Patent Publication Number: US-2022228488-A1

Title: Rotor disc, rotor shaft, turbine rotor, and gas turbine

Description:
TECHNICAL FIELD 
     The present invention relates to a rotor disc, a rotor shaft, a turbine rotor, and a gas turbine. 
     Priority is claimed on Japanese Patent Application No. 2019-097549, filed May 24, 2019, the content of which is incorporated herein by reference. 
     BACKGROUND ART 
     A gas turbine includes a compressor that compresses air to generate compressed air; a combustor that combusts fuel in the compressed air to generate combustion gas; and a turbine that is driven by the combustion gas. The turbine includes a turbine rotor that rotates around an axis, and a turbine casing that covers the turbine rotor. The turbine rotor includes a rotor shaft that rotates around the axis and that extends in an axial direction, and a plurality of rotor blade rows. The plurality of rotor blade rows are arranged in the axial direction. Each of the plurality of rotor blade rows includes a plurality of rotor blades arranged in a circumferential direction with respect to the axis. For example, a plurality of rotor discs are stacked in the axial direction to form the rotor shaft. 
     The following PTL 1 discloses a configuration of a rotor disc. The rotor disc includes a radial outer surface facing a radial outer side; a radial inner surface facing a radial inner side; a plurality of blade root grooves recessed from the radial outer surface to the radial inner side and arranged in a circumferential direction; and a plurality of holes recessed from the radial inner surface to the radial outer side. A blade root of a rotor blade is mounted in each of the plurality of blade root grooves. The plurality of holes includes a plurality of first cooling holes and a plurality of second cooling holes. The first cooling hole is provided for each of the plurality of blade root grooves. The first cooling hole communicates with the blade root groove. Air that has passed through a space on the radial inner side from the radial inner surface of the rotor disc flows through the first cooling hole. The air flows into a cooling air flow path in the rotor blade through the blade root groove to cool the rotor blade. The second cooling hole is provided between the plurality of first cooling holes. 
     When the rotor disc rotates around an axis, tensile stress is generated in the rotor disc. The tensile stress generated in the rotor disc is concentrated in the vicinity of an opening of the cooling hole. When the stress is concentrated in the vicinity of the opening of the cooling hole and the stress concentration factor (=maximum stress/average stress) increases, the durability of the rotor disc is reduced. Therefore, in PTL 1, in order to reduce the stress concentration in the vicinity of the opening of the first cooling hole, the second cooling hole is formed between two first cooling holes adjacent to each other in the circumferential direction, so that the stress concentration in the vicinity of the opening of the first cooling hole is reduced. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Unexamined Patent Application Publication No. 2009-203870 
     SUMMARY OF INVENTION 
     Technical Problem 
     An object of the present invention is to provide a technique capable of reducing stress concentration in the vicinity of an opening of a cooling hole and improving the durability of a rotor disc. 
     Solution to Problem 
     According to one aspect of the invention to achieve the above object, there is provided a rotor disc including: a radial outer surface facing a radial outer side that is a side away from an axis in a radial direction with respect to the axis; a radial inner surface facing a radial inner side that is a side opposite the radial outer side in the radial direction, and located on the radial inner side of the radial outer surface; a plurality of blade root grooves recessed from the radial outer surface to the radial inner side, and arranged in a circumferential direction with respect to the axis; and a plurality of hole groups formed for the plurality of blade root grooves, and arranged in the circumferential direction. Each of the plurality of hole groups includes holes including a cooling hole penetrating from the radial inner surface to the radial outer surface. A width of each of the plurality of hole groups in the circumferential direction is larger than a width of each of the plurality of hole groups in an axial direction in which the axis extends, and is smaller than a minimum interval of intervals between the plurality of hole groups in the circumferential direction. The cooling hole communicates with an inside of the blade root groove. 
     In the rotor disc of this aspect, a cooling medium in a space on the radial inner side of the radial inner surface can be guided to a rotor blade through the cooling hole and the blade root groove. When the rotor disc rotates around the axis, tensile stress is generated in the rotor disc. The tensile stress generated in the rotor disc is concentrated in the vicinity of an opening of the cooling hole. As the interval between openings of two holes is reduced, the stress concentration factor is reduced. The reason is that stress generated around the opening of one hole is dispersed around the opening of the adjacent hole. 
     In this aspect, a circumferential group width that is a width of the hole group in the circumferential direction is larger than an axial group width that is a width of the hole group in the axial direction, and is smaller than a minimum group interval that is the minimum interval of the intervals between the plurality of hole groups in the circumferential direction. It is assumed that the hole group includes a plurality of the holes including the cooling hole. In this case, the hole interval between two holes of one hole group in the circumferential direction is smaller than the minimum group interval. For this reason, in this case, the hole interval between the plurality of holes of one hole group is smaller than when all the holes formed in the rotor disc are arranged at equal intervals in the circumferential direction. Therefore, in this case, the stress concentration in the vicinity of the opening of the cooling hole can be reduced, and the durability of the rotor disc can be improved. 
     In addition, it is assumed that the width of a cross section of the cooling hole of the hole group in the circumferential direction is larger than the width of the cross section of the cooling hole in the axial direction. As described above, as the interval between the openings of the two holes is reduced, the stress concentration factor is reduced. Therefore, when the interval between the openings of the two holes is reduced, and the openings of the two holes are connected to each other to form one opening, stress generated around the opening of the hole is reduced. The reason is that the stress is dispersed in a direction in which the two holes are connected to each other. The cooling hole in this case has a shape in which two holes are connected to each other in the circumferential direction. For this reason, in this case, the stress generated around the opening of the cooling hole is dispersed in the circumferential direction. Therefore, also in this case, the stress concentration in the vicinity of the opening of the cooling hole can be reduced, and the durability of the rotor disc can be improved. 
     Here, in the rotor disc of this aspect, each of the plurality of hole groups may include a plurality of the holes recessed from the radial inner surface to the radial outer side and arranged in the circumferential direction. In this case, at least one of the plurality of holes is the cooling hole. 
     This aspect is the case where the hole group includes the plurality of holes including the cooling hole described above. Therefore, in this aspect, the stress concentration in the vicinity of the opening of the cooling hole can be reduced, and the durability of the rotor disc can be improved. 
     In the rotor disc according to this aspect in which the hole group includes the plurality of holes, a maximum hole interval of intervals between the plurality of holes of the hole group in the circumferential direction may be smaller than the minimum interval of the intervals between the plurality of hole groups in the circumferential direction. 
     In the rotor disc according to one of the above aspects in which the hole group includes the plurality of holes, all the plurality of holes of the hole group may be the cooling holes. 
     In addition, in the rotor disc according to this aspect, a width of an inner opening, which is an opening of the cooling hole on the radial inner surface, in the circumferential direction may be larger than a width of the inner opening in the axial direction. 
     This aspect is basically the same as the case where the width of the cross section of the cooling hole in the circumferential direction is larger than the width of the cross section of the cooling hole in the axial direction described above. Therefore, in this aspect, the stress concentration in the vicinity of the inner opening of the cooling hole can be reduced, and the durability of the rotor disc can be improved. 
     In the rotor disc according to this aspect in which the width of the inner opening of the cooling hole in the circumferential direction is larger than the width of the inner opening in the axial direction, a width of an outer opening, which is an opening of the cooling hole on the radial outer surface, in the circumferential direction may be larger than a width of the outer opening in the axial direction. 
     In this aspect, the stress concentration in the vicinity of the outer opening of the cooling hole can be reduced, and the durability of the rotor disc can be improved. 
     In the rotor disc according to one of the above aspects, a position of an opening of the cooling hole on the radial outer surface in the circumferential direction may be in a region in the circumferential direction in which a groove bottom surface of the blade root groove is present. 
     In the rotor disc according to one of the above aspects, a region of the radial inner surface around an opening of the hole of the hole group on the radial inner surface may be perpendicular to a direction in which the hole extends on a virtual plane including the axis and the hole. 
     It is assumed that the region of the radial inner surface around the opening of the hole on the radial inner surface is an inclined surface (region inner surface) that is inclined with respect to the direction in which the hole extends on the virtual plane including the axis and the hole. In this case, an angle at a corner between a generating line on an axial downstream side of a columnar hole and the inclined surface is an obtuse angle, and an angle at a corner between a generating line on an axial upstream side of the columnar hole and the inclined surface is an acute angle. For this reason, stress is concentrated at an edge of the inner opening on the axial upstream side. In this aspect, both the angle at the corner between the generating line on the axial downstream side of the columnar hole and the region inner surface and the angle at the corner between the generating line on the axial upstream side of the columnar hole and the region inner surface are 90°, so that stress can be prevented from being concentrated at the edge of the inner opening on the axial upstream side. 
     According to one aspect of the invention to achieve the above object, there is provided a rotor shaft including: a plurality of the rotor discs according to one of the above aspects; and a spindle bolt penetrating through the plurality of rotor discs in the axial direction to connect the plurality of rotor discs to each other, the rotor discs being arranged in the axial direction. 
     According to one aspect of the invention to achieve the above object, there is provided a turbine rotor including: the rotor shaft according to this aspect; and a rotor blade mounted in the blade root groove of each of the plurality of rotor discs. 
     According to one aspect of the invention to achieve the above object, there is provided a gas turbine including: the turbine rotor according to this aspect; and a turbine casing covering an outer periphery of the turbine rotor. 
     Advantageous Effects of Invention 
     According to one aspect of the present invention, the stress concentration in the vicinity of the opening of the cooling hole can be reduced, and the durability of the rotor disc can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view illustrating a configuration of a gas turbine as one embodiment according to the present invention. 
         FIG. 2  is a sectional view of a main part of a turbine as one embodiment according to the present invention. 
         FIG. 3  is a perspective view of a main part of a turbine disc as one embodiment according to the present invention. 
         FIG. 4  is a view of a disc body and a rotor blade as a first embodiment according to the present invention as seen from an axial upstream side. 
         FIG. 5  is a sectional view of the disc body taken along line V-V of in  FIG. 4 . 
         FIG. 6  is a sectional view of a main part of a rotor disc as the first embodiment according to the present invention. 
         FIG. 7  is a view of a disc body as the first embodiment according to the present invention as seen from a radial outer side. 
         FIG. 8  is a graph illustrating a relationship between a stress concentration factor and an interval between openings of holes. 
         FIG. 9  is a view of a disc body as a second embodiment according to the present invention as seen from the radial outer side. 
         FIG. 10  is a sectional view taken along line X-X in  FIG. 9 . 
         FIG. 11  is a sectional view of a main part of a turbine disc as a third embodiment according to the present invention. 
         FIG. 12  is a view of a disc body as the third embodiment according to the present invention as seen from the radial outer side. 
         FIG. 13  is a sectional view taken along line XIII-XIII in  FIG. 11 . 
         FIG. 14  is a view of a disc body as a first modification example of the third embodiment according to the present invention as seen from the radial outer side. 
         FIG. 15  is a view of a disc body as a second modification example of the third embodiment according to the present invention as seen from the radial outer side. 
         FIG. 16  is a sectional view of a main part of a rotor disc as a modification example of the first embodiment according to the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of a gas turbine including a rotor disc according to the present invention and various embodiments of the rotor disc will be described with reference to the drawings. 
     [Implementation of Gas Turbine] 
     An embodiment of a gas turbine according to the present invention will be described with reference to the drawings. 
     As illustrated in  FIG. 1 , a gas turbine  10  of the present embodiment includes a compressor  20  that compresses air A; a combustor  30  that combusts fuel F in the air A compressed by the compressor  20  to generate combustion gas G; and a turbine  40  that is driven by the combustion gas G. 
     The compressor  20  includes a compressor rotor  21  that rotates around an axis Ar; a compressor casing  25  that covers the compressor rotor  21 ; and a plurality of stator blade rows  26 . The turbine  40  includes a turbine rotor  41  that rotates around the axis Ar; a turbine casing  45  that covers the turbine rotor  41 ; and a plurality of stator blade rows  46 . Hereinafter, a direction in which the axis Ar extends is referred to as an axial direction Da, a circumferential direction around the axis Ar is simply referred to as a circumferential direction Dc, and a direction perpendicular to the axis Ar is referred to as a radial direction Dr. In addition, one side in the axial direction Da is referred to as an axial upstream side Dau, and an opposite side is referred to as an axial downstream side Dad. In addition, a side approaching the axis Ar in the radial direction Dr is referred to as a radial inner side Dri, and an opposite side is referred to as a radial outer side Dro. 
     The gas turbine  10  of the present embodiment further includes an intermediate casing  16 . The compressor  20  is disposed on the axial upstream side Dau with respect to the turbine  40 . The intermediate casing  16  is disposed between the compressor casing  25  and the turbine casing  45  in the axial direction Da, and connects the compressor casing  25  and the turbine casing  45 . The compressor rotor  21  and the turbine rotor  41  are located on the same axis Ar, and are connected to each other to form a gas turbine rotor  11 . For example, a rotor of a generator GEN is connected to the gas turbine rotor  11 . In addition, the compressor casing  25 , the intermediate casing  16 , and the turbine casing  45  are connected to each other to form a gas turbine casing  15 . 
     The compressor rotor  21  includes a rotor shaft  22  extending in the axial direction Da and having the axis Ar as a center, and a plurality of rotor blade rows  23  attached to the rotor shaft  22 . The plurality of rotor blade rows  23  are arranged in the axial direction Da. Each of the rotor blade rows  23  is formed of a plurality of rotor blades arranged in the circumferential direction Dc. The stator blade row  26  is disposed on the axial downstream side Dad of each of the plurality of rotor blade rows  23 . Each of the stator blade rows  26  is provided inside the compressor casing  25 . Each of the stator blade rows  26  is formed of a plurality of stator blades arranged in the circumferential direction Dc. 
     The turbine rotor  41  includes a rotor shaft  42  extending in the axial direction Da and having the axis Ar as a center, and a plurality of rotor blade rows  43  attached to the rotor shaft  42 . The plurality of rotor blade rows  43  are arranged in the axial direction Da. Each of the rotor blade rows  43  is formed of a plurality of rotor blades  44  arranged in the circumferential direction Dc. The stator blade row  46  is disposed on the axial upstream side Dau of each of the plurality of the rotor blade rows  43 . Each of the stator blade rows  46  is provided on the radial inner side of the turbine casing  45 . Each of the stator blade rows  46  is formed of a plurality of stator blades  47  arranged in the circumferential direction Dc. 
     As illustrated in  FIG. 2 , the turbine casing  45  includes an outer casing  45   a  having a tubular shape and forming an outer shell of the turbine casing  45 ; an inner casing  45   b  fixed to a radial inner side of the outer casing  45   a ; a plurality of heat shield rings  45   c  fixed to a radial inner side of the inner casing  45   b ; and a ring segment  45   d  fixed to a radial inner side of each of the plurality of heat shield rings  45   c . Each of a plurality of the ring segments  45   d  is provided at a position between the plurality of stator blade rows  46 . Therefore, the rotor blade row  43  is disposed on the radial inner side Dri of each of the ring segments  45   d . In addition, the stator blade  47  is also fixed to the radial inner side Dri of each of the plurality of heat shield rings  45   c.    
     An annular space between an outer peripheral side of the rotor shaft  42  and an inner peripheral side of the turbine casing  45 , in which the stator blades  47  and the rotor blades  44  are disposed in the axial direction Da, forms a combustion gas flow path  49  through which the combustion gas G from the combustor  30  flows. 
     As illustrated in  FIG. 1 , the gas turbine  10  of the present embodiment is provided with a cooling device  50 . The cooling device  50  is a device that cools high-temperature components in contact with the high-temperature combustion gas among gas turbine components. The cooling device  50  includes an air bleeding line  51  that bleeds compressed air in the intermediate casing  16 ; a cooler  52  that is provided in the air bleeding line  51 ; a cooling air line  53  that guides the compressed air which has been cooled by the cooler  52 , as cooling air, to the turbine rotor  41  that is one of the high-temperature components; and a booster  54  that is provided in the cooling air line  53  and that boosts the cooling air. A cooling air flow path  42   p  is formed in the rotor shaft  42  of the turbine  40 . The cooling air flow path  42   p  extends to the plurality of rotor blades  44  attached to the rotor shaft  42 . 
     As illustrated in  FIG. 2 , the rotor shaft  42  includes a plurality of rotor discs  42   d  arranged in the axial direction Da, and a spindle bolt  42   s  that penetrates through the plurality of rotor discs  42   d  in the axial direction Da to connect the plurality of rotor discs  42   d  to each other. The plurality of rotor blades  44  forming one rotor blade row  43  are attached to one rotor disc  42   d . The rotor blade  44  includes a blade body  44   b  having a blade shape; a platform  44   f  formed on the radial inner side Dri of the blade body  44   b ; and a blade root  44   r  formed on the radial inner side Dri of the platform  44   f . A cooling air passage  44   p  through which the cooling air flows is formed in the rotor blade  44 . An inlet opening of the cooling air passage  44   p  is formed in a bottom surface of the blade root  44   r , the bottom surface facing the radial inner side Dri. 
     As illustrated in  FIG. 1 , the compressor  20  compresses the air A to generate the compressed air. The compressed air flows from the compressor  20  into the intermediate casing  16 . Some of the compressed air that has flowed into the intermediate casing  16  flows into the combustor  30 . The fuel F is supplied to the combustor  30 . In the combustor  30 , the fuel F is combusted in the compressed air to generate the high-temperature and high-pressure combustion gas G. The combustion gas G is delivered from the combustor  30  into the combustion gas flow path  49  inside the turbine  40 . The combustion gas G rotates the turbine rotor  41  in the process of flowing through the combustion gas flow path  49  to the axial downstream side Dad. The rotor of the generator GEN connected to the gas turbine rotor  11  is rotated by the rotation of the turbine rotor  41 . As a result, the generator GEN generates electricity. 
     The rotor blade  44  or the stator blade  47  of the turbine  40  is exposed to the high-temperature combustion gas G. For this reason, the rotor blade  44  or the stator blade  47  is cooled by a cooling medium. The rotor blade  44  of the present embodiment is cooled by the cooling air from the cooling device  50 . Some of the compressed air generated by the compressor  20  is bled from the intermediate casing  16 . The compressed air flows into the cooler  52  through the air bleeding line  51 , and is cooled here. The compressed air that has been cooled by the cooler  52  is boosted by the booster  54 , and then flows into the cooling air flow path  42   p  of the rotor shaft  42  through the cooling air line  53  as cooling air Ac. The cooling air Ac flows from the cooling air flow path  42   p  of the rotor shaft  42  into the cooling air passage  44   p  of the rotor blade  44  to cool the rotor blade  44 . 
     The rotor disc  42   d  described above is a rotor disc to be described in any one of the following embodiments and modification examples. 
     First Embodiment of Rotor Disc 
     Hereinafter, a rotor disc of the present embodiment will be described with reference to  FIGS. 2 to 8 . 
     As illustrated in  FIGS. 2 and 3 , a rotor disc  60  of the present embodiment includes a disc body  61 , a seal ring  85 , and a seal cap  88 . 
     The disc body  61  includes a large-diameter portion  62 , a small-diameter portion  72 , and a plurality of extension portions  81  and  83 . Both the large-diameter portion  62  and the small-diameter portion  72  have a substantially columnar shape around the axis Ar. A radius of the large-diameter portion  62  is larger than a radius of the small-diameter portion  72 . The small-diameter portion  72  is provided on the axial upstream side Dau of the large-diameter portion  62 . The extension portions  81  and  83  include an upstream extension portion  81  extending from a surface on the axial upstream side Dau of the small-diameter portion  72  to the axial upstream side Dau, and a downstream extension portion  83  extending from a surface on the axial downstream side Dad of the large-diameter portion  62  to the axial downstream side Dad, respectively. 
     The large-diameter portion  62  includes an outer peripheral surface  63  facing the radial outer side Dro, and a plurality of blade root grooves  64  recessed from the outer peripheral surface  63  toward the radial inner side Dri. The plurality of blade root grooves  64  are arranged at equal intervals in the circumferential direction Dc. The blade root  44   r  of the rotor blade  44  is mounted in each of the plurality of blade root grooves  64 . 
     As illustrated in  FIGS. 3 to 6 , the small-diameter portion  72  includes an outer peripheral surface  73  facing the radial outer side Dro; a front surface  74  facing the axial upstream side Dau; a plurality of communication grooves  75  recessed from the outer peripheral surface  73  to the radial outer side Dro; and an annular groove  76  recessed from the front surface  74  to the axial downstream side Dad and extending in the circumferential direction Dc with respect to the axis Ar. Each of the plurality of communication grooves  75  is formed at the same position as that of one blade root groove  64  of the plurality of blade root grooves  64  in the circumferential direction Dc. A distance from the axis Ar to a groove bottom surface  75   b  of the communication groove  75  is substantially equal to a distance from the axis Ar to a groove bottom surface  64   b  of the blade root groove  64 . For this reason, each of the plurality of communication grooves  75  communicates with one blade root groove  64  of the plurality of blade root grooves  64 . The annular groove  76  includes an inner groove side surface  76   i  facing the radial outer side Dro; an outer groove side surface  76   o  facing the radial inner side Dri; and a groove bottom surface  76   b  facing the axial upstream side Dau. The inner groove side surface  76   i  is located on the radial inner side Dri of the outer groove side surface  76   o . A part of the inner groove side surface  76   i  of the annular groove  76  forms the groove bottom surface  75   b  of the communication groove  75 . For this reason, the annular groove  76  communicates with the plurality of communication grooves  75 . All of the inner groove side surface  76   i  of the annular groove  76 , the groove bottom surface  75   b  of the communication groove  75 , and the groove bottom surface  64   b  of the blade root groove  64  are radial outer surfaces facing the radial outer side Dro. 
     The small-diameter portion  72  further includes a plurality of hole groups  77  arranged in the circumferential direction Dc. The plurality of hole groups  77  are provided for the plurality of blade root grooves  64 . Namely, one hole group  77  is provided for one blade root groove  64 . One hole group  77  includes a plurality of holes, two holes in the present embodiment, which are recessed from a radial inner surface  82  of the upstream extension portion  81  toward the radial outer side Dro. The radial inner surface  82  of the upstream extension portion  81  is located on the radial inner side Dri of the radial outer surfaces  76   i ,  75   b  of the small-diameter portion  72 , and the groove bottom surface  64   b  of the blade root groove  64 . In the present embodiment, all the plurality of holes form respective cooling holes  78  penetrating from the radial inner surface  82  of the upstream extension portion  81  to the radial outer surface that is the groove bottom surface  75   b  of the communication groove  75  (inner groove side surface  76   i  of the annular groove  76 ). Two cooling holes  78  are open in the groove bottom surface  75   b  of one communication groove  75 . Hereinafter, the opening will be referred to as an outer opening  780 . As described above, the communication groove  75  communicates with the blade root groove  64 . Therefore, the cooling hole  78  communicates with a space in the blade root groove  64  through a space in the communication groove  75 . The cross-sectional shape of the cooling hole  78  is a circle. The cross section referred to here is a plane extending in a direction perpendicular to a direction in which the cooling hole  78  extends. A plurality of the cooling holes  78  are arranged in the circumferential direction Dc. 
     The radial inner side Dri of the upstream extension portion  81  serves as a cooling air space (refer to  FIG. 2 ) into which the cooling air from the cooling device  50  flows. Therefore, the cooling air that has flowed into the cooling air space flows into the blade root groove  64  through the cooling hole  78  and the space in the communication groove  75 . The cooling air that has flowed into the space in the blade root groove  64  flows into the cooling air passage  44   p  of the rotor blade  44 . Therefore, in the present embodiment, the cooling air flow path  42   p  of the rotor shaft  42  described with reference to  FIG. 2  includes the cooling air space, the cooling hole  78 , the space in the communication groove  75 , and the space in the blade root groove  64 . 
     The seal ring  85  includes a ring piece  86  extending in the circumferential direction Dc, and a plurality of partition pieces  87 . The ring piece  86  closes a part of an opening in the circumferential direction Dc of the annular groove  76 . The partition piece  87  protrudes from a surface on the axial downstream side Dad of the ring piece  86  to the axial downstream side Dad to partition the inside of the annular groove  76  in the circumferential direction Dc. 
     As illustrated in  FIGS. 3 and 6 , the seal cap  88  closes an opening of the communication groove  75 . The seal cap  88  is in contact with the seal ring  85  and the blade root  44   r  to fill a gap therebetween. 
     As illustrated in  FIGS. 3 and 7 , the position of the outer opening  78   o  of the cooling hole  78  in the circumferential direction Dc is in a region Rb in the circumferential direction Dc in which the groove bottom surface  64   b  of the blade root groove  64  is present. In the circumferential direction Dc, each of the plurality of partition pieces  87  of the seal ring  85  is located between the region Rb and the region Rb in the circumferential direction Dc in which the groove bottom surface  64   b  of another blade root groove  64  adjacent to the blade root groove  64  is present. For this reason, a space in the annular groove  76  is partitioned by the partition piece  87  in the circumferential direction Dc into a space of a portion in which a blade root groove  64  is present and a space of a portion in which another blade root groove  64  adjacent to the blade root groove  64  in the circumferential direction Dc is present. 
     A circumferential group width dgc that is a width of each of the plurality of hole groups  77  in the circumferential direction Dc is larger than an axial group width dga that is a width of each of the plurality of hole groups  77  in the axial direction Da, and is smaller than a minimum group interval dg of group intervals, each of which is an interval between the plurality of hole groups  77  in the circumferential direction Dc. The axial group width dga coincides with the diameter of the circular outer opening  78   o  of the cooling hole  78 . The circumferential group widths dgc, each of which is the width of each of the plurality of hole groups  77  in the circumferential direction Dc, are the same as each other. The group interval dg between two hole groups  77  adjacent to each other in the circumferential direction Dc is the same as the group interval dg between another two hole groups  77  adjacent to each other in the circumferential direction Dc. Namely, the group intervals dg are the same as each other. Therefore, in the present embodiment, each of the group intervals dg is also the minimum group interval dg. A hole interval dh that is an interval in the circumferential direction Dc between two cooling holes  78  forming one hole group  77  is a dimension obtained by subtracting twice the diameter (=dga) of the cooling hole  78  from the circumferential group width dgc (=dgc−2·dga). Therefore, a magnitude relationship between the dimensions is as follows. 
         Dg&gt;Dgc &gt;( Dga,Dh ) 
     The magnitude relationship between dga and dh does not matter. 
     The magnitude relationship between the dimensions described above is a magnitude relationship in the radial outer surface  76   i  in which the outer opening  78   o  of the cooling hole  78  is formed. However, in the present embodiment, a magnitude relationship between the dimensions in the radial inner surface  82  in which an inner opening  78   i  of the cooling hole  78  is formed, and a magnitude relationship between the dimensions at a position between the radial outer surface  76   i  and the radial inner surface  82  are the same as the magnitude relationship between the dimensions in the radial outer surface  76   i.    
     As illustrated in  FIG. 8 , as the interval between the openings of the two holes is reduced, the stress concentration factor is reduced. The reason is that stress generated around the opening of one hole is dispersed around the opening of the adjacent hole. The stress concentration factor is a value obtained by dividing a maximum stress σmax generated in a member by an average stress cave generated in the member (=σmax/σave). 
     In the present embodiment, as described above, the circumferential group width dgc is smaller than the minimum group interval dg. Therefore, the hole interval dh between two cooling holes  78  forming one hole group  77  is smaller than the minimum group interval dg. For this reason, in the present embodiment, the hole interval dh between two cooling holes  78  forming one hole group  77  is smaller than when all the cooling holes  78  formed in the small-diameter portion  72  are arranged at equal intervals in the circumferential direction Dc. Therefore, in the present embodiment, the stress concentration in the vicinity of the opening of the cooling hole  78  can be reduced, and the durability of the rotor disc  60  can be improved. 
     Second Embodiment of Rotor Disc 
     Hereinafter, a rotor disc of the present embodiment will be described with reference to  FIGS. 9 and 10 . 
     A rotor disc  60   a  of the present embodiment has a different configuration of a plurality of hole groups from that of the rotor disc  60  of the first embodiment, and has basically the same other configurations as those of the rotor disc  60  of the first embodiment. Therefore, hereinafter, a plurality of hole groups in the rotor disc  60   a  of the present embodiment will be mainly described. 
     As illustrated in  FIG. 9 , in the present embodiment, each of a plurality of hole groups  77   a  includes three holes recessed from the radial inner surface  82  of the upstream extension portion  81  toward a radial outer side. Two holes of the three holes form the cooling hole  78  penetrating from the radial inner surface  82  of the upstream extension portion  81  to the radial outer surface that is the groove bottom surface  75   b  of the communication groove  75  (inner groove side surface  76   i  of the annular groove  76 ). Similarly to the cooling hole  78 , one remaining hole  79  also penetrates from the radial inner surface  82  of the upstream extension portion  81  to the radial outer surface that is the inner groove side surface  76   i  of the annular groove  76 . The hole  79  further penetrates from the outer groove side surface  76   o  of the annular groove  76  to the outer peripheral surface  73  of the small-diameter portion  72 . The hole  79  is a dummy hole that does not function as a hole through which the cooling air Ac passes. As illustrated in  FIG. 10 , an opening of the dummy hole  79  in the outer peripheral surface  73  of the small-diameter portion  72  is closed with a plug  89 . 
     Similarly to the cooling hole  78  of the first embodiment, the position of the outer opening  78   o  of the cooling hole  78  in the circumferential direction Dc is in the region Rb in the circumferential direction Dc in which the groove bottom surface  64   b  of the blade root groove  64  is present. The position of an outer opening  790  of the dummy hole  79  (radial outer surface) of the annular groove in the inner groove side surface  76   i  in the circumferential direction Dc is shifted from the region Rb in the circumferential direction Dc in which the groove bottom surface  64   b  of the blade root groove  64  is present. One partition piece  87  of the plurality of partition pieces  87  of the seal ring  85  is located between the cooling hole  78  of two cooling holes  78  of one hole group  77   a  and the dummy hole  79  in the circumferential direction Dc, the cooling hole  78  being closest to the dummy hole  79  of the hole group  77   a . In addition, the other partition piece  87  is located between the hole group  77   a  and another hole group  77   a  adjacent to the hole group  77   a  in the circumferential direction Dc. For this reason, a space in the annular groove  76  is partitioned by the partition piece  87  in the circumferential direction Dc into a space of a portion in which the blade root groove  64  is present (portion in which the outer openings  78   o  of two cooling holes  78  are present) and a space of a portion in which the blade root groove  64  is not present. Therefore, the dummy hole  79  and the inside of the blade root groove  64  do not communicate with each other. For this reason, even if the cooling air Ac flows into the dummy hole  79 , the cooling air Ac is not capable of flowing into the cooling air passage  44   p  of the rotor blade  44  through the blade root groove  64 . 
     Similarly to the first embodiment, the circumferential group width dgc of each of the plurality of hole groups  77   a  is larger than the axial group width dga of each of the plurality of hole groups  77   a , and is smaller than a minimum group interval dg 1  between the plurality of hole groups  77   a . The axial group width dga coincides with the diameter of the circular outer opening  78   o  of the cooling hole  78 . Also in the present embodiment, the circumferential group widths dgc of the plurality of hole groups  77   a  are the same as each other. A group interval dg 1  between two hole groups  77   a  adjacent to each other in the circumferential direction Dc is the same as the group interval dg 1  between another two hole groups  77   a  adjacent to each other in the circumferential direction Dc. Namely, the group intervals dg 1  are the same as each other. Therefore, in the present embodiment, each of the group intervals dg 1  is also the minimum group interval dg 1 . The hole interval between two cooling holes  78  of one hole group  77   a  in the circumferential direction Dc is a first hole interval dh 1 . In addition, the hole interval between the cooling hole  78  of two cooling holes  78  of one hole group  77   a , the cooling hole  78  being closest to the dummy hole  79  of the hole group  77   a , and the dummy hole  79  in the circumferential direction Dc is a second hole interval dh 2 . One of the first hole interval dh 1  and the second hole interval dh 2  is a maximum hole interval, and the other is a minimum hole interval. 
         dg  1&gt; dgc &gt;( dga,dh 1, dh 2) 
     The magnitude relationship between dga, dh 1 , and dh 2  does not matter. 
     The magnitude relationship between the dimensions described above is a magnitude relationship in the radial outer surface  76   i  in which the outer opening  78   o  of the cooling hole  78  and the outer opening  78   o  of the dummy hole are formed. However, in the present embodiment, a magnitude relationship between the dimensions in the radial inner surface  82  in which the inner opening  78   i  of the cooling hole  78  and the inner opening  78   i  of the dummy hole are formed, and a magnitude relationship between the dimensions at a position between the radial outer surface  76   i  and the radial inner surface  82  are the same as the magnitude relationship between the dimensions in the radial outer surface  76   i.    
     In the present embodiment, as described above, the circumferential group width dgc is smaller than the minimum group interval dg 1 . Therefore, the hole intervals dh 1  and dh 2  between three holes forming one hole group  77   a  are smaller than the minimum group interval dg 1 . For this reason, also in the present embodiment, the stress concentration in the vicinity of the opening of the cooling hole  78  can be reduced, and the durability of the rotor disc  60   a  can be improved. In particular, in the present embodiment, since the number of the holes forming the hole group  77   a  is larger than the number of the holes forming the hole group  77  of the first embodiment, the stress concentration in the vicinity of the opening of the cooling hole  78  can be reduced more than in the first embodiment. 
     The hole group  77   a  of the present embodiment and the hole group  77  of the first embodiment includes two cooling holes  78 ; however, when the flow rate of the cooling air which is necessary and sufficient to cool the rotor blade  44  is satisfied with one cooling hole  78 , one of the two cooling holes  78  may function as one dummy hole  79 . In addition, the hole group  77   a  of the present embodiment includes one of three holes as the dummy hole  79 , but all three holes may function as the cooling holes  78 . Further, the hole group  77   a  of the present embodiment includes three holes, but may include four or more holes. In this case, at least one of the four or more holes needs to be the cooling hole  78 . 
     The above-described tensile stress generated in the rotor disc  60   a  is larger in the radial inner surface  82  than in the radial outer surface  76   i  of the small-diameter portion  72 . For this reason, stress generated around the inner opening  78   i  of the cooling hole  78  is also larger than stress generated around the outer opening  78   o  of the cooling hole  78 . Therefore, all the plurality of holes of the hole group  77   a  need to be open in the radial inner surface  82  of the small-diameter portion  72 . On the other hand, all the plurality of holes of the hole group  77   a  do not need to be open in the radial outer surface  76   i  of the small-diameter portion  72 . For this reason, the dummy hole  79  that is one of the plurality of holes of the hole group  77   a  may not be open in the radial outer surface  76   i  of the small-diameter portion  72 . 
     Third Embodiment of Rotor Disc 
     Hereinafter, a rotor disc of the present embodiment will be described with reference to  FIGS. 11 to 13 . 
     A rotor disc  60   b  of the present embodiment has a different configuration of a plurality of hole groups from that of the rotor disc  60  of the first embodiment, and has basically the same other configurations as those of the rotor disc  60  of the first embodiment. Therefore, hereinafter, a plurality of hole groups  77   b  in the rotor disc  60   b  of the present embodiment will be mainly described. 
     As illustrated in  FIGS. 11 to 13 , in the present embodiment, each of the plurality of hole groups  77   b  includes one hole recessed from the radial inner surface  82  of the upstream extension portion  81  toward a radial outer side. The hole forms a cooling hole  78   b  penetrating from the radial inner surface  82  of the upstream extension portion  81  to the radial outer surface that is the groove bottom surface  75   b  of the communication groove  75  (inner groove side surface  76   i  of the annular groove  76 ). The cooling hole  78   b  has an oval cross-sectional shape in a plane perpendicular to the radial direction Dr in which the cooling hole  78   b  extends. Therefore, the shape of the inner opening  78   o  of the cooling hole  78   b  is also an oval as illustrated in  FIG. 13 , and the shape of the outer opening  78   i  of the cooling hole  78   b  is also an oval as illustrated in  FIG. 12 . The oval is the shape of a running track, and is a shape in which two semi-arcs facing each other with an interval therebetween are connected by two straight lines parallel to each other. In the present embodiment, a longitudinal direction of the oval is the circumferential direction Dc. For this reason, a circumferential opening width dhc that is a width of the inner opening  78   i  of the cooling hole  78   b  in the circumferential direction Dc is larger than an axial opening width dha that is a width of the inner opening  78   i  in the axial direction Da. In addition, the circumferential opening width dhc that is the width of the outer opening  78   o  of the cooling hole  78   b  in the circumferential direction Dc is larger than the axial opening width dha that is the width of the outer opening  78   o  in the axial direction Da. 
     Similarly to the cooling hole  78  of the first embodiment, the position of the outer opening  78   o  of the cooling hole  78   b  in the circumferential direction Dc is in the region Rb in the circumferential direction Dc in which the groove bottom surface  64   b  of the blade root groove  64  is present. In the circumferential direction Dc, each of the plurality of partition pieces  87  of the seal ring  85  is located between the region Rb and the region Rb in the circumferential direction Dc in which the groove bottom surface  64   b  of another blade root groove  64  adjacent to the blade root groove  64  is present. For this reason, a space in the annular groove  76  is partitioned by the partition piece  87  in the circumferential direction Dc into a space of a portion in which a blade root groove  64  is present and a space of a portion in which another blade root groove  64  adjacent to the blade root groove  64  in the circumferential direction Dc is present. 
     The circumferential group width dgc of each of the plurality of hole groups  77   b  is equal to the circumferential opening width dhc of the cooling holes  78   b . The circumferential group width dgc and the circumferential opening width dhc are larger than the axial group width dga of each of the plurality of hole groups  77   b , and are smaller than a minimum group interval dg 2  of group intervals, each of which is an interval between the plurality of hole groups  77   b  in the circumferential direction Dc. The axial group width dga is equal to the axial opening width dha of the cooling hole  78   b . The circumferential group widths dgc of the plurality of hole groups  77   b  are the same as each other. A group interval dg 2  between two hole groups  77   b  adjacent to each other in the circumferential direction Dc is the same as the group interval dg 2  between another two hole groups  77   b  adjacent to each other in the circumferential direction Dc. Namely, the group intervals dg 2  are the same as each other. Therefore, in the present embodiment, each of the group intervals dg 2  is also the minimum group interval dg 2 . A magnitude relationship between the dimensions is as follows. 
         dg 2&gt; dgc=dhc&gt;dga=dha    
     The magnitude relationship between the dimensions described above is a magnitude relationship in the radial outer surface  75   b  in which the outer opening  78   o  of the cooling hole  78   b  is formed. However, in the present embodiment, a magnitude relationship between the dimensions in the radial inner surface  82  in which the inner opening  78   i  of the cooling hole  78   b  is formed, and a magnitude relationship between the dimensions at a position between the radial outer surface  75   b  and the radial inner surface  82  are the same as the magnitude relationship between the dimensions in the radial outer surface  75   b.    
     In the present embodiment, the outer opening  78   o  and the inner opening  78   i  of the cooling hole  78   b  have a shape in which openings of two holes are connected to each other in the circumferential direction Dc. For this reason, in the present embodiment, stress generated around the outer opening  78   o  of the cooling hole  78   b  and stress generated around the inner opening  78   i  are dispersed in the circumferential direction Dc. Therefore, also in the present embodiment, the stress concentration in the vicinity of the opening of the cooling hole  78   b  can be reduced, and the durability of the rotor disc  60   b  can be improved. 
     In the cooling hole  78   b  of the present embodiment, both the shape of the inner opening  78   i  and the shape of the outer opening  78   o  are an oval. As described above, stress generated around the inner opening  78   i  of the cooling hole  78   b  is larger than stress generated around the outer opening  78   o  of the cooling hole  78   b . For this reason, the shape of the inner opening  78   i  of the cooling hole  78   b  may be an oval, and the shape of the outer opening  78   o  of the cooling hole  78   b  may be a circle. In addition, the shape of the opening of the cooling hole  78   b  does not need to be an oval which is long in the circumferential direction Dc as long as the circumferential opening width dhc of the cooling hole  78   b  is larger than the axial opening width dha. Specifically, as illustrated in  FIG. 14 , the shape of an opening of a cooling hole  78   c  may be a shape in which two circles partly overlap each other in the circumferential direction Dc. In addition, as illustrated in  FIG. 15 , the shape of an opening of a cooling hole  78   d  may be an elliptical shape that is long in the circumferential direction Dc. 
     In addition, similarly to the second embodiment, the hole group  77   b  of the present embodiment may also include the dummy hole  79  in addition to the cooling hole  78   b.    
     Various Modification Examples 
     The rotor discs of the first and third embodiments include the annular groove  76 . However, the rotor discs of the first and third embodiments may not include the annular groove  76 . When no annular groove  76  is provided, the seal ring  85  that closes the opening of the annular groove  76  is not required. 
     In each of the embodiments described above, the plurality of hole groups are arranged at equal intervals in the circumferential direction Dc. However, the plurality of hole groups may be arranged in the circumferential direction Dc but not at equal intervals. 
     In each of the embodiments described above, a region around the inner opening of the hole on the radial inner surface  82  is inclined in a direction in which the hole extends on a virtual plane including the axis Ar and the hole, namely, in the radial direction. In other words, as illustrated in  FIG. 6  and the like, the region around the inner opening on the radial inner surface  82  is an inclined surface that gradually approaches the radial inner side Dri toward the axial downstream side Dad. In this case, an angle θd at a corner between a generating line on the axial downstream side Dad of the columnar hole and the inclined surface is an obtuse angle, and an angle θu at a corner between a generating line on the axial upstream side Dau of the columnar hole and the inclined surface is an acute angle. For this reason, stress is concentrated at an edge of the inner opening on the axial upstream side Dau. Therefore, as illustrated in  FIG. 16 , it is preferable that a region inner surface  82   a  around the inner opening of the hole on the radial inner surface  82  is perpendicular to the direction in which the hole extends on the virtual plane including the axis Ar and the hole. In such a manner, both the angle at the corner between the generating line on the axial downstream side Dad of the columnar hole and the region inner surface  82   a  and the angle at the corner between the generating line on the axial upstream side Dau of the columnar hole and the region inner surface  82   a  are 90°, so that stress can be prevented from being concentrated at the edge of the inner opening  78   i  on the axial upstream side Dau. 
     In each of the embodiments described above, the direction in which the hole extends is the radial direction perpendicular to the axis Ar. However, the direction in which the hole extends may be gradually inclined toward the axial downstream side Dad toward the radial outer side Dro. When the direction in which the hole extends is inclined in such a manner, as described above, even in the case where the region around the inner opening  78   i  on the radial inner surface  82  is an inclined surface that gradually approaches the radial inner side Dri toward the axial downstream side Dad, the direction in which the hole extends can be made perpendicular to the inclined surface. In addition, when the direction in which the hole extends is inclined in such a manner, the cooling hole  78   b  is capable of directly communicating with the space in the blade root groove  64  without having to go through the space in the communication groove  75 . 
     INDUSTRIAL APPLICABILITY 
     According to one aspect of the present invention, the stress concentration in the vicinity of the opening of the cooling hole can be reduced, and the durability of the rotor disc can be improved. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10 : Gas turbine 
               11 : Gas turbine rotor 
               15 : Gas turbine casing 
               16 : Intermediate casing 
               20 : Compressor 
               21 : Compressor rotor 
               22 : Rotor shaft 
               23 : Rotor blade row 
               25 : Compressor casing 
               26 : Stator blade row 
               30 : Combustor 
               40 : Turbine 
               41 : Turbine rotor 
               42 : Rotor shaft 
               42   p : Cooling air flow path 
               42   d ,  60 ,  60   a ,  60   b : Rotor disc 
               42   s : Spindle bolt 
               43 : Rotor blade row 
               44 : Rotor blade 
               44   b : Blade body 
               44   f : Platform 
               44   r : Blade root 
               44   p : Cooling air passage 
               45 : Turbine casing 
               45   a : Outer casing 
               45   b : Inner casing 
               45   c : Heat shield ring 
               45   d : Ring segment 
               46 : Stator blade row 
               47 : Stator blade 
               49 : Combustion gas flow path 
               50 : Cooling device 
               51 : Air bleeding line 
               52 : Cooler 
               53 : Cooling air line 
               54 : Booster 
               61 : Disc body 
               62 : Large-diameter portion 
               63 : Outer peripheral surface (or radial outer surface) 
               64 : Blade root groove 
               64   b : Groove bottom surface (or radial outer surface) 
               72 : Small-diameter portion 
               73 : Outer peripheral surface 
               74 : Front surface 
               75 : Communication groove 
               75   b : Groove bottom surface (or radial outer surface) 
               76 : Annular groove 
               76   i : Inner groove side surface (or radial outer 
             surface) 
               76   o : Outer groove side surface 
               76   b : Groove bottom surface 
               77 ,  77   a ,  77   b : Hole group 
               78 ,  78   b ,  78   c ,  78   d : Cooling hole 
               78   i : Inner opening 
               78   o : Outer opening 
               79 : Dummy hole 
               81 : Upstream extension portion 
               82 : Radial inner surface 
               82   a : Region inner surface 
               83 : Downstream extension portion 
               85 : Seal ring 
               86 : Ring piece 
               87 : Partition piece 
               88 : Seal cap 
               89 : Plug 
             A: Air 
             Ac: Cooling air 
             F: Fuel 
             G: Combustion gas 
             dh: Hole interval 
             dhc: Circumferential opening width 
             dha: Axial opening width 
             dgc: Circumferential group width 
             dga: Axial group width 
             dg: Minimum group interval 
             Ar: Axis 
             Da: Axial direction 
             Dau: Axial upstream side 
             Dad: Axial downstream side 
             Dc: Circumferential direction 
             Dr: Radial direction 
             Dri: Radial inner side 
             Dro: Radial outer side