Multi-stage turbomachine

Abstract

To inhibit peeling of a boundary layer in a return passage.SOLUTION: A multistage turbo machine 2 includes: a first impeller and a second impeller; a shroud 13; a hub 14; a diffuser passage 15 which is defined between the shroud 13 and the hub 14 and located at the radial outer side of the first impeller and in which a fluid from the first impeller flows in a radially outward direction; and a return passage 16 which is defined between the shroud 13 and the hub 14 and connected to the diffuser passage 15 and in which a fluid from the diffuser passage 15 is bent in a radially inward direction toward the second impeller. A width w of the return passage 16 has a maximal value in a first position P1 located at the downstream side of an outermost part 16a as seen in a radial direction on a predetermined cross section obtained along a center axis.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present disclosure relates to a multistage turbomachine.

Background Art

[0002] In this technical field, various configurations have been proposed to suppress the separation of the boundary layer in the return passage. For example, Patent Document 1 discloses a multistage centrifugal compressor. This multistage centrifugal compressor includes two impellers. A diffuser and a return guide vane are provided between the two impellers. In this multistage centrifugal compressor, in order to suppress the separation of the boundary layer, the return guide vane is formed such that the blade angle measured from the circumferential direction at the leading edge of the return guide vane is larger on the shroud side than on the hub side.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As described above, in this technical field, it is desirable to suppress the separation of the boundary layer in the return passage.

[0005] An object of the present disclosure is to provide a multistage turbomachine capable of suppressing the separation of the boundary layer in the return passage.

Means for Solving the Problems

[0006] A multi-stage turbomachine according to one aspect of the present disclosure includes a first impeller and a second impeller that are spaced apart from each other along a central axis direction on the same central axis, a shroud that houses the first impeller and the second impeller, a hub disposed radially inside the shroud, a diffuser flow path defined between the shroud and the hub and located radially outside the first impeller, in which fluid from the first impeller flows radially outward, and a return flow path defined between the shroud and the hub and connected to the diffuser flow path, in which the fluid from the diffuser flow path is bent radially inward toward the second impeller, and the width of the return flow path has a maximum value at a first position located downstream of the outermost portion in the radial direction in a predetermined cross section obtained along the central axis. The return flow path includes a plurality of return guide vanes arranged along the circumferential direction. The first position is formed between the outermost part and the return guide vanes. The width of the return flow path includes a minimum value at a second position downstream of the first position, and the minimum value is smaller than the width of the return flow path at the outermost part. 。

[0010] Ri The turn flow path includes a section in which the angle between the surface of the shroud and the radial direction is larger than the angle between the surface of the hub and the radial direction in a region where the fluid flows radially inward in a predetermined cross section obtained along the central axis. This may be the case, and this section may be formed in a region including the leading edge of the return guide vane. 。

[0011] Shi The shroud may include at least one depression formed adjacent to at least one of the positive pressure surface or the negative pressure surface with respect to each return guide vane.

[0012] The shroud may include two depressions formed adjacent to both the positive pressure surface and the negative pressure surface with respect to each return guide vane.

[0013] The depression may extend along the flow from upstream of the return guide vane to the region between adjacent return guide vanes.

[0014] Shi The shroud may include a ridge protruding toward the return flow path between adjacent return guide vanes.

Advantages of the Invention

[0015] According to the present disclosure, separation of the boundary layer in the return flow path can be suppressed. [Brief Description of the Drawings]

[0016]

Figure 1

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[0017] Embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. Specific dimensions, materials, numerical values, etc. shown in such embodiments are merely examples for facilitating understanding and do not limit the present disclosure unless otherwise specified. In the present specification and drawings, elements having substantially the same functions and configurations are denoted by the same reference numerals to omit redundant descriptions, and elements not directly related to the present disclosure are not shown.

[0018] [First Embodiment] FIG. 1 is a schematic diagram showing a power generation system 100 including a multistage compressor 2 according to the first embodiment. In the present embodiment, the multistage compressor (multistage turbomachine) 2 is applied to the power generation system 100 including the power generation device 1. The application destination of the multistage compressor 2 is not limited to this, and the multistage compressor 2 may be applied to various facilities other than the power generation system. Further, in other embodiments, the multistage turbomachine may not be a compressor, and may be other turbomachines such as a blower, a fan, or a pump. Hereinafter, the multistage compressor 2 may be simply referred to as a "compressor", and the power generation system 100 may be simply referred to as a "system". The system 100 includes a power generation device 1, a compressor 2, a combustor 3, a shaft 4, and a turbine 5.

[0019] The compressor 2 pressurizes the inhaled air. The pressurized air is supplied to the combustor 3 and mixed with fuel. The mixed gas of air and fuel is burned in the combustor 3. The exhaust gas from the combustor 3 is supplied to the turbine 5. The shaft 4 partially passes through each of the power generation device 1, the compressor 2, and the turbine 5.

[0020] When the exhaust gas passes between the blades in the turbine 5, the blades are rotated together with the shaft 4. The rotational force of the shaft 4 is used for power generation in the power generation device 1. Further, the rotational force of the shaft 4 is used for pressurizing air in the compressor 2. The exhaust gas that has passed through the turbine 5 may be used in various facilities such as a boiler.

[0021] Subsequently, the compressor 2 will be described in detail.

[0022] FIG. 2 is a schematic partial cross-sectional view showing the compressor 2 in FIG. 1, and shows a certain cross-section among a plurality of cross-sections obtained along the central axis Ax of the shaft 4. The compressor 2 includes a first impeller 11 and a second impeller 12. That is, in the present embodiment, the compressor 2 has two stages. In other embodiments, the compressor 2 may include more than two impellers, that is, more than two stages.

[0023] The compressor 2 partially includes the above-described shaft 4. The first impeller 11 and the second impeller 12 are arranged on the shaft 4, that is, on the same central axis Ax, spaced apart from each other along the central axis direction. The first impeller 11 and the second impeller 12 are rotated together with the shaft 4. Therefore, the central axis direction, the radial direction, and the rotational direction of the shaft 4 are also common to the first impeller 11 and the second impeller 12. In the present disclosure, the central axis direction, the radial direction, and the rotational direction of the shaft 4, the first impeller 11, and the second impeller 12 can be simply referred to as the central axis direction, the radial direction, and the rotational direction, respectively, unless otherwise indicated.

[0024] The compressor 2 includes a shroud 13 and a hub 14. The shroud 13 is located radially outside the first impeller 11 and the second impeller 12 and houses the first impeller 11 and the second impeller 12. The shroud 13 includes a first space SP1 that houses the first impeller 11 and a second space SP2 that houses the second impeller 12. Further, the shroud 13 includes an air inlet 13a. The inlet 13a is connected to the first space SP1 and is in fluid communication with the first space SP1. Air is inhaled from the inlet 13a by the rotation of the impeller 11 and is guided to the first space SP1.

[0025] The hub 14 is located radially inside the shroud 13 and is housed in the shroud 13. The hub 14 divides the first space SP1 and the second space SP2 in the central axis direction. The shroud 13 and the hub 14 define a diffuser flow path 15 and a return flow path 16 therebetween.

[0026] The diffuser flow path 15 is located radially outside the first impeller 11. The diffuser flow path 15 has an annular shape. The diffuser flow path 15 is connected to the first space SP1 and is in fluid communication with the first space SP1. The air guided to the first space SP1 is accelerated and pressurized by the first impeller 11 and is guided to the diffuser flow path 15. The fluid from the first impeller 11 flows radially outward in the diffuser flow path 15. The diffuser flow path 15 decelerates the air flow and converts the velocity energy of the air into static pressure.

[0027] The return channel 16 is connected to the outermost end of the diffuser channel 15 and is in fluid communication with the diffuser channel 15. The return channel 16 has an annular shape. The return channel 16 is connected to the second space SP2 and is in fluid communication with the second space SP2. The air from the diffuser channel 15 flows in the central axis direction within the return channel 16 and then radially inward, and is guided to the second space SP2. The return channel 16 includes a bend portion 17 and a straight portion 18.

[0028] In the cross-section shown in FIG. 2, the bend portion 17 generally has an inverted U shape. One end of the inverted U shape is connected to the outermost end of the diffuser channel 15. The bend portion 17 changes the radially outward flow from the diffuser channel 15 into a radially inward flow.

[0029] In the cross-section shown in FIG. 2, the straight portion 18 generally has a straight shape. One end of the straight shape is connected to the other end of the inverted U shape of the bend portion 17, and the other end of the straight shape is connected to the second space SP2. In the present embodiment, the width of the straight portion 18 expands as it moves downstream. In other embodiments, the width of the straight portion 18 may be constant throughout the length along the flow. The air from the bend portion 17 flows radially inward within the straight portion 18 toward the second space SP2. The straight portion 18 includes a plurality of return guide vanes 19 arranged along the circumferential direction.

[0030] FIG. 3 is a schematic enlarged cross-sectional view of part A in FIG. 2, and FIG. 4 is a schematic enlarged cross-sectional view of part A in FIG. 2 at a position different from FIG. 3 in the circumferential direction. FIGS. 3 and 4 show the details of the return channel 16. In FIG. 4, for better understanding, a part of the contour of the bend portion 17 in FIG. 3 is shown by a dashed line.

[0031] The width w of the return channel 16 has maximum and minimum values downstream of the outermost portion 16a of the return channel 16 in some of the plurality of cross-sections obtained along the central axis Ax. In the present disclosure, the width w means the distance from the position on the surface of the hub 14 to the intersection between the perpendicular line to the surface of the hub 14 at that position and the surface of the shroud 13 in the cross-section obtained along the central axis Ax.

[0032] Specifically, referring to FIG. 3, in the cross-section shown in FIG. 3, the bend portion 17 of the return channel 16 has a generally constant width w. Therefore, the width w of the return channel 16 does not include maximum and minimum values in the cross-section shown in FIG. 3.

[0033] Referring to FIG. 4, in the cross-section shown in FIG. 4, the shroud 13 includes a depression g downstream of the outermost portion 16a of the return channel 16. Specifically, the depression g extends from the position between the outermost portion 16a and the return guide vane 19 beyond the leading edge LE of the return guide vane 19 to the region between adjacent return guide vanes 19. A plurality of depressions g are formed along the circumferential direction.

[0034] FIG. 5 is a schematic cross-sectional view taken along line V-V in FIG. 3 and shows one of the plurality of return guide vanes 19. The depression g is formed adjacent to each of the suction surface SS and the pressure surface PS of the return guide vane 19. In other embodiments, the depression g may be formed adjacent to only one of the suction surface SS or the pressure surface PS.

[0035] Referring to FIG. 4, in the cross-section including the depression g, the width w of the return channel 16 increases along the depression g as it moves along the air flow from the outermost portion 16a to the straight portion 18. The width w has a maximum value at the position (first position) P1 where the depression g is deepest. The position P1 is formed between the outermost portion 16a and the return guide vane 19.

[0036] Subsequently, the width w decreases along the depression g as it moves from the position P1 towards the straight portion 18. That is, the width w has a maximum value at the position P1. The width w has a minimum value at the position (second position) P2 at the end of the depression g. Also, as described above, the width of the straight portion 18 expands as it moves downstream. Therefore, the width w increases again as it moves downstream from the position P2. Thus, the width w has a minimum value at the position P2. In the present embodiment, the width w at the position P2 is smaller than the width w at the outermost portion 16a.

[0037] From another perspective, the return channel 16 includes a section in some of the cross-sections obtained along the central axis Ax where the angle α1 between the surface of the shroud 13 and the radial direction Vt is larger than the angle α2 between the surface of the hub 14 and the radial direction Vt in the region where air flows radially inwards. Such a section is formed in the region spanning the outlet of the bend portion 17 and the inlet of the straight portion 18. More specifically, such a section is formed in the region including the leading edge LE of the return guide vane 19.

[0038] Specifically, referring to FIG. 3, the cross-section shown in FIG. 3 does not include the depression g. For this reason, in the region where air flows radially inwards, the angle α1 between the surface of the shroud 13 and the radial direction Vt is generally constant. Also, in this region, the angle α2 between the surface of the hub 14 and the radial direction Vt is also generally constant. Furthermore, since the width of the straight portion 18 expands as it moves downstream, in the region where air flows radially inwards, the angle α1 is smaller than the angle α2. Therefore, in the cross-section shown in FIG. 3, the return channel 16 does not include a section where the angle α1 is larger than the angle α2 in the region where air flows radially inwards.

[0039] Referring to FIG. 4, the cross-section shown in FIG. 4 includes the depression g. Therefore, in the region including the depression g, the angle α1 between the surface of the shroud 13 and the radial direction Vt changes along the depression g. In the depression g, the angle α1 is the largest at the position P2 at the end of the depression g. The angle α1 at this position P2 is larger than the angle α2 between the surface of the hub 14 and the radial direction Vt. Therefore, in the cross-section shown in FIG. 4, the return flow path 16 includes a section where the angle α1 is larger than the angle α2 in the region where the air flows radially inward.

[0040] In the return flow path 16 as described above, the radially outward flow from the diffuser flow path 15 is changed to a radially inward flow. In particular, in the region spanning from the outlet of the bend portion 17 to the inlet of the straight portion 18, the air flows such that the boundary layer separates from the surface of the inner hub 14.

[0041] However, as described above, the width w of the return flow path 16 has a maximum value at the position P1 downstream of the outermost portion 16a. That is, the width w of the return flow path 16 narrows from the position P1 having the maximum value toward the region where the boundary layer is likely to occur. Therefore, the boundary layer attempting to separate from the surface of the inner hub 14 is pressed against the surface of the hub 14. Thus, the separation of the boundary layer can be suppressed.

[0042] Also, the width w of the return flow path 16 has a minimum value at the position P2 downstream of the position P1 having the maximum value, and the minimum value at this position P2 is smaller than the width w at the outermost portion 16a. Therefore, the width w of the return flow path 16 is the narrowest at the position P2, and the boundary layer attempting to separate from the surface of the hub 14 is pressed by the surface of the hub 14. Thus, the separation of the boundary layer can be suppressed.

[0043] From another perspective, as described above, the return flow path 16 includes a section in the region where air flows radially inward, where the angle α1 between the surface of the shroud 13 and the radial Vt is larger than the angle α2 between the surface of the hub 14 and the radial Vt. In this section, the flow along the surface of the shroud 13 is bent so as to flow toward the surface of the hub 14. Therefore, this flow presses the boundary layer that tends to separate from the surface of the hub 14 against the surface of the hub 14. Thus, the separation of the boundary layer can be suppressed.

[0044] Various dimensions of the return flow path 16, such as the maximum value, minimum value and their positions of the width w, as well as the depth, length and width of the depression g, etc., may be determined by simulations such as numerical fluid dynamics (CFD) so as to be able to suppress the separation of the boundary layer.

[0045] The compressor 2 as described above includes a first impeller 11 and a second impeller 12 that are spaced apart from each other along the central axis direction on the same central axis Ax, a shroud 13 that houses the first impeller 11 and the second impeller 12, a hub 14 disposed radially inside the shroud 13, a diffuser flow path 15 defined between the shroud 13 and the hub 14 and located radially outside the first impeller 11, in which fluid from the first impeller 11 flows radially outward, and a return flow path 16 defined between the shroud 13 and the hub 14 and connected to the diffuser flow path 15, in which the fluid from the diffuser flow path 15 is bent radially inward toward the second impeller 12. The width w of the return flow path 16 includes a maximum value at a position P1 downstream of the outermost portion 16a of the return flow path 16 in the cross section of FIG. 4 obtained along the central axis Ax. According to such a configuration, the width w of the return flow path 16 narrows from the position P1 having the maximum value toward the region where the boundary layer is likely to occur, so that the boundary layer that tends to separate from the surface of the hub 14 is pressed against the surface of the hub 14. Thus, the separation of the boundary layer can be suppressed.

[0046] Further, in the compressor 2, the width w of the return passage 16 includes a minimum value at a position P2 downstream of the position P1. According to such a configuration, the width w of the return passage 16 is the narrowest at the position P2. Therefore, the boundary layer that tries to separate from the surface of the hub 14 is pressed most closely against the surface of the hub 14 at the position P2. Thus, separation of the boundary layer can be suppressed.

[0047] Further, in the compressor 2, the minimum value at the position P2 is smaller than the width w of the return passage 16 at the outermost 16a. According to such a configuration, the boundary layer that tries to separate from the surface of the hub 14 can be pressed by the surface of the hub 14. Thus, separation of the boundary layer can be suppressed.

[0048] Further, in the compressor 2, the return passage 16 includes a plurality of return guide vanes 19 arranged along the circumferential direction, and the position P1 is located upstream of the return guide vanes 19. The boundary layer may separate near the inlet of the return guide vanes 19. In such a case, by positioning the position P1 having a maximum value upstream of the return guide vanes 19, the width w of the return passage 16 narrows as it moves toward the inlet of the return guide vanes 19. Thus, separation of the boundary layer can be suppressed.

[0049] Further, in the compressor 2, the shroud 13 includes at least one recess g formed adjacent to at least one of the positive pressure surface PS or the negative pressure surface SS with respect to each return guide vane 19. And the recess g extends from upstream of the return guide vane 19 to the region between adjacent return guide vanes 19. According to these configurations, simply by forming the recess g along the return guide vane 19, a cross section having a maximum value as described above can be easily produced. Also, in the present embodiment, the shroud 13 includes two recesses g formed adjacent to both the positive pressure surface PS and the negative pressure surface SS with respect to each return guide vane 19. Thus, more flow can be formed to press the boundary layer against the surface of the hub 14.

[0050] From another perspective, in the cross-section of FIG. 4 obtained along the central axis Ax, the return flow path 16 includes a section where the angle α1 between the surface of the shroud 13 and the radial direction Vt is larger than the angle between the surface of the hub 14 and the radial direction Vt in the region where air flows radially inward. As described above, in this section, the flow along the surface of the shroud 13 is bent so as to flow toward the surface of the hub 14. Therefore, this flow presses the boundary layer that tends to separate from the surface of the hub 14 against the surface of the hub 14. Thus, the separation of the boundary layer can be suppressed.

[0051] Subsequently, other embodiments will be described.

[0052] [Second Embodiment] FIG. 6 is a schematic cross-sectional view showing a multi-stage compressor 2A according to the second embodiment, and shows an enlarged cross-sectional view at a position corresponding to part A in FIG. 2. In the compressor 2A, a cross-section including the position P1 having a maximum value is formed over the entire circumferential direction. That is, the compressor 2A does not include the depression g. Further, since the depression g is not formed in the compressor 2A, the position P2 having the minimum value of the width w is formed at the same position or substantially the same position as the leading edge LE of the return guide vane 19. In other respects, the compressor 2A may be the same as the compressor 2 according to the first embodiment.

[0053] In such a compressor 2A, the width w of the return flow path 16 increases as it moves from the outermost portion 16a toward the straight portion 18 in any cross-section obtained along the central axis Ax. The width w has a maximum value at the position P1 between the outermost portion 16a and the return guide vane 19.

[0054] Subsequently, the width w decreases as it moves from the position P1 toward the straight portion 18. That is, the width w has a maximum value at the position P1. The width w has a minimum value at the position P2. Also, the width of the straight portion 18 expands as it moves downstream. Therefore, the width w increases again as it moves downstream from the position P2. Thus, the width w has a minimum value at the position P2.

[0055] From another perspective, in any cross-section obtained along the central axis line Ax, the return flow path 16 includes a section where the angle α1 between the surface of the shroud 13 and the radial direction Vt is larger than the angle α2 between the surface of the hub 14 and the radial direction Vt in the region where air flows radially inward. Such a section is formed in the region including the leading edge LE of the return guide vane 19. The angle α1 is the largest at the position P2. The angle α1 at this position P2 is larger than the angle α2 between the surface of the hub 14 and the radial direction Vt.

[0056] The compressor 2A as described above can achieve the same effects as the compressor 2 according to the first embodiment. In particular, in the compressor 2A, since the cross-section including the position P1 having the maximum value is formed throughout the circumferential direction, the flow that presses the boundary layer against the surface of the hub 14 can be formed throughout the circumferential direction. Therefore, the separation of the boundary layer can be more effectively suppressed.

[0057] [Third Embodiment] FIG. 7 is a schematic cross-sectional view showing a multistage compressor 2B according to the third embodiment, and shows an enlarged cross-sectional view of a position corresponding to part A in FIG. 2. In the compressor 2B, the width of the straight portion 18 is constant over the entire length. In other respects, the compressor 2B may be the same as the compressor 2A according to the second embodiment.

[0058] In such a compressor 2B, the width w of the return flow path 16 has a minimum value at the position P2. However, the width of the straight portion 18 is constant downstream of the position P2. Therefore, the width w does not increase as it moves downstream from the position P2. Thus, the width w does not have a minimum value at the position P2. In this embodiment, the width w at the position P2 may be the same as the width w at the outermost portion 16a.

[0059] The compressor 2B as described above can achieve substantially the same effects as the compressor 2 according to the first embodiment and the compressor 2A according to the second embodiment.

[0060] [Fourth Embodiment] FIG. 8 is a schematic cross-sectional view showing the multi-stage compressor 2C according to the third embodiment, and shows an enlarged cross-sectional view at a position corresponding to part A in FIG. 2. FIG. 9 is a schematic cross-sectional view taken along line IX-IX in FIG. 8, and shows two adjacent return guide vanes 19. In FIG. 9, for better understanding, the connection portion between the hub 14 and the return guide vane 19 in the compressors 2, 2A, and 2B is shown by a broken line. Referring to FIGS. 8 and 9, in the compressor 2C, the shroud 13 includes a ridge b that protrudes toward the return flow path 16 between adjacent return guide vanes 19. Further referring to FIG. 9, in the compressor 2C, the radius of curvature of the connection portion between the hub 14 and the return guide vane 19 is larger than that of the compressors 2, 2A, and 2B described above. In other respects, the compressor 2C may be the same as the compressor 2B according to the third embodiment.

[0061] Referring to FIG. 8, the ridge b extends from the position P2 of the leading edge LE of the return guide vane 19 toward the region between adjacent return guide vanes 19. Referring to FIG. 9, for example, the position P3 of the apex of the ridge b may be provided at or approximately at the center between adjacent return guide vanes 19 in the circumferential direction. Referring to FIG. 8, in the cross-section including the position P3, the width w of the return flow path 16 has a minimum value at the position P3. Specifically, the width w decreases as it moves from the position P2 toward the position P3 Decrease and increases as it moves downstream from the position P3 Increase Accordingly, the width w has a minimum value at the position P3.

[0062] Referring to FIG. 9, as described above, in the compressor 2C, the radius of curvature of the connection portion between the hub 14 and the return guide vane 19 is larger than that of the compressors 2, 2A, and 2B described above. Therefore, the boundary layer is less likely to separate from the surface of the hub 14. In the present embodiment, such a connection portion having such a radius of curvature is formed for both the positive pressure surface PS and the negative pressure surface SS. In other embodiments, such a connection portion may be formed for only one of the positive pressure surface PS or the negative pressure surface SS.

[0063] The compressor 2C as described above can achieve the same effects as the compressors 2, 2A, and 2B. In particular, in the compressor 2C, the shroud 13 includes a ridge b that protrudes toward the return flow path 16 between adjacent return guide vanes 19. According to such a configuration, the flow along the ridge b presses the boundary layer that tends to separate from the surface of the hub 14 against the surface of the hub 14. Therefore, the separation of the boundary layer can be further suppressed.

[0064] As described above, the embodiments have been described with reference to the accompanying drawings, but the present disclosure is not limited to the above embodiments. It is obvious that those skilled in the art can conceive various modification examples or correction examples within the scope described in the claims, and it is naturally understood that those also belong to the technical scope of the present disclosure.

Description of Reference Numerals

[0065] 2 Multistage compressor (multistage turbomachine) 2A Multistage compressor (multistage turbomachine) 2B Multistage compressor (multistage turbomachine) 2C Multistage compressor (multistage turbomachine) 11 First impeller 12 Second impeller 13 Shroud 14 Hub 15 Diffuser flow path 16 Return flow path 16a Outermost 19 Return guide vane Ax Central axis b Ridge g Depression P1 First position P2 Second position PS Positive pressure surface SS Negative pressure surface Vt Radial direction w Width of the return flow path α1 Angle between the surface of the shroud and the radial direction in the return flow path α2 Angle between the surface of the hub and the radial direction in the return flow path

Claims

1. A first impeller and a second impeller that are spaced apart from each other along a central axis on the same central axis line, a shroud that houses the first impeller and the second impeller, a hub disposed radially inside the shroud, a diffuser flow path defined between the shroud and the hub and located radially outside the first impeller, wherein fluid from the first impeller flows radially outward in the diffuser flow path, the diffuser flow path; a return flow path defined between the shroud and the hub and connected to the diffuser flow path, wherein the fluid from the diffuser flow path is bent radially inward toward the second impeller in the return flow path, and the width of the return flow path has a maximum value at a first position located downstream of the outermost portion in a predetermined cross section obtained along the central axis, the return flow path; comprising, the return flow path includes a plurality of return guide vanes arranged along the circumferential direction, the first position is formed between the outermost portion and the return guide vane, the width of the return flow path includes a minimum value at a second position downstream of the first position, the minimum value is smaller than the width of the return flow path at the outermost portion, a multistage turbomachine.

2. In a predetermined cross section obtained along the central axis, the return flow path includes a section where the angle between the surface of the shroud and the radial direction is larger than the angle between the surface of the hub and the radial direction in a region where the fluid flows radially inward, The multistage turbomachine according to claim 1, wherein the section is formed in a region including the leading edge of the return guide vane.

3. The shroud includes at least one depression formed adjacent to at least one of the positive pressure surface or the negative pressure surface for each return guide vane, The multistage turbomachine according to any one of claims 1 or 2.

4. The shroud includes two depressions formed adjacent to both the positive pressure surface and the negative pressure surface for each return guide vane, The multistage turbomachine according to claim 3.

5. The multistage turbomachine according to claim 3 or 4, wherein the depression extends from upstream of the return guide vane to a region between adjacent return guide vanes.

6. The shroud according to any one of claims 1 to 5, wherein the shroud includes a ridge protruding toward the return flow path between adjacent return guide vanes.

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