Abstract:
A vane doublet for a turbine nozzle includes a radially inner band; a radially outer band; and a pair of airfoils extending between the inner band and the outer band. One of the inner and outer bands is split in a generally axial direction along a split line passing between the pair of airfoils, and a seal is secured between or adjacent to facing edges of the split line.

Description:
This invention relates generally to gas turbine technology, and more specifically, to the manufacture or repair of turbine nozzles. 
     BACKGROUND OF THE INVENTION 
     Turbine nozzles are commonly cast as vane doublets, i.e., arcuate segments with two vanes or airfoils per segment, in order to lower the cost of the casting. The doublet configuration, however, leads to high stress/strain in certain areas of the doublet, which can manifest itself in the form of cracks. Once cracks grow to a critical size, the component part must be removed from service and repaired to prevent it from failing catastrophically in the turbine. 
     In more modern designs of gas turbine nozzles, a common approach is to use a singlet configuration, i.e., one airfoil per segment, rather than the doublet configuration. The use of a singlet configuration eliminates much of the stress/strain that drives cracking in the doublet configuration, but typically, singlets are considerably more expensive to produce. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In an exemplary, but nonlimiting embodiment of the present invention, an otherwise conventional vane doublet turbine nozzle is modified to behave more like a singlet nozzle by splitting one or the other of the inner and outer bands (i.e., sidewalls) of the doublet. 
     Accordingly, in one aspect, the present invention relates to a vane doublet for a turbine nozzle comprising: a radially inner band; a radially outer band; and a pair of airfoils extending between the inner band and the outer band, wherein one of the inner and outer bands is split in a generally axial direction, along a split line passing between the pair of airfoils; and wherein a seal is secured between or adjacent to facing edges of the split line. 
     In another aspect, the present invention relates to a method of repairing a turbine nozzle doublet vane to reduce occurrence of cracking and extend service intervals wherein the doublet vane comprises inner and outer bands with a pair of airfoils extending therebetween, the method comprising: a) removing the doublet vane from the turbine nozzle assembly; and b) splitting one of the inner band and the outer band from a leading end to a trailing end, between the pair of airfoils and securing a seal between or adjacent to facing edges of the split line. 
    
    
     
       The invention will now be described in connection with the drawings identified below. 
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a turbine nozzle doublet in accordance with an exemplary but nonlimiting embodiment of the invention; 
         FIG. 2  is a schematic bottom plan view of the doublet shown in  FIG. 1 ; 
         FIG. 3  is a schematic bottom plan view of a variation of the doublet shown in  FIG. 2 ; 
         FIG. 4  is a perspective view of a turbine nozzle doublet in accordance with another exemplary but nonlimiting embodiment of the invention; 
         FIG. 5  is a schematic top plan view of the doublet shown in  FIG. 4 ; and 
         FIG. 6  is a schematic top plan view of a variation of the doublet shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference initially to  FIGS. 1 and 2 , a doublet nozzle segment (or simply, vane doublet)  10  for a gas turbine nozzle includes a radially inner band or sidewall  12  and a radially outer band or sidewall  14 . The nozzle doublet essentially forms one arcuate segment of a plurality of such nozzle doublet segments secured within an annular diaphragm, such that the vanes surround the turbine rotor (not shown), with rows of blades or airfoils on the rotor located on opposite sides of the stationary nozzle in conventional fashion. 
     In this exemplary embodiment, a pair of vanes or airfoils  16 ,  18  extend radially between the inner and outer bands  12 ,  14 . The airfoils have respective leading edges  20 ,  22  and respective trailing edges  24 ,  26 . As oriented in  FIG. 1 , the nozzle doublet inner band  12  may be considered to have a forward or leading end  28  and an aft or trailing end  30 , i.e., the flow of air is in a direction from end  28  to end  30 . With this understanding as to orientation, the airfoil  18  may be regarded as the right-hand airfoil, and airfoil  20  as the left-hand airfoil. 
     The above-described doublet configuration is essentially a four-bar linkage, which in the environment of a gas turbine nozzle, creates undesirably high stresses as mentioned above, particularly in the airfoil  18  on the right hand side of the doublet. In order to alleviate the stress on the airfoil  18 , which has a tendency to grow thermally in a radial direction, it has been discovered that the resultant cracking problem can be mitigated by relieving the stress on the airfoil. In the exemplary implementation, this is achieved by splitting the inner band  12  and thus decoupling the four bar linkage. Specifically, a split line  32  is formed which progresses from the leading end  28  to the trailing end  30  between the airfoils  16  and  18 . In the configuration exemplary in  FIG. 1 , where the outer band is secured to a retaining ring that prevents radial expansion of the outer band, the split allows the airfoil  18  to push the inner band inward in a radial direction, as the airfoil expands due to thermal growth. The movement is no more than a few thousandths of an inch, but is sufficient to alleviate the thermally induced stress. In the preferred arrangement, split line  32  follows substantially the curvature of the suction side  37  of airfoil  20  or the pressure side  38  of the airfoil  18  to facilitate the cutting process. 
     The split inner band  12  may be achieved by any suitable technique such as by wire EDM or carbon electrode cutting. If the split line  32  is cut at an angle and/or if sufficiently thin, flow leakage through the inner band  12  in a radial direction will be insignificant. Preferably, however, side notches or grooves  34  may be cut in the facing inner band edges that define the split line, and a flexible seal, such as a flexible woven metal cloth  36 , may be inserted within the aligned grooves  34  to provide a more substantial seal against leakage. Other seals, such as sheet metal seals, may be employed, with the seals tack-welded in place in a location such that the pressures within the engine will provide sufficient sealing without binding along the split line. 
       FIG. 3  illustrates an alternative path or shape for the inner band split line. In this instance, the split line  40  is composed of two straight-line segments  42 ,  44  with segment  42  substantially parallel to the side edges  46 ,  48  of the inner band or sidewall  12 , and segment  44  substantially parallel to one of the angled side edges  50  of the inner band  12 . Otherwise, the nozzle doublet illustrated in  FIG. 3  is substantially identical to that shown in  FIGS. 1 and 2 . It will be appreciated that, in this split line arrangement, the seal  36  may be composed of two discrete seal strips, inserted from opposite ends of the split line. The seal strips would then meet at the intersection of the segments  42 ,  44 . 
       FIGS. 4 and 5  illustrate an alternative nozzle vane doublet configuration where the outer band of the doublet is split. For purposes of this disclosure, the doublet may be considered otherwise identical to the doublet  10  and, for convenience, similar reference numerals are used to designate corresponding elements, surfaces and the like, but with the prefix “1” added. Accordingly, the doublet vane  110  includes a radially inner band or sidewall  112  and a radially outer band or sidewall  114 . 
     In this exemplary embodiment, a pair of airfoils  116 ,  118  extend radially between the inner and outer bands  112 ,  114 . The airfoils have respective leading edges  120 ,  122  and respective trailing edges  124 ,  126  ( FIG. 5 ). As oriented in  FIG. 4 , the nozzle doublet inner band  112  may be considered to have a forward or leading end  128  and an aft or trailing end  130 , and here again, the flow of air is in a direction from end  128  to end  130 . In this exemplary embodiment, a split line  132  is formed in the outer band  114 , progressing from the leading end  128  to the trailing end  130  between the airfoils  116  and  118 . In the preferred arrangement, split line  132  follows substantially the curvature of the split line in  FIGS. 1 and 2 . 
     The side edges of the split line  132  may also be notched or grooved to receive a seal  36  as in the previously described embodiment. 
       FIG. 6  illustrates a variation where the split line  138  is composed of two straight-line segments  140 ,  142  and, here again, the seal (not shown in  FIG. 6 ) may be composed of discrete segments that engage at the juncture of seal line segments  140 ,  142 . 
     To further reduce thermally-induced stresses in the airfoils, the pressure side of the airfoils may be coated with a conventional thermal barrier coating. This is especially useful on the right hand airfoil  18  (or  118 ) since the stresses are greater on the right side of each doublet. By coating the high temperature side of the airfoil, it is cooled and there is thus less tendency to grow thermally. It is also advantageous to cool the airfoils  16 ,  18 ,  116  or  118  more efficiently by, for example, adding cooling apertures  52  along the trailing edge of one or both airfoils, and adding film cooling holes  54  upstream of the trailing edge holes (visible only in  FIG. 1 ). It will be appreciated that the aperture and hole locations may vary as needed to optimize results. Such cooling reduces the temperature difference between the airfoils and the inner and outer bands or sidewalls. Of course, combinations of coatings and cooling apertures along with the split lines may be used to enhance stress reduction and thus reduce or at least delay the onset of cracking. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.