TURBINE IMPELLER AND VARIABLE GEOMETRY TURBINE

Abstract

A turbine impeller includes: a hub portion coupled to an end of a rotational shaft; a plurality of main blades disposed at intervals on a peripheral surface of the hub portion; and a short blade disposed between two adjacent main blades among the plurality of main blades. An inter-blade flow channel is formed between the two adjacent main blades so that a fluid flows through the inter-blade flow channel from an outer side toward an inner side of the turbine impeller in a radial direction. In a meridional plane, a hub-side end of a leading edge of the short blade is disposed on an inner side, in the radial direction, of a hub-side end of a leading edge of the main blade.

Claims

1-12. (canceled)

13. A turbine impeller, comprising: a hub portion coupled to an end of a rotational shaft; a plurality of main blades disposed at intervals on a peripheral surface of the hub portion; and a short blade disposed between two adjacent main blades among the plurality of main blades, wherein an inter-blade flow channel is formed between the two adjacent main blades so that a fluid flows through the inter-blade flow channel from an outer side toward an inner side of the turbine impeller in a radial direction, wherein a hub-side end of a leading edge of the short blade is disposed on an inner side, in the radial direction, of a hub-side end of a leading edge of the main blade, in a meridional plane, and wherein, in the meridional plane, a hub-side end of a trailing edge of the short blade is positioned on a same position, in an axial direction, as a hub-side end of a trailing edge of the main blade.

14. The turbine impeller according to claim 13, wherein the hub-side end of the leading edge of the short blade is positioned in a region which satisfies 0.30<Lh2/Lh1<0.89, provided that Lh1 is a distance from the hub-side end of the leading edge of the main blade to the hub-side end of the trailing edge of the main blade in the meridional plane, and Lh2 is a distance from the hub-side end of the leading edge of the main blade to the hub-side end of the leading edge of the short blade in the meridional plane.

15. The turbine impeller according to claim 14, wherein the hub-side end of the leading edge of the short blade is positioned in a region which satisfies 0.52<Lh2/Lh1<0.84.

16. The turbine impeller according to claim 15, wherein the hub-side end of the leading edge of the short blade is positioned in a region which satisfies 0.60<Lh2/Lh1<0.80.

17. The turbine impeller according to claim 13, wherein the hub-side end of the trailing edge of the short blade is positioned downstream, in the flow direction of the fluid, of the hub-side end of the trailing edge of the main blade.

18. The turbine impeller according to claim 17, wherein a shroud-side end of the trailing edge of the short blade is positioned on the trailing edge of the main blade in the meridional plane.

19. The turbine impeller according to claim 13, wherein a blade height of the short blade is lower than a blade height of the main blade.

20. A turbine impeller, comprising: a hub portion coupled to an end of a rotational shaft; a plurality of main blades disposed at intervals on a peripheral surface of the hub portion; and a short blade disposed between two adjacent main blades among the plurality of main blades, wherein an inter-blade flow channel is formed between the two adjacent main blades so that a fluid flows through the inter-blade flow channel from an outer side toward an inner side of the turbine impeller in a radial direction, wherein, in a meridional plane, a hub-side end of a leading edge of the short blade is disposed on an inner side, in the radial direction, of a hub-side end of a leading edge of the main blade, wherein, in the meridional plane, a hub-side end of a trailing edge of the short blade is positioned on a same position, in an axial direction, as a hub-side end of a trailing edge of the main blade, or positioned downstream, in a flow direction of the fluid, of the hub-side end of the trailing edge of the main blade, and wherein the hub-side end of the leading edge of the short blade is positioned in a region which satisfies 0.30<Lh2/Lh1<0.89, provided that Lh1 is a distance from the hub-side end of the leading edge of the main blade to the hub-side end of the trailing edge of the main blade in the meridional plane, and Lh2 is a distance from the hub-side end of the leading edge of the main blade to the hub-side end of the leading edge of the short blade in the meridional plane.

21. A turbine impeller, comprising: a hub portion coupled to an end of a rotational shaft; a plurality of main blades disposed at intervals on a peripheral surface of the hub portion; and a short blade disposed between two adjacent main blades among the plurality of main blades, wherein an inter-blade flow channel is formed between the two adjacent main blades so that a fluid flows through the inter-blade flow channel from an outer side toward an inner side of the turbine impeller in a radial direction, wherein, in a meridional plane, a hub-side end of a leading edge of the short blade is disposed on an inner side, in the radial direction, of a hub-side end of a leading edge of the main blade, wherein, in the meridional plane, a hub-side end of a trailing edge of the short blade is positioned on a same position, in an axial direction, as a hub-side end of a trailing edge of the main blade, or positioned downstream, in a flow direction of the fluid, of the hub-side end of the trailing edge of the main blade, and wherein a shroud-side end of the trailing edge of the short blade is positioned on the trailing edge of the main blade in the meridional plane.

22. A variable geometry turbine, comprising: the turbine according to claim 13, a turbine housing for housing the turbine impeller; and a variable nozzle mechanism for controlling a flow direction of a fluid which flows toward the turbine impeller.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0029] FIG. 1 is a cross-sectional view of a variable geometry turbine according to an embodiment of the present invention.

[0030] FIG. 2 is a diagram of a turbine impeller according to an embodiment of the present invention.

[0031] FIG. 3 is a diagram showing the meridional shape of a turbine impeller according to an embodiment of the present invention.

[0032] FIG. 4 is a diagram showing the meridional shape of a turbine impeller according to an embodiment of the present invention.

[0033] FIG. 5 is a diagram showing the meridional shape of a turbine impeller according to an embodiment of the present invention.

[0034] FIG. 6 is a diagram showing the meridional shape of a turbine impeller according to an embodiment of the present invention.

[0035] FIG. 7 is a diagram showing the meridional shape of a turbine impeller according to an embodiment of the present invention.

[0036] FIG. 8A is a chart showing the relationship between the leading-edge position of a short blade of a turbine impeller and the turbine efficiency, according to an embodiment of the present invention. FIG. 8B is a chart showing the relationship between the leading-edge position of a short blade of a turbine impeller and the inertia moment, according to an embodiment of the present invention.

[0037] FIG. 9 is an explanatory diagram for describing a flow of a fluid at a high flow rate and at a low flow rate, in a variable geometry turbine according to an embodiment of the present invention.

[0038] FIG. 10A is a diagram of a turbine impeller according to the first comparative example.

[0039] FIG. 10B is a diagram of a turbine impeller according to the second comparative example.

DETAILED DESCRIPTION

[0040] Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shapes, relative positions and the like of components described in the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention.

[0041] For instance, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function.

[0042] For instance, an expression of an equal state such as “same” “equal” and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function.

[0043] Further, for instance, an expression of a shape such as a rectangular shape or a cylindrical shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved.

[0044] On the other hand, an expression such as “comprise”, “include”, “have”, “contain” and “constitute” are not intended to be exclusive of other components.

[0045] Further, in the description below, some the same features are associated with the same reference numerals and not described again.

[0046] Further, in the following description of the comparative examples, some o of the same features are associated with the same reference numerals and an apostrophe suffixed there to, and not described again.

[0047] FIG. 1 is a schematic cross-sectional view of a variable geometry turbine according to an embodiment of the present invention.

[0048] As depicted in FIG. 1, a variable geometry turbine 10 according to an embodiment of the present invention includes a turbine impeller 1, a turbine housing 2 housing the turbine impeller 1, and a variable nozzle mechanism 3 for controlling a flow direction of a fluid (e.g. exhaust gas discharged from a non-depicted engine) that flows toward the turbine impeller 1.

[0049] FIG. 2 is a diagram of a turbine impeller according to an embodiment of the present invention. FIG. 2A is a perspective view of the turbine impeller 1, and FIG. 2B is a diagram of a flow of a fluid that flows through the turbine impeller 1.

[0050] As depicted in FIG. 2, the turbine impeller 1 includes a hub portion 11 to be coupled to an end side of the rotational shaft 4, a plurality of main blades 12 disposed at intervals on the peripheral surface of the hub portion 11, and short blades 13 each of which is disposed between two adjacent main blades 12, 12 of the plurality of main blades 12. An inter-blade flow channel 14 is formed between two adjacent main blades 12, 12 as shown by arrow R in FIG. 2. A fluid flows through the inter-blade flow channel 14 from the outer side toward the inner side in the radial direction of the turbine impeller 1.

[0051] In the embodiment depicted in FIG. 2, one short blade 13 is disposed between two adjacent main blades 12, 12. Eight main blades 12 and eight short blades 13 are disposed alternately at regular intervals. However, the turbine impeller 1 according to an embodiment of the present invention is not limited to this example, and may have two or more short blades 13 between two adjacent main blades 12, 12. Further, for instance, the short blades 13 may be not necessarily disposed between every adjacent pair of main blades 12, 12. For instance, the short blades 13 may be provided between every other pair of main blades 12, 12.

[0052] Further, in the embodiment depicted in FIG. 1, the turbine housing 2 has, inside the turbine housing 2, a scroll flow path 21 formed on the radially outer side of the turbine impeller 1, and an outlet flow path 22 formed along the extending direction of the rotational axis K of the rotational shaft 4. The turbine housing 2 is coupled to the bearing housing 5 at the opposite side from the opening end of the outlet flow path 22. The bearing housing 5 houses a bearing 51 that supports the rotational shaft 4 rotatably.

[0053] FIGS. 3 to 7 are each a diagram showing the meridional shape of a turbine impeller according to an embodiment of the present invention.

[0054] As depicted in FIGS. 2, and 3 to 7, the turbine impeller 1 according to an embodiment of the present invention is configured such that a hub-side end 131a of a leading edge 131 of the short blade 13 is disposed on the inner side, in the radial direction, of a hub-side end 121a of a leading edge 121 of the main blade 12, in a meridional plane.

[0055] FIG. 10A is a diagram of a turbine impeller according to the first comparative example. FIG. 10A (a) is a perspective view of a turbine impeller 1′, and FIG. 10A (b) is a diagram of a flow of a fluid that flows through the turbine impeller 1′. FIG. 10B is a diagram of a turbine impeller according to the second comparative example. FIG. 10B (a) is a perspective view of the turbine impeller 1′, and FIG. 10B (b) is a diagram of a flow of a fluid that flows through the turbine impeller 1′.

[0056] A fluid that flows into the turbine impeller 1 from the scroll flow path 21 flows diagonally with respect to the leading edge 121 of the main blade 12, from the outer side toward the inner side, in the radial direction, of the turbine impeller 1, as shown by arrow f in FIG. 2B. Thus, as depicted in FIG. 10A, the fluid flowing into the turbine impeller 1′ collides with the leading edge 121′ of the main blade 12′ and separates, thus causing loss. Further, according to findings of the present inventors, separation due to collision between the fluid flowing into the turbine impeller 1′ and the leading edge 121′ of the main blade 12′ provokes a secondary flow sf′ having a swirl component inside the inter-blade flow channel 14′, and this secondary flow sf′ also generates loss.

[0057] Further, according to findings of the present inventors, as depicted in FIG. 10B, the short blades 13′ are disposed between two adjacent main blades 12′, 12′ so that the position of the leading edges 131′ of the short blades 13′ in the radial direction is the same as the position of the leading edges 121′ of the main blades 12′, and thereby it is possible to suppress separation at the leading edges 121′ of the main blades 12′ and at the leading edges 131′ of the short blades 13′. However, the effect to reduce loss due to the secondary flow sf′ flowing through the inter-blade flow channel 14′ is limited. This is because, the secondary flow sf1′ flowing between the main blade 12′ and a surface 13a′ (e.g. suction surface) of the short blade 13′ collides with the secondary flow sf2′ flowing between an adjacent main blade 12′ and another surface 13b′ (e.g. pressure surface) of the short blade 13′, at the downstream side of the short blade 13′ in the inter-blade flow channel 14′, thus generating loss. Further, with the short blades 13′ disposed on the outer section of the turbine impeller 1′ in the radial direction, the inertia moment increases, and the transient responsiveness deteriorates.

[0058] In contrast, the turbine impeller 1 according to an embodiment of the present invention includes short blades 13 disposed between two adjacent main blades 2, and the hub-side ends 131a of the leading edges 131 of the short blades 13 are disposed on the inner side, in the radial direction, of the hub-side ends 121a of the leading edges 121 of the main blades 12, in a meridional plane. According to findings of the present inventors, with this configuration, it is possible to reduce loss due to the secondary flows sf1, sf2 that flow through the inter-blade flow channel 14 considerably, compared to the turbine impeller 1′ of the second comparative example. Further, with the short blades 13 disposed on the inner section of the turbine impeller 1 in the radial direction, it is possible to suppress an increase in the inertia moment due to provision of the short blades 13, compared to the above described second example.

[0059] Moreover, with the turbine impeller 1 according to an embodiment of the present invention, the short blades 13 are disposed on the inner section of the turbine impeller 1 in the radial direction, and thereby it is possible to reduce the number of main blades compared to the turbine impeller 1′ (FIG. 10A) of the first comparative example having no short blades, and to the turbine impeller 1′ (FIG. 10B) of the second comparative example having the short blades 13′ on the outer section in the radial direction. While the turbine impeller 1 has eight main blades 12 and eight short blades 13 in the embodiment depicted in FIG. 2, the turbine impeller 1′ has eleven main blades 12′ in the first comparative example. In the second comparative example, the turbine impeller 1′ has ten main blades 12′ and ten short blades 13′. With the short blades 13 disposed on the inner section of the turbine impeller 1 in the radial direction, the short blades 13 cross the minimum width section of the inter-blade flow channels 14. Thus, to optimize the throat width of the minimum-width portion (throat portion) between the main blades 12 and the short blades 13, the number of the main blades 12 is reduced.

[0060] Accordingly, with the turbine impeller 1 according to an embodiment of the present invention, while greater loss is generated due to separation at the leading edge 121 of each main blade 12, it is possible to suppress loss due to separation at the leading edges 121 of the main blades 12 for the turbine impeller 1 as a whole. Further, the inertia moment can be reduced by reducing the number of the main blades 12.

[0061] As depicted in FIGS. 2, and 3 to 7, in some embodiments, the hub-side ends 132a of the trailing edges 132 of the short blades 13 of the turbine impeller 1 are disposed, in a meridional plane, on the same position in the axial direction as the hub-side ends 122a of the trailing edges 122 of the main blades 12, or are disposed downstream of the hub-side ends 122a of the trailing edges 122 of the main blades 12 with respect to the flow direction of the fluid (closer to the tip of the hub portion 11 in the axial direction).

[0062] In the embodiment depicted in FIGS. 2, 3 and 6, the hub-side ends 132a of the trailing edges 132 of the short blades 13 of the turbine impeller 1 are disposed on the same position in the axial direction as the hub-side ends 122a of the trailing edges 122 of the main blades 12, in a meridional plane. In the embodiment depicted in FIGS. 4, 5 and 7, the hub-side ends 132a of the trailing edges 132 of the short blades 13 of the turbine impeller 1 are disposed closer to the tip of the hub portion 11 in the axial direction than the hub-side ends 122a of the trailing edges 122 of the main blades 12, in a meridional plane.

[0063] According to the above embodiments, it is possible to prevent collision, inside the inter-blade flow channel 14, of the secondary flow sf1 flowing between the main blade 12 and a surface 13a of the short blade 13 and the secondary flow sf2 flowing between an adjacent main blade 12 and another surface 13b of the short blade 13. Accordingly, it is possible to reduce the loss due to the secondary flows sf1, sf2 that flow through the inter-blade flow channel 14.

[0064] FIG. 8A is a chart showing the relationship between the leading-edge position of a short blade of a turbine impeller and the turbine efficiency, according to an embodiment of the present invention. FIG. 8B is a chart showing the relationship between the leading-edge position of a short blade of a turbine impeller and the inertia moment, according to an embodiment of the present invention. In FIGS. 8A and 8B, the x-axis is a ratio of Lh2 to Lh1 (Lh2/Lh1), provided that, in a meridional plane, Lh1 is a distance from the hub-side end 121a of the leading edge 121 of the main blade 12 to the hub-side end 122a of the trailing edge 122 of the main blade 12, and Lh2 is a distance from the hub-side end 121a of the leading edge 121 of the main blade 12 to the hub-side end 131a of the leading edge 131 of the short blade 13. The y-axis in FIG. 8A is a change in the turbine efficiency with respect to a reference turbine impeller. The y-axis in FIG. 8B is a change in the inertia moment with respect to a reference turbine impeller.

[0065] In the present embodiment, as depicted in FIGS. 8A and 8B, changes in the turbine efficiency and in the inertia moment, when Lh2/Lh1 is changed from “0” to “0.2”, “0.4”, “0.6”, “0.8”, “1”, are analyzed and approximate curves of the changes are obtained. In the present embodiment, the turbine impeller includes eight main blades and eight short blades. If Lh2/Lh1 of the x-axis is “0” in FIGS. 8A and 8B, the turbine impeller includes only the main blades, corresponding to the turbine impeller having sixteen main blades only. If Lh2/Lh1 of the x-axis is “1”, the turbine impeller includes no short blades, and corresponds to a turbine impeller having eight main blades alone. Further, in the present embodiment, the above mentioned reference turbine impeller is the turbine impeller 1′ having eleven main blades alone, depicted in FIG. 10A. Further, the blade height of the short blades is the same as the blade height of the main blades. Further, while computation fluid dynamics (CFD) is used for the analysis in the present embodiment, the method for the analysis is not particularly limited to CFD.

[0066] As depicted in FIG. 8A, if the hub-side end 121a of the leading edge 121 of the main blade 12 and the hub-side end 131a of the leading edge 131 of the short blade 13 are at a small distance (if Lh1/Lh1 is small), the turbine efficiency does not improve much. This may be because, if the distance from the hub-side end 121a of the leading edge 121 of the main blade 12 to the hub-side end 131a of the leading edge 131 of the short blade 13 is too close, a separation flow caused by collision with the leading edge 121 of the main blade 12 hits the leading edge 131 of the short blade 13, thus generating loss. On the other hand, as shown in FIG. 8B, if the distance between the hub-side end 121a of the leading edge 121 of the main blade 12 and the hub-side end 131a of the leading edge 131 of the short blade 13 is too great (if Lh2/Lh1 is close to 1), the loss due to the secondary flows sf1, sf2 through the inter-blade flow channel 14 cannot be sufficiently suppressed.

[0067] The present inventors found that, as depicted in FIG. 8A, with the hub-side end 131a of the leading edge 131 of the short blade 13 being disposed in a region satisfying an expression 0.30<Lh2/Lh1<0.89, it is possible to reduce the loss due to the secondary flows sf1, sf2 flowing through the inter-blade flow channel 14, and to improve the turbine efficiency by at least 1% compared to the reference turbine impeller 1′ having only the above described eleven main blades.

[0068] Furthermore, as depicted in FIG. 8A, the present inventors found that, with the hub-side end 131a of the leading edge 131 of the short blade 13 being disposed in a region satisfying an expression 0.52<Lh2/Lh1<0.84, it is possible to improve the turbine efficiency by at least 1.4% compared to the above mentioned turbine impeller 1′.

[0069] Furthermore, as depicted in FIG. 8A, the present inventors found that, with the hub-side end 131a of the leading edge 131 of the short blade 13 being disposed in a region satisfying an expression 0.60<Lh2/Lh1<0.80, it is possible to improve the turbine efficiency by at least 1.6% compared to the above mentioned turbine impeller 1′.

[0070] In some embodiments, as depicted in FIGS. 4, 5, and 7, the hub-side end 131a of the trailing edge 131 of the short blade 13 is disposed downstream with respect to the flow direction of the fluid (on the inner section in the radial direction) of the hub-side end 122a of the trailing edge 122 of the main blade 12.

[0071] According to this embodiment, it is possible to shift the position of collision between the secondary flow sf1 flowing between the main blade 12 and the surface 13a of the short blade 13 and the secondary flow sf2 flowing between the adjacent main blade 12 and the other surface 13b of the short blade 13, to the downstream side away from the trailing edge 122 of the main blade 12. Accordingly, it is possible to further suppress the loss due to the secondary flows sf1, sf2 that flow through the inter-blade flow channel 14.

[0072] As depicted in FIGS. 5 and 7, in some embodiments, the shroud-side end 132b of the trailing edge 132 of the short blade 13 is disposed on the trailing edge 122 of the main blade 12, in a meridional plane.

[0073] The effect in this embodiment, which is to shift the position of collision between the secondary flow sf1 flowing between the main blade 12 and the surface 13a of the short blade 13 and the secondary flow sf2 flowing between the adjacent main blade 12 and the other surface 13b of the short blade 13 to the downstream side away from the trailing edge 122 of the main blade 12, is greater at the hub-side portion than at the shroud-side portion of the short blade 13. Thus, with the above embodiment, compared to a case in which the entire trailing edge 132 of the short blade 13 is disposed downstream of the trailing edge 122 of the main blade 12 (FIG. 4), it is possible to reduce the size of the short blade 13 and thereby to reduce the inertia moment, while reducing the loss due to the secondary flows sf1, sf2.

[0074] As depicted in FIGS. 6 and 7, in some embodiments, the blade height H2 of the short blade 13 is smaller than the blade height H1 of the main blade 12.

[0075] As described above, the effect of providing the short blade 13 to reduce the loss due to the secondary flows sf1, sf2 flowing through the inter-blade flow channel 14 is greater at the hub-side portion than at the shroud portion of the short blade 13. Thus, with the above embodiment, the blade height H2 of the short blade 13 is smaller than the blade height H1 of the main blade 12, and thereby it is possible to reduce the inertia moment while reducing the loss due to the secondary flows sf1, sf2.

[0076] In some embodiments, the blade height H2 of the short blade 13 is within a range expressed by ⅓H1≦H2<⅔H1. Thus, with the above embodiment, it is possible to reduce the inertia moment suitably while reducing the loss due to the secondary flows sf1, sf2.

[0077] As depicted in FIG. 1, in some embodiments, the variable geometry turbine 10 according to an embodiment of the present invention includes the turbine housing 2 housing the turbine impeller 1, and the variable nozzle mechanism 3 for controlling the flow direction of the fluid that flows toward the turbine impeller 1.

[0078] In the embodiment depicted in FIG. 1, the variable nozzle mechanism 3 includes a nozzle mount 31, a nozzle plate 32, a nozzle support 33, and a nozzle vane 34. The nozzle mount 31 is a disc-shaped member having an opening in the center. The outer circumferential portion of the nozzle mount 31 is nipped by the turbine housing 2 and the bearing housing 5, and thereby the nozzle mount 31 is fixed between the turbine housing 2 and the bearing housing 5. The nozzle plate 32 is a disc-shaped member having an opening in the center, and is fixed to the shroud portion 23 of the turbine housing 2 in a position facing the nozzle mount 31. The nozzle mount 31 and the nozzle plate 32 are coupled by a plurality of nozzle supports 33. A plurality of nozzle vanes 34 is disposed at intervals in the circumferential direction between the nozzle mount 31 and the nozzle plate 32. A nozzle flow path 34a is formed between adjacent nozzle vanes 34, 34. The nozzle vanes 34 are configured such that the vane angle of the nozzle vanes 34 changes as a drive mechanism 36 drives nozzle shafts 35 to revolve about the axis.

[0079] FIG. 9 is an explanatory diagram for describing a flow of a fluid at a high flow rate and at a low flow rate, in a variable geometry turbine according to an embodiment of the present invention. When the flow rate is high as depicted in FIG. 9B, each of the plurality of nozzle vanes 34 revolves in a direction that widens the nozzle flow path 34a formed between two adjacent nozzle vanes 34. When the flow rate is low as depicted in FIG. 9A, each of the plurality of nozzle vanes 34 revolves in a direction that narrows the nozzle flow path 34a formed between two adjacent nozzle vanes 34. The flow directions fa, fb of the fluid flowing into the turbine impeller 1 form smaller angles with the tangent direction of the turbine impeller 1 when the flow rate is low (fa) than when the flow rate is high (fb). Thus, the loss due to separation caused by collision of the fluid flowing into the turbine impeller 1 with the leading edge 121 of the main blade 12 has a greater effect when the flow rate is low than when the flow rate is high.

[0080] Thus, with the variable geometry turbine 10 being provided with the above described turbine impeller 1, it is possible to improve the turbine efficiency when the flow rate is low.

[0081] The embodiments of the present invention have been described above. However, the present invention is not limited thereto, and various modifications may be applied as long as they do not depart from the object of the present invention.

[0082] For instance, while the variable geometry turbine 10 in FIG. 1 is a radial turbine in which the fluid flows into the turbine impeller 1 in the radial direction, the variable geometry turbine 10 according to an embodiment of the present invention is not limited to this, and may be configured as a mixed flow turbine in which the fluid flows into the turbine impeller 1 in a diagonal direction.

DESCRIPTION OF REFERENCE NUMERALS

[0083] 1 Turbine impeller [0084] 2 Turbine housing [0085] 3 Variable nozzle mechanism [0086] 4 Rotational shaft [0087] 5 Bearing housing [0088] 10 Variable geometry turbine [0089] 11 Hub portion [0090] 12 Main blade [0091] 121 Leading edge of main blade [0092] 121a Hub-side end of leading edge of main blade [0093] 122 Trailing edge of main blade [0094] 122a Hub-side end of trailing edge of main blade [0095] 13 Short blade [0096] 131 Leading edge of short blade [0097] 131a Hub-side end of leading edge of short blade [0098] 132 Trailing edge of short blade [0099] 132a Hub-side end of trailing edge of short blade [0100] 132b Shroud-side end of trailing edge of short blade [0101] 13a Surface of short blade [0102] 13b Another surface of short blade [0103] 14 Inter-blade flow channel [0104] 21 Scroll flow path [0105] 22 Outlet flow path [0106] 23 Shroud portion [0107] 31 Nozzle mount [0108] 32 Nozzle plate [0109] 33 Nozzle support [0110] 34 Nozzle vane [0111] 34a Nozzle flow path [0112] 36 Drive mechanism [0113] 51 Bearing