Abstract:
A steam turbine is provided having a rotor which rotates around a machine axis, and a stator which concentrically encompasses the rotor with clearance, between which an annular passage is formed which is exposed to throughflow by steam in the axial direction and in which a multiplicity of rotor blades, which are fastened on the rotor, and fixed stator blades are arranged in a plurality of stages, wherein the stator blades of the last stage have a sweep with a sweep angle which changes in sign over the relative blade height.

Description:
FIELD OF INVENTION 
     The present invention refers to the field of rotating thermal machines, especially to a steam turbine, the stator blades of which have sweep angles which change in their sign over the blade height. 
     BACKGROUND 
     On the stator blades in the last stage of very large steam turbines, a supersonic flow predominantly prevails on the outlet side. The influence of the flow angle, which is customarily brought about as a result of the incidence angle of the stator blades, in this case is determined to a great extent as a result of the supersonic expansion and the shock fronts. In addition, the design engineer is confronted with the situation that the stage reaction degree is undesirably subjected to sharp radial changes. There are different ways of influencing these parameters, such as by applying a twisting of the blade profile over the blade height (“twist” in English), an inclination of the blade airfoil in the tangential or circumferential direction (“lean” in English), and the use of special supersonic profiles. Furthermore, the blade airfoil can have a sweep, or inclination in the meridional plane (“sweep” in English), in order to influence the flow. The latter inclination is typically specified by a sweep angle in the meridional plane (“sweep angle”) which is included by a radial to the rotor axis and by a tangent which lies against the blade airfoil in the meridional plane, wherein the sign of the angle measured from the radial is critical. However, other inherently dependent flow parameters are also altered in this case. In the case of large steam turbines, large variations of the hub radius and casing radius additionally lead to special flow structures which are to be implemented with a local inclination and incidence of the blades only with difficulty in order to achieve the set aims with regard to the radial flow angle variation and reaction degree variation. 
     For increasing the efficiency in the case of the stator blades in the last stage of a steam turbine, it has been proposed in U.S. Pat. No. 5,906,474 to provide rounded corners at the transitions between the blade airfoil and the base. 
     In contrast, in WO-A1-2005/005784 it is proposed to provide the turbine blades (both stator blades and rotor blades) with a sweep which is negative on the rotor-side and stator-side edge zones of the blade, and which changes into positive in the middle region in between. 
     SUMMARY 
     The present disclosure relates to a steam turbine with a rotor, which rotates around a machine axis, and a stator, which concentrically encompasses the rotor with clearance, between which an annular passage is formed which is exposed to throughflow by steam in the axial direction. A multiplicity of rotor blades, which are fastened on the rotor, and fixed stator blades are arranged in a plurality of stages, wherein the stator blades of the last stage have a sweep with a sweep angle that changes over the relative blade height. The sweep is formed with an S-shape with a sweep angle which both adjacent to the stator and adjacent to the rotor is positive, and at mid-height of the stator blade is negative. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is subsequently explained in more detail based on exemplary embodiments in conjunction with the drawing. In the drawings: 
         FIG. 1  shows a detail of the last stage of a developed steam turbine with a stator blade with S-shaped sweep according to an exemplary embodiment of the invention; 
         FIG. 2  shows the curves of the dependence of different sweep angles upon the relative height of the stator blades, with an S-shaped sweep, according to an exemplary embodiment of the invention; 
         FIG. 3   a  shows ranges for the inclination angle δ in dependence upon the relative height of the stator blades, with an S-shaped sweep, according to an exemplary embodiment of the invention; 
         FIG. 3   b  shows a view of the inclination angle of a chosen line of a blade of a steam turbine; 
         FIG. 3   c  shows values of the inclination angle δ at the trailing edge of a stator blade in a steam turbine according to the invention in dependence upon the blade height; 
         FIG. 4   a  shows the pitch ratio of a stator blade of a steam turbine according to the invention in dependence upon the relative blade height; 
         FIG. 4   b  shows a view of the pitch ratio as is applied in  FIG. 4   a.    
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Introduction to the Embodiments 
     It is the object of the invention to optimize a steam turbine of the type mentioned in the introduction to the effect that the stage reaction degree is improved. 
     The object is achieved by the features of claim  1 . It is a feature of the invention that the sweep of the stator blades of the last stage is formed with an S-shape by the sweep angle being positive both on the casing side (i.e. adjacent to the stator) and on the hub side (i.e. adjacent to the rotor), and by being negative at mid-height of the stator blade. In this case, a positive sweep angle is defined by the measurement direction in the clockwise sense from a radial to the rotor axis towards the tangent to the stator blade in the meridional plane, and a negative sweep angle is correspondingly defined by a measurement direction in the counterclockwise sense from a radial to the rotor axis towards the tangent to the stator blade. 
     The S-shaped sweep of the stator blades in the last stage of a steam turbine according to the invention is especially suitable for effectively controlling the pressure between the blades and the residual kinetic energy. The principal aerodynamic effect is that the stage reaction at the hub is reduced and on the other hand increased at the blade tip by the entry angle to the rotor blades being altered. The delivered output and the pressure distribution at the outlet of the subsequent rotor blade of the last stage follow the same pattern in this case. The kinetic energy is advantageously reduced. A feature of the invention provides an improved blade design without increase of the secondary flows. 
     In an embodiment, the stator blades of the last stage have a leading edge, a trailing edge and a center of gravity line, wherein the leading edge, the trailing edges and the center of gravity line have a sweep in each case, which according to the invention is S-shaped. In particular, the S-shaped progression of the sweep angle in dependence upon the relative blade height is homogeneous for the leading edge, the trailing edge and the center of gravity line in the sense of the progression of the sweep angle over the blade height from the positive to the negative, and to the positive again. 
     In a further embodiment, the stator blades of the last stage have a leading edge line, a trailing edge line and center of gravity line, wherein two of these three lines have a sweep which according to the invention is S-shaped. In this case, the S-shaped progression of the sweep of two of the three lines is again homogeneous in the aforementioned sense. 
     In another embodiment, the stator blades of the last stage have a leading edge line, a trailing edge line and a center of gravity line, wherein either the leading edge, trailing edge or the center of gravity line has a sweep which according to the invention is S-shaped. 
     In a further embodiment, the trailing edge of the stator blades has a sweep in each case which according to the invention is S-shaped. This development has the special advantage that the effect with regard to stage reaction is extremely great. 
     In a still further embodiment, the sweep angle on the hub side, i.e. adjacent to the rotor, and on the casing side, i.e. adjacent to the stator, lies in the range of between 10° and 25°, and at mid-height lies in the range of between 0° and −15°. Furthermore, the sweep angle is preferably negative in a range of the relative blade height of 30 to 80%. 
     In another embodiment, the sweep angle on the hub side, i.e. adjacent to the rotor, and on the casing side, i.e. adjacent to the stator, lies in the range of between 13° and 20°, and at mid-height lies in the range of between −6° and −10°. Furthermore, the sweep angle is preferably negative in a range of the relative blade height of 30 to 80%. 
     For simultaneous influencing of the flow parameters, an embodiment, in addition to the sweep in the stator blades of the last stage, has an inclination in the circumferential direction, i.e. in the direction tangentially to the rotor of the steam turbine (“lean”), wherein in particular the inclination angle varies over the blade height, for example having a particularly increasingly negative inclination angle which decreases from the hub towards the casing. In this case, positive inclination angles, as seen in the direction of the steam flow through the turbine, are defined by the angle from a radial to the rotor axis in the clockwise sense to the tangent to the stator blade, and negative inclination angles are correspondingly defined by the angle from a radial in the counterclockwise sense to the rotor axis to the tangent to the stator blade. 
     In a further embodiment, the inclination angle of the trailing edge line varies in the tangential or circumferential direction, and with regard to a radial to the turbine rotor axis varies between +5° and −5° on the hub side and between −10° and +5° on the casing side. 
     In a further embodiment, the stator blade additionally has a pitch ratio which varies radially to the rotor axis, wherein this pitch ratio increases as the blade height increases. The pitch ratio is equal to the ratio of pitch or distance between adjacent stator blades in a blade ring along the circumferential line to the length of the chord of a stator blade. In particular, with a blade height of zero at the hub the pitch ratio has a value in the range of 0.4 to 0.5, and with a maximum blade height at the casing at the blade root has a value in the range of 0.45 to 0.65. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a detail of the last stage of a steam turbine  10  which comprises a rotor  11 , with rotor blades  14 ,  15 , which is rotatable around a turbine rotor axis  17  and concentrically encompassed with a clearance by a stator  12 . Between consecutively following rows of rotor blades  14 ,  15 , a row of fixed stator blades  16  is arranged, by which the steam flow  20  which flows in the annular passage  13  between rotor  11  and stator  12  is deflected for the following rotor blade row  15 . 
     According to the invention, the stator blades  16  have a sweep with a sweep angle α 1 , α 2 , α 3  which changes over the relative blade height (the blade height h begins at the hub and refers to the maximum height h m ). The angles are measured beginning on the radial which is perpendicular to the turbine rotor axis  17  and ending on the meridional tangent to a reference line on the stator blade  16 , in this case the trailing edge  16   a  of the blade. The angles are positive if the measurement direction is in the clockwise sense, and negative if the measurement direction is in the counterclockwise sense. According to the invention, the sweep angle α 1  adjacent to the rotor (Position P 1 ) and α 3  adjacent to the stator (Position P 3 ) is positive, wherein the sweep angle α 2  at a mid-height of the stator blade  16  (Position P 2 ) is negative. As a result of the changing sign of the angles over the blade height, a sweep formed with an S-shape is created. In the exemplary embodiment which is shown, the sweep is formed with an S-shape both on the trailing edge  16   a , on the leading edge  16   b , and along the center of gravity line of the stator blade  16 . 
     In further embodiments, as mentioned previously, just one of the three reference lines (trailing edge, leading edge, center of gravity line) of the blade, or two of the three reference lines, can have a sweep of this type. 
     A preferred development of the sweep is reproduced in  FIG. 2  based on the sweep angle of the leading edge (β), of the trailing edge (α) and of the center of gravity line (ε) in dependence upon the relative blade height h/h m . The sweep is S-shaped in all three cases. In particular, the curve progression of the sweep angle β, or γ, or α is homogeneous in dependence upon the relative blade height h/h m  for the leading edge  16   b , for the trailing edge  16   a  and for the center of gravity line. The sweep angles α, β and ε adjacent to the rotor  11  (Position P 1 ) and adjacent to the stator  12  (Position P 3 ) lie in the range of between 13° and 20°. At mid-height (Position P 2 ), the sweep angle lies in the range of between −6° and −10°. As can be seen from  FIG. 2 , the sweep angle is negative in a range of the relative blade height h/h m  of 30% to 80%. 
     In a further development, the sweep angle at mid-height (Position P 2 ) is in the range of between 0° and 15°, and the sweep angle in the edge zones (Positions P 1  and P 3 ) is in the range of between 10° and 25°. 
     While the sweep of the stator blades  16  are in reference to a meridional plane which passes through the machine axis or turbine rotor axis  17 , the stator blades  16  are inclined in the circumferential direction, that is to say in a plane which is perpendicular to the machine axis  17 , and with regard to a radial to the turbine rotor axis  17 . In the case of an inclination of this type which varies over the blade height and changes in sign, a curvature is created.  FIG. 3   a  shows the ranges in which the inclination angle over the relative blade height can move for an exemplary embodiment of the invention. With a blade height of zero (at the hub), the inclination angles are in a range of between −5° and +5°, and with a full blade height are in the range of between −10° and +5°.  FIG. 3   b  shows inclination angles δ 1 , δ 2 , δ 3  and δ 4 , shown at different blade heights, for a chosen line or edge of a blade. The angles are measured in each case beginning on a radial through a point on the selected line and ending on the tangent to the line at the point concerned, wherein an angle which is measured in the clockwise sense is positive (δ 4 ) and angles which are measured in the counterclockwise sense are negative (δ 1 , δ 2 , δ 3 ). In one exemplary embodiment of the invention, the local inclination angles δ are reproduced in  FIG. 3   c  in dependence upon the relative height h/h m  for the trailing edge  16   a.    
     In a further exemplary embodiment of the invention, the stator blade of the steam turbine additionally has a pitch ratio which varies with the blade height and is defined by the ratio of the pitch between adjacent stator blades and the chord length.  FIG. 4   a  shows some stator blades of a blade ring projected onto the plane, and also the pitch T between adjacent stator blades  16 , and also the chord length S, which are critical for the pitch ratio T/S.  FIG. 4   b  shows the value ranges of the pitch ratio T/S as a function of the relative blade height h/h m . With a minimum blade height, the pitch ratio moves in a range of between 0.4 and 0.5, and with a maximum blade height moves in a range of between 0.45 and 0.65. 
     LIST OF DESIGNATIONS 
     
         
           10  Steam turbine 
           11  Rotor 
           12  Stator 
           13  Annular passage 
           14 ,  15  Rotor blade 
           16  Stator blade 
           16   a  Trailing edge of the stator blade 
           16   b  Leading edge of the stator blade 
           17  Machine axis 
           20  Steam flow 
         h Blade height 
         h m  Max. blade height 
         δ 1 , δ 2 , δ 3 , δ 4  Inclination angle 
         S Chord length 
         P 1 , . . . , P 3  Position 
         β Sweep angle (leading edge) 
         ε Sweep angle (center of gravity line) 
         α, α 1 , α 2 , α 3  Sweep angle (trailing edge)