Abstract:
In a method for influencing, in particular damping or suppressing, mechanical vibrations occurring during operation in a turbomachine blade ( 10 ), the mechanical vibratory energy of the turbomachine blade ( 10 ) is first converted into electrical energy and the electrical energy generated is then converted into heat loss. Effective damping which can be used especially simply and in a versatile way is achieved in that the piezoelectric effect is used in order to convert mechanical vibratory energy into electrical energy.

Description:
This application claims priority under 35 U.S.C. §119 to Swiss application number 01966/10, filed 24 Nov. 2010, the entirety of which is incorporated by reference herein. 
     BACKGROUND 
     1. Field of Endeavor 
     The present invention relates to the field of turbo-machines, such as, for example, wind turbines, steam turbines, gas turbines, and compressors. It relates to a method for influencing, in particular damping or suppressing, mechanical vibrations occurring during operation in a turbomachine blade. It also relates to a turbomachine blade useful for carrying out the method and to a piezoelectric damping element to be installed in such a turbomachine blade. 
     2. Brief Description of the Related Art 
     When turbines (or compressors) are in operation, asynchronous and synchronous blade vibrations may be generated due to aerodynamic effects (for example, fluttering) or to mechanical effects (for example, on account of the friction of the blades against the casing). Resonances in the blade may lead to problems of vibratory crack formation (“high-cycle fatigue” HCF) which constitutes a critical type of failure for turbine and compressor blades. 
     In order to protect turbomachine blades against such HCF faults, the blade leaves are coupled to integral shroud elements or winglets which increase the rigidity of the blade arrangement of a turbine stage and damp or suppress vibrations due to friction between the adjacent blades. Arrangements of this type are known, for example, from the publication EP 0 214 393 A1 or U.S. Pat. No. 3,752,599. 
     If the turbine concept requires free-standing blades, friction-generating devices may be arranged on the blades beneath the platforms or between the blades as friction pins or may be accommodated inside the blades. Such solutions are known, for example, from the publication EP 1 538 304 A2 or U.S. Pat. Nos. 4,460,314 and 6,979,180 B2. 
     The damping effect of such friction-generating devices and frictional couplings depends, however, upon the optimal normal force and rigidity of the coupling contact which have to be coordinated suitably with the relevant resonant frequency (to be damped). In other words, frictional damping can be used effectively only for a specific vibratory frequency of the blade, whereas other frequencies are damped inadequately or not at all. 
     Frictional dampers are highly dependent upon amplitude, and any variation in rigidity or in the mass system involved on account of abrasion or other modifying processes in the system results in changed resonant frequencies which are detrimental to the effectiveness of frictional dampers. 
     However, it has already been proposed (see, for example, the publication EP 0 727 564 A1), to damp vibrations of turbine blades in that, as a result of the interaction of permanent magnets with the blades, eddy currents are generated which are converted into heat loss. The range of use of such solutions is narrowly limited, however, because interaction is restricted to the region between the blade tip and the opposite casing wall. Vibrations occurring inside the blade leaf therefore cannot be effectively damped. 
     Furthermore, it is known (see, for example, the publication JP 2003138904), for actively controlling frictional coupling between adjacent turbine blades, to insert in the blades piezoelectric elements by which frictional contact during operation can be optimized and readjusted. This solution is not suitable for free-standing blades. However, piezoelectric elements of this type may also be used in order to measure and monitor the contact pressure of such frictional couplings (JP 2003138910). 
     Overall, the friction-based damping systems are complicated in terms of setup and use and can be frequency-tuned only with difficulty, while the principle based on the generation of eddy currents can be used to only a very limited extent. 
     SUMMARY 
     One of numerous aspects of the present invention includes a method of the aforementioned type which can avoid disadvantages of known methods and can be distinguished by simple and very broad applicability. 
     Another aspect includes a turbomachine blade useful for carrying out the method. 
     Yet another aspect includes a piezoelectric damping element to be installed in a turbomachine blade. 
     Another aspect includes influencing, in particular damping or suppressing, mechanical vibrations occurring during operation in a turbomachine blade. Mechanical vibratory energy of the turbomachine blade is first converted into electrical energy and the electrical energy generated is then converted into heat loss. Methods embodying principles of the present invention can be distinguished in that the piezoelectric effect is used in order to convert mechanical vibratory energy into electrical energy. 
     A refinement can include that, in order to convert mechanical vibratory energy into electrical energy, at least one piezoelectric damping element is installed firmly in the turbomachine blade to be damped, the damping element being deformed as a result of the mechanical vibrations of the turbomachine blade and generating an electrical voltage, and ohmic heat loss is produced by the generated electrical voltage in a connected electrical network. 
     In particular, the electrical network includes as a shunt a series connection composed of an inductance and of an ohmic resistance, the piezoelectric damping element including in the manner of a capacitance, a piezo body arranged between electrodes, and the resonant frequency of the resulting oscillatory circuit being tuned to the mechanical vibration, to be damped, of the turbomachine blade. 
     Furthermore, it is advantageous if the electrical oscillations in the piezoelectric damping element are measured and evaluated in order to determine and monitor the mechanical vibrations in the turbomachine blade. 
     Another refinement includes that a cavity is introduced into the turbomachine blade in order to install the piezoelectric damping element, and in that the piezoelectric damping element is inserted into the cavity and is firmly coupled there to the turbomachine blade, in particular by materially integral connection or mechanical bracing. 
     Turbomachine blades embodying principles of the present invention can be characterized in that at least one piezoelectric damping element is firmly installed in the turbomachine blade. 
     A refinement includes that the at least one piezoelectric damping element is designed as a piezo element which is connected to an electrical network and, in particular, is damped electrically by a shunt containing an ohmic resistance. 
     In particular, the electrical network or the shunt includes a series connection composed of the ohmic resistance and of an inductance. 
     In this case, the electrical network or the shunt may preferably include a coil. 
     According to another refinement, the piezo element includes a stack of piezo bodies and electrodes. 
     Another refinement includes that the turbomachine blade has formed in it a cavity in which the at least one piezoelectric damping element is accommodated. 
     In particular, the at least one piezoelectric damping element is surrounded by a dedicated casing which is preferably composed of an upper and a lower subcasing. 
     According to another refinement, the at least one piezoelectric damping element is firmly coupled to the turbomachine blade, in particular by materially integral connection or mechanical bracing. 
     A further refinement is characterized in that a plurality of piezoelectric damping elements are arranged in the turbomachine blade so as to be distributed over said turbomachine blade. 
     Yet another refinement is distinguished in that the at least one piezoelectric damping element is connected wirelessly to a measuring device for receiving and evaluating the signals occurring at the piezoelectric damping element. 
     A piezoelectric damping element to be installed in a turbomachine blade is characterized in that the piezoelectric damping element is designed as a piezo element which is connected to an electrical network and, in particular, is damped electrically by a shunt containing an ohmic resistance. 
     A refinement of the piezoelectric damping element is characterized in that the piezo element includes a stack of piezo bodies and electrodes. 
     Another refinement is characterized in that the electrical network or the shunt includes a series connection composed of the ohmic resistance and of an inductance. 
     In particular, the electrical network or the shunt includes a coil which may have any desired form. 
     According to another refinement, the piezoelectric damping element is surrounded by a casing which is preferably composed of an upper and of a lower subcasing. 
     Finally, a time-variable resistance may be provided in the electrical network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail below by reference to exemplary embodiments, in connection with the drawings, in which: 
         FIG. 1  shows a perspective view of an exemplary turbine blade with piezoelectric damping elements arranged inside it, according to an exemplary embodiment of the invention; 
         FIG. 2  shows an equivalent circuit diagram of an installed piezo element with shunt, with one mechanical degree of freedom; 
         FIG. 3  shows a sectional view of an exemplary embodiment of a piezoelectric damping element according to principles of the invention; 
         FIG. 4  shows an example of the installation of a piezoelectric damping element according to  FIG. 3  in a turbine blade; 
         FIG. 5  shows a perspective view of a piezoelectric damping element according to  FIG. 3 , including a coil in the shunt, said damping element being accommodated in a two-part casing; 
         FIG. 6  shows an example of the installation of the element from  FIG. 5  in a turbine blade, comparable to  FIG. 4 ; 
         FIG. 7  shows several sub-figures ( a ) to ( c ) of various steps in the installation of a piezo-electric damping element in a turbine blade according to an exemplary embodiment of the invention, and 
         FIG. 8  shows a basic arrangement for the transmission and evaluation of signals from piezoelectric damping elements installed in a turbine blade. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Principles of the present invention are based primarily on the piezoelectric effect. The piezoelectric effect is the ability of a material to form an electrical charge when it is acted upon by external forces, that is to say, ultimately, to convert mechanical energy into electrical energy. If the locations at which the voltage generated occurs are connected to an electrical network with ohmic resistance, part of the electrical energy generated can be converted into heat loss. So that energy does not have to be fed into the rotating turbomachine blades or turbine blades from outside, the electrical network, which forms a shunt, should be of passive design. A passive network which is especially effective in damping terms is composed of an inductance L in series with an electrical resistance R (see the shunt  21  in  FIG. 2 ). Such a shunt is often designated as an “LR shunt” and may generally include the most diverse possible combinations of inductances L and resistances R in a series and/or parallel connection, self-supplied switching networks (of the type “self-powered synchronized switch damping on inductor” (SSDI)) and the like. It is also conceivable, however, to use active solutions with negative capacitance, or, in general, to provide a time-variable resistance in the network. 
       FIG. 2  shows an equivalent circuit diagram  17  of a piezoelectric damping element SPE installed with one mechanical degree of freedom (perpendicularly to the reference plane) and having a shunt  21 . The piezoelectric damping element SPE lies between a reference plane  18  and a mass  20  of size m which is coupled resiliently to the reference plane  18  (spring  19  with spring constant c). The piezo element (gray box in  FIG. 2 ), provided with electrodes on opposite surfaces, may be considered in electrical terms as a capacitance of size C p  which lies parallel to an internal voltage source I and at which a voltage v is present on the outside under mechanical load. If the piezo element is provided with a shunt  21  having a series connection composed of a resistance R and inductance L (see the coil  27  in  FIG. 5 ), an oscillatory circuit is obtained which behaves in the same way as a frequency-tuned vibration damper. In a similar way to a tuned vibration damper, the oscillatory circuit has to be tuned to the oscillations of the system  18 ,  19 ,  20 , in order to have a damping action. However, the oscillatory circuit may also retroact in the manner of an absorber upon the blade and its vibrations and may oscillate with an independent frequency which is then damped to a greater or lesser extent. 
     When, according to  FIG. 1 , such piezoelectric damping elements SPE 1 , SPE 2  are installed inside a turbine blade  10  which, in the example illustrated, includes a blade root  11 , a platform  12 , a blade leaf  13  with a leading edge  14  and trailing edge  15 , and a blade tip  16  which vibrates in the way indicated by the double arrows, the vibrations of the turbine blade  10  generate in the piezo element deformations which charge the capacitance C p  of the piezoelectric damping element SPE with a corresponding voltage. The shunt  21  then converts the vibratory energy of the blade into lost energy as a function of the predefined parameters and of the position of the damping element on the vibrating blade. 
     The advantage of such a piezoelectric damping element SPE is that damping is independent of the normal force and the rigidity of mechanical contact which play an essential part in frictional damping. Furthermore, the damping action in this arrangement can be set for more than one resonant frequency of the blade, in that, for example, a plurality of LR shunts with correspondingly different parameters are provided. However, a plurality of piezoelectric damping elements may also be connected to one another by a common electrical network. 
     As compared with the conventional friction damper, the piezoelectric damping element has the advantage, furthermore, that its action is independent of phase shifts and magnitudes of the vibration amplitudes of adjacent blades at the resonant frequency. Also the damping action of the piezoelectric damping element can easily be adapted to another resonant frequency simply by varying the electrical connection. Thus, for example, if there is appropriate access to the inside of the blade, the parameters of the shunt  21  can be varied at a later stage by laser beam (for example by changing the geometry of a resistance path) or the like. 
     Furthermore, as compared with the conventional friction damper, there is the technical advantage that, by using the proposed damper, the resonant frequency of the undisturbed blade is shifted toward another value to a lesser extent, this being an important criterion in the design of the turbine. Also, the piezoelectric damper does away with problems of abrasion which have to be taken into account in frictional damping. 
     A piezoelectric damping element embodying principles of the present invention may be used, for example, for turbine and compressor blades of steam-, gas- and wind-powered machines. The piezoelectric damping element is preferably installed in a cavity in the blade or in the associated blade carrier. The configuration and location of the cavity may in this case be selected optimally in a largely independent way. 
     An exemplary embodiment of a piezoelectric damping element according to principles of the invention is illustrated in sections in  FIG. 3 . The piezoelectric damping element SPE of  FIG. 3  is constructed as a stack from two sheet-like, disk-shaped piezo bodies  22  and  23  which are composed of a material suitable for the piezoelectric effect and the operating temperature of the turbine and which alternate in the stack with contact elements or electrodes  24 . The choice of material and connection technique for the elements  22 - 24  are basically known to a person skilled in the art. 
     The piezoelectric damping element SPE of  FIG. 3  may, according to  FIG. 4 , be installed directly in a cavity  25  provided for it inside a blade  26  or may be assembled there. The opposite outsides of the piezoelectric damping element SPE are coupled firmly to the inner walls of the cavity  25 . This may take place in various ways, such as for example, by adhesive bonding, soldering or mechanical clamping, based on friction or thermal expansion. After the element has been installed, the cavity  25  can be protected by a cover which is connected to the margin of the cavity  25 , for example, by soldering or welding. After closing, the surface of the arrangement can be re-machined in order to fulfill aerodynamic requirements demanded of the blade. It is also conceivable, however, to fill up or grout the cavity  25  with the installed piezoelectric damping element SPE with a material which solidifies, for example, as a result of the action of heat and forms a fixed unit with the element and the blade. 
     In addition to the direct installation of the piezoelectric damping element SPE in the cavity  25  of the blade  26  according to  FIG. 4 , however, it is also possible to insert the piezoelectric damping element SPE according to  FIG. 5  first between an upper subcasing  28   a  and a lower subcasing  28   b  which are composed of a metal or of another material suitable for the application and which together form a casing  28 . According to  FIG. 6 , the piezoelectric damping element SPE, together with its casing  28 , can then be installed in the cavity  25  of the blade  26 . By a suitable choice of material for the casing  28 , if appropriate, connection techniques other than when the piezoelectric damping element SPE is installed directly without the casing may be used for connection between the casing  28  and blade  26 . 
     The outer contour of the casing  28  can be adapted exactly to the outer profile of the turbine blade, so that re-machining is unnecessary in this case. An adapted contour of this type may be produced, for example, by selective laser melting (SLM) according to the three-dimensionally sensed geometry of the corresponding turbine blade. The use of other rapid manufacturing techniques may likewise be envisaged. 
     Another possibility is, according to  FIG. 7 , to cut out a cutout  30  from the turbine blade  29  locally at the place of use ( FIG. 7(   a )), then to divide this cutout  30  into an upper part  30   a  and a lower part  30   b  ( FIG. 7(   b )), to introduce a recess  31  or  32  in each case into the two parts  30   a, b  and then to use the two parts as subcasings (as in  FIGS. 5 and 6)  for a piezoelectric damping element SPE. The arrangement composed of the piezoelectric damping element SPE and of the casing ( 30   a, b ) is then installed in the blade  29  again ( FIG. 7(   c )). 
     Here, too, various techniques for the firm connection between the casing ( 30   a, b ) having the piezoelectric damping element SPE located in it and the turbine blade may again be used. For example, a brazing process may be employed. In this process, the brazing alloy is heated to a melting temperature of above 450° C., so that it is then distributed between the casing ( 30   a, b ) and the turbine blade  29 . It is important in this case that the piezoelectric damping element is not heated to above its Curie temperature, because otherwise it loses its piezoelectric properties. 
     The position of the piezoelectric damping element SPE is defined with regard to as high a damping as possible of the respective vibrations of the turbine blade, thus it is important that the useful life of the blade, overall, is not reduced. In particular, as illustrated in  FIG. 1 , a plurality of piezoelectric damping elements SPE 1 , SPE 2  may be arranged at various locations in the turbine blade  10  if corresponding expansions occur there. 
     In practice, all the blades of a turbine stage (rotor disk) may be equipped with piezoelectric damping elements SPE. It is also conceivable however, to equip only selected blades or blade leaves of a turbine stage with such elements. Selection may in this case take place on the basis of a predetermined deliberate detuning pattern or measured detuning pattern in an existing turbine stage. If desired, piezoelectric damping elements which are arranged on various turbine blades of a turbine stage may be connected to one another in an (electrical) network, in order to intensify and optimize the damping effect in the turbine stage. 
     The permissible operating temperature and other load variables are determined or limited solely by the choice of piezo material and connection technique. Piezoelectric materials having a high Curie temperature are obtainable. For use in turbomachines, for example, barium titanate is a good choice, this having a high operating temperature of approximately 500° C. Unfortunately, however, barium titanate has a comparatively low piezoelectric modulus and low permittivity. Lead metaniobate is therefore to be preferred up to temperatures of 350° C. Materials of the PZT type (lead-zirconate-titanate) have the highest piezoelectric modulus, but can be used only up to temperatures of 180° C. 
     It is furthermore conceivable to use the proposed piezoelectric damping elements together with conventional friction dampers, in order to optimize overall damping in the blade. 
     Moreover, it is possible, according to  FIG. 8 , to use the piezoelectric damping elements SPE at the same time as sensors for measuring the blade vibrations. The measured signals can be transmitted wirelessly (telemetrically) from the rotating turbine to a stationary measuring device  34  which is equipped with a (reception) antenna  35 , with a signal receiver  36  and with an evaluation unit  37 . The data obtained can be used for monitoring the turbine, in particular its vibration behavior. The two functions of damping and vibration measurement may in this case be performed simultaneously in the system. 
     The described use of piezoelectric damping elements SPE may be extended to various components of gas turbines, steam turbines and compressors in aviation, in shipping, in industry and in large engines, insofar as the operating temperature does not overshoot the Curie temperature of the piezoelectric material used. 
     LIST OF REFERENCE SYMBOLS 
     
         
           10  Turbine blade 
           11  Blade root 
           12  Platform 
           13  Blade leaf 
           14  Leading edge 
           15  Trailing edge 
           16  Blade tip 
           17  Equivalent circuit diagram 
           18  Reference plane 
           19  Spring 
           20  Mass 
           21  Shunt 
           22 ,  23  Piezo body 
           24  Contact element (electrode) 
           25  Cavity 
           26 ,  29  Turbine blade 
           27  Coil (inductance) 
           28  Casing 
           28   a, b  Subcasing 
           30  Cutout 
           30   a  Upper part 
           30   b  Lower part 
           31 ,  32  Recess 
           34  Measuring device 
           35  Antenna 
           36  Signal receiver 
           37  Evaluation unit 
         SPE Piezoelectric damping element 
         SPE 1 , 2  Piezoelectric damping element 
       
    
     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.