Abstract:
According to one aspect of the invention, a turbine assembly includes an airfoil extending from a blade and a dovetail located on a lower portion of the blade, wherein the dovetail has a dovetail contact surface. The turbine assembly also includes a member with a slot configured to couple to the airfoil via the dovetail, the slot having a slot contact surface to contact the dovetail contact surface, wherein the dovetail contact surface is reduced by a relief to alter a fundamental frequency of an assembly of the blade and member.

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to rotating and stationary components of turbomachinery and, more particularly, to a blade and disk dovetail design for turbine systems. 
     Certain turbine rotor disks include a plurality of circumferentially spaced dovetail slots about the outer periphery of the disk. Each of the dovetail slots receives a blade formed with an airfoil portion and a blade dovetail having a male portion complementary to the female portion of the dovetail slots. The blade dovetail is received by the dovetail slot in an axial direction. 
     During operation of the turbine, movement of certain components and flow of compressed air and hot gas through the turbine can cause vibration in the turbine system. For example, the vibration of rotating blades can be driven by air or gas flowing through adjacent static vanes. Specifically, during operation of the turbine system, driving frequencies are caused by pulses formed as fluid passes through blades in the compressor or turbine. It is desirable for blades to be designed such that their fundamental natural frequencies either avoid the driving frequencies or can withstand the vibration caused by them, otherwise wear, high cycle fatigue, and other damage to components can occur. Repair and/or replacement of components due to vibration induced fatigue can be costly and time consuming 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention, a turbine assembly includes an airfoil extending from a blade and a dovetail located on a lower portion of the blade, wherein the dovetail has a dovetail contact surface. The turbine assembly also includes a member with a slot configured to couple to the airfoil via the dovetail, the slot having a slot contact surface to contact the dovetail contact surface, wherein the dovetail contact surface is reduced by a relief to alter a fundamental frequency of an assembly of the blade and member. 
     According to another aspect of the invention, a method for altering a fundamental frequency of a turbine assembly includes flowing hot gas across an airfoil extending from a blade, the blade coupled to a rotor disk by a dovetail on the blade and a slot on the rotor disk and altering a fundamental frequency of an assembly of the rotor disk and blade via a reduced area of contact between a dovetail contact surface and a slot contact surface of the slot. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a perspective view of a turbine disk segment and a turbine blade according to an embodiment; 
         FIG. 2  is a perspective view of the turbine blade shown in  FIG. 1 ; 
         FIG. 3  is a detailed perspective view of a dovetail portion of a turbine blade according to an embodiment; 
         FIG. 4  is a detailed side view of a portion of the dovetail shown in  FIG. 3 ; and 
         FIG. 5  is a detailed view of a portion of the dovetail shown in  FIGS. 3 and 4 . 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a perspective view of an exemplary turbine disk segment  110  in which a turbine blade  112  is secured. Embodiments may include applications for gas turbines, steam turbines, axial flow compressors, or other devices involving a plurality of rotating blades secured by dovetails. The disk  110  includes a dovetail slot  114  that receives a correspondingly shaped blade dovetail  116  to secure the blade  112  to the disk  110 . In an embodiment, the blade dovetail  116  has three tangs  121  to retain the blade  112  in the dovetail slot  114 . Embodiments may include as few as one and as many as eight or more tangs  121 .  FIG. 2  shows a bottom section of the blade  112  including an airfoil  218  and the blade dovetail  116 . In an embodiment, a hot gas flows across the airfoil  218 , thereby creating a pressure side  222  (i.e., leading edge) and a suction side  224  (i.e., trailing edge) of the blade  112 . As described in further detail below, a plurality of reliefs  226  are formed in the tangs  121  to alter a fundamental frequency of an assembly of the blade  112  and disk segment  110  (also referred to as “member” or “turbine member”). The fundamental frequency is altered or shifted away from one or more driving frequencies of the turbine system, thereby reducing incidence of wear and fatigue for the components. 
     The dovetail slots  114  are typically termed “axial entry” slots in that the dovetails  116  of the blades  112  are inserted into the dovetail slots  114  in a generally axial direction, i.e., generally parallel but skewed to the axis of the disk  110 . The features described herein are generally applicable to any airfoil and disk interface. The structure depicted in  FIGS. 1 and 2  is merely representative of many different disk and blade designs across different classes of turbines. In an embodiment, reliefs  226  are formed by any suitable method for removal of material from the dovetail  116  to form a recess in the surface such as casting, cutting and machining. For example, the reliefs  226  may include a cut or machined recess in the dovetail surface that produces a gradual or gentle rounded slope in the recess. 
     As used herein, “downstream” and “upstream” are terms that indicate a direction relative to the flow of working fluid through the turbine. As such, the term “downstream” refers to a direction that generally corresponds to the direction of the flow of working fluid, and the term “upstream” generally refers to the direction that is opposite of the direction of flow of working fluid. The term “radial” refers to movement or position perpendicular to an axis or center line. It may be useful to describe parts that are at differing radial positions with regard to an axis. In this case, if a first component resides closer to the axis than a second component, it may be stated herein that the first component is “radially inward” of the second component. If, on the other hand, the first component resides further from the axis than the second component, it can be stated herein that the first component is “radially outward” or “outboard” of the second component. The term “axial” refers to movement or position parallel to an axis. Finally, the term “circumferential” refers to movement or position around an axis. Although the following discussion primarily focuses on gas turbines, the concepts discussed are not limited to gas turbines and may apply to any suitable machinery, including steam turbines. Accordingly, the discussion herein is directed to gas turbine embodiments, but may apply to other turbine systems. 
       FIG. 3  is a perspective view of a portion of an embodiment of a blade including a dovetail  300 . The dovetail  300  includes reliefs  302 ,  306 ,  310  and  314  formed in tangs  304 ,  308 ,  312  and  316 , respectively. The reliefs remove material from the dovetail  300 , thereby reducing an area of a contact surface  317  that is in contact with a receiving dovetail slot, such as a slot formed in a turbine or compressor disk. In an embodiment, reliefs are formed in a first lateral side  318  and a second lateral side  320  of the dovetail  300 . In addition, reliefs are formed in a leading edge  322  (i.e., pressure side) and a trailing edge  324  (i.e., suction side) of the dovetail  300 . Various configurations of the dovetail, tangs and reliefs are contemplated. In embodiments, one or more reliefs may be formed in as few as one tang or as many as all tangs  304 ,  308 ,  312  and  316 . Further, one or more reliefs may be formed one or both of the leading edge  322  and trailing edge  324 . In addition, one or more reliefs may be formed in one or both of the first lateral side  318  and second lateral side  320  of the dovetail  300 . 
     In one embodiment, the reduced contact surface  317  provided by the reliefs  302 ,  306 ,  310  and  314  alters a fundamental frequency of an assembly of the blade and receiving member (e.g., turbine disk segment or compressor casing). Thus, the fundamental frequency of the assembly is shifted away from one or more driving frequencies of the turbine system, thereby reducing fatigue and improving the life of the components. In one embodiment, one or more of the reliefs shift the fundamental frequency of the blade and disk assembly by 1-2% or more, thus shifting the fundamental frequency away from driving frequencies. In embodiments, the reliefs may be one of a plurality of techniques used to alter the fundamental frequency of the blade and disk segment assembly. The reliefs  302 ,  306 ,  310  and  314  may be formed by any suitable method, such as by machining the dovetail after it is cast. For example, the blade and dovetail may be cast from an alloy and tested to determine the fundamental frequency of the blade and disk segment assembly, where the number, location and size of the reliefs are determined by the tests and subsequently formed by machining the dovetail. 
       FIG. 4  is a detailed side view of a portion of the exemplary dovetail  300  shown in  FIG. 3 . The illustrated view shows the second lateral side  320  of the dovetail  300  in detail. As depicted, the relief  302  has a first axial length  400 , the relief  306  has a second axial length  402 , the relief  310  has a third axial length  404  and the relief  314  has a fourth axial length  406 . In an embodiment, the dimension of axial lengths  400 ,  402 ,  404  and  406  are different. In another embodiment, one or more of the axial lengths  400 ,  402 ,  404  and  406  are the same dimension. The length, cut depth (i.e., lateral depth of cut into the surface  317 ) and location of the one or more reliefs may be altered depending on the application and desired changes to the fundamental frequency for the blade and receiving member. 
       FIG. 5  is a detailed view of a portion of the exemplary dovetail  300  shown in  FIGS. 3 and 4 . The illustration shows the reliefs  302  and  306  formed in the tangs  304  and  308  of the dovetail  300 . The reliefs  302  and  306  reduce the contact surface  317  to alter a fundamental frequency for the blade (including the dovetail) and the receiving member (e.g., disk) assembly. Specifically, the area of contact between contact surface  317  of dovetail  300  and the contact surface of the receiving dovetail slot is reduced by the reliefs  302  and  306 . In embodiments, the area of contact between the dovetail  300  and the dovetail slot may be reduced by any suitable method, such as cuts, grooves and recesses formed in the contact surface of the dovetail and/or dovetail slot. The depicted embodiment of the blade dovetail and receiving member improve the life span of the receiving member and/or blade and increase robustness of the assembly by altering a fundamental frequency of the assembly away from a driving frequency of the turbine system. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.