EP3976931B1 - Turbine blade having a fragilization cavity of a frangible section - Google Patents

Description

Technical field of the invention

The present invention relates to the turbine blades of a turbomachine and, in particular, to a stilt arrangement of a turbine blade. The prop of a blade is a support part of the blade of the blade which extends radially between a lower attachment part of the blade called "fir tree foot" and a platform of the blade.

More particularly, the invention relates to the turbine blades of a free-turbine turbomachine.

The invention also relates to a turbomachine turbine comprising such blades.

State of the prior art

Classically, as illustrated in figure 1 , a free-turbine turbomachine comprises a gas generator 1, comprising at least one compressor which comprises one or more compression stages 2, a combustion chamber 3, and a turbine in which the pressurized hot gases from the combustion chamber expand and in which the kinetic and thermal energy of the gases is transformed into mechanical energy to rotate a shaft which connects the turbine to the compressor, in order to also drive the compressor. A turbine 4, called a free turbine, which comprises one or more turbine stages, is arranged downstream of the turbine of the gas generator 1, and mechanically decoupled from the latter. The free turbine 4 is driven in rotation by the gases from the gas generator 1.

In free-turbine turbine engines, used for example, but not exclusively, in helicopter propulsion systems, the free turbine is mechanically independent of the rotor of the helicopter, a reduction gear being interposed between the shaft line and the main rotor .

In the event of a break in the power transmission line, for example in the event of a break in the shaft line or the transmission line connected to the gearbox, the turbine may be in a overspeed situation due to the disappearance of the resistive torque applied to the turbine blades.

This overspeed situation can be particularly dangerous, leading to the breakage of at least one rotating disk which supports the blades of one stage of the turbine, under the effect of centrifugal force, and causing the release of debris at very high energy that cannot be contained by the shield provided on the motor.

It is therefore necessary to provide protection systems in the turbines which prevent overspeed.

It has already been proposed, in the state of the art, protection systems against overspeed, known under the name of "blade-shedding" in the Anglo-Saxon terminology, which consist in creating in the blades a frangible zone of so that they break at a predetermined rotational speed avoiding any risk of the disc breaking which would be caused by centrifugal forces. In this regard, reference may be made to the document GB 881, 850 which describes a turbine intended for driving accessories in which holes are made at the base of the blades of the blades.

Thus, in the event of a risk of overspeed, after the blades have broken, the turbine, having lost its aerodynamic profiles, naturally slows down and may stop rotating. Slowing down the free turbine to return to acceptable speeds thus avoids the risk of the disc breaking due to centrifugal forces.

In this regard, it has been proposed to machine the leading edge of the root of the blade of the turbine in order to adjust the section of the throat of the blade so that the latter breaks at a desired speed, while retaining a sufficient length of contact between the tree root of the blade and the corresponding cavity of the disk which receives the tree root to guarantee the mechanical strength of the blade attachment on the disk.

We represented on the figures 2 and 3 a blade with a frangible section intended to break to prevent overspeed.

As can be seen, the stilt 4 of the dawn, which extends between the fir tree base 6 and a platform which forms the base of a profile aerodynamic 5 or blade of the blade, comprises a concave leading edge 7 making it possible to form in the stilt a frangible zone of minimum cross-section capable of allowing the blade to detach from the disc from a threshold speed protection.

We represented on the figure 4 the radial stress which is applied in the blade shank under the effect of thermomechanical forces. As can be seen, the creation of a concave leading edge 7 locally reducing the section of the stilt generates the appearance of a zone Z of maximum stress on the leading edge, in the corners of the reduced section . This increase in the maximum stress is accompanied by the appearance of an additional moment due to the decentering of the aerodynamic profile with respect to the minimum section of the neck of the stilt.

It has been observed that this maximum stress conditions the fatigue life of the blade so that the production of this concave zone in the leading edge to locally weaken the blade requires relatively complex design work in order to define the threshold value from which the stilts break while limiting the increase in the maximum stress harmful to the fatigue life of the blade.

Furthermore, the use of materials having an increased resistance for the production of the blades generates, for the same frangible section, an increase in the threshold speed from which the stilts break.

The production of a concave zone of increased dimensions in the leading edge of the blade so as to locally reduce the section of the blade shank would not make it possible to reduce the value of the ultimate breaking speed of the stilt without prohibitively increasing the maximum stress on the minimum section of the stilt and consequently reducing the fatigue life of the blade. It is therefore desirable to be able to reduce the breaking speed value of the stilt when using a material of increased strength for the production of the blade.

Finally, we can refer to the documents CN 109 139 123 And US 5,435,694 .

Disclosure of Invention

In view of the foregoing, the invention aims to provide a turbine blade provided with a frangible section making it possible to adjust the breaking speed value of the blade without increasing the maximum stress in the blade.

The subject of the invention is therefore a turbine blade comprising a blade and a root, the root comprising a strut having lateral flanks with a curvilinear profile, the said strut comprising a frangible zone adapted to undergo breakage of the strut if radial forces greater than a threshold are exerted on the blade, in particular centrifugal forces during a state of overspeed of the turbine.

The frangible zone comprises at least one oblong frangibility cavity formed on at least one of the lateral sides of the stilt, said oblong cavity extending in an axial direction of the stilt along a longitudinal axis parallel to or included in a plane of minimum section in which a minimum cross-section of the stilt lies.

This cavity thus makes it possible to weaken the frangible section of the stilt by increasing the average stress which is exerted in the neck of the stilt, without significantly increasing the maximum stress generated locally under the action of the thermomechanical forces. It therefore makes it possible to optimize the adjustment of the limit speed from which the blades break.

Advantageously, the blade is mounted on a disk, the longitudinal axis of the or each oblong cavity being included in a plane of frangibility located at a distance from an axis of rotation of the disk comprised between h+0.06h and h- 0.06h, preferably between h+0.04h and h-0.04h, h designating the distance between the axis of rotation and the plane of minimum section, the frangibility plane and the plane of minimum section being parallel between them and to the axis of rotation.

According to the invention, the frangible zone of the stilt is formed by a concave zone of the stilt produced on a front face and on at least one of the lateral sides of the stilt, the zone furthest depth of the oblong cavity being intersected by the minimum section plane of the stilt.

For example, the maximum depth of the oblong cavity is between 9% and 35% of the width of the stilt, preferably between 10% and 25% of the width of the stilt, considered at the most deep in the cavity.

In one embodiment, the maximum depth of the oblong cavity is between 10% and 25% of its length, preferably between 14% and 20% of the length of the cavity

In one embodiment, in which each lateral flank of the stilt comprises an oblong frangibility cavity, the distance between the barycenter of the cavities and the projection of the center of gravity of the blade on the plane of minimum section is between 0 and 20% of the axial length of the stilt, preferably between 0 and 15% of said width of the stilt.

Advantageously, the oblong cavity has a curvilinear cross-section.

Preferably, the oblong cavity has a cross-section in the form of an arc of a circle.

Another subject of the invention is a turbomachine turbine, comprising a rotor comprising at least one disk and a set of turbine blades mounted on the disk, each blade being a blade as defined above.

Advantageously, the longitudinal axis of the or each oblong cavity of each blade is included in a plane of frangibility situated at a distance from an axis of rotation of the disk comprised between h+0.06h and h-0.06h, preferably between h+0.04h and h-0.04h, h designating the distance between the axis of rotation of the disc and the minimum section plane, the frangibility plane and the minimum section plane being parallel to each other and to l 'rotation axis.

Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of example, and made with reference to the appended drawings.

Brief description of the drawings

• [ Fig 1 ], which has already been mentioned, illustrates the general structure of a free turbine gas turbine according to the state of the art;
• [ Fig 2 ]
• [ Fig.3 ], already mentioned, are respectively front and perspective views of a blade according to the state of the art;
• [ Fig 4 ] which has already been mentioned, shows the stress field exerted on the stilt of the dawn of the figures 2 and 3 ;
• [ Fig.5 ] And
• 10 [ Fig 6 ] are respectively front and perspective views of a blade according to the invention;
• [ Fig 7 ] is a larger scale detail view of the dawn of the figure 6 ;
• [ Fig.8 ] is a cross-sectional view of the dawn stilt of the figures 5 and 6 at the deepest part of the cavities;
• [ Fig.9 ] shows the dawn of figures 5 and 6 mounted on a rotor disc;
• [ Fig. 10 ] is a perspective view of the figure 8 ; And
• [ Fig.11 ] shows the stress field exerted on the stilt of the dawn of the figures 5 and 6 .

Detailed description of at least one embodiment

We represented on the figures 5 and 6 a turbomachine blade, in particular a free turbine blade, designated by the general reference numeral 10.

This blade 10 comprises a blade 11, a tree root 12 intended for fixing the blade on a rotor disc, by engagement of the root 12 in a housing also called a "cell" of corresponding shape made in the disc, a stilt 13 extending the tree stand 12 and a platform 14.

The tree root extends along a longitudinal axis, which in a manner known per se can form an angle with the axis of rotation AA' of the turbine disc, in order to increase the contact length between the tree root and the disc. The axis of the tree stand once dawn has risen on the disk extends in the direction of the corresponding cell in the disk. The cells of a free turbine disk can each be provided more or less obliquely in a plane tangential to the disk, with respect to the axial direction of the disk. In other words, an angle in a plane tangential to the disk is formed between the direction of a cell and the axis of the disk.

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

It comprises, on its front face, on the side of the leading edge of the blade, a concave shape 15 and also concave side flanks 16 in order to locally reduce the section of the stilt to delimit a frangible zone in the stilt .

The blade 10 also comprises oblong cavities 17, that is to say having a longitudinal dimension greater than their lateral dimension, which are formed in the side flanks of the stilt 13. Each cavity 17 extends along a longitudinal axis X-X'parallel or substantially parallel to the longitudinal axis of the tree stand. The axis XX' of each cavity can therefore, like the axis of the fir tree, form an angle with the axis of rotation AA' of the turbine disc, visible on the figure 9 .

Each cavity constitutes a pocket locally reducing the cross-section of the neck of the stilt in order to weaken the frangible zone of the stilt and to adjust the overspeed limit speed from which the blade detaches from the disc.

For example, as shown, each side flank of the stilt has at least one cavity. Each lateral flank here comprises a cavity, the stilt comprising a pair of cavities made symmetrically.

As shown on the figure 7 , each cavity has a concave cross-section, considered perpendicular to the longitudinal axis of the cavity, preferably a round cross-section, without edges. The radius R of the cavity is preferably between 10 and 25% of the length of the cavity, advantageously between 14% and 20% of the length of the cavity.

Its depth, which can for example correspond to the radius of the cavity, is advantageously between 9% and 35% of the minimum width Imin of the stilt, considered at the level of the deepest point of the cavity ( figure 8 ). The depth of the cavity is preferably between 10% and 25% of the width Imin of the stilt.

With reference to the figure 9 , which illustrates a vane mounted on a portion of a disc D, each cavity 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.

Considering the distance h between the axis of rotation A-A' of the disk of the turbine and the plane P, the axis X-X' of each cavity is included in a plane, subsequently designated plane of frangibility, which is coincident with the plane P, either is parallel to the plane P and is located slightly above or above the plane P. More precisely, the plane of frangibility is located at a distance from the axis of rotation A-A' of the disc comprised between h- 0.06h and h+0.06h, preferably between h-0.04h and h+0.04h. Furthermore, if the stilt comprises a pair of cavities made symmetrically, the frangibility plane comprises the two respective X-X' axes of the two cavities.

Moreover, as seen in the figure 10 , which illustrates a perspective and cross-sectional view at the level of the deepest points of the cavities, the distance d between the barycenter B of all the cavities and the radial projection of the center of gravity G of the blade on the plane cutting is between 0 and 20% of the axial length L of the stilt at the location of its minimum section, preferably between 0 and 15% of this length L. The axial length L is measured in a parallel direction to the axis of the foot of the fir tree, which can advantageously make an angle with the axis of rotation AA' of the disk of the turbine. This angle is for example between 5° and 20°.

The length of the cavities is for example approximately 40% of the total length of the tree stand at the location of the minimum section and their depth is approximately 20% of the width of the neck.

Each lateral flank of the stilt may comprise any number of cavities in order to locally reduce the section of the stilt and thereby adjust the speed limit of rotation of the blades.

As indicated above, the cavities have no sharp corners so as not to induce a concentration of stresses greater than those which are already induced by the concave shape made in the front face, on the side of the leading edge.

These cavities make it possible to adjust the breaking speed of the blade by increasing the average stress which is exerted in the neck of the stilt, without significantly increasing the maximum stress generated under the action of the thermomechanical forces harmful to the duration. of dawn life.

Indeed, as shown by figure 11 , which illustrates the field of radial stresses exerted in the blade under the action of the thermomechanical forces, the introduction of a cavity in the frangible zone of the stilt does not generate a significant increase in the maximum stress which remains localized in the zone Z' of the edge of the concavity of the leading edge of the blade. By way of example, the introduction of an oblong cavity in each of the two lateral sides of the stilt, in the case represented on this figure 11 , locally increased the maximum stress by only 1%.

Claims (9)

1. A turbine vane (10) for a turbomachine, including a blade (11) and a root (12), said root comprising a stilt (13) having lateral flanks with a curvilinear profile, said stilt comprising a frangible zone adapted to undergo breaking of the stilt if radial forces greater than a threshold are exerted on the vane, in particular centrifugal forces during an overspeed condition of the turbine, the turbine vane being characterised in that the frangible zone comprises at least one frangibility oblong cavity (17) provided on at least one of the lateral flanks of the stilt, said oblong cavity extending in an axial direction of the stilt along a longitudinal axis (X-X') parallel to or included in a plane of minimum cross-section area (P) in which a minimum transverse cross-section area of the stilt is located, the frangible zone of the stilt (13) being formed by a concave zone (15) of the stilt made on a front face and on at least one of the lateral flanks of the stilt, the deepest zone of the oblong cavity (17) being intersected by the plane of minimum cross-section area (P) of the stilt.
2. The vane according to claim 1, wherein the longitudinal axis (X-X') of the or each oblong cavity (17) is included in a frangibility plane located at a distance from an axis of rotation (A-A') of a turbomachine rotor disc (D) of between h+0.06h and h-0.06h, preferably of between h+0.04h and h-0.04h, h referring to the distance between the axis of rotation (A-A') and the plane of minimum cross-section area (P), the frangibility plane and the plane of minimum cross-section area (P) being parallel to each other and to the axis of rotation (A-A').
3. The vane according to one of claims 1 to 2, wherein the maximum depth (R) of the oblong cavity (17) is between 9 % and 35 % of the width (lmin) of the stilt, preferably between 10 % and 25 % of the width (lmin) of the stilt, considered at the deepest location in the cavity.
4. The vane according to any of claims 1 to 3, wherein the maximum depth (R) of the oblong cavity (17) is between 10 % and 25 % of its length, preferably between 14 % and 20 % of the length of the cavity.
5. The vane according to any of claims 1 to 4, wherein each lateral flank of the stilt comprises a frangibility oblong cavity (17) and wherein the distance between the barycentre (B) of the cavities and the projection of the centre of gravity G of the blade onto the plane of minimum cross-section area (P) is between 0 and 20 % of the axial length (L) of the stilt, preferably between 0 and 15 % of said length of the stilt.
6. The vane according to any of claims 1 to 5, wherein the oblong cavity (17) includes a curvilinear transverse cross-section.
7. The vane according to any of claims 1 to 6, wherein the oblong cavity (17) includes an arc-of-circle transverse cross-section.
8. A turbine of a turbomachine, comprising a rotor including at least one disc and a set of turbine vanes mounted to the disc, characterised in that each vane is a vane (10) according to any of claims 1 to 7.
9. The turbine according to claim 8, wherein the longitudinal axis (X-X') of the or each oblong cavity (17) of each vane is included in a frangibility plane located at a distance from an axis of rotation (A-A') of the disc (D) comprised between h+0.06h and h-0.06h, preferably comprised between h+0.04h and h-0.04h, h referring to the distance between the axis of rotation of the disc (A-A') and the plane of minimum cross-section area (P), the frangibility plane and the plane of minimum cross-section area (P) being parallel to each other and to the axis of rotation (A-A').

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