Patent Publication Number: US-10323526-B2

Title: Blade with tip shroud

Description:
TECHNICAL FIELD 
     The present disclosure relates to blades with tip shrouds, and particularly to blades with tip shrouds comprising a fin and a platform. 
     BACKGROUND OF THE INVENTION 
     Turbine blades for heavy duty gas turbines are required to operate reliably at high speeds in a hostile environment. This is particularly true for end-stage turbine blades, which tend to be very long, placing considerable strain on the blades. Existing blade designs can provide damping of vibrations, lightweight design, or minimal over-tip leakage flow, but struggle to provide good performance in all of these areas at once, especially in end-stage turbine blades where the centrifugal forces are highest and the need for stiff and lightweight designs is particularly acute. As a result, it has been appreciated that the design of turbine blades could be refined to provide improved turbine blade performance, particularly in end-stage turbine blades. 
     SUMMARY OF THE INVENTION 
     The invention is defined in the appended independent claims to which reference should now be made. Advantageous features of the invention are set forth in the dependent claims. 
     A first aspect of the invention provides a blade comprising a leading edge, a trailing edge, a pressure surface, a suction surface, a root end and a tip end, the blade comprising a tip shroud attached to the tip end, the tip shroud comprising a platform and a fin, wherein the fin comprises a leading edge side facing towards the leading edge of the blade, a trailing edge side facing towards the trailing edge of the blade, a back end and a front end, the leading edge side and the trailing edge side extending between the back end and the front end, the fin extending across the tip end of the blade at an angle to the chord of the blade at the tip end of the blade, wherein the platform comprises a first platform portion and a second platform portion, wherein the first platform portion extends from the leading edge side of the fin between the front end of the fin and the suction surface at the tip end of the blade, to the suction surface at the tip end of the blade between the leading edge of the blade and the leading edge side of the fin, wherein the second platform portion extends from the trailing edge side of the fin between the back end of the fin and the pressure surface at the tip end of the blade, to the pressure surface at the tip end of the blade between the trailing edge of the blade and the trailing edge side of the fin, and wherein the tip shroud is delimited by the suction surface at the tip end of the blade between the trailing edge and the trailing edge side of the fin, and by the trailing edge side of the fin between the suction surface at the tip end of the blade and the front end of the fin. 
     This can provide an overall improvement in performance compared to existing blades. In particular, it can provide good performance in terms of minimising over-tip leakage flow and minimising the amount of material in the tip shroud to reduce the amount of material that has to pass through the narrow blade space in the mould during casting of the blade and to reduce centrifugal load on the blade aerofoil, blade root and rotor. The platform portion at the trailing edge-pressure surface area can reduce leakage and provide stiffness. The platform portion at the leading edge-suction surface area can increase stiffness. 
     In one embodiment, the fin comprises a fin platform end adjacent to the platform and a fin tip end distal from the platform, and the distance from the leading edge side of the fin to the trailing edge side of the fin is greater at the fin platform end than at the fin tip end. Variable thickness of fin can provide minimum weight whilst maintaining stiffness and minimising leakage. 
     In one embodiment, the distance from the leading edge side of the fin to the trailing edge side of the fin is greater near the front end of the fin, near the back end of the fin and/or near the tip end of the blade than elsewhere on the fin. Providing a thicker portion of the fin near the front and/or back end can provide sufficient stiffness despite contact between the fin and the next fin when in use. Being able to couple adjacent blades can improve damping. Providing a thicker portion near the tip end of the blade can provide good structural strength and stiffness while minimising the amount of material in the fin. 
     In one embodiment, the portion of the fin near the front end of the fin where the width between the leading edge side and the trailing edge side is greater is closer to the front end of the fin on the leading edge side of the fin than on the trailing edge side of the fin. This can provide the thicker portion in the most useful position on both sides of the fin. 
     In one embodiment, the platform comprises a third platform portion, wherein the third platform portion extends from the point where the pressure surface and the leading edge side of the fin meet, wherein the third platform portion extends along the pressure surface at the tip end of the blade between the leading edge of the blade and the leading edge side of the fin, and along the leading edge side of the fin between the suction surface at the tip end of the blade and the back end of the fin. This can improve structural strength and stiffness and reduce leakage. In one embodiment, the area of the third platform portion is at least three times smaller than the area of the first platform portion. 
     In one embodiment, the tip shroud is delimited by the pressure surface at the tip end of the blade between the leading edge and the leading edge side of the fin, and by the leading edge side of the fin between the pressure surface and the back end of the fin. This can minimise the amount of material in the tip shroud. 
     In one embodiment, an edge of the first platform portion extends in a straight or substantially straight line from the leading edge to the front end of the fin, and/or an edge of the second platform portion extends in a straight or substantially straight line from the trailing edge to the back end of the fin. This can minimise leakage and improve the stiffness of the tip shroud. 
     In one embodiment, a hole through the first and/or second platform portions is provided. This can minimise material usage and weight whilst still providing stiffness and structural strength. 
     In one embodiment, a first dust hole in the tip end of the blade between the leading edge side of the fin and the leading edge, and/or a second dust hole in the tip end of the blade between the trailing edge side of the fin and the trailing edge is provided. 
     In one embodiment, the second dust hole leads to a dust hole channel that extends through the blade in a longitudinal direction of the blade, wherein the dust hole channel is further away from the trailing edge at the tip end of the blade than elsewhere in the blade. Arranging the second dust hole in this way can have structural advantages, particularly in that the dust hole can be provided in a comparatively wide part of the blade tip whilst providing most of the dust hole channel closer to the trailing edge. 
     In one embodiment, a fillet radius where the fin meets the tip end of the blade is larger than a fillet radius where the platform meets the tip end of the blade. Fillet radius variation can help reduce weight. 
     A second aspect of the invention provides a gas turbine comprising a blade as described above. A third aspect of the invention provides a method of manufacture of a blade as described above, wherein the blade is formed by casting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: 
         FIG. 1  shows a top view of an exemplary blade; 
         FIG. 2  shows a partially see-through perspective view of the blade of  FIG. 1 , including lines showing the shape of the surfaces; 
         FIG. 3  shows two cross-sections  60 ,  62  through the fin of the blade of  FIG. 2 ; and 
         FIG. 4  shows a partially see-through perspective view of three blades according to the embodiment of  FIGS. 1 to 3  adjacent to one another in a gas turbine. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 and 2  show a blade  10  comprising a leading edge  12 , a trailing edge  14 , a pressure surface  16 , a suction surface  18 , a root end to be attached to a blade root  20  and a tip end  24 . A tip shroud  22  is attached to the tip end  24 , the tip shroud  22  comprising a platform  31 ,  32 ,  33  and a fin  40 . 
     The fin  40  comprises a leading edge side  43  facing towards the leading edge  12  of the blade, a trailing edge side  44  facing towards the trailing edge  14  of the blade, a back end  42  and a front end  41 , the leading edge side  43  and the trailing edge side  44  extending between the back end and the front end. The fin extends across the tip end  24  of the blade at an angle to the chord (the line from the leading edge to the trailing edge) of the blade  10  at the tip end  24  of the blade. 
     The platform comprises a first platform portion  31  and a second platform portion  32 , wherein the first platform portion  31  extends from the leading edge side  43  of the fin between the front end  41  of the fin and the suction surface  18  at the tip end  24  of the blade, to the suction surface  18  at the tip end  24  of the blade between the leading edge  12  of the blade and the leading edge side  43  of the fin. 
     Similarly on the other side of the fin, the second platform portion  32  extends from the trailing edge side  44  of the fin between the back end  42  of the fin and the pressure surface  16  at the tip end  24  of the blade, to the pressure surface  16  at the tip end  24  of the blade between the trailing edge  14  of the blade and the trailing edge side  44  of the fin. 
     The tip shroud  22  is delimited by the suction surface  18  at the tip end  24  of the blade between the trailing edge  14  and the trailing edge side  44  of the tip shroud  22 , and by the trailing edge side  44  of the fin between the suction surface  18  at the tip end  24  of the blade and the front end  41  of the fin. 
     The platform portions of the tip shroud can be thought of as extending over four separate areas delimited by the fin  40  and the tip end  24  of the blade. The first area (leading edge-suction side) contains the first platform portion  31  and the second area (trailing edge-pressure side) contains the second platform portion  32 . The third area (leading edge-pressure side), extending between the pressure surface  16  of the tip end  24  of the blade and the leading edge side  43  of the fin, can optionally contain a third platform portion  33 . The third platform portion  31  (see  FIGS. 1 and 2 ) extends from the pressure surface  16  at the tip end  24  of the blade between the leading edge  12  of the blade and the leading edge side  43  of the fin, to the leading edge side  43  of the fin between the suction surface  18  at the tip end  24  of the blade and the back end  42  of the fin. In the fourth area (trailing edge-suction side), which extends from the trailing edge side  44  of the fin  40  between the back end  42  of the fin and the suction surface  18  of the tip end, to the suction surface  18  of the tip end between the trailing edge side  44  of the fin and the trailing edge  14 , there is no platform. 
     In embodiments where the third area does not contain a third platform portion  33 , the tip shroud  22  is delimited by the pressure surface  16  at the tip end  24  of the blade between the leading edge  12  and the leading edge side  43  of the fin, and by the leading edge side  43  of the fin between the pressure surface  16  at the tip end  24  of the blade and the back end  42  of the fin. 
     The first platform portion  31  extends over a roughly triangular area, with its first and second sides following the fin  40  and the suction surface  18  of the blade as described above. The third side extends from the front end  41  of the fin to the leading edge  12 . The third side could be straight, or can have a slight concave or convex curve. The embodiment shown in the Figures has a slight concave curve, curving towards the point where the fin  40  and the tip end  24  of the blade meet. The curvature is generally gentle, with a radius of curvature greater than the length of the third side. An exception may be present on the portion of the third side closest to the fin (e.g. the 10 or 20% of the third side closest to the fin), where the curvature is considerably smaller (e.g. smaller than the width of the thinnest part of the fin). This portion of greater curvature may be necessary for manufacturing in embodiments such as those shown in the Figures, where the end of the first platform portion  31  is set back slightly from the front end  41  of the fin. 
     Similarly, the second platform portion  32  extends over a roughly triangular area, with its first and second sides following the fin  40  and the pressure surface  16  as described above. The third side could be straight (as in the Figures) or have a slight concave or convex curve. As with the first platform portion  31 , the second platform portion  32  has a portion close to the fin  40  (e.g. the 10 or 20% of the third side closest to the fin  40 ), where the curvature is considerably smaller (e.g. smaller than the width of the thinnest part of the fin  40 ). This portion of greater curvature may be necessary for manufacturing in embodiments such as those shown in the Figures, where the end of the second platform portion  32  is set back slightly from the back end  42  of the fin  40 . 
     In other embodiments, the curvature may also be different to that described, for example with a greater curvature. 
     The third platform portion  33  is preferably much smaller than the first and second platform portions  31 ,  32  (preferably at least three times smaller by area than the first platform, more preferably less than 20% of the area of the first platform, and most preferably less than 10% of the area of the first platform; preferably the ratios of the third platform area relative to the second platform area are similar or the same), and does not extend to the leading edge  12  or to the back end  42  of the fin. Nevertheless it may optionally extend in a similar way to the first and second platform portions, all the way from the leading edge to the back end of the fin. 
     The fin  40  is described briefly above and will now be discussed in more detail. The fin  40  extends beyond the tip end  24  of the blade in a direction parallel or substantially parallel to the longitudinal direction of the blade  10 . The fin  40  is generally cuboidal, with the longest dimension being between the front end  41  and the back end  42 , and the shortest dimension being between the leading edge side  43  and the trailing edge side  44 . 
     The fin tip end  46  is generally substantially straight, with a slight concave curvature to mirror the curvature of the adjacent gas turbine parts when in use. The distance between the leading edge side  43  and the trailing edge side  44  at the fin tip end  46  is generally the same along the length of the fin tip end  46  from the front end  41  to the back end  42 . 
     The distance between the leading edge side  43  and the trailing edge side  44  is constant along the fin tip end  46 . The distance between the leading edge side  43  and the trailing edge side  44  is greater at the fin platform end  45  than at the fin tip end  46 . The distance between the leading edge side  43  and the trailing edge side  44  at the fin platform end  45  is variable; this is discussed in more detail below. 
     The front end  41  and the back end  42  of the fin may be angled; that is, the cross-section of the fin  40  perpendicular to the longitudinal direction of the blade  10  is not a rectangle but instead a parallelogram with corner angles that do not equal 90° (as shown in  FIG. 3  in particular). Similarly, the fin tip end  46  would generally also describe a parallelogram with corner angles that do not equal 90°. The faces of the front end  41  and the back end  42  of the fin should normally be parallel to one another, as they will interact with corresponding faces on adjacent blades  10  (the back end  42  of one fin interacts with the front end  41  of the fin of the adjacent blade  10 ), as shown in  FIG. 4 . 
     A portion of the leading edge side  43  and the trailing edge side  44  adjacent to the fin tip end  46  is preferably planar or substantially planar, as shown in the Figures. The shape of the leading edge side  43  and the trailing edge side  44  further from the fin tip end  46  transitions into a non-planar surface, with several protrusions deviating from a planar surface. That is, the shape of the leading edge side  43  and the trailing edge side  44  further from the fin tip end  46  comprise several wider portions extending from the planar surface. Firstly, the distance between the leading edge side  43  and the trailing edge side  44  increases close to the front end  41  and back end  42  of the fin. Secondly, the distance between the leading edge side  43  and the trailing edge side  44  increases close to the tip end  24  of the blade. This is closer to the front end  41  on the leading edge side  43  compared to on the trailing edge side  44 , which results in an offset in the wider portion on the leading edge side  43  compared to the trailing edge side  44 , and in an asymmetrical fin  40 . 
     In  FIG. 2  in particular, it can be seen that the increase in width from the fin tip end  46  to the fin platform end  45  is not necessarily smooth. In  FIG. 2 , the increase is smooth at the back end  42  and front end  41 , but not at the portion close to the tip end  24  of the blade in the longitudinal direction of the blade, where a stepwise or substantially stepwise increase in width can be seen. Nevertheless, the increase in width would typically not be stepwise in the longitudinal direction of the fin (which is normally also the blade rotation direction), as can be seen in the Figures. 
     To further illustrate the variations in width and the features of the fin  40  described above,  FIG. 3  shows two cross-sections  60 ,  62  of the fin  40  superimposed on each other for comparison. The cross-sections  60 ,  62  are marked on  FIG. 2 . The cross-section  60  is shown as lines in one direction and the cross-section  62  is shown as a lines in a second direction. This results in hatching over most of the area of the cross-sections in  FIG. 3 , where both sets of lines are present. The cross-section  62  is closer to the fin platform end  45 , and is always wider or as wide as the cross-section  60 —that is, the width of the fin  40  (from the leading edge side  43  to the trailing edge side  44 ) at cross-section  60  is always thinner or the same as the width of the fin  40  at cross-section  62 . 
     Two dust holes  50 ,  52  are also present in the tip end  24  of the blade. The first dust hole  50  is between the leading edge  12  and the fin  40 , and the second dust hole  52  is between the fin  40  and the trailing edge  14 . The first dust hole  50 , and the second dust hole  52  lead to a dust hole channel  53 . The dust hole channel mostly extends through the blade in the longitudinal direction  72  of the blade (and/or parallel to the trailing edge  14 ), but near the tip end  24  of the blade the dust hole channel curves away from the trailing edge  14 . The dust hole channel  53  is preferably smooth because it is loaded by the shroud overhangs and also during casting (a purpose of the dust holes can be to hold the core during casting). 
     The blade described above is preferably a (rotating) turbine blade for use in a gas turbine. The blade could alternatively be used in a steam turbine.  FIG. 4  shows how the blades would be arranged adjacent to one another in a gas turbine. The back end  42  of the fin of one blade  10  is adjacent to the front end  41  of the fin of the adjacent blade  10 . Generally, the blades are not attached to one another, but are able to move slightly relative to one another. 
     Preferably, the blade root  20 , the main blade portion  21  and the tip shroud  22  are integrally cast as a single component, as this can provide an extremely strong component. Alternatively, some or all of the various parts may be manufactured separately and attached to the blade. 
     The blade shape described above is merely an example, and other blade shapes could be used. For example, the blade root  20  is optional, and may be a separate part. The specific features such as the leading edge  12 , trailing edge  14  and so on may be another shape from that shown in the Figures. The blade may comprise internal cooling. 
     The end surface of the tip end  24  of the blade, on which the fin is mounted, may be perpendicular to the longitudinal direction  72  of the blade. Alternatively, the end surface may be slightly inclined from parallel, particularly on the portion of the tip end of the blade between the trailing edge side of the fin and the trailing edge. 
     The platform is described in detail in the embodiments described above, but variation in the shape and structure of the platform is possible. The first and second platform portions may extend to the front/back end of the fin, or one or more may be set back slightly from the front/back end of the fin as shown in the Figures. Similarly, the first and second platform portions may extend to the leading edge/trailing edge of the blade tip, or one or more may be set back slightly from the leading/trailing edge of the blade tip. The platform may be of a variable thickness (in the longitudinal direction of the blade). Optionally, a hole may be provided through the first and/or second platform portions, particularly through the second platform portion. The hole/s extends through the first and/or second platform portions in the longitudinal blade direction  72 . Dotted line  55  in  FIG. 1  shows the possible location and extent of a second platform hole, although other shapes such as circular or oval holes may also be used. Where the hole is triangular or substantially triangular in shape, or another shape with sharp corners, the corners may be rounded. Alternatively, rather than a hole at this location, this area of the platform could be thinner than the rest of the platform (for example a blind hole). 
     The delimitations in the tip shroud as discussed above can be thought of as being in a plane perpendicular to a longitudinal direction  72  of the blade, or in a plane containing the tip end  24  of the blade. The tip shroud can be thought of as extending out from the fin up to the delimitation point (which is only out to the edge of the fin itself for some parts of the fin). 
     As with the blade and the platform, various embodiments of the fin are possible besides those described above and may be combined with various different embodiments of the blade and the platform. For example, the trailing edge side and/or leading edge side may be planar. The thicker portions (where the distance from the leading edge to the trailing edge is greater) may extend part-way between the fin platform end and the fin tip end, or may extend all the way from the fin platform end to the fin tip end (up to and including the fin tip end). The distance between the trailing edge side and the leading edge side may be the same at the fin platform edge and the fin tip edge. In general, changes in width of the fin (width from the trailing edge side to the leading edge side) are gradual, but stepwise changes in width are also possible. Although the fin generally widens from the fin tip end to the fin platform end, in some embodiments there may be portions of the fin where the opposite is the case, or where the fin is wider at the fin tip end than the fin platform end. 
     Preferably, the fin is angled such that when in use, the longitudinal extent of the fin (from the back end to the front end) is parallel to the blade rotation direction  70 . Preferably only a single fin is provided for any given blade. Preferably the back end and the front end are the same width (from the trailing edge side to the leading edge side) 
     An embodiment with two dust holes is described above, but one, three or more dust holes may alternatively be provided. 
     Preferably, the fillet radius where the fin meets the tip end of the blade is larger than the fillet radius where the platform meets the tip end of the blade. 
     Various modifications to the embodiments described are possible and will occur to those skilled in the art without departing from the invention which is defined by the following claims. 
     
       
         
           
               
             
               
                   
               
               
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                 10 
                 blade 
               
               
                 12 
                 leading edge 
               
               
                 14 
                 trailing edge 
               
               
                 16 
                 pressure surface (pressure side) 
               
               
                 18 
                 suction surface (suction side) 
               
               
                 20 
                 blade root 
               
               
                 21 
                 main blade portion 
               
               
                 22 
                 tip shroud 
               
               
                 23 
                 root end of the blade 
               
               
                 24 
                 tip end of the blade 
               
               
                 31 
                 first platform portion 
               
               
                 32 
                 second platform portion 
               
               
                 33 
                 third platform portion 
               
               
                 40 
                 fin 
               
               
                 41 
                 front end of the fin 
               
               
                 42 
                 back end of the fin 
               
               
                 43 
                 leading edge side of the fin 
               
               
                 44 
                 trailing edge side of the fin 
               
               
                 45 
                 fin platform end 
               
               
                 46 
                 fin tip end 
               
               
                 50 
                 first dust hole 
               
               
                 52 
                 second dust hole 
               
               
                 53 
                 dust hole channel 
               
               
                 55 
                 second platform hole 
               
               
                 60 
                 fin cross-section 
               
               
                 62 
                 fin cross-section 
               
               
                 70 
                 blade rotation direction 
               
               
                 72 
                 longitudinal direction of the blade