Abstract:
A rotor blade for a turbomachine, particularly a steam turbine is provided. The rotor blade includes a turbine blade and a blade base, wherein the turbine blade has a suction side and a pressure side, as well as an inflow edge and an outflow edge, wherein an erosion protection shield is disposed at a distance in front of the outflow edge in order to avoid liquid impingement erosion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2008/066307, filed Nov. 27, 2008 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 07024843.0 EP filed Dec. 20, 2007. All of the applications are incorporated by reference herein in their entirety. 
       FIELD OF INVENTION 
       [0002]    The invention relates to a rotor blade comprising a main blade part and a blade root, wherein the main blade part has a suction side and a pressure side and also a leading edge and a trailing edge. 
       BACKGROUND OF INVENTION 
       [0003]    Rotor blades and guide vanes, inter alia, are used in turbomachines. The collective term “turbomachines” encompasses water turbines, steam and gas turbines, wind wheels, centrifugal pumps and centrifugal compressors and also propellers. All these machines share the common factor that they serve to extract power from a fluid in order to thereby drive another machine, or vice versa to supply power to a fluid in order to increase the pressure thereof. 
         [0004]    In a steam turbine, as an embodiment of a turbomachine, the fluid used is steam. This fluid is also referred to as flow medium. It is conventional for the steam to initially flow into a high-pressure partial turbine, where this steam is at a temperature of up to 620° C. and a pressure of up to 320 bar. After it has flowed through the high-pressure partial turbine, the flow medium flows through a medium-pressure partial turbine and finally through a low-pressure partial turbine. Here, the pressure and the temperature of the steam decrease. During expansion of the steam in the low-pressure partial turbine, droplets of mist may form as a result of spontaneous condensation; these are also referred to as primary droplets and are very small. Primary droplets of this type grow to diameters of about 0.2 μm. These primary droplets accumulate on the guide vanes and rotor blades and, owing to a film of water, form a larger secondary droplet having a diameter of up to about 400 μm. Even larger water droplets are not stable in the steam turbine flow since they are atomized again. 
         [0005]    These droplets cause what is known as drop impingement erosion, during which material may be removed when a droplet impinges on the rotor blade. 
         [0006]    In addition, locally negative axial speeds may arise in the region of the rear edge of the rotor blade for various operating points of the low-pressure partial turbine, for example during part-load operation. This movement of the steam has the result that water droplets present in the steam flow back into the blading. Here, the water droplets have such a low circumferential component that they impinge on the rear edge of the rotor blade profile on the suction side at a high relative speed and thereby lead to considerable erosion damage. This results in considerable damage to the blading. 
         [0007]    In order to prevent damage of this type, it is firstly known to minimize the water droplets present in the steam by appropriate operating ranges. In addition, thickening of the rear edges of the blade may be considered. It is likewise known to carry out grinding measures if there is a relatively large amount of erosion damage in the region of the rear edges. It is also known to harden the rear edge in order to increase the resistance of the blade. Finally, it is also known to prevent the formation of the secondary droplets by means of suction devices and targeted axial gap design. 
         [0008]    It would be desirable for there to be a simple way of preventing damage to the rotor blades caused by the water droplets which occur during part-load operation and have locally negative axial speeds. 
       SUMMARY OF INVENTION 
       [0009]    This is where the invention becomes relevant. It is an object of the invention to specify a simply way of preventing damage to a rotor blade burdened with water droplets which, during part-load operations, have a locally negative axial speed in the region of the rear edge of the rotor blade and thereby impinge on the rear edge of the rotor blade. 
         [0010]    This object is achieved by a rotor blade comprising a main blade part and a blade root, wherein the main blade part has a suction side and a pressure side and also a leading edge and a trailing edge, wherein an anti-erosion shield is arranged in front of the trailing edge in order to prevent drop impingement erosion. 
         [0011]    The invention proposes the use of a further component in addition to the rotor and the rotor blades. This further component is an anti-erosion shield, which is arranged in front of the trailing edge in such a way that the water droplets which occur during part-load operation do not impinge on the trailing edge of the rotor blade, but instead on the anti-erosion shield. Accordingly, the water droplets can no longer cause damage to the trailing edge of the rotor blade since they are prevented from impinging on the rotor blade from the outset, since they impinge on the anti-erosion shield and are thereby decelerated or dissolved. These measures according to the invention mean that the measures known in the prior art for preventing damage caused by water droplets can be dispensed with. In particular, the invention has the advantage that next to no changes have to be made to the existing rotor blade. The only change to the rotor blade consists in making it possible to accommodate or arrange the anti-erosion shield therein. In this context, the anti-erosion shield is arranged in such a way that water droplets which occur during part-load operation and have a locally negative axial speed in the region of the rear edge of the rotor blade cannot impinge on the rear edge of the rotor blade. Damage is thus prevented from the outset. 
         [0012]    The anti-erosion shield is spaced apart from the trailing edge of the blade root. In this case, the distance between the anti-erosion shield and the trailing edge should be 
         [0013]    selected such that the flow of the steam does not experience any losses as it expands in a turbine stage 
         [0014]    In one advantageous development of the invention, the anti-erosion shield is oriented along the longitudinal orientation of the main blade part. The damage appears primarily near the blade root and propagates in the longitudinal direction of the blade. Orientation of the anti-erosion shield along the longitudinal orientation therefore prevents further damage. 
         [0015]    In one advantageous development, the rotor blade has a length L, the length of the anti-erosion shield being selected such that the length is 1%-100% of the length L of the rotor blade. 
         [0016]    In one advantageous development, the main blade part has a chord length S, the anti-erosion shield being configured in such a way that the width of the anti-erosion shield is about 5% to 75% of the chord length S. Accurate adaptation to the operating conditions of the turbomachine is possible as a result of the targeted selection of the size of the anti-erosion shield. 
         [0017]    In one advantageous development, the main blade part has a pressure side and a suction side, the anti-erosion shield being arranged in front of the suction side. It has been found that the most damage occurs on the suction side of the main blade part. It is therefore expediently proposed to arrange the anti-erosion shield in front of said suction side. 
         [0018]    In one advantageous development, the anti-erosion shield is connected to the blade root in a nonpositively locking manner. It is likewise expedient to connect the anti-erosion shield to the blade root integrally or in a positively locking manner. 
         [0019]    A nonpositively locking connection is obtained by the application of force produced by means of a suitable prestress. By way of example, the cohesion of the nonpositively locking connection can be ensured purely by static friction. By contrast, positively locking connections are obtained by at least two connection partners engaging one into the other. Here, the positive locking is brought about by forces which arise as a result of operating conditions. Integral connections are defined by connection partners which are held together by atomic or molecular forces. 
         [0020]    In one advantageous development, the anti-erosion shield and the rotor blade are formed as a single, integral component. As a result of this measure, the anti-erosion shield can be connected to the turbine blade by way of a comparatively large binding force or retaining force. 
         [0021]    In a further advantageous development, the anti-erosion shield has a rear edge and a front edge, the rear edge protruding beyond the trailing edge of the rotor blade. This has the result that the anti-erosion shield covers, as it were, a larger region, as a result of which the droplets are prevented from impacting on subsequent rotor blades. This makes it possible to reduce the anti-erosion shields over the entire blade ring. It would thereby be possible to reduce the number of anti-erosion shields in order to thereby produce a turbomachine at lower cost. 
         [0022]    In one advantageous development, the anti-erosion shield has a turbine profiling with a suction side and a pressure side. Like the rotor blade, the anti-erosion shield is thereby able to convert the thermal energy of the steam into kinetic energy. 
         [0023]    The anti-erosion shield is preferably formed from an erosion-resistant material, e.g. stellite, Ultimet, α-titanium or β-titanium or hardened steel. 
         [0024]    The anti-erosion shield advantageously has a dovetail root, the blade root being designed to receive the dovetail root. This is a very simple and inexpensive way of securing the anti-erosion shield to the turbine blade root. 
         [0025]    In a further advantageous development, the anti-erosion shield is curved around the longitudinal axis. Depending on the flow conditions of the flow medium which are present, this may improve the degree of efficiency. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The invention is explained in more detail, by way of example, with reference to the drawings. Identical reference symbols have the same meaning in the various figures. In the drawings, in a partially schematic manner and not to scale: 
           [0027]      FIG. 1  shows a perspective view of part of a turbine stage, 
           [0028]      FIG. 2  shows a further perspective view of part of a turbine stage, 
           [0029]      FIG. 3  shows a side view of a rotor blade with anti-erosion shield, 
           [0030]      FIG. 4  shows a perspective view of part of a rotor blade with blade root, and 
           [0031]      FIG. 5  shows a side view of the anti-erosion shield. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0032]      FIG. 1  shows a perspective view of part of a turbine stage  1 . The turbine stage  1  comprises a plurality of rotor blades  2  which are arranged about a common axis of rotation (not shown in more detail in  FIG. 1 ) in a rotor  3 . During operation, the rotor blades  2  rotate at a rotational speed of up to 3600 revolutions per minute. The rotor blade  2  has a main blade part  4  and also a blade root  5 . The main blade part  4  is profiled and has a suction side and a pressure side  7  (not visible in  FIG. 1 ). Furthermore, the rotor blade  2  has a leading edge  8  (not visible in  FIG. 1 ) and a trailing edge  9 . The blade root  5  is held on the rotor  3  by way of a Laval root, straddle root, plug-in root, inverted T root, saw tooth root or fir-tree root.  FIGS. 1 and 2  show a fir-tree root as an example. 
         [0033]    An anti-erosion shield  10  is arranged on the rotor blade  2  at the blade root  5 . The anti-erosion shield  10  is formed from an erosion-resistant material, e.g. stellite, Ultimet, α-titanium or β-titanium or hardened steel, the anti-erosion shield  10  being arranged in front of the trailing edge  9  in order to prevent drop impingement erosion. 
         [0034]    The rotor blade  2  is formed along a longitudinal orientation  11 , the anti-erosion shield  10  likewise being oriented along this longitudinal orientation  11 . The longitudinal orientation  11  is substantially identical to the radial direction perpendicular to the axis of rotation (not shown in more detail). 
         [0035]    The anti-erosion shield  10  is spaced apart from the trailing edge  9  by a distance d. Here, the distance d is selected such that it results in small flow losses in the turbine stage  1 . 
         [0036]    The rotor blade  2  has a length L. Here, the length of the anti-erosion shield  10  is 1% to 100% of the length L. 
         [0037]    The main blade part  4  has a chord length S, the width B of the anti-erosion shield  10  being 5% to 75% of the chord length S. 
         [0038]    The anti-erosion shield  10  is connected to the blade root  5  in a nonpositively locking manner. For this purpose, the blade root  5  has a dovetail-root-like recess  12  into which the anti-erosion shield  10 , which has a dovetail root  13 , can be inserted. 
         [0039]    In alternative embodiments, the anti-erosion shield  10  is connected to the blade root  5  integrally or in a positively locking manner. 
         [0040]    As can be seen in  FIGS. 1 ,  2  and  4 , the anti-erosion shield  10  is arranged in front of the suction side  6  of the main blade part  4 . The dovetail root  13  has a straight form. In alternative embodiments, the dovetail root  13  can have a curved form (not shown in  FIG. 4 ). In  FIG. 4 , the recess  12  for the dovetail root  13  has a straight form and is directed substantially virtually parallel to the suction side  6  on the trailing edge  9 . 
         [0041]    In an alternative embodiment, the anti-erosion shield  10  and the rotor blade  2  can be formed from a single, integral component. This can be carried out by close-tolerance finish forging, investment casting, envelope forging with subsequent milling, milling, erosion or other known processes. 
         [0042]      FIG. 2  shows a perspective view of part of a turbine stage  1 . The anti-erosion shield  10  is shown in the installed state. 
         [0043]      FIG. 3  shows a side view of the turbine stage  1 . The anti-erosion shield  10  has a front edge  14  and a rear edge  15 . Here, the anti-erosion shield  10  is arranged on the blade root  5  in such a manner that the rear edge  15  protrudes beyond the trailing edge  9 . 
         [0044]      FIG. 5  shows a side view of the anti-erosion shield  10 . The anti-erosion shield  10  is formed with a rectangular or a triangular profile, as seen in cross section, in longitudinal orientation  11 . In an alternative embodiment, the anti-erosion shield  10  has a turbine profiling with a suction side and a pressure side (not shown in  FIG. 5 ). The anti-erosion shield  10  can be formed such that it curves around the longitudinal orientation  11 , and this results in a curved dovetail root  13  which is arranged in a curved recess  12 . 
         [0045]    The rear edge  15  of the anti-erosion shield  10  protrudes beyond the trailing edge  9  by a distance  1 . 
         [0046]    In an alternative embodiment, the anti-erosion shield  10  can be arranged directly on the rotor  3  (not shown in  FIGS. 1 to 5 ). 
         [0047]    In an alternative embodiment, the anti-erosion shield  10  can be equipped with support fins. The support fins are formed in such a manner that they are supported on the blade profile. This increases the range in which the anti-erosion shield  10  can be used. The support fins are not shown in more detail in the figures.