CENTRIFUGAL COMPRESSOR

CROSS REFERENCE TO RELATED APPLICATIONS

[0001]This application is a continuation application of International Application No. PCT/JP2024/021636, filed on Jun. 14, 2024, which claims priority to Japanese Patent Application No. 2023-153480 filed on Sep. 20, 2023, the entire contents of which are incorporated herein by reference.

BACKGROUND ART

Technical Field

[0002]The present disclosure relates to a centrifugal compressor.

[0003]A centrifugal compressor may include a circulation flow path positioned radially outside a main flow path to curb surging (e.g., Patent Literatures 1 and 2).

CITATION LIST

Patent Literature

SUMMARY

Technical Problem

[0006]The present inventor found that while a centrifugal compressor may curb surging as intended during standalone testing, surging may not be curbed when a centrifugal compressor is connected to an engine.

[0007]The present disclosure aims to provide a centrifugal compressor that can curb surging.

Solution to Problem

[0008]In order to solve the above problem, a centrifugal compressor according to one aspect of the present disclosure includes a compressor impeller and a compressor housing that accommodates the compressor impeller, wherein the compressor housing includes a main flow path that accommodates the compressor impeller and a circulation flow path that is positioned outside the main flow path in a radial direction of the compressor impeller and that is connected to the main flow path, the circulation flow path includes a downstream slit that is connected to the main flow path at a position facing the compressor impeller in the radial direction, an upstream slit that is connected to the main flow path at a position upstream of the downstream slit in the main flow path, and an intermediate flow path that extends along an axial direction of the compressor impeller and that connects the downstream slit to the upstream slit, the intermediate flow path is expanded as the intermediate flow path moves from the downstream slit toward the upstream slit at least in a part continuous with the downstream slit, an outer peripheral surface of the intermediate flow path is inclined radially outward as the outer peripheral surface moves from the downstream slit toward the upstream slit at least in a part continuous with the downstream slit, an inner peripheral surface of the intermediate flow path is inclined radially inward as the inner peripheral surface moves from the downstream slit toward the upstream slit at least in a part continuous with the downstream slit, and an angle of the inner peripheral surface with respect to a straight line passing through the intermediate flow path and parallel to a central axis of the compressor impeller is greater than an angle of the outer peripheral surface with respect to the straight line.

[0009]A spread angle 20 expressed by the following equation (3) may be greater than 20°.

[0016]The spread angle 20 may be greater than 30°.

Effects

[0017]According to the present disclosure, surging can be curbed.

BRIEF DESCRIPTION OF DRAWINGS

[0018]FIG. 1 is a schematic cross-sectional view of a turbocharger including a centrifugal compressor according to an embodiment.

[0019]FIG. 2 is a schematic enlarged cross-sectional view showing area A in FIG. 1.

[0020]FIG. 3 is a graph showing a relationship between a coefficient of flow and a flow rate of air in a circulation flow path.

[0021]FIG. 4 is a schematic enlarged cross-sectional view of a centrifugal compressor according to another embodiment.

[0022]FIG. 5 is a schematic enlarged cross-sectional view of a centrifugal compressor according to yet another embodiment.

[0023]FIG. 6 is a schematic enlarged cross-sectional view of a centrifugal compressor according to yet another embodiment.

DESCRIPTION OF EMBODIMENTS

[0024]Embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. Specific dimensions, materials, and numerical values described in the embodiments are merely examples for better understanding, and do not limit the present disclosure unless otherwise specified. In this specification and the drawings, duplicate explanations are omitted for elements having substantially the same functions and configurations by assigning the same sign. Furthermore, elements not directly related to the present disclosure are omitted from the figures.

[0025]FIG. 1 is a schematic cross-sectional view of a turbocharger TC including a centrifugal compressor C according to an embodiment. FIG. 1 shows a cross-section that is parallel to a central axis of a compressor impeller 4 of the centrifugal compressor C and that passes through the central axis. In the present embodiment, the centrifugal compressor C is incorporated into the turbocharger TC. In another embodiment, the centrifugal compressor C may be incorporated into other devices or may be a standalone device.

[0026]The turbocharger TC includes a housing 1, a shaft 2, a turbine impeller 3, and the compressor impeller 4. As described later, the turbine impeller 3 and

[0027]the compressor impeller 4 rotate integrally with the shaft 2. Accordingly, in the present disclosure, an axial direction, a radial direction, and a circumferential direction of the shaft 2, the turbine impeller 3, and the compressor impeller 4 may simply be referred to as the “axial direction,” the “radial direction,” and the “circumferential direction,” respectively, unless otherwise instructed. Also, in the present disclosure, the center axis of the shaft 2, the turbine impeller 3, and the compressor impeller 4 may simply be referred to as the “center axis” unless otherwise instructed.

[0028]The housing 1 includes a bearing housing 5, a turbine housing 6, and a compressor housing 7. In the axial direction, one end of the bearing housing 5 is connected to the turbine housing 6 by a fastener such as bolts. In the axial direction, the other end of the bearing housing 5 is connected to the compressor housing 7 by a fastener such as bolts.

[0029]The bearing housing 5 includes a bearing hole 5a. The bearing hole 5a extends in the axial direction within the bearing housing 5. The bearing hole 5a accommodates a bearing B. The bearing B rotatably supports the shaft 2. In the present embodiment, a pair of full-floating bearings is used as the bearing B. In another embodiment, other radial bearings, such as a semi-floating bearing or a rolling bearing, may be used as the bearing B.

[0030]The turbine impeller 3 is provided at a first end of the shaft 2 in the axial direction. The turbine impeller 3 rotates integrally with the shaft 2. The turbine housing 6 accommodates the turbine impeller 3 in a rotatable manner. The compressor impeller 4 is provided at a second end that is opposite to the first end of the shaft 2 in the axial direction. The compressor impeller 4 rotates integrally with the shaft 2. The compressor housing 7 accommodates the compressor impeller 4 in a rotatable manner.

[0031]The compressor housing 7 includes an inlet 71 at an end opposite to the bearing housing 5 in the axial direction. The inlet 71 is connected to an air cleaner (not shown).

[0032]The compressor housing 7 includes a main flow path 72. The main flow path 72 is connected to the inlet 71. The compressor impeller 4 is arranged in the main flow path 72. The main flow path 72 extends along the axial direction. The main flow path 72 has a circular cross-sectional shape perpendicular to the axial direction.

[0033]The compressor housing 7 includes a circulation flow path 73. The circulation flow path 73 is positioned radially outside the main flow path 72. The circulation flow path 73 is connected to the main flow path 72. The circulation flow path 73 will be described in detail later.

[0034]The bearing housing 5 and the compressor housing 7 define a diffuser flow path 74 therebetween. The diffuser flow path 74 has an annular shape. The diffuser flow path 74 is positioned radially outside the compressor impeller 4. The diffuser flow path 74 is fluidly connected to the main flow path 72 and the inlet 71.

[0035]The compressor housing 7 includes a compressor scroll flow path 75. The compressor scroll flow path 75 is positioned radially outside the diffuser flow path 74. The compressor scroll flow path 75 is connected to the diffuser flow path 74. The compressor scroll flow path 75 is also fluidly connected to an intake manifold of an engine (not shown).

[0036]As the compressor impeller 4 rotates, air is sucked from the inlet 71 into the main flow path 72. The air is accelerated and pressurized by centrifugal force while passing through the compressor impeller 4. The air is further pressurized while passing through the diffuser flow path 74 and the compressor scroll flow path 75. The pressurized air flows out from an outlet opening (not shown) and is directed to the intake manifold of the engine. In the turbocharger TC, a portion including the compressor impeller 4 and the compressor housing 7 functions as the centrifugal compressor C.

[0037]The turbine housing 6 includes an outlet 61 at an end opposite to the bearing housing 5 in the axial direction. The outlet 61 is connected to an exhaust gas purifier (not shown).

[0038]The turbine housing 6 includes a connecting flow path 62. The connecting flow path 62 has an annular shape. The connecting flow path 62 is positioned radially outside the turbine impeller 3. The connecting flow path 62 is fluidly connected to the outlet 61.

[0039]The turbine housing 6 includes a turbine scroll flow path 63. The turbine scroll flow path 63 is positioned radially outside the connecting flow path 62. The turbine scroll flow path 63 is connected to the connecting flow path 62. Furthermore, the turbine scroll flow path 63 is connected to a gas inlet (not shown). The gas inlet receives exhaust gas discharged from an exhaust manifold of the engine.

[0040]The exhaust gas is directed from the gas inlet into the turbine scroll flow path 63 and further directed through the connecting flow path 62 and the turbine impeller 3 to the outlet 61. The exhaust gas passes through the turbine impeller 3, and rotates the turbine impeller 3. Rotational force of the turbine impeller 3 is transmitted to the compressor impeller 4 via the shaft 2. As the compressor impeller 4 rotates, air is pressurized as described above. As such, the pressurized air is directed to the intake manifold of the engine. In the turbocharger TC, a portion including the turbine impeller 3 and the turbine housing 6 functions as a turbine T.

[0041]Next, the circulation flow path 73 will be described.

[0042]FIG. 2 is a schematic enlarged cross-sectional view showing area A in FIG. 1. As described above, the circulation flow path 73 is positioned radially outside the main flow path 72. The circulation flow path 73 includes a downstream slit 76, an upstream slit 77, and an intermediate flow path 78.

[0043]The downstream slit 76 is connected to the main flow path 72 at a position facing the compressor impeller 4 in the radial direction. The downstream slit 76 has a substantially annular shape, and expands from a radially inner side toward an outer side.

[0044]The upstream slit 77 is connected to the main flow path 72 at a position upstream of the downstream slit 76 in the main flow path 72. The upstream slit 77 is spaced apart from the downstream slit 76 in the axial direction. The upstream slit 77 does not face the compressor impeller 4 in the radial direction. The upstream slit 77 has a substantially annular shape, and expands from a radially inner side toward an outer side.

[0045]The intermediate flow path 78 extends along the axial direction. The intermediate flow path 78 has an annular cross-sectional shape perpendicular to the axial direction. The intermediate flow path 78 includes a first end 78c and a second end 78d in the axial direction. For example, in the present disclosure, the intermediate flow path 78 may refer to a space enclosed by both an outer peripheral surface 78a and an inner peripheral surface 78b in the radial direction. In the axial direction, the inner peripheral surface 78b is shorter than the outer peripheral surface 78a. Accordingly, the first end 78c and the second end 78d correspond to both ends of the inner peripheral surface 78b in the axial direction. The first end 78c is connected to the downstream slit 76. The second end 78d is connected to the upstream slit 77. The intermediate flow path 78 connects the downstream slit 76 to the upstream slit 77.

[0046]Regarding formation of the circulation flow path 73 as described above, for example, the compressor housing 7 according to the present embodiment includes a main body 8, a first ring 9, and a second ring 10.

[0047]The main body 8 includes the outer peripheral surface 78a of the circulation flow path 73.

[0048]The first ring 9 is positioned radially inside the main body 8. The radial gap between the main body 8 and the first ring 9 corresponds to the intermediate flow path 78. In other words, an outer peripheral surface of the first ring 9 corresponds to the inner peripheral surface 78b of the circulation flow path 73. An axial gap between the main body 8 and the first end 78c on the first ring 9 corresponds to the downstream slit 76. For example, the first ring 9 is fixed to the main body 8 by a plurality of vanes 11. The plurality of vanes 11 connect the inner peripheral surface 78b to the outer peripheral surface 78a.

[0049]Referring to FIG. 1, the second ring 10 is fitted into the main body 8. The second ring 10 includes the inlet 71 described above. Referring to FIG. 2, an axial gap between the second end 78d on the first ring 9 and the second ring 10 corresponds to the upstream slit 77.

[0050]In the present embodiment, the intermediate flow path 78 is expanded as the intermediate flow path moves from the downstream slit 76 toward the upstream slit 77. In the present embodiment, the intermediate flow path 78 is expanded over its entire axial length as the intermediate flow path moves from the downstream slit 76 toward the upstream slit 77.

[0051]Specifically, in the cross-section of FIG. 2, the outer peripheral surface 78a of the intermediate flow path 78 is inclined radially outward as the outer peripheral surface moves along the axial direction from the downstream slit 76 toward the upstream slit 77. In other words, a radius of the outer peripheral surface 78a increases as the outer peripheral surface moves along the axial direction from the downstream slit 76 toward the upstream slit 77. In the present embodiment, the outer peripheral surface 78a is inclined radially outward over its entire axial length as the outer peripheral surface moves from the downstream slit 76 toward the upstream slit 77. The outer peripheral surface 78a has a truncated conical shape that tapers from the upstream slit 77 toward the downstream slit 76. In the present embodiment, in the cross-section of FIG. 2, the outer peripheral surface 78a has a straight shape. In another embodiment, in the cross-section of FIG. 2, the outer peripheral surface 78a may have a curved shape.

[0052]In the cross-section of FIG. 2, the inner circumferential surface 78b of the intermediate flow path 78 is inclined radially inward as the inner circumferential surface moves from the downstream slit 76 toward the upstream slit 77. In other words, a radius of the inner circumferential surface 78b decreases as the inner circumferential surface moves along the axial direction from the downstream slit 76 toward the upstream slit 77. In the present embodiment, the inner circumferential surface 78b is inclined radially inward along its entire axial length as the inner circumferential surface moves from the downstream slit 76 toward the upstream slit 77. The inner peripheral surface 78b has a truncated conical shape that tapers from the downstream slit 76 toward the upstream slit 77. In the present embodiment, in the cross-section of FIG. 2, the inner peripheral surface 78b has a straight shape. In another embodiment, in the cross-section of FIG. 2, the inner peripheral surface 78b may have a curved shape.

[0053]In the cross-section of FIG. 2, a straight line (imaginary line) parallel to the central axis and passing through the intermediate flow path 78 is indicated by reference sign X.

[0054]In the cross-section of FIG. 2, an angle of the outer peripheral surface 78a with respect to the straight line X is indicated by reference sign α1. For example, when the outer peripheral surface 78a has a curved shape in the cross-section of FIG. 2, the angle α1 may be defined as an angle between a straight line connecting a point at the first end 78c and a point at the second end 78d on the outer peripheral surface 78a, and the straight line X.

[0055]In the cross-section of FIG. 2, an angle of the inner peripheral surface 78b with respect to the straight line X is indicated by reference sign α2. For example, when the inner peripheral surface 78b has a curved shape in the cross section of FIG. 2, the angle α2 may be defined as an angle between a straight line connecting a point at the first end 78c and a point at the second end 78d on the inner peripheral surface 78b, and the straight line X.

[0056]The angle α2 of the inner circumferential surface 78b with respect to the straight line X is greater than the angle α1 of the outer circumferential surface 78a with respect to the straight line X. In other words, an absolute value of decrease in the radius of the inner circumferential surface 78b from the first end 78c to the second end 78d is greater than an absolute value of increase in the radius of the outer circumferential surface 78a from the first end 78c to the second end 78d. In yet other words, the absolute value of the increase in the radius of the outer circumferential surface 78a from the first end 78c to the second end 78d is smaller than the absolute value of the decrease in the radius of the inner circumferential surface 78b from the first end 78c to the second end 78d. Since the increase in the radius of the outer peripheral surface 78a is smaller, the entire outer peripheral surface 78a can be positioned further radially outward. Consequently, the cross-sectional area of the intermediate flow path 78 can be enlarged, increasing a flow rate of air within the circulation flow path 73. In this case, a room for expanding the intermediate flow path 78 radially outward from the downstream slit 76 toward the upstream slit 77 is small. However, since the decrease in the radius of the inner circumferential surface 78b is large, it is easier to expand the intermediate flow path 78 further radially inward from the downstream slit 76 toward the upstream slit 77.

[0057]In the present embodiment, a spread angle 20 expressed by the following equation (3) is used as an indicator showing the degree to which the intermediate flow path 78 is expanded.

[0064]In the present embodiment, the spread angle 20 is greater than 20°. Furthermore, in the present embodiment, the spread angle 20 may be greater than 30°.

[0065]Next, operation of the centrifugal compressor C according to the present embodiment will be described.

[0066]In the centrifugal compressor C, when a flow rate of air flowing through the main flow path 72 decreases, a part of the air flows backward in an area around ends of blades of the compressor impeller 4. The air flowing backward flows into the circulation flow path 73 through the downstream slit 76. The air flows from the first end 78c to the second end 78d through the intermediate flow path 78, and returns to the main flow path 72 through the upstream slit 77. In this way, an effect of backflow on the compressor impeller 4 can be reduced, and surging can be curbed.

[0067]However, the present inventor found that while a centrifugal compressor can curb surging as intended during standalone testing, surging may not be curbed when a centrifugal compressor is connected to an engine. Specifically, when a centrifugal compressor is connected to an engine, a flow rate of air within a main flow path fluctuates. Consequently, a flow rate of air within a circulation flow path also fluctuates. The inventor found that when a flow rate of air in a circulation flow path fluctuates significantly, surge cannot be curbed as intended.

[0068]In this regard, in the present embodiment, since the intermediate flow path 78 is expanded from the downstream slit 76 toward the upstream slit 77, a flow of air is likely to separate from the outer peripheral surface 78a and the inner peripheral surface 78b. In other words, in the present embodiment, the intermediate flow path 78 is expanded from the downstream slit 76 toward the upstream slit 77 so that the flow of air separates from the outer peripheral surface 78a and the inner peripheral surface 78b. As such, in the intermediate flow path 78, the flow of air is likely to turbulence, and energy of the flow is easily lost. Accordingly, even if pressure that pushes air from the main flow path 72 into the circulation flow path 73 increases, the flow rate of air in the intermediate flow path 78 does not easily increase. In other words, even if the flow rate of air in the main flow path 72 fluctuates, the flow rate of air in the circulation flow path 73 does not easily fluctuate. Accordingly, the circulation flow path 73 is likely to operate as intended. As a result, when the centrifugal compressor C is connected to an engine, surging can be curbed.

[0069]In particular, in the present embodiment, the angle α2 of the inner peripheral surface 78b with respect to the straight line X is larger than the angle α1 of the outer peripheral surface 78a with respect to the straight line X, as described above. According to such a configuration, the intermediate flow path 78 can largely be expanded radially inward from the downstream slit 76 toward the upstream slit 77, as described above. As such, the flow of air easily separates from the outer peripheral surface 78a and the inner peripheral surface 78b, and the flow rate of air in the circulation flow path 73 is less likely to fluctuate. As a result, when the centrifugal compressor C is connected to the engine, surging can be curbed.

[0070]Furthermore, in the present embodiment, the spread angle 2θ is greater than 20°. The present inventor found that when the spread angle 2θ is greater than 20°, the flow of air is more likely to separate from the outer peripheral surface 78a and the inner peripheral surface 78b, and the flow rate of air in the circulation flow path 73 is less likely to fluctuate. Furthermore, in the present embodiment, the spread angle 2θ may be greater than 30°. In this case, the flow of air is even more likely to separate from the outer peripheral surface 78a and the inner peripheral surface 78b, and the flow rate of air in the circulation flow path 73 is even less likely to fluctuate.

[0071]FIG. 3 is a graph showing a relationship between a coefficient of flow and the flow rate of air in the circulation flow path 73. FIG. 3 shows a result of analysis for the embodiment (triangular plots) and a result of analysis for a comparative example (square plots). In the analyses of FIG. 3, a rotational speed of the compressor impeller 4 was kept constant. In FIG. 3, the horizontal axis indicates the coefficient of flow. The coefficient of flow relates to the pressure pushing the air into the circulation flow path 73, i.e., a difference between pressure at the downstream slit 76 and pressure at the upstream slit 77. Specifically, since the rotational speed of the compressor impeller 4 was kept constant, i.e., energy applied to the air is constant, as the coefficient of flow increases (the coefficient of flow moves to the right), the pressure pushing the air into the circulation flow path 73 decreases, and as the coefficient of flow decreases (the coefficient of flow moves to the left), the pressure pushing the air into the circulation flow path 73 increases. In other words, fluctuation in the coefficient of flow corresponds to fluctuation in the pressure pushing the air into the circulation flow path 73. The vertical axis indicates the flow rate of air in the circulation flow path 73. This flow rate is expressed as a percentage (%) with respect to a flow rate of air in the main flow passage 72.

[0072]In the centrifugal compressor C of the embodiment (triangular plots), the circulation flow path 73 is expanded from the downstream slit 76 toward the upstream slit 77, as described above. In contrast, in the centrifugal compressor C of the comparative example (square plots), the circulation flow path does not expand from the downstream slit toward the upstream slit.

[0073]As shown in FIG. 3, overall, the flow rate of air in the circulation flow path 73 of the embodiment is lower than that of the comparative example. However, a slope of the flow rate of air in the circulation flow path 73 of the embodiment is smaller than that of the comparative example. This means that in the centrifugal compressor C of the embodiment, even if the coefficient of flow fluctuates, i.e., even if the pressure pushing the air into the circulation flow path 73 fluctuates, the flow rate or air in the circulation flow path 73 is less likely to fluctuate, compared to the comparative example. It is understood also from these analysis results that the flow rate of air in the circulation flow path 73 is less likely to fluctuate in the centrifugal compressor C of the embodiment.

[0074]The centrifugal compressor C according to the present embodiment as described above includes the compressor impeller 4 and the compressor housing 7 that accommodates the compressor impeller 4. The compressor housing 7 includes the main flow path 72 that accommodates the compressor impeller 4, and the circulation flow path 73 that is positioned outside the main flow path 72 in the radial direction and that is connected to the main flow path 72. The circulation flow path 73 includes the downstream slit 76 that is connected to the main flow path 72 at the position facing the compressor impeller 4 in the radial direction, the upstream slit 77 that is connected to the main flow path 72 at the position upstream of the downstream slit 76 in the main flow path 72, and the intermediate flow path 78 that extends along the axial direction and that connects the downstream slit 76 to the upstream slit 77. The intermediate flow path 78 is expanded as the intermediate flow path moves from the downstream slit 76 toward the upstream slit 77. The outer peripheral surface 78a of the intermediate flow path 78 is inclined radially outward as the outer peripheral surface moves from the downstream slit 76 toward the upstream slit 77, and the inner peripheral surface 78b is inclined radially inward as the inner peripheral surface moves from the downstream slit 76 toward the upstream slit 77. The angle α2 of the inner peripheral surface 78b with respect to the straight line X that passes through the intermediate flow path 78 and that is parallel to the central axis is greater than the angle α1 of the outer peripheral surface 78a with respect to the straight line X. According to such a configuration, it is easier to expand the intermediate flow path 78 further radially inward from the downstream slit 76 toward the upstream slit 77, as described above. Accordingly, the flow of air is likely to separate from the outer peripheral surface 78a and the inner peripheral surface 78b, and the flow rate of air in the circulation flow path 73 does not easily fluctuate. As a result, when the centrifugal compressor C is connected to the engine, surging can be curbed.

[0075]Furthermore, in the centrifugal compressor C, the spread angle 2θ is greater than 20°. In this case, the flow of air is likely to separate from the outer peripheral surface 78a and the inner peripheral surface 78b, and the flow rate of air within the circulation flow path 73 does not easily fluctuate.

[0076]Furthermore, in centrifugal compressor C, the spread angle 2θ may be greater than 30°. In this case, the flow of air is more likely to separate from the outer peripheral surface 78a and the inner peripheral surface 78b, and the flow rate of air within the circulation flow path 73 fluctuates even less.

[0077]Although an embodiment of the present disclosure has been described above with reference to the accompanying drawings, the present disclosure is not limited thereto. It is obvious that a person skilled in the art can conceive of various examples of variations or modifications within the scope of the claims, which are also understood to belong to the technical scope of the present disclosure.

[0078]For example, in the above embodiment, the intermediate flow path 78 is expanded along its entire axial length as it moves from the downstream slit 76 toward the upstream slit 77, the outer peripheral surface 78a of the intermediate flow path 78 is inclined radially outward along its entire axial length as it moves from the downstream slit 76 toward the upstream slit 77, and the inner peripheral surface 78b is inclined radially inward along its entire axial length as it moves from the downstream slit 76 toward the upstream slit 77. In another embodiment, the intermediate flow path 78 may be expanded as it moves from the downstream slit 76 toward the upstream slit 77 at least in a part that is continuous with the downstream slit 76, the outer peripheral surface 78a of the intermediate flow path 78 may be inclined radially outward as it moves from the downstream slit 76 toward the upstream slit 77 at least in a part that is continuous with the downstream slit 76, and the inner peripheral surface 78b may be inclined radially inward as it moves from the downstream slit 76 toward the upstream slit 77 at least in a part that is continuous with the downstream slit 76.

[0079]FIG. 4 is a schematic enlarged cross-sectional view of a centrifugal compressor according to another embodiment. For example, in the embodiment of FIG. 4, the outer peripheral surface 78a of the intermediate flow path 78 is parallel to the straight line X near the second end 78d connected to the upstream slit 77. In other words, the outer peripheral surface 78a may not be inclined radially outward as the outer peripheral surface moves from the downstream slit 76 toward the upstream slit 77 in some area except for the part continuous with the downstream slit 76. In this case, the angle α1 may be defined as an angle between a straight line connecting a point on the first end 78c and a point on the second end 78d of the outer peripheral surface 78a, and the straight line X.

[0080]FIG. 5 is a schematic enlarged cross-sectional view of a centrifugal compressor according to yet another embodiment. For example, in the embodiment of FIG. 5, in addition to the outer peripheral surface 78a, the inner peripheral surface 78b is also parallel to the straight line X near the second end 78d connected to the upstream slit 77. In other words, the inner peripheral surface 78b may not be inclined radially inward as the inner peripheral surface moves from the downstream slit 76 toward the upstream slit 77 in some area except for the part continuous with the downstream slit 76. In this case, the angle α2 may be defined as an angle between a straight line connecting a point on the first end 78c and a point on the second end 78d of the inner peripheral surface 78b, and the straight line X. Note that in the embodiment of FIG. 5, the outer peripheral surface 78a may not include the part parallel to the straight line X.

[0081]FIG. 6 is a schematic enlarged cross-sectional view of a centrifugal compressor according to yet another embodiment. For example, in the embodiment of FIG. 6, the outer peripheral surface 78a is parallel to the straight line X near the second end 78d connected to the upstream slit 77 and includes a step between the part parallel to the straight line X and the part inclined with respect to the straight line X. In this case, the angle α1 may be defined as an angle between a line connecting a point on first end 78c and a point on second end 78d of outer peripheral surface 78a, and the straight line X. Similarly, the inner peripheral surface 78b is parallel to the straight line X near the second end 78d connected to the upstream slit 77 and includes a step between the part parallel to the straight line X and the part inclined with respect to straight line X. In this case, the angle α₂ may be defined as an angle between the straight line connecting a point on the first end 78c and a point on the second end 78d of the inner circumferential surface 78b, and the straight line X. Note that in the embodiment of FIG. 6, one of the outer circumferential surface 78a and the inner circumferential surface 78b may not include the step.