阅读说明：本技术 离心压缩机以及具备该离心压缩机的涡轮增压器 (Centrifugal compressor and turbocharger provided with same ) 是由 岩切健一郎 于 2017-11-06 设计创作，主要内容包括：至少一个凹状圆弧部分中的在叶轮的半径方向上处于最外侧的凹状圆弧部分的叶轮的半径方向外侧的端缘的切线方向与垂直于旋转轴线的方向所成的倾斜角度沿着涡旋流路的周向具有分布,当用以舌状部为基准而以旋转轴线为中心的中心角来表示从涡旋部的舌状部朝向涡旋流路的出口的涡旋流路内的周向位置时,倾斜角度的分布在从30°到210°的中心角的范围内具有极小值或最小值。(An inclination angle formed by a tangential direction of an edge of the concave arc portion located outermost in a radial direction of the impeller, the edge being located on an outer side in the radial direction of the impeller, of the at least one concave arc portion, and a direction perpendicular to the rotation axis has a distribution along a circumferential direction of the scroll flow path, and when a circumferential position in the scroll flow path from the tongue portion of the scroll portion toward an outlet of the scroll flow path is expressed by a central angle centered on the rotation axis with respect to the tongue portion, the distribution of the inclination angle has a minimum value or a minimum value in a range of the central angle from 30 ° to 210 °.)

1. A centrifugal compressor comprises an impeller and a housing, wherein,

The housing includes:

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

a diffuser portion including a pair of flow path walls provided at an interval in an extending direction of a rotation axis of the impeller, the diffuser portion forming a diffuser flow path communicating with the scroll flow path along a circumferential direction of the scroll flow path on an inner side in a radial direction of the impeller between the pair of flow path walls,

The pair of flow path walls includes:

A first flow path wall; and

A second flow path wall located on a scroll center side of the scroll flow path with respect to the first flow path wall in an extending direction of the rotation axis,

The second flow path wall includes a part of an inner wall surface located on the inner side in the radial direction among inner wall surfaces of the scroll flow path, the inner wall surface of the second flow path wall constitutes at least one concave circular arc portion having a radius of curvature in the scroll flow path in a cross section of the housing formed by a plane including the rotation axis,

An inclination angle formed by a tangential direction of an end edge of the concave arc portion located outermost in a radial direction of the impeller and a direction perpendicular to the rotation axis of the impeller, among the at least one concave arc portion, and a direction perpendicular to the rotation axis of the impeller, is distributed along a circumferential direction of the scroll flow path,

When a circumferential position within the scroll flow path from the tongue portion of the scroll portion toward an outlet of the scroll flow path is expressed by a central angle centered on the rotation axis with reference to the tongue portion, the distribution of the inclination angles has a minimum value or a minimum value in a range of the central angle from 30 ° to 210 °.

2. the centrifugal compressor of claim 1,

The distribution of the inclination angles has a minimum value or a minimum value in a range of the central angle from 30 ° to 120 °.

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

The second flow path wall includes:

A flat inner wall surface that defines the diffuser flow path and is perpendicular to the rotation axis;

A convex inner wall surface that defines the scroll flow path and is curved in a convex shape with respect to the scroll flow path;

At least one concave inner wall surface that defines the scroll flow path and constitutes the at least one concave circular arc portion in a cross section of the casing formed by a plane including the rotation axis, the concave inner wall surface that is outermost in a radial direction of the impeller being connected to the convex circular arc portion; and

An end surface connecting the flat inner wall surface and the convex inner wall surface at an outermost side of the flat inner wall surface in a radial direction of the impeller.

4. a centrifugal compressor according to any one of claims 1 to 3,

An outer diameter of the diffuser flow path centered on the rotation axis has a distribution in a circumferential direction of the diffuser flow path, the distribution of the outer diameter of the diffuser flow path having a maximum value or a maximum value in a range of the center angle from 30 ° to 210 °.

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

A distance from the rotation axis to a vortex center of the vortex flow path has a distribution in a circumferential direction of the vortex flow path, the distribution of the distance having a minimum value or a minimum value in a range of the center angle from 30 ° to 210 °.

6. a turbocharger comprising the centrifugal compressor according to any one of claims 1 to 5.

Technical Field

The present invention relates to a centrifugal compressor and a turbocharger provided with the centrifugal compressor.

Background

In recent years, it has been required to expand the operating region of a centrifugal compressor. For example, in an automobile engine, it is required to improve fuel efficiency and acceleration performance in a low speed region, and along with this, the turbocharger is also required to improve efficiency at a low speed and low flow rate operating point. Such an operating region is a region in which the centrifugal compressor of the turbocharger operates in a stall state, and in this region, it is confirmed that large-scale separation occurs in the scroll flow path. Patent document 1 describes that peeling occurs in a scroll flow path due to a recirculation flow from the end of winding to the start of winding.

Prior art documents

Patent document

Patent document 1: international publication No. 2017/109949

Disclosure of Invention

Problems to be solved by the invention

However, the present inventors have conducted extensive studies and, as a result, have clarified the occurrence of peeling caused by another factor different from the recirculation flow described in patent document 1. That is, the compressed air discharged from the diffuser flow path in the vicinity of the tongue portion of the scroll portion in which the scroll flow path is formed flows through the scroll flow path while swirling along the inner wall surface of the scroll flow path, and such a swirling flow interferes with the compressed air discharged from the diffuser flow path in the vicinity of just one revolution along the inner wall surface of the scroll flow path. This causes separation in the scroll flow path.

In view of the above circumstances, an object of at least one embodiment of the present invention is to provide a centrifugal compressor in which efficiency at a low-flow operation point is improved, and a turbocharger equipped with the centrifugal compressor.

Means for solving the problems

(1) a centrifugal compressor according to at least one embodiment of the present invention includes an impeller and a casing, wherein the casing includes:

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

A diffuser portion including a pair of flow path walls provided at an interval in an extending direction of a rotation axis of the impeller, the diffuser portion forming a diffuser flow path communicating with the scroll flow path along a circumferential direction of the scroll flow path on an inner side in a radial direction of the impeller between the pair of flow path walls,

The pair of flow path walls includes:

a first flow path wall; and

A second flow path wall located on a scroll center side of the scroll flow path with respect to the first flow path wall in an extending direction of the rotation axis,

The second flow path wall includes a part of an inner wall surface located on the inner side in the radial direction among inner wall surfaces of the scroll flow path, the inner wall surface of the second flow path wall constitutes at least one concave circular arc portion having a radius of curvature in the scroll flow path in a cross section of the housing formed by a plane including the rotation axis,

An inclination angle formed by a tangential direction of an end edge of the concave arc portion located outermost in a radial direction of the impeller and a direction perpendicular to the rotation axis of the impeller, among the at least one concave arc portion, and a direction perpendicular to the rotation axis of the impeller, is distributed along a circumferential direction of the scroll flow path,

When a circumferential position within the scroll flow path from the tongue portion of the scroll portion toward an outlet of the scroll flow path is expressed by a central angle centered on the rotation axis with reference to the tongue portion, the distribution of the inclination angles has a minimum value or a minimum value in a range of the central angle from 30 ° to 210 °.

According to the configuration of the above (1), since the angle formed by the direction of the swirling flow and the flow direction of the compressed fluid discharged from the diffuser flow path is reduced in the vicinity of the swirling flow that circulates along the inner wall surface of the scroll flow path by just one revolution along the inner wall surface of the scroll flow path, interference between the swirling flow and the flow of the compressed fluid discharged from the diffuser flow path can be suppressed, and the occurrence of separation in the scroll flow path can be reduced. As a result, the efficiency of the centrifugal compressor at the low flow operating point can be improved.

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

the distribution of the inclination angles has a minimum value or a minimum value in a range of the central angle from 30 ° to 120 °.

the flow path area of the scroll flow path decreases from the outlet side toward the tongue portion. Due to the shape of the scroll flow path, the inclination angle of the concave arc portion tends to be larger as the tongue portion is closer. According to the configuration of the above (2), when the scroll flow path is formed without being aware of the magnitude of the inclination angle, the scroll flow path is formed so that the inclination angle has the minimum value or the minimum value in the range of the central angle from 30 ° to 120 ° where the inclination angle tends to become large, and the inclination angle can be reduced in the range of the central angle from 30 ° to 210 °, so that interference between the swirling flow and the compressed fluid flow discharged from the diffuser flow path can be further suppressed, and the occurrence of separation in the scroll flow path can be further reduced. As a result, the efficiency of the centrifugal compressor at the low flow operating point can be improved.

(3) in several embodiments, in the structure of the above (1) or (2),

The second flow path wall includes:

A flat inner wall surface that defines the diffuser flow path and is perpendicular to the rotation axis;

A convex inner wall surface that defines the scroll flow path and is curved convexly with respect to the scroll flow path;

At least one concave inner wall surface that defines the scroll flow path and constitutes the at least one concave circular arc portion in a cross section of the casing formed by a plane including the rotation axis, the concave inner wall surface that is outermost in a radial direction of the impeller being connected to the convex circular arc portion; and

An end surface connecting the flat inner wall surface and the convex inner wall surface at an outermost side of the flat inner wall surface in a radial direction of the impeller.

According to the configuration of the above (3), interference between the swirling flow and the compressed fluid flow discharged from the diffuser flow path can be suppressed, occurrence of separation in the scroll flow path can be reduced, and processing of the diffuser flow path can be facilitated by making the inner wall that partitions the diffuser flow path perpendicular to and flat with respect to the rotation axis.

(4) In some embodiments, in any of the structures (1) to (3) above,

An outer diameter of the diffuser flow path centered on the rotation axis has a distribution in a circumferential direction of the diffuser flow path, the distribution of the outer diameter of the diffuser flow path having a maximum value or a maximum value in a range of the center angle from 30 ° to 210 °.

According to the configuration of the above (4), since the inclination angle of the concave arc portion can be made extremely small or minimum within the range of the central angle from 30 ° to 210 °, interference between the swirling flow and the compressed fluid flow discharged from the diffuser flow path can be suppressed, and occurrence of separation in the scroll flow path can be reduced. As a result, the efficiency of the centrifugal compressor at the low flow operating point can be improved.

(5) in some embodiments, in any of the structures (1) to (4) above,

a distance from the rotation axis to a vortex center of the vortex flow path has a distribution in a circumferential direction of the vortex flow path, the distribution of the distance having a minimum value or a minimum value in a range of the center angle from 30 ° to 210 °.

according to the configuration of the above (5), since the inclination angle of the concave arc portion can be made extremely small or minimum within the range of the central angle from 30 ° to 210 °, interference between the swirling flow and the compressed fluid flow discharged from the diffuser flow path can be suppressed, and occurrence of separation in the scroll flow path can be reduced. As a result, the efficiency of the centrifugal compressor at the low flow operating point can be improved.

(6) a turbocharger according to at least one embodiment of the present invention includes any one of the centrifugal compressors described in (1) to (5) above.

According to the configuration of the above (6), since the occurrence of separation in the scroll flow path can be reduced, the efficiency of the turbocharger at a low-speed and low-flow operating point can be improved.

ADVANTAGEOUS EFFECTS OF INVENTION

According to at least one embodiment of the present invention, since the angle between the direction of the swirling flow and the flow direction of the compressed fluid discharged from the diffuser flow path is reduced in the vicinity of the swirling flow that circulates along the inner wall surface of the scroll flow path in the vicinity of just one revolution along the inner wall surface of the scroll flow path, interference between the swirling flow and the flow of the compressed fluid discharged from the diffuser flow path can be suppressed, and the occurrence of separation in the scroll flow path can be reduced. As a result, the efficiency of the centrifugal compressor at the low flow operating point can be improved.

Drawings

fig. 1 is a view schematically showing an example of a cross section perpendicular to a rotation axis of a centrifugal compressor according to an embodiment of the present invention.

Fig. 2 is a partial sectional view of a casing of the centrifugal compressor formed by a plane including a rotation axis of the centrifugal compressor according to the embodiment of the present invention.

Fig. 3 is a flow chart showing a state in which compressed air discharged from the diffuser flow path swirls along the inner wall surface of the scroll flow path in the centrifugal compressor according to the embodiment of the present invention.

Fig. 4 is a schematic diagram for explaining the principle in which the swirling flow interferes with the compressed air flow discharged from the diffuser flow path in the scroll flow path.

Fig. 5 is a schematic cross-sectional view showing a cross-sectional shape of a scroll flow path of a centrifugal compressor according to an embodiment of the present invention.

Fig. 6 is a graph showing the distribution of the inclination angle α in the centrifugal compressor according to the embodiment of the present invention.

Fig. 7 is a graph showing the distribution of the outer diameter of the diffuser flow path and the distribution of the inclination angle α in the centrifugal compressor according to the embodiment of the present invention.

Fig. 8 is a graph showing the distribution of the distance between the rotation axis and the scroll center and the distribution of the inclination angle α in the centrifugal compressor according to the embodiment of the present invention.

Fig. 9 is a partially enlarged sectional view of a second flow path wall of the centrifugal compressor according to the embodiment of the present invention.

Detailed Description

Hereinafter, several embodiments of the present invention will be described 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 components described in the following embodiments are not intended to limit the scope of the present invention to these, and are merely illustrative examples.

A centrifugal compressor according to an embodiment of the present invention will be described with reference to a centrifugal compressor of a turbocharger as an example. However, the centrifugal compressor in 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 any fluid may be used instead.

As shown in fig. 1, the centrifugal compressor 1 includes a casing 2 and an impeller 3 provided rotatably about a rotation axis L in the casing 2. As shown in fig. 2, the housing 2 has: a scroll portion 4 having a scroll-like scroll flow path 5 formed on the outer peripheral side of the impeller 3; a diffuser portion 6 including a pair of flow path walls 7, i.e., a first flow path wall 7a and a second flow path wall 7b, which are provided at intervals in the extending direction of the rotation axis L; and a cylindrical air inlet portion 9. The second flow path wall 7b is located at the scroll center O of the scroll flow path 5 with respect to the first flow path wall 7a in the extending direction of the rotation axis L_SAnd (3) side. A diffuser flow path 8 that communicates with the scroll flow path 5 along the circumferential direction of the scroll flow path 5 on the radially inner side of the impeller 3 is formed between the first flow path wall 7a and the second flow path wall 7 b.

The air flowing into the centrifugal compressor 1 through the air inlet 9 is compressed by the impeller 3 to become compressed air. The compressed air flows through the diffuser flow path 8, flows into the scroll flow path 5, then flows through the scroll flow path 5, and is discharged from the centrifugal compressor 1.

When the amount of air flowing into the centrifugal compressor 1 is small as in the case where the turbocharger operates at a low speed, the centrifugal compressor 1 operates in a stall state, and the efficiency decreases. In such a working region, it was confirmed that large-scale separation occurred in the scroll flow path 5. The present inventors have conducted extensive studies and, as a result, have found one of the main causes of such peeling. The principle of occurrence of peeling caused by this factor is described below.

As shown in fig. 1, the circumferential position in the scroll flow path 5 from the tongue-shaped portion 4a of the scroll portion 4 (see fig. 2) toward the outlet of the scroll flow path 5 is represented by a central angle θ centered on the rotation axis L with respect to the tongue-shaped portion 4 a. Therefore, the center angle θ indicating the circumferential position of the tongue-shaped portion 4a is 0 °.

as shown in fig. 3, the compressed air flow f discharged from the diffuser flow path 8 in the vicinity of the tongue-shaped portion 4a₁Flows through the scroll flow path 5 while swirling along the inner wall surface of the scroll flow path 5. The swirling flow f of the compressed air₂In the vicinity of just one revolution (in fig. 3, the vicinity of the central angle θ of 30 °) along the inner wall surface of the scroll flow path 5, the compressed air f discharged from the diffuser flow path 8 is mixed with the compressed air₃and (5) interference. This interference is one of the main causes of separation in the scroll flow path 5.

as shown in fig. 4(a), in the cross section of the casing 2 (see fig. 2) including the rotation axis L, as the inclination angle α between the tangential direction a of the portion 5a1 of the inner wall surface 5a of the scroll passage 5 connected to the second passage wall 7B and the direction B perpendicular to the rotation axis L is larger, that is, as closer to 90 °, the swirling flow f of the compressed air flowing along the inner wall surface 5a of the scroll passage 5 is as shown in fig. 4(B)₂With the compressed air flow f discharged from the diffuser flow path 8₃The larger the angle beta. Thus, due to the swirling flow f₂to block the compressed air flow f from the diffuser flow path 8 into the scroll flow path 5₃The interference is caused, and therefore, peeling occurs at a portion where the interference occurs.

Therefore, in order to suppress the occurrence of such interference, the cross-sectional shape of the scroll flow path 5 whose inclination angle α is made smaller is required. Fig. 5 shows an example of the cross-sectional shape of the scroll flow path 5 in which the inclination angle α is reduced. The second flow path wall 7b has: a flat inner wall surface 21 that defines the diffuser flow path 8 and is perpendicular to the rotation axis L and flat, a flat end surface 22 that is connected to the flat inner wall surface 21 at the outermost side in the radial direction of the flat inner wall surface 21 and is perpendicular to the flat inner wall surface 21, and a convex inner surface that is connected to the end surface 22 and is curved in a convex manner with respect to the scroll flow path 5The wall surface 23, and a concave inner wall surface 24 that is connected to the convex inner wall surface 23 and is curved concavely with respect to the scroll flow path 5. Here, it is assumed that the center of the vortex O passes_SAnd an imaginary line L 'parallel to the rotation axis L divides the inner wall surface 5a of the scroll passage 5 into a radially inner portion 5a2 and a radially outer portion 5a3 with respect to the imaginary line L'. The end surface 22, the convex inner wall surface 23, and the concave inner wall surface 24 are part of the portion 5a2 of the inner wall surface 5 a.

The convex curvature with respect to the scroll flow path 5 means that the center of curvature of the convex arc portion 23a formed by the convex inner wall surface 23 is located outside the scroll flow path 5 in the cross section of the casing 2 (see fig. 2) including the rotation axis L, and the concave curvature with respect to the scroll flow path 5 means that the center of curvature of the concave arc portion 24a formed by the concave inner wall surface 24 is located inside the scroll flow path 5 in the cross section of the casing 2 including the rotation axis L.

In the cross section of the housing 2 including the rotation axis L, when the inclination angle α formed by the tangential direction a of the radially outer end edge 24a1 of the concave arc portion 24a and the direction B perpendicular to the rotation axis L is decreased, the swirling flow f₂And a compressed air flow f which is about to flow into the scroll flow path 5 from the diffuser flow path 8₃The angle β becomes smaller. Thus, the swirling flow f can be suppressed₂with a flow of compressed air f₃therefore, the occurrence of peeling can be reduced. Therefore, the cross-sectional shape of the scroll flow path 5 is such that the inclination angle α is reduced at the portion where such interference occurs, whereby the occurrence of separation can be reduced.

The present inventors have also obtained a result that peeling is likely to occur in the range of the central angle θ from 30 ° to 210 ° by performing CFD analysis. This is because, when a stable swirling flow is formed in the scroll flow path 5, the swirling flow in the scroll flow path 5 and the compressed air flow discharged from the diffuser flow path 8 gradually do not interfere with each other any more, and therefore such interference tends to occur on the upstream side of the scroll flow path 5. Therefore, by making the cross-sectional shape of the scroll flow path 5a shape in which the upstream-side inclination angle α becomes smaller, the occurrence of separation can be effectively reduced.

The cross-sectional shape of the scroll flow path 5 shown in fig. 5 is a shape in one cross-section of the casing 2 (see fig. 2). The cross-sectional shape of the scroll flow path 5 actually changes in the circumferential direction. Therefore, the inclination angle α varies according to the circumferential direction. That is, the inclination angle α has a distribution along the circumferential direction of the scroll flow path 5. Therefore, according to the above findings obtained by the present inventors, as shown in fig. 6, the distribution of the inclination angle α has the minimum value in the range of the circumferential position of the scroll flow path 5 in which the central angle θ is in the range from 30 ° to 210 °, and the occurrence of separation can be effectively reduced. The distribution of the inclination angle α may have a minimum value, that is, a distribution of minimum values, in a range of the central angle θ from 30 ° to 210 °, instead of the minimum value in the above range. In other words, the distribution of the inclination angle α may also have a value smaller than the minimum value within the range of the central angle θ after 210 °.

Next, several embodiments of the structure of the housing 2 (see fig. 2) for obtaining the minimum value or the minimum value of the distribution of the inclination angle α in the range of the central angle θ from 30 ° to 210 ° will be described.

in one embodiment, the outer diameter of the diffuser channel 8 (see fig. 1) is locally increased in the circumferential direction. That is, the distribution of the outer diameters of the diffuser channels 8 in the circumferential direction is made to have maximum values or maximum values within the range of the central angle θ from 30 ° to 210 °. Referring to fig. 5, in a portion of the diffuser channel 8 where the outer diameter is locally large, the position of the end surface 22 of the second channel wall 7b is located radially outward of the other portions. Thus, since the width of the concave inner wall surface 24 in the radial direction can be increased, the inclination of the portion 5a1 in the tangential direction a is closer to the horizontal direction, and the inclination angle α is smaller.

Fig. 7 shows a graph showing the distribution of the outer diameters of the diffuser channels 8 in the circumferential direction and a graph showing the distribution of the inclination angle α in this case. When the outer diameter of the diffuser flow path 8 is maximized within the range of the central angle θ from 30 ° to 210 °, the inclination angle α is minimized within the range of the central angle θ from 30 ° to 210 °. If the inclination angle α is not the minimum value but takes the minimum value within this range, the outside diameter of the diffuser flow path 8 may be maximized within the range of the central angle θ from 30 ° to 210 °.

In another embodiment, the rotation axis L is directed to the scroll center O of the scroll flow path 5_SThe distance R (see fig. 2) is locally reduced in the circumferential direction. That is, the distribution of the distance R in the circumferential direction is made to take a minimum value or a minimum value in the range of the central angle θ from 30 ° to 210 °. Referring to fig. 5, in a portion where the distance R is locally small, the cross section of the scroll passage 5 is located radially inward of the other portions, although the position of the outlet of the diffuser passage 8 is the same. Then, the inclination of the tangential direction a of the portion 5a1 is closer to the horizontal direction, and therefore, the inclination angle α becomes smaller.

fig. 8 shows a graph showing the distribution of the distance R in the circumferential direction and a graph showing the distribution of the inclination angle α in this case. When the distance R is the smallest in the range of the center angle θ from 30 ° to 210 °, the inclination angle α is the smallest in the range of the center angle θ from 30 ° to 210 °. If the inclination angle α is not the minimum value but takes an extremely small value within this range, the distance R is extremely small within the range of the central angle θ from 30 ° to 210 °.

In another embodiment, the outer diameter of the diffuser flow path 8 (see fig. 1) is locally increased in the circumferential direction and the rotation axis L is extended to the scroll center O of the scroll flow path 5_SIs locally reduced in the circumferential direction (see fig. 2). If the diffusion flow path 8 is separately formed, the outer diameter is locally too large or the distance R is locally too small, which may make the manufacturing difficult or adversely affect the compressed air flow. However, by combining the two, local changes in the outer diameter and the distance R of the diffuser channel 8 can be alleviated.

thus, the swirling flow f that circulates while swirling along the inner wall surface 5a of the scroll passage 5 in the scroll passage 5 flows₂The swirling flow f is formed in the vicinity of just one revolution along the inner wall surface 5a of the scroll passage 5₂And the compressed fluid flow f discharged from the diffuser flow path 8₃Becomes smaller because of the angle beta formed by the directions ofThis can suppress the swirling flow f₂With the flow f of compressed fluid discharged from the diffuser flow path₃The occurrence of separation in the scroll flow path 5 can be reduced. As a result, the efficiency of the centrifugal compressor 1 at the low flow rate operation point can be improved.

In the above embodiment, the second flow path wall 7b has the flat end surface 22 connected to the flat inner wall surface 21 perpendicularly, the convex inner wall surface 23 connected to the end surface 22 and curved convexly with respect to the scroll flow path 5, and the concave inner wall surface 24 connected to the convex inner wall surface 23 and curved concavely with respect to the scroll flow path 5, but is not limited to this form. The end surface 22 may not be perpendicular to the flat inner wall surface 21, or the end surface 22 may be curved without being flat. The concave inner wall surface 24 may be connected to the end surface 22 without the convex inner wall surface 23.

The number of the concave inner wall surfaces 24 may be two or more. Fig. 9 exemplarily shows a case where the concave inner wall surface 24 includes two concave inner wall surfaces. In a cross section of the housing 2 (see fig. 2) including the rotation axis L, the two concave inner wall surfaces respectively constitute a first concave arc portion 241 and a second concave arc portion 242. The second concave arc portion 242 has a radially inner edge 242a and a radially outer edge 242b, the edge 242a being connected to the first concave arc portion 241, and the edge 242b being connected to the convex arc portion 23 a. In such an embodiment, the inclination angle α is an angle formed by a tangential direction a of a radially outer end edge 242B of the second concave arc portion 242, which is a concave arc portion located at the outermost side in the radial direction, and a direction B perpendicular to the rotation axis L.

In the above embodiment, the distribution of the inclination angles α has the minimum value or the minimum value in the range of the central angle θ from 30 ° to 210 °, but may have the minimum value or the minimum value in the range of the central angle θ from 30 ° to 120 ° (see fig. 6). As shown in fig. 1, the flow passage area of the scroll flow passage 5 decreases from the outlet side toward the tongue portion 4 a. Due to the shape of the scroll flow path 5, the inclination angle α (see fig. 5) of the concave arc portion 24a (see fig. 5) tends to be larger as the tongue portion 4a is closer. In thatWhen the scroll flow path 5 is formed without being aware of the magnitude of the inclination angle α, the scroll flow path 5 is formed so that the inclination angle α has a minimum value or a minimum value in a range of the central angle from 30 ° to 120 ° in which the inclination angle α tends to increase, and thus the inclination angle α can be reduced in a range of the central angle from 30 ° to 210 °, and therefore, the swirling flow f can be further suppressed₂With the flow f of compressed fluid discharged from the diffuser flow path 8₃The interference of (3) can further reduce the occurrence of separation in the scroll flow path 8. As a result, the efficiency of the centrifugal compressor 1 at the low flow rate operation point can be improved.

Further, although the diffuser flow path 8 is usually formed by cutting, in the above embodiment, the flat inner wall surface 21 defining the diffuser flow path 8 is flat and perpendicular to the rotation axis L, and the diffuser flow path 8 can be easily processed.

Description of the reference numerals

1 centrifugal compressor

2 casing

3 impeller

4 scroll part

4a tongue part

5 scroll flow path

5a (of the scroll flow path) inner wall surface

5a1 (of the inner wall surface)

5a2 (of the inner wall surface) at the radially inner side

5a3 (of inner wall surface) at the radially outer side

6 diffusion part

7 flow path wall

7a first flow path wall

7b second flow path wall

8 diffusion flow path

9 air inlet part

21 flat inner wall surface

22 end face

23 convex inner wall surface

23a convex arc part

24 concave inner wall surface

24a concave arc part

24a1 (concave arc portion) end edge

241 first concave arc portion

242 second concave arc portion

242a (of the second concave arc portion)

242b (of the second concave arc portion)

A tangential direction

B direction perpendicular to the rotation axis

l (of the impeller) axis of rotation

L' imaginary line

O_sCentre of vortex

Angle of inclination alpha

Angle beta

central angle of theta

f₁Compressed air flow discharged from diffusion flow path near tongue part

f₂Swirling flow

f₃Compressed air flow discharged from diffuser flow path