Patent Description:
The steam turbine includes a diffuser on the downstream side of a a final stage for recovering static pressure and discharging steam to the outside. For example, <CIT> discloses a configuration in which a turbine casing has a diffuser formed by a double structure of an outer casing and an inner casing. The diffuser is formed such that a cross-sectional area of a flow passage thereof gradually expands from the upstream side to the downstream side. With such a diffuser, the static pressure can be recovered by guiding the flow of steam discharged from the final stage.

Other known steam turbines are disclosed in <CIT>, <CIT>, <CIT>, <CIT>, <CIT> or <CIT>.

However, in the steam turbine as described above, when the flow velocity of the steam flowing inside is high, the flow velocity of the steam at an outlet of the final stage may become a transonic speed or a subsonic speed. When the flow velocity of steam becomes the transonic speed or the subsonic speed, the steam may cause a shock wave or peeling in the diffuser. Therefore, it is desired to recover the static pressure more effectively in the diffuser and improve the efficiency of the steam turbine.

The present invention provides a steam turbine capable of efficiently recovering the static pressure of steam in a diffuser.

The aforementioned advantage is reached by a steam turbine as claimed in the appended set of claims.

Hereinafter, an embodiment of a steam turbine according to the present disclosure will be described below with reference to the accompanying drawings. However, the present disclosure is not limited to the embodiment.

As shown in <FIG>, a steam turbine 1A of the present embodiment has a rotor <NUM> that rotates about an axis O and a casing <NUM> that covers the rotor <NUM>.

For the convenience of the following description, a direction in which the axis O extends is referred to as an axial direction Da. In addition, a radial direction in the rotor <NUM> with respect to the axis O is simply referred to as a radial direction Dr. Further, a circumferential direction of the rotor <NUM> about the axis O is simply referred to as a circumferential direction Dc.

The rotor <NUM> has a rotor shaft <NUM> and a rotor blade row <NUM>. The rotor shaft <NUM> extends in the axial direction Da about the axis O. The rotor shaft <NUM> is rotatable about the axis O. The rotor shaft <NUM> has a shaft core portion <NUM> and a plurality of disc portions <NUM>. The shaft core portion <NUM> is formed in a columnar shape about the axis O and extends in the axial direction Da. The plurality of disc portions <NUM> are disposed at intervals in the axial direction Da. Each disc portion <NUM> is integrally formed with the shaft core portion <NUM> so as to constitute an outer peripheral portion of the rotor shaft <NUM>. Each disc portion <NUM> is disposed so as to extend from the shaft core portion <NUM> to the outer side Dro in the radial direction Dr.

The rotor blade row <NUM> is fixed to the outer side Dro of the rotor shaft <NUM> in the radial direction Dr. A plurality of rows of rotor blade row <NUM> are disposed at intervals along the axial direction Da of the rotor shaft <NUM>. In the case of the present embodiment, the rotor blade rows <NUM> are disposed in four rows, for example. Therefore, in the case of the present embodiment, the rotor blade rows <NUM> are disposed from the first row to the fourth row of rotor blade rows <NUM>.

As shown in <FIG>, the rotor blade rows <NUM> of each row have a plurality of rotor blades <NUM> arranged side by side in the circumferential direction Dc. The plurality of rotor blades <NUM> are attached side by side on an outer circumference of the disc portion <NUM>. Each rotor blade <NUM> has a rotor blade main body <NUM>, a shroud <NUM>, and a platform <NUM>.

Each rotor blade main body <NUM> extends in the radial direction Dr. The shroud <NUM> is disposed on the outer side Dro in the radial direction Dr with respect to the rotor blade main body <NUM>. The platform <NUM> is disposed on an inner side Dri in the radial direction Dr with respect to the rotor blade main body <NUM>. The platform <NUM> is fixed to the disc portion <NUM>. In the rotor blade <NUM>, a part of a steam main flow passage <NUM> which is a flow passage through which steam S flows is formed between the shroud <NUM> and the platform <NUM>. That is, the steam main flow passage <NUM> is formed between the shroud <NUM> positioned on an outer peripheral edge of the rotor blade row <NUM> and the platform <NUM> positioned on an inner peripheral edge of the rotor blade row <NUM>. By disposing a plurality of rotor blades <NUM> side by side in the circumferential direction Dc, the steam main flow passage <NUM> is formed in an annular shape on the outer peripheral portion of the rotor <NUM>.

The casing <NUM> is formed so as to cover the rotor shaft <NUM>, the plurality of rotor blade rows <NUM>, that is, the rotor <NUM>. A stator vane row <NUM> is fixed to the inner side Dri of the radial direction Dr in the casing <NUM>. A plurality of stator vane rows <NUM> are disposed at intervals along the axial direction Da. In the present embodiment, the number of rows of the stator vane rows <NUM> is disposed in the same four rows as that of the rotor blade rows <NUM>. The stator vane rows <NUM> are disposed so as to be arranged side by side at intervals on a first side Dau in the axial direction Da with respect to each rotor blade row <NUM>. The stator vane row <NUM> constitutes one compression stage together with the rotor blade row <NUM>. Therefore, in the present embodiment, the four rows of rotor blade rows <NUM> and the stator vane rows <NUM> constitute four compression stages having the fourth stages as the final stage.

The stator vane row <NUM> of each row has a plurality of stator vanes <NUM> arranged side by side in the circumferential direction Dc. The stator vane row <NUM> has an outer ring <NUM>, a stator vane main body <NUM>, and an inner ring <NUM>. The outer ring <NUM> is formed in an annular shape. The outer ring <NUM> is disposed on the outer side Dro of the stator vane main body <NUM> in the radial direction Dr. The inner ring <NUM> is formed in an annular shape. The inner ring <NUM> is disposed on the inner side Dri of the stator vane main body <NUM> in the radial direction Dr. An annular space between the outer ring <NUM> and the inner ring <NUM> forms a part of the steam main flow passage <NUM> through which the steam S flows.

The steam main flow passage <NUM> extends in the axial direction Da across a plurality of rotor blade rows <NUM> and stator vane rows <NUM>. Here, the first side Dau in the axial direction Da is the upstream side in the flow direction of the steam S in the steam main flow passage <NUM>. In addition, a second side Dad in the axial direction Da is on the opposite side to the first side Dau, and is the downstream side in the flow direction of the steam S in the steam main flow passage <NUM>. That is, the steam S flows in the casing <NUM> from the first side Dau to the second side Dad in the axial direction Da.

The casing <NUM> includes an exhaust casing <NUM> and a diffuser <NUM>. The exhaust casing <NUM> is connected to the outside of the casing <NUM>. The exhaust casing <NUM> discharges the steam S flowing through the steam main flow passage <NUM> to the outside of the casing <NUM>. The exhaust casing <NUM> is disposed on the second side Dad farthest in the axial direction Da in the casing <NUM>. An exhaust port <NUM> (refer to <FIG>) that opens downward is formed in a lower portion of the exhaust casing <NUM>. The exhaust casing <NUM> exhausts the steam S whose static pressure has been recovered by the diffuser <NUM>, which will be described later, to the outside through the exhaust port <NUM>.

The diffuser <NUM> guides the steam S flowing out from the rotor blade row 31F of the final stage, which is disposed on the second side Dad farthest in the axial direction Da among a plurality of the rotor blade rows <NUM>, to the outside of the casing <NUM> via the exhaust casing <NUM>. The diffuser <NUM> is disposed between the rotor blade row 31F of the final stage and the exhaust casing <NUM>. The diffuser <NUM> of the present embodiment has an outer guide <NUM> and an inner guide <NUM>.

The outer guide <NUM> is disposed on the second side Dad in the axial direction Da with respect to the rotor blade row 31F of the final stage. The outer guide <NUM> is formed so as to gradually expand to the outer side Dro in the radial direction Dr from the first side Dau to the second side Dad in the axial direction Da. The outer guide <NUM> of the present embodiment is curved so as to be convex toward the inner side Dri in the radial direction Dr. The outer guide <NUM> has a first diameter-expanded portion <NUM> and a second diameter-expanded portion <NUM>.

The first diameter-expanded portion <NUM> is disposed on the first side Dau farthest in the axial direction Da in the outer guide <NUM>. That is, in the present embodiment, the first diameter-expanded portion <NUM> is disposed at a position closest to the rotor blade row 31F of the final stage in the outer guide <NUM>. The first diameter-expanded portion <NUM> is formed so as to gradually expand to the outer side Dro in the radial direction Dr at a first radius of curvature R1 from the first side Dau to the second side Dad in the axial direction Da. The first diameter-expanded portion <NUM> is formed in a curved plate shape with the first radius of curvature R1 in a cross-sectional view parallel to and orthogonal to the axis O. Specifically, the first diameter-expanded portion <NUM> is formed by curving, in a cross-sectional view parallel to and orthogonal to the axis O, such that an intermediate portion <NUM> of the diameter-enlarged portion in the axial direction Da extends to the inner side Dri in the radial direction Dr with respect to a first end <NUM> of the first diameter-enlarged portion of the first side Dau in the axial direction Da and a second end <NUM> of the second diameter-enlarged portion of the second side Dad in the axial direction Da.

The second diameter-expanded portion <NUM> is disposed on the second side Dad in the axial direction Da with respect to the first diameter-expanded portion <NUM>. The second diameter-expanded portion <NUM> is integrally formed so as to be connected to the first diameter-expanded portion <NUM> by the second side Dad in the axial direction Da. In the present embodiment, the second diameter-expanded portion <NUM> is formed so as to gradually expand to the outer side Dro in the radial direction Dr from the first side Dau to the second side Dad in the axial direction Da. The second diameter-expanded portion <NUM> has a second radius of curvature R2 larger than the first radius of curvature R1 and gradually expands to the outer side in the radial direction Dr. The second diameter-expanded portion <NUM> is formed in a curved plate shape with the second radius of curvature R2 in a cross-sectional view parallel to and orthogonal to the axis O. Specifically, the second radius of curvature R2 is preferably as large as possible with respect to the first radius of curvature R1. That is, the second diameter-expanded portion <NUM> expands more slowly than the first diameter-expanded portion <NUM>. In the present embodiment, the second diameter-expanded portion <NUM> linearly expands in diameter from the first side Dau to the second side Dad in the axial direction Da.

The inner guide <NUM> is disposed at intervals in the inner side Dri in the radial direction with respect to the outer guide <NUM>. As a result, an annular flow passage <NUM>, which is the flow passage through which the steam S can flow, is defined between the outer guide <NUM> and the inner guide <NUM>. The annular flow passage <NUM> is defined between the outer guide <NUM> and the inner guide <NUM> so as to form an annular shape when viewed from the axial direction Da. The annular flow passage <NUM> is connected to the steam main flow passage <NUM> by the second side Dad in the axial direction Da. The inner guide <NUM> is formed so as to gradually expand to the outer side Dro in the radial direction Dr from the first side Dau to the second side Dad in the axial direction Da with a third radius of curvature R3. The inner guide <NUM> has an inner curved diameter-expanded portion <NUM>. The inner curved diameter-expanded portion <NUM> is formed in a curved plate shape with the third radius of curvature R3 in a cross-sectional view parallel to and orthogonal to the axis O. Specifically, the inner curved diameter-expanded portion <NUM> is formed by curving, in a cross-sectional view parallel to and orthogonal to the axis O, such that an intermediate portion <NUM> of the inner guide between a first end <NUM> of the inner guide and a second end <NUM> of the inner guide extends to the inner side Dri in the radial direction Dr with respect to the first end <NUM> of the inner guide of the first side Dau in the axial direction Da and the second end <NUM> of the inner guide of the second side Dad in the axial direction Da. The third radius of curvature R3 is preferably set be larger than the first radius of curvature R1 of the first diameter-expanded portion <NUM>. In the present embodiment, the third radius of curvature R3 is larger than the first radius of curvature R1 and smaller than the second radius of curvature R2. The third radius of curvature R3 is not limited to being smaller than the second radius of curvature R2 as long as the third radius of curvature R3 is larger than the first radius of curvature R1. Accordingly, the third radius of curvature R3 may be the same as the second radius of curvature R2.

In addition, the diffuser <NUM> is divided into a first region P1 positioned on the first side Dau in the axial direction Da and a second region P2 positioned on the second side Dad in the axial direction Da.

The first region P1 is a region closest to the rotor blade row 31F of the final stage in the axial direction Da. The first diameter-expanded portion <NUM> is disposed in the first region P1. In the first region P1, a part of the inner curved diameter-expanded portion <NUM> including the first end <NUM> of the inner guide is disposed.

The second region P2 is a region connected to the first region P1 by a second side Dad in the axial direction Da. The second diameter-expanded portion <NUM> is disposed in the second region P2. A part of the inner curved diameter-expanded portion <NUM> including the second end <NUM> of the inner guide is disposed in the second region P2.

In addition, a length L2 of the second region P2 in the axial direction Da is preferably, for example, about <NUM> to <NUM> times a length L1 of the first region P1 in the axial direction Da. Here, the length L1 of the first region P1 and the length L2 of the second region P2 are the lengths near the center of the annular flow passage <NUM> in the radial direction Dr in each region. Further, the length L2 of the second region P2 is preferably about <NUM> to <NUM> times the length L1 of the first region P1. In particular, the length L2 of the second region P2 is further preferably about <NUM> to <NUM> times the length L1 of the first region P1.

Generally, when the steam turbine 1A is in rated operation, the flow velocity (average flow velocity) of the steam S flowing out from the rotor blade row 31F of the final stage may be the transonic speed. Further, the flow velocity distribution of the steam S flowing out from the rotor blade row 31F of the final stage gradually increases from the inner side Dri to the outer side Dro in the radial direction Dr due to the influence of the centrifugal force by the rotor blade row <NUM>. Therefore, when the flow velocity of the steam S flowing out from the rotor blade row 31F of the final stage is transonic speed, the flow velocity of the steam S is further increased in a region close to the shroud <NUM>. Accordingly, in the annular flow passage <NUM>, the steam S flows obliquely toward the outer side Dro in the radial direction Dr with respect to the axis O. As a result, the steam S flowing in the diffuser <NUM> is easily peeled off from a wall surface forming the diffuser <NUM> before flowing into the exhaust casing <NUM>. When the peeling occurs, the exhaust loss increases.

On the other hand, in the steam turbine 1A having the above-described configuration, the inner curved diameter-expanded portion <NUM> is curved. Accordingly, the steam S flowing out from the rotor blade row 31F of the final stage flows along the inner curved diameter-expanded portion <NUM> in the portion close to the inner guide <NUM> in the radial direction Dr. As a result, in the vicinity of the inner curved diameter-expanded portion <NUM>, the steam S flows such that the flow direction is changed to the outer side Dro in the radial direction Dr while suppressing the peeling from the inner curved diameter-expanded portion <NUM>. In addition, the first diameter-expanded portion <NUM> is curved with the first radius of curvature R1. Therefore, the steam S flowing out from the rotor blade row 31F of the final stage flows, in the first region P1, along the first diameter-expanded portion <NUM> in the portion close to the outer guide <NUM> in the radial direction Dr. By flowing the steam S along the curved surface, the steam S flowing out from the rotor blade row 31F of the final stage can be efficiently guided. After that, the steam S flowing in the portion close to the outer guide <NUM> in the radial direction Dr flows, in the second region P2, along the second diameter-expanded portion <NUM>. The second diameter-expanded portion <NUM> slowly expands to the outer side Dro in the radial direction Dr as compared with the first diameter-expanded portion <NUM>. Thus, the second diameter-expanded portion <NUM> is formed along the direction in which the steam S flowing from the first diameter-expanded portion <NUM> peels off. Therefore, the flow of the steam S can be suppressed to the inner side Dri in the radial direction Dr as compared with when the second diameter-expanded portion <NUM> is formed with the first radius of curvature R1 such that the first diameter-expanded portion <NUM> is extended as it is. Therefore, the steam S flowing along the first diameter-expanded portion <NUM> flows along the second diameter-expanded portion <NUM> without causing peeling. In this way, by increasing the radius of curvature on the downstream side (second side Dad) of the outer guide <NUM>, it is possible to suppress the occurrence of peeling in the flow of the steam S on the outer side Dro in the radial direction Dr. In this way, the diffuser <NUM> can reduce the flow velocity while suppressing the peeling of the steam S. Therefore, even when the flow velocity (average flow velocity) of the steam S flowing out from the rotor blade row 31F of the final stage is transonic speed, the occurrence of peeling can be suppressed. Accordingly, it is possible to efficiently recover the static pressure of the steam S in the diffuser <NUM>.

In addition, in the steam turbine 1B, the third radius of curvature R3 of the inner curved diameter-expanded portion <NUM> is larger than the first radius of curvature R1 of the first diameter-expanded portion <NUM>. Thus, the occurrence of peeling can be efficiently suppressed even on the inner side Dri in the radial direction Dr. As a result, it becomes possible to more efficiently recover the static pressure of the steam S in the diffuser <NUM>.

In addition, in the steam turbine 1A, the length L2 of the axial direction Da of the second region P2 is <NUM> to <NUM> times the length L1 of the axial direction Da of the first region P1. That is, the length of the second diameter-expanded portion <NUM> in the axial direction Da is <NUM> to <NUM> times the length of the first diameter-expanded portion <NUM> in the axial direction Da. As a result, the flow velocity of the steam S can be adjusted in a well-balanced manner in the first region P1 and the second region P2. Therefore, it is possible to efficiently recover the static pressure.

In the second embodiment described below, the same reference numerals are given in the drawings to the configurations common to the first embodiment, and the description thereof will be omitted.

As shown in <FIG>, in the steam turbine 1B of the second embodiment, the structure of the diffuser <NUM> is different from that of the first embodiment.

The diffuser <NUM> of the second embodiment has an outer guide <NUM> and an inner guide <NUM>.

The outer guide <NUM> is disposed on the second side Dad in the axial direction Da with respect to the rotor blade row 31F of the final stage. The outer guide <NUM> is formed so as to gradually expand to the outer side Dro in the radial direction Dr from the first side Dau to the second side Dad in the axial direction Da. The outer guide <NUM> has a first inclined portion <NUM> and a second inclined portion <NUM>.

The first inclined portion <NUM> is disposed on the first side Dau farthest in the axial direction Da in the outer guide <NUM>. That is, in the present embodiment, the first inclined portion <NUM> is disposed at a position closest to the rotor blade row 31F of the final stage in the outer guide <NUM>. The first inclined portion <NUM> is formed so as to gradually expand to the outer side Dro in the radial direction Dr from the first side Dau to the second side Dad in the axial direction Da. The first inclined portion <NUM> is inclined at a first inclination angle θ1 with respect to the axis O in a cross-sectional view parallel to and orthogonal to the axis O. The first inclined portion <NUM> is formed in a flat plate shape in a cross-sectional view parallel to and orthogonal to the axis O. That is, the first inclined portion <NUM> is formed linearly in a cross-sectional view parallel to and orthogonal to the axis O.

The second inclined portion <NUM> is disposed on the second side Dad in the axial direction Da with respect to the first inclined portion <NUM>. The second inclined portion <NUM> is formed integrally with the first inclined portion <NUM>. The second inclined portion <NUM> is formed so as to gradually expand to the outer side Dro in the radial direction Dr from the first side Dau to the second side Dad in the axial direction Da. The second inclined portion <NUM> is inclined at a second inclination angle θ2 larger than the first inclination angle θ1 with respect to the axis O in a cross-sectional view parallel to and orthogonal to the axis O. The second inclined portion <NUM> is formed in a flat plate shape in a cross-sectional view parallel to and orthogonal to the axis O. That is, the second inclined portion <NUM> is formed linearly in a cross-sectional view parallel to and orthogonal to the axis O.

The inner guide <NUM> is disposed at intervals in the inner side Dri in the radial direction Dr with respect to the outer guide <NUM>. As a result, an annular flow passage <NUM>, which is the flow passage through which the steam S can flow, is defined between the outer guide <NUM> and the inner guide <NUM>. The inner guide <NUM> is formed so as to gradually expand to the outer side Dro in the radial direction Dr from the first side Dau to the second side Dad in the axial direction Da. The length of the axial direction Da of the inner guide <NUM> is formed to be longer than the length of the axial direction Da of the outer guide <NUM>. The inner guide <NUM> extends to be longer than the outer guide <NUM> on the second side Dad in the axial direction Da. The inner guide <NUM> is inclined at a third inclination angle θ3 with respect to the axis O in a cross-sectional view parallel to and orthogonal to the axis O. The inner guide <NUM> is formed in a flat plate shape in a cross-sectional view parallel to and orthogonal to the axis O. Accordingly, the inner guide <NUM> is formed so as to extend linearly and straight without bending even once from the first end <NUM> of the inner guide of the first side Dau in the axial direction Da toward the second end <NUM> of the inner guide of the second side Dad in the axial direction Da in a cross sectional view parallel and perpendicular to the axis O. The angle of the third inclination angle θ3 with respect to the axis O is larger than that of the first inclination angle θ1 and smaller than that of the second inclination angle θ2.

The first inclined portion <NUM> is disposed in the first region P11 of the second embodiment. A part of the inner guide <NUM> including the first end <NUM> of the inner guide is disposed in the first region P11. In the first region P11, a cross-sectional area of the annular flow passage <NUM>, which is a flow passage defined between the outer guide <NUM> and the inner guide <NUM>, gradually decreases from the first side Dau to the second side Dad in the axial direction Da. That is, in the first region P11, the cross-sectional area of the annular flow passage <NUM> is maximum on the first side Dau in the axial direction Da and minimum on the second side Dad in the axial direction Da. The minimum cross-sectional area A1 min of the annular flow passage <NUM> in the first region P11 is formed to be larger than a cross-sectional area Aw of the steam main flow passage <NUM> in the rotor blade row 31F of the final stage.

The second inclined portion <NUM> is disposed in the second region P12 of the second embodiment. A part of the inner guide <NUM> including the second end <NUM> of the inner guide is disposed in the second region P12. In the second region P12, a cross-sectional area A2 of the annular flow passage <NUM> is formed so as to gradually increase toward the second side Dad in the axial direction Da. That is, in the second region P12, the cross-sectional area of the annular flow passage <NUM> is the minimum on the first side Dau in the axial direction Da and maximum on the second side Dad in the axial direction Da. The maximum cross-sectional area A2max of the annular flow passage <NUM> in the second region P12 is formed so as to be larger than the maximum cross-sectional area A1 max of the annular flow passage <NUM> in the first flow passage <NUM>.

In addition, a length L2 of the second region P12 in the axial direction Da is preferably, for example, about <NUM> to <NUM> times a length L1 of the first region P11 in the axial direction Da. Further, the length L2 of the second region P12 is preferably about <NUM> to <NUM> times the length L1 of the first region P11. In particular, the length L2 of the second region P12 is further preferably about <NUM> to <NUM> times the length L1 of the first region P11.

In a case where the flow velocity (average flow velocity) of the steam S flowing out from the rotor blade row 31F of the final stage when the steam turbine 1B is in rated operation is subsonic speed, the flow velocity of the steam S may further increase in a region close to the shroud <NUM> to become supersonic speed. On the other hand, in the present embodiment, in the first region P11 of the diffuser <NUM>, the cross-sectional area A1 of the annular flow passage <NUM> gradually becomes smaller toward the second side Dad in the axial direction Da. The annular flow passage <NUM> is narrowed, so that the flow velocity (mach number) of the steam S flowing out from the rotor blade row 31F of the final stage is entirely reduced in the first region P11. As a result, the flow velocity of the steam S in a region close to the outer guide <NUM> in the radial direction Dr in the first region P11 is reduced from supersonic speed to subsonic speed. After that, the steam S flows from the first region P11 to the second region P12. The flow velocity of the steam S in the second region P12 is further reduced by gradually increasing the cross-sectional area A2 of the annular flow passage <NUM> toward the second side Dad in the axial direction Da while being reduced to the subsonic speed. Thus, the static pressure can be recovered. Accordingly, even when the flow velocity of the steam S flowing out from the rotor blade row 31F of the final stage is subsonic speed, it is possible to efficiently recover the static pressure of the steam S in the diffuser <NUM>.

In addition, in the steam turbine 1B, the minimum cross-sectional area A1 min of the annular flow passage <NUM> in the first region P11 is larger than the cross-sectional area Aw of the steam main flow passage <NUM> formed between the outer peripheral edge and the inner peripheral edge of the rotor blade row 31F of the final stage. As a result, it is possible to suppress the flow of steam S flowing out from the rotor blade row 31F of the final stage from being choked in the first region P11 (the flow rate does not change even if the pressure ratio is large).

In addition, in the steam turbine 1B, the maximum cross-sectional area A2max of the annular flow passage <NUM> in the second region P12 is larger than the maximum cross-sectional area A1 max of the annular flow passage <NUM> in the first region P11. As a result, the flow velocity of the steam S flowing into the second region P12 after the flow velocity is reduced in the first region P11 can be surely reduced.

In addition, in the steam turbine 1B, the inner guide <NUM> is formed so as to extend linearly from the first end <NUM> of the inner guide of the first side Dau in the axial direction Da toward the second end <NUM> of the inner guide of the second side Dad in the axial direction Da. Further, the inner guide <NUM> is inclined at the third inclination angle θ3 that is larger than the first inclination angle θ1 of the first inclined portion <NUM> and smaller than the second inclination angle θ2 of the second inclined portion <NUM>. As a result, in the annular flow passage <NUM>, the turbulence of the flow of the steam S at the inner side Dri in the radial direction Dr can be suppressed.

In addition, in the steam turbine 1B, the length L2 of the axial direction Da of the second region P12 is <NUM> to <NUM> times the length L1 of the axial direction Da of the first region P11. As a result, the flow velocity of the steam S can be adjusted in a well-balanced manner in the first region P11 and the second region P12. Therefore, it is possible to efficiently recover the static pressure.

The embodiments of the present disclosure have been described in detail with reference to the drawings, but the specific configuration is not limited to the embodiments, and includes design changes and the like within the scope of the invention as defined in the appended set of claims.

Claim 1:
A steam turbine (1B) comprising:
a rotor shaft (<NUM>) that is configured to rotate about an axis (O);
a plurality of rotor blade rows (<NUM>) that are fixed to an outer side of the rotor shaft (<NUM>) and disposed at intervals in an axial direction along which the axis extends;
a casing (<NUM>) that covers the rotor shaft (<NUM>) and the plurality of rotor blade rows (<NUM>); and
stator vane rows (<NUM>) that are fixed to the casing (<NUM>), wherein each of the stator vane rows (<NUM>) is disposed at intervals axially upstream of a respective rotor blade row of the plurality of rotor blade rows (<NUM>), wherein
the casing (<NUM>) has a diffuser (<NUM>) that is configured to guide steam flowing out from a rotor blade row (31F) of a final stage, that is disposed axially most downstream among the plurality of rotor blade rows (<NUM>), to an outside of the casing (<NUM>),
the diffuser (<NUM>) including
an outer guide (<NUM>) that gradually expands radially outwards from its upstream end to its downstream end,
an inner guide (<NUM>) that is disposed radially inwards of the outer guide (<NUM>) and gradually expands radially outwards from its upstream end to its downstream end, wherein an annular flow passage (<NUM>) is defined between the outer guide (<NUM>) and the inner guide (<NUM>), and characterized in that:
the diffuser (<NUM>) includes
a first region (P11) that is a region closest to the rotor blade row (31F) of the final stage in the axial direction in which a cross-sectional area of the flow passage defined between the outer guide (<NUM>) and the inner guide (<NUM>) gradually decreases from one end to the other end in an axial downstream direction, and
a second region (P12) that is connected to the first region (P11) on the downstream side in the axial direction, in which the cross-sectional area of the flow passage gradually increases from one end to the other end in an axial downstream direction.