Abstract:
One or more protuberances are provided on a turbine blade on its suction side in a vicinity of its trailing edge. The protuberances serve to reduce a separation zone in a vicinity of a blade surface that is responsible for disturbances that affect efficiency of the turbine.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a turbine blade, in particular a blade for a moving wheel of a low pressure turbine for an airplane turbojet, with the aerodynamic behavior of the blade being improved so as to avoid the boundary layer of the air stream separating from the surface of the blade, mainly at its rear in the region of its suction side. 
     2. Description of the Related Art 
     In the design of novel turbine blade shapes, in particular for the moving blades fitted to a wheel of a given turbine stage, it is desirable to increase performance by modifying certain structural parameters. In particular, in order to reduce the weight of the turbine, one possible solution consists in reducing the number of blades, thereby requiring the blade profiles to be re-drawn so as to comply with the outlet angles and so as to compensate as much as possible for the loss of efficiency. 
     By proceeding in this way, it is possible to find that there is a harmful risk of the air flow “separating” from the suction side. Such turbulent disturbances begin in the vicinity of certain zones of the suction side of the blade and they are very damaging to performance. The invention serves to attenuate those separation phenomena. 
     BRIEF SUMMARY OF THE INVENTION 
     More particularly, the invention provides a turbine blade, characterized in that it includes at least one protuberance in the vicinity of its trailing edge on its suction side. 
     In order to determine the location of such a protuberance along the trailing edge, the starting point is a representation of the separation zone on the suction side (which representation may be obtained by calculation), and then it is decided to place such a protuberance in the vicinity of the maximum disturbance zone as determined without using such a protuberance. 
     By proceeding in this way, it generally happens that at least one above-mentioned protuberance is placed substantially halfway up the trailing edge. Other protuberances may advantageously be placed in the vicinity of the inner radial end of said trailing edge and/or in the vicinity of the radial outer end of said trailing edge. 
     As a general rule, the calculations lead to placing a plurality of the above-mentioned protuberances so that they are distributed along the outer radial third of said trailing edge. 
     The shape of such a protuberance is preferably generally that of a rounded stud projecting from the suction side surface and connecting smoothly therewith. 
     Preferably, a middle section of the protuberance, taken perpendicularly to the trailing edge, is in the form of a half-wave that connects smoothly with the suction side surface. 
     In an embodiment, another section of said protuberance, taken perpendicularly to said middle section, has the shape of an undulation presenting a central extremum with damped side waves. In other words, as seen in this section, said protuberance is similar in shape to the wave that is caused in the plane surface of a liquid by a falling drop of liquid, this wave shape nevertheless not being circumferentially isomorphic around the central point. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention can be better understood and other advantages thereof appear more clearly in the light of the following description given purely by way of example and made with reference to the accompanying drawings, in which: 
         FIG. 1  is a fragmentary perspective view of a turbine rotor wheel provided with blades in accordance with the invention; 
         FIG. 2  is a fragmentary section of a blade, showing the profile of a protuberance in accordance with the invention; 
         FIG. 3  is a fragmentary section in another plane showing the profile of said protuberance; 
         FIG. 3A  is a view analogous to  FIG. 3  showing a variant; and 
         FIG. 4  is a diagrammatic view showing a step in the method of determining the number and the locations of the protuberances along the trailing edge. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a plurality of rotor blades  11 , specifically the moving blades extending generally radially from the periphery of a rotor disk  13 . Conventionally, each blade, presenting a certain amount of thickness that varies from front to back, is curved between a leading edge  15  and a trailing edge  17 . The concave portion or pressure side  19  is smooth. The convex portion or suction side  21  is smooth over the major fraction of its surface with the exception of a few protuberances  25  in accordance with the invention that are distributed in the vicinity of and along the trailing edge  17 . In general, such a protuberance  25  is preferably generally in the shape of a rounded stud projecting from the suction side surface  21  and smoothly connecting therewith. 
     Preferably, the profile of the section of the protuberance, perpendicularly to the suction side surface where it is located, varies between the shapes shown in  FIGS. 2 and 3 . 
     Thus, as shown in  FIG. 2 , a middle section through said protuberance  25  taken perpendicularly to the trailing edge  17  presents the shape of a simple half-wave that connects smoothly with the surface of the suction side  21 . It should be observed that the slope of this half-wave is shallower towards the front and steeper towards the rear. In this section, the protuberance connects to the trailing edge with continuity. 
     In contrast, if consideration is given to another section of the same protuberance that is perpendicular to the preceding section, i.e. parallel to the trailing edge, as shown in  FIG. 3 , then it can be seen that the protuberance presents a shape that is more complex, namely one or more undulations presenting a central extremum  27  and damped side waves  28 . In the variant of  FIG. 3A , it can be seen that the section of the protuberance  25  comprises a plurality of damped undulations on either side of the extremum. As mentioned above, this section is comparable to the wave that is generated on a calm liquid surface by a falling drop of liquid. On “turning” about the protuberance, the section varies in continuous manner from one of those sections to the other over one-fourth of a turn. 
     With the preferred shape for the protuberance being as defined above, there follows a description of the locations for such protuberances and the way in which these locations are determined. 
       FIG. 4  is a diagram showing from left to right the various stages in positioning the protuberances on a blade  11  shown looking at the surface of its suction side  21 . The grayed portion represents the “separation” zone  30  on the suction side in the vicinity of the trailing edge. 
     It can be seen that without any protuberance, this separation zone  30  extends over practically the entire height of the blade from the trailing edge, with a maximum width substantially halfway up. An analysis of this shape leads to placing a first protuberance  25   a  in the vicinity of the zone of maximum disturbance, i.e. halfway up the blade, close to the trailing edge. The result of this first simulation (not shown) reveals a reduction in the area of the disturbance at half-height, but also reveals persistent disturbances at the inner and outer radial ends. This leads to placing another protuberance  25   b  in the vicinity of the inner radial end of the trailing edge and/or in the vicinity of the outer radial end  25   c  of the trailing edge. Thus, by way of example, when three protuberances are put into place, as shown, this also corresponding to the embodiment that can be seen in  FIG. 1 , then the width of the zone of disturbances or separation zone is reduced over practically the entire radial height of the blade, while nevertheless maintaining a persistent pronounced disturbance zone between the central protuberance and the outer protuberance. 
     It is found that in application of the method developed in the context of the invention, placing a fourth protuberance  25   d  along the outer radial third of the trailing edge between the central protuberance  25   a  and the outer protuberance  25   c  serves to reduce this last separation zone.