JP2002364587A - Impeller of centrifugal compressor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an impeller for centrifugal compressor capable of lessening a loss of a fluid flow by making equal the rate of flow between its full vane and half vane. SOLUTION: Full vanes 17 and half vanes 18 are installed radially in alternation on the hub surface 14 of the impeller 11, and a fluid flowing in a passage TA formed between two adjoining full vanes 17 is divided by half vane 18 into a suction surface side passage TS and a pressure surface side passage TP. The hub surface 14 is formed from a suction surface side hub surface 19 constituting the suction surface side passage TS and a pressure surface side hub surface 20 constituting the pressure surface side passage. In a position S on the way of the passage TA, the suction surface side hub surface 19 is curved convex outward in the radial direction of the impeller 11, while the pressure surface side hub surface 20 is curved concave outward in the radial direction of the impeller 11. Thereby the section area of the passage TS becomes smaller than that of the passage TP.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an impeller for a centrifugal compressor.

[0002]

2. Description of the Related Art It is desired that a turbocharger used for an automobile engine maintain high efficiency in a wide operating range. As one of the improvement methods, it is required to improve the efficiency of a turbocharger compressor. I have.
A centrifugal compressor is generally used as a compressor for a turbocharger, and in order to achieve high efficiency, a reduction in size and an increase in flow rate are desired. For that purpose, a device for improving the shape of the impeller has been devised.

[0003] As an impeller of a conventional centrifugal compressor, for example, one disclosed in Japanese Patent Application Laid-Open No. 10-213094 is known. FIG. 5 is a diagram showing the shape and positional relationship between all the wings and half wings on a cylindrical section centered on the rotation axis of the impeller of this conventional centrifugal compressor. As shown in FIG.
The flow path TA2 is formed by the second suction surface 32a in the rotation direction and the pressure surface 32b in the rotation direction. Then, the half wing 3 located downstream of the flow path TA2
3, the fluid flowing through the flow path TA2 is divided into a suction surface side flow path TS2 and a pressure surface side flow path TP2.

Generally, in order to realize high efficiency of the centrifugal compressor, it is desired that the flow rate of the fluid discharged from the suction surface side flow path TS2 and the pressure surface side flow path TP2 is uniform. Then, the suction surface side flow path TS2 of the impeller 31 has a negative pressure, and the fluid entering the flow path TA2 of the impeller 31 has a distribution in which a fast flow mainly gathers on the suction surface 32a. For this reason, in this impeller 31, if the flow path cross-sectional areas of the flow paths TS2 and TP2 are geometrically made equal, the suction surface side flow path TS2 has a larger flow velocity than the pressure surface side flow path TP2.
The flow rate increases, and the flow rate of the fluid discharged from each outlet of the impeller 31 becomes uneven.

Therefore, in the impeller 31 of the conventional centrifugal compressor shown in FIG. 5, the cross-sectional areas of the flow paths TS2 and TP2 are not made geometrically equal but the tip portion 33a of the half wing 33 is suctioned. It is disclosed that the flow path is curved toward the surface 32a so that the flow path width DP2 of the pressure surface flow path TP2 is larger than the flow path width DS2 of the suction surface flow path TS2. That is, the tip portion 3 of the half wing 33
By bending 3a to the suction surface 32a side,
The cross-sectional area of the pressure-side flow path TP2 increases, and the cross-sectional area of the suction-side flow path TS2 decreases. Thereby, by making the flow path cross-sectional area of the pressure flow path TP2 having a low flow velocity larger than that of the suction flow path TS2 having a high flow velocity, the suction flow path TS2 and the pressure flow path TP2 are formed. The flow rate of the fluid flowing through TP2 is made equal.

[0006]

However, in the impeller 31 of the above-mentioned conventional centrifugal compressor, the tip portion 3 of the half wing 33 is not provided.
Since only 3a is curved to the suction surface 32a side,
The flow direction F of the fluid flowing through the flow path TA2 is determined by the tip portion 33a.
And the shape does not match. That is, the tip portion 33a
And the flow direction F, the inflow angle α, which is an angle formed by the flow direction F As a result, the collision loss of the fluid occurs at the tip portion 33a, and the separation of the flow easily occurs on the forward side in the rotation direction of the half wing 33, which causes the friction loss of the flow. Further, if a collision loss or friction loss of the flow occurs in the upstream, the flow velocity distribution in the downstream becomes non-uniform, and a mixing loss of the flow for uniforming the fluid occurs in the downstream.

Therefore, in the conventional impeller of the centrifugal compressor, it is difficult to make the flow rate of the fluid between the full blades 32 and the half blades 33 equal in the entire impeller and to reduce the loss of the fluid flow at the same time. Has become.

An object of the present invention is to provide an impeller of a centrifugal compressor capable of equalizing the flow rate between all the wings and the half wings and reducing the loss of fluid flow in order to solve the above problems. It is in.

[0009]

In order to solve the above-mentioned problems, the invention according to claim 1 is directed to a whole blade provided radially from a fluid inlet to a fluid outlet on a hub surface around a rotation axis. And half wings radially provided on the hub surface from a middle position of a flow path formed between the adjacent all wings to the outlet portion, and are sucked from the inlet portion along with the rotation. In an impeller of a centrifugal compressor that compresses a fluid and discharges the fluid to the outlet, the hub surface includes:
A suction surface side hub surface located between a full wing suction surface that is a surface on the rotation direction rear side of the all wings and the half wing located behind the all wing suction surface in the rotation direction, and rotation of the all wings A pressure surface-side hub surface positioned between the whole wing pressure surface that is a surface on the front side in the direction and the half wing that is positioned forward in the rotation direction of the all-wing pressure surface, and the suction surface-side hub surface is The gist is that the distance from the rotation shaft is larger at the intermediate position than the hub surface on the pressure surface side, and coincides with the hub surface on the pressure surface side at the outlet portion.

According to a second aspect of the present invention, in the impeller of the centrifugal compressor according to the first aspect, the hub surface on the suction surface side is curved in a convex shape radially outward of the impeller at the halfway position. The gist is that the pressure surface side hub surface is configured to be concavely curved toward the radial direction outside of the impeller at the intermediate position.

[0011] The invention described in claim 3 is the invention according to claim 1 or 2.
In the impeller of the centrifugal compressor described in the above, the suction surface side hub surface, the distance from the rotation shaft, at an intermediate position between the intermediate position and the outlet portion, compared with the pressure surface side hub surface. Make it small.

According to a fourth aspect of the present invention, there is provided an impeller for a centrifugal compressor according to any one of the first to third aspects,
The suction surface side hub surface is curved concavely outward in the radial direction of the impeller at an intermediate position between the intermediate position and the outlet portion, and the pressure surface side hub surface is an intermediate position between the intermediate position and the outlet portion. The gist of the present invention is that the impeller is configured to be curved in a convex shape outward in the radial direction of the impeller.

(Operation) According to the first aspect of the present invention,
Fluid flowing between two adjacent blades of the impeller of the centrifugal compressor is rotated by a half-blade provided between the two blades, on the suction surface side flow path on the rotation direction front side and on the rotation direction rear side. It is distributed to the pressure surface side channel. And
Since the suction surface side hub surface forming the suction surface side flow path is located further away from the impeller rotation axis at an intermediate position than the pressure surface side hub surface forming the pressure surface side flow path, the suction surface side flow The cross-sectional area of the passage is smaller than that of the pressure-side passage.

Therefore, the fluid flows into the suction surface side flow passage having a relatively small cross-sectional area and the pressure surface side flow passage having a relatively large cross-sectional area. Further, at the outlet portion, the suction surface side hub surface and the pressure surface side hub surface are located at the same distance from the rotation axis of the impeller, so that the flow passage cross-sectional area of the suction surface side flow passage and the pressure surface side flow passage is obtained. Are equal. Therefore, the fluid flows through the flow passages having the same flow passage cross-sectional area at the outlet portion, and is discharged from the outlet portion.

Therefore, at the intermediate position, since the cross-sectional area of the suction surface side flow path is smaller than that of the pressure surface side flow path, the suction surface side flow path whose flow velocity is relatively large due to the negative pressure, It is possible to make the flow rates of the fluid flowing through the pressure-side flow path having a relatively small flow velocity equal. Further, at the intermediate position, the fluid distributed at the same flow rate to the pressure surface side flow path and the suction surface side flow path passes through the flow path having the same flow path cross-sectional area at the outlet portion, so that the fluid at the impeller outlet is Flow rate becomes uniform. As a result, it is possible to reduce the mixing loss when the uneven flow at the impeller outlet is made uniform.

That is, by changing the difference between the distance between the hub surface on the suction surface side and the hub surface on the pressure surface side and the rotation axis, the cross-sectional area of the flow passage between the suction surface side flow passage and the pressure surface side flow passage can be freely adjusted. Can be changed. Therefore, it is not necessary to move the position of the half wing or the like in order to equalize the cross-sectional areas of the suction surface side flow passage and the pressure surface side flow passage, and the collision of the flow caused by the inconsistency of the flow and the blade shape. Loss, peeling, and the like can be suppressed.

According to the second and fourth aspects of the present invention, the suction surface side hub surface and the pressure surface side hub surface form a curved surface that changes gradually, and the fluid flows without a sudden change in the flow. .

Therefore, the flow does not separate due to a sudden change in the flow path, so that the friction loss and the like are small, and the efficiency of the centrifugal compressor is improved. According to the third aspect of the present invention, the fluid has a suction surface side flow path in which the flow path cross-sectional area increases from the intermediate position to the intermediate position, and a pressure surface side in which the flow path cross-sectional area is reduced from the intermediate position to the intermediate position. It flows separately into the flow path.

Therefore, the flow having a relatively high flow velocity at the middle position of the suction surface side flow path is decelerated from the middle position to the intermediate position, and the flow having the relatively low flow velocity at the middle position of the pressure surface side flow path is intermediate from the middle position. The speed is increased toward the position. Thereby, the flow velocity distribution of the fluid between the suction surface side flow path and the pressure surface side flow path is made uniform, and the efficiency of the centrifugal compressor is improved.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is a front view of the centrifugal compressor of the present embodiment, in which a lower half of the impeller is omitted, and FIG. 2 is a cross-sectional view taken along line AA of FIG. FIG. 3 is a sectional view taken along line BB of FIG. 2, and FIG. 4 is a sectional view taken along line CC of FIG.

As shown in FIG. 1 and FIG. 2, the impeller 11 of the centrifugal compressor of the present embodiment is an open impeller, and a rotary shaft 12a is penetrated through a central rotary shaft hole 12. Numeral 11 rotates in the direction of the arrow around the rotating shaft 12a with the rotating shaft 12a penetrating therethrough. A substantially conical hub 13 is provided around the rotation shaft hole 12 around the rotation shaft hole 12.
On a hub surface 14 which is an outer surface of the hub 13, a plurality of all blades 17 are provided at equal intervals radially along a flow direction of the fluid from a fluid inlet 15 to a fluid outlet 16. The whole wing 17 is constituted by a whole wing suction surface 17a on the rear side in the rotation direction of the impeller 11 and a whole wing pressure surface 17b on the front side in the rotation direction. Then, a flow path TA is formed by the whole wing suction surface 17a and the whole wing pressure surface 17b of the two adjacent wings 17, and the fluid flows into the flow path TA from the inlet 15 and from the outlet 16. The fluid is discharged.

Further, from the intermediate position S of the flow passage TA to the outlet portion 16, a half wing 18 is
Are provided along the flow direction of the fluid. That is,
The half wing 18 has a shorter length in the fluid flow direction than the whole wing 17, and the whole wing 17 and the half wing 18 are alternately arranged along the flow direction of the fluid on the hub surface 14 of the impeller 11. It is provided in such a manner. And the half wing 18 is the whole wing 17
A suction surface side flow path TS is formed by the whole wing suction surface 17a, and a pressure surface side flow path TP is formed by the whole wing pressure surface 17b. The half wings 18 are arranged so that the flow path width DS of the flow path TS is equal to the flow path width DP of the flow path TP.

The hub surface 14 is connected to the suction surface side flow path T
S, a suction surface side hub surface 19 and a pressure surface side flow passage TP.
It is constituted by and. As shown in FIG. 3, at a middle position S of the flow path TA, the suction surface side hub surface 19 forms a curved surface having a convex shape on the radial outside of the impeller 11,
The pressure side hub surface 20 forms a curved surface having a concave shape on the radially outer side of the impeller 11. Therefore, the suction surface side hub surface 19 and the pressure surface side hub surface 20 form a continuous curved surface with the whole wing 17 and the half wing 18 interposed therebetween. At the intermediate position S, the suction surface side hub surface 19 is located farther from the rotation axis 12a than the pressure surface side hub surface 20, and the suction surface side flow path TS is compared with the pressure surface side flow path TP. As a result, the cross-sectional area of the flow path is reduced.

As shown in FIG. 4, at the intermediate position M between the intermediate position S and the outlet portion 16, the suction surface side hub surface 19 forms a curved surface having a concave shape radially outward of the impeller 11, and Surface side hub surface 20 is impeller 1
A curved surface having a convex shape on the outside in the radial direction is formed. That is, at the intermediate position M, the suction surface side hub surface 1
9 is the rotating shaft 12a more than the pressure side hub surface 20.
It is located more distant, the suction surface side flow path TS,
The flow path cross-sectional area is larger than the pressure surface side flow path TP.

In the outlet section 16, the suction surface side hub surface 19 and the pressure surface side hub surface 20 have the same distance from the rotary shaft 12a, and the flow paths TS and TP have the same flow sectional area.

Therefore, due to the shape of the suction surface side hub surface 19, the suction surface side flow path TS has a relatively small flow path cross-sectional area at the intermediate position S, and the flow path from the intermediate position S to the intermediate position M It expands to have a relatively large channel cross-sectional area at the intermediate position M. Further, due to the shape of the pressure surface side hub surface 20, the pressure surface side flow path TP has a relatively large flow path cross-sectional area at the intermediate position S, and the flow path is narrowed from the intermediate position S to the intermediate position M, At the intermediate position M, the flow path has a relatively small cross-sectional area. Then, from the intermediate position M to the exit 1
The suction surface side flow path TS narrows down the flow path toward 6, while the pressure surface side flow path TP expands the flow path,
The cross-sectional areas of the flow paths TS and TP at the outlet 16 finally become equal.

Next, the operation of the impeller 11 of the centrifugal compressor according to the present embodiment configured as described above will be described. First, the fluid that has flowed in from the inlet portion 15 of the impeller 11 passes through the channel TA, and at the intermediate position S, the channels TS, T
The stream is separated into P. At this time, the flow velocity distribution in the rotation direction of the impeller 11 around the intermediate position S decreases from the whole-blade suction surface 17a to the whole-blade pressure surface 17b because the pressure of the fluid on the whole-blade suction surface 17a is a negative pressure. There is a tendency. Therefore, the velocity of the fluid flowing through the suction surface side flow path TS is higher than the velocity of the fluid flowing through the pressure surface side flow path TP.
At this time, the suction surface side flow path TS at the intermediate position S
Is narrower than the flow path cross-sectional area of the pressure surface side flow path TP, and the flow rates of the fluids flowing through the flow paths TS and TP become equal.

Next, the fluid flowing from the intermediate position S at a flow rate equal to the flow paths TS and TP is decelerated in the suction surface side flow path TS as the suction side flow path TS expands, and the pressure is reduced. In the surface-side flow path TP, the flow speed increases with the restriction of the pressure-side flow path TP. At the intermediate position M, the cross-sectional area of the suction-side flow path TS is larger than the cross-sectional area of the pressure-side flow path TP, and the flow paths TS and TP have the same flow rate. Since the fluid is flowing, the velocity of the fluid in the pressure surface side flow path TP is higher than the velocity of the fluid flowing in the suction surface side flow path TS.

The flow of the fluid passing through the intermediate position M is increased in the suction surface side flow path TS according to the restriction of the suction surface side flow path TS, and is increased in the pressure side flow path TP. The flow decelerates as the flow path TP expands. Then, the flow rates of the fluids flowing through the flow paths TS and TP become equal at the outlet section 16 where the flow path cross-sectional areas of the flow paths TS and TP are equal, and
The flow of the fluid flowing out of 6 becomes uniform.

According to the above embodiment, the following features can be obtained. (1) In the above embodiment, the fluid flowing between the adjacent two full wings 17 of the impeller 11 is rotated by the half wing 18 provided between the two full wings 17 on the suction surface side on the rotation direction front side. It is distributed to the flow path TS and the pressure surface side flow path TP on the rear side in the rotation direction. The suction surface side hub surface 19 forming the suction surface side flow channel TS is the pressure surface side hub surface 2 forming the pressure surface side flow channel TP.
0, the rotation shaft 12 of the impeller
a, the cross-sectional area of the suction-side flow path TS is smaller than the pressure-side flow path TP.

Therefore, the fluid flows into the suction surface side flow path TS having a relatively small cross-sectional area and the pressure surface side flow path TP having a relatively large cross-sectional area. In the outlet portion 16, the suction surface side hub surface 19 and the pressure surface side hub surface 20 are connected to the rotation shaft 1 of the impeller 11.
Since they are located at the same distance from 2a, the flow path cross-sectional areas of the suction surface side flow path TS and the pressure surface side flow path TP become equal. As a result, the fluid flows through the flow passages TS and TP having the same flow passage cross-sectional area at the outlet portion 16, respectively.
Is more exhausted.

Therefore, at the intermediate position S, the cross-sectional area of the suction surface side flow path TS is smaller than the pressure surface side flow path T
Since it is smaller than P, the flow rate of the fluid flowing through the suction surface side flow path TS having a relatively high flow rate due to the negative pressure and the flow rate of the fluid flowing through the pressure surface side flow path TP having a relatively low flow rate can be equalized. Further, at the intermediate position S,
Since the fluid distributed at the same flow rate as the pressure side flow path TP and the suction side flow path TS passes through the flow path having the same flow path cross-sectional area at the outlet portion 16, respectively.
The flow velocity of the fluid at the outlet 16 of the impeller 11 can be made uniform. Therefore, the outlet 16 of the impeller 11
Can reduce the mixing loss caused by the non-uniform flow in the above.

That is, by changing the difference between the distance between the rotary shaft 12a and the suction surface side hub surface 19 and the pressure surface side hub surface 20, the suction surface side flow path TS
And the cross-sectional area of the pressure surface side flow path TP can be freely changed. Further, it is not necessary to move the position of the half wing 18 or the like in order to equalize the cross-sectional areas of the suction surface side flow path TS and the pressure surface side flow path TP, and the flow generated because the flow does not match the blade shape. Collision loss, separation, and the like can be suppressed.

(2) The hub surface 19 on the suction surface side and the hub surface 20 on the pressure surface side form a curved surface that changes gently, and the fluid flows without a sudden change in the flow. Therefore, the flow does not separate due to a sudden change in the flow path, so that the friction loss and the like are small and the efficiency of the centrifugal compressor is improved.

(3) The fluid flows from the intermediate position S to the intermediate position M
Flow path TS whose flow path cross-sectional area increases toward
And a pressure surface side flow path TP whose flow path cross-sectional area is reduced from the intermediate position S to the intermediate position M.

Accordingly, in the suction surface side flow path TS, the flow having a relatively large flow velocity at the intermediate position S is decelerated from the intermediate position S to the intermediate position M, and the flow velocity at the intermediate position S of the pressure surface side flow path TP is relatively low. The speed of the small flow is increased from the intermediate position S to the intermediate position M. Thereby, the flow velocity distribution of the fluid between the suction surface side flow path TS and the pressure surface side flow path TP is made uniform, and the efficiency of the centrifugal compressor is improved.

The above embodiment may be modified as follows. In the above embodiment, the half wing 18 has a flow path width DS of the flow path TS.
And the flow path width DP of the flow path TP is described as being the same, but the position of the half wing 18 may be arbitrarily moved so that the half wing shape matches the flow angle. in this case,
The difference in distance between the suction surface side hub surface 19 and the pressure surface side hub surface 20 from the rotation shaft 12a may be changed according to the difference between the flow path widths DS and DP.

In the above embodiment, the suction surface side hub surface 19 and the pressure surface side hub surface 20 are connected to the impeller 1
The shape of the channel cross-sectional area of the suction surface side flow channel TS and the pressure surface side flow channel TP was changed by curving in the radial direction convex or concave shape. However, instead of being curved in a convex or concave shape, the hub surface 19 on the suction surface side and the hub surface 20 on the pressure surface side are used.
May be changed to other shapes to change the cross-sectional area of the suction surface side flow path TS and the pressure surface side flow path TP. For example, the suction surface side hub surface 19
And the pressure-side hub surface 20 are formed in a plane shape, and the distance between the plane-shaped suction surface-side hub surface 19 and the pressure-side hub surface 20 from the rotation axis 12a is changed to thereby increase the size of each flow path cross-sectional area. May be changed.

In the above embodiment, due to the shape of the suction surface side hub surface 19, the suction surface side flow path TS having a relatively small flow path cross-sectional area at the intermediate position S is obtained after the flow path is expanded toward the intermediate position M. The flow path was narrowed from the intermediate position M to the outlet 16. Further, due to the shape of the pressure surface side hub surface 20, the pressure surface side flow path TP having a relatively large flow path cross-sectional area at the intermediate position S is formed from the intermediate position M after the flow path is narrowed down to the intermediate position M. The flow channel was expanded toward 16.

However, the suction surface side flow path TS is made to gradually expand from the middle position S to the outlet portion 16, and the pressure surface side flow passage TP is gradually flowed from the middle position S to the outlet portion 16. You may comprise so that a road may be narrowed.

In the above embodiment, the impeller 11 has been described as an open impeller, but may be a closed impeller. Next, technical ideas that can be grasped from the above embodiment and other examples will be additionally described below together with their effects.

(1) In the impeller of the centrifugal compressor according to claim 1 or 2, the hub surface is defined by a distance between the suction surface side hub surface from the rotation axis of the impeller and the pressure surface side hub surface. An impeller for a centrifugal compressor, wherein a difference from a distance from the rotation shaft is gradually reduced from the intermediate position to the outlet, and is made to coincide at the outlet.

Therefore, according to the invention described in the above (1),
Since the passage cross-sectional area of the suction-surface-side flow passage formed by the suction-surface-side hub surface gradually expands, it is possible to reduce the occurrence of loss such as flow separation caused by the rapid expansion.

[0044]

As described in detail above, according to the first to fourth aspects of the present invention, the flow rate of the fluid between the full wing and the half wing is made equal in the entire impeller, and Loss can be reduced.

[Brief description of the drawings]

FIG. 1 is a partial front view of an impeller according to an embodiment.

FIG. 2 is a sectional view taken along line AA in FIG.

FIG. 3 is a sectional view taken along line BB in FIG. 2;

FIG. 4 is a sectional view taken along line CC in FIG. 2;

FIG. 5 is a partial sectional view of an impeller according to the related art.

[Explanation of symbols]

S: middle position, M: middle position, TA: flow path, 11: impeller, 12a: rotating shaft, 15: inlet, 16: outlet
17: whole wing, 17a: whole wing suction surface, 17b ... whole wing pressure surface, 18 ... half wing, 19 ... suction surface side hub surface, 20 ... pressure surface side hub surface

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3H033 AA02 AA17 BB03 BB06 CC01 DD12 EE08 EE19

Claims (4)

[Claims] 1. A whole wing provided radially from a fluid inlet to an outlet on a hub surface around a rotation axis, and the outlet from a middle position of a flow path formed between adjacent ones of the wings. And a half wing radially provided on the hub surface.The impeller of the centrifugal compressor that compresses fluid sucked in from the inlet portion along with its rotation and discharges the fluid to the outlet portion. A suction surface side hub surface located between a full wing suction surface that is a surface on the rotation direction rear side of the all wings and the half wing that is positioned rearward in the rotation direction of the all wing suction surfaces; A suction surface side hub surface located between the whole wing pressure surface which is a surface on the rotation direction front side and the half wing positioned in the rotation direction front of the all wing pressure surface; The centrifugal compressor, wherein a distance from the rotation axis is larger than the pressure surface side hub surface at the intermediate position, and coincides with the pressure surface side hub surface at the outlet portion. Impeller. 2. The impeller of a centrifugal compressor according to claim 1, wherein the suction surface-side hub surface is curved in a radially outward shape of the impeller at the intermediate position, and the pressure surface-side hub surface is the pressure surface-side hub surface. An impeller for a centrifugal compressor, wherein the impeller is configured to be curved in a concave shape outward in the radial direction of the impeller at an intermediate position. 3. The impeller of a centrifugal compressor according to claim 1, wherein the suction surface side hub surface is arranged such that the distance from the rotation shaft is an intermediate position between the intermediate position and the outlet portion. An impeller for a centrifugal compressor, wherein the impeller is smaller than the hub surface on the pressure surface side. 4. The impeller of a centrifugal compressor according to claim 1, wherein said suction surface side hub surface is radially outside of said impeller at a position intermediate between said intermediate position and said outlet portion. A centrifugal compressor characterized in that the pressure-side hub surface is curved so as to protrude radially outward of the impeller at an intermediate position between the intermediate position and the outlet portion. Impeller.