Patent Publication Number: US-11643934-B2

Title: Trailing edge tip cooling of blade of a gas turbine blade

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to European Patent Application No. 20 207 341.7, filed on Nov. 13, 2020, the disclosure of which is incorporated herein by reference in its entirety. 
     FIELD 
     Apparatuses and methods consistent with exemplary embodiments relate to gas turbines, and more particularly, to techniques for cooling a trailing edge tip of a gas turbine blade. 
     BACKGROUND 
     A gas turbine is a power engine that mixes air compressed by a compressor with fuel for combustion and rotates a turbine with high-temperature gas produced by the combustion. The gas turbine is used to drive a generator, an aircraft, a ship, a train, and so forth. 
     Referring to  FIGS.  4  and  7   , a related art gas turbine blade  1 ′ include an airfoil  100  extending radially outward from a blade platform with respect to a rotational axis of the gas turbine. The airfoil  100  includes a leading edge  106  and a trailing edge  108 , a pressure side  102  and a suction side  1004  each of which extends from the leading edge  106  and trailing edge  108 , and an airfoil tip  100   a  disposed at radially outer end of the airfoil  100 . The airfoil tip  100   a  faces a surface of a stator disposed radially more outward of the airfoil  100  and defining an outer surface of high temperature combustion gas path through the gas turbine. The surface of the stator toward which the airfoil tip  100   a  faces may be an inner surface of a casing or an inner surface of a turbine shroud. 
     The airfoil tip  100   a  is spaced apart from the opposing stator surface (i.e., in a non-contact manner). In other words, a radial clearance or gap is included between the airfoil tip  100   a  of the airfoil  100  and the opposing stator surface to avoid collision or friction between the airfoil tip  100   a  of the airfoil  100  and the opposing stator surface when the gas turbine is operated. However, a portion of the hot gas flowing through the hot gas path (i.e., combustion products) does not flow over the turbine blade airfoil  100  but leaks through the radial clearance, reducing efficiency. 
     Therefore, it is desirable that the radial clearance between the airfoil tip  100   a  of the airfoil  100  and the opposing stator surface be kept as small as possible to minimize leakage of hot gas. 
     To keep the radial clearance small, and to protect the airfoil body from structural damage in case of accidental contact between the airfoil tip  100   a  of the airfoil  100  and the opposing stator surface during operation of the gas turbine, it is well known in the art of gas turbines to employ a squealer tip structure  60 ′ disposed at the airfoil tip  100   a  of the airfoil  100  and extending radially outwardly towards the opposing stator surface. 
     The squealer tip  60 ′ has a shape of a rail and is positioned at and extending along a periphery of the airfoil tip  100   a . For example, the squealer tip  60 ′ may have a suction side rail  64 ′ positioned at and extending along a periphery of the suction side  104  at the airfoil tip  100   a  and a pressure side rail  62 ′ positioned at and extending along a periphery of the pressure side  102  at the airfoil tip  100   a . The suction side rail  64 ′ and the pressure side rail  62 ′ of the squealer tip  60 ′ meet at a trailing edge tip portion  70 ′ of the squealer tip  60 ′. 
     Because the squealer tip  60 ′ is immersed in hot combustion products (i.e., the hot gas), cooling of the squealer tip  60 ′ is particularly required at the trailing edge tip portion  70 ′. For example, cooling holes  102   h′  are disposed on side surfaces of the airfoil  100 , i.e., at the pressure side  102  and/or the suction side  104  of the airfoil  100 , in the vicinity of the trailing edge tip portion  70 ′. However, the related art cooling technique does not provide efficient cooling of the trailing edge tip portion  70 ′ of the squealer tip  60 ′ of the blade  1 ′. 
     SUMMARY 
     Aspects of one or more exemplary embodiments provide a turbine blade for effectively cooling the trailing edge tip portion of the squealer tip of a gas turbine blade. 
     Additional aspects will be set forth in part in the description which follows and, in part, will become apparent from the description, or may be learned by practice of the exemplary embodiments. 
     According to an aspect of an exemplary embodiment, there is provided a turbine blade including: an airfoil having an airfoil tip, a leading edge, a trailing edge, and a pressure side and a suction side extending from the leading edge to the trailing edge and defining an airfoil cavity therein; a squealer tip arranged at the airfoil tip and including a trailing edge tip portion disposed at the trailing edge of the airfoil and a pressure side rail and a suction side rail meeting at the trailing edge tip portion and defining a squealer tip pocket at the airfoil tip; and at least one tip cooling hole disposed at the squealer tip pocket to provide cooling air from the airfoil cavity to the squealer tip pocket. The trailing edge tip portion of the squealer tip may include a chamfer disposed towards the pressure side of the airfoil and a groove extending from the squealer tip pocket to the chamfer to provide cooling air from the squealer tip pocket to the chamfer. 
     The groove and the chamfer may extend in a longitudinal direction along a camber line of the airfoil. 
     The chamfer may have a concave shape at the groove-side of the chamfer. The chamfer may have a planar or convex shape at a trailing-edge-side of the chamfer. 
     A shape of the chamfer may be formed by gradually transitioning from a concave shape at the groove-side of the chamfer to a planar or convex shape at the trailing-edge-side of the chamfer. 
     The trailing edge tip portion may include at least one chamfer-cooling hole disposed at the chamfer to provide cooling air from the airfoil cavity to the chamfer. 
     The trailing edge tip portion may include a plurality of chamfer-cooling holes spaced apart from each other at regular intervals. 
     An outlet of the groove may be spaced apart from the chamfer along a radially outward direction of the airfoil. 
     The at least one tip cooling hole may include a first tip cooling hole disposed adjacent to the inlet of the groove. 
     A length of the chamfer may be greater than or equal to 0.01 times a chord length of the airfoil and less than or equal to 0.2 times the chord length of the airfoil. Preferably, the length of the chamfer may be greater than or equal to 0.02 times the chord length of the airfoil and less than or equal to 0.15 times the chord length of the airfoil. 
     The chamfer may be defined between a first position of the trailing edge tip portion corresponding to the trailing edge of the airfoil and a second position of the trailing edge tip portion corresponding to a distance equal to or less than 0.2 times the chord length of the airfoil from the first position measured along a chord of the airfoil. 
     A width of the groove may decrease along a radially inward direction of the airfoil. A floor of the groove may be inclined downward towards the pressure side of the airfoil. 
     A length of the groove may be greater than or equal to 0.01 times a chord length of the airfoil and less than or equal to 0.20 times the chord length of the airfoil. Preferably, the length of the groove may be greater than or equal to 0.02 times the chord length of the airfoil and less than or equal to 0.15 times the chord length of the airfoil. 
     A length of the groove may be smaller than a length of the chamfer. 
     According to an aspect of another exemplary embodiment, there is provided a turbine blade assembly including: a rotor disk configured to be rotatable and a plurality of turbine blades installed on the rotor disk. Each of the turbine blade may include: an airfoil having an airfoil tip, a leading edge, a trailing edge, and a pressure side and a suction side extending from the leading edge to the trailing edge and defining an airfoil cavity therein; a squealer tip arranged at the airfoil tip and comprising a trailing edge tip portion disposed at the trailing edge of the airfoil and a pressure side rail and a suction side rail meeting at the trailing edge tip portion and defining a squealer tip pocket at the airfoil tip; and at least one tip cooling hole disposed at the squealer tip pocket to provide cooling air from the airfoil cavity to the squealer tip pocket, wherein the trailing edge tip portion of the squealer tip may include: a chamfer disposed towards the pressure side of the airfoil; and a groove extending from the squealer tip pocket to the chamfer to provide cooling air from the squealer tip pocket to the chamfer. 
     According to an aspect of another exemplary embodiment, there is provided a gas turbine including: a compressor configured to compress air introduced thereinto from an outside; a combustor configured to mix fuel with air compressed by the compressor for combustion; and a turbine including a plurality of turbine blades rotated by combustion gas produced by the combustor. Each of the turbine blade may include: an airfoil having an airfoil tip, a leading edge, a trailing edge, and a pressure side and a suction side extending from the leading edge to the trailing edge and defining an airfoil cavity therein; a squealer tip arranged at the airfoil tip and comprising a trailing edge tip portion disposed at the trailing edge of the airfoil and a pressure side rail and a suction side rail meeting at the trailing edge tip portion and defining a squealer tip pocket at the airfoil tip; and at least one tip cooling hole disposed at the squealer tip pocket to provide cooling air from the airfoil cavity to the squealer tip pocket, wherein the trailing edge tip portion of the squealer tip may include: a chamfer disposed towards the pressure side of the airfoil; and a groove extending from the squealer tip pocket to the chamfer to provide cooling air from the squealer tip pocket to the chamfer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects will become more apparent from the following description of the exemplary embodiments with reference to the accompanying drawings, in which: 
         FIG.  1    is a sectional view of a part of a gas turbine including a turbine blade according to an exemplary embodiment; 
         FIG.  2    is a schematical view illustrating a turbine blade assembly according to an exemplary embodiment; 
         FIG.  3    is a vertical cross-sectional view illustrating a turbine blade according to an exemplary embodiment; 
         FIG.  4    is a perspective view illustrating a part of a related art airfoil with a squealer tip; 
         FIG.  5    is a perspective view illustrating a part of an airfoil of a blade according to an exemplary embodiment; 
         FIG.  6    is a perspective view of a portion N of  FIG.  5   ; 
         FIG.  7    is another perspective view illustrating a part of the related art airfoil with the squealer tip of  FIG.  4   ; 
         FIG.  8    is another perspective view illustrating a part of the airfoil of the blade of  FIG.  5   ; 
         FIG.  9 A  is a schematical view illustrating a chamfer of a trailing edge tip portion of the blade according to an exemplary embodiment; 
         FIG.  9 B  is a schematical cross-sectional view of the chamfer of the trailing edge tip portion along the line C of  FIG.  9 A ; 
         FIG.  9 C  is a schematical cross-sectional view of the chamfer of the trailing edge tip portion along the line V of  FIG.  9 A ; 
         FIG.  10    is a schematical illustration of a groove of a trailing edge tip portion of the blade according to an exemplary embodiment; 
         FIG.  11 A  is a schematical cross-sectional view illustrating the groove of the trailing edge tip portion along the line G-G′ of  FIG.  10   ; 
         FIG.  11 B  is a schematical cross-sectional view illustrating the groove of the trailing edge tip portion along the line G-G′ of  FIG.  10    according to another exemplary embodiment; 
         FIG.  12 A  is a schematical cross-sectional view illustrating the groove of the trailing edge tip portion along the line G-G′ of  FIG.  10    according to another exemplary embodiment; and 
         FIG.  12 B  is a schematical view illustrating the groove of  FIG.  12 A . 
     
    
    
     DETAILED DESCRIPTION 
     Various modifications and various embodiments will be described below in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the disclosure. It should be understood, however, that the various embodiments are not for limiting the scope of the disclosure to the specific embodiment, but they should be interpreted to include all modifications, equivalents, and alternatives of the embodiments included within the spirit and scope disclosed herein. 
     The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the scope of the disclosure. The singular expressions “a”, “an”, and “the” are intended to include the plural expressions as well unless the context clearly indicates otherwise. In the disclosure, terms such as “comprises”, “includes”, or “have/has” should be construed as designating that there are such features, integers, steps, operations, components, parts, and/or combinations thereof, not to exclude the presence or possibility of adding of one or more of other features, integers, steps, operations, components, parts, and/or combinations thereof. 
     Hereinafter, exemplary embodiments will be described below in detail with reference to the accompanying drawings. It should be noted that like reference numerals refer to like parts throughout the various figures and exemplary embodiments. In certain embodiments, a detailed description of functions and configurations well known in the art may be omitted to avoid obscuring appreciation of the disclosure by a person of ordinary skill in the art. For the same reason, some components may be exaggerated, omitted, or schematically illustrated in the accompanying drawings. 
       FIG.  1    is a sectional view of a part of a gas turbine  10  including a turbine blade according to an exemplary embodiment. Referring to  FIG.  1   , the gas turbine  10  may include an inlet  12 , a compressor section  14 , a combustion section  16  and a turbine section  18  arranged in the direction of a rotational axis  20 . The gas turbine  10  may further include a shaft  22  rotatable about the rotational axis  20  and extending in a longitudinal direction. The shaft  22  may connect the turbine section  18  to the compressor section  14 . 
     The compressor section  14  may suck air  24  through the air inlet  12 , compress the air, and supply the compressed air to the combustion section  16 . The combustion section  16  may include 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  may be located inside the burner plenum  26 . The compressed air passing through the compressor section  14  may enter a diffuser  32  and exit the burner plenum  26 , where a portion of the air may enter the burner  30  and mix with a gas or liquid fuel. The air/fuel mixture is burned and combustion gas  34  discharged from the combustion section  16  is supplied to the turbine section  18  via a transition duct  17 . 
     A plurality of combustors constituting the combustion section  16  may be arranged in a form of a shell in a housing. Each of the combustors may include the burner  30  having a fuel injection nozzle and the like, a combustor liner defining the combustion chamber  28 , and the transition duct  17  serving as a connector between the combustion section  16  and the turbine section  18 . 
     The turbine section  18  may include a plurality of blade carrying disks  36  attached to the shaft  22 .  FIG.  1    shows two disks  36  each carrying an annular array of turbine blades  38 , and it is understood that more or less than two disks may be included in one or more other embodiments. In addition, turbine vanes  40 ,  44  fixed to a stator  42  of the gas turbine  10  may be disposed between the turbine blades  38  to guide a flow direction of the combustion gas passing through the turbine blades  38 . 
     The combustion gas discharged from the combustion chamber  28  is supplied to the turbine section  18 . The supplied combustion gas expands and applies impingement or reaction force to turbine blades  38  to generate rotational torque. That is, the supplied combustion gas drives the turbine blades  38  which in turn rotates the shaft  22 . A portion of the rotational torque is transmitted to the compressor section  14 , and remaining portion which is the excessive torque is used to drive a generator or the like. 
     The compressor section  14  may be driven by some of power output from the turbine section  18 . The compressor section  14  may include an axial series of vane stages  46  and rotor blade stages  48 . The rotor blade stages  48  may include a rotor disc supporting an annular array of blades. The compressor section  14  may further include a casing  50  that surrounds the rotor stages and supports the vane stages  48 . The vane stages  46  may include an annular array of radially extending compressor vanes mounted to the casing  50  in such a way that the compressor vanes form each stage. The compressor vanes guide the compressed air transferred from compressor blade disposed at a preceding stage, to compressor blade disposed at a following stage. In an exemplary embodiment, at least some of the compressor vanes may be mounted so as to be rotatable within a predetermined range, e.g., to adjust the inflow rate of air. The casing  50  may define a radially outer surface  52  of a passage  56  of the compressor section  14 . A radially inner surface  54  of the passage  56  may be defined at least in part by a rotor drum  53  of the rotor which may be defined in part by the annular array of blades  48 . 
     The exemplary embodiment shows gas turbine having a single shaft connecting single/multi-stage compressor and single/one or more stage turbine, and it is understood that two or three shaft engines may be included in one or more other embodiments. 
     The terms upstream and downstream refer to the flow direction of the airflow and/or working gas flow through the engine. The terms forward and rearward refer to the flow direction of hot gas through the engine. The terms axial, radial and circumferential are made with reference to the rotational axis  20  of the gas turbine  10 . 
       FIG.  2    is a schematical view illustrating a turbine blade assembly according to an exemplary embodiment.  FIG.  3    is a vertical cross-sectional view illustrating a turbine blade according to an exemplary embodiment.  FIG.  5    is a perspective view illustrating a part of an airfoil of a turbine blade according to an exemplary embodiment.  FIG.  6    is a perspective view of a portion N of  FIG.  5   .  FIG.  8    is another perspective view illustrating a part of the airfoil of the blade of  FIG.  5   . 
     Referring to  FIGS.  2  and  3   , the turbine blade assembly may include the turbine blades  38 , also referred to as turbine blade  1 , arranged on and coupled to the rotor disk  36 . The turbine blade  1  may include a platform  200 , an airfoil  100  extending radially outwardly from the platform  200  which may extend circumferentially, and a root  300  extending radially inwardly from the platform  200 . The turbine blade  1  may be fixed to the rotor disk  36  via the root  300 . The airfoil  100  may be formed of an airfoil-shaped curved plate and have an optimized shape according to specification of the gas turbine  10 . 
     Referring to  FIGS.  2 ,  3 ,  5 ,  6  and  8   , the airfoil  100  includes a pressure side  102  (also referred to as pressure surface or concave surface/side) and a suction side  104  (also referred to as suction surface or convex surface/side). The pressure side  102  and the suction side  104  meet at a leading edge  106  and a trailing edge  108  of the airfoil  100 . 
     The airfoil  100  may have a base part  100   b  adjacent to the platform  200  and a tip part  100   a  (also referred to as an airfoil tip) spaced apart from the base part  100   b  along a radial direction  9   r  of the airfoil  100 . 
     The pressure side  102 , the suction side  104 , the leading edge  106  and the trailing edge  108  define an airfoil cavity  100   s  of the airfoil  100 . The airfoil cavity  100   s  of the airfoil  100  may be limited by a wall of the airfoil tip  100   a  disposed at the radially outermost end of the airfoil  100 . 
     The airfoil tip  100   a  may be formed as a wall having an outer surface and an inner surface. 
     The turbine blade  1  includes a squealer tip  60 . The squealer tip  60  may be disposed at the airfoil tip  100   a , e.g., may extend outward radially from the outer surface of the airfoil tip  100   a.    
     Referring to  FIGS.  5 ,  6  and  8   , the squealer tip  60  may be formed as a rail that surrounds continuously or intermittently along a periphery of the airfoil tip  100   a.    
     The squealer tip  60  may include a suction side rail  64  positioned at and extending along a periphery of the suction side  104  at the airfoil tip  100   a , a pressure side rail  62  positioned at and extending along a periphery of the pressure side  102  at the airfoil tip  100   a , and a trailing edge tip portion  70  disposed at the trailing edge  108  of the airfoil  100 . 
     The pressure side rail  62  and the suction side rail  64  meet at the trailing edge tip portion  70 . The trailing edge tip portion  70  may be formed as a unitary rail which spans between the pressure side  102 , the suction side  104  and the trailing edge  108 . The trailing edge tip portion  70  does not extend to the leading edge  106  of the airfoil  100 . 
     The pressure side rail  62 , the suction side rail  64 , the trailing edge tip portion  70  and the airfoil tip  100   a  define a squealer tip pocket  60   s  disposed at the airfoil tip  100   a  in a radially outward direction. 
     A plurality of airfoil tip cooling holes  65   a ,  65   b  may be disposed in the squealer tip pocket  60   s  to conduct a flow of cooling air from the airfoil cavity  100   s  to the squealer tip pocket  60   s.    
     An outlet of the airfoil tip cooling holes  65   a ,  65   b  may be positioned at the airfoil tip  100   a  within the squealer tip pocket  60   s , and an inlet of the airfoil tip cooling holes  65   a ,  65   b  may be positioned at the airfoil cavity  100   s.    
     The trailing edge tip portion  70  includes a chamfer  90  and a groove  80 . The chamfer  90  is arranged towards the pressure side  102  of the airfoil  100 . The groove  80  extends between the chamfer  90  and the squealer tip pocket  60   s . At least a part of the cooling air from the squealer tip pocket  60   s  flows to the chamfer  90  via the groove  80 . 
     An inlet  82  of the groove  80  may be positioned at the squealer tip pocket  60   s  to receive cooling air from the squealer tip pocket  60   s . An outlet  84  of the groove  80  may be positioned at or adjacent to the chamfer  90  to allow cooling air from the outlet  84  to flow to the chamfer  90 . 
     The trailing edge tip portion  70  may include a pressure-side surface  72  corresponding to the pressure side  102  of the airfoil  100 , a suction-side surface  74  corresponding to the suction side  104  of the airfoil  100 , a side surface facing the squealer tip pocket  60   s  and extending between the pressure rail  62  and the suction rail  64 , and an upper surface  76  which is a radially outer surface of the trailing edge tip portion  70 . 
     Here, the chamfer  90  is disposed between the upper surface  76  and the pressure side surface  72  of the trailing edge tip portion  70  or between the upper surface  76  and the pressure side  102  of the airfoil  100 . 
     The groove  80  is formed as a flow channel for cooling air from the squealer tip pocket  60   s  to the chamfer  90 , for example, is formed as an indentation or cavity or recess or notch formed in the trailing edge tip portion  70 . For example, the groove  80  may be formed in a radially inward direction with respect to axis  9   r  of the airfoil  100  in the radially upper surface  76  of the trailing edge tip portion  70 . 
     The groove  80  may extend in a longitudinal direction along a camber line of the airfoil  100 . That is, a shape of the groove  80  may be aligned with or corresponding to the camber line of the airfoil  100 . The chamfer  90  may extend in the longitudinal direction along the camber line of the airfoil  100 . That is, a shape of the chamfer  90  may be aligned with or corresponding to the camber line of the airfoil  100 . However, it is understood that the shapes are not limited to examples described above and may be changed or vary according to one or more other exemplary embodiments. 
     For example, the groove  80  may be disposed between the squealer tip pocket  60   s  and the chamfer  90  along a chordwise direction  9   c  of the airfoil  100 . 
     As shown in  FIG.  6   , the chamfer  90  may include a first portion disposed adjacent to the trailing edge  108  of the airfoil  100 . The first portion may be a position or portion of the chamfer  90  where the chamfer  90  starts at or adjacent the trailing edge  108  of the airfoil  100 . The first portion may be referred to as a trailing-edge-side  91  of the chamfer  90 . 
     The chamfer  90  may further include a second portion disposed adjacent to the groove  80 . The second portion may be a position or portion of the chamfer  90  where the chamfer  90  starts at or adjacent the groove  80 . The second portion may be referred to as a groove-side  92  of the chamfer  90 . The groove-side  92  of the chamfer  90  faces the trailing-edge-side  91  of the chamfer  90  in the chordwise direction  9   c  of the airfoil  100 . Another embodiment regarding the shape of the chamfer  90  is described with reference to  FIGS.  8  and  9 A to  9 C . 
     Along the chordwise direction  9   c  of the airfoil  100 , when moving from the leading edge  106  to the trailing edge  108  of the airfoil  100 , the squealer tip pocket  60   s , the groove  80  and the chamfer  90  may be arranged consecutively or sequentially. 
     Hereinafter, another exemplary embodiments regarding the shape of the chamfer  90  are described with reference to  FIG.  8    and  FIGS.  9 A to  9 C .  FIG.  9 A  is a schematical view illustrating a chamfer of a trailing edge tip portion of the blade according to an exemplary embodiment.  FIG.  9 B  is a schematical cross-sectional view of the chamfer of the trailing edge tip portion (i.e., the groove-side  92 ) along the line C of  FIG.  9 A , when viewed in the chordwise direction  9   c  from the leading edge  106  towards the trailing edge  108  of the airfoil  100 .  FIG.  9 C  is a schematical cross-sectional view of the chamfer of the trailing edge tip portion (i.e., the trailing-edge-side  91 ) along the line V of  FIG.  9 A , when viewed in the chordwise direction  9   c  from the trailing edge  108  towards the leading edge  106  of the airfoil  100 . 
     Referring to  FIGS.  8  and  9 B , the chamfer  90  may be concave at the groove-side  92  of the chamfer  90 . For example, entire chamfer  90 , that is, from the groove-side  92  to the trailing-edge-side  91 , may have a concave shape. 
     Referring to  FIGS.  8  and  9 C , the chamfer  90  may be convexly shaped at the trailing-edge-side  91  of the chamfer  90 . For example, entire chamfer  90 , that is, from the groove-side  92  to the trailing-edge-side  91 , may have a convex shape. 
     Alternatively, the chamfer may be planar in shape at the trailing-edge-side  91  of the chamfer  90 . For example, entire chamfer  90 , that is, from the groove-side  92  to the trailing-edge-side  91 , may have a planar shape. 
     Referring to  FIG.  8    and  FIGS.  9 A to  9 C , the chamfer  90  may be concave at the groove-side  92  of the chamfer  90  and may be planar or convex at the trailing-edge-side  91  of the chamfer  90 . Also, the shape of the chamfer  90  may be formed by gradually transitioning from the concave shape at the groove-side  92  to the planar shape or the convex shape at the trailing-edge-side  91  of the chamfer  90 . 
     Referring to  FIG.  9 A , a length L of the chamfer  90  may be greater than or equal to 0.01 times a chord length of the airfoil  100 , and may be less than or equal to 0.2 times the chord length of the airfoil  100 . Preferably, the length L of the chamfer  90  may be greater than or equal to 0.02 times the chord length of the airfoil  100  and less than or equal to 0.15 times the chord length of the airfoil  100 . 
     The chamfer  90  may be defined between a first position of the trailing edge tip portion  70  and a second position of the trailing edge tip portion  70  of the squealer tip  60 . 
     The position at line V in  FIG.  9 A  may be the first position of the trailing edge tip portion  70 . For example, the first position may radially overlap the trailing edge  108  of the airfoil  100  as shown in  FIG.  8   . Alternatively, as shown in  FIG.  9 A , the first position may be spaced apart from the trailing edge  108  of the airfoil  100  by a distance Lv in the chordwise direction. The distance Lv may be less than or equal to 0.1 times the length L of the chamfer  90  measured along the chordwise direction from the trailing edge  108  of the airfoil  100 . The part of the chamfer  90  disposed at the first position is the trailing-edge-side  91  of the chamfer  90 . 
     The position at line C in  FIG.  9 A  may be the second position of the trailing edge tip portion  70 . The second position may correspond to a distance equal to or less than 0.2 times the chord length of the airfoil  100  from the first position measured along a chord of the airfoil  100 . The part of the chamfer  90  disposed at the second position is the groove-side  92  of the chamfer  90 . 
     Referring to  FIGS.  9 B and  9 C , the chamfer  90  may include a first planar portion  901 , a second planar portion  902  and an intermediate fillet portion  903  between the first and second planar portions  901 ,  902 , i.e., an arc shaped portion connecting the first and second planar portions  901 ,  902 . The first planar portion  901 , the fillet portion  903  and the second planar portion  902  may be arranged along the radial direction  9   r  of the airfoil  100 . In other words, the fillet portion  903  may be disposed radially outward with respect to the radial direction  9   r  of the first planar portion  901 , and the second planar portion  902  may be disposed radially outward with respect to the radial direction  9   r  of the fillet portion  903 . The shape including the first planar portion  901 , the fillet portion  903  and the second planar portion  902  is easy to manufacture with precision. 
     In  FIGS.  9 B and  9 C , reference sign ‘A’ indicates a maximum width of the chamfer  90 , reference sign ‘B’ indicates a maximum height/depth of the chamfer  90 , and reference sign ‘Q’ indicates a maximum width of the trailing edge tip portion  70  at the groove-side  92 . The widths A and Q are measured along a thickness direction of the airfoil  100 , i.e., a direction extending vertically between the pressure side  102  and the suction side  104  of the airfoil  100 . In other words, the widths A and Q are measured perpendicular to each other with respect to directions  9   r  and  9   c . The height or depth B is measured along the radial direction  9   r  of the airfoil  100 . 
     The width Q of the trailing edge tip portion  70  at the groove-side  92  may be greater than or equal to 0.02 times the chord length, and less than or equal to 0.1 times the chord length. 
     For example, the width A of the chamfer  90 , i.e., at the groove-side  92  or at the trailing-edge-side  91  of the chamfer  90  between the groove-side  92  and the trailing-edge-side  91  of the chamfer  90 , may be greater than or equal to 0.2 times the width Q of trailing edge tip portion  70  at the groove-side  92 . 
     Here, a ratio (i.e., a ratio B/A) of the height/depth B and the width A of the chamfer  90 , i.e., at the groove-side  92  or at the trailing-edge-side  91  of the chamfer  90  between the groove-side  92  and the trailing-edge-side  91  of the chamfer  90 , may be greater than or equal to 0.5, and less than or equal to 2. 
     The width A and/or height/depth B of the chamfer  90  may be constant, i.e., may be same from the groove-side  92  to the trailing-edge-side  91  of the chamfer  90 . 
     Here, the ratio B/A may be constant from the groove-side to the trailing-edge-side. For example, one or both of width W and height B may remain the same or vary, but the ratio B/A may be constant from the groove-side to the trailing-edge-side. 
     In  FIGS.  9 B and  9 C , reference sign ‘rC’ indicates a maximum radius of the fillet portion  903  at groove-side  92  of chamfer  90 , and reference sign ‘rV’ indicates a maximum radius of the fillet portion  903  at trailing-edge-side  91  of chamfer  90 . 
     For example, the radius of the fillet portion  903  of the chamfer  90 , i.e., radius rC at the groove-side  92  or radius rV at the trailing-edge-side  91  of the chamfer  90  between the groove-side  92  and the trailing-edge-side  91  of the chamfer  90 , may be greater than or equal to 0.005 times the chord length of the airfoil  100 , and preferably may be greater than or equal to 0.01 times the chord length of the airfoil  100 . 
     In  FIG.  9 B , reference sign ‘αC’ indicates an angle between the first planar surface  901  of the chamfer  90  and the airfoil pressure side  102  and/or the pressure-side surface  72  of the trailing edge tip portion  70  at the groove-side  92  of the chamfer  90 , and reference sign ‘βC’ indicates an angle between the second planar surface  902  of the chamfer  90  and the upper surface  76  of the trailing edge tip portion  70  at the groove-side  92  of the chamfer  90 . 
     For example, the angles αC and βC may be greater than or equal to 10° and less than or equal to 85°. Preferably, the angles αC and βC may be greater than or equal to 45° and less than or equal to 85°. 
     In  FIG.  9 C , reference sign ‘αV’ indicates an angle between the first planar surface  901  of the chamfer  90  and the airfoil pressure side  102  and/or the pressure-side surface  72  of the trailing edge tip portion  70  at the trailing-edge-side  91  of the chamfer  90 , and reference sign ‘βV’ indicates an angle between the second planar surface  902  of the chamfer  90  and the upper surface  76  of the trailing edge tip portion  70  at the trailing-edge-side  91  of the chamfer  90 . 
     For example, the angles αV and βW may be greater than or equal to 5° and less than or equal to 85°. Preferably, the angles αV and βW may be greater than or equal 10° and less than or equal to 60°, more preferably the angles αV and βW may be greater than or equal 10° and less than or equal to 45°. 
     Here, the angles α and β, i.e., at any position of the chamfer  90  between the groove-side  92  and the trailing-edge-side  91 , may be equal to or less than the angle αC, βC at the groove-side  92  of the airfoil  100 , and may be equal to or greater than the angle αV, βV at the trailing-edge-side  91  of the airfoil  100 . 
     As shown in  FIGS.  5 ,  6  and  8   , the trailing edge tip portion  70  may include at least one chamfer-cooling hole  95  disposed at the chamfer  90 . The chamfer cooling holes  95  provide cooling air from the airfoil cavity  100   s  to the chamfer  90 .  FIG.  6    shows three chamfer-cooling holes  95 , and it is understood that more or less than 3 chamfer-cooling holes  95  may be included in one or more other embodiments. 
     The trailing edge tip portion  70  may include a plurality of chamfer-cooling holes  95  spaced apart from each other. The chamfer-cooling holes  95  may be arranged at equal intervals along the chordwise direction  9   c  of the airfoil  100  or along the camber line of the airfoil  100 . 
     Referring to  FIG.  6   , the outlet  84  of the groove  80  may be spaced apart from the chamfer  90  along a spanwise direction or radial direction  9   r , i.e., a floor  88  (shown in  FIGS.  11 A- 12 B ) may be disposed radially outward of the chamfer  90 . 
     Hereinafter, another exemplary embodiments regarding the shape of the groove  80  are described with reference to  FIGS.  10  to  12 B .  FIG.  10    is a schematical illustration of a groove of a trailing edge tip portion of the blade according to an exemplary embodiment.  FIG.  11 A  is a schematical cross-sectional view illustrating the groove of the trailing edge tip portion along the line G-G′ of  FIG.  10   , when viewed in the chordwise direction  9   c  from the trailing edge  108  towards the leading edge  106  of the airfoil  100 .  FIG.  11 B  is a schematical cross-sectional view illustrating the groove of the trailing edge tip portion along the line G-G′ of  FIG.  10    according to another exemplary embodiment.  FIG.  12 A  is a schematical cross-sectional view illustrating the groove of the trailing edge tip portion along the line G-G′ of  FIG.  10    according to another exemplary embodiment.  FIG.  12 B  is a schematical view illustrating the groove of  FIG.  12 A . 
     Referring to  FIG.  10   , the at least one tip cooling hole  65   a ,  65   b  may include a first tip cooling hole  65   a  disposed adjacent to the inlet  82  of the groove  80 . The first cooling hole  65   a  may be positioned from the inlet  82  of the groove  80  within a distance equal to a length LG of the groove  80  measured along chordwise direction  9   c  from the inlet  82  of the groove  80 . Preferably, the first cooling hole  65   a  may be disposed from the inlet  82  of the groove  84  within a distance equal to half the length LG of the groove  80 . 
     Referring to  FIGS.  11 A to  12 B , the groove  80  may include side walls facing each other and a floor  88  forming a bottom surface of the groove  80 . The floor  88  may be planar or may be curved or rounded. The planar floor  88  may be disposed horizontally, i.e., along the thickness direction of the airfoil  100 . Alternatively, the planar floor  88  may be inclined with respect to the pressure side  102  or the pressure-side surface  72 . Preferably, the planar floor  88  may be inclined downward towards the pressure side  102  or the pressure-side surface  72 . 
     As shown in  FIG.  11 A , the groove  80  may be formed such that a width W of the groove  80 , i.e., a spacing between the opposite side walls, may be constant or unchanged along a spanwise direction of the airfoil  100  i.e., the radially inward direction  9   r  of the airfoil  100 . This ensures increased volume or amount of cooling air to flow to the chamfer  90 . 
     Alternatively, as shown in  FIGS.  11 B,  12 A and  12 B , the groove  80  may be formed such that a width W of the groove  80 , i.e., a spacing between the opposite side walls, may decrease along a spanwise direction of the airfoil  100  i.e., the radially inward direction  9   r  of the airfoil  100 . For example, a width W 1  at an opening of the groove  80  formed at the upper surface  76  may be greater than a width W 2  at the floor  88  of the groove  80 . 
     As shown in  FIG.  10   , the length LG of the groove  80  may be greater than or equal to 0.01 times the chord length of the airfoil  100 , and may be less than or equal to 0.20 times the chord length of the airfoil  100 . Preferably, the length LG of the groove  80  may be greater than or equal to 0.02 times the chord length of the airfoil  100  and less than or equal to 0.15 times the chord length of the airfoil  100 . 
     In  FIG.  12 B , reference sign ‘H’ denotes a height or depth of the groove  80  measured along the radial direction  9   r  of the airfoil  100  from the upper surface  76  of the trailing edge tip portion  70  of the squealer tip  60 . 
     A maximum width W 1  of the groove  80 , i.e., width W 1  at the opening of the groove  80  formed at the upper surface  76 , may be less than or equal to the maximum width A of the chamfer  90 . 
     The maximum width W 1  of the groove  80  may be greater than or equal to 0.5 mm, and may be less than or equal to the maximum width A of the chamfer  90 . 
     A maximum depth/height H of the groove  80  may be less than or equal to the maximum depth/height B of the chamfer  90 . 
     The maximum depth/height H of the groove  80  may be greater than or equal to 0.5 mm, and may be less than or equal to the maximum depth/height B of the chamfer  90 . 
     The depth/height H of the groove  80  may be constant from the inlet  82  to the outlet  84  of the groove  80 . If the floor  88  is inclined or the floor  88  is non-planar or curved, the depth/height H of the groove  80  may be a mean depth/height H of the groove  80 . 
     In  FIG.  12 B , reference sign ‘γ’ denotes an inclination angle of the floor  88  of the groove  80  with respect to the thickness direction of the airfoil  100 . 
     The angle γ may be greater than or equal to 0° and less than or equal to 75°. The angle γ of the floor  88  of the groove  80  may be constant from the inlet  82  to the outlet  84  of the groove  80 . 
     The side walls and the floor  88  of the groove  80  may be filleted, i.e., may be connected by an arc-shaped edge as shown by dashed line Fr in  FIG.  12 B . 
     A radius of the filleted part between the side walls and the floor  88  of the groove  80  may be greater than or equal to 0.1 mm and less than or equal to half of W 2 , i.e., a minimum width W 2  of the groove  80  or a width W 2  at the floor  88  of the groove  80 . 
     While one or more exemplary embodiments have been described with reference to the accompanying drawings, it will be apparent to those skilled in the art that various variations and modifications may be made by adding, changing, or removing components without departing from the spirit and scope of the disclosure as defined in the appended claims, and these variations and modifications fall within the spirit and scope of the disclosure as defined in the appended claims. Accordingly, the description of the exemplary embodiments should be construed in a descriptive sense only and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.