Centrifugal compressor and turbocharger

Abstract

A centrifugal compressor 10 includes a diffuser 13 that comprises: a shroud wall surface 131 that extends in the radial direction of a rotating shaft 3; and a hub wall surface 132 that extends in the radial direction and opposes the shroud wall surface 131 on the downstream side of a flow in the axial direction of the rotating shaft 3, the hub wall surface having a gap between itself and the shroud wall surface 131, and with that gap, forming an annular diffuser flow path 130 through which a fluid flows. A hub-side convex portion 132b is formed over the entire periphery of the hub wall surface 132, the hub-side convex portion 132b protruding toward the shroud wall surface 131 side relative to a straight line L1 that connects a starting end 132s on an inlet 130a side of the diffuser flow path 130 and a terminating end 132e on an outlet 130b side of the diffuser flow path 130.

Claims

1. A centrifugal compressor comprising: an impeller boosting a fluid by rotation about an axis of a rotating shaft; and. a diffuser converting a dynamic pressure of the fluid boosted by the impeller into a static pressure, wherein the diffuser has an inlet, an outlet, a shroud wall surface extending in a radial direction of the centrifugal compressor and a hub wall surface extending in the radial direction and opposing the shroud wall surface on a downstream side of a flow in an axial direction of the centrifugal compressor, the diffuser having a gap between the hub wall surface and the shroud wall surface and forming an annular diffuser flow path with the gap, the fluid flowing through the diffuser flow path, a hub-side convex portion is formed over an entire periphery of the hub wall surface, the hub-side convex portion protruding toward the shroud wall surface side relative to a straight line connecting a starting end on an inlet side of the diffuser flow path and a terminating end on an outlet side of the diffuser flow path, the hub-side convex portion is formed so as to have a size at which an annular area as a product of a circumferential length and a width of the diffuser flow path at any radial position is greater than an annular area as a product of a circumferential length and a width of the diffuser flow path at the inlet, the shroud wall surface has a shroud-side concave portion provided so as to oppose the hub-side convex portion and be concave to a side opposite to the hub wall surface, and a vertex of the hub-side convex portion is provided in a range inward in the radial direction from a central portion of the hub-side convex portion in the radial direction.

2. The centrifugal compressor according to claim 1, wherein the vertex of the hub-side convex portion is formed at a radial position 1.05 to 1.4 times a radius from the rotating axis at the inlet of the diffuser flow path.

3. The centrifugal compressor according to claim 2, wherein the hub-side convex portion is provided inside a position in the radial direction, the position having a radius of 0.9 or less times a radius from the rotating axis at the outlet of the diffuser flow path.

4. The centrifugal compressor according to claim 2, wherein the hub-side convex portion has a distance from the straight line to the vertex in the axial direction ranging from 0.1 times to 0.3 times a width of the diffuser flow path at the outlet.

5. The centrifugal compressor according to claim 1, wherein the hub-side convex portion is provided inside a position in the radial direction, the position having a radius of 0.9 or less times a radius from the rotating axis at the outlet of the diffuser flow path.

6. The centrifugal compressor according to claim 5, wherein the hub-side convex portion has a distance from the straight line to the vertex in the axial direction ranging from 0.1 times to 0.3 times a width of the diffuser flow path at the outlet.

7. The centrifugal compressor according to claim 1, wherein the hub-side convex portion has a distance from the straight line to the vertex in the axial direction ranging from 0.1 times to 0.3 times a width of the diffuser flow path at the outlet.

8. The centrifugal compressor according to claim 1, wherein the shroud-side concave portion is formed so as to have a size, as a maximum, at which a width of the diffuser flow path becomes constant in the radial direction between the hub-side convex portion and the shroud-side concave portion.

9. The centrifugal compressor according to claim 1 wherein the impeller has an impeller hub rotating integrally with the rotating shaft and a blade attached to the impeller hub, the impeller hub includes a linear portion extending in a direction orthogonal to the rotating shaft to an impeller outlet, and the hub wall surface forming the diffuser flow path extends obliquely toward the downstream side in the axial direction from the starting end toward the terminating end.

10. The centrifugal compressor according to claim 1, wherein the shroud wall surface has an asymptotic portion that is asymptotic toward the hub wall surface side radially outward from the inlet.

11. A turbocharger comprising: the centrifugal compressor according to claim 1; and a turbine that is coaxially connected to the centrifugal compressor via the rotating shaft.

12. A centrifugal compressor comprising: an impeller boosting a fluid by rotation about an axis of a rotating shaft; and a diffuser converting a dynamic pressure of the fluid boosted by the impeller into a static pressure, wherein the diffuser has an inlet, an outlet, a shroud wall surface extending in a radial direction of the centrifugal compressor and a hub wall surface extending in the radial direction and opposing the shroud wall surface on a downstream side of a flow in an axial direction of the centrifugal compressor, the diffuser having a gap between the hub wall surface and the shroud wall surface and forming an annular diffuser flow path with the gap, the fluid flowing through the diffuser flow path, a hub-side convex portion is formed over an entire periphery of the hub wall surface, the hub-side convex portion protruding toward the shroud wall surface side relative to a straight line connecting a starting end on an inlet side of the diffuser flow path and a terminating end on an outlet side of the diffuser flow path, the hub-side convex portion is formed so as to have a size at which an annular area as a product of a circumferential length and a width of the diffuser flow path at any radial position is greater than an annular area as a product of a circumferential length and a width of the diffuser flow path at the inlet, the impeller has an impeller hub rotating integrally with the rotating shaft and a blade attached to the impeller hub, the impeller hub includes an inclined portion extending obliquely toward the downstream side in the axial direction toward the hub wall surface forming the diffuser flow path, and the hub wall surface forming the diffuser flow path has a hub-side concave portion that is concave toward a side opposite to the shroud wall surface and has an inclination angle conforming to an inclination angle of the inclined portion of the impeller hub radially inside the hub-side convex portion.

13. The centrifugal compressor according to claim 12, wherein thea vertex of the hub-side convex portion is provided in a range inward in the radial direction from a central portion of the hub-side convex portion in the radial direction.

14. The centrifugal compressor according to claim 12, wherein a vertex of the hub-side convex portion is formed at a radial position 1.05 to 1.4 times a radius from the rotating axis at the inlet of the diffuser flow path.

15. The centrifugal compressor according to claim 12, wherein the hub-side convex portion is provided inside a position in the radial direction, the position having a radius of 0.9 or less times a radius from the rotating axis at the outlet of the diffuser flow path.

16. The centrifugal compressor according to claim 12, wherein the hub-side convex portion has a distance from the straight line to a vertex in the axial direction ranging from 0.1 times to 0.3 times a width of the diffuser flow path at the outlet.

17. The centrifugal compressor according to claim 12, wherein the shroud wall surface has an asymptotic portion that is asymptotic toward the hub wall surface side radially outward from the inlet.

18. A turbocharger comprising: the centrifugal compressor according to claim 12; and a turbine that is coaxially connected to the centrifugal compressor via the rotating shaft.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic configuration diagram illustrating a turbocharger according to a first embodiment.

(2) FIG. 2 is a front view illustrating a centrifugal compressor according to the first embodiment.

(3) FIG. 3 is a cross-sectional view illustrating the centrifugal compressor according to the first embodiment.

(4) FIG. 4 is a cross-sectional view illustrating a centrifugal compressor as a comparative example.

(5) FIG. 5 is an explanatory diagram illustrating an example of the flow rate-pressure characteristics of the centrifugal compressor according to the first embodiment and an example of the flow rate-pressure characteristics of the centrifugal compressor as the comparative example.

(6) FIG. 6 is a cross-sectional view illustrating a centrifugal compressor according to a modification example of the first embodiment.

(7) FIG. 7 is a cross-sectional view illustrating a centrifugal compressor according to another modification example of the first embodiment.

(8) FIG. 8 is a cross-sectional view illustrating a centrifugal compressor according to a second embodiment.

(9) FIG. 9 is a cross-sectional view illustrating a centrifugal compressor according to a third embodiment.

(10) FIG. 10 is a cross-sectional view illustrating a centrifugal compressor according to a fourth embodiment.

DESCRIPTION OF EMBODIMENTS

(11) Hereinafter, embodiments of a centrifugal compressor and a turbocharger according to the present invention will be described in detail with reference to accompanying drawings. It should be noted that this invention is not limited by the embodiments.

First Embodiment

(12) FIG. 1 is a schematic configuration diagram illustrating a turbocharger according to a first embodiment. A turbocharger (exhaust turbocharger) 1 according to the first embodiment is provided with a centrifugal compressor (compressor) 10 and a turbine 2. The turbocharger 1 is provided adjacent to an internal combustion engine (not illustrated). In the turbocharger 1, the centrifugal compressor 10 and the turbine 2 are coaxially connected via a rotating shaft 3. In the turbocharger 1, the turbine 2 is rotationally driven by exhaust gas exhausted from the internal combustion engine (not illustrated) and the centrifugal compressor 10 is driven by the rotating shaft 3. As a result, a fluid such as air taken into the centrifugal compressor 10 from the outside is compressed and pumped toward the internal combustion engine (not illustrated).

(13) FIG. 2 is a front view illustrating the centrifugal compressor according to the first embodiment, and FIG. 3 is a cross-sectional view illustrating the centrifugal compressor according to the first embodiment. FIG. 3 illustrates a meridional section including the rotating shaft 3 along line A-A of FIG. 2 (hereinafter, simply referred to as “meridional section”). As illustrated in FIGS. 2 and 3, the centrifugal compressor 10 according to the first embodiment is provided with a casing 11, an impeller 12, and a diffuser 13. The centrifugal compressor 10 is formed in an axisymmetric structure that has the rotating shaft 3 as the center of the axisymmetric structure.

(14) The casing 11 has a shroud 111 and a hub 112. As illustrated in FIG. 3, the shroud 111 has a tubular portion 111a extending in the axial direction of the rotating shaft 3 (hereinafter, simply referred to as “axial direction”) and a disk-shaped portion 111b extending in the radial direction of the rotating shaft 3 of the tubular portion 111a (hereinafter, simply referred to as “radial direction”). In FIGS. 3, 4 and 7-10 the axis of the rotating shaft 3A is shown. The tubular portion 111a forms a suction passage 14 along the axial direction. The disk-shaped portion 111b extends radially outward substantially along a direction orthogonal to the rotating shaft 3 after extending while curving radially outward from the tubular portion 111a. The hub 112 is an annular disk disposed so as to oppose the disk-shaped portion 111b of the shroud 111. The hub 112 rotatably supports the rotating shaft 3.

(15) The impeller 12 has an impeller hub 12a integrally attached to the rotating shaft 3 and a plurality of blades 12b provided at equal intervals on the outer periphery of the impeller hub 12a. The outer periphery of the impeller 12 is covered with the curved part of the disk-shaped portion 111b and the tubular portion 111a of the shroud 111 except for an impeller outlet 12c, which is the position of the peripheral edge of the blade 12b. The impeller 12 is capable of taking in a fluid via the suction passage 14 of the shroud 111. As illustrated in FIG. 3, in the present embodiment, the impeller hub 12a has a back plate portion 121a as a part of the outer peripheral surface of the impeller hub 12a to which the blade 12 is attached, the back plate portion 121a extends radially outward, and the back plate portion 121a includes a linear portion 121b extending to the impeller outlet 12c in the direction orthogonal to the rotating shaft 3.

(16) In the first embodiment, the diffuser 13 is a vaneless diffuser. The diffuser 13 is disposed on the downstream side of the impeller 12. The diffuser 13 is an annular space formed by the hub 112 and the disk-shaped portion 111b of the shroud 111 and communicating with the impeller outlet 12c. In other words, the diffuser 13 has a shroud wall surface 131 formed by a part of the disk-shaped portion 111b of the shroud 111 and a hub wall surface 132 formed by the hub 112. The shroud wall surface 131 is a part of the inner wall surface of the disk-shaped portion 111b and extends radially outward radially outside the impeller outlet 12c. The hub wall surface 132 is a part of the inner wall surface of the hub 112 and extends radially outward, while opposing the shroud wall surface 131, radially outside the impeller outlet 12c. The hub wall surface 132 has a gap between the hub wall surface 132 and the shroud wall surface 131. An annular diffuser flow path 130 is formed by the gap between the shroud wall surface 131 and the hub wall surface 132. A fluid discharged from the impeller outlet 12c flows through the diffuser flow path 130.

(17) Once the rotating shaft 3 rotates as the turbine 2 is driven, the impeller 12 rotates and a fluid is suctioned into the casing 11 through the suction passage 14. After the suctioning into the casing 11, the fluid is boosted during passage through the impeller 12 rotating about the rotating shaft 3. Subsequently, the fluid is discharged from the impeller outlet 12c toward the diffuser 13. The fluid discharged from the impeller outlet 12c toward the diffuser 13 flows radially outward as indicated by the solid-line arrows in FIG. 3 while turning in the circumferential direction of the rotating shaft 3 (hereinafter, simply referred to as “circumferential direction”) in the diffuser flow path 130 as indicated by a two-dot chain line in FIG. 2. At this time, the fluid is decelerated by the frictional force of the shroud wall surface 131 and the hub wall surface 132. In addition, the flow velocity of the fluid in the turning direction is decreased as the radius of the diffuser flow path 130 (hereinafter, simply referred to as “radius”) increases from the rotating shaft 3. Further, as the fluid heads radially outward, the fluid is decelerated with an increase in the cross-sectional area of the diffuser flow path 130. As a result, the fluid undergoes dynamic-to-static pressure conversion while passing through the diffuser 13 and the static pressure rises (recovers). The centrifugal compressor 10 supplies the internal combustion engine (not illustrated) with the fluid boosted in this manner. It should be noted that a mechanism such as a scroll may be provided in the outer peripheral portion of the diffuser 13.

(18) Next, the diffuser 13 of the centrifugal compressor 10 according to the first embodiment will be described in detail. As illustrated in FIG. 3, the shroud wall surface 131 of the diffuser 13 has an asymptotic portion 131a asymptotic toward the hub wall surface 132 side radially outward from an inlet 130a of the diffuser flow path 130 and a linear portion 131b extending in the direction orthogonal to the rotating shaft 3 from the asymptotic portion 131a to an outlet 130b of the diffuser flow path 130.

(19) As illustrated in FIG. 3, the hub wall surface 132 of the diffuser 13 has a first linear portion 132a extending in the direction orthogonal to the rotating shaft 3 radially outward from the inlet 130a of the diffuser flow path 130, a hub-side convex portion 132b extending radially outward from the first linear portion 132a, and a second linear portion 132c extending in the direction orthogonal to the rotating shaft 3 from the hub-side convex portion 132b to the outlet 130b of the diffuser flow path 130.

(20) Here, a straight line connecting a starting end 132s of the hub wall surface 132 on the inlet 130a side of the diffuser flow path 130 and a terminating end 132e of the hub wall surface 132 on the outlet 130b side of the diffuser flow path 130 is defined as a straight line L1. In the first embodiment, the straight line L1 is the same direction as the direction orthogonal to the rotating shaft 3 and the first linear portion 132a and the second linear portion 132c of the hub wall surface 132 extend along the straight line L1.

(21) The hub-side convex portion 132b is a part that protrudes toward the shroud wall surface 131 side relative to the straight line L1 connecting the starting end 132s and the terminating end 132e of the hub wall surface 132. As described above, the centrifugal compressor 10 is formed in an axisymmetric structure having the rotating shaft 3 as the center of the axisymmetric structure, and thus the hub-side convex portion 132b is formed over the entire periphery of the hub wall surface 132. In the first embodiment, the hub-side convex portion 132b is formed in the shape of a smooth curve that has a curvature continuously changing between the first linear portion 132a and the second linear portion 132c. The hub-side convex portion 132b extends, while approaching the shroud wall surface 131 side, radially outward from an innermost peripheral portion 132i on the first linear portion 132a side and approaches the shroud wall surface 131 most at a vertex 132t. The hub-side convex portion 132b extends away from the shroud wall surface 131 radially outward from the vertex 132t to an outermost peripheral portion 132o on the second linear portion 132c side.

(22) In the first embodiment, the innermost peripheral portion 132i of the hub-side convex portion 132b is provided radially outside the starting end 132s and the outermost peripheral portion 132o of the hub-side convex portion 132b is provided radially inside the terminating end 132e. Preferably, the outermost peripheral portion 132o of the hub-side convex portion 132b is provided radially inside the position that has a radius of 0.9 times or less an outlet radius r2 at the outlet 130b of the diffuser flow path 130. In other words, it is preferable that the hub-side convex portion 132b is provided radially inside the position that has a radius of 0.9 times or less the outlet radius r2.

(23) Preferably, the vertex 132t of the hub-side convex portion 132b is provided in the range that is radially inward from the central portion of the hub-side convex portion 132b in the radial direction, that is, the intermediate position between the innermost peripheral portion 132i and the outermost peripheral portion 132o in the radial direction.

(24) More specifically, it is preferable that the vertex 132t of the hub-side convex portion 132b is formed at the radial position that is 1.1 times or more and 1.4 times or less an inlet radius r1 at the inlet 130a of the diffuser flow path 130. More preferably, the vertex 132t of the hub-side convex portion 132b is formed at the radial position that is 1.05 times or more and 1.4 times or less the inlet radius r1. In a case where the value that is obtained by an inlet width b1 of the diffuser flow path 130 at the inlet 130a being divided by the inlet radius r1 is approximately 0.05, it is preferable that the vertex 132t is formed at the radial position that is 1.1 times or more and 1.2 times or less the inlet radius r1. In a case where the value that is obtained by the inlet width b1 of the diffuser flow path 130 at the inlet 130a being divided by the inlet radius r1 is approximately 0.2, it is preferable that the vertex 132t is formed at the radial position that is 1.3 times or more and 1.4 times or less the inlet radius r1.

(25) As for the hub-side convex portion 132b, it is preferable that a distance D from the straight line L1 to the vertex 132t in the axial direction is 0.1 times or more and 0.3 times or less an outlet width b2 of the diffuser flow path 130 at the outlet 130b.

(26) Preferably, the inlet width b1 and the inlet radius r1 of the diffuser flow path 130 at the inlet 130a and a width b and a radius r of the diffuser flow path 130 at any radial position in the range where the hub-side convex portion 132b is formed satisfy the relationship of the following Equation (1). The left side in Equation (1) represents the annular area that is the product of circumferential length “2πr” and the width b of the diffuser flow path 130 at any radial position. The right side in Equation (1) represents the annular area that is the product of circumferential length “2πr1” and the width b1 of the diffuser flow path 130 at the inlet 130a. In other words, it is preferable that the hub-side convex portion 132b is formed so as to have a size at which the annular area that is the product of circumferential length “2πr” and the width b of the diffuser flow path 130 at any radial position increases as compared with the annular area that is the product of circumferential length “2πr1” and the width b1 of the diffuser flow path 130 at the inlet 130a.
b.Math.2πr>b1.Math.2πr1  (1)

(27) Next, the action of the centrifugal compressor 10 according to the first embodiment will be described based on comparison with a comparative example. FIG. 4 is a cross-sectional view illustrating a centrifugal compressor as the comparative example. FIG. 5 is an explanatory diagram illustrating an example of the flow rate-pressure characteristics of the centrifugal compressor according to the first embodiment and an example of the flow rate-pressure characteristics of the centrifugal compressor as the comparative example. The solid line in FIG. 5 is an example of the flow rate-pressure characteristics of the centrifugal compressor 10 according to the first embodiment, and the dashed line in FIG. 5 is an example of the flow rate-pressure characteristics of a centrifugal compressor 10A as the comparative example. It should be noted that the two-dot chain line in FIG. 5 indicates ideal flow rate-pressure characteristics in a case where it is assumed that there is no pressure loss in the impeller 12 and the diffuser 13 and the one-dot chain line in FIG. 5 indicates the flow rate-pressure characteristics in a case where it is assumed that there is no pressure loss in the diffuser 13 with the pressure loss in the impeller 12 taken into account.

(28) Regarding the centrifugal compressor 10A as the comparative example, the solid-line arrows in FIG. 4 indicate the radial component of the flow velocity in the diffuser flow path 130 at a time when the centrifugal compressor 10A operates at a low flow operation point 101A (see FIG. 5), which is lower in flow rate than a normal operation point 100A (see FIG. 5) near the maximum efficiency point. It should be noted that a flow angle θ2 in the turning direction decreases by approximately ⅔ to ½ from a flow angle θ1 in the case of the normal operation point 100A as illustrated in, for example, FIG. 2 when the centrifugal compressor 10A operates at the low flow operation point 101A.

(29) As illustrated in FIG. 4 and unlike the centrifugal compressor 10 according to the first embodiment, the hub wall surface 132 of the diffuser 13 of the centrifugal compressor 10A as the comparative example does not have the hub-side convex portion 132b. In the centrifugal compressor 10A as the comparative example, the hub wall surface 132 of the diffuser 13 extends perpendicularly in the radial direction along the direction orthogonal to the rotating shaft 3. The other constituent elements of the centrifugal compressor 10A and the size and the like of each constituent element are similar to those of the centrifugal compressor 10, and thus will not be described. Hereinafter, the flow of the fluid in the diffuser flow path 130 will be described first with reference to FIG. 4 regarding the centrifugal compressor 10A as the comparative example.

(30) As illustrated in FIG. 4, in the centrifugal compressor 10A as the comparative example, the radial component of the flow velocity of the fluid that has flowed into the diffuser flow path 130 has a boundary layer in the vicinity of the shroud wall surface 131 and the hub wall surface 132. In general, in the vicinity of the inlet 130a, the force with which the flow heads toward the downstream side in the axial direction (right side in FIG. 4, hereinafter, simply referred to as “axially downstream side”) after passing through the impeller 12 remains, and thus the boundary layer on the hub wall surface 132 side becomes thin and the boundary layer on the shroud wall surface 131 side becomes thick. As the flow in the diffuser flow path 130 heads toward the outlet 130b side, the force toward the axially downstream side decreases. Accordingly, in general, the boundary layer on the shroud wall surface 131 side and the boundary layer on the hub wall surface 132 side gradually become uniform as the flow in the diffuser flow path 130 heads toward the outlet 130b side in a case where the centrifugal compressor 10A operates at the flow rate at the normal operation point 100A.

(31) A reverse flow may occur at the boundary layer of the flow on the hub wall surface 132 side as illustrated in FIG. 4 in a case where the centrifugal compressor 10A operates at the flow rate at the low flow operation point 101A. This is because the circumferential component of the flow velocity is smaller (that is, the centrifugal force of the flow is smaller) on the hub wall surface 132 side than on the shroud wall surface 131 side and it may become impossible to resist the radially inward force acting on the fluid in the diffuser flow path 130, where the static pressure of the fluid increases as the radius increases.

(32) In FIG. 4, the range that is closer to the hub wall surface 132 side than the line indicated by the two-dot chain line is a reverse flow region where the reverse flow has occurred. In a general vaneless diffuser, the reverse flow region is usually generated from the radial position that is 1.1 times or more and 1.2 times or less the inlet radius r1 in a case where the value that is obtained by the inlet width b1 of the diffuser flow path 130 at the inlet 130a being divided by the inlet radius r1 of the inlet 130a is approximately 0.05. In addition, the reverse flow region is usually generated from the radial position that is 1.1 times or more and 1.2 times or less the inlet radius r1 in a case where the value that is obtained by the inlet width b1 of the diffuser flow path 130 at the inlet 130a being divided by the inlet radius r1 is approximately 0.2. In other words, in a general vaneless diffuser, the reverse flow region is usually generated from the radial position that is 1.1 times or more and 1.4 times or less the inlet radius r1 of the inlet 130a of the diffuser flow path 130.

(33) Once a reverse flow occurs on the hub wall surface 132 side in the diffuser flow path 130, a flow center line Lc (center line of flow rate bisection in the width direction of the diffuser flow path 130) moves toward the shroud wall surface 131 side in the vicinity of the reverse flow region as the center line Lc heads radially outward from the inlet 130a. Then, the flow rate in the vicinity of the shroud wall surface 131 relatively increases, and thus no reverse flow is likely to occur at the boundary layer on the shroud wall surface 131 side. Subsequently, the center line Lc of the flow heading toward the outlet 130b from the vicinity of the reverse flow region gradually moves toward the hub wall surface 132 side, and thus the center line Lc draws an S shape as a whole.

(34) The reverse flow region at the boundary layer on the hub wall surface 132 side expands in a case where the centrifugal compressor 10A is operated with the flow rate further reduced from the example that is illustrated in FIG. 4. Once the reverse flow region reaches the outlet 130b of the diffuser flow path 130, a flow with a small turning-direction energy flows from the outlet 130b into the diffuser flow path 130 (reverse flow region). As a result, the reverse flow region expands over the entire width of the diffuser flow path 130 in the vicinity of the outlet 130b, boosting of the fluid by the diffuser 13 becomes impossible, and a stall (surge) of the diffuser 13 occurs. The flow rate at which the stall of the diffuser 13 occurs is defined as a surge point 103A in FIG. 5.

(35) Once a reverse flow occurs on the hub wall surface 132 side in the diffuser flow path 130 as described above, the width of the diffuser flow path 130 is substantially narrowed by the reverse flow region. Then, it may be impossible to sufficiently reduce the flow velocity. In addition, the pressure loss at the diffuser 13 increases due to the reverse flow. As a result, the static pressure of the fluid cannot be sufficiently raised by the diffuser 13, which leads to a decline in the efficiency of the centrifugal compressor 10A and, in turn, a decline in the efficiency of the turbocharger 1. In addition, the expansion of the reverse flow occurring in the diffuser flow path 130 leads to a stall (surge) of the diffuser 13 as described above. Accordingly, it is necessary to maintain a flow rate at which no stall occurs, which is an obstacle to the industrial demand of expansion of a surge margin as the difference between the flow rate at the normal operation point 100A and the flow rate at the stall-occurring surge point 103A.

(36) In order to solve this problem, the hub wall surface 132 of the diffuser 13 of the centrifugal compressor 10 according to the first embodiment has the hub-side convex portion 132b. The hub-side convex portion 132b is formed in a reverse flow-prone region at the boundary layer on the hub wall surface 132 side. Accordingly, the region on the hub wall surface 132 side where a reverse flow is likely to occur in the diffuser flow path 130 during operation at a low flow rate in particular is closed in advance by the hub-side convex portion 132b. In addition, as illustrated in FIG. 3, the boundary layer of the flow on the hub wall surface 132 side in the vicinity of the hub-side convex portion 132b becomes thinner than in the centrifugal compressor 10A of the comparative example by the hub-side convex portion 132b. Narrowed as a result is the range in which a fluid having a low circumferential flow velocity and a small centrifugal force is incapable of resisting the radially inward force that acts on the fluid in the diffuser flow path 130. Further, since the width of the diffuser flow path 130 is narrowed by the hub-side convex portion 132b, the main flow velocity in the diffuser flow path 130 becomes higher than in the centrifugal compressor 10A of the comparative example. As a result, a reverse flow is prevented from occurring at the boundary layer of the flow on the hub wall surface 132 side in the diffuser flow path 130. As a result, the boundary layer on the shroud wall surface 131 side and the boundary layer on the hub wall surface 132 side gradually become uniform as the flow in the diffuser flow path 130 heads toward the outlet 130b side, as illustrated in FIG. 3, even in a case where the centrifugal compressor 10 is operated at a low flow operation point 101 (see FIG. 5), which is equal in flow rate to the low flow operation point 101A. In other words, it is possible to form a stable flow in the diffuser flow path 130 even in a case where the centrifugal compressor 10 is operated at the low flow operation point 101.

(37) As a result, it is possible to prevent a reverse flow from occurring on the hub wall surface 132 side of the diffuser flow path 130, and thus the flow velocity of the flow can be sufficiently reduced by the diffuser 13 and it is possible to prevent a pressure loss in the diffuser 13. As a result, the static pressure of the fluid can be sufficiently raised, as illustrated in FIG. 5, by the diffuser 13 even during operation at a low flow rate as compared with the centrifugal compressor 10A of the comparative example and it is possible to improve the efficiency of the centrifugal compressor 10 and, in turn, the efficiency of the turbocharger 1. In addition, it is possible to improve the output of the internal combustion engine (not illustrated) by improving the efficiency of the centrifugal compressor 10 and the efficiency of the turbocharger 1.

(38) In addition, a stall of the diffuser 13 attributable to a reverse flow can be prevented as a result of the reverse flow prevention on the hub wall surface 132 side. As described above, in the centrifugal compressor 10A of the comparative example, the reverse flow region expands up to the outlet 130b of the diffuser flow path 130 and a stall of the diffuser 13 occurs once a reverse flow occurs at the low flow operation point 101A illustrated in FIG. 5 and the flow rate decreases to the surge point 103A. In the centrifugal compressor 10 according to the first embodiment, in contrast, a reverse flow begins to occur when the flow rate has further decreased as compared with the low flow operation point 101, which is equal in flow rate to the low flow operation point 101A, and a stall of the diffuser 13 occurs when the flow rate has decreased to a surge point 103 illustrated in FIG. 5. In this manner, in the centrifugal compressor 10 according to the first embodiment, a reverse flow is less likely to occur and the reverse flow region is less likely to expand, even when the operation point is changed to a lower flow rate side, than in the centrifugal compressor 10A of the comparative example by the hub wall surface 132 being provided with the hub-side convex portion 132b. In other words, the flow rate at the surge point 103 at which a stall of the diffuser 13 occurs can be lower than the flow rate at the surge point 103A. Accordingly, the surge margin of the centrifugal compressor 10 can be expanded and the centrifugal compressor 10 can be operated at a lower flow rate.

(39) As described above, according to the centrifugal compressor 10 and the turbocharger 1 according to the first embodiment, it is possible to prevent a reverse flow from occurring on the hub wall surface 132 side forming the diffuser flow path 130, the efficiency of the centrifugal compressor 10 can be improved, the efficiency of the turbocharger 1 can be improved, and the surge margin of the centrifugal compressor 10 can be expanded.

(40) The vertex 132t of the hub-side convex portion 132b is provided in the range that is radially inward from the central portion of the hub-side convex portion 132b in the radial direction, that is, the intermediate position between the innermost peripheral portion 132i and the outermost peripheral portion 132o in the radial direction.

(41) With this configuration, the vertex 132t of the hub-side convex portion 132b can be brought close to the inlet 130a side of the diffuser flow path 130, and thus it is possible to satisfactorily prevent a reverse flow on the hub wall surface 132 side that is likely to occur at the front half part of the diffuser flow path 130 on the inlet 130a side.

(42) The vertex 132t of the hub-side convex portion 132b is formed at the radial position that is 1.05 times or more and 1.4 times or less the inlet radius r1 at the inlet 130a of the diffuser flow path 130.

(43) With this configuration, it is possible to satisfactorily prevent a reverse flow on the hub wall surface 132 side that is likely to occur at the radial position which is 1.05 times to 1.4 times the inlet radius r1 at the inlet 130a of the diffuser flow path 130.

(44) The hub-side convex portion 132b is provided radially inside the radial position that is 0.9 times or less the outlet radius r2 at the outlet 130b of the diffuser flow path 130.

(45) With this configuration, a reverse flow on the hub wall surface 132 side that is likely to occur at the front half part of the diffuser flow path 130 on the inlet 130a side can be satisfactorily prevented and, at the same time, it is possible to prevent the width of the diffuser flow path 130 from being narrowed in an excessive radius region (region in the radial direction) in the region where the hub-side convex portion 132b reaches the vicinity of the outlet 130b. As a result, it is possible to achieve a sufficient deceleration of the flow by means of the diffuser 13.

(46) As for the hub-side convex portion 132b, the distance D from the straight line L1 to the vertex 132t in the axial direction ranges from 0.1 times to 0.3 times the outlet width b2 of the diffuser flow path 130 at the outlet 130b.

(47) With this configuration, it is possible to prevent the diffuser flow path 130 from being excessively narrowed in the width direction by the hub-side convex portion 132b, and thus it is possible to achieve a sufficient deceleration of the flow by means of the diffuser 13.

(48) The hub-side convex portion 132b is formed so as to have a size at which the annular area that is the product of circumferential length “2πr” and the width b of the diffuser flow path 130 at any radial position increases as compared with the annular area that is the product of circumferential length “2πr1” and the width b1 of the diffuser flow path 130 at the inlet 130a.

(49) With this configuration, an excessive decrease in the annular area of the diffuser flow path 130 attributable to the hub-side convex portion 132b can be prevented, and thus it is possible to achieve a sufficient deceleration of the flow by means of the diffuser 13.

(50) The shroud wall surface 131 has the asymptotic portion 131a asymptotic to the hub wall surface 132 side radially outward from the inlet 130a.

(51) With this configuration, the width of the diffuser flow path 130 in the vicinity of the inlet 130a can be narrowed by the asymptotic portion 131a of the shroud wall surface 131, and thus the boundary layer of the flow on the shroud wall surface 131 side, which is likely to become thick in the vicinity of the inlet 130a, can be reduced in thickness. As a result, the thickness of the boundary layer of the flow on the shroud wall surface 131 side and the thickness of the boundary layer of the flow on the hub wall surface 132 side can become uniform in the vicinity of the inlet 130a of the diffuser flow path 130 and the flow can be pushed out toward the hub wall surface 132 side as a whole. As a result, the boundary layer of the flow on the hub wall surface 132 side can be further reduced in thickness and it is possible to prevent a reverse flow at the boundary layer of the flow on the hub wall surface 132 side.

(52) It should be noted that the shroud wall surface 131 may not have the asymptotic portion 131a in the first embodiment. In other words, the shroud wall surface 131 may have only a linear portion extending radially outward in the direction orthogonal to the rotating shaft 3.

(53) FIG. 6 is a cross-sectional view illustrating a centrifugal compressor according to a modification example of the first embodiment. As illustrated in FIG. 6, in a centrifugal compressor 10B according to the modification example, the linear portion 131b of the shroud wall surface 131 extends obliquely to the axially downstream side radially outward from the asymptotic portion 131a. As illustrated in FIG. 6, in the centrifugal compressor 10B according to the modification example, the second linear portion 132c of the hub wall surface 132 extends obliquely to the axially downstream side radially outward from the hub-side convex portion 132b. In the present embodiment, the inclination angle of the linear portion 131b of the shroud wall surface 131 and the inclination angle of the second linear portion 132c of the hub wall surface 132 are substantially equal to each other. Preferably, the inclination angle of the linear portion 131b of the shroud wall surface 131 and the inclination angle of the second linear portion 132c of the hub wall surface 132 are approximately five degrees to 10 degrees relative to the direction orthogonal to the rotating shaft 3.

(54) Also with the centrifugal compressor 10B, in which the linear portion 131b of the shroud wall surface 131 and the second linear portion 132c of the hub wall surface 132 are inclined radially outward to the axially downstream side as described above, effects similar to those of the centrifugal compressor 10 can be achieved by the hub-side convex portion 132b being formed on the hub wall surface 132.

(55) FIG. 7 is a cross-sectional view illustrating a centrifugal compressor according to another modification example of the first embodiment. Although only the second linear portion 132c of the hub wall surface 132 is inclined radially outward to the axially downstream side in the example that is illustrated in FIG. 6, the first linear portion 132a and the hub-side convex portion 132b of the hub wall surface 132 may be inclined at the same angle as the second linear portion 132c as in a centrifugal compressor 10C illustrated in FIG. 7. In other words, in the centrifugal compressor 10C, the hub wall surface 132 may extend obliquely toward the axially downstream side from the starting end 132s toward the terminating end 132e. Also in this case, it is preferable that the inclination angle of the linear portion 131b of the shroud wall surface 131 and the inclination angle of the hub wall surface 132 are substantially equal to each other and the angles are approximately five degrees to 10 degrees relative to the direction orthogonal to the rotating shaft 3.

(56) In this configuration, the hub wall surface 132 inclined toward the axially downstream side from the starting end 132s toward the terminating end 132e is capable of smoothly guiding, into the diffuser flow path 130, a flow in which a force toward the axially downstream side remains in the vicinity of the inlet 130a of the diffuser flow path 130, that is, the impeller outlet 12c. In the present embodiment, the shroud wall surface 131 has the asymptotic portion 131a as described above. The shroud wall surface 131 having the asymptotic portion 131a is another reason why it is possible to smoothly guide, into the diffuser flow path 130, the flow in which the force toward the axially downstream side remains in the vicinity of the inlet 130a of the diffuser flow path 130, that is, the impeller outlet 12c. As a result, a pressure loss at the inlet 130a of the diffuser flow path 130 can be prevented, the rate of static pressure recovery by the diffuser 13 can be further increased, and the efficiency of the centrifugal compressor 10C and the efficiency of the turbocharger 1 can be further improved.

Second Embodiment

(57) Next, a centrifugal compressor 20 according to a second embodiment will be described. FIG. 8 is a cross-sectional view illustrating the centrifugal compressor according to the second embodiment. The centrifugal compressor 20 according to the second embodiment is provided with a diffuser 23 in place of the diffuser 13 of the centrifugal compressor 10 according to the first embodiment. The diffuser 23 has a shroud wall surface 231 in place of the shroud wall surface 131 of the diffuser 13 of the centrifugal compressor 10 according to the first embodiment. The other configurations of the centrifugal compressor 20 and the diffuser 23 are similar to those of the centrifugal compressor 10 and the diffuser 13, and thus will not be described. It should be noted that the centrifugal compressor 20 according to the second embodiment is also applied to the turbocharger 1 described in the first embodiment.

(58) In the diffuser 23, the shroud wall surface 231 has an asymptotic portion 231a asymptotic toward the hub wall surface 132 radially outward from the inlet 130a of the diffuser flow path 130, a shroud-side concave portion 231b extending radially outward from the asymptotic portion 231a, and a linear portion 231c extending in the direction orthogonal to the rotating shaft 3 from the shroud-side concave portion 231b to the outlet 130b of the diffuser flow path 130.

(59) In the second embodiment, the outermost peripheral portion of the asymptotic portion 231a of the shroud wall surface 231 and the innermost peripheral portion of the linear portion 231c are formed side by side in the axial direction. The shroud-side concave portion 231b is a part concave to the side that is opposite to the hub wall surface 132 (left side in FIG. 8) beyond a straight line L2 connecting the outermost peripheral portion of the asymptotic portion 231a and the innermost peripheral portion of the linear portion 231c. The shroud-side concave portion 231b is formed over the entire periphery of the shroud wall surface 231. In the second embodiment, the shroud-side concave portion 231b is formed in the shape of a smooth curve that has a curvature continuously changing between the asymptotic portion 231a and the linear portion 231c. As illustrated in FIG. 8, the shroud-side concave portion 231b is provided at a position opposing the hub-side convex portion 132b.

(60) In the second embodiment, the shroud-side concave portion 231b is formed so as to have a size at which the width of the diffuser flow path 130 becomes constant between the shroud-side concave portion 231b and the hub-side convex portion 132b as a limit. In other words, in the second embodiment, the shroud-side concave portion 231b is concave toward the side that is opposite to the hub wall surface 132 in a shape conforming to the shape of the hub-side convex portion 132b with the shroud-side concave portion 231b having the same radial starting end as the innermost peripheral portion 132i of the hub-side convex portion 132b and having the same radial terminating end as the outermost peripheral portion 132o of the hub-side convex portion 132b.

(61) With this configuration, an excessive decrease in the width of the diffuser flow path 130 can be prevented by the shroud-side concave portion 231b even with the hub wall surface 132 provided with the hub-side convex portion 132b. Accordingly, it is possible to prevent an excessive increase in the main flow velocity in the diffuser flow path 130 attributable to the provided hub-side convex portion 132b. As a result, a pressure loss attributable to wall surface friction can be prevented and adjustment can be performed in a more appropriate manner such that flow deceleration by the diffuser 23 and, in turn, the rate of recovery of the static pressure of the fluid reaches a desired value. Accordingly, the efficiency of the centrifugal compressor 20 and the efficiency of the turbocharger 1 can be further improved as compared with the centrifugal compressor 10 according to the first embodiment.

(62) The shroud-side concave portion 231b is formed so as to have a size at which the width of the diffuser flow path 130 becomes constant between the shroud-side concave portion 231b and the hub-side convex portion 132b as a limit.

(63) With this configuration, it is possible to prevent an excessive increase in the width of the diffuser flow path 130 between the hub-side convex portion 132b and the shroud-side concave portion 231b and it is possible to prevent the flow from losing uniformity in the diffuser flow path 130. As a result, it becomes possible to more appropriately adjust the rate of recovery of the static pressure of the fluid by the diffuser 23.

(64) In the second embodiment, the shroud-side concave portion 231b may not have exactly the same radial starting end as the innermost peripheral portion 132i of the hub-side convex portion 132b and may not have exactly the same radial terminating end as the outermost peripheral portion 132o of the hub-side convex portion 132b insofar as the shroud-side concave portion 231b opposes the hub-side convex portion 132b. In addition, the shroud-side concave portion 231b may not be concave toward the side opposite to the hub wall surface 132 in the shape that conforms to the shape of the hub-side convex portion 132b. In this case, the shroud-side concave portion 231b may not be formed so as to have the size at which the width of the diffuser flow path 130 becomes constant between the shroud-side concave portion 231b and the hub-side convex portion 132b as a limit.

(65) In the second embodiment, the linear portion 231c of the shroud wall surface 231 and the second linear portion 132c of the hub wall surface 132 (or the entire hub wall surface 132) may be inclined radially outward to the axially downstream side as in the centrifugal compressor 10B according to the modification example of the first embodiment.

(66) Also in the second embodiment, the shroud wall surface 231 may not have the asymptotic portion 231a. In other words, the shroud wall surface 231 may have a linear portion extending radially outward in the direction orthogonal to the rotating shaft 3, the linear portion 231c, and the shroud-side concave portion 231b concave toward the side opposite to the hub wall surface 132 between the linear portion 231c and the linear portion.

Third Embodiment

(67) Next, a centrifugal compressor 30 according to a third embodiment will be described. FIG. 9 is a cross-sectional view illustrating the centrifugal compressor according to the third embodiment. The centrifugal compressor 30 according to the third embodiment is provided with an impeller 32 in place of the impeller 12 of the centrifugal compressor 10 according to the first embodiment. In addition, the centrifugal compressor 30 according to the third embodiment is provided with a diffuser 33 in place of the diffuser 13 of the centrifugal compressor 10 according to the first embodiment. The other configurations of the centrifugal compressor 30 are similar to those of the centrifugal compressor 10, and thus will not be described. It should be noted that the centrifugal compressor 30 according to the third embodiment is also applied to the turbocharger 1 described in the first embodiment.

(68) As illustrated in FIG. 9, the impeller 32 has an impeller hub 32a rotating integrally with the rotating shaft 3 and a plurality of blades 32b attached to the impeller hub 32a. The blade 32b is attached to the outer peripheral surface of the impeller hub 32a. A back plate portion 321a, which is a part of the outer peripheral surface and extends radially outward, includes an inclined portion 321b extending obliquely toward the axially downstream side and a hub wall surface 332. In the third embodiment, the inclined portion 321b is inclined at an inclination angle φ1 relative to the direction orthogonal to the rotating shaft 3 at the impeller outlet 12c. Here, the impeller 32 is referred to as a back plate-inclined impeller.

(69) As illustrated in FIG. 9, the diffuser 33 has the hub wall surface 332 in place of the hub wall surface 132 of the diffuser 13. In addition, the hub wall surface 332 has a hub-side concave portion 332a in place of the first linear portion 132a of the hub wall surface 132. The other configurations of the diffuser 33 and the hub wall surface 332 are similar to those of the diffuser 13 and the hub wall surface 132, and thus will not be described.

(70) In the third embodiment, the hub-side concave portion 332a extends radially outward from the inlet 130a of the diffuser flow path 130 and is connected to the innermost peripheral portion 132i of the hub-side convex portion 132b. The hub-side concave portion 332a is a part concave toward the side that is opposite to the shroud wall surface 131 relative to the straight line L1 connecting the terminating end 132e and the starting end 132s of the hub wall surface 332. The hub-side concave portion 332a is formed over the entire periphery of the hub wall surface 332. In the third embodiment, the hub-side concave portion 332a is formed in the shape of a smooth curve that has a curvature continuously changing between the hub-side convex portion 132b and the starting end 132s of the hub wall surface 332.

(71) The hub-side concave portion 332a is concave toward the side opposite to the shroud wall surface 131 at an inclination angle conforming to the inclination angle φ1 of the back plate portion 321a of the impeller hub 32a. In other words, in the third embodiment, the part of the hub-side concave portion 332a that extends away from the shroud wall surface 131 radially outward from the starting end 132s is inclined with respect to the direction orthogonal to the rotating shaft 3 at the angle that is equal to the inclination angle φ1.

(72) In this configuration, the back plate portion 321a of the impeller hub 32a is inclined at the inclination angle φ1 at the impeller outlet 12c and the hub-side concave portion 332a formed at an inclination angle conforming to the inclination angle φ1 of the impeller hub 32a is capable of smoothly guiding the flow into the diffuser flow path 130 even in a case where the force toward the downstream side in the axial direction of the flow becomes stronger in the vicinity of the inlet 130a of the diffuser flow path 130. As a result, a pressure loss at the inlet 130a of the diffuser flow path 130 can be prevented, the rate of static pressure recovery by the diffuser 33 can be further increased, and the efficiency of the centrifugal compressor 30 and the efficiency of the turbocharger 1 can be further improved.

(73) It should be noted that the inclination angle of the hub-side concave portion 332a may not be exactly the same as the inclination angle φ1 and the inclination angle of the hub-side concave portion 332a may be smaller or larger in value than the inclination angle φ1 insofar as a fluid can be smoothly guided into the diffuser flow path 130 from the impeller hub 32a.

(74) In the third embodiment, the shroud wall surface 131 has the asymptotic portion 131a, which is asymptotic to the hub wall surface 332 side radially outward from the inlet 130a of the diffuser flow path 130, as in the first and second embodiments. Accordingly, an excessive increase in the width of the diffuser flow path 130 in the vicinity of the inlet 130a can be prevented by the asymptotic portion 131a of the shroud wall surface 131 even with the hub-side concave portion 332a formed in the hub wall surface 332. As a result, the thickness of the boundary layer of the flow on the shroud wall surface 131 side and the thickness of the boundary layer of the flow on the hub wall surface 332 side can become uniform in the vicinity of the inlet 130a of the diffuser flow path 130 and the flow can be pushed out toward the hub wall surface 332 side as a whole. As a result, an increase in the thickness of the boundary layer of the flow on the hub wall surface 332 side can be prevented even in a case where the hub wall surface 332 is provided with the hub-side concave portion 332a, and thus it is possible to prevent a reverse flow at the boundary layer of the flow on the hub wall surface 332 side.

(75) It should be noted that the shroud wall surface 131 may not have the asymptotic portion 131a in the third embodiment. In other words, the shroud wall surface 131 may have only a linear portion extending radially outward in the direction orthogonal to the rotating shaft 3. In addition, the asymptotic portion 131a may be formed in the shape of a convex portion that is closer to the hub wall surface 332 side than in the example illustrated in FIG. 9. Then, it is possible to further prevent the boundary layer of the flow on the hub wall surface 332 side from becoming thick even in a case where the hub wall surface 332 is provided with the hub-side concave portion 332a, and thus it is possible to prevent a reverse flow at the boundary layer of the flow on the hub wall surface 332 side.

(76) In the third embodiment, the linear portion 131b of the shroud wall surface 131 and the second linear portion 132c of the hub wall surface 332 (or the entire hub wall surface 132) may be inclined radially outward to the axially downstream side as in the centrifugal compressor 10B according to the modification example of the first embodiment.

Fourth Embodiment

(77) Next, a centrifugal compressor 40 according to a fourth embodiment will be described. FIG. 10 is a cross-sectional view illustrating the centrifugal compressor according to the fourth embodiment. The centrifugal compressor 40 according to the fourth embodiment is provided with the impeller 32 of the third embodiment in place of the impeller 12 of the centrifugal compressor 10 according to the first embodiment. In addition, the centrifugal compressor 40 according to the fourth embodiment is provided with a diffuser 43 in place of the diffuser 13 of the centrifugal compressor 10 according to the first embodiment. The other configurations of the centrifugal compressor 40 are similar to those of the centrifugal compressor 10, and thus will not be described. It should be noted that the centrifugal compressor 40 according to the fourth embodiment is also applied to the turbocharger 1 described in the first embodiment.

(78) The diffuser 43 has the shroud wall surface 231 of the diffuser 23 of the second embodiment in place of the shroud wall surface 131 of the diffuser 13 of the first embodiment. In addition, the diffuser 43 has the hub wall surface 332 of the diffuser 33 of the third embodiment in place of the hub wall surface 132 of the diffuser 13 of the first embodiment.

(79) In the centrifugal compressor 40 according to the fourth embodiment, the diffuser 43 has the shroud wall surface 231 of the second embodiment and the hub wall surface 332 of the third embodiment. Accordingly, the centrifugal compressor 40 according to the fourth embodiment is capable of achieving the effects of both the centrifugal compressor 20 according to the second embodiment and the centrifugal compressor 30 according to the third embodiment.

(80) In the fourth embodiment, the linear portion 231c of the shroud wall surface 231 and the second linear portion 132c of the hub wall surface 332 (or the entire hub wall surface 132) may be inclined radially outward to the axially downstream side as in the centrifugal compressor 10B according to the modification example of the first embodiment.

(81) The shroud wall surface 231 may not have the asymptotic portion 231a in the fourth embodiment. In other words, the shroud wall surface 231 may have a linear portion extending radially outward in the direction orthogonal to the rotating shaft 3, the linear portion 231c, and the shroud-side concave portion 231b concave toward the side opposite to the hub wall surface 332 between the linear portion 231c and the linear portion. In addition, the asymptotic portion 231a may be formed in the shape of a convex portion that is closer to the hub wall surface 332 side than in the example illustrated in FIG. 10. Then, it is possible to further prevent the boundary layer of the flow on the hub wall surface 332 side from becoming thick even in a case where the hub wall surface 332 is provided with the hub-side concave portion 332a, and thus it is possible to prevent a reverse flow at the boundary layer of the flow on the hub wall surface 332 side.

(82) Although the hub-side convex portion 132b is formed in the shape of a smooth curve that has a curvature continuously changing between the second linear portion 132c and the first linear portion 132a or the hub-side concave portion 332a in the first to fourth embodiments, the shape of the hub-side convex portion 132b is not limited thereto. The hub-side convex portion 132b may have, for example, a circular arc shape or a parabolic shape. In addition, the hub-side convex portion 132b may include a linear part at a part of the hub-side convex portion 132b.

(83) The hub-side convex portion 132b may be connected to the first linear portion 132a or the hub-side concave portion 332a in the shape of a smooth curve or while being refracted. The hub-side convex portion 132b may be connected to the second linear portion 132c in the shape of a smooth curve or while being refracted. In a case where the hub-side convex portion 132b and the second linear portion 132c are connected while being refracted, a linear portion extending in the axial direction may be included between the second linear portion 132c and the outermost peripheral portion 132o of the hub-side convex portion 132b.

(84) The hub-side convex portion 132b may be formed from the starting end 132s of the hub wall surface 132 at the inlet 130a of the diffuser flow path 130 or may be formed from the terminating end 132e of the hub wall surface 132 at the outlet 130b of the diffuser flow path 130. In other words, the innermost peripheral portion 132i of the hub-side convex portion 132b may coincide with the starting end 132s and the outermost peripheral portion 132o of the hub-side convex portion 132b may coincide with the terminating end 132e.

(85) Although the present invention is applied to a vaneless diffuser in the first to fourth embodiments, the present invention may also be applied to a so-called small-chord ratio diffuser in which a vane (blade) is disposed in the range of up to approximately ½ of the radius gap from the inlet 130a to the outlet 130b in the radial direction from the inlet 130a of the diffuser flow path 130. In addition, the present invention may be applied to a so-called vaned diffuser in which a vane (blade) is disposed in the range of approximately 80% to 90% of the radius gap from the inlet 130a to the outlet 130b in the diffuser flow path 130.

REFERENCE SIGNS LIST

(86) 1 Turbocharger 2 Turbine 3 Rotating shaft 10, 10A, 10B, 10C Centrifugal compressor 100A Normal operation point 101, 101A Low flow operation point 103, 103A Surge point 11 Casing 111 Shroud 111a Tubular portion 111b Disk-shaped portion 112 Hub 12 Impeller 12a Impeller hub 121a Back plate portion 121b Linear portion 12b Blade 12c Impeller outlet 13 Diffuser 130 Diffuser flow path 130a Inlet 130b Outlet 131 Shroud wall surface 131a Asymptotic portion 131b Linear portion 231b Shroud-side concave portion 132, 332 Hub wall surface 132a First linear portion 132b Hub-side convex portion 132c Second linear portion 132e Terminating end 132i Innermost peripheral portion 132o Outermost peripheral portion 132s Starting end 132t Vertex 14 Suction passage 20 Centrifugal compressor 23 Diffuser 231 Shroud wall surface 231a Asymptotic portion 231b Shroud-side concave portion 231c Linear portion 30 Centrifugal compressor 32 Impeller 32a Impeller hub 32b Blade 321a Back plate portion 321b Inclined portion 33 Diffuser 332a Hub-side concave portion 43 Diffuser b Width b1 Inlet width b2 Outlet width D Distance L1, L2 Straight line Lc Center line r Radius r1 Inlet radius r2 Outlet radius θ1, θ2 Flow angle φ1 Inclination angle