Abstract:
In an axial flow gas turbine efficient cooling and a long life-time can be achieved by providing the outer blade platforms ( 45 )with a plurality of outer teeth ( 46   a - c ) running parallel to each other in the circumferential direction and being arranged one after the other in the direction of the hot gas flow. The teeth ( 46   a - c ) are divided into first and second teeth (46 a;    46   b - c ), the second teeth ( 46   b - c ) being located downstream of the first teeth ( 46   a ), the first teeth ( 46   a ) are opposite to a downstream projection ( 33 ) of the adjacent vanes ( 21 ) of the turbine stage (TS), and the second teeth ( 46   b - c ) are opposite to the respective stator heat shields ( 27 ).

Description:
[0001]    This application claims priority under 35 U.S.C. 119 to Russian Federation application no. 2010148720, filed 29 Nov. 2010, the entirety of which is incorporated by reference herein. 
       BACKGROUND 
       [0002]    1. Field of Endeavor 
         [0003]    The present invention relates to gas turbines, and more specifically to an axial flow gas turbine 
         [0004]    Yet more specifically, the invention relates to a stator heat shield protecting the vane carrier of an axial-flow turbine used in a gas turbine unit. 
         [0005]    2. Brief Description of the Related Art 
         [0006]    An example of an axial flow gas turbine is shown in  FIG. 1 . The gas turbine  10  of  FIG. 1  operates according to the principle of sequential combustion. It includes a compressor  11 , a first combustion chamber  14  with a plurality of burners  13  and a first fuel supply  12 , a high-pressure turbine  15 , a second combustion chamber  17  with the second fuel supply  16 , and a low-pressure turbine  18  with alternating rows of blades  20  and vanes  21 , which are arranged in a plurality of turbine stages arranged along the machine axis MA. 
         [0007]    The gas turbine  10  according to  FIG. 1  includes a stator and a rotor. The stator includes a vane carrier  19  with the vanes  21  mounted therein; these vanes  21  are necessary to form profiled channels where hot gas developed in the combustion chamber  17  flows through. Gas flowing through the hot gas path  22  in the required direction hits against the blades  20  installed in shaft slits of a rotor shaft and causes the turbine rotor to rotate. To protect the stator housing against the hot gas flowing above the blades  20 , stator heat shields installed between adjacent vane rows are used. High temperature turbine stages require cooling air to be supplied into vanes, stator heat shields, and blades. 
         [0008]    The stator heat shields are installed in gas turbine housings above blade rows. The stator heat shields preclude hot gas penetration into the cooling air cavity and form the outer surface of the turbine flow path  22 . For the purposes of economy, sometimes a cooling air supply between a vane carrier and a stator heat shield is not used. However, in this case stator heat shields are also necessary to protect the vane carrier. 
       SUMMARY 
       [0009]    One of numerous aspects of the present invention includes a gas turbine with an improved and highly efficient cooling scheme 
         [0010]    Another aspect includes a gas turbine that comprises a rotor with alternating rows of air-cooled blades and rotor heat shields, and a stator with alternating rows of air-cooled vanes and stator heat shields mounted on a vane carrier, whereby the stator coaxially surrounds the rotor to define a hot gas path in between, such that the rows of blades and stator heat shields, and the rows of vanes and rotor heat shields are opposite to each other, respectively, and a row of vanes and the next row of blades in the downstream direction define a turbine stage, and whereby the blades are provided with outer blade platforms at their tips. 
         [0011]    Yet another aspect includes that the outer blade platforms comprise on their outside a plurality of teeth running parallel to each other in the circumferential direction and being arranged one after the other in the direction of the hot gas flow, said teeth are divided into first and second teeth, whereby the second teeth are located downstream of the first teeth, the first teeth are opposite to a downstream projection of the adjacent vanes of the turbine stage, and the second teeth are opposite to the respective stator heat shields. With such an axially “shortened” version of the stator heat shields it especially becomes possible to feed air used up in the adjacent vane airfoil to simultaneously protect the stator heat shield and cool the outer blade platform. 
         [0012]    Another aspect includes that the blade platforms comprise, on their outside, three teeth, the first teeth comprise the first tooth in the downstream direction, and the second teeth comprise the second and third tooth in the downstream direction. 
         [0013]    In yet another aspect, the adjacent vanes of the turbine stage are cooled with cooling air, and the utilized air from the adjacent vanes effuses between the stator heat shields and the adjacent vanes into the hot gas path to flow along and externally cool the stator heat shields and opposite outer blade platforms. 
         [0014]    Another aspect includes that the stator heat shields are mounted on an inner ring, which on its part is mounted on the vane carrier with a first cavity being provided between the inner ring and the vane carrier, and the vanes are mounted on the vane carrier with a second cavity being provided between the vanes and the vane carrier, which second cavity is supplied with cooling air from a plenum, whereby a leakage of cooling air from the first and second cavities exists between the stator heat shields and the adjacent vanes with their downstream protections, and whereby the leaked cooling air flows along the outside of the outer blade platforms in the downstream direction. 
         [0015]    Another aspect includes that the stator heat shields are each mounted on an inner ring with the possibility of extending freely under action of heat in both axial and circumferential directions by a forward hook and a rear hook being integral with the stator heat shields and extending in the circumferential direction, and the rear hooks are each chamfered at both ends over a predetermined length to reduce high stress concentrations due to high temperature deformation of the stator heat shields. 
         [0016]    Another aspect includes that the stator heat shields are fixed in a circumferential slot of the inner ring in the axial direction by a radial projection, and in the circumferential direction by a pin, which enters into an axial slot under the action of the spring. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings. 
           [0018]      FIG. 1  shows a well-known basic design of a gas turbine with sequential combustion, which may be a starting point for practicing the invention; 
           [0019]      FIG. 2  shows mounting and cooling details of a turbine stage of a gas turbine according to an embodiment of the invention; and 
           [0020]      FIG. 3  shows in a perspective view a single stator heat shield according to  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0021]      FIG. 2  shows mounting and cooling details of a turbine stage TS of a gas turbine  30  according to an exemplary embodiment of the invention. The turbine stage TS, with its hot gas path  22  and hot gas  24  flowing in the axial direction, includes a row of blades  20 , each equipped on its tip with an outer blade platform  45 , and a row of adjacent vanes  21 . The vanes  21  are mounted to a vane carrier  25 . Cooling air from the plenum  23  enters a cavity  31  located between the vanes  21  and the vane carrier  25 . From the cavity  31  cooling air is supplied to the airfoils of a vanes  21  with the utilized air  35  exiting the airfoil and the vane above a rear or downstream projection  33  (see the arrows in  FIG. 2 ). 
         [0022]    Opposite to the row of blades  20  there is positioned a ring of segmented stator heat shields  27 , which are each mounted to an inner ring  26 . A single stator heat shield  27  is shown in a perspective view in  FIG. 3 . The inner ring  26  itself is mounted to the vane carrier  25  with the cavity  29  in between. Another cavity  32  is provided between the stator heat shields  27  and the inner ring  26 . To seal the cavity  32  between adjacent stator heat shields  27  in the circumferential direction, sealing plates  28  ( FIG. 2 ) are provided in respective slots  40  ( FIG. 3 ). 
         [0023]    The stator heat shields  27  can have diverse shapes depending on the design of the vane carrier  25  and the outer blade platform  45 . The shape disclosed in  FIGS. 2 and 3  demonstrates a proposed design of the stator heat shield positioned above a blade  20  with three teeth  46   a - c  arranged on the outside of the outer blade platform  45 . 
         [0024]    The inner ring  26 , which carries the stator heat shields  27 , is mounted in respective slots of the vane carrier  25 . The stator heat shields  27  are fixed in a slot in the inner ring  26  in the axial direction by a radial projection  36  (see  FIG. 3 ), and in the circumferential direction by a pin  44  (see  FIG. 2 ), which during mounting of the stator heat shield  27  enters into an (axial) slot  37  (see  FIG. 3 ) under the action of a spring (see  FIGS. 2 ). 
         [0025]    Thus, due to this kind of mounting, the stator heat shields  27  can extend freely under action of heat in both the axial and circumferential directions. As can be seen in  FIG. 2 , the stator heat shields  27  of this embodiment are only provided with honeycombs ( 41  in  FIG. 3 ) for the second and third blade teeth  46   b  and  46   c,  while the first tooth  46   a  is not covered by the stator heat shield. Opposite to the first tooth  46   a  is a rear or downstream projection  33  (with a respective honeycomb) provided at the adjacent vanes  21 . 
         [0026]    Such a design makes it possible to avoid both additional cooling air supply into the cavity  32  to cool the stator heat shields  27  and further transportation of this air through holes within the stator heat shields to cool the opposite outer blade platforms  45 . 
         [0027]    Thus, a non-cooled stator heat shield is proposed. Furthermore, the outer blade platform  45  is assumed to be cooled by air used up in the vane airfoil (utilized air  35 ). In so doing, turbine efficiency increases due to this double cooling air utilization. 
         [0028]    As shown in  FIG. 3 , the stator heat shield  27  has a rear hook  38  and a forward hook  39  running in the circumferential direction. In connection with the cooling scheme explained above, it is advantageous to provide the stator heat shields  27  in accordance with  FIG. 3  with special chamfers made in outer surfaces at both ends of the rear hooks  38  within zones  42  over a predetermined length L. This chamfer is helpful from the viewpoint of mechanical integrity, since when a stator heat shield is operated under high temperature conditions, the edges  43  of the rear hook  38  strive to displace in the radial direction relative to the inner ring  26 . If there were no chamfers over the length L, a very high stress concentration would occur at the edges  43 , and the life-time of the stator heat shields  27  would decrease drastically. 
         [0029]    On the other hand, no chamfers are provided at the forward hook  39 , since with regard to shape of the outer blade platform, the stator heat shield  27  is provided there with a flexure to increase its stiffness in its forward portion. 
         [0030]    Some characteristics and advantages can be summarized as follows: 
         [0031]    1. The “shortened” version of the stator heat shields, provided with honeycomb above the last two outer blade platform teeth  46   b,c , provides the possibility of using air, which has already been utilized in the vane airfoil, for simultaneous protection of the stator heat shields and cooling the outer blade platform  45  (see  FIG. 2 ). The shortened stator heat shield shape enables a honeycomb to be arranged on the vane projection  33  above the first tooth  46   a  of the outer blade platform  45 , which precludes any possibility for leakage of utilized air in front of the first tooth  46   a  of the outer blade platform  45 . 
         [0032]    2. The shortened version of the stator heat shield  27 , provided with honeycombs above the last blade platform teeth  46   b, c , provides the possibility of using cooling air leakages  34  from cavities  29  and  31  for additional cooling of the platform  45  since the projection  33  rules out any possibility for air leakage upstream of the first tooth  46   a  of blade platform  45 . 
         [0033]    3. Chamfers in the rear hook  38  of the stator heat shield  27  reduce the stress level in the stator heat shield  27  to a sufficient extent, and increase its life-time considerably, when it is operated in the gas turbine. 
         [0034]    The combination of stress-decreasing chamfers and a shortened part shape in the same stator heat shield simultaneously makes it possible to create a non-cooled stator heat shield with a long-term lifespan, and increase turbine efficiency due to air saving. 
       LIST OF REFERENCE NUMERALS 
       [0035]      10 , 30  gas turbine 
         [0036]      11  compressor 
         [0037]      12 , 16  fuel supply 
         [0038]      13  burner 
         [0039]      14 , 17  combustion chamber 
         [0040]      15  high-pressure turbine 
         [0041]      18  low-pressure turbine 
         [0042]      19  vane carrier (stator) 
         [0043]      20  blade 
         [0044]      21  vane 
         [0045]      22  hot gas path 
         [0046]      23  plenum 
         [0047]      24  hot gas 
         [0048]      25  vane carrier 
         [0049]      26  inner ring 
         [0050]      27  stator heat shield 
         [0051]      28  sealing plate 
         [0052]      29 , 31 , 32  cavity 
         [0053]      33 , 36  projection 
         [0054]      34  leakage 
         [0055]      35  utilized air 
         [0056]      37  slot 
         [0057]      38  rear hook 
         [0058]      39  forward hook 
         [0059]      40  slot (for sealing plates) 
         [0060]      41  honeycomb 
         [0061]      42  zone 
         [0062]      43  edge 
         [0063]      44  pin 
         [0064]      45  blade outer platform 
         [0065]      46   a - c  tooth 
         [0066]    L length 
         [0067]    MA machine axis 
         [0068]    TS turbine stage 
         [0069]    While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.