Patent Publication Number: US-2011070074-A1

Title: Gas turbine with a shroud and labyrinth-type sealing arrangement

Description:
This application claims priority to German Patent Application DE102009042857.7 filed Sep. 24, 2009, the entirety of which is incorporated by reference herein. 
     This invention relates to a gas turbine having a shroud and labyrinth-type sealing arrangement. 
     More particularly, the present invention relates to an axial-flow turbine of a gas-turbine engine provided with at least one bladed rotor having—arranged on its outer periphery—a segmental shroud joining the blade tips. 
     The term “casing” as used in this invention refers to a non-rotating component including any tubular component which is not a casing as such. The component referred to as a casing in the present invention can be an arrangement of structural elements, a ring or a part of the actual turbine casing. 
     On the turbine of a gas-turbine engine, power is extracted from the gas flow released from the combustion chamber to thereby produce a torque to drive a compressor. For this purpose, the turbine has a number of stationary vanes and at least one rotor provided with a blade row. The number of these components is variable according to the present invention. 
     The rotor moves relatively to the casing, with the required free movability of the rotor being provided by a gap between the rotor and the casing. This gap invariably involves a certain amount of leakage flow entailing, in particular, two negative effects compromising the efficiency of the turbine. Firstly, the leakage flow reduces the gas mass flowing through the rotor, thereby decreasing the power extracted. Secondly, the flow caused by the gap affects the actual flow through the turbine as they have different angles and different local velocities. This results in a mixing of both flows and a reduction of the aerodynamic efficiency. The leakage flow also leads to a degradation of the inflow into a subsequent stator vane row. 
     From the state of the art it is known to reduce the leakage flow by use of suitable sealing measures in the form of a labyrinth-type sealing arrangement.  FIGS. 3 and 4  schematically show a blade root  7  that is part of a rotor  9 . Arranged on the respective blade root  7  is a blade  2 . The radially outward end areas of the blades  2  are joined by a shroud  3  being essentially annular and composed of, for example, individual segments. 
     The shroud  3  is provided with several, essentially radially outwardly extending sealing fins  5 .  FIGS. 3 and 4  each show three such sealing fins  5 . 
     The sealing fins  5  together with the wall of a casing  1  form a labyrinth seal  4 . The dashed lines in  FIGS. 3 and 4  show the flow through the labyrinth seal. While a stepped casing contour with casing steps  8  is provided in  FIG. 3 ,  FIG. 4  shows a conically widening casing cross-section (in axial cross-sectional view). Accordingly,  FIGS. 3 and 4  show a meridional section of the turbine rotor and the casing. 
     The annular fins  5  can be oriented purely radially, but are also axially inclinable, as shown in  FIGS. 3 and 4 . Furthermore, it is known from the state of the art to provide varying numbers of such annular fins  5 . 
     The dashed lines in the representations of  FIGS. 3 and 4  show that the leakage flow, although being obstructed in the labyrinth seal  4 , still passes the latter to an amount that is capable of producing the detriments described earlier. This applies in particular where design or thermal operating conditions entail a larger radial spacing between the shroud and the casing. 
     In a broad aspect, the present invention provides a gas turbine with a shroud and labyrinth-type sealing arrangement, which, while being simply designed and cost-effectively producible, avoids the disadvantages of the state of the art and ensures optimum sealing efficiency. 
     According to the present invention, it is therefore provided that, on the casing, at least one sealing fin is arranged which extends essentially radially inwardly to the shroud at least over a part of the circumference and at least partly in a space between the annular fins. 
     The sealing fin, which according to the present invention can be a closed axial ring or circumferentially segmented, therefore is a flow barrier additionally obstructing the leakage flow. Thus, the flow mass of the leakage flow is reduced. 
     The sealing fins according to the present invention can have a circumferentially constant or varying cross-section. The cross-section can, for example, be rectangular with sharp edges, but it is also possible to provide a rounded cross-section or an additional leakage flow barrier by way of a special cross-sectional configuration. Furthermore, the cross-section of the sealing fins can be such that the sealing fins are also arranged axially inclined. Here, one side or both sides are angularly inclinable relative to the axial direction. Also, the radially situated end face can be slanted or straight. Also providable is a stepped cross-section or a parallelogrammatic cross-section. 
     According to the present invention, the sealing fins can be either integral with the casing or provided as separate items connected to the casing, for example as annular segments or the like. 
     The present invention is applicable to both conically smooth, widening casing cross-sections and stepped casing cross-sections (in each case relative to an axial sectional view). 
     Accordingly, the present invention is advantageous in that the leakage flow through the annular gap between the surface of the shroud and the casing is considerably obstructed. The jet-type flow, which is accelerated upon passing the annular fins, is significantly obstructed and deflected with regard to the flow direction as it impinges on the sealing fins according to the present invention. Thus, kinetic energy of the leakage flow is dissipated, resulting in a reduced mass of the leakage flow. Similarly, the leakage flow passing the sealing fins is obstructed upon impinging on the axially subsequent sealing fin of the casing or shroud, with flow energy here also being dissipated. Accordingly, a considerable obstruction of the leakage flow is provided by one sequence or several sequences of annular fins and sealing fins, resulting in an improvement of the sealing efficiency. Thereby, the negative effects described earlier are reduced, allowing the blade areas to be flown with less disturbance and better efficiency. This provides for an overall higher efficiency of the turbine. 
    
    
     
       The present invention is more fully described in light of the accompanying drawings, showing preferred embodiments. In the drawings, 
         FIG. 1  is a schematic axial sectional view of a first embodiment of the present invention; 
         FIG. 2  is a view, analogically to  FIG. 1 , of a modified embodiment of the present invention; 
         FIG. 3  (Prior Art) is an illustration applicable to the state of the art, analogically to  FIG. 2 ; and 
         FIG. 4  (Prior Art) is an illustration applicable to the state of the art, analogically to  FIG. 2 . 
     
    
    
     In the embodiments, like parts carry the same reference numerals. 
       FIGS. 1 and 2  show, analogically to  FIGS. 3 and 4 , a casing  1  which can be provided with casing steps  8  ( FIG. 1 ) or have a smooth, conically widening surface ( FIG. 2 ). 
     As regards the description of rotor  9 , the blades  2 , the shroud  3  and the annular fins  4 , reference is made to the description of  FIGS. 3 and 4  applicable to the state of the art. 
       FIGS. 1 and 2  show in dashed representation the flow through the gap between the shroud  3  and the surface of the casing  1 . 
     According to the present invention, sealing fins  6  extending essentially in the radially inward direction are arranged in the spaces between axially spaced annular fins  5 . Accordingly, in one embodiment, a sealing fin  6  is disposed in each space. According to the examples of  FIGS. 1 and 2 , the sealing fins  6  have rectangular cross-section. However, the cross-section is also variable, both circumferentially and in departure from the rectangular cross-section. 
     The representations in  FIGS. 1 and 2  convey that the sealing fins  6  effect a considerable obstruction of the flow and a significant mass flow reduction of the leakage flow through the labyrinth seal  4 . 
     According to the present invention, the sealing fins  6  are provided such that they obstruct the flow without contacting the shroud  3  and/or the annular fins  5 . Accordingly, they do not form elements of a rubbing-contact sealing arrangement, but rather a defined distance is provided between the sealing fins  6  and the shroud  3  and/or the annular fins  5 . 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Casing 
           2  Blade 
           3  Shroud 
           4  Labyrinth seal 
           5  Annular fin/shroud fin/shroud sealing fin 
           6  Sealing fin/casing fin/casing sealing fin 
           7  Blade root 
           8  Casing step 
           9  Rotor