阅读说明：本技术 离心压缩机以及具有该离心压缩机的涡轮增压器 (Centrifugal compressor and turbocharger having the same ) 是由 林良洋 于 2017-11-16 设计创作，主要内容包括：本发明提供一种离心压缩机以及具有该离心压缩机的涡轮增压器。离心压缩机具有壳体,该壳体包括：涡旋部,其在叶轮的外周侧形成有涡卷状的涡旋流路；扩散器部,其形成有扩散器流路,且扩散器流路在涡旋流路的径向内侧沿涡旋流路延伸并与涡旋流路连通。扩散器流路包括：内侧流路部,其从扩散器流路的入口部到喉部使流路高度减小并延伸；外侧流路部,其从喉部延伸到扩散器流路的出口部。在以涡旋部的舌部为基准的周向的角度范围为240°到300°的范围内的从旋转轴线到喉部的平均距离比240°到300°的范围外的从旋转轴线到喉部的平均距离大。(The invention provides a centrifugal compressor and a turbocharger with the same. The centrifugal compressor has a housing comprising: a scroll portion having a scroll-like scroll flow path formed on an outer peripheral side of the impeller; and a diffuser portion in which a diffuser flow path is formed, the diffuser flow path extending along the scroll flow path on a radially inner side of the scroll flow path and communicating with the scroll flow path. The diffuser flow path includes: an inner flow path portion which reduces and extends a flow path height from an inlet portion to a throat portion of the diffuser flow path; an outer flow path portion extending from the throat portion to an outlet portion of the diffuser flow path. An average distance from the rotation axis to the throat in a circumferential angular range of 240 ° to 300 ° with respect to the tongue of the scroll portion is larger than an average distance from the rotation axis to the throat outside the range of 240 ° to 300 °.)

1. A centrifugal compressor having an impeller and a casing, characterized in that,

the housing has:

A scroll portion having a scroll-like scroll flow path formed on an outer peripheral side of the impeller;

a diffuser portion in which a diffuser flow path is formed, the diffuser flow path extending along the scroll flow path on a radially inner side of the scroll flow path and communicating with the scroll flow path;

the diffuser flowpath includes:

An inner flow path portion extending with a flow path height reduced from an inlet portion of the diffuser flow path to a throat portion located more radially outward than the inlet portion;

An outer flow path portion extending from the throat to an outlet portion of the diffuser flow path;

An average distance from the rotation axis to the throat in a range where an angular range of a circumferential direction with respect to a tongue of the scroll portion is 240 ° to 300 ° is larger than an average distance from the rotation axis to the throat outside the range where the angular range is 240 ° to 300 °.

2. The centrifugal compressor according to claim 1,

The distance from the axis of rotation to the throat varies at least partially in the circumferential direction and is greatest in the angular range of 240 ° to 300 °.

3. The centrifugal compressor according to claim 1 or 2,

An average flow path height of the throat portion in the range of the angular range of 240 ° to 300 ° is above an average flow path height of the throat portion outside the range of the angular range of 240 ° to 300 °.

4. The centrifugal compressor according to claim 3,

The diffuser portion includes a first inner wall surface and a second inner wall surface that define the inner flow path portion therebetween, the first inner wall surface being perpendicular to the rotation axis, the second inner wall surface being inclined at an inclination angle that forms an acute angle with respect to a plane perpendicular to the rotation axis so as to approach the first inner wall surface from the inlet portion to the throat portion,

The average value of the inclination angles in the range of the angle range of 240 ° to 300 ° is smaller than the average value of the inclination angles outside the range of the angle range of 240 ° to 300 °.

5. the centrifugal compressor according to any one of claims 1 to 4,

The outer flow path portion has a circumferential range in which a flow path height from the throat portion to at least a partial region of the outlet portion in a radial direction decreases toward a circumferential downstream in a range in which the angular range is 270 ° to 360 °.

6. The centrifugal compressor according to claim 5,

The diffuser portion includes a third inner wall surface and a fourth inner wall surface that define the outer flow path portion therebetween, the third inner wall surface being perpendicular to the rotation axis,

the fourth inner wall surface has the circumferential range that is inclined with respect to a plane perpendicular to the rotation axis so as to approach the third inner wall surface toward a circumferential downstream in a range where the angular range is 270 ° to 360 °.

7. The centrifugal compressor according to claim 5 or 6,

When the height of the flow path of the at least one partial region on the most upstream side in the circumferential direction is h in the circumferential range_CAnd the height of the flow path in the at least one partial region on the most downstream side in the circumferential direction is set to h_DWhen is, is

0.6≦h_D/h_C≦0.9

8. A turbocharger, characterized in that,

A centrifugal compressor according to any one of claims 1 to 7.

Technical Field

The present invention relates to a centrifugal compressor and a turbocharger having the same.

background

in recent years, the operating region of centrifugal compressors is required to be expanded. Patent document 1 discloses a centrifugal compressor in which the radial position of the throat portion of the diffuser flow path in the region near the winding start end of the scroll flow path is located radially outward of the radial position of the throat portion of the diffuser flow path in the region near the winding end of the scroll flow path. With this configuration, since a change in the outlet static pressure of the diffuser passage in the circumferential direction can be reduced on the small flow rate side, the operating region can be expanded to the small flow rate side.

Disclosure of Invention

Technical problem to be solved by the invention

However, patent document 1 does not describe a case where the operation region is expanded to the large flow rate side. Generally, at a large flow rate operating point of a centrifugal compressor, it is known that a static pressure tends to decrease toward the outlet side of a scroll passage as a static pressure becomes lower due to an increased velocity flow caused by an excessively large volume flow rate flowing into the scroll passage. As a result of studies by the inventors of the present invention, it has been found that, in the vicinity of the outlet of the swirl flow path, dynamic pressure is locally excessive in the adjacent diffuser flow path due to the influence of the static pressure field, and the amount of decrease in efficiency increases with the pressure loss at that position.

the present invention has been made in view of the above problems, and an object of at least one embodiment of the present invention is to provide a centrifugal compressor and a turbocharger having the same, which suppress a decrease in efficiency on the large flow rate side.

Technical solution for solving technical problem

(1) The centrifugal compressor of at least one embodiment of the present invention is a centrifugal compressor having an impeller and a casing,

the housing includes:

a scroll portion having a scroll-like scroll flow path formed on an outer peripheral side of the impeller;

A diffuser portion in which a diffuser flow path is formed, the diffuser flow path extending along the scroll flow path on a radially inner side of the scroll flow path and communicating with the scroll flow path;

The diffuser flowpath includes:

An inner flow path portion extending with a flow path height reduced from an inlet portion of the diffuser flow path to a throat portion located more radially outward than the inlet portion;

An outer flow path portion extending from the throat to an outlet portion of the diffuser flow path;

An average distance from the rotation axis to the throat in a range of an angular range of 240 ° to 300 ° in a circumferential direction with respect to a tongue of the scroll portion is larger than an average distance from the rotation axis to the throat outside the range of the angular range of 240 ° to 300 °.

according to the configuration of the above (1), since the average distance from the rotation axis of the impeller to the throat portion in the region near the winding end portion of the scroll flow path is made larger than the average distance from the rotation axis to the throat portion in the region other than the region near the winding end portion of the scroll flow path, the flow path area of the diffuser flow path in the region near the winding end portion is enlarged and the pressure loss is reduced, so that the efficiency decrease of the centrifugal compressor on the large flow rate side can be suppressed.

(2) In several embodiments, based on the structure of (1) above,

The distance from the axis of rotation to the throat varies at least partially in the circumferential direction and is greatest in the angular range of 240 ° to 300 °.

According to the configuration of the above (2), since the distance from the rotation axis to the throat portion is maximized in the region near the winding end portion of the scroll flow path, the flow path area of the diffuser flow path in the region near the winding end portion is enlarged and the pressure loss is reduced, so that the efficiency of the centrifugal compressor on the large flow rate side can be suppressed from being lowered.

(3) In several embodiments, based on the structure of the above (1) or (2),

An average flow path height of the throat portion in the range of the angular range of 240 ° to 300 ° is above an average flow path height of the throat portion outside the range of the angular range of 240 ° to 300 °.

In the case of a configuration in which only the throat portion is located radially outward of the other region in the region near the winding end portion of the scroll flow path, the flow path height of the throat portion is lower than that of the other region, and the flow path area decreases. However, according to the configuration of the above (3), even if the throat portion is positioned radially outward of the other region in the region near the winding end portion of the scroll flow path, the average flow path height of the throat portion is equal to or greater than the average flow path height of the throat portion in the other region, and therefore, a configuration in which the flow path area is reduced can be avoided.

(4) In several embodiments, based on the structure of (3) above,

the diffuser portion includes a first inner wall surface and a second inner wall surface that define the inner flow path portion therebetween, the first inner wall surface being perpendicular to the rotation axis, the second inner wall surface being inclined at an inclination angle that forms an acute angle with respect to a plane perpendicular to the rotation axis so as to approach the first inner wall surface from the inlet portion to the throat portion,

the average value of the inclination angles in the range of the angle range of 240 ° to 300 ° is smaller than the average value of the inclination angles outside the range of the angle range of 240 ° to 300 °.

According to the configuration of the above (4), the throat portion is located radially outward of the other region in the region near the winding end portion of the scroll flow path, whereby the configuration of the above (3) can be realized, and the formation of the configuration in which the flow path area is reduced can be avoided.

(5) In some embodiments, based on any of the structures (1) to (4) above,

The outer flow path portion has a circumferential range in which a flow path height from the throat portion to at least a partial region of the outlet portion in a radial direction decreases toward a circumferential downstream in a range in which the angular range is 270 ° to 360 °.

on the small flow rate side, the flow velocity of the compressed fluid decreases as the inside of the scroll flow path approaches the downstream side, and therefore the compressed fluid may flow back from the vicinity of the winding end portion of the scroll flow path to the diffuser flow path, and the stall region may expand in the direction from the vicinity of the winding end portion toward the vicinity of the winding end portion. According to the configuration of the above (5), since the diffuser flow path is formed in the region from the vicinity of the winding end portion to the vicinity of the winding end portion of the scroll flow path so that the circumferential range in which the flow path height in the radial direction from the throat portion to at least a partial region of the outlet portion decreases downstream in the circumferential direction, the expansion of the stall region from the vicinity of the winding end portion toward the vicinity of the winding end portion can be reduced, and therefore, the reduction in efficiency of the centrifugal compressor on the low flow rate side can be suppressed.

(6) In several embodiments, based on the structure of (5) above,

The diffuser portion includes a third inner wall surface and a fourth inner wall surface that define the outer flow path portion therebetween, the third inner wall surface being perpendicular to the rotation axis,

The fourth inner wall surface has the circumferential range that is inclined with respect to a plane perpendicular to the rotation axis so as to approach the third inner wall surface toward a circumferential downstream in a range where the angular range is 270 ° to 360 °.

according to the configuration of the above (6), the configuration of the above (5) can be realized, which can reduce the expansion of the stall region from the vicinity of the winding end portion toward the vicinity of the winding end portion.

(7) in several embodiments, based on the structure of (5) or (6) above,

when the height of the flow path of the at least one partial region on the most upstream side in the circumferential direction is h in the circumferential range_CAnd the height of the flow path in the at least one partial region on the most downstream side in the circumferential direction is set to h_DWhen is, is

0.6≦h_D/h_C≦0.9。

According to the configuration of the above (7), the reduction in the flow path area of the outer flow path portion can be minimized, the expansion of the stall region can be reduced, and the reduction in the efficiency of the centrifugal compressor on the low flow rate side can be suppressed.

(8) A turbocharger according to at least one embodiment of the present invention includes:

The centrifugal compressor according to any one of the above (1) to (7).

According to the configuration of the above (8), a decrease in efficiency of the centrifugal compressor on the large flow rate side can be suppressed.

ADVANTAGEOUS EFFECTS OF INVENTION

According to at least one embodiment of the present invention, since the average distance from the rotation axis of the impeller to the throat portion in the region near the winding end portion of the scroll flow path is made larger than the average distance from the rotation axis to the throat portion in the region other than the region near the winding end portion of the scroll flow path, the flow path area of the diffuser flow path in the region near the winding end portion is enlarged and the pressure loss is reduced, so that it is possible to suppress a decrease in efficiency of the centrifugal compressor on the large flow rate side.

Drawings

Fig. 1 is a sectional view of a centrifugal compressor according to a first embodiment of the present invention.

fig. 2 is a sectional view of a diffuser portion of a centrifugal compressor according to a first embodiment of the present invention.

fig. 3 is a schematic sectional view of a diffuser portion of a centrifugal compressor according to a first embodiment of the present invention.

Fig. 4 is a sectional view of a centrifugal compressor according to a second embodiment of the present invention.

Fig. 5 is a partially sectional schematic view of an outer flow path portion of a diffuser flow path of a centrifugal compressor according to a second embodiment of the present invention.

Detailed Description

Several embodiments of the present invention will be described below with reference to the drawings. However, the scope of the present invention is not limited to the following embodiments. The dimensions, materials, shapes, relative arrangements, and the like of the constituent members described in the following embodiments are merely illustrative examples, and the scope of the present invention is not intended to be limited thereto.

A centrifugal compressor according to several embodiments of the present invention will be described below with reference to a centrifugal compressor of a turbocharger as an example. However, the centrifugal compressor of the present invention is not limited to the centrifugal compressor of the turbocharger, and may be any centrifugal compressor that operates alone. In the following description, the fluid compressed by the compressor is air, but may be replaced with any fluid.

(first embodiment)

As shown in fig. 1, a centrifugal compressor 1 according to a first embodiment of the present invention includes: a casing 2, and an impeller 3 rotatably provided in the casing 2 centering on a rotation axis L. The housing 2 includes: a scroll portion 4 having a spiral scroll flow path 5 and a diffuser portion 6 having a diffuser flow path 7 are formed on the outer peripheral side of the impeller 3.

In the present invention, the circumferential position with respect to the tongue portion 4a of the scroll portion 4 is represented by the central angle θ around the rotation axis L. Therefore, the central angle θ indicating the circumferential position of the tongue 4a is 0 °. However, the position of the tongue 4a, which means that the tongue 4a makes one turn along the scroll flow path 5 from the tongue 4a and returns to the tongue 4a, is represented by a central angle θ of 360 °. In addition, an arbitrary range in the circumferential direction can be represented by a range of the center angle θ, and the range represented by the range of the center angle θ is defined as an angular range.

the inventors of the present invention found out that, as a result of determining the dynamic pressure distribution in the diffuser flow path 7 when the centrifugal compressor 1 is operated on the large flow rate side by CFD analysis, the dynamic pressure increases in the vicinity of the winding end of the scroll portion 4, that is, in the vicinity of the circumferential position (region a in fig. 1) where the center angle θ is 270 °, and the efficiency decrease due to the pressure loss increases. Therefore, the efficiency of the centrifugal compressor 1 on the large flow rate side can be improved by reducing the pressure loss in the region a.

As shown in fig. 2, the diffuser flow path 7 includes: an inner flow path portion 11 extending from the inlet portion 7a of the diffuser flow path 7 to a throat portion 10 located radially outward of the inlet portion 7a while reducing the flow path height; and an outer flow path portion 12 extending from the throat portion 10 to the outlet portion 7b of the diffuser flow path 7. Here, the flow path height means the width of the diffuser flow path 7 in the direction in which the rotation axis L extends.

The inner flow path portion 11 is defined between the first inner wall surface 11a and the second inner wall surface 11b of the diffuser portion 6 facing each other in the direction in which the rotation axis L extends. The outer flow path portion 12 is defined between the third inner wall surface 12a and the fourth inner wall surface 12b of the diffuser portion 6 facing each other in the direction in which the rotation axis L extends. The first inner wall surface 11a is perpendicular to a plane P perpendicular to the rotation axis L, and the second inner wall surface 11b is inclined at an inclination angle α which is an acute angle with respect to the plane P so as to approach the first inner wall surface 11a from the inlet portion 7a to the throat portion 10. Thus, the inner flow path portion 11 is configured to extend while reducing the flow path height from the inlet portion 7a to the throat portion 10.

As shown in fig. 1, the centrifugal compressor 1 is configured such that a distance R from the rotation axis L to the throat portion 10 in an angular range B in which the central angle θ is 240 ° to 300 ° is greater than a distance R' from the rotation axis L to the throat portion 10 outside the angular range B. In other words, the radial position of the throat 10 within the angular range B is located more radially outward than the radial position of the throat 10 outside the angular range B. In particular, the maximum value R of the distance R_maxat a circumferential position having a center angle θ of 270 °. With the above configuration, the flow passage area of the diffuser flow passage 7 in the region near the winding end portion of the scroll portion 4 is increased and the pressure loss is reduced, so that a decrease in the efficiency of the centrifugal compressor 1 on the large flow rate side can be suppressed.

However, as shown in fig. 3, the inclination angle α in the angle range B is set to be_BMaintaining the angle of inclination alpha outside the angular range B₀When the throat portion 10 is configured to be displaced radially outward in the same manner, the flow path height h of the throat portion 10 in the angular range B is set to be equal to_B' flow path height h of throat 10 outside lower angle range B₀And the flow path area is reduced. Therefore, the inclination angles α (see fig. 2) are distributed in the circumferential direction, and the inclination angle α in the angular range B is set to be larger_BAngle of inclination alpha outside the specific angle range B₀small, but can be the flow path height h of the throat 10 within the angular range B_Bflow path height h from throat 10 outside angle range B₀Flow path height h of throat 10 in the same or angular range B_BFlow path height h of throat 10 outside of specific angle range B₀The large structure can suppress the reduction of the flow path area.

In this way, by making the distance R from the rotation axis L to the throat portion 10 of the impeller 3 in the region near the winding end portion of the scroll flow path 5 larger than the distance R' from the rotation axis L to the throat portion 10 in the region other than the region near the winding end portion of the scroll flow path 5, the flow path area of the diffuser flow path 7 in the region near the winding end portion is enlarged and the pressure loss is reduced, so that it is possible to suppress a decrease in the efficiency of the centrifugal compressor 1 on the large flow rate side.

In the first embodiment, the distance R in the entire angular range B is larger than any distance R' other than the angular range B, and the maximum value R of the distance R is set_maxthe center angle θ is 270 ° in the circumferential direction, but the present invention is not limited to this. Even if there is a distance R smaller than the distance R' of any portion other than the angular range B within the angular range B, or there is a distance R larger than the maximum value outside the angular range B_maxThe distance R 'may be larger than the average value of the distances R'.

In the first embodiment, the flow path height h of the entire angular range B is set_Bany flow path height h outside the angle range B₀Height h of flow path in the same or the whole angle range B_BArbitrary flow path height h outside the specific angle range B₀However, the method is not limited to this method. Even if the flow path height h of any part outside the angle range B exists in the range of the angle range B₀small flow path height, i.e., flow path height h_Bis less than the flow path height h₀The average value of (2) is large.

In the first embodiment, the names of the first inner wall surface 11a and the second inner wall surface 11b may be replaced with each other. That is, the inner wall surfaces facing each other may be inclined with respect to the plane P. In addition, the first inner wall surface 11a may be inclined at the same inclination angle or different inclination angles with respect to the plane P, instead of being perpendicular to the plane P, both the first inner wall surface 11a and the second inner wall surface 11b may be inclined.

(second embodiment)

Next, a centrifugal compressor according to a second embodiment will be described. The centrifugal compressor according to the second embodiment has a structure for suppressing a decrease in efficiency of the centrifugal compressor on the low flow rate side, compared to the first embodiment. In the second embodiment, the same components as those of the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted.

as shown in fig. 4, the inventors of the present invention found that the stall region expands in the direction (the direction of arrow F) from the vicinity of the winding end portion of the scroll flow path 5 toward the vicinity of the winding end portion by CFD analysis to obtain the radial velocity distribution in the diffuser flow path 7 when the centrifugal compressor 1 is operated on the small flow rate side. On the low flow rate side, the flow velocity of the compressed air decreases as the interior of the scroll flow path 5 approaches the downstream side, and therefore the compressed air flows back to the diffuser flow path 7 from the vicinity of the winding end portion of the scroll flow path 5, and the stall starts from this point. Since the return flow from the scroll flow path 5 has a velocity component in the rotational direction about the rotation axis L, the stall region expands in the direction from the vicinity of the winding end portion toward the vicinity of the winding release end portion. Therefore, the efficiency of the centrifugal compressor 1 on the low flow rate side can be improved by preventing the backflow.

Fig. 5 is a schematic cross-sectional view of the outer flow path portion 12 along the circumferential direction from a circumferential position (position C in fig. 4) at a center angle θ of 270 ° to a circumferential position (position D in fig. 4) at a center angle θ of 360 °. In the outer flow path portion 12, the flow path height of the throat portion 10 (see fig. 4) decreases toward the circumferential downstream in the range from the position C to the position D. When the flow path height of the throat part 10 at the position C is set to h_Cand the flow path height of the throat 10 at the position D is h_Dwhen is h_C＞h_D。

In a cross section along the circumferential direction from the position C to the position D, the third inner wall surface 12a is perpendicular to the plane P perpendicular to the rotation axis L, and the fourth inner wall surface 12b is inclined at an acute angle β with respect to the plane P from the position C to the position D, that is, to approach the third inner wall surface 12a toward the circumferential downstream. Thus, the outer flow path portion 12 has a structure in which the flow path height of the throat portion 10 decreases toward the downstream in the circumferential direction. The other structure is the same as that of the first embodiment.

In the above description, the flow path height of the throat portion 10 is reduced from the position C to the position D, but since the third inner wall surface 12a and the fourth inner wall surface 12b face each other, the flow path height at any position in the radial direction of the outer flow path portion 12 is reduced in the circumferential region where the flow path height of the throat portion 10 is reduced. Therefore, in the following description, the flow path height of the outer flow path portion 12 is described as "decreased".

In the centrifugal compressor 1, the fourth inner wall surface 12b is inclined with respect to the plane P so as to approach the third inner wall surface 12a in the direction in which the stall region expands (arrow F), whereby the flow path height of the outer flow path portion 12 is reduced, so that expansion of the stall region in this direction can be reduced. As a result, a decrease in the efficiency of the centrifugal compressor 1 on the low flow rate side can be suppressed.

In the second embodiment, 0.6 ≦ h is preferable_D/h_C0.9% or less. This can minimize the reduction in the flow path area of the outer flow path portion 12, reduce the expansion of the stall region, and suppress the reduction in the efficiency of the centrifugal compressor 1 on the low flow rate side.

In the second embodiment, the flow path height of the outer flow path portion 12 is continuously decreased in the circumferential direction over the entire range from the position C to the position D, but the present invention is not limited to this embodiment. The range from the position C to the position D may include a circumferential range in which the flow path height decreases downstream in the circumferential direction. That is, the flow path height may be reduced in only a partial region (circumferential range) in the circumferential direction in the range from the position C to the position D, and the flow path height may be constant in the other region. In addition, a plurality of circumferential ranges in which the flow path height is reduced in the range from the position C to the position D may be provided, and the flow path height between the plurality of circumferential ranges in which the flow path height is reduced may be constant.

In the second embodiment, the height of the entire flow path from the throat portion 10 to the outlet portion 7b in the radial direction decreases toward the downstream in the circumferential direction, but the present invention is not limited to this embodiment. It may be that, although the flow path height of a region from the throat portion 10 to at least a part of the outlet portion 7b in the radial direction decreases toward the circumferential downstream, the flow path height of the other region is constant.

In the second embodiment, the names of the third inner wall surface 12a and the fourth inner wall surface 12b may be replaced with each other. That is, the inner wall surfaces facing each other may be inclined with respect to the plane P. In addition, the third inner wall surface 12a may be inclined at the same inclination angle or different inclination angles with respect to the plane P, instead of being perpendicular to the plane P, both the third inner wall surface 12a and the fourth inner wall surface 12b may be inclined.

In the second embodiment, a configuration is added in which the flow path height of the outer flow path portion 12 decreases toward the downstream in the circumferential direction in the angular range of 270 ° to 360 ° compared to the first embodiment, but the present invention is not limited to this embodiment. The structure of the first embodiment may not be provided, and for example, the structure of the second embodiment may be provided together with the structure in which the radial position of the throat portion 10 is constant in the circumferential direction. In this case, although the effect of suppressing the decrease in efficiency of the centrifugal compressor 1 on the large flow rate side cannot be obtained, the effect of suppressing the decrease in efficiency of the centrifugal compressor 1 on the small flow rate side can be obtained.

Description of the reference numerals

1a centrifugal compressor; 2, a shell; 3, an impeller; 4a vortex part; 4a tongue portion; 5 a vortex flow path; 6 a diffuser section; 7a diffuser flow path; 7a (of the diffuser flow path) inlet; 7b (of the diffuser flow path); 10 throat parts; 11 an inner flow path portion; 11a first inner wall surface; 11b a second inner wall surface; 12 an outer flow path portion; 12a third inner wall surface; 12b a fourth inner wall surface; h is₀Flow path height (of throat); h is_BFlow path height (of throat); h is_CFlow path height (of throat); h is_DFlow path height (of throat); a region A; the angle range B; position C; position D; l (of the impeller) axis of rotation; a P plane; the R distance; r_maxMaximum value (of distance R); an alpha tilt angle; a beta tilt angle; theta central angle.