Diffuser for centrifugal compressor and method of producing the same

Abstract

At least the throat of the suction surface of a conventionally-used diffuser blade having a two-dimensional shape is cut into a concave shape with a ball end mill. In the throat, the nearer to the bottom of the concave shape, the higher the gas pressure becomes, and therefore the gas flows from the concave bottom toward the hub end and the shroud end thereby to reduce the thickness of the boundary layer formed on the hub end and the shroud end.

Description

TECHNICAL FIELD

The present invention relates to a diffuser for a centrifugal compressor and a fabricating method and, in particular, to a diffuser for a centrifugal compressor and a fabricating method capable of reducing both the incidence loss.

BACKGROUND ART

A centrifugal compressor is well known as a type of compressor for compressing and increasing the pressure of a gas.

FIG. 1 is a sectional view of a centrifugal compressor comprising an impeller 12 mounted on a rotary shaft 11 and a casing 13. The casing 13 has formed therein an impeller accommodating portion, a diffuser portion and a spiral scroll.

A blade 14 is mounted on the impeller 12, and the gas flowing in along the direction of arrow 15 is accelerated by the impeller 12 and flows out along the radial direction 16 of the centrifugal compressor. The diffuser portion on the outer periphery of the impeller 12 includes a diffuser blade 17 for converting the dynamic pressure of the fluid flowing out along the radial direction 16 into a static pressure. Specifically, the diffuser blade 17 is mounted on an annular disk 18 fitted in the front inner wall of the diffuser portion of the casing 13, and the diffuser blade 17 extends toward the rear inner wall (along the height) of the diffuser portion in the casing.

FIG. 2 is a perspective view of the conventional diffuser comprising a plurality of two-dimensional diffuser blade 17 mounted on the annular disk 18. The cross section A-A of the diffuser blade 17 is rectangular. The connection between each side wall of the diffuser blade 17 and the annular disk 18 is defined by an arcuate surface, and the curves connecting the center of the radius of curvature of the arcuate surfaces extend smoothly along the diffuser blade 17 and intersect to each other. At the upstream side of the diffuser blade 17, a ridge line is formed by the intersection of the arcuate surfaces to prevent stalling and surging. This conventional diffuser blade is proposed and disclosed in, for example, Japanese Unexamined Patent Publication No. 10-77997.

The gas flowing out of the impeller 12 flows into the diffuser portion in the direction of arrow 20. The angle between the velocity vector of the gas and an axis tangent to the circumference of the annular disk 18 is referred to as flow angle α. The distribution of the flow angle α along the height of the diffuser blade has a parabolic shape wherein the angle is substantially zero at one side where the diffuser blade 17 is mounted to the annular disk 18 (shroud end S) and at the other side rear the inner wall of the casing (hub end H), while the flow angle α is positive in the intermediate area therebetween.

FIG. 3 is a graph showing the distribution of the flow angle along the height of the diffuser blade, in which the abscissa is the height of the diffuser blade and the ordinate is the flow angle α. Each diffuser blade 17 is mounted obliquely on the annular disk 18 in a direction (length direction) from the impeller of the diffuser mounted portion toward the scroll. The surface impinged by the gas flowing into the diffuser 17 is called a pressure surface, and the surface opposite to the pressure surface a suction surface. The angles of the suction surface and the pressure surface relative to the axis tangent to the circumference of the annular disk 18 are referred to as a suction surface blade angle α_ksuc and a pressure surface blade angle α_kpre, respectively. The suction surface blade angle α_ksuc and the pressure surface blade angle α_kpre are substantially constant over the height of the diffuser blade 17.

The difference (α−α_ksuc) between the flow angle α and the suction surface blade angle α_ksuc is defined as an incidence In. The incidence In becomes negative near the hub end H and the shroud end S (hatched portions in FIG. 3) of the diffuser blade 17. As a result, the boundary layer thickness increases, which results in increase in a loss on the suction surface near the hub end H and the shroud end S of the diffuser blade 17.

DISCLOSURE OF THE INVENTION

The present invention has been achieved in view of the problems described above, and the object thereof is to provide a diffuser for the centrifugal compressor and a fabrication method capable of reducing the pressure loss by suppressing the generation of the boundary layer at the suction surface near the hub end and the shroud end of each diffuser blade.

According to a first aspect of the invention, there is provided a diffuser for a centrifugal compressor comprising a diffuser blade including a suction surface which is concave toward a pressure surface in a cross section, perpendicular to the flow direction.

According to a second aspect of the invention, there is provided a diffuser for a centrifugal compressor comprising a diffuser blade including a suction surface which defines an acute angle, in a cross section perpendicular to the flow direction, in relation to at least one of a shroud surface and a hub surface.

According to a third aspect of the invention, there is provided a diffuser for a centrifugal compressor comprising a diffuser blade including a suction surface which is concave toward a pressure surface in a cross section, perpendicular to the flow direction at least in a throat portion and a portion upstream thereof.

According to a fourth aspect of the invention, there is provided a diffuser for a centrifugal compressor comprising a suction surface which defines an acute angle, in a cross section perpendicular to the flow direction at least in a throat portion and a portion upstream thereof, in relation to at least one of a shroud surface and a hub surface.

According the first to fourth aspects of the invention, the suction surface is formed into a concave surface at least in the throat portion to suppress the generation of a boundary layer near the shroud surface and the hub surface.

According to a fifth aspect of the invention, there is provided a method of fabricating a diffuser for a centrifugal compressor, comprising the steps of fabricating a two-dimensional blade; and cutting one of the side surfaces of the blade, fabricated in the preceding step, into a concave shape with a ball end mill.

In the fifth aspect of the invention, the diffuser blade is fabricated by cutting one side surface of the diffuser blade having a two-dimensional shape with a ball end mill.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a centrifugal compressor.

FIG. 2 is a perspective view of a diffuser conventionally used.

FIG. 3 is a graph showing the distribution of the flow angle along the height of the diffuser blade.

FIG. 4A is a perspective view of a diffuser blade according to the invention.

FIG. 4B is a sectional view of a diffuser blade taken in line I in FIG. 4A.

FIG. 4C is a sectional view of a diffuser blade taken in line II in FIG. 4A.

FIG. 4D is a sectional view of a diffuser blade taken in line III in FIG. 4A.

FIG. 4E is a sectional view of a diffuser blade taken in line IV in FIG. 4A.

FIG. 4F is a sectional view of a diffuser blade taken in line V in FIG. 4A.

FIG. 5 is a graph showing the distribution of the flow angle along the height of a diffuser blade according to the invention.

FIG. 6 is a sectional view of a second diffuser blade used with a diffuser according to the invention.

FIG. 7A is a top plan view of a diffuser according to the invention.

FIG. 7B is a sectional view of a diffuser blade taken in line B-B in FIG. 7A.

FIGS. 8A to 8C are diagrams showing the steps of fabricating a diffuser blade used with a diffuser according to the invention.

FIG. 8D is a sectional view showing the portion indicated by III in FIG. 8C.

FIG. 8E is a sectional view showing the portions indicated by II and IV in FIG. 8C.

FIG. 8F is a sectional view showing the portions indicated by I and V in FIG. 8C.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 4A is a perspective view of a diffuser blade used in a diffuser according to the invention, and FIGS. 4B to 4F are sectional views from the upstream side taken in lines I to V, respectively. A suction surface 401 of a diffuser blade 4 is formed into a concave surface toward an opposite pressure surface 402. Specifically, at least one of angle δH which is defined by the suction surface and inner wall of the casing near a hub end H and angle δS which is defined by the suction surface and, an annular disk near a shroud end S of the diffuser blade 4 is acute.

In FIGS. 4A to 4F, the suction surface is formed as a concave surface toward the pressure surface.

FIG. 5 is a graph showing the distribution of the flow angle along the height of the diffuser blade. The suction surface angle α_ksuc decreases near the hub end H and the shroud end S of the diffuser blade 4 and, on the other hand, increases at the central portion along the height of the diffuser blade 4. Therefore, the incidence In=(α_ksuc−α) of the diffuser blade according to the invention assumes a negative value near the hub end H and the shroud end S of the diffuser blade 4, while the absolute value of the incidence In is smaller than the conventional diffuser blade.

This indicates that the boundary layer thickness of the suction surface near the hub end H and the shroud end S of the diffuser blade 4 is reduced thereby leading to a smaller incidence loss than the conventional diffuser blade.

FIG. 6 is a sectional view of a second diffuser blade according to the invention, which is configured of not a curved surface but of bent flat surfaces.

FIG. 7A is a top plan view of the diffuser according to the invention, and FIG. 7B a sectional view of the diffuser blade along line B-B in FIG. 7A. The constant-pressure curves of the gas between the two diffuser blades 41, 42 are shown in FIG. 7A. Area between the normal line C-C to the center line along the length of the first diffuser blade 41 from the upstream end of the second diffuser blade 42 and the upstream end of the first diffuser blade 41 is defined as a throat, and the length from the upstream end of the first diffuser blade 41 to the normal line C-C is defined as a throat length XT. In the throat area from the upstream end of the first diffuser blade 41 to the throat line C-C, the pressure distribution is in the shape of a fan centered at the upstream end of the second diffuser blade 42. The pressure in the area “a” connecting the upstream ends of the blade is low, while the pressure in the area “b” connecting the upstream end of the second diffuser blade 42 and the center of the throat area of the first diffuser blade 42 is high.

FIG. 7B is a sectional view along line B-B perpendicular to the center line along the length of the first diffuser blade 41 in the throat area. The gas pressure distribution is such that the higher the pressure, the nearer to the first diffuser blade 41. Thus, the gas develops a secondary flow 43 from the suction surface of the first diffuser blade 41 toward the pressure surface of the second diffuser blade 42. As a result, the boundary layer thickness on the suction surface of the first diffuser blade 41 near the hub end H and the shroud end S is reduced while. Therefore the incidence loss is reduced.

In the area downstream of the throat area where the first diffuser blade 41 and the second diffuser blade 42 overlap each other, the constant-pressure curves assume a form perpendicular to the center line along the length of the first diffuser blade 41 and the second diffuser blade 42. Therefore, the gas is prevented from flowing from the suction surface of the first diffuser blade 41 toward the pressure surface of the second diffuser blade 42. Thus, it is especially important to form a throat and a concave suction surface at upstream of the throat, while the portion downstream of the throat is not required to be formed into a concave shape.

FIGS. 8A to 8F are diagrams for explaining the steps of fabricating a diffuser blade according to this invention. First, a two-dimensional diffuser blade having a rectangular cross section is fabricated (FIG. 8A).

Next, the suction surface of the two-dimensional diffuser blade is cut with the ball end mill thereby to form a concave suction surface. After cutting the two-dimensional diffuser blade along one end thereof (FIG. 8B), a concave suction surface is formed by cutting along the other end (FIG. 8C). The angles δH and δS that the suction surface forms with the hub surface and the shroud surface, respectively, of the diffuser blade are acute.

The cutting depth of the ball end mill being constant, the steps of fabricating the diffuser blade according to this invention are simplified.

Normally, a two-dimensional diffuser blade has the greatest thickness at the longitudinally central portion thereof, and therefor the cross section of the diffuser blade according to the invention is symmetric longitudinally about the central portion (III) thereof. Specifically, the cross section of the central portion (III) is shaped as shown in FIG. 8D. The cross sections of the portions (II) and (IV) are shaped as shown in FIG. 8E, and the cross sections of the portions (I) and (V) as shown in FIG. 8F.

The embodiment of the invention described above refers to an application of the diffuser according to the invention to a centrifugal compressor. It is apparent, however, that the diffuser according to this invention is also applicable to a centrifugal blower and a centrifugal pump with equal effect.

With the diffuser for a centrifugal compressor according to this invention, the boundary layer generated on the suction surface near the shroud end and the hub end of the diffuser blade is reduced, whereby the incidence loss is reduced.

In the method of fabricating the diffuser for the centrifugal compressor according to this invention, the diffuser blade can be easily fabricated by cutting a two-dimensional diffuser blade with a ball end mill.

Claims

1. A diffuser for a centrifugal compressor comprising a diffuser blade including a suction surface which is concave toward a pressure surface in a cross section, perpendicular to the flow direction.

2. A diffuser for a centrifugal compressor comprising a diffuser blade including a concave suction surface which defines an acute angle, in a cross section perpendicular to the flow direction, in relation to at least one of a shroud surface and a hub surface.

3. A diffuser for a centrifugal compressor comprising a diffuser blade including a suction surface which is concave toward a pressure surface in a cross section, perpendicular to the flow direction at least in a throat portion or a portion upstream thereof.

4. A diffuser for a centrifugal compressor comprising a concave suction surface which defines an acute angle, in a cross section perpendicular to the flow direction at least in a throat portion or a portion upstream thereof, in relation to at least one of a shroud surface and a hub surface.

5. A method of fabricating a diffuser for a centrifugal compressor comprising the steps of fabricating a two-dimensional blade, and cutting one of the side surfaces of the blade, fabricated in the preceding step, into a concave shape with a ball-end mill.