Regenerative pump

Abstract

A regenerative pump includes a casing having a fluid passage connecting an inlet with an outlet. An impeller has vanes faced to the fluid passage that includes an arc passage. An outlet passage communicates with the outlet. The outlet passage is substantially constant in cross sectional area. A communication passage connects the arc passage with the outlet passage. The casing has an inner wall that defines the communication passage. The inner wall is in a curved shape in an axial section that includes a center axis of the communication passage and extends along a center axis of the casing. The center axis of the communication passage and the outer circular profile of the impeller define an intersection therebetween. The communication passage has a vertical section, which includes the intersection, being perpendicular to the center axis of the communication passage. The inner wall reduces a crosssectional area of the communication passage in the vertical section.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and incorporates herein by reference Japanese Patent Application No. 2005-356088 filed on Dec. 9, 2005.

FIELD OF THE INVENTION

The present invention relates to a regenerative pump.

BACKGROUND OF THE INVENTION

Conventionally, a regenerative pump is used for supplying fuel into an engine, or for supplying air into exhaust gas for reducing emission of an engine. According to U.S. Pat. No. 5,498,124 (JP-A-6-288381), an impeller is rotatable along an arc-shaped fluid passage defined in a casing. The impeller is rotated, so that vanes of the impeller draw fluid through an inlet. The vanes pressurize the fluid, and discharge the fluid through an outlet.

The outlet has a connecting portion, which connects with an end of the fluid passage. In general, the inner wall of the casing defining the connecting portion is in a simplified linear shape. In this structure, the sectional area of the connecting portion drastically decreases, compared with the fluid passage and the outlet. In particular, the sectional area of the connecting portion drastically increases. Consequently, pressure loss is caused in the connecting portion.

SUMMARY OF THE INVENTION

The present invention addresses the above disadvantage.

According to one aspect of the present invention, a regenerative pump includes a casing that has an inlet and an outlet. The casing defines a fluid passage that is in a substantially arc shape connecting the inlet with the outlet. The regenerative pump further includes an impeller that is rotatable in the casing. The impeller has a plurality of vanes each having a radial end defining an outer circular profile of the impeller. The plurality of vanes is faced to the fluid passage. The fluid passage includes an arc passage, an outlet passage, and a communication passage. The arc passage is in a substantially arc shape along a circumferential direction of the casing through a predetermined angular range. The outlet passage communicates with the outlet, and has a cross sectional area that is substantially constant with respect to a flow direction of fluid. The communication passage connects the arc passage with the outlet passage. The casing has an inner wall that defines the communication passage. The inner wall is in a substantially curved shape in an axial section that includes a center axis of the communication passage and extends along a center axis of the casing. The center axis of the communication passage and the outer circular profile of the impeller define an intersection therebetween. The communication passage has a vertical section, which includes the intersection, being perpendicular to the center axis of the communication passage. The inner wall reduces a cross-sectional area of the communication passage by extending through the vertical section.

According to another aspect of the present invention, a regenerative pump includes a casing that has an inlet and an outlet. The casing defines a fluid passage that is in a substantially arc shape connecting the inlet with the outlet. The regenerative pump further includes an impeller that is rotatable in the casing. The impeller has a plurality of vanes each having a radial end defining an outer circular profile of the impeller. The plurality of vanes is faced to the fluid passage. The fluid passage includes an arc passage, an outlet passage, and a communication passage. The arc passage is in a substantially arc shape along a circumferential direction of the casing through a predetermined angular range. The outlet passage communicates with the outlet, and has a cross sectional area that is substantially constant with respect to a flow direction of fluid. The communication passage connects the arc passage with the outlet passage. The casing has an inner wall that defines the communication passage. The inner wall is in a substantially curved shape on a radially outer side in an axial section that includes a center axis of the communication passage and extends perpendicularly to a center axis of the casing. The center axis of the communication passage and the outer circular profile of the impeller define an intersection therebetween. The communication passage has a vertical section, which includes the intersection, being perpendicular to the center axis of the communication passage. The inner wall reduces a cross-sectional area of the communication passage by extending through the vertical section.

According to another aspect of the present invention, a regenerative pump includes a casing that has an inlet communicating with an outlet through an arc passage, a communication passage, and an outlet passage. The arc passage extends from the inlet circumferentially in the casing. The communication passage connects the arc passage with the outlet passage. The outlet passage communicates with the outlet, and has a cross sectional area that is substantially constant with respect to a flow direction of fluid. The regenerative pump further includes an impeller that is rotatable in the casing. The impeller has a plurality of vanes each having a radial end defining an outer circular profile of the impeller. The plurality of vanes is faced to the fluid passage. The communication passage has a center axis extending substantially along the flow direction. The center axis of the communication passage and the outer circular profile of the impeller define an intersection therebetween. The communication passage has a vertical section, which includes the intersection, being perpendicular to the center axis of the communication passage. The casing has an inner wall that defines the communication passage. The communication passage has a height when being viewed from an axial section that includes the center axis of the communication passage and extends along a center axis of the casing. The inner wall is curved to reduce the height of the communication passage by extending through the vertical section.

According to another aspect of the present invention, a regenerative pump includes a casing that has an inlet communicating with an outlet through an arc passage, a communication passage, and an outlet passage. The arc passage extends from the inlet circumferentially in the casing. The communication passage connects the arc passage with the outlet passage. The outlet passage communicates with the outlet, and has a cross sectional area that is substantially constant with respect to a flow direction of fluid. The regenerative pump further includes an impeller that is rotatable in the casing. The impeller has a plurality of vanes each having a radial end defining an outer circular profile of the impeller. The plurality of vanes is faced to the fluid passage. The communication passage has a center axis extending substantially along the flow direction. The center axis of the communication passage and the outer circular profile of the impeller define an intersection therebetween. The communication passage has a vertical section, which includes the intersection, being perpendicular to the center axis of the communication passage. The casing has an inner wall that defines the communication passage. The inner wall is in a substantially curved shape on a radially outer side when being viewed from an axial section that includes a center axis of the communication passage and extends perpendicularly to a center axis of the casing. The inner wall is curved to reduce the cross sectional area of the communication passage by extending through the vertical section.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:

FIG. 1 is a longitudinal sectional view showing a regenerative pump according to a first embodiment;

FIG. 2 is a sectional view taken along the line II-II in FIG. 1;

FIG. 3 is a sectional view taken along the line III-III in FIG. 2;

FIG. 4 is a graph showing a relationship between a distance from a point A and a sectional area of the communication passage;

FIG. 5 is a graph showing a relationship between discharge pressure of the regenerative pump and pump efficiency rip;

FIG. 6 is a sectional view, which corresponds to FIG. 3, showing a regenerative pump according to a comparative example;

FIG. 7 is a sectional view, which corresponds to FIG. 2, showing a regenerative pump according to a second embodiment;

FIG. 8 is a sectional view, which corresponds to FIG. 2, showing a regenerative pump according to a third embodiment; and

FIG. 9 is a sectional view taken along the line IX-IX in FIG. 8.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

First Embodiment

As shown in FIG. 1, a regenerative pump 10 is provided to supply air into an exhaust passage of an engine mounted in a vehicle, for example. The regenerative pump 10 includes a pump device 20 and a motor device 50. The pump device 20 pressurizes fluid drawn into the pump device 20. The motor device 50 drives the pump device 20. The regenerating pump 10 includes a housing 11 that is in a substantially cylindrical shape. The housing 11 has an inner periphery, to which permanent magnets 12 are annually provided along the circumferential direction thereof. A rotor 51 is provided to the inner periphery of the permanent magnets 12 such that the rotor 51 is substantially coaxial with respect to the permanent magnets 12.

The pump device 20 includes a casing body 21, a casing cover 22, and an impeller 23. The casing body 21 and the casing cover 22 serve as casing members. Each of the casing body 21, the casing cover 22, and the impeller 23 has the center axis that is substantially coaxial with respect to the center axis of the regenerative pump 10. As shown in FIG. 2, the casing body 21 and the casing cover 22 define a pump passage 30 that is in a substantially C-shape with respect to the circumferential direction thereof. The pump passage 30 defines a fluid passage. As referred to FIG. 1, the casing body 21 and the casing cover 22 rotatably accommodate the impeller 23 therebetween. The casing body 21 and the casing cover 22 are formed of aluminum die-cast, for example. The casing body 21 is press-inserted into one axial end of the housing 11.

A bearing 13 is provided to a center of the casing body 21. The casing cover 22 is fixed by crimping, for example, to one end of the housing 11 in a condition, in which the casing cover 22 is covered with the casing body 21. A thrust bearing 14 is provided to a center of the casing cover 22. The rotor 51 has a shaft 52 that is rotatably supported radially at one end thereof by a bearing 13. The shaft 52 is axially supported by the thrust bearing 14. The shaft 52 has the other end that is rotatably supported radially by a bearing 15.

As referred to FIG. 2, the impeller 23 is in a substantially disc shape. The impeller 23 has the radially outer periphery that is provided with vanes 24, which are circumferentially arranged around the radially outer periphery of the impeller 23. The vanes 24 are arranged at substantially regular intervals along the rotative direction of the impeller 23.

As referred to FIG. 1, the casing cover 22 has an inlet port 25. The impeller 23 having the vanes 24 at the radially outer periphery thereof rotates in the pump passage 30, so that fluid is drawn into the pump passage 30 through the inlet port 25. The fluid in the pump passage 30 is pressurized by rotation of the impeller 23, so that the fluid is discharged into a pump chamber 53 of the motor device 50 through a discharge port 26 (FIG. 2). As referred to FIG. 2, the casing body 21 defines the discharge port 26.

The housing 11 has the other end on the opposite side of the casing body 21 and the casing cover 22. The other end of the housing 11 is provided with a motor casing 54 and a discharge cover 60. The motor casing 54 is interposed between the discharge cover 60 and the housing 11. The discharge cover 60 is crimped, thereby being fixed to the housing 11. The motor casing 54 has a communication passage 55 that communicates the pump chamber 53 with a fuel passage 61 of the discharge cover 60.

The discharge cover 60 includes a discharge portion 62 and a connector 63 on the radially outer side of the shaft 52. The discharge portion 62 has a fluid passage 64 and a pressure control valve 65. The pressure control valve 65 communicates and blocks the fluid passage 64. The pressure control valve 65 communicates the fluid passage 64 when pressure of fluid in the regenerative pump 10 becomes greater than a predetermined threshold.

An electric power source (not shown) supplies electricity to a coil of the rotor 51 via the connector 63, a brush, and a commutator (nor shown). The rotor 51 rotates, so that the impeller 23 rotates together with the rotor 51 and the shaft 52. The impeller 23 rotates so that fluid is drawn into the pump passage 30 through the inlet port 25. Fluid drawn into the pump passage 30 is discharged from the pump passage 30 into the pump chamber 53 through the discharge port 26 by being applied with kinetic energy from the vanes 24 of the impeller 23. The fluid discharged into the pump chamber 53 is supplied to the outside of the regenerative pump 10 after passing through the space around the rotor 51 and the discharge portion 62.

Next, the pump passage 30 is described in detail.

As referred to FIG. 2, the pump passage 30 includes an arc passage 31, an outlet passage 32, and a communication passage 33. The arc passage 31 defines a main portion of the pump passage 30. The arc passage 31 circumferentially extends by a predetermined angle with respect to the circumferential direction of the casing body 21 and the casing cover 22. The outlet passage 32 defines a space in the vicinity of the discharge port 26. The outlet passage 32 defines the end of the pump passage 30 on the side of the discharge port 26. The communication passage 33 communicates the arc passage 31 with the outlet passage 32. The end of the arc passage 31 on the opposite side of the inlet port 25 communicates with the communication passage 33. The end of the arc passage 31 on the side of the discharge port 26 communicates with the communication passage 33. In this structure, the communication passage 33 communicates with the arc passage 31 on one end thereof, and communicates with the outlet passage 32 on the other end thereof.

The casing body 21 and the casing cover 22 define the pump passage 30 therebetween. FIG. 3 depicts a cross section that is taken along a surface, which includes a center axis L1 of the communication passage 33 and extends along the center axis of the regenerative pump 10. That is, the cross section in FIG. 3 is taken along the surface that extends perpendicularly to the plane in FIG. 2.

As shown in FIG. 3, the communication passage 33 is defined between an inner wall 41 of the casing body 21 on the side of the casing cover 22 and an inner wall 42 of the casing cover 22 on the side of the casing body 21. The cross-sectional area is substantially constant through the arc passage 31 and the outlet passage 32 along the flow direction of fluid. The inner walls of the casing body 21 and the casing cover 22 defining the arc passage 31 and the outlet passage 32 are substantially in parallel with the center axis L1 of the fluid passage.

As referred to FIG. 3, the inner wall 41 of the casing body 21 has a first curved surface 41a, and the inner wall 42 of the casing cover 22 has a second curved surface 42a. The first curved surface 41a and the second curved surface 42a define the communication passage 33 therebetween. As referred to FIG. 2, the radial ends of the vanes 24 of the impeller 23 define an imaginary outer circular profile (shown by the dotted line in FIG. 2), as the impeller 23 rotates.

As referred to FIGS. 2, 3, the center axis L1 of the communication passage 33 and the imaginary outer circular profile define an intersection P therebetween. The communication passage 33 has a vertical section, which includes the intersection P and is perpendicular to the center axis L1 of the communication passage 33. The first curved surface 41a and the second curved surface 42a reduce the cross-sectional area of the communication passage 33 such that the first curved surface 41a and the second curved surface 42a include the location of the vertical section. That is, the first curved surface 41a and the second curved surface 42a reduce the cross-sectional area of the communication passage 33 in the location of the vertical section.

In this structure, in the downstream of the point in which the communication passage 33 departs from the impeller 23 is formed in the curved shape such that the cross sectional area of the communication passage 33 is reduced, i.e., throttled. Therefore, the communication passage 33 does not drastically increase in cross-sectional area, so that pressure loss can be reduced.

The inner wall 41 of the casing body 21 and the inner wall 42 of the casing cover 22, which define the communication passage 33 therebetween, are in curved shapes in the cross section depicted by FIG. 3. That is, the inner wall 41 of the casing body 21 and the inner wall 42 of the casing cover 22, which define the communication passage 33 therebetween, are in the curved shapes with respect to the flow direction of fluid. The inner wall 41 of the casing body 21 and the inner wall 42 of the casing cover 22, which define the communication passage 33 therebetween, are in curved arc shapes in the cross section depicted by FIG. 3. The inner wall 41 of the casing body 21 and the inner wall 42 of the casing cover 22 are not limited to be in the curved arc shapes in the cross section depicted by FIG. 3. The inner wall 41 of the casing body 21 and the inner wall 42 of the casing cover 22 may be in cycloid curved shapes, parabolic curved shapes, involute curved shapes, and logarithmic curved shapes in the cross section for defining the communication passage.

The inner wall 41 of the casing body 21 and the inner wall 42 of the casing cover 22, which define the communication passage 33 therebetween, are in curved shapes in the cross section depicted by FIG. 3, so that change in cross section in the communication passage 33 becomes moderate. As referred to FIG. 2, the distance between the radial ends of the vanes 24 and the communication passage 33 becomes large toward the outlet passage 32. Therefore, the cross section of the communication passage 33 excluding the impeller 23 becomes large toward the outlet passage 32. As referred to FIG. 3, the inner wall 41 of the casing body 21 and the inner wall 42 of the casing cover 22 are in curved shapes, such that the cross section of the communication passage 33 excluding the impeller 23 becomes small toward the outlet passage 32. The cross section in the communication passage 33 is reduced in FIG. 3 by defining the inner walls 41, 42 in the curved shape. By contrast, the cross section in the communication passage 33 is increased in FIG. 2 by increase in the distance between the casings 21, 22, which define the communication passage 33, and the impeller 23. In the above structure, the reduction in the cross section in the communication passage 33 caused by defining the inner walls 41, 42, is balanced out by increase in the cross section in the communication passage 33 caused by increase in the distance between the casings 21, 22 and the impeller 23. Therefore, change in the cross-sectional area becomes totally small through the communication passage 33.

As shown in FIG. 4, the cross-sectional area of the communication passage 33 does not greatly change from the point A to the point B in FIGS. 2, 3, as the distance from A becomes large. For example, the cross section in the communication passage 33 is predetermined at a value S. In this present structure (first embodiment), variation in the cross-sectional area of the communication passage 33 is within ±5% of the value S, regardless of the location through the communication passage 33. As referred to FIGS. 1, 2, the point A is a boundary between the arc passage 31 and the communication passage 33. The point B is a boundary between the communication passage 33 and the outlet passage 32. The variation in the cross-sectional area of the communication passage 33 is determined to be within ±5% of the value S, so that, as shown in FIG. 5, the relationship between hydraulic pressure of fluid discharged from the regenerative pump 10 and the pump efficiency ηp can be improved in this present structure, compared with a comparative example.

FIG. 6 depicts the cross section of a pump passage of a regenerative pimp according to the comparative example. The cross section of FIG. 6 corresponds to the cross section depicted in FIG. 3. In this comparative example, in the cross section same as that in the first embodiment, the inner wall 41 of the casing body 21 and the inner wall 42 of the casing cover 22, which define the communication passage 33 therebetween, are in linear shapes extending from the arc passage 31 toward the outlet passage 32.

In this structure, the distance between the inner wall defining the communication passage 33 and the vanes 24 of the impeller 23 becomes large through the communication passage 33 toward the outlet passage 32. Accordingly, as referred to FIG. 4, the cross-sectional area in the communication passage 33 once drastically increases from the point A toward the point B, as the distance from A becomes large. Consequently, as referred to FIG. 5, the pump efficiency ηp decreases in the comparative embodiment, compared with the present structure. In particular, as fluidic pressure of discharged fluid increases, the pump efficiency ηp decreases in the comparative embodiment.

As described above, in this first embodiment, the inner wall 41 of the casing body 21 and the inner wall 42 of the casing cover 22, which define the communication passage 33 therebetween, are in the curved shapes such as the arc shapes, in cross section. In this structure, change in cross-sectional area becomes small in the communication passage 33, even as the vanes of the impeller 23 become distant from the arc passage 31 toward the outlet passage 32 through the communication passage 33. Therefore, pressure loss can be reduced in fluid, which is pumped through the arc passage 31, while passing through the communication passage 33, so that pressure of the discharged fluid can be enhanced, and the pump efficiency ηp can be enhanced.

Second Embodiment

FIG. 7 depicts a cross section that is taken along a surface, which includes the center axis L1 of the communication passage 33. This cross section is perpendicular to the center axis of the regenerative pump 10. In the second embodiment, as shown in FIG. 7, the casing body 21 and the casing cover 22, which define the communication passage 33 therebetween, have an inner wall 43 on the radially outer side. The inner wall 43 is in a curved shape. The cross-sectional area changes through the communication passage 33, as the distance between the impeller 23 and both the casing body 21 and the casing cover 22, which define the communication passage 33 therebetween, increases. In this structure, change in cross-sectional area of the communication passage 33 can be adjusted. Change in cross-sectional area of the communication passage 33 can be reduced by adjusting the inner wall 43, even in a structure in which the impeller 23 becomes distant from the inner wall 43 of both the casing body 21 and the casing cover 22. Thus, pressure loss in fluid passing through the communication passage 33 can be reduced, so that the pump efficiency ηp can be enhanced.

Third Embodiment

As shown in FIGS. 8, 9, the structure of the third embodiment is constructed by combining the structures of the first and second embodiments. In the third embodiment, the inner wall 41 of the casing body 21 and the inner wall 42 of the casing cover 22, which define the communication passage 33 therebetween, are in curved shapes in cross section depicted in FIG. 9. In addition, as shown in FIG. 8, the inner wall 43 of both the casing body 21 and the casing cover 22 is in a curved shape. The inner wall 43 is located on the radially outer side of the communication passage 33. Change in cross-sectional area of the communication passage 33 can be accurately adjusted by combining the inner wall 41 of the casing body 21, the inner wall 42 of the casing cover 22, and the inner wall 43 on the radially outer side, as appropriate. Thus, pressure loss in fluid passing through the communication passage 33 can be reduced, so that the pump efficiency rip can be enhanced.

The fluid discharged from the regenerative pump 10 is not limited to gas such as air. The fluid discharged from the regenerative pump 10 may be any other liquid such as fuel and water. The fluid discharged from the regenerative pump 10 may be two-phase fluid.

Various modifications and alternations may be diversely made to the above embodiments without departing from the spirit of the present invention.

Claims

1. A regenerative pump comprising:

a casing that has an inlet and an outlet, the casing defining a fluid passage that is in a substantially arc shape connecting the inlet with the outlet; and

an impeller that is rotatable in the casing, the impeller having a plurality of vanes each having a radial end defining an outer circular profile of the impeller, the plurality of vanes being faced to the fluid passage,

wherein the fluid passage includes an arc passage, an outlet passage, and a communication passage,

the arc passage is in a substantially arc shape along a circumferential direction of the casing through a predetermined angular range,

the outlet passage communicates with the outlet, and has a cross sectional area that is substantially constant with respect to a flow direction of fluid,

the communication passage connects the arc passage with the outlet passage, the casing has an inner wall that defines the communication passage,

the inner wall is in a substantially curved shape in an axial section that includes a center axis of the communication passage and extends along a center axis of the casing,

the center axis of the communication passage and the outer circular profile of the impeller define an intersection therebetween,

the communication passage has a vertical section, which includes the intersection, being perpendicular to the center axis of the communication passage, and

the inner wall reduces a cross-sectional area of the communication passage by extending through the vertical section.

2. The regenerative pump according to claim 1, wherein the inner wall defining the communication passage is in an arc shape.

3. The regenerative pump according to claim 1, wherein the inner wall defining the communication passage is in a cycloid curved shape.

4. The regenerative pump according to claim 1, wherein the inner wall defining the communication passage is in a parabolic curved shape.

5. A regenerative pump comprising:

a casing that has an inlet and an outlet, the casing defining a fluid passage that is in a substantially arc shape connecting the inlet with the outlet; and

an impeller that is rotatable in the casing, the impeller having a plurality of vanes each having a radial end defining an outer circular profile of the impeller, the plurality of vanes being faced to the fluid passage,

wherein the fluid passage includes an arc passage, an outlet passage, and a communication passage,

the arc passage is in a substantially arc shape along a circumferential direction of the casing through a predetermined angular range,

the outlet passage communicates with the outlet, and has a cross sectional area that is substantially constant with respect to a flow direction of fluid,

the communication passage connects the arc passage with the outlet passage, the casing has an inner wall that defines the communication passage,

the inner wall is in a substantially curved shape on a radially outer side in an axial section that includes a center axis of the communication passage and extends perpendicularly to a center axis of the casing,

the center axis of the communication passage and the outer circular profile of the impeller define an intersection therebetween,

the communication passage has a vertical section, which includes the intersection, being perpendicular to the center axis of the communication passage, and

the inner wall reduces a crosssectional area of the communication passage by extending through the vertical section.

6. The regenerative pump according to claim 5, wherein the inner wall defining the communication passage is in an arc shape.

7. The regenerative pump according to claim 5, wherein the inner wall defining the communication passage is in a cycloid curved shape.

8. The regenerative pump according to claim 5, wherein the inner wall defining the communication passage is in a parabolic curved shape.

9. A regenerative pump comprising:

a casing that has an inlet communicating with an outlet through an arc passage, a communication passage, and an outlet passage, the arc passage extending from the inlet circumferentially in the casing, the communication passage connecting the arc passage with the outlet passage, the outlet passage communicating with the outlet, and having a cross sectional area that is substantially constant with respect to a flow direction of fluid; and

an impeller that is rotatable in the casing, the impeller having a plurality of vanes each having a radial end defining an outer circular profile of the impeller, the plurality of vanes being faced to the fluid passage,

wherein the communication passage has a center axis extending substantially along the flow direction,

the center axis of the communication passage and the outer circular profile of the impeller define an intersection therebetween,

the communication passage has a vertical section, which includes the intersection, being perpendicular to the center axis of the communication passage,

the casing has an inner wall that defines the communication passage,

the communication passage has a height when being viewed from an axial section that includes the center axis of the communication passage and extends along a center axis of the casing, and

the inner wall is curved to reduce the height of the communication passage by extending through the vertical section.

10. A regenerative pump comprising:

a casing that has an inlet communicating with an outlet through an arc passage, a communication passage, and an outlet passage, the arc passage extending from the inlet circumferentially in the casing, the communication passage connecting the arc passage with the outlet passage, the outlet passage communicating with the outlet, and having a cross sectional area that is substantially constant with respect to a flow direction of fluid; and

an impeller that is rotatable in the casing, the impeller having a plurality of vanes each having a radial end defining an outer circular profile of the impeller, the plurality of vanes being faced to the fluid passage,

wherein the communication passage has a center axis extending substantially along the flow direction,

the center axis of the communication passage and the outer circular profile of the impeller define an intersection therebetween,

the communication passage has a vertical section, which includes the intersection, being perpendicular to the center axis of the communication passage,

the casing has an inner wall that defines the communication passage,

the inner wall is in a substantially curved shape on a radially outer side when being viewed from an axial section that includes a center axis of the communication passage and extends perpendicularly to a center axis of the casing, and

the inner wall is curved to reduce the cross sectional area of the communication passage by extending through the vertical section.