Patent Publication Number: US-2020291786-A1

Title: Aerofoil for gas turbine incorporating one or more encapsulated void

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2017/056023 filed Mar. 14, 2017, and claims the benefit thereof. The International Application claims the benefit of United Kingdom Application No. GB 1604525.4 filed Mar. 17, 2016. All of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates to gas turbines, and more particularly to aerofoils for a gas turbine. 
     BACKGROUND OF INVENTION 
     When a gas turbine is operated at a given operational stage for example idle stage, take-off stage, climb stage, cruise stage, etc. aerofoils in the compressor section of the gas turbine are subjected to various excitation frequencies depending upon the stage of operation of the gas turbine. Generally, the excitation frequency which the aerofoil is subjected to is related to a rotational speed of the turbine which in turn depends on the operational stage of the turbine. The excitation frequency may also be dependent on other factors such as disturbances in the airflow around the aerofoil. 
     The aerofoils are capable of vibrating in different vibrational modes for example bending mode, edgewise mode, torsional mode, camber mode, and so on and so forth. The natural frequency for a given vibrational mode of the aerofoil is also sometimes referred to as the vibrational mode frequency for the given vibrational mode of the aerofoil. If the aerofoil experiences an excitation frequency equal to its natural frequency or vibrational mode frequency for a given vibrational mode, the aerofoil, and thus the blade having the aerofoil, is prone to failure as a result of resonant vibrations occurring in the aerofoil. Therefore, it is important to prevent resonant vibration in the aerofoil during operating conditions at a given operational stage. 
     Thus, in the aerofoil it is desirable to tune certain frequencies, i.e. natural frequency or vibrational mode frequency, out of undesirable ranges, i.e. frequencies around the excitation frequency for a given operational stage, to obviate or to at least reduce possibilities of substantial matching of the vibrational mode frequency and the excitation frequency at the given operational stage of the gas turbine, i.e. to reduce the number of times when the aerofoil will experience an excitation frequency equal to one of its natural frequencies at a given running condition. 
     SUMMARY OF INVENTION 
     Thus an object of the present disclosure is to provide an aerofoil for a gas turbine wherein a mass and stiffness of parts of aerofoil are manipulated such that the natural frequency or vibrational mode frequency for a given vibrational mode of the aerofoil is tuned out of undesirable ranges, i.e. tuned out of the frequencies around the excitation frequency for a given operational stage of the turbine. 
     The above objects are achieved by an aerofoil, a compressor for a gas turbine, a method for designing an aerofoil for a gas turbine, and a method for manufacturing an aerofoil for a gas turbine according to the present technique. Advantageous embodiments of the present technique are provided in dependent claims. Features of the independent claims may be combined with features of claims dependent on them respectively, and features of dependent claims can be combined together. 
     In a first aspect of the present technique, an aerofoil for a gas turbine or turbomachine is presented. The aerofoil extends from a platform. The aerofoil includes a generally concave side, also called pressure side, and a generally convex side, also called suction side. The concave side and the convex side meet at a trailing edge on one end and a leading edge on another end. The aerofoil has a tip. Furthermore the aerofoil has one or more voids. Each of the one or more voids is completely encapsulated within the aerofoil such that each of the one or more voids is not fluidly connected with an outside of the aerofoil i.e. no fluid, such as air, gas or a cooling liquid, can flow or flows from the outside of the aerofoil into the voids of the aerofoil. Similarly no fluid such as air, gas or a cooling liquid, can flow or flows from the voids of the aerofoil to the outside of the aerofoil. Moreover, a total volume of the one or more voids i.e. a total volume of all the voids, whether one or multiple, is between 5 percent and 30 percent of a volume of the aerofoil. The volume of the aerofoil is a volume defined by the concave side, the convex side, the leading edge, the trailing edge, the tip and a surface of the platform from which the aerofoil extends radially. 
     For a given vibrational mode of the aerofoil, a vibrational mode frequency corresponding to the given vibrational mode of the aerofoil is dependent on the mass and stiffness of a flexing section or flexing region of the aerofoil i.e. that region in the aerofoil which is subjected to maximum warping or bending and then reverting to shape in the given vibrational mode. The vibrational mode frequency corresponding to the given vibrational mode of the aerofoil is also dependent on the mass and stiffness of regions in the aerofoil around the flexing region of the aerofoil. Alterations of mass and stiffness of the flexing region or the regions surrounding the flexing region in the aerofoil alter the vibrational mode frequency of the given vibrational mode of the aerofoil. By introducing the one or more voids in the aerofoil in the flexing region or in the regions around the flexing region the mass and the stiffness of the flexing region or the regions around the flexing region in the aerofoil are altered and this in turn alters, i.e. lowers or increases, the vibrational mode frequency for the given vibrational mode. 
     Thus by having the one or more voids in the aerofoil with respect to the flexing region, i.e. either in the flexing region or outside the flexing region, the vibrational mode frequency is different as compared to a scenario when none of the one or more voids are present in the aerofoil. If in an aerofoil without the one or more voids the vibrational mode frequency would have been same or substantially similar to an excitation frequency to which the aerofoil may be subjected to in the gas turbine operating at a particular stage of operation, then in that aerofoil but now having the one or more voids the vibrational mode frequency differs from the excitation frequency to which the aerofoil may be subjected to in the gas turbine operating at the particular stage of operation. Thus ensuring reduction in possibility of occurrences of the given vibrational mode of the aerofoil when the gas turbine is operated at the particular stage of operation. 
     In a second aspect of the present technique, a compressor for a gas turbine is presented. The compressor includes an aerofoil as presented according to the first aspect of the present technique. 
     In a third aspect of the present technique, a method for designing an aerofoil for a gas turbine is presented. The method includes a step of identifying a flexing section in the aerofoil, wherein the flexing section corresponds to a predetermined vibrational mode of the aerofoil and a step of determining a vibrational mode frequency of the aerofoil, wherein the vibrational mode frequency corresponds to the predetermined vibrational mode of the aerofoil. The method further includes a step of determining an external excitation frequency for the aerofoil, wherein the external excitation frequency corresponds to an operational stage of the gas turbine. The method finally includes a step of altering the vibrational mode frequency of the aerofoil by introducing one or more voids in the aerofoil positioned inside the aerofoil with respect to the flexing section such that the vibrational mode frequency of the aerofoil after alteration is distinct from the external excitation frequency. In the method each of the one or more voids is completely encapsulated within the aerofoil such that each of the one or more voids is not fluidly connected with an outside of the aerofoil and wherein a total volume of the one or more voids is between 5 percent and 30 percent of a volume of the aerofoil. By introducing the one or more voids in the aerofoil in the flexing region or in the regions around the flexing region the mass and the stiffness of the flexing region or the regions around the flexing region in the aerofoil are altered and this in turn alters, i.e. lowers or increases, the vibrational mode frequency for the a vibrational mode. Thus the method of designing the aerofoil ensures a reduction in possibility of occurrences of the given vibrational mode of the aerofoil when the gas turbine is operated at the particular stage of operation. 
     In an embodiment of the method for designing, in the step of altering the vibrational mode frequency the one or more voids are introduced in the flexing section of the aerofoil to lower the vibrational mode frequency. In another embodiment of the method, in the step of altering the vibrational mode frequency the one or more voids are introduced outside of the flexing section of the aerofoil to increase the vibrational mode frequency. In the method the predetermined vibrational mode of the aerofoil is one of a bending mode, a torsional mode, an extension mode, a camber mode and a combination thereof. 
     In a fourth aspect of the present technique a method of manufacturing an aerofoil for a gas turbine is presented. The method includes a step of designing the aerofoil for the gas turbine according to the third aspect of the present technique and a step of forming the aerofoil according to the aerofoil so designed. In one embodiment of the method the step of forming the aerofoil comprises additive manufacturing technique. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above mentioned attributes and other features and advantages of the present technique and the manner of attaining them will become more apparent and the present technique itself will be better understood by reference to the following description of embodiments of the present technique taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows part of a turbine engine in a sectional view and in which an aerofoil of the present technique is incorporated; 
         FIG. 2  schematically illustrates a front view of an exemplary embodiment of an aerofoil with a void in accordance with aspects of the present technique; 
         FIG. 3  schematically illustrates a cross-section of a side view of the aerofoil with the void of  FIG. 2 ; 
         FIG. 4  schematically illustrates a cross-section of a top view of the aerofoil with the void of  FIGS. 2 and 3 ; 
         FIG. 5  schematically illustrates another exemplary embodiment of the aerofoil with multiple voids; 
         FIG. 6  schematically illustrates another exemplary embodiment of the aerofoil with multiple voids depicting a scheme for determining locations of each of the voids within the aerofoil; 
         FIG. 7  schematically illustrates an exemplary embodiment of the aerofoil with voids depicting a scheme of locations for the voids; 
         FIG. 8  schematically illustrates another exemplary embodiment of the aerofoil with voids depicting another scheme of locations for the voids; 
         FIG. 9  schematically illustrates yet another exemplary embodiment of the aerofoil with voids depicting yet another scheme of locations for the voids; 
         FIG. 10  schematically illustrates an alternative exemplary embodiment of the aerofoil of  FIG. 9  with voids depicting an alternative scheme of locations for the voids as opposed to the scheme of location depicted in  FIG. 9 ; 
         FIG. 11  schematically illustrates a further exemplary embodiment of the aerofoil with voids depicting further scheme of locations for the voids; 
         FIG. 12  schematically illustrates an alternative exemplary embodiment of the aerofoil of  FIG. 11  with voids depicting an alternative scheme of locations for the voids as opposed to the scheme of location depicted in  FIG. 11 ; 
         FIG. 13  schematically illustrates one more exemplary embodiment of the aerofoil with voids depicting one more scheme of locations for the voids; 
         FIG. 14  is a flow chart depicting a method for designing an aerofoil; 
         FIG. 15  schematically illustrates a model of an exemplary embodiment of the aerofoil for the method for designing the aerofoil; 
         FIG. 16  schematically illustrates a model of another exemplary embodiment of the aerofoil for the method for designing the aerofoil; and 
         FIG. 17  is a flow chart depicting a method for manufacturing an aerofoil with voids; in accordance with aspects of the present technique. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Hereinafter, above-mentioned and other features of the present technique are described in details. Various embodiments are described with reference to the drawing, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be noted that the illustrated embodiments are intended to explain, and not to limit the invention. It may be evident that such embodiments may be practiced without these specific details. 
       FIG. 1  shows an example of a gas turbine engine  10  in a sectional view. The gas turbine engine  10  comprises, in flow series, an inlet  12 , a compressor or compressor section  14 , a combustor section  16  and a turbine section  18  which are generally arranged in flow series and generally about and in the direction of a longitudinal or rotational axis  20 . The gas turbine engine  10  further comprises a shaft  22  which is rotatable about the rotational axis  20  and which extends longitudinally through the gas turbine engine  10 . The shaft  22  drivingly connects the turbine section  18  to the compressor section  14 . 
     In operation of the gas turbine engine  10 , air  24 , which is taken in through the air inlet  12  is compressed by the compressor section  14  and delivered to the combustion section or burner section  16 . The burner section  16  comprises a burner plenum  26 , one or more combustion chambers  28  and at least one burner  30  fixed to each combustion chamber  28 . The combustion chambers  28  and the burners  30  are located inside the burner plenum  26 . The compressed air passing through the compressor  14  enters a diffuser  32  and is discharged from the diffuser  32  into the burner plenum  26  from where a portion of the air enters the burner  30  and is mixed with a gaseous or liquid fuel. The air/fuel mixture is then burned and the combustion gas  34  or working gas from the combustion is channeled through the combustion chamber  28  to the turbine section  18  via a transition duct  17 . 
     This exemplary gas turbine engine  10  has a cannular combustor section arrangement  16 , which is constituted by an annular array of combustor cans  19  each having the burner  30  and the combustion chamber  28 , the transition duct  17  has a generally circular inlet that interfaces with the combustor chamber  28  and an outlet in the form of an annular segment. An annular array of transition duct outlets form an annulus for channeling the combustion gases to the turbine  18 . 
     The turbine section  18  comprises a number of blade carrying discs  36  attached to the shaft  22 . In the present example, two discs  36  each carry an annular array of turbine blades  38 . However, the number of blade carrying discs could be different, i.e. only one disc or more than two discs. In addition, guiding vanes  40 , which are fixed to a stator  42  of the gas turbine engine  10 , are disposed between the stages of annular arrays of turbine blades  38 . Between the exit of the combustion chamber  28  and the leading turbine blades  38  inlet guiding vanes  44  are provided and turn the flow of working gas onto the turbine blades  38 . 
     The combustion gas from the combustion chamber  28  enters the turbine section  18  and drives the turbine blades  38  which in turn rotate the shaft  22 . The guiding vanes  40 ,  44  serve to optimise the angle of the combustion or working gas on the turbine blades  38 . 
     The turbine section  18  drives the compressor section  14 . The compressor section  14  comprises an axial series of vane stages  46  and rotor blade stages  48 . The rotor blade stages  48  comprise a rotor disc supporting an annular array of blades. The compressor section  14  also comprises a casing  50  that surrounds the rotor stages and supports the vane stages  48 . The guide vane stages include an annular array of radially extending vanes that are mounted to the casing  50 . The vanes are provided to present gas flow at an optimal angle for the blades at a given engine operational point. Some of the guide vane stages have variable vanes, where the angle of the vanes, about their own longitudinal axis, can be adjusted for angle according to air flow characteristics that can occur at different engine operations conditions. 
     The casing  50  defines a radially outer surface  52  of the passage  56  of the compressor  14 . A radially inner surface  54  of the passage  56  is at least partly defined by a rotor drum  53  of the rotor which is partly defined by the annular array of blades  48 . 
     The present technique is described with reference to the above exemplary turbine engine having a single shaft or spool connecting a single, multi-stage compressor and a single, one or more stage turbine. However, it should be appreciated that the present technique is equally applicable to two or three shaft engines and which can be used for industrial, aero or marine applications. 
     The terms upstream and downstream refer to the flow direction of the airflow and/or working gas flow through the engine unless otherwise stated. The terms forward and rearward refer to the general flow of gas through the engine. The terms axial, radial and circumferential are made with reference to the rotational axis  20  of the engine. 
       FIGS. 2, 3 and 4  schematically illustrate different views of an exemplary embodiment of an aerofoil  1  with a void  70 , in accordance with aspects of the present technique.  FIGS. 2-4  have been explained hereinafter in combination with  FIG. 1 . The aerofoil  1  extends from a platform  60 , and more particularly from a side  62 , hereinafter referred to as the aerofoil side  62 , of the platform  60 . From another side  64 , hereinafter referred to as the root side  64 , of the platform  60  emanates a root  68  or a fixing part  68 . The root  68  or the fixing part  68  may be used to attach the aerofoil  1  to a compressor disc (not shown in  FIGS. 2-4 ) and thus the aerofoil  1  forms a part of the compressor blades  48  in the compressor section  14 . The present technique may be implemented in the aerofoil  1  having an average chord/thickness aspect ratio typically above 8. The root  68  or the fixing part  68  may alternatively be used to attach the aerofoil  1  to the casing  50  and thus the aerofoil  1  forms a part of the compressor vanes  46  in the compressor section  14 . 
     The aerofoil  1  includes a generally convex side  104 , also called suction side  104 , and a generally concave side  102 , also called pressure side  102 . The convex side  104  and the concave side  102  meet at a trailing edge  108  on one end and a leading edge  106  on another end. The aerofoil  1  has a tip  110 . The aerofoil  1  may also include a shroud (not shown) at the tip  110  of the aerofoil  1 . According to the present technique, furthermore, the aerofoil  1  has one or more voids  70 . Each of the one or more voids  70  is completely encapsulated within the aerofoil  1  such that each of the one or more voids  70  is not fluidly connected with an outside  5  of the aerofoil i.e. no fluid, such as air, gas or a cooling liquid, can flow or flows from the outside  5  of the aerofoil  1  into the void  70  of the aerofoil  1 . Similarly no fluid such as air, gas or a cooling liquid, can flow or flows from the void  70  of the aerofoil  1  to the outside  5  of the aerofoil. The outside  5  of the aerofoil  1  may be a space directly outside of the aerofoil  1  or may be a pathway (not shown) such as a cooling channel or an opening that is fluidly connected to the outside  5  of the aerofoil  1 . 
     Moreover, a total volume of the one or more voids  70  i.e. a total volume of all the voids  70 , whether one or multiple, is between 5 percent and 30 percent of a volume of the aerofoil  1 . The volume of the aerofoil  1  is a volume defined by the concave side  102 , the convex side  104 , the leading edge  106 , the trailing edge  108 , the tip  110  and the aerofoil side  62  of the platform  60  from which the aerofoil  1  extends radially. The volume of the aerofoil  1  may be understood as the space enclosed by the aerofoil  1  and includes the total volume of all the voids  70 , and the volume occupied by material of the aerofoil  1  in forming the aerofoil  1 , as well as any other channels or pathways that may be defined within the aerofoil  1 . The volume of the aerofoil  1  does not include a volume of the platform  60  and the root  68 . The aerofoil  1  may be formed of a homogenous material or may be formed of a composite material. 
     It may be noted that the void  70  will not be visible from an outside  5  of the aerofoil  1  and the void  70  has been made schematically visible in  FIG. 2  only for purposes of explanation.  FIG. 2  may be understood as if a part of the concave wall  102  has been removed to show the void  70  which is internal to the convex side  104 , the concave side  102 , the leading edge  106 , the trailing edge  108 , the tip  110  and the aerofoil side  62  of the platform  60  and completely limited within the space defined by the convex side  104 , the concave side  102 , the leading edge  106 , the trailing edge  108 , the tip  110  and the aerofoil side  62  of the platform  60 . Furthermore, the void  70  is physically removed from and does not open at the convex side  104 , the concave side  102 , the leading edge  106 , the trailing edge  108 , the tip  110  and the aerofoil side  62  of the platform  60 .  FIGS. 2 and 3  show a more realistic representation of the void  70  of the aerofoil  1 , and as shown in  FIGS. 2 and 3 , the void  70  is physically removed from external surfaces of the convex side  104 , the concave side  102 , the leading edge  106 , the trailing edge  108 , the tip  110  and the aerofoil side  62  of the platform  60 . The void  70  has a direct effect on mass and stiffness of a part of the aerofoil  1  where the void  70  is present, for example as depicted in  FIG. 2  the void  70  is present towards the middle of the aerofoil  1  and towards the tip  110  of the aerofoil  1  and thus the void  70  decreases the mass and the stiffness of that part of the aerofoil  1  where the void  70  is present, as compared to a respective part in a similar aerofoil (not shown) but without the void  70 . Additionally, the void  70  has also an effect on physical property such as mass of another part which is adjacent to the part of the aerofoil  1  where the void  70  is present, for example as depicted in  FIG. 2  the a part of the aerofoil  1  between the part of the aerofoil where the void  70  is present and the aerofoil side  62  of the platform  60 . 
       FIG. 5  represents an exemplary embodiment of the aerofoil  1  where the void  70  includes at least a first void  71  and a second void  72 , and may include more voids as well. Each of the voids  71 ,  72  may be positioned in the aerofoil  1  in a part of interest. The total volume of the one or more voids  70  i.e. a total volume of all the voids  71 ,  72  forming the void  70 , is between 5 percent and 30 percent of the volume of the aerofoil  1 . 
     Vibration mode frequencies are a function of the mass and stiffness of the aerofoil  1 , particularly of the parts of the aerofoil  1  which undergo maximum flexing in the aerofoil  1 . Mass and stiffness are defined by the shape, volume, strength (modulus) and density of the material forming the aerofoil  1 . 
     Introduction of completely encapsulated voids  70  i.e. for example say voids  71 ,  72 , into an otherwise solid metallic aerofoil  1  whilst using no separate parts, joining techniques or additional materials ensures homogeneity and structural integrity of the aerofoil  1 . The encapsulated void  70  has no fluid flow through it and by means of the absence of material in the void  70  the mass and stiffness of the part of the aerofoil  1  where the void  70  is located is reduced which in turn can alter the vibrational mode frequency of the aerofoil  1 . 
     The void  70 , for example the voids  71 ,  72  may be selectively shaped, scaled and positioned within the aerofoil  1  so as to beneficially affect the vibrational behaviour of the aerofoil  1  by moving a vibrational mode frequency value to a higher or to a lower value to avoid coincidence with an exciting frequency. If it is desired to avoid a vibrational mode of the aerofoil  1  by lowering the vibrational mode frequency of the aerofoil  1 , then the stiffness is reduced in the flexing part, also called as the dynamically strained part, by incorporating the void  70  in the flexing part. 
     Alternatively, if it is desired to avoid a vibrational mode of the aerofoil  1  by increasing or raising the vibrational mode frequency of the aerofoil  1 , then mass of a part or a section outboard or overhanging of the flexing part is reduced by incorporation of the void  70  in the outboard or the overhanging part and thus the influence of the mass of the outboard or overhanging part on the flexing part is reduced and thus the flexing part acts as more stiff and thereby the vibrational mode frequency in increased. 
     The shape, scale, i.e. the volumetric size, and position of the void  70  may differ according to the vibration mode being addressed. The shape of the void  70 , for example the voids  71 ,  72 , may be spherical, cylindrical, horizontal to the platform  60 , vertical to the platform  60  or inclined to the platform  60 , may be parallel sided or tapered sided, and may be straight edged, curved or defined by a spline (eg when following a contour of the aerofoil  1  surfaces such as a surface of the concave side  102  or a surface of the convex side  104 ) or may be freeform i.e. irregular geometric shape. As depicted in  FIGS. 3 and 4 , the void  70  may be positioned at minimum of 10% of the local aerofoil section thickness away from an external surface, such as the surface of the concave side  102  or the convex side  104 . Furthermore at least one of the one or more voids  70  may comprise a support member (not shown) connecting a first inner section (not shown) of the aerofoil  1  and the second inner section (not shown) of the aerofoil  1 , wherein the first and the second inner sections of the aerofoil  1  are adjacent to the void  70  and wherein the support member is disposed in the void  70 . The support member may be understood as a rib or joint or a bar running from one end of the void  70  to another end of the void  70  and formed of the same material as the rest of the aerofoil  1 . In one embodiment, the void  70  may have several such support members and may be visualized as a honeycomb structure of the void  70 . 
       FIG. 6  schematically illustrates another exemplary embodiment of the aerofoil  1  with multiple voids  71 ,  72 , namely the first void  71  and the second void  72 , and depicts a scheme for determining locations of each of the voids  71 ,  72  within the aerofoil  1 . Each of the voids  70  has a centroid, for example a centroid  73  of the first void  71 , a centroid  74  of the second void  72 . The scheme uses ‘radial distance’ and ‘circumferential distance’ to define location of the centroid  73 ,  74  within the aerofoil  1  and thus the location of the voids  71 ,  72  within the aerofoil  1 . The centroid of the void  70 , for example centroids  73  or  74  of the voids  71 ,  72  may be understood as a point that represents a mean position of all the points of the void  70 ,  71 ,  72 . For a symmetrical  3 D shaped void for example a spherical shaped void  70 ,  71 ,  72  the centroid  73 ,  74  will be geometric center of the void  70 ,  71 ,  72 . The void  70 ,  71 ,  72  may have a desired geometric shape such as, but not limited to, a sphere, a parallelepiped, a cone, a cylinder, and so on and so forth. 
     The radial distance ‘h’ is measured from the aerofoil side  62  of the platform  60  to the centroid of the void  70 , for example for the first void  71 , the first radial distance h 1  is measured from the aerofoil side  62  of the platform  60  to the centroid  73  of the first void  71  and for the second void  72 , the second radial distance h 2  is measured from the aerofoil side  62  of the platform  60  to the centroid  74  of the second void  72 . The radial distances are measured substantially perpendicular to the aerofoil side  62  of the platform  60  or to perpendicular to the rotational axis  20 . The circumferential distance ‘c’ is measured from the leading edge  106  or the trailing edge  108 , as specified with the measurement and is performed substantially perpendicular to the radial direction, i.e. substantially perpendicularly to the platform  60 , upto the centroid of the void  70 , for example for the first void  71 , the first circumferential distance c 1  may be measured from the leading edge  106  or the trailing edge  108 , as may be specified with the measurement, to the centroid  73  of the first void  71  and for the second void  72 , the second circumferential distance c 2  may be measured from the leading edge  106  or the trailing edge  108 , as may be specified with the measurement, to the centroid  74  of the second void  72 . The circumferential distances are measured substantially tangential to the rotational axis  20 . Thus from the radial distances h 1 , h 2  and the circumferential distances c 1 , c 2  of the centroid  73 ,  74  of the first void  71  and/or the second void  72 , a location of the first void  71  and/or the second void  72  within the aerofoil  1  is determined. 
     The radial distance h for a centroid of the void  70  is expressed hereinafter as percentages of a height ‘H’ of the aerofoil  1  measured from the aerofoil side  62  of the platform upto the tip  110  of the aerofoil  1  through the centroid for which the radial distance is being measured. For example, the first radial distance h 1  of the centroid  73  of the first void  71  has been expressed hereinafter as percentage of a height ‘H’ of the aerofoil  1  measured from the aerofoil side  62  of the platform upto the tip  110  of the aerofoil  1  through the centroid  73  of the first void  71 . The measurement of the height H in relation to which the first radial distance h 1  is expressed is performed substantially perpendicularly to the aerofoil side  62  of the platform  60 , or in other words measurement of the height H in relation to which the first radial distance h 1  is expressed is performed along the first radial distance h 1 . Similarly, the second radial distance h 2  of the centroid  74  of the second void  72  has been expressed hereinafter as percentage of a height ‘H’ of the aerofoil  1  measured from the aerofoil side  62  of the platform upto the tip  110  of the aerofoil  1  through the centroid  74  of the second void  72 . The measurement of the height H in relation to which the second radial distance h 2  is expressed is performed substantially perpendicularly to the aerofoil side  62  of the platform  60 , or in other words measurement of the height H in relation to which the second radial distance h 2  is expressed is performed along the second radial distance h 2 . 
     The circumferential distance c for a centroid of the void  70  is expressed hereinafter as percentages of a chord length ‘C’ of the aerofoil  1  measured from the leading edge  106  to the trailing edge  108  of the aerofoil  1  through the centroid for which the radial distance is being measured. For example, the first circumferential distance c 1  of the centroid  73  of the first void  71  has been expressed hereinafter as percentage of a chord length ‘C’ of the aerofoil  1  measured from the leading edge  106  to the trailing edge  108  of the aerofoil  1  through the centroid  73  of the first void  71 . The measurement of the chord length C in relation to which the first circumferential distance c 1  is expressed is performed substantially parallelly to the aerofoil side  62  of the platform  60 , or in other words measurement of the chord length C in relation to which the first circumferential distance c 1  is expressed is performed along the circumferential distance c 1 . Similarly, the second circumferential distance c 2  of the centroid  74  of the second void  72  has been expressed hereinafter as percentage of a chord length ‘C’ of the aerofoil  1  measured from the leading edge  106  to the trailing edge  108  of the aerofoil  1  through the centroid  74  of the second void  72 . The measurement of the chord length C in relation to which the second circumferential distance c 2  is expressed is performed substantially parallelly to the aerofoil side  62  of the platform  60 , or in other words measurement of the chord length C in relation to which the second circumferential distance c 2  is expressed is performed along the second circumferential distance c 2 . 
       FIGS. 7 to 13  present various exemplary embodiments of the aerofoil  1  of the present technique depicting different locations for the first void  71  and/or the second void  72  enclosed within the aerofoil  1 . It may be noted that the location of the first void  71  has been expressed by defining the first radial distance h 1  and the first circumferential distance c 1  of the centroid  73  of the first void  71 , although the centroid  73  has not been depicted in  FIGS. 7 to 13  for sake of simplicity. Similarly, the location of the second void  72 , if present, has been expressed by defining the second radial distance h 2  and the second circumferential distance c 2  of the centroid  74  of the second void  72 , although the centroid  74  has not been depicted in  FIGS. 7 to 13  for sake of simplicity. 
     As shown in  FIG. 7 , the one or more voids  70  includes at least the first void  71  having the centroid  73  positioned at the first radial distance h 1  and the first circumferential distance c 1  measured from the leading edge  106 . The first radial distance h 1  of the centroid  73  of the first void  71  is between 60 percent and 90 percent of the height H of the aerofoil  1  measured along the first radial distance h 1  of the centroid  73  of the first void  71  and the first circumferential distance c 1  of the centroid  73  of the first void  71  is between 30 percent and 70 percent of the chord length C of the aerofoil  1  measured along the first circumferential distance c 1  of the centroid  73  of the first void  71 . 
     In vibrational mode  1 F i.e. first bending mode or first flapping mode, bending or vibrating of the aerofoil may be visualized to be along YZ plane in a three-coordinate system depicted in  FIG. 2 , where the X axis is in the direction running on the aerofoil side  62  of the platform  60  along the leading edge  106  and the trailing edge  108 , the Y axis is in the direction running on the aerofoil side  62  of the platform  60  and perpendicular to the X axis, and Z axis is mutually perpendicular to both X axis and Y axis. For the  1 F mode the dynamic stress or the strain in the aerofoil  1  is centered in the aerofoil  1  just above the aerofoil side  62  of the platform  60 . Thus by introducing the first void  71  as shown in  FIG. 7 , a mass of the overhanging or overboard region of the aerofoil  1  outside the flexing region, i.e. the region in the aerofoil  1  just above the aerofoil side  62  of the platform  60 , is decreased and this in turn results in increase of the vibrational mode frequency, corresponding to the  1 F mode, of the aerofoil  1 . 
     In vibrational mode  1 E i.e. first edgewise mode, bending or vibrating of the aerofoil may be visualized to be along XZ plane in a three-coordinate system depicted in  FIG. 2 , as explained earlier. For the  1 E mode the dynamic stress or the strain in the aerofoil  1  is present in the aerofoil  1  just above the aerofoil side  62  of the platform  60  leaning towards the leading edge  106  and the trailing edge  108 , say the flexing section. Thus by introducing the first void  71  as shown in  FIG. 7 , a mass of the overhanging or overboard region of the aerofoil  1  outside the flexing section is decreased and this in turn results in increase of the vibrational mode frequency, corresponding to the  1 E mode, of the aerofoil  1 . 
     Furthermore, the aerofoil  1  may include the second void  72  in addition to the first void  71 . The second void  72  having the centroid  74  is positioned at the second radial distance h 2  and the second circumferential distance c 2  measured from the leading edge  106 . The second radial distance h 2  of the centroid  74  of the second void  72  is between 40 percent and 60 percent of the height H of the aerofoil  1  measured along the second radial distance h 2  of the centroid  74  of the second void  72  and the second circumferential distance c 2  of the centroid  74  of the second void  72  is between 30 percent and 70 percent of the chord length C of the aerofoil  1  measured along the second circumferential distance c 2  of the centroid  74  of the second void  72 . The introduction of the second void  72  aids in decrease in the vibrational mode frequency for  2 F mode of vibration, i.e. second order bending mode vibration. 
     As shown in  FIG. 8 , the one or more voids  70  includes at least the first void  71  having the centroid  73  positioned at the first radial distance h 1  and the first circumferential distance c 1  measured from the leading edge  106 . The first radial distance h 1  of the centroid  73  of the first void  71  is between 5 percent and 20 percent of the height H of the aerofoil  1  measured along the first radial distance h 1  of the centroid  73  of the first void  71  and the first circumferential distance c 1  of the centroid  73  of the first void  71  is between 30 percent and 70 percent of the chord length C of the aerofoil  1  measured along the first circumferential distance c 1  of the centroid  73  of the first void  71 . 
     In vibrational mode  1 F i.e. first bending mode or first flapping mode, bending or vibrating of the aerofoil may be visualized to be along YZ plane in the three-coordinate system depicted in  FIG. 2 . As mentioned earlier, for the  1 F mode the dynamic stress or the strain in the aerofoil  1  is centered in the aerofoil  1  just above the aerofoil side  62  of the platform  60 , say the flexing section. Thus by introducing the first void  71  as shown in  FIG. 8 , a mass and stiffness of the flexing section of the aerofoil  1  is decreased and this in turn results in decrease of the vibrational mode frequency, corresponding to the  1 F mode, of the aerofoil  1 . 
     Furthermore, as shown in  FIG. 8 , the aerofoil  1  may include the second void  72  in addition to the first void  71 . The second void  72  having the centroid  74  is positioned at the second radial distance h 2  and the second circumferential distance c 2  measured from the leading edge  106 . The second radial distance h 2  of the centroid  74  of the second void  72  is between 40 percent and 60 percent of the height H of the aerofoil  1  measured along the second radial distance h 2  of the centroid  74  of the second void  72  and the second circumferential distance c 2  of the centroid  74  of the second void  72  is between 30 percent and 70 percent of the chord length C of the aerofoil  1  measured along the second circumferential distance c 2  of the centroid  74  of the second void  72 . The introduction of the second void  72  aids in decrease in the vibrational mode frequency for  2 F mode of vibration, i.e. second order bending mode vibration. 
     As shown in  FIGS. 9 and 10 , the one or more voids  70  includes at least the first void  71  having the centroid  73  positioned at the first radial distance h 1  and the first circumferential distance c 1  measured from the leading edge  106 . The first radial distance h 1  of the centroid  73  of the first void  71  is between 5 percent and 20 percent of the height H of the aerofoil  1  measured along the first radial distance h 1  of the centroid  73  of the first void  71  and the first circumferential distance c 1  of the centroid  73  of the first void  71  is between 10 percent and 25 percent of the chord length C of the aerofoil  1  measured along the first circumferential distance c 1  of the centroid  73  of the first void  71  as depicted in the exemplary embodiment of the aerofoil  1  of  FIG. 9 , or alternatively, the first circumferential distance c 1  of the centroid  73  of the first void  71  is between 75 percent and 90 percent of the chord length C of the aerofoil  1  measured along the first circumferential distance c 1  of the centroid  73  of the first void  71  as depicted in the exemplary embodiment of the aerofoil  1  of  FIG. 10 . 
     Furthermore, as shown in  FIGS. 9 and 10 , the aerofoil  1  may include the second void  72  in addition to the first void  71 . The second void  72  has the centroid  74  positioned at the second radial distance h 2  and the second circumferential distance c 2  measured from the leading edge  106 . The second radial distance h 2  of the centroid  74  of the second void  72  is between 5 percent and 20 percent of the height H of the aerofoil  1  measured along the second radial distance h 2  of the centroid  74  of the second void  72  and the second circumferential distance c 2  of the centroid  74  of the second void  72  is between 75 percent and 90 percent of the chord length C of the aerofoil  1  measured along the second circumferential distance c 2  of the centroid  74  of the second void  72  as depicted in the exemplary embodiment of the aerofoil  1  of  FIG. 9 , or alternatively, the second circumferential distance c 2  of the centroid  74  of the second void  72  is between 10 percent and 25 percent of the chord length C of the aerofoil  1  measured along the second circumferential distance c 2  of the centroid  74  of the second void  72  as depicted in the exemplary embodiment of the aerofoil  1  of  FIG. 10 . 
     It may be noted that the circumferential distances c 1  and c 2  described in the present disclosure, for example as described above in relation to  FIGS. 9 and 10 , have been expressed as measured from the leading edge  106  or the trailing edge, for example for  FIGS. 9 and 10  have been expressed as measured from the leading edge  106  but as may be appreciated by one skilled in the art, the circumferential distances c 1  and c 2  can also be expressed as measured from the other edge for example the trailing edge  108  for  FIGS. 9 and 10 , such as 75 percent to 90 percent from leading edge  106  may be expressed as 10 percent to 25 percent from the trailing edge  108 . 
     In vibrational mode  1 E i.e. first edgewise mode, as explained earlier, the dynamic stress or the strain in the aerofoil  1  is present in the aerofoil  1  just above the aerofoil side  62  of the platform  60  leaning towards the leading edge  106  and the trailing edge  108 , say the flexing section. Thus by introducing the first void  71 , and optionally the second void  72 , as shown in  FIGS. 9 and 10 , a mass and stiffness of the flexing section is decreased and this in turn results in decrease of the vibrational mode frequency, corresponding to the  1 E mode, of the aerofoil  1 . 
     As shown in  FIGS. 11 and 12 , the one or more voids  70  includes at least the first void  71  having the centroid  73  positioned at the first radial distance h 1  and the first circumferential distance c 1  measured from the leading edge  106 . The first radial distance h 1  of the centroid  73  of the first void  71  is between 80 percent and 90 percent of the height H of the aerofoil  1  measured along the first radial distance h 1  of the centroid  73  of the first void  71  and the first circumferential distance c 1  of the centroid  73  of the first void  71  is between 10 percent and 25 percent of the chord length C of the aerofoil  1  measured along the first circumferential distance c 1  of the centroid  73  of the first void  71  as depicted in the exemplary embodiment of the aerofoil  1  of  FIG. 11 , or alternatively, the first circumferential distance c 1  of the centroid  73  of the first void  71  is between 75 percent and 90 percent of the chord length C of the aerofoil  1  measured along the first circumferential distance c 1  of the centroid  73  of the first void  71  as depicted in the exemplary embodiment of the aerofoil  1  of  FIG. 12 . 
     Furthermore, as shown in  FIGS. 11 and 12 , the aerofoil  1  may include the second void  72  in addition to the first void  71 . The second void  72  has the centroid  74  positioned at the second radial distance h 2  and the second circumferential distance c 2  measured from the leading edge  106 . The second radial distance h 2  of the centroid  74  of the second void  72  is between 80 percent and 90 percent of the height H of the aerofoil  1  measured along the second radial distance h 2  of the centroid  74  of the second void  72  and the second circumferential distance c 2  of the centroid  74  of the second void  72  is between 75 percent and 90 percent of the chord length C of the aerofoil  1  measured along the second circumferential distance c 2  of the centroid  74  of the second void  72  as depicted in the exemplary embodiment of the aerofoil  1  of  FIG. 11 , or alternatively, the second circumferential distance c 2  of the centroid  74  of the second void  72  is between 10 percent and 25 percent of the chord length C of the aerofoil  1  measured along the second circumferential distance c 2  of the centroid  74  of the second void  72  as depicted in the exemplary embodiment of the aerofoil  1  of  FIG. 12 . 
     In vibrational mode  1 T i.e. first torsional mode, bending or vibrating of the aerofoil  1  may be visualized as the aerofoil  1  is fixed at the platform  60  but progressively twists towards the tip  110  as viewed in the XY plane in the three-coordinate system depicted in  FIG. 2 . For the  1 T mode the dynamic stress or the strain in the aerofoil  1  is centered in the aerofoil  1  just above the aerofoil side  62  of the platform  60  centered between the leading edge  106  and the trailing edge  108 , say the flexing section. Thus by introducing the first void  71 , and optionally the second void  72 , as shown in  FIGS. 11 and 12 , a mass of a region outside the flexing section of the aerofoil  1  is decreased and this in turn results in increase of the vibrational mode frequency, corresponding to the  1 T mode, of the aerofoil  1 . 
     As shown in  FIG. 13 , the one or more voids  70  includes at least the first void  71  having the centroid  73  positioned at the first radial distance h 1  and the first circumferential distance c 1  measured from the leading edge  106 . The first radial distance h 1  of the centroid  73  of the first void  71  is between 15 percent and 40 percent of the height H of the aerofoil  1  measured along the first radial distance h 1  of the centroid  73  of the first void  71  and the first circumferential distance c 1  of the centroid  73  of the first void  71  is between 40 percent and 60 percent of the chord length C of the aerofoil  1  measured along the first circumferential distance c 1  of the centroid  73  of the first void  71 . 
     In vibrational mode  1 T i.e. first torsional mode, the flexing section in the aerofoil  1  is in the aerofoil  1  just above the aerofoil side  62  of the platform  60  centered between the leading edge  106  and the trailing edge  108 . Thus by introducing the first void  71 , as shown in  FIG. 13 , a mass and stiffness of the flexing section of the aerofoil  1  is decreased and this in turn results in decrease of the vibrational mode frequency, corresponding to the  1 T mode, of the aerofoil  1 . 
     It may be noted that the vibrational modes addressed in  FIGS. 7 to 13  are for exemplary purposes only, and other vibrational modes for example, camber mode or second order vibrational modes, or combination of different vibration modes can be addressed in similar way within the scope of the present technique. 
     According to the second aspect of the present technique, the aerofoil  1  of the present technique as described in relation to  FIGS. 2 to 13  is incorporated in the compressor  14  as shown in  FIG. 1 . 
       FIG. 14  is a flow chart depicting a method  900  for designing the aerofoil  1 .  FIGS. 15 and 16  schematically illustrates a model of an exemplary embodiment of the aerofoil  1  for the method  900  for designing the aerofoil  1 . 
     The method  900  includes a step  500  of identifying a flexing section  75  (shown in  FIGS. 15 and 16 ) in the aerofoil  1 . The flexing section  75  corresponds to a predetermined vibrational mode of the aerofoil  1 . The flexing section  75  in the aerofoil  1  is the section or region in the aerofoil  1  which is subjected to maximum warping or bending and then reverting to shape in the given vibrational mode. The method  900  also includes a step  600  of determining a vibrational mode frequency of the aerofoil  1 . The vibrational mode frequency corresponds to the predetermined vibrational mode of the aerofoil  1 . The predetermined vibrational mode of the aerofoil  1  may be, but not limited to, one of a bending mode, a torsional mode, an extension mode, a camber mode and a combination thereof. 
     The method  900  further includes a step  700  of determining an external excitation frequency for the aerofoil  1 . The external excitation frequency corresponds to an operational stage of the gas turbine  10 . The method  900  finally includes a step  800  of altering the vibrational mode frequency of the aerofoil  1  by introducing one or more voids  70 ,  71 ,  72  in the aerofoil  1  positioned inside the aerofoil  1  with respect to the flexing section  75  such that the vibrational mode frequency of the aerofoil  1  after alteration is distinct from the external excitation frequency. 
     In the method  900  each of the one or more voids  70 ,  71 ,  72  is completely encapsulated within the aerofoil  1  and may be understood as explained with reference to  FIGS. 2 to 13  hereinabove. Thus each of the one or more voids  70 ,  71 ,  72  is not fluidly connected with the outside  5  of the aerofoil  1  and the total volume of the one or more voids  70 ,  71 ,  72  is between 5 percent and 30 percent of the volume of the aerofoil  1 . 
     As shown in  FIG. 15 , in an embodiment of the method  900  for designing the aerofoil  1 , in the step  800  the one or more voids  70 ,  71 ,  72  are introduced in the flexing section  75  of the aerofoil  1  to lower the vibrational mode frequency. In another embodiment of the method  900 , as shown in  FIG. 16 , in an embodiment of the method  900  for designing the aerofoil  1 , in the step  800  the one or more voids  70 ,  71 ,  72  are introduced outside the flexing section  75  of the aerofoil  1  to increase or raise the vibrational mode frequency. 
       FIG. 17  is a flow chart depicting a method  1000  for manufacturing the aerofoil  1  with voids  70 ,  71 ,  72 ; in accordance with aspects of the present technique. The method  1000  includes a step  900  of designing the aerofoil  1  for the gas turbine  10 . The step  900  is same as the method  900  explained in reference to  FIG. 14  hereinabove. The method  1000  further includes a step  950  of forming the aerofoil  1  according to the aerofoil  1  so designed in the step  900 . In one embodiment of the method  1000  the step  950  of forming the aerofoil  1  includes additive manufacturing technique such as, but not limited to, laser sintering, selective laser sintering, and so on and so forth. 
     While the present technique has been described in detail with reference to certain embodiments, it should be appreciated that the present technique is not limited to those precise embodiments. Rather, in view of the present disclosure which describes exemplary modes for practicing the invention, many modifications and variations would present themselves, to those skilled in the art without departing from the scope and spirit of this invention. The scope of the invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope. 
     LIST OF REFERENCE CHARACTERS 
     
         
           1  aerofoil 
           5  outside of the aerofoil 
           10  gas turbine engine 
           12  inlet 
           14  compressor section 
           16  combustor section or burner section 
           17  transition duct 
           18  turbine section 
           19  combustor cans 
           20  longitudinal or rotational axis 
           22  shaft 
           24  air 
           26  burner plenum 
           28  combustion chamber 
           30  burner 
           32  diffuser 
           34  combustion gas or working gas 
           36  blade carrying discs 
           38  turbine blades 
           40  guiding vanes 
           42  stator 
           44  inlet guiding vanes 
           46  vane stages 
           48  rotor blade stages 
           50  casing 
           52  radially outer surface 
           53  rotor drum 
           54  radially inner surface 
           56  passage 
           60  platform 
           62  aerofoil side of platoform 
           64  root side of platform 
           68  root 
           70  void 
           71  first void 
           72  second void 
           73  centroid of the first void 
           74  centroid of the second void 
           75  flexing section 
           102  concave side 
           104  convex side 
           106  leading edge 
           108  trailing edge 
           110  tip 
           500  step of identifying a flexing section in the aerofoil 
           600  step of determining a vibrational mode frequency 
           700  step of determining an external excitation frequency 
           800  step of altering the vibrational mode frequency 
           900  method for designing the aerofoil 
           950  step of forming the aerofoil 
           1000  method for manufacturing the aerofoil 
         c 1  circumferential distance of the centroid of first void 
         c 2  circumferential distance of the centroid of second void 
         C 1 , C 2  Chord lengths of the aerofoil 
         h 1  radial distance of the centroid of the first void 
         h 2  radial distance of the centroid of the second void 
         H 1 , H 2  Height of the aerofoil