Patent Publication Number: US-8979479-B2

Title: Gas turbine

Description:
This application claims priority to European application no. 09171142.4, filed 23 Sep. 2009, the entirety of which is incorporated by reference herein. 
     BACKGROUND 
     1. Field of Endeavor 
     The present invention relates to a gas turbine. 
     In particular the present invention refers to the sealing of the zone between the guide vane boxes of the high-pressure turbine immediately downstream of the combustion chamber and a fixed frame, such that possible leakages of hot gases flowing in the combustion chamber and/or compressed air used to seal the zone between the combustion chamber and stator airfoil row do not enter the rotor airfoils cooling circuit. 
     2. Brief Description of the Related Art 
     In the following reference, will be made to  FIG. 1  for describing the relevant parts of the gas turbine; in particular reference will be made to a sequential combustion gas turbine, it is anyhow clear that structures embodying principles of the present invention may be implemented in any gas turbine also not being a sequential combustion gas turbine. 
     Sequential combustion gas turbines  1  have a compressor (not shown) compressing air and supplying it to first burners (not shown) where fuel is injected and a mixture to be combusted is formed. 
     Downstream of the first burners a first combustion chamber  2  is provided, where the mixture is combusted to form high pressure hot gases F that are supplied to a high-pressure expansion stage. 
     The high-pressure expansion stage includes a stator airfoil row  4  separated from the combustion chamber  2  by a first gap  5 , and a rotor airfoil row  6  separated from the stator airfoil row  4  by a second gap  7 ; third gaps  8  are provided between the rotor airfoil row  6  and an annular duct  9  feeding a plurality of side-by-side second burners  10 , wherein further fuel is injected in the hot gases (still rich in air) already partially expanded in the high-pressure expansion stage, such that an ignitable mixture is formed. This ignitable mixture is combusted in a second combustion chamber (not shown) and the hot gases produced are further expanded in a low pressure turbine (not shown). 
     The stator airfoil row  4  is made of stator airfoils  15  defining between each other guide vanes and having endwalls  16  connected to guide vane boxes  17 . 
     The guide vane boxes  17  have a box structure and are fed with cooling air A via connections not shown for simplicity. 
     In particular, the cooling air A comes from the compressor at a temperature of about 450-550° C. and is cooled by an external cooler to a temperature of typically 200-400° C. 
     Moreover the guide vane boxes  17  are also provided with nozzles  20  that inject the cooling air A into the second gap  7 . 
     The rotor airfoil row  6  includes a plurality of rotor airfoils  22  having a hollow body provided with an inlet  23  arranged to collect the cooling air A injected from the nozzles  20 . 
     During operation, the hot gases F formed in the first combustion chamber  2  pass through the stator and rotor airfoil row  4 ,  6  such that the rotor airfoil row  6  extracts mechanical power from them. 
     Moreover, the air A from the guide vane boxes  17  is injected through the nozzles  20  in the second gap  7  towards the rotor airfoil inlets  23 . 
     As the rotor airfoil row  6  rotate with high speed, it draws the cooling air A injected from the nozzles  20  and makes it to enter the rotor airfoil  22  via the inlets  23 . 
     The cooling air A entering the rotor airfoils  22  cools the rotor airfoils  22  and is then injected through holes (usually at the leading edge and trailing edge of each rotor airfoil row); the air injected through the leading and trailing edges of the rotor airfoils  22  is indicated by A 2 . 
     In order to prevent the hot gases F from entering the first gap  5  (the hot gases have a temperature of about 1200-1500° C. and would impair the components close to the first gap  5 ), compressed air (the so-called ‘purge air’) is diverted from the compressor and is injected in the first gap  5 . This air has a temperature of about 450-550° C. and thus is not dangerous for the components close to the gaps  5 . 
     In addition, in order to prevent the compressed air (purge air) from reaching the rotor airfoil inlet  23 , seals  25  are provided between the stator airfoil endwalls  16 /guide vane boxes  17  and a fixed frame  26 . 
     Nevertheless, the compressed air diverted from the compressor may leak and pass through the seals  25  and mix with the cooling air A injected in the second gap  7 . 
     For this reason, the cooling air A flow rate is quite large, such that, in all operating conditions, the air entering the rotor airfoil  22  has a correct temperature to safeguard the rotor airfoil integrity and guarantee their lifetime. 
     Nevertheless, since the cooling air A flow rate diverted from the compressor into the guide vane boxes is quite large, efficiency of the gas turbine is reduced. 
     FR 1 351 268 discloses a guide vane box with openings from which compressed air is injected to enter cooling conduits of the rotor airfoils. 
     GB 2 246 836 discloses a guide vane with first and second passages; from these passages cooling air is injected into cooling passages of the rotor airfoils. The second passages can be blocked off by a Belleville washer. 
     EP 0 636 765 discloses guide vane boxes with passages from where a flow is injected into rotor airfoil inlets of a rotor cooling circuit. 
     SUMMARY 
     One of numerous aspects of the present invention includes a gas turbine by which the aforementioned problems of the known art are addressed. 
     Another aspect of the present invention includes a gas turbine having an increased efficiency when compared with traditional gas turbines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages of the invention will be more apparent from the description of a preferred but non-exclusive embodiment of the gas turbine according to the invention, illustrated by way of non-limiting example in the accompanying drawings, in which: 
         FIG. 1  shows a schematic cross section of a portion of a gas turbine according to the prior art; 
         FIG. 2  shows a schematic cross section of a portion of a gas turbine according to the invention; and 
         FIGS. 3 and 4  show a particular of guide vane boxes according to two embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     With reference to the figures, these show a gas turbine  1  having the combustion chamber  2  followed by the stator airfoil row  4  and the rotor airfoil row  6 . 
     The structure of the gas turbine is the same as that already described; it is thus not described again and with the same reference numbers the same elements are indicated. 
     In particular, the guide vane boxes  17  are provided with passages  30  connecting a zone  31  upstream of the guide vanes boxes  17  to a zone  32  of the second gap  7  downstream of the guide vanes boxes  17 . 
     In addition, the mouth  34  of the passages  30  facing the rotor airfoil row  6  is closer to a hot gases path  35  than the nozzles  20 . 
     The mouth  34  of the passages  30  facing the rotor airfoil row  6  is substantially as close as, or it is closer than, the rotor airfoil inlet  23  to the hot gases path  35 . This permits the flow going out from the mouth  34  not to be drawn from the rotor airfoil row  6  to enter the inlet  23 . 
     In a first embodiment ( FIG. 4 ), the passages  30  are defined by slots at sidewalls  36  of the guide vane boxes  17 . 
     In this embodiment, the two contacting sides of two adjacent guide vane boxes may be provided with the slot, such that the passages  34  are defined between two facing slots. 
     Alternatively only one of the two contacting sidewalls  36  of the adjacent guide vane boxes  17  may be provided with the slot, in which case the passages  30  are defined by the slot of a guide vane box  17  and the flat surface of the adjacent guide vane box  17 . 
     In a different embodiment ( FIG. 3 ), the passages  30  extend inside of the guide vane boxes  17  and are defined by pipes. 
     Naturally, in further embodiments the guide vane boxes may be provided with both the slot and the pipes. 
     In addition, a seal  37  is provided downstream of the mouths  38  of the passages  30  opposite the rotor airfoil row  6 , between the guide vane boxes  17  and the fixed frame  26 . This lets the leakage that may overcome the seals  25  be withheld in a zone separate from the rotor airfoil row  6 . 
     The operation of the gas turbine of the invention is apparent from that described and illustrated and is substantially the following. 
     The hot gases pass through the hot gases path  35  and thus they pass through the combustion chamber  2 , the stator airfoil row  4 , and the rotor airfoil  6 . 
     Through the first gap  5  compressed air (purge air) is supplied in the combustion chamber  2 . 
     A part of the compressed air (purge air) may leak, overcoming the seals  25  to enter the zone  31  upstream of the guide vane boxes  17 . 
     Thanks to its high pressure (greater than the pressure inside of the second slot  7 ), the compressed air (purge air) enters the passages  30  through the mouths  38 , passes through the passages  30  and moves out through the mouths  34  entering the second gap  7  in a zone from where it cannot enter the rotor airfoil inlet  23 ; thus the compressed air (purge air) enters the hot gases path  35 . 
     The additional seal  37  keeps this compressed air (purge air) in a zone adjacent to the mouth  38  of the passage and prevents the high temperature compressed air from being drawn from the high speed rotating rotor airfoil row  6 . 
     As shown in  FIG. 2 , the vane includes a platform  28  having an extension which extends to at least partly close the gap separating the rotor airfoil from the stator airfoil row. The mouths of the passages connecting a zone upstream of the guide vane boxes to a zone of the gap downstream of the guide vane boxes are below the platform extension on a side facing away from a hot gas flow path. The guide vane boxes each include an extension  27  which divides the gap into a radially inner section of the guide vane boxes and a radially outer section side facing the gap. Each of the passage mouths are positioned at a gap in a radially inner section. 
     The gas turbine conceived in this manner is susceptible to numerous modifications and variants, all falling within the scope of the inventive concept; moreover all details can be replaced by technically equivalent elements. 
     In practice the materials used and the dimensions can be chosen at will according to requirements and to the state of the art. 
     REFERENCE NUMBERS 
     
         
         
           
               1  gas turbine 
               2  combustion chamber 
               4  stator airfoil row 
               5  first gap 
               6  rotor airfoil row 
               7  second gap 
               8  third gap 
               9  annular gap 
               10  second burner 
               15  stator airfoils 
               16  endwalls of  15   
               17  guide vane boxes 
               20  nozzles 
               22  rotor airfoils 
               23  rotor airfoil inlet 
               25  seals 
               26  fixed frame 
               30  passages 
               31  zone upstream of the guide vane boxes 
               32  zone downstream of the guide vane boxes 
               34  mouth of  30   
               35  hot gases path 
               36  sidewalls of  17   
               37  seal 
               38  mouth of  30   
             A cooling air 
             A 2  air injected through  22   
             F hot gases flow 
           
         
       
    
     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.