Abstract:
A method of tuning a compressor stator blade, having a base portion and an airfoil portion, to achieve a desired natural frequency, comprising: a) identifying the natural frequency of the compressor stator blade; b) determining a different target natural frequency for the compressor stator blade; and c) removing material from at least one of the side surfaces of the base portion of the compressor stator blade in an amount and in a configuration that achieves the target natural frequency.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This present application relates generally to methods, systems, and/or apparatus for frequency tuning blades of turbine engines, which, as used herein and unless specifically stated otherwise, is meant to include all types of turbine engines, including gas turbine engines, aircraft engines, steam turbine engines, and rotary engines. More specifically, but not by way of limitation, the present application relates to methods, systems, and/or apparatus pertaining to the manufacture and/or modification of turbine blades such that the frequency of the blades is changed in a manner that improves one or more operational characteristics. 
         [0002]    In the past, natural frequency tuning of turbine blades has been accomplished by modifying the shape of the airfoil portion of the blade and or making certain significant modifications to the root portion of the blades. It would be desirable, however, to be able to modify the natural frequency of the airfoil of a turbine blade without having to modify the airfoil shape or make such significant modifications to the root. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0003]    The present application thus describes a method of tuning a compressor stator blade, having a base portion and an airfoil portion, to achieve a desired natural frequency, comprising: a) identifying the natural frequency of the compressor stator blade; b) determining a different target natural frequency for the compressor stator blade; and c) removing material from at least one of the side surfaces of the base portion of the compressor stator blade in an amount and in a configuration that achieves the target natural frequency. 
         [0004]    The present application further describes a compressor stator blade that includes an airfoil portion and a base portion that is substantially rectangular with side surfaces that include a pressure side, a suction side, a leading face and a trailing face, and a radially inner surface and a radially outer surface, the compressor stator blade comprising at least one groove formed in at least one the side surfaces that is configured such that the compressor stator blade has a desired natural frequency. 
         [0005]    These and other features of the present application will become apparent upon review of the following detailed description of the preferred embodiments when taken in conjunction with the drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    These and other objects and advantages of this invention will be more completely understood and appreciated by careful study of the following more detailed description of exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
           [0007]      FIG. 1  is a schematic representation of an exemplary turbine engine in which certain embodiments of the present application may be used; 
           [0008]      FIG. 2  is a sectional view of the compressor section of the gas turbine engine of  FIG. 1 ; 
           [0009]      FIG. 3  is a sectional view of the turbine section of the gas turbine engine of  FIG. 1 ; 
           [0010]      FIG. 4  is a view of a conventional stator blade design; 
           [0011]      FIG. 5  is a view of another conventional stator blade design; 
           [0012]      FIG. 6  depicts the general configuration of a base and airfoil; 
           [0013]      FIG. 7  is a illustration of a base with side grooves according to an exemplary embodiment of the present invention; 
           [0014]      FIG. 8  is a illustration of a base with side grooves according to an exemplary embodiment of the present invention; 
           [0015]      FIG. 9  is a illustration of a base with side grooves according to an exemplary embodiment of the present invention; 
           [0016]      FIG. 10  is a illustration of a base with side grooves according to an exemplary embodiment of the present invention; 
           [0017]      FIG. 11  is a cross-sectional view of an exemplary profile of a side groove; 
           [0018]      FIG. 12  is a cross-sectional view of an exemplary profile of a side groove; 
           [0019]      FIG. 13  is across-sectional view of an exemplary profile of a side groove; 
           [0020]      FIG. 14  is a cross-sectional view of an exemplary profile of a side groove; and 
           [0021]      FIG. 15  is a cross-sectional view of an exemplary profile of a side groove. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    Referring now to the figures,  FIG. 1  illustrates a schematic representation of a gas turbine engine  100 . In general, gas turbine engines operate by extracting energy from a pressurized flow of hot gas that is produced by the combustion of a fuel in a stream of compressed air. As illustrated in  FIG. 1 , gas turbine engine  100  may be configured with an axial compressor  106  that is mechanically coupled by a common shaft or rotor to a downstream turbine section or turbine  110 , and a combustor  112  positioned between the compressor  106  and the turbine  110 . Note that the following invention may be used in all types of turbine engines, including gas turbine engines, steam turbine engines, aircraft engines, and others. Hereinafter, the invention will be described in relation to a gas turbine engine. This description is exemplary only and not intended to be limiting in any way. 
         [0023]      FIG. 2  illustrates a view of an exemplary multi-staged axial compressor  118  that may be used in a gas turbine engine. As shown, the compressor  118  may include a plurality of stages. Each stage may include a row of compressor rotor blades  120  followed by a row of compressor stator blades  122 . Thus, a first stage may include a row of compressor rotor blades  120 , which rotate about a central shaft, followed by a row of compressor stator blades  122 , which remain stationary during operation. The compressor stator blades  122  generally are circumferentially spaced one from the other and fixed about the axis of rotation. The compressor rotor blades  120  are circumferentially spaced and attached to the shaft, when the shaft rotates during operation, the compressor rotor blades  120  rotates about it. As one of ordinary skill in the art will appreciate, the compressor rotor blades  120  are configured such that, when spun about the shaft, they impart kinetic energy to the air or working fluid flowing through the compressor  118 . The compressor  118  may have many other stages beyond the stages that are illustrated in  FIG. 2 . Additional stages may include a plurality of circumferential spaced compressor rotor blades  120  followed by a plurality of circumferentially spaced compressor stator blades  122 . 
         [0024]      FIG. 3  illustrates a partial view of an exemplary turbine section or turbine  124  that may be used in the gas turbine engine. The turbine  124  also may include a plurality of stages. Three exemplary stages are illustrated, but more or less stages may present in the turbine  124 . A first stage includes a plurality of turbine buckets or turbine rotor blades  126 , which rotate about the shaft during operation, and a plurality of nozzles or turbine stator blades  128 , which remain stationary during operation. The turbine stator blades  128  generally are circumferentially spaced one from the other and fixed about the axis of rotation. The turbine rotor blades  126  may be mounted on a turbine wheel (not shown) for rotation about the shaft (not shown). A second stage of the turbine  124  also is illustrated. The second stage similarly includes a plurality of circumferentially spaced turbine stator blades  128  followed by a plurality of circumferentially spaced turbine rotor blades  126 , which are also mounted on a turbine wheel for rotation. A third stage is illustrated, and similarly includes a plurality of turbine stator blades  128  and rotor blades  126 . It will be appreciated that the turbine stator blades  128  and turbine rotor blades  126  lie in the hot gas path of the turbine  124 . The direction of flow of the hot gases through the hot gas path is indicated by the arrow. As one of ordinary skill in the art will appreciate, the turbine  124  may have many other stages beyond the stages that are illustrated in  FIG. 3 . Each additional stage may include a row of turbine stator blades  128  followed by a row of turbine rotor blades  126 . 
         [0025]    Note that as used herein, reference, without further specificity, to “rotor blades” is a reference to the rotating blades of either the compressor  118  or the turbine  124 , which include both compressor rotor blades  120  and turbine rotor blades  126 . Reference, without further specificity, to “stator blades” is a reference to the stationary blades of either the compressor  118  or the turbine  124 , which include both compressor stator blades  122  and turbine stator blades  128 . The term “airfoil” will be used herein to refer to either type of blade. Thus, without further specificity, the term “airfoil” is inclusive to all type of turbine engine blades, including compressor rotor blades  120 , compressor stator blades  122 , turbine rotor blades  126 , and turbine stator blades  128 . 
         [0026]    In use, the rotation of compressor rotor blades  120  within the axial compressor  118  may compress a flow of air. In the combustor  112 , energy may be released when the compressed air is mixed with a fuel and ignited. The resulting flow of hot gases from the combustor  112  then may be directed over the turbine rotor blades  126 , which may induce the rotation of the turbine rotor blades  126  about the shaft, thus transforming the energy of the hot flow of gases into the mechanical energy of the rotating blades and, because of the connection between the rotor blades in the shaft, the rotating shaft. The mechanical energy of the shaft may then be used to drive the rotation of the compressor rotor blades  120 , such that the necessary supply of compressed air is produced, and also, for example, a generator to produce electricity. 
         [0027]      FIGS. 4 and 5  illustrate compressor stator blades  128  of conventional or known design. Generally, stator blades  128  include a mounting portion or base  132  and an airfoil portion or airfoil  134 . The base  132  is generally rectangular in shape, with a pair of longer side surfaces  136 ,  138  and a pair of shorter end surfaces  140 ,  142 , along with a radially outer surface  144  and a radially inner surface  146 . When inserted into a properly configured slot (not shown) formed in the turbine casing, it will be appreciated that a pair of tabs or rails  147  along the end surfaces  140 ,  142  may prevent the radial displacement of the stator blades  128  during operation. The base  132  also may be formed in the shape of a parallelogram, i.e., where the parallel end surfaces are not perpendicular to the parallel side surfaces. 
         [0028]    In the past, as with the convention design of  FIG. 4 , to alter the natural frequency of the airfoil  134 , the shape of the airfoil itself had to be modified. As illustrated in  FIG. 5 , conventional design also includes a method of changing natural frequency by creating a single, wide groove  150  in the radially outer surface  144  of the base  132 . The single groove  150  generally includes the removal of a significant amount of material from the base  132  as it extends across the width of the base  132 , i.e., from side surface  136  to side surface  138 , and approximately parallel to end surfaces  140 ,  142 . It can be seen that the width of the groove  150  substantially spans the entire chord length of the airfoil  134 . 
         [0029]      FIG. 6  illustrates a blade  155 , the configuration of which will be referenced in the subsequent figures to describe several embodiments of the present application. It will be appreciated that the blade  155  includes a base  156  and an airfoil  158  (of which only a portion is shown). The base  156  includes a rectilinear shape with rails  160  that are used in securing the blade  155  in slots formed in the casing of the turbine. The base  156  generally includes a radially outer surface  162  and a radially inner surface  164 . The base  156  also includes four radially-oriented side surfaces: a first side surface  171  that is across from and generally parallel to a second side surface  172 , and a third side surface  173  that is across from and generally parallel to a fourth side surface  174 . It will be appreciated that side surfaces of the base  156  may include a pressure side and a suction side, which coincide respectively with the pressure side and the suction side of the airfoil, and a leading face and trailing face, which coincide respectively with the leading edge and trailing edge of the airfoil. For the sake of this example, the first side surface  171  and the second side surface  172  may be the pressure side and suction side, respectively, and the third side surface  173  and the fourth side surface  174  may be the leading face and the trailing face, respectively. 
         [0030]      FIG. 7  illustrates a modified stator blade  155  in accordance with a non-limiting exemplary embodiment of the present application. In this embodiment, the stator blade  155  generally is similar to and includes the same components as that described above for the blade in  FIG. 6 . In addition, though, the stator blade  155  includes a side groove  177  along the third side surface  173  and the fourth side surface  174 . As used herein, reference to a “side groove,” without further specification, is meant to have the broadest interpretation or meaning. That is, reference to a side groove is meant to broadly include any depression, groove, notch, trench, or similar formation that extends, whether continuously or intermittently, across one of the sides  171 ,  172 ,  173 ,  174  of the base  156 . The side groove  177  illustrated in  FIG. 7 , for example, may be described as a groove with a rectangular profile that extends in a continuous manner across the length of the third side surface  173  and the fourth side surface  174 , which, respectively, may be the leading face and trailing face of the base  156 . The side groove  177  of  FIG. 7 , however, is exemplary only. As will be discussed in more detail below, side grooves according to the present invention may come in many shapes, sizes, and/or configurations. 
         [0031]    The particular configuration of the side groove  177  may be determined as follows. After having determined the natural frequency of the blade  155  and after having identified a target natural frequency, one or more of the side surfaces  171 ,  172 ,  173 ,  174  of the stator blade  155  is modified by selectively removing material from one or more of them in the form of a side groove. Material is removed until the target natural frequency for the blade  155  is achieved. A side groove  177  may be formed in the base  156  by cutting or machining to the desired geometry. As illustrated, in preferred embodiments, the groove  177  may extend such that it is parallel to the edges of the side surfaces  173 ,  174 . Also, in preferred embodiments, the groove  177  also may have a constant depth and a constant width. 
         [0032]    It will be appreciated by those skilled in the art that the amount of material removed from the side surfaces is dependent upon the desired or target natural frequency. Thus, the width and the depth of the groove  177  may be altered as necessary to achieve the targeted natural frequency. In some embodiments, the width and depth may not be constant across the the side surface. Further, in some embodiments, the desired frequency may also be achieved by forming one or more additional grooves  177  of the same or different size and shape across the third  173  and fourth side surface  174 . 
         [0033]    The removal of material from the stator blade base or mounting portion for purposes of tuning the natural frequency of the airfoil is a concept that may not only be retrofitted into existing compressor stator blades, but also used in the initial design and manufacture of stator blades. The ability to utilize the invention in existing stator blades provides a relatively quick hardware solution to a frequency related issue as compared to the normal cycle for the production of a new stator blade with a modified airfoil shape. 
         [0034]      FIGS. 8 through 10  illustrate alternative embodiments, any of which may be used to achieve the desired target natural frequency. While these figures illustrate examples of alternative side groove  177  configurations, it will be appreciated that the criteria and design options discussed above apply for each.  FIG. 8  illustrates side grooves  177  that are similar to the ones shown in  FIG. 7  except the location is in the first  171  and second side surfaces  172 , which, respectively, may be the pressure and suction side of the blade  155 .  FIG. 9  illustrates side grooves  177  that wrap around all four of the sides  171 ,  172 ,  173 ,  174  of the base  156 . 
         [0035]      FIG. 10  illustrates an exemplary embodiment, which is similar to  FIG. 7  (i.e., side grooves in the third side surface  173  and the fourth side surface  174 ) except for the addition of tapering rails  185 . It will be appreciated by those skilled in the art that the size and angle of the tapering rails  185  may be manipulated to alter the frequency of the blade  155  to achieve a target natural frequency. Accordingly, in some embodiments, the size and angle of the tapering rails  147  of the groove  177  may be altered as necessary to achieve the targeted natural frequency. This may be done in conjunction with or separately from the side grooves  177  described above. 
         [0036]      FIGS. 11 through 15  illustrate some preferred cross-sectional shapes for the side groove  177 .  FIG. 11  illustrates a semi-circular profile.  FIG. 12  illustrates a semi-oval shape.  FIG. 13  is a conical shape with a rounded end.  FIG. 14  depicts a rectangular shape that terminates into a semi-circular shape.  FIG. 15  is a rectilinear shape that is similar to the examples of  FIGS. 7 through 10 , but has rounded corners or fillets. 
         [0037]    From the above description of preferred embodiments of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims. Further, it should be apparent that the foregoing relates only to the described embodiments of the present application and that numerous changes and modifications may be made herein without departing from the spirit and scope of the application as defined by the following claims and the equivalents thereof.