AXIAL TURBINE

Abstract

Axial turbine, including: a plurality of blades (3, 6) arranged circumferentially; and an element (2, 4, 5) with a circumferential surface (9, 10, 11, 12) which connects the plurality of blades (3, 6) on its inner circumferential side or outer circumferential side and forms a wall surface of a main flow path (8), wherein the circumferential surface of the element has a multitude of recessed sections (18), and each of the recessed sections (18) is formed in a region located on a downstream side of a constriction, where the distance between a suction surface (15) of a blade of a pair of adjacent blades and a pressure surface (16) of another blade of the pair of adjacent blades is shortest, and which lies circumferentially in a region of a constriction position (P1) on the suction surface (15) of one blade to a downstream edge position (P2) of one blade, which includes a downstream edge position of the circumferential surface in the axial direction, Each of the multiple recessed sections (18) is formed along a working medium flow direction on the downstream side of the constriction and gradually deepens along the working medium flow direction to a downstream edge position of the circumferential surface in the axial direction of the element.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The present invention relates to an axial turbine used for a steam turbine, gas turbine or the like in power plants. 2. Description of the state of the art

[0002] An axial turbine, for example, comprises an annular diaphragm outer ring provided on an inner circumferential side of a casing, a plurality of guide vanes provided on an inner circumferential side of the diaphragm outer ring and arranged in a circumferential direction, a diaphragm inner ring provided on an inner circumferential side of the plurality of guide vanes, a rotor, a plurality of movable blades provided on an outer circumferential side of the rotor and arranged on an upstream side of the plurality of guide vanes in the circumferential direction, and a cover plate provided on an outer circumferential side of the plurality of movable blades, see, for example, JP 2017-8 756 A. US 2017 / 0 218 769 A1 describes an axial turbine with an optimized flow geometry. EP 2 696 029 A1 describes a blade cascade of a turbomachine.

[0003] US 2012 / 0 201 663 A1 describes a turbomachine.

[0004] A main flow path of an axial turbine consists of a flow path formed between an inner peripheral surface of the diaphragm outer ring and an outer peripheral surface of the diaphragm inner ring, and a flow path formed between an inner peripheral surface of the shroud and an outer peripheral surface of the rotor. It is configured so that the working fluid flowing through the main flow path is accelerated and caused to rotate by the guide vanes, then exerts a rotational force on the movable blades.

[0005] A first cavity is formed between the diaphragm inner ring and the rotor. A portion of the working fluid flows from the upstream side of the guide vanes in the main flow path into the first cavity and flows from the first cavity to the downstream side of the guide vanes in the main flow path. Since this portion of the working fluid is neither accelerated nor caused to rotate by the guide vanes, a loss occurs. To reduce this loss, the first cavity is equipped with a labyrinth seal.

[0006] A second cavity is formed between the cover plate and the diaphragm outer ring or the housing. A portion of the working fluid flows from an upstream side of the moving blades in the main flow path into the second cavity and flows from the second cavity to a downstream side of the moving blades in the main flow path. Since this portion of the working fluid does not exert any rotational force on the moving blades, a loss occurs. To reduce this loss, the second cavity is equipped with a labyrinth seal. SUMMARY OF THE INVENTION

[0007] Meanwhile, a typical circumferential pressure distribution is generated on an outlet side of the guide vanes or the movable vanes in the main flow path. Specifically, a static pressure is generated in a region located on a downstream side of a throat where a distance between a suction surface or a back surface of one vane of a pair of adjacent vanes and a pressure surface or a front surface of the other vane of the pair of adjacent vanes becomes shortest, and in a region circumferentially from a throat on the suction surface of one vane to a downstream edge of one vane. Accordingly, a flow for flowing out of a cavity toward the main flow path is generated in this region.On the other hand, in a region located downstream of the throat and circumferentially within a throat region, the static pressure becomes relatively higher on the suction surface of one blade to a downstream edge of the other blade. Accordingly, a flow is generated in that region, which flows out from the main flow path toward the cavity. Due to a difference between the flows in the circumferential direction, the interference loss increases, especially the fusion loss on the outlet side of the cavity and the branching loss on the inlet side of the cavity. Furthermore, due to the influence of the difference between the aforementioned flows, the secondary flow loss in blades on the downstream side increases.

[0008] The present invention is to provide an axial turbine that can reduce circumferential pressure differences to reduce losses.

[0009] To achieve the above-mentioned object, the present invention provides an axial flow turbine having a plurality of blades arranged circumferentially and a member having a circumferential surface that connects the plurality of blades to each other on its inner peripheral side or outer peripheral side and forms a wall surface of a main flow path. The circumferential surface of the member has a plurality of depressed portions, and each of the depressed portions is formed in a region located on a downstream side of a throat where a distance between a suction surface of one blade of a pair of adjacent blades and a pressure surface of another blade of the pair of adjacent blades becomes shortest, and which is located in the circumferential direction in a region from a throat on the suction surface of one blade to a downstream edge of the one blade.Furthermore, the region includes a downstream edge of the circumferential surface in the axial direction.

[0010] According to the present invention, it is possible to reduce circumferential pressure differences to reduce losses. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an axial cross-sectional view schematically illustrating a partial structure of a steam turbine in a first embodiment of the present invention; Fig. Figure 2 is a circumferential cross-sectional view taken along a section II-II in Fig. 1 and represents a flow in a main flow path; Fig. 3 is a drawing illustrating a structure of an outer peripheral surface of a diaphragm inner ring in the first embodiment of the present invention; Fig. 4 is a figure from the direction of an arrow IV in Fig. 3; Fig. 5 is a figure illustrating a static pressure distribution of the vane surface in the first embodiment of the present invention and a comparative example; Fig. 6 is a drawing illustrating a structure of an outer peripheral surface of a diaphragm inner ring in a second embodiment of the present invention; and Fig. 7 is a figure from a direction of an arrow VII in Fig. 6. DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, embodiments of the present invention in cases where the present invention is applied to a steam turbine will be explained with reference to the drawings.

[0012] Fig. 1 is an axial cross-sectional view schematically illustrating a partial structure of a steam turbine in a first embodiment of the present invention. Fig. Figure 2 is a circumferential cross-sectional view taken along section II-II in Fig. 1 and represents a flow in a main flow path.

[0013] The steam turbine in the present embodiment includes an annular diaphragm outer ring 2 on the inner peripheral side of a casing 1, a plurality of guide vanes 3 on the inner peripheral side of the diaphragm outer ring 2, and an annular diaphragm inner ring 4 on the inner peripheral side of the guide vanes 3. The plurality of guide vanes 3 are arranged between the diaphragm outer ring 2 and the diaphragm inner ring 4 at predetermined intervals in the circumferential direction.

[0014] Furthermore, the steam turbine includes a rotor 5, a plurality of movable blades 6 provided on the outer peripheral side of the rotor 5, and an annular shroud 7 provided on the outer peripheral side of the movable blades 6. The plurality of movable blades 6 are arranged between the rotor 5 and the shroud 7 at predetermined intervals in the circumferential direction.

[0015] A main flow path 8 of the steam turbine is formed by a flow path between an inner peripheral surface 9 of the diaphragm outer ring 2 and an outer peripheral surface 10 of the diaphragm inner ring 4, and a flow path between an inner peripheral surface 11 of the cover plate 7 and an outer peripheral surface 12 of the rotor 5. That is, the diaphragm outer ring 2 has the inner peripheral surface 9 that connects the plurality of guide vanes 3 on its outer peripheral side and forms a wall surface of the main flow path 8. The diaphragm inner ring 4 has the outer peripheral surface 10 that connects the plurality of guide vanes 3 on its inner peripheral side and forms a wall surface of the main flow path 8. The cover plate 7 has the inner peripheral surface 11 that connects the plurality of movable blades 6 on their outer peripheral side and forms a wall surface of the main flow path 8.The rotor 5 has the outer peripheral surface 12 which connects the plurality of movable blades 6 on its inner peripheral side and forms a wall surface of the main flow path 8.

[0016] In the main flow path 8, the plurality of guide vanes 3, ie, a guide vane row, and the plurality of movable vanes 6, ie, a movable vane row, are arranged on the downstream side of the plurality of guide vanes 3 or the right side in Fig. 1. A combination of these guide vanes 3 and movable vanes 6 forms a section. Note that although only movable vanes 6 of the first section and guide vanes 3 and movable vanes 6 of the second section are arranged in Fig. 1 are shown for reasons of convenience, but the number of sections provided in the axial direction is typically three or more in order to efficiently absorb the inherent energy of the steam or working fluid.

[0017] The steam in the main flow path 8 flows, as the thick arrows in Fig. 1. Then, the inherent energy, ie, pressure energy and the like, of the steam is converted into kinetic energy, ie, velocity energy, at the guide vanes 3, and the kinetic energy of the steam is converted into the rotational energy of the rotor 5 at the movable vanes 6. Furthermore, the configuration is such that a generator (not shown) is connected to one end portion of the rotor 5, and the generator converts the rotational energy of the rotor 5 into electrical energy.

[0018] A steam flow or a main flow in the main flow path 8 is determined by Fig. 2. The steam flows from the upstream edge of the guide vanes 3 or from the top into Fig. 2 with an absolute velocity vector C1, in particular an absolute flow with almost no circumferential velocity components. When flowing between the guide vanes 3, the steam is then accelerated to an absolute velocity vector C2, namely an absolute flow with a large circumferential velocity component, and flows from the downstream edge of the guide vane 3 or from the underside into Fig. 2. Most of the steam exiting the guide vanes 3 impacts the movable blades 6 to rotate the rotor 5 at a speed U. At this time, the steam is decelerated and rotated as it passes through the movable blades 6, and a relative velocity vector W2 rotates a relative velocity vector W3. Accordingly, the steam exiting the movable blades 6 has an absolute velocity vector C3, namely, an absolute flow with almost no peripheral velocity components.

[0019] With further reference to Fig. 1, a cavity 13A is formed between the diaphragm inner ring 4 and the rotor 5. A portion of the steam flows from the upstream side of the guide vanes 3 in the main flow path 8 into the cavity 13A and flows from the cavity 13A to the downstream side of the guide vanes 3 in the main flow path 8. Since this portion of the steam is neither accelerated nor rotated by the guide vanes 3, a loss occurs. To reduce the loss, the cavity 13A is equipped with a labyrinth seal 14A. The labyrinth seal 14A consists, for example, of a plurality of ribs provided on the diaphragm inner ring 4 side and a plurality of projections formed on the rotor 5 side.

[0020] A cavity 13B is formed between the cover plate 7 and the housing 1. A portion of the steam flows from the upstream side of the movable blades 6 in the main flow path 8 into the cavity 13B and flows from the cavity 13B to the downstream side of the movable blades 6 in the main flow path 8. Since this portion of the steam does not exert a rotational force on the movable blades 6, a loss occurs. To reduce the loss, the cavity 13B is equipped with a labyrinth seal 14B. The labyrinth seal 14B consists, for example, of a plurality of ribs provided on the housing 1 side and a plurality of projections formed on the cover plate 7 side.

[0021] Meanwhile, there is typically a circumferential pressure distribution generated on the outlet side of the guide vanes 3 in the main flow path 8. Specifically, the static pressure becomes relatively lower in a region located on the downstream side of a throat 17, where the distance between a suction surface or a back surface 15 of a guide vane 3A of a pair of adjacent blades and a pressure surface or a front surface 16 of a guide vane 3B of the pair of adjacent blades becomes the shortest, and which is circumferentially in the range of a throat position P1 on the suction surface 15 of the guide vane 3A to a downstream edge position P2 of the guide vane 3A, see Fig. 3, which are mentioned below. Accordingly, a flow is generated in the region for flowing out of the cavity 13A to the main flow path 8. On the other hand, the static pressure in a region located on the downstream side of the throat 17 and circumferentially within a range of the throat position P1 on the suction surface 15 of the guide vane 3A to a downstream edge position P3 of the guide vane 3B, see Fig. 3, mentioned below. Accordingly, a flow is generated in this area from the main flow path 8 to the cavity 13A. Due to the difference between the flows in the circumferential direction, the interference loss increases. Furthermore, due to the influence of the difference between the above-mentioned flows, the secondary flow loss at the movable blades 6 on the downstream side increases.

[0022] Against this background, the outer peripheral surface 10 of the diaphragm inner ring 4 in the present embodiment has a structure for reducing the pressure difference in the circumferential direction. The details of the structure will be described with reference to Fig. 3 and Fig. 4 explained. Fig. 3 is a drawing illustrating the structure of the outer peripheral surface of the diaphragm inner ring in the present embodiment. Fig. 4 is a figure from the direction of arrow IV in Fig. 3. Note that dashed lines in Fig. 3 Show contour lines of depressed areas.

[0023] The outer peripheral surface 10 of the diaphragm inner ring 4 in the present embodiment is an approximately cylindrical surface and has a plurality of recessed portions 18 recessed radially inward from this cylindrical surface.

[0024] Each recessed portion 18 is formed in a region located on the downstream side of the throat 17 where the distance between the suction surface 15 of the stationary blade 3A of the pair of adjacent blades and the pressure surface 16 of the stationary blade 3B of the pair of adjacent blades becomes shortest, and which is circumferentially located in a range from the throat position P1 on the suction surface 15 of the stationary blade 3A to the downstream edge position P2 of the stationary blade 3A. Further, the region includes the downstream edge position of the outer peripheral surface 10 in the axial direction and is located in a range including not only the downstream side but also the upstream side of the downstream edge position P2 of the stationary blade 3A.

[0025] Furthermore, each depressed portion 18 is formed along the direction of steam flow on the downstream side of the constriction 17, that is, the direction of the above-mentioned absolute velocity vector C2. Specifically, each cross section of a depressed portion 18 has, for example, an approximately triangular shape in the circumferential direction, and a straight line connecting the bottoms of individual cross sections coincides with the direction of steam flow. Furthermore, each depressed portion 18 is shaped to gradually become deeper along the direction of steam flow, and is configured to reduce the influence on the direction of steam flow.

[0026] In the present embodiment, the width of the main flow path 8 increases in the circumferential direction due to the recessed portion 18 on the outer peripheral surface 10 of the diaphragm inner ring 4. This decreases the steam flow rate in the circumferential direction and increases the static pressure. Accordingly, it is possible to reduce pressure differences in the circumferential direction to reduce differences between the flows in the circumferential direction. This can reduce interference loss and secondary flow loss at the movable vanes 6 on the downstream side.

[0027] Furthermore, in the present embodiment, the recessed portion 18 is formed in a region including not only the downstream side but also the upstream side of the downstream edge position P2 of the guide vane 3A in the axial direction. That is, it is formed to reach a position near the suction surface 15 of the guide vane 3A. At this time, as shown in Fig. 5, the static pressure at the suction surface 15 of the guide vane 3A is reduced compared to a comparative example in which the recessed portion 18 is not formed. Accordingly, it is possible to reduce the pressure difference between the pressure surface and the suction surface of a guide vane to reduce the secondary flow loss at the guide vanes.

[0028] It should be noted that, although in the example explained in the first embodiment, the depressed portion 18 is formed in the region circumferentially ranging from the throat position P1 on the suction surface 15 of the stationary blade 3A to the downstream edge position P2 of the stationary blade 3A, this is not the only example, and the depressed portion 18 only needs to be formed in the aforementioned region. Specifically, the depressed portion 18 may be formed in a region starting from a position shifted from the throat position P1 by about 10% of the inter-blade inclination length L toward the downstream edge position P2. Furthermore, the depressed portion 18 may be formed in a region reaching a position shifted from the downstream edge position P2 toward the throat position P1 by about 10% of the inter-blade inclination length L.In this case, similar effects to those described above can be achieved.

[0029] Alternatively, the depressed portion 18 may be formed to slightly exceed the circumferential region within a range from the throat position P1 on the suction surface 15 of the stationary blade 3A to the downstream edge position P2 of the stationary blade 3A. Specifically, the depressed portion 18 may be formed in a region starting from a position shifted from the throat position P1 by about 10% of the inter-blade inclination length L to a side opposite to the downstream edge position P2. Moreover, the depressed portion 18 may be formed in a region reaching a position shifted from the downstream edge position P2 to a side opposite to the throat position P1 by about 10% of the inter-blade inclination length L. In this case, too, similar effects to those described above can be achieved.

[0030] Although in the example explained in the first embodiment, the recessed portion 18 is formed in an area including not only the downstream side but also the upstream side of the downstream edge position P2 of the stationary blade 3A in the axial direction, this is not the only example. That is, although it is not possible to achieve the effect of attempting to reduce the secondary flow loss in the stationary blades, the recessed portion 18 can be formed in an area including only the downstream side of the downstream edge position P2 of the stationary blade 3A in the axial direction.

[0031] A second embodiment of the present invention will be described with reference to Fig. 6 and Fig. 7. Note that portions in the present embodiment corresponding to those in the first embodiment are denoted by the same symbols, and explanations thereof are omitted where appropriate.

[0032] Fig. 6 is a drawing illustrating the structure of the outer peripheral surface of the diaphragm inner ring in the present embodiment. Fig. 7 is a figure from the direction of arrow VII in Fig. 6. Note that dashed lines in Fig. 6 Show contour lines of recessed and protruding sections.

[0033] Similar to the first embodiment, the outer peripheral surface 10 of the diaphragm inner ring 4 in the present embodiment has a substantially cylindrical surface and a plurality of recessed portions 18 recessed radially inward from this cylindrical surface. The outer peripheral surface 10 of the diaphragm inner ring 4 in the present embodiment further has a plurality of protruding portions 19 projecting radially outward from the cylindrical surface.

[0034] Each protruding portion 19 is formed in a region located on the downstream side of the throat 17 where the distance between the suction surface 15 of the stationary blade 3A of the pair of adjacent blades and the pressure surface 16 of the stationary blade 3B of the pair of adjacent blades is the shortest, and which is circumferentially within a range from the throat position P1 on the suction surface 15 of the stationary blade 3A to the downstream edge position P3 of the stationary blade 3B. Further, the region in the axial direction includes the downstream edge position of the outer peripheral surface 10 and is located in a range including not only the downstream side but also the upstream side of the downstream edge position P3 of the stationary blade 3B.

[0035] Furthermore, each protruding portion 19 is formed along the axial direction. Specifically, each cross section of a protruding portion 19 in the circumferential direction has, for example, an approximately triangular shape, and a straight line connecting the vertices of individual cross sections coincides with the axial direction. Furthermore, each protruding portion 19 is formed to gradually increase in height toward the downstream side of the axial direction.

[0036] In the present embodiment, due to the protruding portion 19 on the outer peripheral surface 10 of the diaphragm inner ring 4, the width of the main flow path 8 decreases in the circumferential direction in the region of the protruding portion 19. As a result, the steam flow rate in the circumferential direction increases and the static pressure decreases. Accordingly, compared to the first embodiment, it is possible to further reduce pressure differences in the circumferential direction to further reduce differences between the circumferential flows. This further reduces interference loss and secondary flow loss at the moving blades 6 on the downstream side.

[0037] It should be noted that, although in the example explained in the second embodiment, the protruding portion 19 is formed in the region circumferentially ranging from the throat position P1 on the suction surface 15 of the stationary blade 3B to the downstream edge position P3 of the stationary blade 3A, this is not the only example, and the protruding portion 19 only needs to be formed in the aforementioned region. Specifically, the protruding portion 19 may be formed in a region starting from a position shifted from the throat position P1 by about 10% of the inter-blade inclination length L toward the downstream edge position P3. Furthermore, the protruding portion 19 may be formed in a region reaching a position shifted from the downstream edge position P3 toward the throat position P1 by about 10% of the inter-blade inclination length L.In this case, similar effects to those described above can be achieved.

[0038] Alternatively, the protruding portion 19 may be formed to slightly extend beyond the area circumferentially extending from the throat position P1 on the suction surface 15 of the stationary blade 3A to the downstream edge position P3 of the stationary blade 3B. Note that the depressed portion 18 must be reduced accordingly. Specifically, the protruding portion 19 may be formed in an area starting from a position shifted from the throat position P1 by about 10% of the inter-blade inclination length L to a side opposite to the downstream edge position P3. Furthermore, the protruding portion 19 may be formed in an area reaching a position shifted from the downstream edge position P3 to a side opposite to the throat position P1 by about 10% of the inter-blade inclination length L.In this case, similar effects to those described above can be achieved.

[0039] Although in the example explained in the second embodiment, the protruding portion 19 is formed in the region including not only the downstream side but also the upstream side of the downstream edge position P3 of the guide vane 3B in the axial direction, this is not the only example. That is, the protruding portion 19 may be formed in a region including only the downstream side of the downstream edge position P3 of the guide vane 3B in the axial direction.

[0040] Although the functions of the present invention are applied to the outer peripheral surface 10 of the diaphragm inner ring 4 in the examples explained in the first and second embodiments, these are not the only examples. That is, the functions can be applied to any of the inner peripheral surfaces 9 of the diaphragm outer ring 2, the inner peripheral surface 11 of the cover plate 7, and the outer peripheral surface 12 of the rotor 5.

[0041] Furthermore, although the present invention is applied to a steam turbine in the examples explained in the first and second embodiments, these are not the only examples. That is, the present invention can be applied to a gas turbine.

Claims

[1] Axial turbine, comprising: a plurality of blades (3, 6) arranged in the circumferential direction; and an element (2, 4, 5) having a peripheral surface (9, 10, 11, 12) connecting the plurality of blades (3, 6) on their inner peripheral side or outer peripheral side and forming a wall surface of a main flow path (8), wherein the peripheral surface of the element has a plurality of recessed portions (18), and each of the recessed portions (18) is formed in a region located on a downstream side of a throat where a distance between a suction surface (15) of one blade of a pair of adjacent blades and a pressure surface (16) of another blade of the pair of adjacent blades is shortest and which is circumferentially located in a range from a throat position (P1) on the suction surface (15) of one blade to a downstream edge position (P2) of the one blade, which includes a downstream edge position of the circumferential surface in the axial direction, each of the plurality of depressed portions (18) is formed along a working medium flow direction on the downstream side of the throat and gradually becomes deeper along the working medium flow direction up to a downstream edge position of the peripheral surface in the axial direction of the element. [2] The axial flow turbine according to claim 1, wherein the axial flow turbine of each of the plurality of recessed portions is formed in a region including not only a downstream side but also an upstream side of the downstream edge position of the one blade in the axial direction. [3] Axial turbine according to claim 1, wherein the peripheral surface of the element has a plurality of projecting portions (19), and each of the projecting portions (19) is formed in a region located on the downstream side of the throat and circumferentially within a range of the throat position on the suction surface (15) of one blade to a downstream edge position of the other blade, which includes the downstream edge position of the circumferential surface of the element in the axial direction. [4] Axial turbine according to claim 1, wherein the element is one of: a diaphragm inner ring (4) having an outer peripheral surface (10) connecting a plurality of guide vanes (3) on its inner peripheral side and forming a wall surface of the main flow path; a diaphragm outer ring (2) having an inner peripheral surface (9) connecting the plurality of guide vanes (3) on their outer peripheral side and forming a wall surface of the main flow path; a rotor (5) having an outer peripheral surface (12) connecting a plurality of movable blades (6) on its inner peripheral side and forming a wall surface of the main flow path (8); and a cover plate (7) having an inner peripheral surface (11) connecting a plurality of movable blades (6) on its outer peripheral side and forming a wall surface of the main flow path (8).

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