Axial-flow pump

Abstract

An axial-flow pump employs an increased boss ratio Dh/Dt, wherein Dh is the hub diameter of a rotary shaft of a rotor, and Dt is the tip diameter of impeller vanes of the rotor. Specifically, the boss ratio is 0.65 to 0.85, preferably 0.7 to 0.8.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an axial-flow pump having a stable pressure characteristic; for example, an axial-flow pump that can be favorably applied as an artificial heart pump.

2. Description of the Related Art

Conventionally, pumps of various kinds, such as pulsating pumps, turbo pumps, and roller pumps, have been used as artificial heart pumps. Turbo pumps are suited for reduction in size. Particularly, axial-flow pumps are most suited for reduction in size.

FIG. 4 is a schematic, sectional, structural view of a conventional axial-flow pump. As shown in FIG. 4, the axial-flow pump includes a cylindrical housing 1; a rotor 3, which is an impeller and is rotatably supported in the housing 1 in such a manner as to rotate about the axis X of the housing 1; and a drive mechanism for rotating the rotor 3. Rotation of the rotor 3 propels blood under pressure in the axial direction (in FIG. 4, from right to left). In FIG. 4, inline arrows show a major blood flow path.

The specific configuration of the conventional axial-flow pump will be described. A plurality of straightening vanes 4 are projectingly joined to the inner wall surface of the housing 1 at a position located upstream of the rotor 3. A cylindrical, upstream stationary member 5 is joined to the radially inner edges of the straightening vanes 4 coaxially with the axis X. A plurality of plate-like blades of a diffuser 6 are projectingly joined to the inner wall surface of the housing 1 at a position located downstream of the rotor 3. A cylindrical, downstream stationary member 7 is joined to the radially inner edges of the blades of the diffuser 6 coaxially with the axis X. The upstream end of the upstream stationary member 5 assumes the form of a round nose so as to smoothly divide and lead blood to the straightening vanes 4. The downstream end of the downstream stationary member 7 assumes the form of a round nose so as to smoothly merge divided flows of blood from the diffuser 6.

The downstream stationary member 7 contains a motor 10 (drive mechanism), which rotates about the axis X. A rotary shaft 8 is coupled with the motor 10, so that the rotary shaft 8 rotates about the axis X. A plurality of impeller vanes 9 are projectingly joined to the outer circumferential surface of the rotary shaft 8. The radially outer edges of the impeller vanes 9 closely oppose the inner wall surface of the housing 1. The rotary shaft 8 and the impeller vanes 9 constitute the rotor 3.

When the motor 10 is energized, the rotary shaft 8 and the impeller vanes 9, which constitute the rotor 3, rotate unitarily about the axis X in the interior of the housing 1. Accordingly, blood is taken into the housing 1, its flow is straightened by the straightening vanes 4, and its pressure is increased by the impeller vanes 9, whereby the blood becomes blood having dynamic pressure. Then, the diffuser 6 causes most of the blood having dynamic pressure to be restored to blood having static pressure, which is discharged from the housing 1. In this manner, the axial pump propels blood under pressure.

However, detailed study on the discharge pressure characteristic of the axial-flow pump have revealed that the discharge pressure characteristic is still not as desired. FIG. 5 is a schematic, partial, structural view of the conventional axial-flow pump. The outside diameter of the rotary shaft 8 of the rotor is called a hub diameter Dh; the outside diameter of the impeller vanes 9 of the rotor is called a tip diameter Dt; and their ratio Dh/Dt is called a boss ratio. In the conventional axial-flow pump, the boss ratio of the rotor is at most 0.5. Generally, the boss ratio tends to be further reduced in order to improve power efficiency, air diffusion efficiency, water supply efficiency, or a like factor (refer to Japanese Patent Application Laid-Open (kokal) Nos. H08-33896 and H05-253592).

FIG. 6 is a pair of conceptual views showing how fluid flows along the rotor (impeller) of the conventional axial-flow pump. As shown in FIG. 6(a), when the boss ratio is low; i.e., when the impeller vanes 9 occupy a large portion of the rotor, reverse flows arise in an outer circumferential portion of the inlet region of the impeller vanes 9 in operation at low flow rate. As a result, the pump head drops.

As shown in FIG. 6(b), when the flow rate lowers further from the level of FIG. 6(a), reverse flows arise not only in an outer circumferential portion of the inlet region of the impeller vanes 9, but also in an inner circumferential portion of the outlet region of the impeller vanes 9. As a result, a main flow of fluid is biased toward the outer circumference, whose rotational speed is high, of the impeller vanes 9, so that the centrifugal effect, as seen in a centrifugal pump or the like, causes an increase in pump head.

FIG. 7 is a graph showing a flow rate vs. pump head characteristic of the conventional axial-flow pump. As shown in FIG. 7, in operation at a flow rate lower than the design flow rate (a design point of operation), reverse flows in the inlet region cause a drop in pump head (see FIG. 6(a)), or the centrifugal effect causes an increase in pump head (see FIG. 6(b)). As a result, the axial-flow pump exhibits an unstable flow rate vs. pump head characteristic curve. Particularly, the centrifugal effect tends to greatly increase the non-discharge pump head as compared with the rated pump head (a pump head at the design point of operation).

As mentioned above, because of an unstable pressure characteristic, the conventional axial-flow pump has encountered difficulty in application to such a working condition as to require constant discharge pressure regardless of flow rate (for example, use as an artificial heart pump).

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is to provide an axial-flow pump having a stable pressure characteristic.

To achieve the above object, the present invention provides an axial-flow pump for propelling fluid under pressure by means of impeller vanes, the axial-flow pump having such a boss ratio as to suppress biased flow of the fluid caused by reverse flows arising in an outer circumferential portion of an inlet region of the impeller vanes and reverse flows arising in an inner circumferential portion of an outlet region of the impeller vanes.

Preferably, the boss ratio is 0.65 to 0.85.

The present invention further provides an axial-flow pump comprising a housing, an impeller provided rotatably in the housing, and a drive mechanism for rotating the impeller, and adapted to axially propel fluid under pressure by means of the drive mechanism rotating the impeller. In the axial-flow pump, the drive mechanism is fixedly attached to a downstream stationary member which, in turn, is fixedly attached to a diffuser located downstream of the impeller and projecting from an inner wall surface of the housing; the impeller comprises a rotary shaft coupled with the drive mechanism and impeller vanes projecting from the outer circumferential surface of the rotary shaft; and a boss ratio, which is the ratio between the outside diameter of the rotary shaft and the outside diameter of the impeller vanes, is 0.65 to 0.85.

The axial-flow pump of the present invention can exhibit a stable pressure characteristic and can reduce the degree of change in discharge pressure associated with a change in flow rate. Accordingly, the axial-pump of the present invention can be applied as, for example, an artificial heart pump, which requires a stable discharge pressure characteristic. Furthermore, application of the axial-flow pump of the present invention as an artificial heart pump can implement an artificial heart pump of reduced size.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiment when considered in connection with the accompanying drawings, in which:

FIG. 1 is a schematic, partial, structural view of an axial-flow pump according to an embodiment of the present invention;

FIG. 2 is a pair of conceptual views showing how fluid flows along the rotor (impeller) of the axial-flow pump according to the embodiment;

FIG. 3 is a graph showing a flow rate vs. pump head characteristic of the axial-flow pump according to the embodiment;

FIG. 4 is a schematic, sectional, structural view of a conventional axial-flow pump;

FIG. 5 is a schematic, partial, structural view of the conventional axial-flow pump;

FIG. 6 is a pair of conceptual views showing how fluid flows along the rotor (impeller) of the conventional axial-flow pump; and

FIG. 7 is a graph showing a flow rate vs. pump head characteristic of the conventional axial-flow pump.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will next be described in detail with reference to the drawings.

An axial-flow pump according to the present embodiment, whose schematic, partial structure is shown in FIG. 1, has the same structure as that of the conventional axial-flow pump shown in FIG. 4. Thus, repeated description of the structure is omitted. As compared with the conventional axial-flow pump, the axial-flow pump of the present embodiment has a higher boss ratio. FIG. 1 corresponds to FIG. 5 and shows a partial, structural arrangement of a diffuser 6′, a downstream stationary member 7′, a rotary shaft 8′, and impeller vanes 9′ in the axial-flow pump of the present embodiment.

In the present embodiment, the rotary shaft 8′ and the impeller vanes 9′ constitute a rotor, and the ratio between the hub diameter Dh of the rotary shaft 8′ and the tip diameter Dt of the impeller vanes 9′; i.e., the boss ratio Dh/Dt, is 0.65 to 0.85.

As shown in FIG. 2, even at a high boss ratio of 0.65 to 0.85; i.e., even when a region of the rotor that is occupied by the impeller vanes 9′ is relatively small, reverse flows arise in an outer circumferential portion of the inlet region of the impeller vanes 9′ in operation at low flow rate (FIG. 2(a)). As a result, as shown in FIG. 3, even in the present embodiment, the pump head drops.

When the flow rate lowers further from the level of FIG. 2(a), reverse flows arise not only in an outer circumferential portion of the inlet region of the impeller vanes 9′, but also in an inner circumferential portion of the outlet region of the impeller vanes 9′ (FIG. 2(b)). As a result, a main flow of fluid is biased toward the outer circumference, whose rotational speed is high, of the impeller vanes 9′, so that the centrifugal effect causes an increase in pump head. However, since the boss ratio is relatively high, the biasing degree of fluid flow (the degree of a change in fluid flow associated with a change in flow rate) is small as compared with the conventional axial-flow pump, as shown in FIG. 2(b). This indicates that the axial-flow pump of the present embodiment suppresses occurrence of the centrifugal effect.

A boss ratio of 0.65 or higher, preferably 0.7 or higher, is effective for suppressing occurrence of the centrifugal effect and thus for stabilizing the pressure characteristic of an axial-flow pump. As shown in FIG. 3, the axial-flow pump having such a boss ratio exhibits a stable pressure characteristic over a wide flow rate range including a design flow rate (a design point of operation).

As the boss ratio increases, the centrifugal effect is suppressed more reliably, and thus a pressure characteristic becomes stabler. However, increasing the boss ratio reduces the cross-sectional area of a fluid flow path in the axial-flow pump. Thus, at high flow rate, friction loss increases, causing a drop in pump head. As a result, when the boss ratio becomes too high, the ratio between the non-discharge pump head and the rated pump head increases, and thus a pressure characteristic becomes unstable. Therefore, the boss ratio must be 0.85 or less, preferably 0.8 or less.

In selection of the boss ratio Dh/Dt, there are selected the hub diameter Dh of the rotary shaft 8′ of the rotor and the tip diameter Dt of the impeller vanes 9′ of the rotor. In order to select a high boss ratio, the hub diameter Dh and the tip diameter Dt are selected in the following manners: while the tip diameter Dt is held constant, the hub diameter Dh is increased; while the hub diameter Dh is held constant, the tip diameter Dt is decreased; both of the hub diameter Dh and the tip diameter Dt are increased in such a manner that the hub diameter Dh is increased at a higher rate than the tip diameter Dt; and both of the hub diameter Dh and the tip diameter Dt are decreased in such a manner that the tip diameter Dt is decreased at a higher rate than the hub diameter Dh. A high boss ratio may be selected in any of the above manners in view of the degree of the above-mentioned effect of an increase in the boss ratio, the size of an axial-flow pump, and the cross-sectional area of a fluid flow path in the axial-flow pump.

For example, in design of an axial-flow pump, when the hub diameter Dh is increased while the tip diameter Dt is held constant (first manner), there is obtained a small-sized axial-flow pump that is equivalent in size to the conventional axial-flow pump while having a high boss ratio and a stable pressure characteristic. In this case, since the cross-sectional area of a fluid flow path in the axial-flow pump is smaller than the conventional axial-flow pump, the axial-flow pump is suited for such an application that accepts an increase in frictional loss.

When the tip diameter Dt is decreased while the hub diameter Dh is held constant (second manner), there is obtained an axial-flow pump that is smaller in size than the conventional axial-flow pump while having a high boss ratio and a stable pressure characteristic. In this case, the cross-sectional area of a fluid flow path in the axial-flow pump becomes smaller than in the case of the first manner.

When both of the hub diameter Dh and the tip diameter Dt are increased in such a manner that the hub diameter Dh is increased at a higher rate than the tip diameter Dt (third manner), there is obtained an axial-flow pump that is larger in the cross-sectional area of a flow path than the conventional axial-flow pump while having a high boss ratio and a stable pressure characteristic. In this case, since a large tip diameter Dt is selected, the size of the axial-flow pump becomes large to some extent.

When both of the hub diameter Dh and the tip diameter Dt are decreased in such a manner that the tip diameter Dt is decreased at a higher rate than the hub diameter Dh (fourth manner), there is obtained an axial-flow pump that is smaller in size than the conventional axial-flow pump while having a high boss ratio and a stable pressure characteristic. In this case, the cross-sectional area of a fluid flow path in the axial-flow pump becomes smaller than in the case of the third manner.

Furthermore, by means of increasing both of the hub diameter Dh and the tip diameter Dt, a higher boss ratio can be selected while the cross-sectional area of a flow path in an axial-flow pump is held unchanged. For example, in the case of the hub diameter Dh=7 mm and the tip diameter Dt=10 mm, the boss ratio is 0.7, and the cross-sectional area of the flow path is about 51π mm² (=10²π−7²π) . By contrast, in the case of the hub diameter Dh=9.64 mm and the tip diameter Dt=12 mm, the boss ratio is 0.803, and the cross-sectional area of the flow path is about 51π mm² (=12²π−9.64²π). In other words, the-boss ratio-can be increased while the cross-sectional area of a fluid flow path is held unchanged. In this case, since the larger tip diameter Dt is selected, the size of the axial-flow pump becomes larger to some extent.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.

Claims

1. An axial-flow pump for propelling fluid under pressure by means of impeller vanes, the axial-flow pump having such a boss ratio as to suppress biased flow of the fluid caused by reverse flows arising in an outer circumferential portion of an inlet region of the impeller vanes and reverse flows arising in an inner circumferential portion of an outlet region of the impeller vanes.

2. An axial-flow pump according to claim 1, wherein the boss ratio is 0.65 to 0.85.

3. An axial-flow pump comprising a housing, an impeller provided rotatably in the housing, and a drive mechanism for rotating the impeller, and adapted to axially propel fluid under pressure by means of the drive mechanism rotating the impeller, wherein the drive mechanism is fixedly attached to a downstream stationary member which, in turn, is fixedly attached to a diffuser located downstream of the impeller and projecting from an inner wall surface of the housing; the impeller comprises a rotary shaft coupled with the drive mechanism and impeller blades projecting from an outer circumferential surface of the rotary shaft; and a boss ratio, which is the ratio between an outside diameter of the rotary shaft and an outside diameter of the impeller vanes, is 0.65 to 0.85.