TURBINE VANE PROVIDED WITH A RECESS FOR EMBRITTLEMENT OF A FRANGIBLE SECTION

Abstract

A turbine vane of a turbine engine is described. The turbine vane includes a blade and a root. The root includes a stilt having lateral flanks with a curvilinear profile. The stilt includes a frangible zone suitable for undergoing a breakage of the stilt if radial forces higher than a threshold are exerted on the vane, in particular centrifugal forces during an overspeed state of the turbine. The frangible zone includes at least one oblong frangibility recess formed on at least one of the lateral flanks of the stilt, the oblong recess extending in an axial direction of the stilt along a longitudinal axis parallel to or included in a minimum cross-sectional plane which contains a minimum cross-section of the stilt.

Claims

1. A turbine vane of a turbine engine, including a blade and a root, said root comprising a stilt having lateral flanks with a curvilinear profile, said stilt comprising a frangible zone adapted to undergo a breakage of the stilt if radial forces greater than a threshold are exerted on the vane wherein the frangible zone comprises at least one oblong frangibility recess formed on at least one of the lateral flanks of the stilt, said oblong recess extending in an axial direction of the stilt along a longitudinal axis parallel to or comprised in a minimum cross-sectional plane in which a minimum cross-section of the stilt is located, the frangible zone of the stilt being formed by a concave zone of the stilt formed on a front face and on at least one of the lateral flanks of the stilt, the deepest zone of the oblong recess being intersected by the minimum cross-sectional plane of the stilt.

2. The turbine vane according to claim 1, wherein the turbine vane is mounted on a turbine engine rotor disk, wherein the longitudinal axis of the or each oblong recess is comprised in a frangibility plane located at a distance from an axis of rotation of the disk comprised between h+0.06 h and h−0.06 h, h being the distance between the axis of rotation and the minimum cross-sectional plane, the frangibility plane and the minimum cross-sectional plane being parallel to one another and to the axis of rotation.

3. The turbine vane according to claim 1, wherein the maximum depth of the oblong recess is between 9% and 35% of the width of the stilt, considered at the deepest point of the recess.

4. The turbine vane according to claim 1, wherein the maximum depth of the oblong recess is between 10% and 25% of the length thereof.

5. The turbine vane according to claim 1, wherein each lateral flank of the stilt comprises an oblong frangibility recess and wherein the distance between the barycentre of the recesses and the projection of the centre of gravity of the vane on the minimum cross-sectional plane is between 0 and 20% of the axial length (L) of the stilt.

6. The turbine vane according to claim 1, wherein the oblong recess has a curvilinear cross-section.

7. The turbine vane according to claim 1, wherein the oblong recess has a cross-section in the arc of a circle.

8. A turbine engine turbine, comprising a rotor including at least one disk and a set of turbine vanes mounted on the disk, wherein each vane of the set of turbine vanes is a turbine vane according to claim 1.

9. The turbine engine turbine according to claim 8, wherein the longitudinal axis of the or each oblong recess of each vane is comprised in a frangibility plane located at a distance from an axis of rotation of the disk between h+0.06 h and h−0.06 h, h being the distance between the axis of rotation of the disk and the minimum cross-sectional plane, the frangibility plane and the minimum cross-sectional plane being parallel to one another and to the axis of rotation.

10. The turbine vane according to claim 1, wherein said radial forces greater than a threshold are centrifugal forces exerted on the vane during an overspeed state of the turbine.

11. The turbine vane according to claim 2, wherein the longitudinal axis of the or each oblong recess is comprised in a frangibility plane located at a distance from an axis of rotation of the disk comprised between h+0.04 h and h−0.04 h.

12. The turbine vane according to claim 3, wherein the maximum depth of the oblong recess is between 10% and 25% of the width of the stilt.

13. The turbine vane according to claim 4, wherein the maximum depth of the oblong recess is between 14% and 20% of the length thereof.

14. The turbine vane according to claim 5, wherein the distance between the barycentre of the recesses and the projection of the centre of gravity of the vane on the minimum cross-sectional plane is between 0 and 15% of the axial length of the stilt.

15. The turbine engine turbine according to claim 9, wherein the longitudinal axis of the or each oblong recess of each vane is comprised in a frangibility plane located at a distance from an axis of rotation of the disk between h+0.04 h and h−0.04 h.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1, which has been mentioned above, illustrates the general structure of a free-turbine gas turbine according to the prior art;

[0033] FIG. 2

[0034] FIG. 3, mentioned above, are respectively front and perspective views of a vane according to the prior art;

[0035] FIG. 4 which has been mentioned above, shows the stress field exerted on the stilt of the vane in FIGS. 2 and 3;

[0036] FIG. 5

[0037] FIG. 6 are respectively front and perspective views of a vane according to the invention;

[0038] FIG. 7 is a detailed view on a larger scale of the vane of FIG. 6;

[0039] FIG. 8 is a cross-sectional view of the stilt of the vane of FIGS. 5 and 6 at the deepest point of the recesses;

[0040] FIG. 9 shows the vane of FIGS. 5 and 6 mounted on a rotor disk;

[0041] FIG. 10 is a perspective view of FIG. 8; and

[0042] FIG. 11 shows the stress field exerted on the stilt of the vane in FIGS. 5 and 6.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT

[0043] In FIGS. 5 and 6, a turbine engine vane, in particular a free turbine vane, represented by the general reference number 10, is shown.

[0044] This vane 10 includes a blade 11, a fir-tree root 12 intended to fasten the vane onto a rotor risk, by engaging the root 12 into a housing also known as “receptacle” of corresponding shape formed in the disk, a stilt 13 extending the fir-tree root 12 and a platform 14.

[0045] The fir-tree root extends along a longitudinal axis, which in a manner known per se can form an angle with the axis of rotation A-A′ of the turbine disk, in order to increase the contact length between the fir-tree root and the disk. The axis of the fir-tree root once the vane is mounted on the disk extends along the direction of the corresponding receptacle in the disk. The receptacles of a free turbine disk can be provided each more or less sloping in a tangential plane to the disk, with respect to the axial direction of the disk. In other words, an angle in a tangential plane to the disk is formed between the direction of a receptacle and the axis of the disk.

[0046] As can be seen, the stilt 13 has a curvilinear shape.

[0047] It includes, on the anterior face thereof, on the side of the leading edge of the vane, a concave shape 15 and lateral flanks 16, also concave, in order to reduce the cross-section of the stilt locally to delimit a frangible zone in the stilt.

[0048] The vane 10 also includes recesses 17 that are oblong, i.e. having a longitudinal dimension greater than the lateral dimension thereof, which are formed in the lateral flanks of the stilt 13. Each recess 17 extends along a longitudinal axis X-X′ parallel or substantially parallel to the fir-tree root. The axis X-X′ of each recess can, therefore, like the axis of the fir-tree root, form an angle with the axis of rotation A-A′ of the turbine disk, seen in FIG. 9.

[0049] Each recess forms a pocket locally reducing the cross-section of the neck of the stilt in order to embrittle the frangible zone of the stilt and set the overspeed limit speed from which the blade is detached from the disk.

[0050] For example, as illustrated, each lateral flank of the stilt includes at least one recess. Each lateral flank includes here a recess, the stilt comprising a pair of recesses formed symmetrically.

[0051] As illustrated in FIG. 7, each recess includes a concave cross-section, considered perpendicular to the longitudinal axis of the cavity, preferably a round cross-section, with no edges. The radius R of the recess is preferably between 10 and 25% of the length of the recess, advantageously between 14% and 20% of the length of the recess.

[0052] The depth thereof, which can for example correspond to the radius of the recess, is advantageously between 9% and 35% of the minimum width 1 min of the stilt, considered at the deepest point of the recess (FIG. 8). The depth of the recess is preferably between 10% and 25% of the width 1 min of the stilt.

[0053] With reference to FIG. 9, which illustrates a vane mounted on a portion of a disk D, each recess is formed in the concave surface of a lateral flank of the stilt and extends parallel to a longitudinal plane P which coincides with the minimum width of the neck of the stilt.

[0054] Considering the distance h between the axis of rotation A-A′ of the turbine disk and the plane P, the axis X-X′ of each recess is comprised in a plane, hereinafter referred to as frangibility plane, which either coincides with the plane P, or is parallel to the plane P and is located slightly above or above the plane P. More specifically, the frangibility plane is located at a distance from the axis of rotation A-A′ of the disk between h−0.06 h and h+0.06 h, preferably between h−0.04 h and h+0.04 h. Moreover, if the stilt comprises a pair of recesses formed symmetrically, the frangibility plane comprises the two respective axes X-X′ of the two recesses.

[0055] Moreover, as seen in FIG. 10, which illustrates a perspective and cross-sectional view at the deepest points of the recesses, the distance d between the barycentre B of all the recesses and the radial projection of the centre of gravity G of the vane on the cross-section is between 0 and 20% of the axial length L of the stilt at the point of the minimum cross-section thereof, preferably between 0 and 15% of this length L. The axial length L is measured along a direction parallel to the axis of the fir-tree root, which can advantageously form an angle with the axis of rotation A-A′ of the turbine disk. This angle is for example between 5° and 20°.

[0056] The length of the recesses is for example about 40% of the total length of the fir-tree root at the point of the minimum cross-section and the depth thereof is about 20% of the neck width.

[0057] Each lateral flank of the stilt can include any number of recesses in order to reduce the cross-section of the stilt locally and as such set the limit rotational speed of the vanes.

[0058] As stated above, the recesses are free from sharp angles so as not to induce concentration of higher stress than that already induced by the concave shape formed in the anterior face, on the leading edge side.

[0059] These recesses make it possible to set the breakage speed of the vane by increasing the mean stress exerted in the neck of the stilt, without significantly increasing the maximum stress induced under the action of thermomechanical forces harmful for the lifetime of the vane.

[0060] Indeed, as shown in FIG. 11, which illustrates the radial stress field exerted in the vane under the action of thermomechanical forces, introducing a recess into the frangible zone of the stilt does not induce a significant increase in the maximum stress which remains localised in the zone Z′ of the edge of the concavity of the leading edge of the vane. For example, introducing an oblong recess into each of the two lateral flanks of the stilt, in the case represented in FIG. 11, increased the maximum stress locally by merely 1%.