Abstract:
A cooled turbine vane for a turbine engine, that includes a blade mounted on a platform carried by a base, the blade including one or more cavities formed therein for cooling air circulation, the cavity extending along the trailing edge and being supplied with cooling air by a supply duct connecting an air intake located in a lower portion of the base and the cavity of the trailing edge by defining a bend within the base. The duct includes, on an axis substantially radial relative to the air intake a bell-shaped niche located under the platform, the niche being open at a top thereof via a dusting hole extending through the platform and being defined at a foot of the base by walls extending substantially radially from the platform to close the platform laterally.

Description:
The field of the present invention is that of turbomachines and, in particular, that of cooled turbine vanes for these turbomachines. 
     BACKGROUND OF THE INVENTION 
     Modern turbomachines are conventionally produced in the form of an assembled collection of modules. They first of all comprise one or more compressor modules arranged in series and which compress air drawn into an air inlet. The air is then let into a combustion chamber where it is mixed with a fuel and burnt. The combustion gases pass through one or more turbine modules which drive the compressor or compressors. The gases are finally discharged either into a jet pipe nozzle to produce a propulsive force or onto a free turbine to produce power which is recovered on a transmission shaft. 
     These modern turbomachines, because of the high temperatures of the gases leaving the combustion chamber, are cooled, which means to say that a fresh-air ventilation circuit is installed in order to cool the metal of the turbine vanes when the turbomachine is operating. The ventilation air is bled off the outlet side of the compressor stages and conveyed to the turbine. The air generally enters via the root of the vanes, passes through these vanes via cavities cut into the thickness of the blade and is then discharged through perforations made in the wall or tip of the vanes. The cavities are formed longitudinally inside the vanes, extending between the root and the tip of the vane, so as to carry this air and cool the metal. In general, several cavities are positioned one beside the other along the chord of the vane, so as to equalize the flow rates and cool the metal uniformly. 
     Various types of dust (particularly fine sand) may be drawn into the air intake at the same time as the air needed for the operation of the engine. While most of the dust passes through the engine without causing any damage, some of it reaches the turbine vane cooling circuits; if it builds up inside these circuits there is then the risk of it blocking the cavity outlet discharge orifices and thus endangering the integrity of the vane. To prevent this from occurring, turbine vanes are equipped with calibrated dusting holes in addition to the cooling drillings and vents, these dusting holes being situated at the tops of the cavities and having the role of removing the dust. 
     Dusting holes are installed for each of the cavities. The sizing of the hole is dictated by criteria concerned with the size of the dust and is therefore independent of the overall size of the vanes. Installing a dusting hole in the case of a small-sized vane proves to be particularly tricky and may become a decisive factor in the sizing of the circuit because it is likely to govern the thermal performance of the cooling circuit. 
     The problem that has to be addressed is a particularly acute one in the case of a high pressure (HP) turbine vane the size of which is very small and in which the cavity that cools the trailing edge is of very slender thickness. Installing the trailing edge cavity dusting hole on a short-chord vane may lead to an abnormal increase in the size of this one cavity at the expense of the others. The trailing edge cavity may in such instances represent 25% of the total chord length whereas it usually represents only 15 to 20% on larger vanes. That means that small-sized vanes that have this type of dusting hole will require more cold air to cool them via the trailing edge cavity, at the expense of the cooling using the other cavities. It is therefore important to be able to reduce the cross section of the trailing edge cavity so as to reduce its cooling flow rate and thus be able to maintain a satisfactory overall thermal performance. The problem that has to be addressed in order to achieve this objective is that of finding a way of removing the dust from the trailing edge cavity without installing the dusting hole in the tip of the vane. 
     It is an object of the present invention to solve these problems by proposing a turbine vane that does not have some of the disadvantages of the prior art and that, in particular, has no trailing edge cavity dusting hole situated in the vane tip. Another object of the invention is to optimize the use of the dusting air, by making it contribute to the cooling of the vane. 
     BRIEF SUMMARY OF THE INVENTION 
     To this end, one subject of the invention is a cooled turbine vane for a turbomachine comprising a blade mounted on a platform supported by a root, said blade having one or more cavities cut into it for the circulation of cooling air, the cavity extending along the trailing edge being supplied with cooling air by a supply duct connecting an air inlet situated in the lower part of the root to the trailing edge cavity, making a bend within said root, characterized in that the duct comprises, along an axis that is substantially radial with respect to the air inlet, a niche situated under the platform and in the shape of a bell, said niche opening at its top via a dusting hole that passes through said platform and being delimited inside the root by walls extending substantially radially from the platform in order to close it laterally. 
     The bell shape and the fact that the niche is positioned facing the air inlet allows this niche to act as a particle trap trapping particles that might arrive with the cooling air and that would enter the vane trailing edge cooling circuit. These particles are then removed via the perforation, which is sized according to the predicted size of these particles. 
     In one particular embodiment, the turbine vane further comprises at least one ventilation perforation for ventilating the platform, the perforation being supplied with cooling air from the supply duct and the dusting hole coincides with said ventilation perforation of the platform. 
     This then reduces the number of operations involved in producing the vane. 
     For preference, at least one of the walls of said niche extending radially from the platform is situated in the continuation of one of the walls of the duct. 
     More preferably still, said niche is formed by the walls of the duct and by a ridge positioned across the direction in which the cooling air flows in said duct. This shape is particularly well suited to producing the vane by casting. 
     In an alternative form of the invention, the dusting hole is extended by a recess made in the external face of the platform and having a substantially triangular shape when viewed from above. This arrangement means that the air passing through the dusting hole can be made to contribute to the cooling of the connection between the platform and the blade. 
     For preference, the recess extends from a top positioned on the dusting hole and widens toward the connection between the platform and the blade. 
     In one particular embodiment of this alternative form, the side of the triangle opposite the dusting hole has a length varying between 1.5 and 3 times the diameter of the dusting hole. 
     Advantageously, the two sides extending from the dusting hole have a length of between 6 and 15 times the diameter of said hole. 
     For preference, the median axis of the recess runs in a direction substantially parallel to the edge of the platform which is situated on the trailing edge side of the vane. More preferably still, the recess diverges from the direction of said edge of the platform by 5° at most. 
     In one particular embodiment, the recess in its median part is substantially in the form of an inclined plane which is embedded in the thickness H of the platform in the direction of the dusting hole over a depth h of between 50 and 80% of the value of H. 
     For preference, said inclined plane is inclined by an angle of between 3 and 10° with respect to the plane of the platform. 
     The invention also relates to a turbine module of a turbomachine comprising at least one vane as defined hereinabove or to a turbomachine comprising such a turbine module. 
     The invention will be better understood, and further objects, details, features and advantages thereof will become more clearly apparent during the course of the detailed explanatory description that follows, of one embodiment of the invention given by way of purely illustrative and nonlimiting example and with reference to the attached schematic drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a perspective view of an HP turbine vane; 
         FIG. 2  is a perspective view of an HP turbine vane, showing a trailing edge cavity dusting hole positioned as per the prior art; 
         FIG. 3  is a perspective view of an HP turbine vane according to the prior art, with a cross section taken through the trailing edge cavity; 
         FIG. 4  is a perspective view of an HP turbine vane showing a dusting hole according to one embodiment of the invention, with a cross section taken through the trailing edge cavity; 
         FIG. 5  is a view in a cross section of the trailing edge cavity of an HP turbine vane showing a dusting hole according to the same embodiment of the invention. 
         FIG. 6  is a view from above of a platform of a turbine vane showing an alternative form of dusting hole according to the invention; 
         FIG. 7  is a view in cross section of a platform of a turbine vane showing the shape of the outlet orifice of the same alternative form of a dusting hole according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is made to  FIG. 1  which shows an HP turbine vane  1  comprising a blade  2  extending between a leading edge  3  and a trailing edge  4 , mounted on a platform  6  supported by a vane root  5 ; the platform  6  is connected to the blade  2  by a blend radius  17  of rounded shape. The vane  1  at its trailing edge  4  has vents  7  intended to discharge the cooling air circulating within the trailing edge cavity.  FIG. 2  shows the position, in the prior art, of a perforation  8  acting as a dusting hole for the trailing edge cavity; the hole  8  is positioned on the tip of the vane at this cavity. The vane  1  also comprises perforations  9  on its platform  6 , on the suction face side of the blade  2 , through which perforations air from the root of the blade  5  passes and which perforations have the function of cooling the vane  1  at its connection  17  between the blade  2  and the platform  6 . The perforations  9  depicted in  FIG. 2  are positioned at the trailing edge  4  and fed with air bled off the trailing edge cavity fresh air supply duct. 
       FIGS. 3 and 4  each show a cross section of a turbine vane  1  in the region of the duct  10  that supplies cooling air to the trailing edge cavity  11  thereof. In  FIG. 3 , which represents the prior art, the duct  10  runs from the air inlet  12  situated in the root  5  of the vane as far as its entry into the cavity  11  maintaining a bore section that is substantially constant or alternatively that decreases very gradually in order to take account of the cooling flows tapped from it. It conventionally has a shape with a bend  13 , because it connects the air inlet  12  which is situated in the plane of symmetry of the root  5  to the trailing edge cavity  11  which is for its part offset from this plane because of the curved shape of the vane  1 . 
     In  FIG. 4 , according to the invention, the duct  10  widens from the suction face side of the vane  1  to form a niche  14  positioned under the platform  6  and supplied by the duct  10  and which opens into the stream via a perforation  9  that acts as a dusting hole. This niche  14  is situated substantially along the axis of the air inlet  12  and is in the form of a bell-shaped cavity the roof of which is formed by the lower wall of the platform  6 , which is centered on the dusting hole  19  and which has lateral walls dropping down on each side of the dusting hole  19  to close it laterally. The diameter of the dusting hole  19  is sized to allow through any dust that is likely to reach that far. 
     The niche  14  is essentially created by a diversion of the walls of the cooling duct  10  which diverge more than in the prior art away from the suction face side of the blade  2  to form a space under the platform  6 , and by a ridge  16 , positioned across the direction in which the air circulates, in order to close off this space laterally. Such a configuration is in fact relatively easy to cast during the manufacture of the vane. 
       FIG. 5  is a cross section of the circulation of air through the duct  10  and the path followed by the particles of dust  15  that enter the trailing edge  4  cooling circuit of the vane  1 . 
     It can be seen from  FIGS. 3 and 4  that because the niche  14  is created under the platform  6  and has to be aligned with the air inlet orifice  12 , the dusting hole  19  ( FIG. 4 ) is located a greater distance away from the blend radius  17  than were the cooling perforations  9  in the prior art ( FIG. 3 ). The latter opened at the point at which this radius  17  met the platform  6 , whereas the dusting hole  19  is set away from this point somewhat. If the dusting hole  19  were to take the place of the perforations  9 , this could then result in poorer cooling of this blend region. 
     In order to remedy this and maintain just one perforation for cooling the blend radius  17 ,  FIG. 6  shows an alternative for the bottom part of a vane  1 , with the platform  6  into which the dusting hole  19  opens. This hole is extended by a recess  20 , made in the thickness of the platform, on the outside thereof (which means on the side furthest from the axis of rotation of the engine) and which, when viewed from above, is substantially triangular in shape. The corresponding triangle extends from a vertex positioned on the dusting hole  19  and widens towards the connection  17  between the platform  6  and the blade  2 . In a preferred embodiment, the side opposite the dusting hole  19  has a length varying between 1.5 and three times the diameter of the dusting hole  19 , while the other two sides are between 6 and 15 times this same diameter in length. 
       FIG. 7  shows a view of the recess  20  in cross section on a direction A-A, as illustrated in  FIG. 6 , substantially parallel to that edge of the platform that is situated on the trailing edge side of the vane. The dusting hole  19 , as previously indicated, passes through the entire thickness H of the platform  6  and in its median part the recess  20  has the form of an inclined plane which is embedded into this thickness H. It meets the dusting hole  19  at a depth h of between 50 and 80% of the value of H and is inclined with respect to the plane of the platform by an angle that can vary from 3 to 10°. In the direction transverse to the cross section A-A, the recess  20  has the shape of a U or V which connects it to the plane of the platform  6  along the two long sides of the triangle. 
     The median axis of the triangle constituting the recess  20  is situated in the direction of the flow lines of the stream of gas and for that reason can diverge from the direction of the edge of the platform situated on the trailing edge side of the vane by plus or minus 5°. 
     The cooling of the trailing edge  4  of a vane  1  according to the invention will now be described. 
     The cooling air reaches the root  5  of the vane  1  and enters the cavity  11  supply circuit via the inlet duct  12  in order to cool the trailing edge  4 . Particles of dust  15  are mixed with this air and these need to be eliminated to prevent them from blocking the cooling orifices and disrupting the cooling of certain parts of the vane. 
     The air enters the inside of the vane  1  under pressure and carries the particles  15  along with it. These particles, because of the rotation of the vanes and of the centrifugal force thus generated, shift radially in the cooling duct  10  and migrate toward the niche  14 , positioned for this purpose facing the air inlet  12 . Unlike in the prior art, they do not follow the bend  13  of the duct  10  and remain temporarily below the platform  6 , trapped by the lateral walls of the niche  14 . They accumulate under the roof of this niche and thus find themselves positioned facing the perforation  19 , which acts as a dusting hole. The particles  15  are then carried along in the flow of air passing through the dusting hole  19  and are ejected into the engine gas stream. Because dusting is performed at the perforation  19 , there is no longer any need to drill a hole in the tip of the cavity  11  of the vane  1  in order to perform this function; this then avoids a drilling operation which, as already indicated, was very tricky to perform in the prior art. In the alternative form depicted in  FIGS. 6 and 7 , the cooling air that passes through the dusting hole  19  is diverted as it leaves, in the region of the platform  6 , by the recess  20  and takes the direction substantially parallel to this platform. What is more, it is held tightly against the surface of the platform by the gases flowing in the engine stream which carry it toward the blend radius  17 . The air passing through the dusting hole  19  and which, as a result, is diverted from the vane cooling circuit in order to perform this dusting function, thus still gets used to cool this vane, but this time, at the root thereof. 
     Because the diameter of the dusting hole  19  is greater than that of the perforations  9  of the prior art because it is sized to allow the particles of dust to pass through it, the air flow rate that passes through it is greater than the cooling flow rate which was passing through the perforations  9 . As a result, firstly of the fact that this air flow rate is greater and, secondly, of the triangular shape of the recess  20 , the air leaving the dusting hole  19  sweeps over the blend radius  17  over a certain length. Its thermal efficiency is therefore similar to that of the three perforations  9  in the platform that are found in the prior art and it is possible to dispense with these holes. The improvement achieved in this alternative is therefore twofold, in that the operation of drilling the three holes is avoided and the flow rate of cooling air passing through the trailing edge cavity can be reduced overall. This is because the dusting air in fact performs the functions previously performed by the dusting hole  8  and the functions performed by the cooling perforations  9  for cooling the blade-platform connection  17 . 
     Although the invention has been described in conjunction with two particular embodiments, it is quite obvious that it is not in any way restricted thereto and that it comprises all technical equivalents of the means described and combination thereof where these fall within the scope of the invention.