TITLE OF THE INVENTION
AXIAL FLOW TURBINE
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an axial flow
turbine used as a steam turbine, a gas turbine or the like
for a power-generating plant.
2. Description of the Related Art
An improvement in the power generation efficiency of
the power-generating plant has recently led to a strong
demand for further improved turbine performance. The
turbine performance has a relationship with a stage loss, an
exhaust loss, a mechanical loss and the like associated with
the turbine, and it is considered most effective to reduce
the stage loss among them for further improvement.
The stage loss includes various losses, which are
broadly divided into:
(1) a profile loss attributable to airfoil per se,
(2) a secondary flow loss attributable to a flow not
along with a main flow, and
(3) a leakage loss caused by working fluid (steam,
gas or the like) leaking to outside the main passage.
The above leakage loss includes:
(a) a bypass loss caused by a portion (leaking fluid)
of the working fluid flowing through a clearance passage (a
bypass passage) other than the main passage, making the
energy in the leaking fluid not effectively utilized.
(b) a mixing loss caused when the leaking fluid flows
from the clearance passage into the main passage; and
(c) an interference loss caused by the interference
of the leaking fluid flowing into the main passage with a
blade row on the downstream side thereof.
An important issue in recent years is to reduce not
only the bypass loss but the mixing loss and the
interference loss. In other words, the important issue is
not only to simply reduce the flow rate (a leakage amount)
of the leaking fluid from the main passage into the
clearance passage but how to return the leaking fluid from
the clearance passage into the main passage with no loss.
To solve such problems, it is proposed that a
plurality of guide plates is provided on the downstream side
of the clearance passage so as to change the flowing
direction of the leaking fluid to the main flow direction.
(See JP-2011-106474-A)
SUMMARY OF THE INVENTION
However, the conventional art has room for the
improvement as below. Specifically, the conventional art
described in JP-2011-106474-A only allows the leaking fluid
to pass between the guide plates to change the flowing
direction of the leaking fluid. Therefore, unless the
number of the guide plates is increased to narrow the
interval between the guide plates, an effect of changing the
flowing direction of the leaking fluid cannot sufficiently
be produced, which leads to a possibility that the effect of
reducing the mixing loss cannot be sufficiently obtained.
Contrarily, if the number of the guide plates is increased
to narrow the interval between the guide plates, increase in
a contact area increases a friction loss, which may cancel
out the effect of reducing the mixing loss.
It is an object of the present invention to provide
an axial flow turbine that can enhance an effect of reducing
a mixing loss.
According to one aspect of the present invention, an
axial flow turbine includes: a plurality of stator blades
provided on the inner circumferential side of a stationary
body and circumferentially arranged; a plurality of rotor
blades provided on the outer circumferential side of a
rotating body and circumferentially arranged; a main passage
in which the stator blades and the rotor blades on the
downstream side of the stator blades are arranged, the main
passage through which working fluid flows; a shroud provided
on the outer circumferential side of the rotor blades; an
annular groove portion formed in the stationary body and
housing the shroud therein; a clearance passage formed
between the groove portion and the shroud, wherein a portion
of the working fluid flows from the downstream side of the
stator blades in the main passage into the clearance passage
and flows out toward the downstream side of the rotor blades
in the main passage; a plurality of stages of seal fins
provided in the clearance passage; a circulation flow
generating chamber defined on the downstream side of the
clearance passage; and a plurality of shielding plates
secured to the stationary body in such a manner as to be
located in the circulation flow generating chamber, the
shielding plates extending in axial and radial directions of
the rotating body.
In the aspect of the present invention described
above, a portion of working fluid (leaking fluid) flows into
the clearance passage from the downstream side of the stator
blade (the upstream side of the rotor blade in the main
passage) and flows out toward the downstream side of the
rotor blade in the main passage via the clearance passage.
In this case, the leaking fluid that has flowed into the
clearance passage from the downstream side of the stator
blade in the main passage forms a flow having a large
circumferential velocity component. However, a portion of
the leaking fluid flows into the circulation flow generating
chamber and hits the shielding plates, which can generate a
circulation flow having a suppressed circumferential
velocity component. The interference of the circulation
flow thus generated can effectively reduce the
circumferential velocity component of the flow of the
leaking fluid flowing out from the clearance passage toward
the downstream side of the rotor blade in the main passage.
Thus, the flowing direction of the leaking fluid can
coincide with that of the working fluid (the main-flow
fluid) that has passed the rotor blade, which enhances the
effect of reducing the mixing loss.
The present invention can enhance the effect of
reducing the mixing loss.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a rotor-axial cross-sectional view
schematically illustrating a partial structure of a steam
turbine according to a first embodiment of the present
invention.
Fig. 2 is a partially-enlarged cross-sectional view
of a Il-portion in Fig. 1, illustrating a detailed structure
of a clearance passage according to the first embodiment of
the present invention.
Fig. 3 is a rotor-circumferential cross-sectional
view taken along line III-III in Fig. 1, illustrating the
flow in a main passage.
Fig. 4 is a rotor-circumferential cross-sectional
view taken along line IV-IV in Fig. 1, illustrating the flow
in the clearance passage as well as the flow in the main
passage.
Fig. 5 is a chart illustrating the distribution of
rotor blade outflow angles in the first embodiment of the
present invention and in the conventional art.
Fig. 6 is a chart illustrating the distribution of
rotor blade loss coefficients in the first embodiment of the
present invention and in the conventional art.
Fig. 7 is a partially-enlarged cross-sectional view
illustrating the detailed structure of a clearance passage
according to one modification of the present invention.
Fig. 8 is a partially-enlarged cross-sectional view
illustrating the detailed structure of a clearance passage
according to another modification of the present invention.
Fig. 9 is a partially-enlarged cross-sectional view
illustrating the detailed structure of a clearance passage
according to a second embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of a steam turbine according to
the present invention will now be described with reference
to the accompanying drawings.
Fig. 1 is a schematic cross-sectional view of a
partial structure (a stage structure) of a steam turbine as
viewed in a rotor-axial direction according to a first
embodiment of the present invention. Fig. 2 is a partial
enlarged cross-sectional view of a Il-part in Fig. 1,
illustrating a detailed structure of a clearance passage.
Fig. 3 is a cross-sectional view as viewed in a rotorcircumferential
direction taken along line III-III in Fig. 1,
illustrating a flow in a main passage. Fig. 4 is a crosssectional
view as viewed in the rotor-circumferential
direction taken along line IV-IV, illustrating a flow in the
clearance passage together with the flow in the main passage.
Referring to Figs. 1 to 4, a steam turbine includes
an annular diaphragm outer ring 1 (a stationary body)
provided on the inner circumferential side of a casing (not
shown), a plurality of stator blades 2 provided on the inner
circumferential side of the diaphragm outer ring 1, and an
annular diaphragm inner ring 3 provided on the inner
circumferential side of the stator blades 2. The plurality
of stator blades 2 are arranged at given intervals in a
circumferential direction between the diaphragm outer ring 1
and the diaphragm inner ring 3.
The steam turbine includes a rotor 4 (a rotating
body) rotating around a rotating axis o, a plurality of
rotor blades 5 provided on the outer circumferential side of
the rotor 4, and an annular shroud 6 provided on the outer
circumferential side of the rotor blades 5 (i.e., the bladetip
side of the rotor blades 5). The rotor blades 5 are
arranged between the rotor 4 and the shroud 6 at given
intervals in the circumferential direction.
A main passage 7 for steam (working fluid) is
composed of a passage defined between an inner
circumferential surface 8a of the diaphragm outer ring 1 and
an outer circumferential surface 9 of the diaphragm inner
ring 3, a passage defined between an inner circumferential
surface 10 of the shroud 6 (and an inner circumferential
surface 8b of the diaphragm outer ring 1) and an outer
circumferential surface 11 of the rotor 4, and other
passages. In the main passage 7, the stator blades 2 (i.e.,
a single stator blade row) are arranged. The rotor blades 5
(i.e., a single rotor blade row) are arranged on the
downstream side of the stator blade row. A combination of
the stator blades 2 and the rotor blades 5 constitutes one
stage. Although one stage is illustrated in Fig. 1 for
convenience, a plurality of the stages are provided in the
rotor-axial direction in order to recover the inside energy
of steam efficiently.
The steam in the main passage (i.e. a main flow
steam) flows as indicated by blank arrows in Fig. 1. The
inside energy of steam (i.e. pressure energy or the like) is
converted into kinetic energy (i.e. velocity energy) by the
stator blades 2, and the kinetic energy of steam is
converted into the rotational energy of the rotor 4 by the
rotor blades 5. A generator (not shown) is connected to an
end of the rotor 4. The rotational energy of the rotor 4 is
converted into electric energy by the generator.
A detailed description is given of the flow (main
flow) of the steam in the main passage 7. The steam flows
in from the leading edge side (the left side in Fig. 3) of
the stator blade 2 at an absolute velocity vector CI
(specifically, an axial flow that has almost no
circumferential velocity component). When passing between
the stator blades 2, the steam increases in velocity and
changes in direction to have an absolute velocity vector C2
(specifically, the flow having a large circumferential
velocity component). Then, the steam flows out from the
trailing edge side (the right side in Fig. 3) of the stator
blade 2. A large portion of the steam flowing out from the
stator blade 2 hits the rotor blades 5 to rotate the rotor 4
at velocity U. In this case, when passing the rotor blades
5, the steam decreases in velocity and changes in direction,
so that its relative velocity vector W2 changes into a
relative velocity vector W3. Thus, the steam that flows out
from the rotor blade 5 forms a flow having an absolute
velocity vector C3 (specifically, which is nearly equal to
8
the absolute velocity vector CI and which is an axial flow
that has almost no circumferential velocity component).
An annular groove portion 12 for housing the shroud 6
is formed in the inner circumferential surface of the
diaphragm outer ring 1. A clearance passage (a bypass
passage) 13 is defined between the groove portion 12 and the
shroud 6. As a leaking flow, a portion (leaking steam) of
steam flows from the downstream side of the stator blade 2
(i.e., the upstream side of the rotor blade 5) in the main
passage 13 into the clearance passage 13. The leaking steam
flows out toward the downstream side of the rotor blade 5 in
the main passage 7 via the clearance passage 13. Thus, the
internal energy of the leaking steam is not effectively
utilized, which leads to a bypass loss. To reduce the
bypass loss, that is, to reduce the flow rate (the amount of
leaking) of the leaking steam from the main passage 7 to the
clearance passage 13, a labyrinth seal is provided in the
clearance passage 13.
The labyrinth seal of the present embodiment has
annular seal fins (14A to 14D) provided on the inner
circumferential surface of the groove portion 12. These
seal fins (14A to 14D) are arranged at given intervals in a
rotor-axial direction. The seal fins (14A to 14D) have tip
portions (inner circumferential side edge portions) each of
which is formed into an acute wedge shape. An annular
stepped portion (a raised portion) is formed on the outer
circumferential side of the shroud 6 in such a manner as to
be located between the first-stage seal fin 14A and the
9
fourth-stage seal fin 14D.
A clearance dimension Di between the tip of each seal
fin and the outer circumferential surface of the shroud 6
facing thereto is set so that the flow rate of the leaking
steam is minimized while preventing the contact between the
stationary body side and the rotating body side. A step
dimension D2 of the stepped portion 15 is set, for example,
two to three times the clearance dimension Di mentioned
above. Therefore, the seal fins (14A, 14D) are longer than
the seal fins (14B, 14C) by the above-mentioned step
dimension D2.
The main-flow steam in the main passage 7 on the
downstream side of the stator blade 2 forms the flow having
the large circumferential velocity component (the absolute
velocity vector C2) as mentioned above. In addition, the
leaking steam flowing into the clearance passage 13 forms
the flow having the large circumferential velocity component,
The leaking steam flowing into the clearance passage 13
sequentially passes through clearances (restrictions)
between the tip of the first-stage seal fin 14A and the
outer circumferential surface of the shroud 6, between the
tip of the second-stage seal fin 14B and the outer
circumferential surface of the shroud 6, between the tip of
the third-stage seal fin 14C and the outer circumferential
surface of the shroud 6, and between the tip of the fourthstage
seal fin 14D and the shroud 6. In this case, the
total pressure of the leaking steam lowers due to a loop
loss. Although the axial velocity of the leaking steam
10
increases, the circumferential velocity remains almost
unchanged. In other words, the leaking steam passing
through the clearance between the tip of the final-stage
seal fin 14D and the outer circumferential surface of the
shroud 6 still forms the flow having the large
circumferential velocity component.
On the other hand, the mainstream steam that has
passed the rotor blade 5 in the main passage 5 forms the
flow that has almost no circumferential velocity component
as described above, i.e., the flow having the absolute
velocity vector C3. Therefore, if the leaking steam that
has passed through the clearance between the tip of the
final-stage seal fin 14D and the outer circumferential
surface of the shroud 6 flows out toward the downstream side
of the rotor blade 5 in the main passage 7 while the leaking
steam still has the large circumferential velocity component,
a mixing loss increases.
As the greatest feature of the present embodiment,
provided is an annular projecting portion (a first
projecting portion) 16 projecting toward the downstream-side
end face of the shroud 6 on the downstream-side lateral
surface of the groove portion 12 of the diaphragm outer ring
1. In this way, a circulation flow generating chamber 17 is
defined on the downstream side of the clearance passage 13.
This circulation flow generating chamber 17 is defined by a
portion of the inner circumferential surface of the groove
portion 12 located on the downstream side of the final-stage
seal fin 14D, the downstream-side lateral surface of the
11
groove portion 12 and the outer circumferential surface,
i.e., a radial outside surface of the projecting portion 16.
A portion of the leaking steam that has passed through the
clearance between the tip of the final-stage seal fin 14D
and the outer circumferential surface of the shroud 6 flows
into the circulation flow generating chamber 17 and hits the
downstream-side lateral surface of the groove portion 12 and
other surfaces to form a circulation flow Al.
Further, a plurality of shielding plates arranged at
given intervals in the circumferential direction are secured
to the downstream-side lateral surface of the groove portion
12 (i.e., in the circulation flow generating chamber 17).
The shielding plate 18 extends in the rotor-axial direction
and the rotor-radial direction, and is a flat plate disposed
perpendicularly to the tangential direction of the rotation
of the rotor 4 in the present embodiment. The leaking steam
(i.e., the circulation flow Al) flowing into the circulation
flow generating chamber 17 hits the shielding plates 18,
thereby suppressing the circumferential velocity component
of the circulation flow Al (see Fig. 4). The interference
of the circulation flow Al thus generated can effectively
remove the circumferential velocity component from the flow
Bl of the leaking steam flowing out toward the downstream
side of the rotor blade 5 in the main passage 7 from the
clearance passage 13 (see Fig. 4). In other words, the
circumferential velocity component can effectively be
removed regardless of the magnitude of the circumferential
velocity of the leaking steam compared with the case where
12
leaking steam is allowed to pass between the guide plates as
described in e.g. JP-2011-106474-A.
The tip of the projecting portion 16 is located on
the rotor-radial inside of the outer circumferential surface
of the shroud 6 to which the final-stage seal fin 14D is
opposed. The leaking steam that has passed through the
clearance between the tip of the final-stage seal fin 14D
and the outer circumferential surface of the shroud 6 easily
enters the circulation flow generating chamber 17.
The projecting portion 16 fills the role of
suppressing the radial velocity component of the flow Bl of
the leaking steam flowing out from the clearance passage 13
toward the downstream side of the rotor blade 5 in the main
passage 7. In particular, the inner circumferential surface
of the projecting portion 16 inclines from the outside (the
upside in Fig. 2) toward the inside (the downside in Fig. 2)
in the rotor-radial direction in such a manner as to extend
from the upstream side (the left side in Fig. 2) toward the
downstream side (the right side in Fig. 2) in the rotoraxial
direction. Thus, the leaking steam is directed in the
rotor-axial direction. The projecting portion 16 plays the
role of preventing the steam from flowing back from the main
passage 7 toward the clearance passage 13.
The final-stage seal fin 14D is located to oppose to
the outer circumferential surface of the axially downstream
end portion of the shroud 6. With such arrangement of the
final-stage seal fin, the leaking flow which has passed
through the clearance between the tip of the final-stage
13
seal fin 14D and the outer circumferential surface of the
shroud 6 moves into the circulation flow generating chamber
17 in the state where high velocity is maintained without
the circumferential diffusion of velocity. Thus, the strong
circulation flow Al can be generated. Contrarily, if the
final-stage seal fin is located on the axially upstream side
of the shroud 6, the leaking flow Bl that has passed through
the clearance between the tip of the final-stage seal fin
and the outer circumferential surface of the shroud 6 is
diffused over the full area of the leaking passage and flows
into the circulation flow generating chamber 17 as the
leaking flow having a radially uniform velocity. Therefore,
the circulating flow Al cannot be generated. To generate
the circulation flow Al, it is essential to locate the
final-stage seal fin 14D to oppose to the outer
circumferential surface of the axially downstream end
portion of the shroud 6 so as to allow the leaking flow Bl
to flow into the circulation flow generating chamber 17 in
the state of a high-velocity jet flow.
Advantages of the present embodiment are next
described with reference to Figs. 5 and 6.
Fig. 5 is a chart illustrating the distribution of
rotor blade outflow angles in the present embodiment
indicated by a solid line and the distribution of rotor
blade outflow angles in the conventional art indicated by a
dotted line. A longitudinal axis represents a blade-heightdirectional
position in the main passage 7, and a horizontal
axis represents an outflow angle of the rotor blade (i.e.,
14
an absolute flow angle of steam on the downstream side of
the rotor blade 5). With the rotor-axial direction as a
basis (is set as zero), as the circumferential velocity is
greater in respective with the axial velocity the absolute
value of the outflow angle of the rotor blade 5 gradually
approaches 90 degrees. Fig. 6 is a chart illustrating the
distribution of a rotor blade loss coefficient in the
present embodiment indicated by a solid line and the
distribution of a rotor blade loss coefficient in the
conventional art indicated by a dotted line. A longitudinal
axis represents the blade-height-directional position in the
main passage 7, and a horizontal axis represents the loss
coefficient of the rotor blade 5.
As described above, the leaking steam flows from the
downstream side of the stator blade 2 (i.e., the upstream
side of the rotor blade 5) in the main passage 7 into the
clearance passage 13 and then flows out toward the
downstream side of the rotor blade in the main passage 7
through the clearance passage 13. In this case, the leaking
steam flowing from the downstream side of the stator blade 2
in the main passage 7 into the clearance passage 13 forms a
flow having a large circumferential velocity component. The
leaking steam that has passed the clearance between the tip
of the final-stage seal fin 14D and the outer
circumferential surface of the shroud 6 also forms a flow
having a large circumferential velocity component.
In the conventional art without the above-mentioned
projecting portion 16 and the shielding plates 18, the
15
leaking steam flowing out from the clearance passage 13 into
the main passage 7 has a flow having a large circumferential
velocity component. Meanwhile, the main-flow steam that has
passed the rotor blade 5 in the main passage 7 forms a flow
that has almost no circumferential velocity component as
described above. Therefore, as shown in Fig. 5, a flow
angle in an area other than the vicinity of the blade tip is
nearly equal to zero; however, it comes close to -90 degree
in an area close to the blade tip. In the conventional art,
the projecting portion 16 does not exit; therefore, the
leaking steam flowing out from the clearance passage 13 into
the main passage 7 has relatively large radial velocity.
As shown in Fig. 5, the blade-height-directional area
subjected to the influence of the leaking steam is
relatively large. As shown in Fig. 6, a mixing loss
increases as a result.
In contrast to the conventional art, in the present
embodiment, the circulation flow having the suppressed
circumferential velocity component is generated on the
downstream side of the clearance passage 13. The
interference of the circulation flow can effectively remove
the circumferential velocity component from the flow of the
leaking steam flowing out from the clearance passage 13
toward the downstream side of the rotor blade 5 in the main
passage 7. In other words, the leaking steam flowing out
from the clearance passage 13 into the main passage 7 forms
the flow that has almost no circumferential velocity
component. Therefore, as shown in Fig. 5, the flown angle
16
is nearly equal to zero even in the area close to the blade
tip. In the present embodiment, the existence of the
projecting portion 16 can reduce the radial velocity of the
leaking steam flowing out from the clearance passage 13 into
the main passage 7. Therefore, as shown in Fig. 5, the
blade-height-directional area subjected to the influence of
the leaking steam is relatively small. As shown in Fig. 6,
the mixing loss can be reduced to allow for an improvement
in stage efficiency as a result.
The advantage of the present embodiment is greater in
the case of a plurality of the stages than in the case where
the stage which is a combination of a rotor blade row and a
stator blade row is single. As described above, the
conventional art is such that the flow angle of the area
close to the blade tip is different from that in the other
area, i.e., the flow is twisted in the blade-height
direction. The inlet blade angle of the stator angle does
not largely change in the blade-height direction. Therefore,
if the above-mentioned flow moves toward the downstream side
stator blades, the development of an end face boundary layer
and the growth of a secondary flow are promoted to cause an
interference loss. In the present embodiment, in contrast,
the flow angle in the area close to the blade tip is almost
the same as in the other area as described above, so that
the flow is uniform in the blade-height direction. Even if
the above-mentioned flow moves toward the stator blades, the
incidence of the stator blade is not largely altered, so
that the occurrence of the interference loss can be
17
suppressed. In other words, an increase in the secondary
flow loss of the downstream side stator blade can be
suppressed to allow for improved stage efficiency on the
downstream side.
As shown in Fig. 4, the first embodiment describes as
an example the case where the circumferential intervals
(angle basis) of the shielding plates 18 are almost the same
as the circumferential intervals (angle basis) of the rotor
blades 5. In other words, the number of the shielding
plates 18 is the same as that of the rotor blades 5.
However, the present invention is not limited to this.
Alteration or modification can be done in a range not
departing from the gist and technical concept of the present
invention. Specifically, depending on the circumferential
velocity of the leaking steam flowing into the clearance
passage 13 from the downstream side of the stator blade 2 in
the main passage 7, even if the number of the shielding
plates 18 is made less than that of the rotor blades 5, the
same advantage as that in the first embodiment described
above can be obtained. In such a case, the number of the
shielding plates 18 can be made less than that of the rotor
blades 5.
The first embodiment describes as an example the case
where the circumferential velocity of the main flow steam on
the downstream side of the rotor blade 5 (i.e., on the
upstream side of the stator blade 2) in the main passage is
nearly equal to zero; therefore, the shielding plates 18 are
arranged perpendicularly to the tangential direction of the
18
rotation of the rotor 4. However, the present invention is
not limited to this. Alteration or modification can be done
in a range not departing from the gist and technical concept
of the present invention. Specifically, depending on the
circumferential velocity of the steam on the downstream side
of the rotor blade 5 in the main passage 7, the shielding
plate 18 may slightly be inclined in the circumferential
direction of the rotor. Such a case also can produce the
same effect as that of the first embodiment.
The above first embodiment describes as an example
the case where no projecting portion is provided on the
downstream-side end face of the shroud 6 as illustrated in
Fig. 2. However, the present invention is not limited to
this. A projecting portion may be provided on the
downstream-side end face of the shroud 6. Specifically, as
shown in e.g. an modification in Fig. 7, an inside
projecting portion 19 may be provided on the downstream-side
end face of the shroud 6A in such a manner as to be located
on the rotor radial inside (the downside in the figure) of
the projecting portion 16. The outer circumferential
surface of the inside projecting portion 19 faces the inner
circumferential surface of the projecting portion 16. In
addition, the outer circumferential surface of the
projecting portion 19 inclines from the outside (the upside
in the figure) to the inside (the downside in the figure) in
the rotor-axial direction in such a manner as to extend from
the upstream side (the left side in the figure) toward the
downstream side (the right side in the figure) in the rotor-
19
axial direction. In other words, a guide passage for the
leaking steam is defined between the inner circumferential
surface of the projecting portion 16 and the outer
circumferential surface of the inside projecting portion 19.
As shown by arrow B2 in the figure, the flow direction of
the leaking steam flowing out from the clearance passage 13
into the main passage 7 can be more directed to the rotor
axial direction. Thus, the effect of reducing the mixing
loss and the interference loss can be further enhanced to
improve the stage efficiency as well as the effect of
preventing the backflow of steam.
Alternatively, as in another modification shown in
Fig. 8 an outside projecting portion 20 located on the rotor
radial outside (the upside in the figure) of the projecting
portion 16 may be provided on the downstream-side end face
of a shroud 6B. The outer circumferential surface of the
outside projecting portion 20 inclines from the inside (the
downside in the figure) to the outside (the upside in the
figure) in the rotor-radial direction in such a manner as to
extend from the upstream side (the left side in the figure)
toward the downstream side (the right side in the figure) in
the rotor-axial direction. As shown by arrow B3 in the
figure, the leaking steam that has passed through the
clearance between the tip of the final-stage seal fin 14D
and the outer circumferential surface of the shroud 6B
changes its direction toward the rotor-radial outside and
easily enters the circulation flow generating chamber 17.
Therefore, the circulation flow Al can be strengthened to
20
enhance the effect of removing the circumferential velocity
component due to the interference of the circulation flow Al,
Thus, the effect of reducing the mixing loss and the
interference loss can be further enhanced to allow for an
increase in stage efficiency.
The above-mentioned first embodiment and
modifications describe the labyrinth seal having the fourstage
seal fins (14A to 14D) and one stepped portion 15 by
way of example. However, the present invention is not
limited to this. The labyrinth seal can be modified in a
range not departing from the gist and technical concept of
the present invention. Specifically, the number of the
stages of the seal fins is not limited to four but may be
two, three, five or more. The labyrinth seal may have no
stepped portion or may have two or more stepped portion.
These cases can produce the same effect as above too.
The first embodiment describes as an example the
configuration where the final-stage seal fin 14D is provided
on the inner circumferential surface of the groove portion
12 of the diaphragm outer ring 1. The tip of the projecting
portion 16 is located at the rotor-radial inside of the
outer circumferential surface of the shroud 6 to which the
final-stage seal fin 14D is opposed. However, the present
invention is not limited to this. Such a configuration can
be modified or altered in various ways in a range not
departing from the gist and technical concept of the present
invention. Specifically, the final stage seal fin may be
provided on the outer circumferential surface of the shroud
21
6. The tip of the projecting portion 16 may be located, for
example, at a position on the rotor-radial inside of the tip
(an outer circumferential edge) or a root (an inner
circumferential edge) of the final stage seal fin. Such a
case can produce the same effect as above too.
A second embodiment of the present invention is
described with reference to Fig. 9. In the present
embodiment the same portions as those in the above first
embodiment are denoted by like reference numerals and their
explanations are arbitrarily omitted.
Fig. 9 illustrates a detailed structure of a
clearance passage according to the second embodiment.
In the present embodiment, a cutout is formed in the
downstream side end portion of a shroud 6C. Specifically,
the shroud 6C has an outer circumferential surface 21a to
which a final-stage seal fin 14D is opposed, an outer
circumferential surface 21b which is located on the rotorradial
outside (the upside in the figure) of the outer
circumferential surface 21a and to which the seal fin 14C
anterior to the final stage seal fin is opposed, and a
stepped lateral surface 22 formed between the outer
circumferential surface 21a and the outer circumferential
surface 21b.
A clearance dimension Di between the tip of each seal
fin and the outer circumference of the shroud 6c facing
thereto is set so that similarly to the first embodiment the
flow rate of leaking steam may be minimized while preventing
the stationary body side and the rotating body side from
22
coming into contact with each other. A rotor-radial
dimension D2 (a step dimension) of the stepped lateral
surface 22 is set e.g. five or more times the abovementioned
clearance dimension Di mentioned above (about six
to eight times in the present embodiment). The seal fin 14D
is longer than the seal fin 14C by the above-mentioned step
dimension D2. In other words, the tip of the seal fin 14D is
located on the rotor-radial inside (the downside in the
figure) of the outer circumference 21b.
A rotor-axial dimension H3 between the seal fin 14C
and the seal fin 14D is set two or more times (about two to
three times in the present embodiment) a rotor-axial
dimension Hi between th^ seal fin 14A and the seal fin 14B
or a rotor-axial dimension H2 between the seal fin 14B and
the seal fin 14C. A rotor-axial dimension H4 between the
seal fin 14D and the step lateral surface 22 is greater than
the above-mentioned dimension Hi or H2.
With the above-mentioned structure, a circulation
flow generating chamber 17A is defined by the final stage
seal fin 14D, the seal fin 14C anterior thereto, and a
portion of the inner circumferential surface of the groove
portion 12 located between the seal fins 14C and 14D. The
leaking steam that has passed through the clearance between
the tip of the seal fin 14C and the outer circumferential
surface 21b of the shroud 6C flows into the circulation flow
generating chamber 17A and hits the seal fin 14D and other
surfaces to generate a circulation flow A2.
A plurality of shielding plates 18A arranged
23
circumferentially at given intervals are secured to the
inner circumferential surface of the groove portion 12 in
such a manner as to be located between the seal fins 14C and
14D (to be located in the circulation flow generating
chamber 17A). The shielding plate 18A extends in the rotoraxial
direction and in the rotor-radial direction. In the
present embodiment, the shielding plate 18A is a flat plate
disposed perpendicularly to the tangential direction of the
rotation of the rotor 4. The leaking steam (the circulating
flow A2) that has flowed into the circulating flow
generating chamber 17a hits the shielding plates ISA to
suppress the circumferential velocity component of the
circulating flow A2. The interference of the circulation
flow A2 thus generated can effectively remove the
circumferential velocity component from the flow B4 of the
leaking steam flowing out from the clearance passage 13
toward the downstream side of the rotor blade 5 in the main
passage 7. In other words, the circumferential velocity
component can effectively be removed regardless of the
magnitude of the circumferential velocity of the leaking
steam compared with the case where the leaking steam is
allowed to pass between the guide plates as described in JP-
2011-106474-A.
In the present embodiment, the inner circumferential
surface 8b of the diaphragm outer ring lA is located on the
rotor-radial outside of the tip of the seal fin 14D. Thus,
the leaking steam flowing out from the clearance passage 13
toward the downstream side of the rotor blade 5 in the main
24
passage 7 can be directed to the rotor-axial direction. At
the time of starting a steam turbine, a relative positional
relationship between the stationary body side and the
rotating body side may largely be deviated to the axial
direction due to a thermal expansion difference between the
stationary body side and the rotating body side. Even in
such a case, it is designed that the downstream side end
portion of the shroud 6C and the diaphragm outer ring 1 do
not hit with each other.
The embodiment described above can enhance the effect
of reducing a mixing loss and the like similarly to the
first embodiment.
The above embodiments describe the steam turbine,
which is one of axial flow turbines, as an object to which
the present invention is applied by way of example. However,
the present invention is not limited to this. The present
invention may be applied to a gas turbine and other turbines.
This case can produce the same effect as above too.
25

r
What is claimed is:
1. An axial flow turbine comprising:
a plurality of stator blades provided on the inner
circumferential side of a stationary body and
circumferentially arranged;
a plurality of rotor blades provided on the outer
circumferential side of a rotating body and
circumferentially arranged;
a main passage in which the stator blades and the
rotor blades on the downstream side of the stator blades are
arranged, the main passage through which working fluid
flows;
a shroud provided on the outer circumferential side
of the rotor blades;
an annular groove portion formed in the stationary
body and housing the shroud therein;
a clearance passage formed between the groove portion
and the shroud, wherein a portion of the working fluid flows
from the downstream side of the stator blades in the main
passage into the clearance passage and flows out toward the
downstream side of the rotor blades in the main passage;
a plurality of stages of seal fins provided in the
clearance passage;
a circulation flow generating chamber defined on the
downstream side of the clearance passage; and
a plurality of shielding plates secured to the
stationary body in such a manner as to be located in the
26
circulation flow generating chamber, the shielding plates
extending in axial and radial directions of the rotating
body.
2. The axial flow turbine according to claim 1,
wherein provided is an annular first projecting
portion projecting from a downstream-side lateral surface of
the groove portion toward a downstream-side end face of the
shroud, and
the circulation flow generating chamber is defined by
a portion of an inner circumferential surface of the groove
portion located on the downstream side of a final stage seal
fin of the stages of seal fins, the downstream-side lateral
surface of the groove portion, and an outer circumferential
surface of the first projecting portion.
3. The axial flow turbine according to claim 2,
wherein an annular outside projecting portion is
provided on the downstream-side end face of the shroud in
such a manner so as to be located on a radial outside of the
rotating body with respect to the first projecting portion,
and
an outer circumferential surface of the outside
projecting portion is inclined from the inside toward the
outside in the radial direction of the rotating body in such
a manner as to extend from the upstream side toward the
downstream side in the axial direction of the rotating body.
4. The axial flow turbine according to claim 1,
wherein the stages of seal fins are provided on the
inner circumferential surface of the groove portion;
27
the shroud includes a first outer circumferential
surface to which a final stage seal fin among the plurality
of stages of seal fins is opposed, a second circumferential
surface which is located on a rotor-radial outside of the
first outer circumferential surface and to which a seal fin
of a stage anterior to the final stage is opposed, and a
stepped lateral surface formed between the first outer
circumferential surface and the second outer circumferential
surface,
a tip of the final stage seal fin is located on the
rotor-radial inside of the second outer circumferential
surface,
a rotor-axial dimension between the final stage seal
fin and the seal fin of the stage anterior to the final
stage and a rotor-axial dimension between the final stage
seal fin and the stepped lateral surface are greater than a
rotor-axial dimension between other seal fins, and
the circulation flow generating chamber is defined by
the final stage seal fin, the seal fin of the stage anterior
to the final stage, and a portion of the inner
circumferential surface of the groove portion located
between the final stage seal fin and the seal fin of the
stage anterior to the final stage.
5. An axial flow turbine, substantiall]^''^ herein
described with reference to accompanying drawii