Centrifugal compressor

Abstract

A circulating flow path (73) of this centrifugal compressor includes a downstream slit (76), an upstream slit (77), and an intermediate flow path (78) that connects the downstream slit (76) to the upstream slit (77). At least a portion of the intermediate flow path (78) that is continuous with the downstream slit (76) expands with increasing distance from the downstream slit (76) toward the upstream slit (77), at least a portion of an outer peripheral surface (78a) of the intermediate flow path (78) that is continuous with the downstream slit (76) is inclined radially outward with increasing distance from the downstream slit (76) toward the upstream slit (77), and at least a portion of an inner peripheral surface (78b) that is continuous with the downstream slit (76) is inclined radially inward with increasing distance from the downstream slit (76) toward the upstream slit (77). An angle (α2) of the inner peripheral surface (78b) with respect to a straight line (X) that passes through the intermediate flow path (78) and that is parallel to a central axis is greater than an angle (α1) of the outer peripheral surface (78a) with respect to the straight line (X).

Description

centrifugal compressor

[0001] This application claims the benefit of priority from Japanese Patent Application No. 2023-153480, filed on September 20, 2023, the contents of which are incorporated herein by reference.

[0002] In order to suppress surging, a centrifugal compressor may include a circulation passage located radially outside the main passage (see, for example, Patent Documents 1 and 2).

[0003] Patent No. 6598388 Patent No. 7123029

[0004] The inventors have found that even if a centrifugal compressor can suppress surging as intended in a stand-alone test, surging may not be suppressed when the centrifugal compressor is connected to an engine.

[0005] An object of the present disclosure is to provide a centrifugal compressor capable of suppressing surging.

[0006] In order to solve the above problems, a centrifugal compressor according to one aspect of the present disclosure includes a compressor impeller and a compressor housing that accommodates the compressor impeller, the compressor housing including a main flow path that accommodates the compressor impeller, and a circulation flow path that is located outside the main flow path in a radial direction of the compressor impeller and is connected to the main flow path, the circulation flow path including 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 a slit that extends along the axial direction of the compressor impeller and connects the downstream slit to the upstream slit. and an intermediate flow passage connecting the downstream slit to the upstream slit, wherein the intermediate flow passage expands at least in a portion continuous with the downstream slit as the flow moves from the downstream slit to the upstream slit, the outer circumferential surface of the intermediate flow passage inclines radially outward at least in a portion continuous with the downstream slit as the flow moves from the downstream slit to the upstream slit, and the inner circumferential surface of the intermediate flow passage inclines radially inward at least in a portion continuous with the downstream slit as the flow moves from the downstream slit to the upstream slit, and the angle of the inner circumferential surface with respect to a straight line that passes through the intermediate flow passage and is parallel to the central axis of the compressor impeller is greater than the angle of the outer circumferential surface with respect to the straight line.

[0007] The divergence angle 2θ shown in the following formula (3) may be greater than 20°. However, A ｄｓ A: cross-sectional area of ​​the intermediate flow passage at the first end connected to the downstream slit ｕｓ : Cross-sectional area of ​​the intermediate flow passage at the second end connected to the upstream slit D ｄｓ : equivalent diameter of the intermediate flow passage at the first end D ｕｓ : equivalent diameter of the intermediate flow passage at the second end L: distance between the first end and the second end in the axial direction

[0008] The divergence angle 2θ may be greater than 30°.

[0009] According to the present disclosure, surging can be suppressed.

[0010] Fig. 1 is a schematic cross-sectional view of a turbocharger including a centrifugal compressor according to an embodiment. Fig. 2 is a schematic enlarged cross-sectional view showing part A in Fig. 1. Fig. 3 is a graph showing the relationship between the flow coefficient and the flow rate of air in a circulation flow path. Fig. 4 is a schematic enlarged cross-sectional view of a centrifugal compressor according to another embodiment. Fig. 5 is a schematic enlarged cross-sectional view of a centrifugal compressor according to yet another embodiment. Fig. 6 is a schematic enlarged cross-sectional view of a centrifugal compressor according to yet another embodiment.

[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Dimensions, materials, specific numerical values, etc. shown in the embodiments are merely examples for ease of understanding and do not limit the present disclosure unless otherwise specified. In this specification and drawings, elements having substantially the same functions and configurations are designated by the same reference numerals to avoid redundant explanation. Elements not directly related to the present disclosure are not shown.

[0012] 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 parallel to and passing through the central axis of a compressor impeller 4 of the centrifugal compressor C. In this embodiment, the centrifugal compressor C is incorporated into the turbocharger TC. In other embodiments, the centrifugal compressor C may be incorporated into another device or may be a stand-alone device.

[0013] The turbocharger TC includes a housing 1 , a shaft 2 , a turbine wheel 3 , and a compressor wheel 4 .

[0014] As will be described later, the turbine wheel 3 and the compressor wheel 4 rotate integrally with the shaft 2. Therefore, in this disclosure, the axial direction, radial direction, and circumferential direction of the shaft 2, the turbine wheel 3, and the compressor wheel 4 may be simply referred to as the "axial direction," the "radial direction," and the "circumferential direction," respectively, unless otherwise specified. Also, in this disclosure, the central axis of the shaft 2, the turbine wheel 3, and the compressor wheel 4 may be simply referred to as the "central axis," unless otherwise specified.

[0015] The housing 1 includes a bearing housing 5, a turbine housing 6, and a compressor housing 7. One end of the bearing housing 5 is axially connected to the turbine housing 6 by a fastener such as a bolt. The other end of the bearing housing 5 is axially connected to the compressor housing 7 by a fastener such as a bolt.

[0016] 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 this embodiment, a pair of full-floating bearings are used as the bearings B. In other embodiments, other radial bearings such as semi-floating bearings or rolling bearings may be used as the bearings B.

[0017] A turbine wheel 3 is provided at a first end of the shaft 2 in the axial direction. The turbine wheel 3 rotates integrally with the shaft 2. A turbine housing 6 rotatably accommodates the turbine wheel 3. A compressor wheel 4 is provided at a second end of the shaft 2 opposite the first end in the axial direction. The compressor wheel 4 rotates integrally with the shaft 2. A compressor housing 7 rotatably accommodates the compressor wheel 4.

[0018] The compressor housing 7 includes an air intake port 71 at an end opposite to the bearing housing 5 in the axial direction. The air intake port 71 is connected to an air cleaner (not shown).

[0019] The compressor housing 7 includes a main flow path 72. The main flow path 72 is connected to the intake port 71. The compressor impeller 4 is disposed 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.

[0020] The compressor housing 7 includes a circulation flow path 73. The circulation flow path 73 is located radially outward of 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.

[0021] The bearing housing 5 and the compressor housing 7 define a diffuser passage 74 therebetween. The diffuser passage 74 has an annular shape. The diffuser passage 74 is located radially outward from the compressor impeller 4. The diffuser passage 74 is in fluid communication with the main passage 72 and the intake port 71.

[0022] The compressor housing 7 includes a compressor scroll passage 75. The compressor scroll passage 75 is located radially outward of the diffuser passage 74. The compressor scroll passage 75 is connected to the diffuser passage 74. The compressor scroll passage 75 is also in fluid communication with an intake manifold of the engine (not shown).

[0023] When the compressor impeller 4 rotates, air is drawn into the main flow passage 72 from the intake port 71. The air is accelerated and pressurized by centrifugal force while passing through the compressor impeller 4. The air is further pressurized when passing through the diffuser flow passage 74 and the compressor scroll flow passage 75. The pressurized air flows out from a discharge port (not shown) and is led 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 a centrifugal compressor C.

[0024] The turbine housing 6 includes a discharge port 61 at an end opposite to the bearing housing 5 in the axial direction. The discharge port 61 is connected to an exhaust gas purification device (not shown).

[0025] The turbine housing 6 includes a connecting passage 62. The connecting passage 62 has an annular shape. The connecting passage 62 is located radially outward from the turbine wheel 3. The connecting passage 62 is in fluid communication with the discharge port 61.

[0026] The turbine housing 6 includes a turbine scroll passage 63. The turbine scroll passage 63 is located radially outward of the connecting passage 62. The turbine scroll passage 63 is connected to the connecting passage 62. The turbine scroll passage 63 is also connected to a gas inlet (not shown). The gas inlet receives exhaust gas discharged from an exhaust manifold of the engine.

[0027] Exhaust gas is guided from the gas inlet to the turbine scroll passage 63, and further to the discharge port 61 via the connecting passage 62 and the turbine wheel 3. The exhaust gas rotates the turbine wheel 3 while passing through it. The rotational force of the turbine wheel 3 is transmitted to the compressor wheel 4 via the shaft 2. When the compressor wheel 4 rotates, the air is compressed as described above. The compressed air is then guided to the intake manifold of the engine. In the turbocharger TC, a portion including the turbine wheel 3 and the turbine housing 6 functions as the turbine T.

[0028] Next, the circulation flow path 73 will be described.

[0029] Fig. 2 is a schematic enlarged cross-sectional view showing part A in Fig. 1. As described above, the circulation flow path 73 is located radially outward of 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.

[0030] The downstream slit 76 is connected to the main flow passage 72 at a position radially opposite the compressor impeller 4. The downstream slit 76 has a generally annular shape and expands from the inner side toward the outer side in the radial direction.

[0031] The upstream slit 77 is connected to the main flow passage 72 at a position in the main flow passage 72 that is upstream of the downstream slit 76. The upstream slit 77 is separated 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 generally annular shape and expands from the inner side toward the outer side in the radial direction.

[0032] The intermediate flow passage 78 extends along the axial direction. The intermediate flow passage 78 has a circular cross-sectional shape perpendicular to the axial direction. The intermediate flow passage 78 includes a first end 78c and a second end 78d in the axial direction. For example, in this disclosure, the intermediate flow passage 78 may refer to a space surrounded by both the outer circumferential surface 78a and the inner circumferential surface 78b in the radial direction. The inner circumferential surface 78b is shorter than the outer circumferential surface 78a in the axial direction. Therefore, the first end 78c and the second end 78d correspond to both ends of the inner circumferential 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 passage 78 connects the downstream slit 76 to the upstream slit 77.

[0033] Regarding the formation of the circulation flow path 73 as described above, for example, the compressor housing 7 according to this embodiment includes a main body 8 , a first ring 9 , and a second ring 10 .

[0034] The main body 8 includes an outer peripheral surface 78 a of the circulation flow path 73 .

[0035] The first ring 9 is disposed radially inside the main body 8. The radial gap between the main body 8 and the first ring 9 corresponds to the intermediate flow passage 78. That is, the outer peripheral surface of the first ring 9 corresponds to the inner peripheral surface 78b of the circulation flow passage 73. The axial gap between the main body 8 and a 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.

[0036] 1, the second ring 10 is fitted into the main body 8. The second ring 10 includes the above-mentioned air intake port 71. Referring to FIG. 2, the axial gap between the second end 78d on the first ring 9 and the second ring 10 corresponds to the upstream slit 77.

[0037] In this embodiment, the intermediate flow passage 78 expands as you move from the downstream slit 76 towards the upstream slit 77. In this embodiment, the intermediate flow passage 78 expands over its entire axial length as you move from the downstream slit 76 towards the upstream slit 77.

[0038] Specifically, in the cross section of FIG. 2 , the outer circumferential surface 78 a of the intermediate flow passage 78 slopes radially outward as one moves axially from the downstream slit 76 to the upstream slit 77. In other words, the radius of the outer circumferential surface 78 a increases as one moves axially from the downstream slit 76 to the upstream slit 77. In this embodiment, the outer circumferential surface 78 a slopes radially outward over its entire axial length as one moves from the downstream slit 76 to the upstream slit 77. The outer circumferential surface 78 a has a frusto-conical shape that tapers from the upstream slit 77 to the downstream slit 76. In this embodiment, in the cross section of FIG. 2 , the outer circumferential surface 78 a has a linear shape. In other embodiments, the outer circumferential surface 78 a may have a curved shape in the cross section of FIG. 2 .

[0039] In the cross section of FIG. 2 , the inner circumferential surface 78 b of the intermediate flow passage 78 slopes radially inward as one moves from the downstream slit 76 toward the upstream slit 77. In other words, the radius of the inner circumferential surface 78 b decreases as one moves axially from the downstream slit 76 toward the upstream slit 77. In this embodiment, the inner circumferential surface 78 b slopes radially inward over its entire axial length as one moves from the downstream slit 76 toward the upstream slit 77. The inner circumferential surface 78 b has a frusto-conical shape that tapers from the downstream slit 76 toward the upstream slit 77. In this embodiment, in the cross section of FIG. 2 , the inner circumferential surface 78 b has a linear shape. In other embodiments, the inner circumferential surface 78 b may have a curved shape in the cross section of FIG. 2 .

[0040] In the cross section of FIG. 2, a straight line (imaginary line) that is parallel to the central axis and passes through the intermediate flow passage 78 is indicated by the symbol X.

[0041] 2, the angle of the outer peripheral surface 78a with respect to the line X is indicated by the symbol α1. For example, if the outer peripheral surface 78a has a curved shape in the cross section of FIG. 2, the angle α1 may be defined as the angle between the line X and a line connecting a point on the first end 78c and a point on the second end 78d of the outer peripheral surface 78a.

[0042] 2, the angle of the inner circumferential surface 78b with respect to the line X is indicated by the symbol α2. For example, if the inner circumferential surface 78b has a curved shape in the cross section of FIG. 2, the angle α2 may be defined as the angle between the line X and a line connecting a point on the first end 78c and a point on the second end 78d of the inner circumferential surface 78b.

[0043] The angle α2 of the inner circumferential surface 78b with respect to the line X is greater than the angle α1 of the outer circumferential surface 78a with respect to the line X. In other words, 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 is greater than 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. In 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 increase in the radius of the inner circumferential surface 78b from the first end 78c to the second end 78d. Because the increase in the radius of the outer circumferential surface 78a is small, the entire outer circumferential surface 78a can be positioned further radially outward. This allows the cross-sectional area of ​​the intermediate flow passage 78 to be enlarged, thereby increasing the air flow rate in the circulation flow passage 73. In this case, there is little room to expand the intermediate flow passage 78 radially outward from the downstream slit 76 toward the upstream slit 77. However, since the increase in the radius of the inner circumferential surface 78 b is large, the intermediate flow passage 78 tends to expand significantly radially inward from the downstream slit 76 toward the upstream slit 77 .

[0044] In this embodiment, the expansion angle 2θ expressed by the following equation (3) is used as an index indicating the extent to which the intermediate flow path 78 is expanded. However, A ｄｓ A: Cross-sectional area of ​​the intermediate flow passage 78 at the first end 78c connected to the downstream slit 76 ｕｓ D: Cross-sectional area of ​​the intermediate flow passage 78 at the second end 78d connected to the upstream slit 77 ｄｓ : equivalent diameter of the intermediate flow passage 78 at the first end 78c D ｕｓ : equivalent diameter of the intermediate flow passage 78 at the second end 78d L: distance between the first end 78c and the second end 78d in the axial direction

[0045] In this embodiment, the spread angle 2θ is greater than 20°. Also, in this embodiment, the spread angle 2θ may be greater than 30°.

[0046] Next, the operation of the centrifugal compressor C according to this embodiment will be described.

[0047] In the centrifugal compressor C, when the flow rate of air flowing through the main flow passage 72 decreases, a portion of the air flows back near the blade ends of the compressor impeller 4. The backflowing air flows into the circulation flow passage 73 through the downstream slits 76. The air flows through the intermediate flow passage 78 from the first end 78c to the second end 78d, and returns to the main flow passage 72 through the upstream slits 77. In this way, the effect of the backflow on the compressor impeller 4 can be reduced, and surging can be suppressed.

[0048] However, the present inventors have found that even if a centrifugal compressor can suppress surging as intended in a stand-alone test, surging may not be suppressed when the centrifugal compressor is connected to an engine. Specifically, when a centrifugal compressor is connected to an engine, the flow rate of air in the main flow path fluctuates. Therefore, the flow rate of air in the circulation flow path also fluctuates. The present inventors have found that if the flow rate of air in the circulation flow path fluctuates significantly, surging cannot be suppressed as intended.

[0049] In this regard, in the present embodiment, the intermediate flow passage 78 expands from the downstream slit 76 toward the upstream slit 77, making it easier for the airflow to separate from the outer peripheral surface 78a and the inner peripheral surface 78b. In other words, in the present embodiment, the intermediate flow passage 78 expands from the downstream slit 76 toward the upstream slit 77 so that the airflow separates from the outer peripheral surface 78a and the inner peripheral surface 78b. Therefore, the airflow is easily turbulent in the intermediate flow passage 78, and the energy of the flow is easily lost. Therefore, even if the pressure pushing the air from the main flow passage 72 into the circulation flow passage 73 increases, the airflow rate in the intermediate flow passage 78 is unlikely to increase. In other words, even if the airflow rate in the main flow passage 72 fluctuates, the airflow rate in the circulation flow passage 73 is unlikely to fluctuate. Therefore, the circulation flow passage 73 is likely to operate as intended. As a result, when the centrifugal compressor C is connected to an engine, surging can be suppressed.

[0050] In particular, in this embodiment, as described above, the angle α2 of the inner circumferential surface 78b with respect to the line X is larger than the angle α1 of the outer circumferential surface 78a with respect to the line X. With this configuration, as described above, it is easy to greatly expand the intermediate flow passage 78 radially inward from the downstream slit 76 toward the upstream slit 77. Therefore, the air flow is easily separated from the outer circumferential surface 78a and the inner circumferential surface 78b, and the flow rate of air in the circulation flow passage 73 is less likely to fluctuate. As a result, when the centrifugal compressor C is connected to an engine, surging can be suppressed.

[0051] In this embodiment, the divergence angle 2θ is greater than 20°. The inventors have found that when the divergence angle 2θ is greater than 20°, the air flow is more likely to separate from the outer circumferential surface 78a and the inner circumferential surface 78b, and the flow rate of air in the circulation flow path 73 is less likely to fluctuate. Furthermore, in this embodiment, the divergence angle 2θ may be greater than 30°. In this case, the air flow is more likely to separate from the outer circumferential surface 78a and the inner circumferential surface 78b, and the flow rate of air in the circulation flow path 73 is less likely to fluctuate.

[0052] FIG. 3 is a graph showing the relationship between the flow coefficient and the air flow rate in the circulation flow path 73. FIG. 3 shows the analysis results of the embodiment (triangle plots) and the comparative example (square plots). In the analysis of FIG. 3, the rotation speed of the compressor impeller 4 was constant. In FIG. 3, the horizontal axis represents the flow coefficient. The flow coefficient is related to the pressure that pushes air into the circulation flow path 73, i.e., the difference between the pressure at the downstream slit 76 and the pressure at the upstream slit 77. Specifically, since the rotation speed of the compressor impeller 4 is constant, i.e., the energy imparted to the air is constant, as the flow coefficient increases (the flow coefficient moves to the right), the pressure that pushes air into the circulation flow path 73 decreases, and as the flow coefficient decreases (the flow coefficient moves to the left), the pressure that pushes air into the circulation flow path 73 increases. In other words, fluctuations in the flow coefficient correspond to fluctuations in the pressure that pushes air into the circulation flow path 73. The vertical axis represents the air flow rate in the circulation flow path 73. This flow rate is expressed as a percentage (%) of the air flow rate in the main flow path 72 .

[0053] In the centrifugal compressor C according to the embodiment (triangle plot), as described above, the circulation flow path 73 expands from the downstream slit 76 toward the upstream slit 77. In contrast, in the centrifugal compressor C according to the comparative example (square plot), the circulation flow path does not expand from the downstream slit toward the upstream slit.

[0054] 3, the flow rate of air in the circulation flow path 73 according to the embodiment is generally lower than that of the comparative example. However, the gradient of the flow rate of air in the circulation flow path 73 according to the embodiment is smaller than that of the comparative example. This means that in the centrifugal compressor C according to the embodiment, the flow rate of air in the circulation flow path 73 is less likely to fluctuate compared to the comparative example, even if the flow coefficient fluctuates, i.e., even if the pressure that pushes air into the circulation flow path 73 fluctuates. This analysis result also shows that in the centrifugal compressor C according to the embodiment, the flow rate of air in the circulation flow path 73 is less likely to fluctuate.

[0055] As described above, the centrifugal compressor C according to this embodiment includes the compressor impeller 4 and the compressor housing 7 that accommodates the compressor impeller 4. The compressor housing 7 includes a main flow passage 72 that accommodates the compressor impeller 4 and a circulation flow passage 73 that is located radially outward of the main flow passage 72 and connected to the main flow passage 72. The circulation flow passage 73 includes a downstream slit 76 that is connected to the main flow passage 72 at a position facing the compressor impeller 4 in the radial direction, an upstream slit 77 that is connected to the main flow passage 72 at a position upstream of the downstream slit 76 in the main flow passage 72, and an intermediate flow passage 78 that extends along the axial direction and connects the downstream slit 76 to the upstream slit 77. The intermediate flow passage 78 expands as it moves from the downstream slit 76 toward the upstream slit 77. The outer peripheral surface 78a of the intermediate flow passage 78 inclines radially outward as the flow moves from the downstream slit 76 toward the upstream slit 77, while the inner peripheral surface 78b inclines radially inward as the flow moves from the downstream slit 76 toward the upstream slit 77. The angle α2 of the inner peripheral surface 78b with respect to a line X that passes through the intermediate flow passage 78 and is parallel to the central axis is larger than the angle α1 of the outer peripheral surface 78a with respect to the line X. With this configuration, as described above, the intermediate flow passage 78 is likely to expand significantly radially inward from the downstream slit 76 toward the upstream slit 77. Therefore, the air flow 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 passage 73 is unlikely to fluctuate. As a result, when the centrifugal compressor C is connected to an engine, surging can be suppressed.

[0056] In the centrifugal compressor C, the divergence angle 2θ is greater than 20°. In this case, the air flow is easily separated from the outer circumferential surface 78a and the inner circumferential surface 78b, and the flow rate of air in the circulation flow path 73 is less likely to fluctuate.

[0057] Furthermore, in the centrifugal compressor C, the divergence angle 2θ may be greater than 30°. In this case, the air flow is more likely to separate from the outer circumferential surface 78a and the inner circumferential surface 78b, and the flow rate of air in the circulation flow path 73 is less likely to fluctuate.

[0058] Although the embodiments of the present disclosure have been described above with reference to the accompanying drawings, it goes without saying that the present disclosure is not limited to such embodiments. It is clear that a person skilled in the art can conceive of various modifications and alterations within the scope of the claims, and it is understood that such modifications and alterations also fall within the technical scope of the present disclosure.

[0059] For example, in the above embodiment, the intermediate flow passage 78 expands over its entire axial length as it moves from the downstream slit 76 toward the upstream slit 77, the outer circumferential surface 78a of the intermediate flow passage 78 slopes radially outward over its entire axial length as it moves from the downstream slit 76 toward the upstream slit 77, and the inner circumferential surface 78b slopes radially inward over its entire axial length as it moves from the downstream slit 76 toward the upstream slit 77. In other embodiments, the intermediate flow passage 78 may expand over at least a portion continuous with the downstream slit 76 as it moves from the downstream slit 76 toward the upstream slit 77, the outer circumferential surface 78a of the intermediate flow passage 78 may slope radially outward over at least a portion continuous with the downstream slit 76 as it moves from the downstream slit 76 toward the upstream slit 77, and the inner circumferential surface 78b may slope radially inward over at least a portion continuous with the downstream slit 76 as it moves from the downstream slit 76 toward the upstream slit 77.

[0060] 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 circumferential surface 78a of the intermediate flow passage 78 is parallel to the line X near the second end 78d connected to the upstream slit 77. That is, the outer circumferential surface 78a, except for a portion continuing to the downstream slit 76, does not need to be inclined radially outward as it moves from the downstream slit 76 toward the upstream slit 77. In this case, the angle α1 may be defined as the angle between the line X and a line connecting a point on the first end 78c of the outer circumferential surface 78a to a point on the second end 78d.

[0061] 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 78 a, the inner peripheral surface 78 b is also parallel to the line X near the second end 78 d connected to the upstream slit 77. That is, the inner peripheral surface 78 b does not need to be inclined radially inward as it moves from the downstream slit 76 toward the upstream slit 77, except for a portion connected to the downstream slit 76. In this case, the angle α2 may be defined as the angle between the line X and a line connecting a point on the first end 78 c of the inner peripheral surface 78 b to a point on the second end 78 d. Note that in the embodiment of FIG. 5 , the outer peripheral surface 78 a does not need to include a portion parallel to the line X.

[0062] 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 circumferential surface 78 a is parallel to the line X near a second end 78 d connected to the upstream slit 77 and includes a step between a portion that is inclined with respect to the line X and a portion that is parallel to the line X. In this case, the angle α1 may be defined as the angle between the line X and a line that connects a point on the first end 78 c of the outer circumferential surface 78 a to a point on the second end 78 d. Similarly, the inner circumferential surface 78 b is parallel to the line X near the second end 78 d connected to the upstream slit 77 and includes a step between a portion that is inclined with respect to the line X and a portion that is parallel to the line X. In this case, the angle α2 may be defined as the angle between the line X and a line that connects a point on the first end 78 c of the inner circumferential surface 78 b to a point on the second end 78 d. Note that in the embodiment of FIG. 6 , one of the outer circumferential surface 78 a and the inner circumferential surface 78 b does not necessarily have to include a step.

[0063] 4 Compressor impeller 7 Compressor housing 72 Main flow path 73 Circulation flow path 76 Downstream slit 77 Upstream slit 78 Intermediate flow path 78a Outer circumferential surface 78b Inner circumferential surface 78c First end 78d Second end C Centrifugal compressor X Straight line passing through the intermediate flow path and parallel to the central axis of the compressor impeller α1 Angle of the outer circumferential surface with respect to the line α2 Angle of the inner circumferential surface with respect to the line

Claims

1. A compressor impeller; and a compressor housing accommodating the compressor impeller, the compressor housing including: a main flow passage accommodating the compressor impeller; and a circulation flow passage located outside the main flow passage in a radial direction of the compressor impeller and connected to the main flow passage, the circulation flow passage including: a downstream slit connected to the main flow passage at a position facing the compressor impeller in the radial direction; an upstream slit connected to the main flow passage at a position in the main flow passage upstream of the downstream slit; and an intermediate flow passage extending along the axial direction of the compressor impeller and connecting the downstream slit to the upstream slit, the intermediate flow passage expanding at least in a portion continuous with the downstream slit as it moves from the downstream slit towards the upstream slit, an outer circumferential surface of the intermediate flow passage inclines radially outward at least a portion continuing to the downstream slit as it moves from the downstream slit toward the upstream slit, and an inner circumferential surface of the intermediate flow passage inclines radially inward at least a portion continuing to the downstream slit as it moves from the downstream slit toward the upstream slit, and an angle of the inner circumferential surface with respect to a line that passes through the intermediate flow passage and is parallel to a central axis of the compressor impeller is greater than an angle of the outer circumferential surface with respect to the line.

2. The centrifugal compressor according to claim 1, wherein the spread angle 2θ expressed by the following formula (3) is greater than 20°. However, A ｄｓ A: Cross-sectional area of ​​the intermediate flow passage at the first end connected to the downstream slit ｕｓ D: Cross-sectional area of ​​the intermediate flow passage at the second end connected to the upstream slit ｄｓ : equivalent diameter of the intermediate flow passage at the first end D ｕｓ : equivalent diameter of the intermediate flow passage at the second end L: distance between the first end and the second end in the axial direction 3. A centrifugal compressor according to claim 2, wherein the spread angle 2θ is greater than 30°.

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