Patent Publication Number: US-2022235665-A1

Title: Turbine vane provided with a recess for embrittlement of a frangible section

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to turbine vanes of a turbine engine and, in particular, a turbine vane stilt arrangement. The stilt of a vane is a part supporting the vane blade which extends radially between a lower attachment part known as a “fir-tree root” and a vane platform. 
     More particularly, the invention relates to turbine vanes of a free-turbine turbine engine. 
     The invention also relates to a turbine engine comprising such vanes. 
     PRIOR ART 
     Conventionally, as illustrated in  FIG. 1 , a free-turbine turbine engine includes a gas generator  1 , comprising at least one compressor which includes one or more compression stages  2 , a combustion chamber  3 , and a turbine wherein the pressurised hot gases from the combustion chamber expand and wherein the kinetic and thermal energy of the gases is converted into mechanical energy to rotate a shaft which connects the turbine to the compressor, so as to also actuate the compressor. A turbine  4 , referred to as free turbine, which includes one or more turbine stages, is disposed downstream from the turbine of the gas generator  1 , and mechanically uncoupled therefrom. The free turbine  4  is rotated by the gases from the gas generator  1 . 
     In free-turbine turbine engines, used for example, but not exclusively, in helicopter propulsion units, the free turbine is mechanically independent from the helicopter rotor, a reduction gear being interposed between the shafting and the main rotor. 
     In the event of breakage of the power transmission line, for example in the event of breakage of the shafting or the transmission line connected to the reduction gear, the turbine can find itself in an overspeed scenario due to the disappearance of the resistive torque applied on the turbine vanes. 
     This overspeed scenario can be particularly hazardous, result in the breakage of at least one rotary disk supporting the vanes of a stage of the turbine, under the effect of centrifugal force, and trigger the release of very high-energy debris which cannot be contained by the armour provided on the engine. 
     It is therefore necessary to provide protection systems which prevent overspeed in the turbines. 
     In the prior art, overspeed protection systems have already been proposed, known as “blade-shedding”, which involves creating a frangible zone in the vanes such that they break at a predetermined rotational speed preventing any risk of disk breakage which would be caused by the centrifugal forces. In this regard, reference can be made to GB 881,850 which describes a turbine for driving accessories wherein holes are drilled at the base of the vane blades. 
     Thus, in the event of overspeed risk, after the vanes break, the turbine, having lost the aerodynamic profiles thereof, slows down naturally, and can stop rotating. The deceleration of the free turbine to return to acceptable speeds prevents in this way the risk of breakage of the disk caused by centrifugal forces. 
     In this regard, it has been proposed to machine the leading edge of the turbine vane root in order to adjust the cross-section of the neck of the vane so that it breaks at a required speed, while retaining a sufficient contact length between the fir-tree root of the vane and the corresponding receptacle of the disk which receives the fir-tree root to ensure the mechanical strength of the attachment of the vane on the disk. 
     In  FIGS. 2 and 3 , a vane provided with a frangible section intended to break to prevent overspeed is represented. 
     As can be seen, the stilt  4  of the vane, which extends between the fir-tree root  6  and a platform which forms the base of an aerodynamic profile  5  or blade of the vane, includes a concave leading edge  7  for forming in the stilt a frangible zone of minimal cross-section suitable for enabling the vane to be detached from the disk from a protection threshold speed. 
     In  FIG. 4 , the radial stress which is applied in the vane stilt under the effect of thermomechanical forces is represented. As can be seen, creating a concave leading edge  7  locally reducing the cross-section of the stilt induces the appearance of a maximum stress zone Z on the leading edge, in the corners of the reduced cross-section. This increase in the maximum stress is accompanied by the appearance of an additional moment due to the misalignment of the aerodynamic profile with respect to the neck of the stilt. 
     It has been observed that this maximum stress determines the fatigue lifetime of the vane such that producing this concave zone in the leading edge to embrittle the vane locally requires relatively complex design work in order to define the threshold value from which the stilts break while limiting the increase in maximum stress harmful for the fatigue lifetime of the vane. 
     Moreover, the use of materials having an increased strength for producing the vanes induces, for the same frangible section, an increase in the threshold speed from which the stilts break. 
     Producing a concave zone of increased size in the leading edge of the vane so as to reduce the cross-section of the vane stilt locally would not make it possible to reduce the breakage limit speed of the stilt without increasing the maximum stress on the minimum cross-section on the stilt in a redhibitory way and hence reducing the fatigue lifetime of the vane. It is therefore desirable to be able to reduce the breakage speed of the stilt when using a high-strength material to produce the vane. 
     Disclosure of the Invention 
     In the light of the above, the invention aims to propose a turbine vane provided with a frangible section for setting the vane breakage speed value without increasing the maximum stress in the vane. 
     Therefore, the invention relates to a turbine vane comprising a blade and a root, the 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, in particular centrifugal forces during an overspeed state of the turbine. 
     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. 
     This recess thus helps embrittle the frangible section of the stilt by increasing the mean stress exerted in the neck of the stilt, without significantly increasing the maximum stress induced locally under the action of thermomechanical forces. It hence helps optimise the setting of the limit speed from which the vanes break. 
     Advantageously, the vane is mounted on a disk, the longitudinal axis of the or each oblong recess being 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, preferably between h+0.04 h and h−0.04 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. 
     According to another feature, the frangible zone of the stilt is 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. 
     For example, the maximum depth of the oblong recess 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 deepest point of the recess. 
     In one embodiment, the maximum depth of the oblong recess is between 10% and 25% of the length thereof, preferably between 14% and 20% of the length of the recess. 
     In one embodiment, wherein each lateral flank of the stilt comprises an oblong frangibility recess, 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 of the stilt, preferably between 0 and 15% of said width of the stilt. 
     Advantageously, the oblong recess has a curvilinear cross-section. 
     Preferably, the oblong recess has a cross-section in the arc of a circle. 
     The invention also relates to a turbine engine turbine, comprising a rotor including at least one disk and a set of turbine vanes mounted on the disk, each vane being a vane as defined above. 
     Advantageously, 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, preferably between h+0.04 h and h−0.04 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. 
     Further aims, features and advantages of the invention will emerge on reading the following description, given merely by way of example, with reference to the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , which has been mentioned above, illustrates the general structure of a free-turbine gas turbine according to the prior art; 
       
         FIG. 2 
       
         FIG. 3 , mentioned above, are respectively front and perspective views of a vane according to the prior art; 
         FIG. 4  which has been mentioned above, shows the stress field exerted on the stilt of the vane in  FIGS. 2 and 3 ; 
       
         FIG. 5 
       
         FIG. 6  are respectively front and perspective views of a vane according to the invention; 
         FIG. 7  is a detailed view on a larger scale of the vane of  FIG. 6 ; 
         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; 
         FIG. 9  shows the vane of  FIGS. 5 and 6  mounted on a rotor disk; 
         FIG. 10  is a perspective view of  FIG. 8 ; and 
         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 
     In  FIGS. 5 and 6 , a turbine engine vane, in particular a free turbine vane, represented by the general reference number  10 , is shown. 
     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 . 
     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. 
     As can be seen, the stilt  13  has a curvilinear shape. 
     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. 
     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 . 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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°. 
     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. 
     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. 
     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. 
     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. 
     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%.