Patent Publication Number: US-8540488-B2

Title: Turbine blade damping device with controlled loading

Description:
This invention was made with U.S. Government support under Contract Number DE-FC26-05NT42644 awarded by the U.S. Department of Energy. The U.S. Government has certain rights to this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to and filed on even date with an application having Ser. No. 12/637,066 entitled, “TURBINE BLADE DAMPING DEVICE WITH CONTROLLED LOADING”, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates generally to vibration damping of turbine blades in a turbomachine and, more particularly, to a damping structure comprising a snubber providing a controlled damping force. 
     BACKGROUND OF THE INVENTION 
     A turbomachine, such as a steam or gas turbine is driven by a hot working gas flowing between rotor blades arranged along the circumference of a rotor so as to form an annular blade arrangement, and energy is transmitted from the hot working gas to a rotor shaft through the rotor blades. As the capacity of electric power plants increases, the volume of flow through industrial turbine engines has increased more and more and the operating conditions (e.g., operating temperature and pressure) have become increasingly severe. Further, the rotor blades have increased in size to harness more of the energy in the working gas to improve efficiency. A result of all the above is an increased level of stresses (such as thermal, vibratory, bending, centrifugal, contact and torsional) to which the rotor blades are subjected. 
     In order to limit vibrational stresses in the blades, various structures may be provided to the blades to form a cooperating structure between blades that serves to dampen the vibrations generated during rotation of the rotor. For example, mid-span snubbers, such as cylindrical standoffs, may be provided extending from mid-span locations on the blades for engagement with each other. Two mid-span snubbers are located at the same height on either side of a blade with their respective contact surfaces pointing opposite directions. The snubber contact surfaces on adjacent blades are separated by a small gap when the blades are stationary. However, when the blades rotate at full load and untwist under the effect of the centrifugal forces, snubber surfaces on adjacent blades come in contact with each other. In addition, each turbine blade may be provided with an outer shroud located at an outer edge of the blade and having front and rear shroud contact surfaces that move into contact with each other as the rotor begins to rotate. The engagement between the blades at the front and rear shroud contact surfaces and at the snubber contact surfaces is designed to improve the strength of the blades under the tremendous centrifugal forces, and further operates to dampen vibrations by friction at the contacting snubber surfaces. A disadvantage of snubber damping is that on large diameter blades it is often difficult to achieve the desired contact forces produced between snubbers as a result of the centrifugal untwisting of the blades. In addition, the large mechanical load associated with large diameter blades typically necessitates larger snubber structures for mechanical stability to avoid outward bending of the snubber, resulting in increased aerodynamic losses and flow inefficiencies due to the flow restriction of larger snubbers positioned in the high velocity flow area through the part-span area. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the invention, a damping structure is provided in a turbomachine rotor comprising a rotor disk and a plurality of blades. The damping structure comprises an elongated snubber element including a first snubber end rigidly attached to a first blade and extending toward an adjacent second blade, and an opposite second snubber end positioned adjacent to a cooperating surface at least partly formed on the second blade. The snubber element has a centerline extending radially inwardly in a direction from the first blade toward the second blade along at least a portion of the snubber element between the first and second snubber ends. The cooperating surface defines an axially extending area for accommodating axial movement of the second snubber end along the cooperating surface as the first and second blades untwist during rotor spin-up. Rotational movement of the rotor effects relative movement between the second snubber end and the cooperating surface to position the second snubber end in frictional engagement with the cooperating surface with a predetermined damping force determined by a centrifugal force on the snubber element. 
     The damping structure may be located at a mid-span location between a blade root and a blade tip of the blade. 
     The centerline of the snubber element may comprise a substantially smooth curve with a concave side facing radially outwardly extending from the first snubber end to the second snubber end. 
     The centerline of the snubber element may comprise first and second linear centerline segments and an inflexion angle between the centerline segments at a midway point between the first and second blades, the first centerline segment angling radially inwardly from the first snubber end to the midway point and the second centerline segment angling radially outwardly from the midway point to the second snubber end. 
     The cooperating surface may comprise a circumferentially facing side at least partially formed on a side of the second blade and a radially inwardly facing side formed on a flange extending from the second blade. The circumferentially facing side and the radially inwardly facing side may define a recess for receiving the second snubber end. 
     A midway point is defined between the first and second blades and a radial thickness of the snubber element may decrease extending from each of the blades to the midway point. 
     In accordance with another aspect of the invention, a mid-span damping structure is provided in a turbomachine rotor comprising a rotor disk and a plurality of blades. The damping structure comprises an elongated snubber element including a first snubber end rigidly attached to a first blade and extending toward an adjacent second blade, and an opposite second snubber end positioned adjacent to a cooperating surface at least partly formed on a side surface of the second blade and defining an axially curved bearing surface. The snubber element having a centerline extending radially inwardly in a direction from the first blade toward the second blade along a portion of the snubber element between the first end and a midway point between the first and second blades, and extending radially outwardly from the midway point to the second snubber end. Rotational movement of the rotor effects relative movement between the second snubber end and the cooperating surface to position the second snubber end in frictional engagement with the cooperating surface with a predetermined damping force determined by a centrifugal force on the snubber element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein: 
         FIG. 1  is a partial end view of a rotor, as viewed in an axial flow direction, taken in a plane perpendicular to an axis of rotation and showing an embodiment of the invention; 
         FIG. 1A  is an enlarged view of a contact location between a snubber end and a cooperating surface of a blade; 
         FIG. 2  is view taken on the plane indicated by the line  2 - 2  in  FIG. 1 ; 
         FIG. 3  is a partial end view showing an alternative configuration of the embodiment of  FIG. 1 ; 
         FIG. 4  is a partial end view of a rotor taken in a plane perpendicular to an axis of rotation and showing an alternative embodiment of the invention; and 
         FIG. 5  is a partial end view showing an alternative configuration of the embodiment of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific preferred embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. 
     Referring to  FIG. 1 , a section of a rotor  10  is illustrated for use in a turbomachine (not shown), such as for use in a gas or steam turbine. The rotor  10  comprises a rotor disk  12  and a plurality of blades  14 , illustrated herein as a first blade  14   a  and an adjacent second blade  14   b . The blades  14  comprise radially elongated structures extending from a blade root  16 , engaged with the rotor disk  12 , to a blade tip  18 . Each of the blades  14   a ,  14   b  includes a pressure side surface  20  and a suction side surface  22 . The rotor  10  further includes a damping structure  24  extending between the first and second blades  14   a ,  14   b , and located mid-span between the blade root  16  and the blade tip  18  of the blades  14   a ,  14   b.    
     The damping structure  24  comprises an elongated snubber element  26  including a first snubber end  28  rigidly attached to the suction side surface  22  of the first blade  14   a  and extending toward the adjacent pressure side surface  20  of the second blade  14   b . The snubber element  26  additionally includes an opposite second snubber end  30  positioned adjacent to a cooperating surface  32  associated with the second blade  14   b . The cooperating surface  32  is at least partially formed on the pressure side surface  20  of the second blade  14   b.    
     The snubber element  26  defines a centerline  34  extending radially inwardly in a direction from the first blade  14   a  toward the second blade  14   b  along a first portion  36  of the snubber element  26  between the first snubber end  28  and a midway point  38  between the first and second blades  14   a ,  14   b . The centerline  34  extends radially outwardly along a second portion  40  of the snubber element  26  from the midway point  38  to the second snubber end  30 . The midway point  28  may be defined as any point that is generally at a central region of the snubber element  26  located spaced circumferentially from both the first and second blades  14   a ,  14   b . In the embodiment illustrated in  FIG. 1 , the centerline  34  comprises a substantially smooth curve that is bowed inwardly, e.g., in the manner of a classical Roman arch, from a circumferential line  42  extending between upper edges of the first and second snubber ends  28 ,  30 , and having a concave side that faces radially outwardly extending from the first snubber end  28  to the second snubber end  30 . In addition, the centerline  34  passes through centroids C of the first and second blades  14   a ,  14   b.    
     Referring further to  FIG. 1A , the second snubber end  30  is normally positioned with a small snubber gap G between a snubber end surface  44  and the cooperating surface  32  when the rotor  10  is stationary. The cooperating surface  32  comprises a circumferentially facing side  46  that may be angled circumferentially inwardly in a radial outward direction and faces a similarly angled circumferentially facing portion  44   a  of the snubber end surface  44 . The cooperating surface  32  additionally includes a radially inwardly, facing side  48  formed on a flange  50  extending from the suction side  22  of the second blade  14   b . The circumferentially facing side  46  and the radially inwardly facing side  48  define a recess  52  for receiving the second snubber end  30 . The circumferentially facing side  46  is preferably angled such that it is substantially normal to the centerline  34  of the snubber element  26 , and is generally parallel to the circumferentially facing portion  44   a . A radially outer portion  44   b  of the snubber end surface  44  is located adjacent to the radially inwardly facing side  48  of the flange  50 . 
     As seen in  FIG. 2 , the circumferentially facing side  46  of the cooperating surface  32  extends in an axial direction for engaging the corresponding circumferentially facing portion  44   a  on the snubber end surface  44 . Further, both the circumferentially facing side  46  of the cooperating surface and the circumferentially facing portion  44   a  of the snubber end surface  44  may be formed with a curvature in the axial direction to accommodate relative movement between these members during blade untwist. 
     During spin-up of the rotor  10 , a centrifugal force exerted on the snubber member  26  causes the second snubber end  30  to move radially outwardly and into frictional engagement with the cooperating surface  32 . Specifically, the during rotation of the rotor  10 , the snubber element  26  pivots about the first snubber end  28  and radial outward movement of the second snubber end  30  causes the sloping or angled surfaces  44   a  and  46  of the snubber end surface  44  and cooperating surface  32 , respectively, to engage each other with a predetermined force in a direction generally parallel or tangent to the centerline  34  and extending through the centroid C. Further, the radially outer portion  44   b  of the snubber end surface  44  engages the radially inwardly facing side  48  of the flange  50 , defining a socket area, to limit outward movement of the second snubber end  30  and maintain the second snubber end  30  within the recess  52 . 
     In addition, since the first snubber end  28  is rigidly attached to the first blade  14   a , snubber element  26  will pivot with the first blade  14   a  in a plane generally parallel to the axial and circumferential directions as the first blade untwists during spin-up of the rotor  10 . As illustrated in  FIG. 2 , pivoting movement of the snubber element  26  during blade untwist, depicted by directional arrow  54 , will cause the second snubber end  30  to move axially in an arc, as depicted by arrow  56 . As noted above, the curvature in the axial direction of the circumferentially facing side  46  of the cooperating surface  32  and the circumferentially facing portion  44   a  of the snubber end surface  44  accommodates or guides the movement of the second snubber end  30  as the blades  14  untwist. Also, the snubber gap G provided between the snubber end surface  44  and the cooperating surface  32  provides a reduced friction interface for relative movement between these components before centrifugal forces create an engagement force to lock the snubber end surface  44  to the cooperating surface  32 . 
     The second snubber end  30  engages the cooperating surface  32  with a predetermined minimum damping force, where the damping force may be controlled by the inward angle and mass of the snubber element  26 . It should be noted that it is desirable to configure the snubber element  26  to produce a damping force that is sufficient to produce damping at the interface between the second snubber end  30  and the cooperating surface  32  to control blade vibration without substantially exceeding this minimum damping force. An excess force at this location may lead to excessive wear and stress on the snubber element  26  and cooperating surface  32 . 
     The inward angle formed by the curvature of the snubber element  26 , as defined by the centerline  34 , substantially alters the damping force produced by centrifugal force on the snubber element  26 . The centrifugal force exerted on the snubber element  26  causes the snubber element  26  to bend outwardly and become less concave, producing the damping force between the blades  14 . A larger centerline curvature will produce a greater centrifugal load on the snubber element  26  and a greater damping force applied between the second snubber end  30  and the cooperating surface  32 . For example, it is believed that a snubber element  26  having a curvature that matches a catenary curve would cause the snubber element  26  to produce a substantially greater damping force between the blades  14  than would be required to dampen vibrations. Further, it is believed that a snubber element  26  configured with a centerline  34  having a relatively shallow curve may be sufficient to produce an adequate centrifugal force on the snubber element  26  and provide the necessary damping force to reduce blade vibration while effectively controlling the level of force applied. 
     In order to minimize or reduce inertial loads on the snubber element  26 , the snubber element  26  may be formed with a taper extending from either snubber end  28 ,  30  toward the midway point  38 , as seen in  FIG. 1 . That is the radial thickness of the snubber element  26  may progressively decrease from the snubber ends  28 ,  30  toward the midway point  38 . In addition, the taper may reduce aerodynamic resistance by providing the snubber element  26  with a reduced cross-sectional area, facilitating flow through the turbine between the blades  14 . 
     It should be noted that although a particular configuration for accommodating axial movement of the second snubber end  30  is disclosed, other engagement structure may be provided to accommodate blade untwist. For example, a ball and socket configuration may be provided where the cooperating surface  32  may be formed as rounded socket surface for receiving a ball or partial spherical surface formed on the second snubber end  30 . 
     Referring to  FIG. 3 , an alternative configuration is illustrated comprising a variation of the embodiment shown in  FIG. 1 . Elements in  FIG. 3  corresponding to elements in  FIG. 1  are labeled with the same reference number increased by 100. 
     In  FIG. 3 , the snubber element  126  includes a first snubber end  128  rigidly affixed to a first blade  114   a  and a second snubber end  130  supported adjacent to a cooperating surface  132  on a second blade  114   b . The snubber element  126  is formed with first and second linear portions  136 ,  140  wherein the centerline  134  of the snubber element  126  comprises a first linear centerline segment  134   a  and a second linear centerline segment  134   b . The centerline segments  134   a ,  134   b  meet at an inflexion angle θ at a midway point  138  between the first and second blades  114   a ,  114   b . The first centerline segment  136  angles radially inwardly from the first snubber end  128  to the midway point  138 , and the second centerline segment  140  angles radially outwardly from the midway point  138  to the second snubber end  130 . 
     The configuration of  FIG. 3  provides a damping structure  124  having a triangular configuration that includes a snubber element  126  extending radially inwardly from the circumferential line  142 . In a preferred embodiment, the first and second centerline segments  134   a  and  134   b  each angle inwardly from the circumferential line  142  at an angle α. The angle α may be in the range of from about 3° to about 20°, and preferably is about 6°, such that the inflexion angle θ is about 178°. The damping structure  124  operates in the manner described above for the damping structure  24  wherein centrifugal forces applied on the snubber element  126  cause the second snubber end  130  to engage the cooperating surface  132  with a predetermined force to provide a controlled damping force for damping blade vibrations. Further, a cooperating surface structure similar to the axially extending cooperating surface  32  of  FIG. 2  may be provided to accommodate relative axial movement between the second snubber end  130  and the cooperating surface  132 . 
     Referring to  FIG. 4 , an additional embodiment of the invention is described where elements in  FIG. 4  corresponding to elements in  FIG. 1  are labeled with the same reference number increased by 200. A rotor  210  including a damping structure  224  is illustrated. The damping structure  224  includes a snubber element  226  comprising an elongated first snubber element  260  extending from a first blade  214   a  toward an adjacent second blade  214   b . The first snubber element  260  includes a first snubber end  262  rigidly attached to the first blade  214   a , and an opposite second snubber end  264  extending to a midway point  238 . An elongated second snubber element  266  extends from the second blade  214   b  toward the first blade  214   a  and includes a first snubber end  268  rigidly attached to the second blade  214   b , and an opposite second snubber end  270  extending to a midway point  238 . 
     The second snubber end  264  of the first snubber element  260  defines an engagement surface  272  located adjacent to a cooperating surface  274  on the second snubber end  270  of the second snubber element  266  at the midway point  238  between the first and second blades  214   a ,  214   b . A snubber gap G is defined between the adjacent surfaces  272 ,  274  when the rotor  210  is stationary, i.e., with no centrifugal forces acting on the first and second snubber elements  260 ,  266 . 
     The first and second snubber elements  260 ,  266  define a centerline  234  extending radially inwardly in a direction from the first blade  214   a  toward the midway point  238  and extending radially inwardly in a direction from the second blade  214   b  toward the midway point  238 . The centerline  234  defined by the first and second snubber elements  260 ,  266  comprises a substantially smooth curve with a concave side facing radially outwardly toward a circumferential line  242  extending between radially outer edges of the first snubber end  262  of the first snubber element  260  and the first snubber end  268  of the second snubber element  266 . 
     Rotational movement of the rotor  210  effects relative movement between the second snubber ends  264 ,  270  of the first and second snubber elements  260 ,  266  to close the snubber gap G and position the engagement surface  272  in frictional engagement with the cooperating surface  274  with a predetermined damping force determined by a centrifugal force acting on the first and second snubber elements  260 ,  266 . In particular, the centrifugal force acting on the first and second snubber elements  260 ,  266  effect a movement of the snubber elements  260 ,  266  radially outwardly, causing them to pivot toward each other and the snubber gap G to be closed. In addition, it should be noted that the second ends  264 ,  270  of the snubber elements  260 ,  266  are located to define the snubber gap G at a location between the blades  214   a ,  214   b  where the second ends  264 ,  270  will remain at substantially the same position relative to each other during rotor spin-up and corresponding blade untwist. Hence, the engagement surface  272  will remain in facing relation to the cooperating surface  274  regardless of blade untwist during rotor spin-up and will be positioned in locking frictional engagement during operation of the turbine. 
     Referring to  FIG. 5 , an alternative configuration is illustrated comprising a variation of the embodiment shown in  FIG. 4 . Elements in  FIG. 5  corresponding to elements in  FIG. 4  are labeled with the same reference number increased by 100. 
     In  FIG. 5 , a rotor  310  including a damping structure  324  is illustrated. The damping structure  324  includes a snubber element  326  comprising an elongated first snubber element  360  extending from a first blade  314   a  toward an adjacent second blade  314   b . The first snubber element  360  includes a first snubber end  362  rigidly attached to the first blade  314   a , and an opposite second snubber end  364  extending to a midway point  338 . An elongated second snubber element  366  extends from the second blade  314   b  toward the first blade  314   a  and includes a first snubber end  368  rigidly attached to the second blade  314   b , and an opposite second snubber end  370  extending to the midway point  338 . 
     The second snubber end  364  of the first snubber element  360  defines an engagement surface  372  located adjacent to a cooperating surface  374  on the second snubber end  370  of the second snubber element  366  at the midway point  338  between the first and second blades  314   a ,  314   b . A snubber gap G is defined between the adjacent surfaces  372 ,  374  when the rotor  310  is stationary, i.e., with no centrifugal forces acting on the first and second snubber elements  360 ,  366 . The first and second snubber elements  360 ,  366  define a centerline  334  wherein the centerline  334  comprises a first linear centerline segment  334   a  and a second linear centerline segment  334   b  extending along the first and second snubber elements  360 ,  366  respectively. The centerline segments  334   a ,  334   b  meet at an inflexion angle θ at the midway point  338  between the first and second blades  314   a ,  314   b.    
     The configuration of  FIG. 5  provides a damping structure  324  having a triangular configuration that includes the first and second snubber elements  360 ,  366  extending radially inwardly from a circumferential line  342  connecting radially outer edges of the first snubber end  362  of the first snubber element  360  and the first snubber end  368  of the second snubber element  366 . In a preferred embodiment, the first and second centerline segments  334   a  and  334   b  each angle inwardly from the circumferential line  342  at an angle α. The angle α may be in the range of from about 3° to about 20°, and preferably is about 6°, such that the inflexion angle θ is about 178° when the rotor  310  is stationary. The damping structure  324  operates in the manner described above for the damping structure  224  of  FIG. 4  wherein rotational movement of the rotor  310  produces a centrifugal force on the first and second snubber elements  360 ,  366  to move the snubber elements  360 ,  366  radially outwardly. As the snubber elements  360 ,  366  move outwardly, they pivot toward each other and close the snubber gap G. As the snubber gap G is closed the engagement surface  372  is positioned in frictional engagement with the cooperating surface  374  with a predetermined damping force determined by the centrifugal force loading the first and second snubber elements  360 ,  366 . It is believed that the damping structure  324 , including the first and second snubber elements  360 ,  366  positioned at the described angle of 6°, may produce a force at the snubber gap G of approximately 500 N, above any forces that may occur as a result of movements of the blades  314   a ,  314   b , such as may result from blade untwist. 
     In the embodiments of the invention described with reference to  FIGS. 4 and 5 , in order to minimize or reduce the inertial loads on the first and second snubber elements  260 ,  266  ( 360 ,  366 ) these elements may be tapered extending from the respective first and second blades  214   a ,  214   b  ( 314   a ,  314   b ) toward the snubber gap G at the midway point  238  ( 338 ). That is, the radial thickness may progressively decrease from the snubber ends  262 ,  268  ( 362 ,  368 ) toward the midway point  238  ( 338 ). In addition, the taper may reduce aerodynamic resistance by providing the snubber elements  260 ,  266  ( 360 ,  366 ) with a reduced cross-sectional area to flow through the turbine between the blades. 
     In each of the above-described embodiments, it should be noted that structure is provided for controlling the damping force at a snubber gap between a snubber element and a cooperating surface using a radially inwardly extending configuration to produce a predetermined outwardly directed centrifugal force and a corresponding circumferentially directed damping force at the engaging surfaces. 
     The present invention is particularly applicable to large diameter, cooled turbine blades designed for high temperature (i.e., 850° C.) applications, such as may be used in industrial gas turbines. The present invention enables application of a controlled damping force through a mid-span snubber structure such as may be required for vibration damping of large diameter blades subjected to increased aerodynamic vibrations wherein the damping structure may provide a greater or lesser force, as required, at the snubber gap by utilizing a predetermined centrifugal force acting on the inwardly angled snubber element or elements. 
     While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.