Patent Publication Number: US-9890644-B2

Title: Foundry core assembly for manufacturing a turbomachine blade, associated method of manufacturing a blade and associated blade

Description:
BACKGROUND OF THE INVENTION 
     Field of The Invention 
     The invention relates generally to the field of turbomachines, and more particularly that of turbine blades of these turbomachines and to their manufacture. 
     Description of the Related Art 
     Turbine blades are subjected to strong thermal stresses due to the heat in gases in which they are plunged at the outlet of the combustion chamber, and need to be cooled to support these temperatures. They are accordingly hollow and traversed by internal cavities in which cooling gas circulates, taken at the outlet of a stage of one of the compressors. 
     More precisely, a turbine blade of a turbomachine comprises an aerodynamic surface (or blade) extending between a blade foot and a blade tip. The blade has a leading edge arranged opposite the flow of hot gases coming from the combustion chamber of the turbomachine, as well as a trailing edge opposite the leading edge and the lateral intrados and extrados walls which connect the leading edge to the trailing edge. 
     The internal cavities extend over the height of the blade, and comprise, from upstream to downstream in the direction of the flow of gases from the combustion chamber, a leading edge cavity and a trailing edge cavity, adjacent to the leading edge and the trailing edge of the blade respectively, and at least one central cavity, extending between the leading edge cavity and the trailing edge cavity. These cavities are fed with cooling gas via tubing connecting them to the foot of the blade. 
     The blade also comprises, at the level of its tip, a hollow form or bath, which is defined by the extension of the intrados and extrados walls, as well as by a bottom wall which close off the internal cavities. 
     To make these different cavities, which have complex forms and whereof the geometry must be respected with great precision, the blades are classically produced by a technique known under the name of lost wax smelting. This technique consists schematically of making a blade draft of wax in which cores made of ceramic which reproduce the resulting cavities are embedded. The wax blade is then embedded in a carapace, for example made of refractory material, then the cores are eliminated chemically, leaving in their place the preferred internal cavities and bath. Embodiments of this method are described in particular in documents FR 2 875 425, FR 2 874 186, or FR 2 957 828 in the name of the applicant. 
     The cores for these modern turbine blades are constituted by internal cavity cores, having classically the form of columns, which are positioned side by side and held together by conventional means. 
     These cores have increasingly more complex forms, as the specifications required for cooling of blades grow and the blades diversify. It is necessary to position them in the carapace with extreme precision. 
     A core generally comprises a first core element designed to form the cavities and a second core element designed to form the bath, the second core element being connected to the first core element by linking rods made of alumina or quartz. 
     The aim of these rods is to hold between them the parts of the core and stiffen the resulting assembly, and they are involved in making dedusting holes in the upper part of the blade. These rods are stored in the holes which they tend to make in the bottom of the bath. The dedusting holes enable circulation of the cooling gas in the cavities and evacuation of various particles entering the turbomachine. 
     To improve the aerodynamic performance of the blade and minimise energy losses, it has been proposed to use turbine blades for turbomachines having an advanced blade tip of the type “offset of tip sections” according to the French patent application registered on Nov, 17 2011 No. FR 11 60465 in the name of the applicant. 
     Such turbine blades are adapted to minimise energy losses. They comprise a blade which can be broken down into blade sections stacked according to a stacking direction along the blade. In the case of the blade tip with tip section offset, the stacking of the sections at the level of the tip of the blade is offset in the direction of the intrados wall, preferably progressively. 
     For this, as described in patent application No. FR 11 60465, the blade can comprise a cavity at the level of its tip, open in the direction of its free end and delimited by the bottom wall and a rim which extends between the leading edge and the trailing edge. The stacking of the blade sections of the blade at the level of this rim presents offset in the direction of the intrados, this offset increasing as the free end of the tip of the blade is approached. The blade also comprises cooling channels, inclined relative to the intrados, and connecting the internal cavities to the intrados wall. 
     The intrados wall of the blade can also present a projecting portion, whereof the outer face is inclined relative to the rest of the intrados of the blade and has at its end a terminal face, turned towards the rim. The bottom wall is connected to the intrados wall at the level of the terminal face of the projecting portion, and the cooling channels can be arranged in the projecting portion of the intrados wall such that they terminate on the terminal face of the projecting portion, the distance between the axis of the cooling channels and the outer limit of the free end of the rim of the intrados side being greater than zero. 
     However, this tip section offset and the small size of the blade, and therefore of the cores used for its manufacture, make it difficult to hold the rods of the second core element which is designed to former the bath on the first core element. 
     It has therefore been proposed to orient the rods individually, with big angles relative to the main direction of the blade. However, the cores are complex to produce due to the strong inclination of the linking rods relative to the main direction of the cores (and therefore of the injection of ceramic), which can raise problems of wear of the cores at the level of the bottom of the bath. Also, executing this manufacturing method needs knowhow and experience which are accessible to all those skilled in the art, specifically the founders, as here. 
     It has also been proposed to use linking rods anchored conventionally in the core elements, but with anchoring strongly reduced in comparison with conventional techniques, due to the minimal dimension of the internal cavities of the blade. But, the anchoring depth and the thickness of the cores (generally made of ceramic) about the linking rods cause problems of cracking (partial ruptures which occur under the action of forces resulting from uneven withdrawal) in the core elements, and therefore an excessive rate of discard. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention proposes as such an assembly forming a core for the manufacture of a turbomachine blade cooled by circulation of fluid in internal cavities, comprising a first core element of elongated form for the formation of different internal cavities and a second core element for the formation of a bath cavity, the second core element being designed to be arranged in the extension of the first core element. The first core element comprises an internal core of a leading edge cavity, at least one central cavity internal core and a trailing edge cavity internal core designed to form respectively, from upstream to downstream in the direction of flow of gases in the turbine, a leading edge cavity, at least one central cavity and a trailing edge cavity of the blade. The core of the central internal cavity adjacent to the internal core of the trailing edge cavity has, in the immediate vicinity of the second core element, a bulge which extends in the direction of the core of the leading edge cavity. 
     Such a solution enables anchoring of at least one linking rod at the level of an enlarged area of the first core element (at the level of the bulge), and consequently producing turbine blades for turbomachines having an offset of tip sections according to a reliable method with a minimal rate of discard of cores. 
     The invention also relates to a blade produced by means of such an assembly forming a foundry core, as well as a manufacturing method using such an assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other characteristics, aims and advantages of the present invention will emerge more clearly from the detailed following description, given in reference to the attached drawings given by way of non-limitation and in which: 
         FIG. 1  is a general view in section and in semi-perspective of an example of a turbine blade with tip section offset according to the invention, 
         FIG. 2  is a representation in perspective illustrating a foundry core according to an embodiment of the invention, 
         FIG. 3  is a representation of a detail of the upper part of the foundry core of  FIG. 2  and the linking rods between the different core elements, 
         FIG. 4 a    is a side elevation of an example of a blade with tip section offset in keeping with the prior art, 
         FIG. 4 b    is a view of the top of the blade of  FIG. 4 a   , showing the cavities according to the planes X 1  and X 2 , 
         FIG. 5 a    is a side elevation of an example of a blade with tip section offset in keeping with the invention, and 
         FIG. 5 b    is a view of the top of the blade of  FIG. 5 a   , showing the cavities according to the planes Y 1  and Y 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In reference to  FIGS. 1 and 5   a , these show a turbine blade  1  for a turbomachine comprising a blade  10  extending between a blade foot  11  and a blade tip  12 , and comprising a leading edge  13 , a trailing edge  14  opposite the leading edge, lateral intrados and extrados faces  15 , and internal cavities  19   a - 19   e  separated by intercavity walls  20  which extend according to the height of the blade  1 . 
     The blade can for example present an advanced blade apex of the type “tip section offset” in keeping with French patent application No. FR 11 60465 registered on Nov. 17, 2011 in the name of the applicant. 
     Especially, from the leading edge  13  to the trailing edge  14 , the blade  1  comprises a leading edge cavity  19   a , one or more central cavities  19   b ,  19   c ,  19   d , (in this case three for the blade  1  shown in the figure, specifically a first rising central cavity  19   b , a descending central cavity  19   c , and a second rising central cavity  19   d , which together form assembly an internal cavity “trombone”, and a trailing edge cavity  19   e . The blade  1  also comprises, at the level of its tip  11 , a bath  18 , whereof the bottom wall  17  closes off the internal cooling cavities  19   a - 19   e.    
     The intercavity wall  20  separating the central cavity  19   d  adjacent to trailing edge the cavity  19   e  and the following cavity in the direction of the leading edge  13  (that is, in the case of the blade of  FIG. 1 , the central cavity  19   c ) complies, in the vicinity of the bottom wall  17  of the bath  18 , with an offset  16  in the direction of the leading edge  13 . 
     As a variant, when the blade  1  comprises only a single central cavity  19   d , it is the intercavity wall  20  which separates this single central cavity  19   d  from the leading edge cavity  19   a  which can match, in the vicinity of the bottom wall  17  of the bath, with an offset  16  in the direction of the leading edge  13 . 
     Due to this offset  16  relative to the rest of the intercavity wall  20 , the central cavity  19   d  which is adjacent to the trailing edge cavity  19   e  has a bulge  34  in the vicinity of the bottom wall  17 , said cavity  19   d  being wider at the level of the blade tip  12  than at the level of the blade foot  11 . The particular form of this central cavity  19   d  simplifies the manufacturing of the blade  1 . 
     Also, as illustrated in  FIGS. 1 and 5   a , the intercavity wall  20  is offset at the level of the offset  16  as far as the bottom wall  17  to enlarge the entire upper part of this central cavity  19   d.    
     The assembly forming a foundry core  30  for the manufacture of such a turbomachine blade ( FIGS. 2 and 3 ) has an adapted complementary form and comprises a first core element  31 , repeating the form of the cavities  19   a - 19   e , and a second core element  32 , repeating per se the form of the bath  18 . The two core elements  31  and  32  are for example made of ceramic. 
     The second core element  32  is connected to the first core element  31  by linking rods  40  which can for example be made of aluminium or quartz. 
     The first core element  31  has an overall elongated form according to the height of the blade  1  and comprises a series of internal cores (or columns)  31   a ,  31   b ,  31   c ,  31   d  and  31   e , designed to form respectively the cavities of a leading edge  19   a , the central cavity/cavities  19   b ,  19   c  and  19   d , and the trailing edge cavity  19   e  respectively. 
     The second core element  32  is arranged above the first core element  31 , and is separated from the latter by linking rods  40  positioned so as to form dedusting holes  35  in the internal cores  31   a  to  31   e.    
     The internal core  31   d  defining the central cavity  19   d  adjacent to the core  31   e  of the trailing edge cavity has, at least in an area immediately near the second core element  32 , a bulge  34  in the direction of the internal core  31   a  of the leading edge cavity. The internal core  31   c  of the corresponding central cavity is therefore wider at this level, for example from 30% to 60% wider, as is evident in  FIGS. 4 a  to 5 b   . In fact, on a conventional blade with tip section offset, the maximal width d x  of the core of the internal cavity  31   d  adjacent to the core of the trailing edge cavity  31   e  at the level of the plane is of the order of 3.4 mm. In comparison, the maximal width d y  of the core of the internal cavity  31   d  which is adjacent to the trailing edge cavity internal core  31   e  at the level of the plane Y 2 , for a similar blade with tip section offset but comprising a bulge  34  according to the invention, can be of the order of 5 mm (or around 50% wider). The space available for implanting a linking rod  40  in this cavity core  31   d  is therefore much greater than in the blades of the prior art, which increases the anchoring depth and the thickness of the cores about the linking rods  40  and prevents the formation of cracking in the core elements  31 ,  32 , and therefore significantly reduces the rate of discard of blades during manufacture. 
     Also, the central cavity internal core  31   c  immediately adjacent in the direction of the cavity  31   a  of a leading edge as such matches a complementary counterform  33  such that the intercavity wall  20  made between these two internal cores  31   c  and  31   d  has the offset  16  described hereinabove in the direction of the leading edge  13  of the blade  1 . 
     As a variant, when the blade  1  comprises only a single central cavity  19   d , the first core  31  comprises only a single central cavity internal core  31   c , and it is the internal core  31   a  of the leading edge cavity which is immediately adjacent to this central core internal core  31   d . It is therefore the internal core  31   a  of a leading edge which matches the complementary counterform such that the intercavity wall  20  made between these two internal cores  31   a  and  31   d  has the offset  16  described hereinabove in the direction of the leading edge  13  of the blade  1 . 
     The bulge  34  and the counterform  33  are local, and extend only at the level of the upper part of the internal cores  31   b - 31   d  (respectively  31   a , in the case of a blade comprising a single central cavity), the core  31   d  adjacent to the core  31   e  of the trailing edge cavity being wider at the level of this bulge  34  than at the level of its lower part. 
     The height of the bulge  34  is sufficient to allow anchoring of the linking rods  40  at the level of the bulge  34 , and making dedusting holes  35  in the wall  17  forming the bottom of the bath  18  without formation of cracks in the internal cores  31   b - 31   d . Also, the bulge  34  extends as far as the upper wall of the central cavity internal core  31   e.    
     Making these dedusting holes  35  is made easier by modification of the geometry of the internal core  31   d  adjacent to the trailing edge core  31   e  and more particularly by the existence of the bulge  34  in its upper part. In particular, because of the aerodynamic form of the blade  10 , which has an increasing transversal cross-section between the trailing edge  14  and the leading edge  13 , the presence of the bulge  34  anchors the linking rods  40  in an area of the core  31  wider than with a configuration of a classic core, and consequently limits the angle formed between the linking rods  40  and the main axis of the cores. Making the holes  35  is therefore more favourable for foundry and also improves the possibilities for anchoring the linking rods  40 . 
     Also, the diameter of the retaining rods  40  can be selected so as to be equal to the preferred diameter for the dedusting holes  35  in the final item to avoid an extra step for finishing the blade  10  (capping of holes) after the step of lost wax smelting. 
     As illustrated in  FIGS. 2 and 3 , the dedusting holes  35  are oblique and can for example be oriented towards the leading edge  13  of the blade  1 . This orientation is however not limiting, with respecting of the diameter of the retaining rods  40  being considerable to ensure evacuation of dust in the blade  1 . 
     The linking rods  40  are accordingly oriented obliquely relative to the general direction according to which the second core element  32  they pass through extends to form the dedusting holes  35  of the second core element  32 . 
     The second core element  32  can also comprise bosses  36  ( FIG. 2 ) in the upper part to improve the anchoring of the linking rods  40 . 
     The assembly forming a foundry core  30  such as illustrated in  FIGS. 2 and 3  is then used to make a wax draft, which is then embedded in a carapace, then the cores are eliminated to form the different cavities  19   a - 19   e  and the bath  18 .