TURBOMACHINE BLADE, COMPRISING A ROOT WITH REDUCED STRESS CONCENTRATIONS

Abstract

The flange (11) of a blade root platform (10) is separated from an adjacent edge (31) of the blade (4) by a groove (18), that prevents direct transmission of forces created by the bolted attachment of the platform flange (11) to the adjacent part of the blade (4) and reduces stress concentrations.

Claims

1. Turbomachine blade comprising an airfoil (4) and a root (7), the root including a platform (10) and a flange (11) through which bolt holes (26) are formed for bolting to a case (8) of the turbomachine, the airfoil (4) and the flange (11) forming a single piece with the platform (10) and extending along opposite directions from the platform along a radial direction (Y-Y) of the turbomachine, the airfoil comprising an edge (31) connected to the platform, the platform extending from one end (34) along an essentially axial direction (X-X) of the turbomachine, characterised in that the root comprises a groove (18) extending from said end (34), between the platform (10) and the flange (11) and penetrating in said essentially axial direction towards a bottom (21) beyond a connection point (32) of said edge of the airfoil and the platform and beyond the entire flange (11); said connection point of the edge of the airfoil and the platform is located along said essentially axial direction (X-X), between said end (34) of the platform and at least one face (25) of the flange (11) facing said end (34); the flange (11) being connected to the platform by a neck (23) delimited by a face (19) of the groove (18) and extending mainly along said essentially axial direction (X-X) from the flange to the heel, and having a thickness less than the minimum thickness (24) of the flange (11).

2. Blade according to claim 1, characterised in that the platform comprises a heel (22) corresponding to a thickening of the platform (10) in the radial direction, extending beyond the bottom (21) of the groove (18) along the essentially axial direction (X-X), the neck (23) connecting the heel (22) to the flange (11).

3. Blade according to claim 2, characterised in that the flange (11) is connected to the heel (22) through a rounded surface (29).

4. Blade according to claim 1, characterised in that the groove (18) opens up towards said end of the platform and is delimited by two faces (19, 20) converging towards the bottom (21), and the bottom is formed by a rounded surface joining said faces.

5. Blade according to claim 4, characterised in that said faces comprise a cylindrical face (20) delimiting the platform (10) and a conical face (19) delimiting the flange (11).

6. Blade according to claim 1, characterised in that the flange comprises a thinned central portion (25) between two concentric conical bearing surfaces (27, 28), the central portion being drilled with bolt holes (26), the conical bearing surfaces facing said end (34).

7. Blade according to claim 1, characterised in that said edge (31) is a trailing edge of the blade.

8. Blade according to claim 1, characterised in that it is a fixed blade of the turbomachine.

Description

[0012] We will now give a complete description of a particular embodiment of the invention, that will be sufficient to clearly understand its different aspects, characteristics and advantages with reference to the following figures:

[0013] FIG. 1, already described, represents a blade according to prior art;

[0014] FIG. 2 is an enlargement of FIG. 1 showing an end region of the blade platform;

[0015] FIG. 3 represents the same region for a blade characteristic of the invention;

[0016] FIG. 4 is an enlargement of FIG. 3;

[0017] and FIGS. 5 and 6 represent two design variants of the blade.

[0018] Refer to FIGS. 3 and 4. Unlike the design shown on FIG. 2, the flange 11 is now separated from the platform 10 by a groove 18. This groove 18 is delimited by a conical face 19 on one side of the flange 11, and another face 20 that is conical or cylindrical as in this case, on the side of the platform 10, the faces 19 and 20 converging towards each other and connecting to each other at a rounded groove bottom 21. The groove 18 is fairly deep, from its opening to the bottom 21, to extend over the entire thickness of the flange 11 and beyond it, and thus to separate it from the airfoil 4 in a radial direction Y-Y of the turbomachine. The bottom 21 extends projecting in the axial direction X-X and in the upstream direction (along the direction of flow of fluid around the blade 1) beyond the connection point 32 of the edge 31 of the airfoil 4 to the platform 10. Upstream from the bottom 21 of the groove 18 in the axial direction X-X, the platform 10 is thickened in the radial direction Y-Y, by a heel 22 that is connected to the flange 11 by a neck 23. The neck 23, delimited by face 19, is less thick than the minimum thickness 24 of the flange 11 (in this case a thinned central portion 25 in which the bolt holes 26 are formed, and limited by two concentric conical bearing surfaces 27 and 28, as is usual for this type of flange). Other rounded parts 29 and 30 are formed on concave fillets between the flange 11 and the heel 22, and between the heel 22 and the platform 10 itself.

[0019] Note that unlike usual designs, the edge 31 of the airfoil 4 on the side of the flange 11 is moved towards the end 34 of the platform 10, such that its connection point 32 with the platform 10 extends beyond at least one of the faces of the flange 11 (in this case the face of the thinned central portion 25) that face towards said end 34 along the axial direction X-X.

[0020] The flange 11 is mounted flexibly on the platform 10 by the thin neck 23. The corner of the airfoil 4 adjacent to the connection point 32 is also mounted flexibly on the platform 10, the end of which above the groove 18 forms a projection 33 that is also thin on which this corner and therefore the connection point 32 extends. The increased flexibility of the blade 1 at these locations can reduce stress concentrations, by distributing forces towards adjacent areas with lower loads. Therefore it is advantageous if the groove 18 is relatively wide between the faces 19 and 20 to accentuate the flexibility at immediately adjacent locations of the blade 1 (the neck 23 and the projection 33). It is also advantageous if the groove 18 is sufficiently deep to the bottom 21 so that the neck 23 and the projection 33 can be extended with the same effect of increased flexibility, and to make the transmission path of forces between the flange 11 and the corner of the airfoil 4 more sinuous, and thus reduce their magnitude. The heel 22 helps in distributing stresses and therefore reducing their concentration at the end 34 of the airfoil 1; the rounded parts 29 and 30, and the rounded bottom 21, also tend to reduce local stress concentrations.

[0021] If the edge 31 of the airfoil 4 is pushed towards the end 34 of the platform 10, its profile from the platform 10 is not critical concerning the reduction of stress concentrations: the layouts in FIGS. 5 and 6 have both given good results in this respect.

[0022] Unlike the profile of the trailing edge 13 in FIG. 2 that; starting from the platform 10 and working in a radially outwards direction, bends firstly briefly forwards and then backwards, the profile in FIG. 5 according to the invention bends towards the upstream direction by a height greater than or equal to the distance between the end 34 and an upstream face 35 of the heel 22, with a steep slope in the upstream direction (about 30°) close to the platform 10, and then progressively decreasing; and the profile in FIG. 6, also according to the invention, bends in the upstream direction with a steep (more than 30°) and approximately constant slope over a height equal to approximately the distance between the end 34 and the upstream face 35; and then radially further outwards, it bends slightly in the downstream direction with a shallow slope (about 10°), with a marked inflection point 36 from the previous slope.