward the inside are avoided by the cores being supported on one another. For the production of the cores, this means that only the outside facing the inside of the outer mold has to be produced with high precision. Due to the two cores being supported on one another, the accuracy to size of the further outsides is only of secondary importance. Greater rigidity than in the known devices and methods is also achieved due to the cores being supported on one another. Displacements or deformations of the cores during the casting are therefore reduced. The tolerance range for the wall thickness of the outer walls can therefore be markedly reduced, so that thinner walls overall may be provided. 
     The projections serving for the support on the outer mold advantageously taper starting from the cores. In particular, they may be of conical design. This ensures that only point-like openings are produced in the outer walls, through which openings only minimum cooling medium escapes. Despite the support on the inside of the outer mold, the desired high cooling efficiency is therefore maintained. 
     The cores may be fixed at one or both ends in a receptacle of the outer mold in the longitudinal direction of the blade. Fixing solely in the longitudinal direction is sufficient if the support is effected in the transverse direction by the projections on the cores. The position of the cores during the production of the wax tool and during the casting is thereby ensured. 
     The outer walls are advantageously connected to one another via a plurality of ribs for forming a plurality of cavities. This results in specific cooling of individual regions of the blade with increased strength. 
     According to an advantageous development, a cavity at a leading edge and/or a trailing edge of the blade is free of the center web. The reason for this is that an increased cooling effect is required in the region of the leading edge. The cooling effect would be impaired in the junction region of the center web. This also correspondingly applies to the trailing edge. 
     In an advantageous configuration, the wall thickness of the center web is greater than the wall thickness of the outer walls. The requisite strength of the blade is then ensured by the center web and possibly the ribs. The wall thickness of the outer walls can accordingly be reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in more detail below with reference to an exemplary embodiment which is shown schematically in the drawings, in which: 
     FIG. 1 shows a schematic longitudinal section through a gas turbine; 
     FIG. 2 shows a cross section through a moving blade of the turbine; 
     FIG. 3 shows a cross section through the device provided according to an embodiment of the invention for producing the blade; 
     FIG. 4 shows a schematic side view of the mounting of the cores in the device according to an embodiment of the invention; 
     FIG. 5 shows a view similar to FIG. 4 in a further configuration; and 
     FIG. 6 shows a plan view of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic longitudinal section through a gas turbine  10  having a casing  11  and a rotor  12 . Guide blades  13  are attached to the casing  11  and moving blades  14  are attached to the rotor  12 . A hot medium, in particular a gas, flows through the turbine  10  in arrow direction  15 . On account of this flow, the rotor  12  is set in rotation about an axis  16  relative to the casing  11 . The blades  13 ,  14  must be cooled from inside on account of the high prevailing temperature. 
     FIG. 2 shows a cross section through a moving blade  14  of the turbine  10 . The guide blades  13  are essentially constructed in a similar manner. The moving blade  14  has two outer walls  17 ,  18  which are connected via three ribs  19 ,  20 ,  21 . The ribs  19 ,  20 ,  21  are approximately perpendicular to the outer walls  17 ,  18 . At their two ends, the outer walls  17 ,  18  merge into a leading edge  22  and a trailing edge  23 , respectively. The flow against the blade  14  according to arrow direction  15  takes place from the leading edge  22  to the trailing edge  23 . 
     The intermediate space between the outer walls  17 ,  18  is subdivided into a plurality of cavities  24 ,  25 ,  26 ,  27  by the ribs  19 ,  20 ,  21 . The cavities  26 ,  27  lying in the center of the moving blade  14  are each divided into two passages  26   a ,  26   b ,  27   a ,  27   b  by a center web  28 . In this case, the passages  26   a ,  27   a  are arranged between the first outer wall  17  and the center web  28 . The further passages  26   b ,  27   b  are located between the center web  28  and the second outer wall  18 . The cavities  24 ,  25  in the region of the leading edge  22  and the trailing edge  23  are free of the center web  28 . 
     The wall thickness D of the center web  28  is greater than the wall thickness d of the outer walls. The center web  28  runs from the front rib  19  via the center rib  20  up to the rear rib  21 . It is arranged approximately in the axial profile center of the moving blade  14 . The center web  28 , together with the ribs  19 ,  20 ,  21 , provides the strength required by the moving blade  14  for operation. The outer walls  17 ,  18  may therefore be of thin design. 
     FIG. 3 shows a cross section through a device  29  according to the invention for producing a blade  13 ,  14 . An outer mold  30  having two mold parts  31 ,  32  is provided, it being possible for the mold parts  31 ,  32  to be moved away from one another and toward one another according to arrow direction  33 . Two cores  34 ,  35  formed separately from one another are inserted between the two mold parts  31 ,  32 . The first core  34  has three sections  36   a ,  37   a ,  38   a . The sections  36   a ,  37   a  serve to form the passages  26   a ,  27   a . The section  38   a  forms the cavity  24  in the region of the leading edge  22 . 
     The second core  35  is designed essentially in a similar manner with sections  36   b ,  37   b ,  38   b . Here, too, two sections  36   b ,  37   b  are provided for forming the passages  26   b ,  27   b . The cavity  25  in the region of the trailing edge  23  is formed by the section  38   b . The individual sections  36   ab ,  37   ab ,  38   ab  of the cores  34 ,  35  are connected to one another. 
     The sections  36   ab ,  37   ab  for forming the passages  26   a ,  26   b ,  27   a ,  27   b  have projections  39  for supporting on an inside  40  of the outer mold  30 . The projections  39  taper and are of conical design. They provide the minimum distance between the inside  40  of the outer mold and a respectively associated outside  46   a ,  47   a ,  46   b ,  47   b  of the sections  36   a ,  36   b ,  37   a ,  37   b . This distance essentially corresponds to the wall thickness d of the outer walls  17 ,  18 . The wall thickness D of the center web  28  is established by the distance between the sections  36   a ,  37   a  and the sections  36   b ,  37   b.    
     For the production, only the outsides  46   a ,  47   a ,  46   b ,  47   b  of the sections  36   a ,  37   a ,  36   b ,  37   b  and also the outsides  48   a ,  48   b  of the sections  38   a ,  38   b  have to be machined with high precision. The further surfaces of the cores  34 ,  35  may have comparatively large tolerances, since they are not important for establishing the wall thickness d of the outer walls  17 ,  18 . 
     FIGS. 4 and 5 show the mounting of the cores  34 ,  35  in the device  29 . At both ends, each of the cores  34 ,  35  has projections  41 ,  42  for fastening in a receptacle  43  (shown by broken lines) of the device  29  according to the invention. The two cores  34 ,  35  are supported on one another via spacers  44 ,  45 . As a result, the projections  39  are pressed against the inside  40  of the outer mold  30 . The use of rigid spacers  44  is shown in FIG.  4  and the use of elastic spacers  45 , in particular of spring-like design, is shown in FIG.  5 . 
     In the device according to an embodiment of the invention, the minimum wall thickness d of the outer walls  17 ,  18  is ensured by the cores  34 ,  35  being supported with the projections  39  on the inside  40 . On account of the taper of the projections  39 , only a point-like opening is produced in the outer walls  17 ,  18  of the completed blade  13 ,  14 . Displacement of the cores  34 ,  35  toward one another is prevented by the spacers  44 ,  45 . It is thus ensured that the desired wall thickness d of the outer walls  17 ,  18  is reliably maintained. The tolerances of the wall thickness d which occurred hitherto can be substantially reduced. The wall thickness d can therefore be reduced right from the beginning at the design stage compared with the known blades  13 ,  14  and devices  29 . 
     A further advantage is that the wall thicknesses d of the outer walls  17 ,  18  no longer depend on one another. A displacement or deformation of the core  34  does not lead to a change in the wall thickness d of the outer wall  18 . A displacement or deformation of the core  35  also does not lead to a change in the wall thickness d of the outer wall  17 . 
     FIG. 6 schematically shows a plan view of FIG.  5 . The individual sections  36   a ,  36   b ,  37   a ,  37   b ,  38   a ,  38   b  of the cores  34 ,  35  are rigidly connected to one another as shown. The cores  34 ,  35  are supported on one another via the elastic spacers  45  and are pressed against the inside  40 . If a plurality of spacers  45  distributed over the entire length of the cores  34 ,  35  are used, displacements and deformations during the casting can be substantially reduced. 
     To produce the blade  13 ,  14 , first of all the desired cores  34 ,  35  are preformed in a suitable mold (not shown) and then fired. They are then inserted into the prepared outer mold  30 . The projections  39  of the sections  36   a ,  36   b ,  37   a ,  37   b  of the two cores  34 ,  35  are brought to bear against the inside  40  of the outer mold  30 . For this purpose, either rigid or elastic spacers  44 ,  45  are inserted between the two cores  34 ,  35 . After that, the two cores  34 ,  35  are fixed in the receptacles  43 . 
     A suitable material, for example wax, is poured into the intermediate space between the cores  34 ,  35  and the inside  40  of the outer mold  30 . After the wax has solidified, the outer mold is removed and the wax body is provided with a protective coating. This protective coating, as well as the cores  34 ,  35 , may be made of a ceramic material. 
     The wax tool provided with the protective coating is fired again. The castable material for the blade  13 ,  14  is then introduced into the intermediate space between the protective coating and the cores  34 ,  35 . After this material has solidified, the protective coating and the cores  34 ,  35  are removed in a suitable manner, for example flushed out with an acid or an alkaline solution. 
     The production and assembly tolerances which are present in the known methods and devices during the production and fixing of the cores  34 ,  35 , of the wax tool and of the protective coating can be substantially reduced. The wall thickness of the outer walls  17 ,  18  of the blade  13 ,  14  can therefore be markedly reduced. This results in an improved cooling effect. The requisite strength of the blade  13 ,  14  is ensured by the center web  28 . 
     The invention thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included the scope of the following claims.