Abstract:
A turbine blade including an open cavity at its distal tip, the cavity being defined by a bottom wall and a side wall extending along the perimeter of the distal tip in an extension of the upper and lower walls of the blade, the side wall of the cavity including an opening in a vicinity of the leading edge of the blade opening into the cavity. A deflector extends at least in the middle portion of the cavity between the leading edge and the trailing edge.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns a turbine blade and, more particularly, can concern a hollow blade of a gas turbine rotor, of the high-pressure type, of a turbojet engine. 
     2. Description of the Related Art 
     As illustrated in  FIGS. 1 and 2 , it is known to provide, at the distal tip  3  of a hollow blade  2 , an open cavity  5 , or “bathtub”, defined by a bottom wall  7  that extends over the entire tip of the blade and by a side wall made up of two rims  9  and  10 . These rims extend between the leading edge  12  and the trailing edge  14  of the blade. The rim  9  extends the wall  8  on the lower surface of the blade and the other rim  10  extends the wall  11  on the upper surface of the blade. These rims are hereinafter called upper surface and lower surface rims. 
     The rims  9  and  10  make it possible to ensure a wear zone between the bottom wall  7  and the casing  16  that makes it possible to absorb the contact between the distal tip of the blade  3  and the casing  16 . Moreover, they make it possible to limit the passage of gas from the upper surface toward the lower surface generating aerodynamic losses that are detrimental to the yield. Due to the high temperatures of the gases passing through the turbine and the high rotational speeds of the blade, the walls of the blade and of the cavity  5  can locally reach critical temperatures. 
     Document EP 1 221 537 proposes cooling these walls by forming an opening on the side wall of the cavity in the vicinity of the leading edge of the blade, preferably on the lower surface side. Moreover, this opening may potentially have a small inner rim that channels the gases entering the cavity through said opening. However, the solution proposed by this document does not allow sufficient cooling of several zones of the cavity. 
     Indeed, the gradient of the temperatures T of the gases in the vicinity of the blade depending on the height H of the vein of the gases ( FIG. 3 ) is particularly damaging for several zones of the cavity. Thus, this gradient of the temperatures combined with the topology of the gas flows along the blade makes the hottest gases impact the critical zone C of the blade. This zone C is situated at the tip  3  of the blade in the vicinity of the trailing edge  14 , on the lower surface side  8 . 
     BRIEF SUMMARY OF THE INVENTION 
     The aim of the present invention is to improve the cooling of said zone and of the entire open cavity of the distal tip of a turbine blade. More particularly, the present invention proposes to improve the local cooling of the zone situated at the apex of the blade, in the vicinity of the trailing edge, on the lower surface side. 
     To achieve this aim, the object of the invention is a turbine blade having a cavity open at its distal tip, said cavity being defined by a bottom wall and a side wall extending along the perimeter of said distal tip in the extension of the lower surface and upper surface walls of the blade, the side wall of the cavity having an opening in the vicinity of the leading edge of the blade opening into the cavity, and a deflector extending at least in the median portion of the cavity between the leading edge and the trailing edge. 
     The advantage of such a blade is that the deflector makes it possible to direct the relatively cooler gas flows (see  FIG. 3 ), having entered the cavity through the opening of the side wall, directly toward a preferential zone of the walls of the cavity. This gas current impacts this preferential zone and allows cooling by increased convection of the latter. The aim is to direct an optimized quantity of gas on a particularly stressed zone from a thermal perspective. Thus, in addition to correctly cooling the latter zone, it is possible to obtain homogenous cooling of all of the walls of the cavity. 
     The invention also concerns a turbine comprising at least one blade according to the invention and a turbomachine, such as an airplane turbojet engine, comprising at least one such turbine. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention and its advantages will be better understood upon reading the following detailed description of different embodiments shown as non-limiting examples. This description refers to the appended figures, in which: 
         FIG. 1  is a perspective view of the distal tip of a known blade, 
         FIG. 2  is a view of the blade of  FIG. 1  along cutting plane II, 
         FIG. 3  is a graphic representation of the evolution of the temperature gradient T of the gases over the height H of the vein of the gases in the vicinity of a blade, 
         FIG. 4  is a perspective view of a first embodiment of a blade according to the invention, 
         FIG. 5  is a perspective view of a second embodiment of a blade according to the invention, 
         FIG. 6  is a perspective view of a third embodiment of a blade according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment of the invention is described in reference to  FIG. 4 . In this embodiment, the opening  20  of the side wall  9 ,  10  of the cavity  5  is situated in the vicinity of the leading edge  12  of the blade  2 , on the lower surface side  8 . Without being restrictive, this opening position  20  makes it possible to withdraw the cooling gases (i.e. essentially less hot) in the outer portion of the vein of the gases particularly effectively. Of course, depending on the geometry of the blade and the angle of incidence of the gases, the opening  20  may be placed further downstream on the lower rim  9  or on the other side on the upper rim  10 . 
     In this example, the opening  20  of the side wall  9 ,  10  of the cavity  5  is a recess. The height of this recess can vary according to different embodiments. Here, the recess extends over the entire height of the side wall  9 ,  10  of the cavity  5 . This recess height has the advantage, for a given width, of providing the cavity with a maximum gas flow. 
     In this first embodiment, the deflector  21  has several interesting features. The deflector  21  is essentially perpendicular to the bottom wall  7  of the cavity  5 . This positioning of the deflector  21  relative to the bottom wall  7  of the cavity  5  improves the orientation of the gas toward the zone C to be cooled. 
     The active face  24  of the deflector  21  is concave, the concavity of the deflector  21  having essentially the same orientation as the concavity of the blade  2 . In this way, the deflector  21  has a dual advantage. First, for a given length, a concave deflector fits optimally into the cavity  5 . Then, this allows the deflector  21  to impose a stream line on the gas flow that best fits the shape of the lower rim  9  of the cavity  5 . Thus, while orienting the cooling gas flow toward the zone C, other zones of the cavity  5  are also cooled by this cooling gas current, which circulates along the lower rim  9 . Of course the concavity of the deflector  21  is not limited to the illustrated example and can advantageously be adjusted to each type of blade. 
     The upstream tip  18  of the deflector  21  is distant from the leading edge  12  of the blade  2 . Likewise, the lower tip  19  of the deflector  21  is distant from the trailing edge  14  of the blade  2 . These distances relative to the leading and trailing edges make it possible to adjust the direction of the flows and the quantity of cooling gas guided. The relative distances of the deflector relative to the upper  9  and lower  10  rims are also important parameters that make it possible to optimize such a system. 
     Still in reference to  FIG. 4 , the upstream portion  22  of the deflector  21  is oriented toward the leading edge  12  of the blade  2 . Likewise, the downstream portion  23  of the deflector  21  is oriented toward the trailing edge  14  of the blade  2 . These orientations of the upstream  22  and downstream  23  portions of the deflector  21  are advantageous for guiding the cooling gas flow toward the zone C. Moreover, these orientations make it possible to impose stream lines on the gas flow that are particularly effective for the homogeneity of the cooling of all of the walls of the cavity  5 . 
     A second embodiment of the invention is described in reference to  FIG. 5 . In this example, the opening  20  of the side wall  9 ,  10  of the cavity  5  is a hole. The cylindrical shape of the hole illustrated in this example is not limiting. The hole can have an oblong or triangular shape, among others. As side wall opening of the cavity  5 , the hole has the advantage of being able to precisely adjust the flow of cooling gas entering the cavity  5 . 
     This second example illustrates a hollow blade  2  supplied with cooling gas, and the deflector  21  includes cooling holes  26  communicating with at least one hollow portion of the blade. These cooling holes are radial bores in the deflector  21 . They open into an inner cavity of the blade  2  situated under the deflector  21 . The cooling of the deflector  21  is thus ensured by pumping and heat conduction. Moreover, the cooling of the deflector  21  makes it possible to lower, by convection, the temperature of the gases deviated by the latter, consequently increasing the heat efficiency of the system. 
     A third embodiment of the invention is described in reference to  FIG. 6 . This third example shows a hollow blade  2  supplied with cooling gas, and the deflector  21  includes dust extraction holes  25  communicating with at least one hollow portion of the blade. These dust extraction holes are similar to the cooling holes and ensure, like the latter, the cooling of the cavity  5 . However, they have a larger diameter than that of the traditional cooling holes. The larger diameter of the dust extraction holes allows the evacuation of dust that may be present in the inner cavity(ies) of the blade. Thus, on a deflector having both types of holes, the dust will preferably pass through the dust extraction holes rather than through the narrower cooling holes. One therefore avoids covering the cooling holes of small diameter. 
     Because of the significant diameter of these dust extraction holes, it is necessary for the deflector  21  to have a greater thickness than in the preceding examples.