Abstract:
A turbine vane with an enhanced profile including a trailing edge is curved inwards towards the suction face in the vicinity of the head, as if it was twisted, which opens the vane and limits leakage currents above the head. The flow is thus improved since the eddies formed by the irregular flows in front of the vane are reduced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a turbine vane with an enhanced profile. 
     2. Discussion of the Background 
     A conventional turbine vane comprises, as shown in FIG. 1, a mobile blade  1  in an annular section  2 , a platform  3  confining the section  2  and a projection  4  inserted in a groove of a hub  5  belonging to the rotor. The blade  1 , which is the essential part of the vane, may be broken down into stacked sections  6 , located at increasing radial distances from the rotation axis of the machine and running, curving inwards, from a leading edge  7  to a trailing edge  8  of the flow of gases circulating in the section  2 . The surfaces of the blade  1  consist of a blade face  9  and a suction face  10  each running between the leading and trailing edges  7  and  8 , as illustrated in FIG.  2 . The gases from the machine blow onto the blade face  9  and induce a rotational movement of the hub  5  and the vanes attached to it, to the right in the figures. 
     The profile of the blades  1  generally varies as illustrated in FIG. 3, the sections  6  being longer and curving less towards the platform  3 , which is supposed to give a good flow and a satisfactory mechanical resistance. 
     However, the reason for the invention is the need to reduce the leakage current between the head  11  of blade  1  and the housing  12  around the section  2 , between which a clearance must remain. These leakage currents run from the blade face  9  to the suction face  10  along the arrows  13  in FIG. 2; they produce an eddy downstream from the vane, which interferes with the gas flow. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a selected solution which consists of deforming the sections  6  closer to the head  11  of the vane to reduce the leakage currents at this point. Some patents mention other deformations which may be applied to this part of the vane in relation to the typical profile in FIG.  3 . In this manner, the international application WO 96 14469 describes a vane in which the blade is curved inwards towards the suction face in the vicinity of the head, resulting in an overhanging suction face. The purpose of this arrangement is to drive back the gases blown onto the suction face radially, to the rotation axis, to prevent them from reaching the blade head and moving to other side of the vane, to the blade face, by passing through the clearance adjacent to the housing  12 . The pressure on the suction face increases close to the head  11 , which reduces the difference in pressure between the two sides of the head  11  and the leakages over the head from the blade face to the suction face. It is also possible to reduce said leakage currents in this case in different ways and under different conditions, by bending or favoring the flow at the blade face of the vane in the tangential direction close to the head  11 , which diverts it from the radial direction of its leakage. 
     The measures planned in this case consist of curving the trailing edge outwards at the tip end of the blade such that the blade sections closest to the end of the head are increasingly opened, i.e. pointing in the direction of the flow close to the trailing edge and, therefore, divert the flow less at this point. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail using the following figures: 
     FIG. 1,  2  and  3  described above are various views of a typical vane; 
     FIG. 4 is a view of the vane according to the invention; 
     FIG. 5, similar to FIG. 2, shows the consequences of adopting the invention on the flow; and 
     FIGS. 6 and 7 illustrate the pressures in front of the vanes in FIGS. 3 and 4, respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The references in FIGS. 1 to  3  increased by 100 are used to desognate the corresponding parts of the invention. 
     The vane according to the invention, mainly illustrated in FIG. 4 differs from the prior vane in the vicinity of the head  111  in that the leading edge  108  is curved outward of the tip and towards the suction face  110  at this point, as if the corner of the blade  1  had been twisted outwards, while it may remain approximately rectilinear or approximately rectilinear closer to the platform  103 , as in the prior design. This results in the sections  106  closer to the head  111  opening more closer to the trailing edge  108  than the other sections  106 , i.e. they block the free flow of the gases in front of the vane less, since they are less curved than in the prior solution and they have a profile running approximately in the direction of the flow close the trailing edge  108 . 
     FIG. 5 shows that, in such a situation, the flow rate of the gases passing over the head  11  and diverted to the suction face  110  along the arrows  13  is lower, a larger portion of the flow passing along the vane to the trailing edge  108 . It is possible to envisage, to restore the overall aerodynamic load or lift surface of the vane, to curve the sections  106  near the platform  3  and the median sections slightly more than in the conventional design, i.e. to close them. 
     The shape of the vane may be defined more strictly as follows: along a median line  120  of vane section, there is a point  121 , defined by a curvilinear abscissa from the leading edge  107 , after which the vane sections are curved to a decreasing degree, or are increasingly straight, distinguishing it from the design according to the application WO 96 14496 where the deformation of the vane does not affect the shape of the sections, which remains the same, particularly near the trailing edge. A vane section at a distance from the head  111  is designated with the reference  122 . 
     FIGS. 6 and 7 illustrate the eddies produced downstream from the vanes: one of them, called the clearance eddy, is referenced  14  and  114  in FIGS. 6 and 7, respectively, where the blades,  1  and  101 , respectively, are viewed from the rear in the direction of their trailing edge  8  and  108 . It can be seen that the eddy  114  has a smaller surface than the eddy  14 ; the vacuum produced is also weaker in intensity: the gas flow becomes more regular downstream from the vane. This is due to the reduction in leakages; however, the method used in the invention to reduce leakages also has an original effect that is very favourable for the regularity of the flow and the machine output. 
     The different profile of the trailing edge  108  affects other eddies, called passage eddies, which are two in number and located, in one case ( 15  or  115 , respectively), closer to the housing  12  and, in the other ( 16  or  116 , respectively), near the. hub  5 . The first  15  or  115  remains approximately located at the same point (close to the clearance eddy  4  or  114 ), but the eddy  116  closer to the hub  5  is displaced significantly with reference to the eddy  6 , and is located approximately in front of the trailing edge  108  instead of being on the side of the suction face  110 , as for the other eddy  6 . This results in the passage eddies  115  and  116  being further apart than the eddies  15  and  16  according to the prior art and, therefore, less likely to reinforce each other. Although they occupy a surface area similar to that of the conventional design, they are also not as strong since the vacuum applied is lower. 
     This illustrates the advantage of bending the leakage currents in the main (axial) direction of gas flow.