Abstract:
In order to reduce the rotating stall onset flowrate, this centrifugal compressor comprises: a vaneless diffuser ( 12 ) provided on a discharge outlet ( 6 B) side of an impeller ( 6 ); and a fluid circulation flow path ( 21 ) having an inlet ( 21 A) that opens into a first wall section ( 22 ) of a hub casing ( 2 B) forming the vaneless diffuser ( 12 ) and also having an outlet ( 21 B) that opens into a second wall section ( 23 ) of the hub casing ( 2 B) facing a hub disk ( 6 C) rear surface in the impeller ( 6 ). As a result of causing the first wall section ( 22 ) to protrude on to a wall section ( 31 ) side of the shroud casing ( 2 A), the flow path width (D) of the vaneless diffuser ( 12 ) is smaller than the width (W) of the discharge outlet ( 6 B) in the impeller ( 6 ). In addition, the hub casing ( 2 B) comprises an inclined wall section ( 24 ) having a protruding end ( 24 A) between the discharge outlet ( 6 B) of the impeller ( 6 ) and the inlet of the vaneless diffuser ( 12 ), said inclined wall section ( 24 ) connecting the second wall section ( 23 ) and the first wall section ( 22 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a centrifugal compressor in which a vaneless diffuser is provided on an outlet side of an impeller. 
       BACKGROUND ART 
       [0002]    In general, an industrial centrifugal compressor is used in a petrochemical plant, a natural gas plant, or the like. In this kind of centrifugal compressor, a configuration is widely adopted, which uses a vaneless diffuser in which a vane is not provided in a diffuser portion on the outlet side of an impeller. The configuration which uses the vaneless diffuser has advantages in which the structure is simple, a loss decreases if a flow angle is appropriate, an operation range is wide, and a fluid vibrating force with respect to the impeller does not occur. However, in the vaneless diffuser, if the flow angle increases, the loss increases, a rotating stall occurs in which flows are nonuniform in the circumferential direction, and variation in pressures, shaft vibrations, vibrations of a discharge pipe, or the like occurs due to the rotating stall. Particularly, it is known that the rotating stall occurs in a case where the centrifugal compressor is operated at a small flow-rate region. 
         [0003]    Accordingly, in the related art, in order to prevent occurrence of the rotating stall, a centrifugal compressor is suggested in which a fluid circulation flow path having an inlet which opens into an outlet side of a vaneless diffuser and an outlet which opens into a wall surface of a casing facing a back face of a hub disk of the impeller is provided, and a portion of a high-pressure fluid is recirculated via the fluid circulation flow path (for example, refer to PTL 1). 
       CITATION LIST 
     Patent Literature 
       [0004]    [PTL 1] Japanese Unexamined Patent Application Publication No. 2008-38894 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    By providing the above-described fluid circulation flow path, a decrease in efficiency of the centrifugal compressor is prevented, and it is possible to decrease a rotating stall onset flow-rate when the rotating stall is initially generated in the vaneless diffuser. Meanwhile, a configuration for further decreasing the rotating stall onset flow-rate is required. 
         [0006]    The present invention is made in consideration of the above-described circumstances, and an object thereof is to provide a centrifugal compressor capable of further decreasing the rotating stall onset flow-rate in the configuration in which the fluid circulation flow path is provided. 
       Solution to Problem 
       [0007]    In order to solve the above-described problems and achieve the object, according to an aspect of the present invention, there is provided a centrifugal compressor, including: a vaneless diffuser which is provided on an outlet side of an impeller; and a fluid circulation flow path which includes a fluid inlet which opens into a first wall section of a hub casing forming the vaneless diffuser and a fluid outlet which opens into a second wall section of the hub casing facing a back face of a hub disk in the impeller, in which in the vaneless diffuser, at least the first wall section of the hub casing protrudes toward a wall section side of a shroud casing facing the first wall section, a flow path width of the vaneless diffuser is smaller than an outlet width of the impeller, the hub casing includes an inclined wall section which has a portion between the outlet of the impeller and the inlet of the vaneless diffuser as a protruding end and connects the second wall section and the first wall section. 
         [0008]    According to this configuration, since the fluid circulation flow path is provided, which includes the fluid inlet which opens into the first wall section of the hub casing forming the vaneless diffuser and the fluid outlet which opens into the second wall section of the hub casing facing the back face of the hub disk in the impeller, a portion of a high-pressure fluid is circulated to the inlet side of the vaneless diffuser through the fluid circulation flow path, and it is possible to increase the flow rate of the fluid which flows through the vaneless diffuser. Accordingly, it is possible to shift a rotating stall generation point to a small flow rate side by the ratio of the increased flow rate, and it is possible to prevent occurrence of the rotating stall. In addition, in the vaneless diffuser, at least the first wall section of the hub casing protrudes toward the wall section side of the shroud casing facing the first wall section, the flow path width of the vaneless diffuser is smaller than the outlet width of the impeller, the hub casing includes an inclined wall section which has a portion between the outlet of the impeller and the inlet of the vaneless diffuser as a protruding end and connects the second wall section and the first wall section. Accordingly, the circulating flow blown out from the outlet of the fluid circulation flow path flows along the inclined wall section and can smoothly combine with the main flow of the fluid blown out from the outlet of the impeller, and disturbance of the fluid in the vaneless diffuser is not generated. In addition, since the flow path width of the vaneless diffuser is narrowed by the inclined wall section, even in the time of a small flow rate in which the flow rate of the fluid is small, the flow in the vaneless diffuser becomes smooth, and it is possible to further shift the rotating stall generation point to the small flow rate side. 
         [0009]    In this configuration, preferable, in the inclined wall section, the trailing end which is continued to the first wall section is disposed to be closer to the outside in a radial direction than an end section on the inside in the radial direction of the impeller in the wall section of the shroud casing. According to this configuration, since an inclination angle of the inclined wall section on the hub casing side can be more gently formed than the shrouding casing side, it is possible to smoothly combine the circulating flow with the main flow of the fluid without disturbing the flow in the main flow of the fluid blown out from the outlet of the impeller. 
         [0010]    In addition, preferably, the first wall section of the hub casing further protrudes toward the wall section side of the shroud casing than a hub-side extension line which extends to the outside in the radial direction from the outlet of the impeller, and a hub-side protrusion length from the hub-side extension line to the first wall section is 50% or less of the outlet width of the impeller. According to this configuration, it is possible to prevent a decrease in efficiency of the centrifugal compressor while shifting the rotating stall generation point to the small flow rate side. 
         [0011]    Moreover, preferably, the wall section of the shroud casing further protrudes toward the first wall section side of the hub casing than a shroud-side extension line which extends to the outside in the radial direction from the outlet of the impeller, and a shroud-side protrusion length from the shroud-side extension line to the wall section is 20% or less of the outlet width of the impeller. Within this range, effects capable of shifting the rotating stall generation point to the small flow rate side are exerted. 
       Advantageous Effects of Invention 
       [0012]    According to the centrifugal compressor of the present invention, in the vaneless diffuser, at least the first wall section of the hub casing protrudes toward the wall section side of the shroud casing facing the first wall section, the flow path width of the vaneless diffuser is smaller than the outlet width of the impeller, the hub casing includes an inclined wall section which has a portion between the outlet of the impeller and the inlet of the vaneless diffuser as a protruding end and connects the second wall section and the first wall section. Accordingly, the circulating flow blown out from the outlet of the fluid circulation flow path flows along the inclined wall section and can smoothly combine with the main flow of the fluid blown out from the outlet of the impeller, and disturbance of the fluid in the vaneless diffuser is not generated. In addition, since the flow path width of the vaneless diffuser is narrowed by the inclined wall section, even in the time of a small flow rate in which the flow rate of the fluid is small, the flow in the vaneless diffuser becomes smooth, and it is possible to further shift the rotating stall generation point to the small flow rate side. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a longitudinal sectional view of a centrifugal compressor according to the present invention. 
           [0014]      FIG. 2  is a main portion sectional view showing a configuration in the vicinity of a vaneless diffuser. 
           [0015]      FIG. 3  is a graph showing a relationship among a flow rate coefficient, a pressure coefficient, and efficiency in Comparative Examples 1 to 8, and Example. 
           [0016]      FIG. 4  is a graph showing a relationship between a flow rate coefficient of a rotating stall starting point and efficiency of a design point. 
           [0017]      FIG. 5  is a main portion sectional view showing a configuration in the vicinity of a vaneless diffuser according to another embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    Hereinafter, embodiments according to the present invention will be described with reference to the drawings. In addition, the present invention is not limited by the following embodiments. Moreover, components in the following embodiments include components which can be easily replaced by a person skilled in the art and the approximately same components. 
         [0019]      FIG. 1  is a longitudinal sectional view of a centrifugal compressor according to the present embodiment. A centrifugal compressor  1  includes a casing  2  which is configured by combining a plurality of parts, a rotary shaft  5  which is rotatably supported around an axis L via a bearing (not shown) in the casing  2 , and closed type impellers  6  and  6  which are fixed to the rotary shaft  5  and are provided so as to integrally rotate with the rotary shaft  5 . That is, the centrifugal compressor  1  of the present embodiment is a two-stage centrifugal compressor. 
         [0020]    In the centrifugal compressor  1 , the rotary shaft  5  is driven by a driving device (not shown) so as to rotate the impellers  6  and  6 , and a fluid such as gas or air which is an object to be compressed is suctioned via a suction port  10  which is provide in the casing  2 . A suction flow path  11  is connected to the suction port  10  via a suction space  10 A formed in the casing  2 , and the suction flow path  11  is bent along the axis L direction (axial direction) of the rotary shaft  5  and opens so as to face an intake port  6 A of the first-stage impeller  6 . 
         [0021]    A centrifugal force is applied to the fluid suctioned into the suction port  10  by the rotation of the first-stage impeller  6 , and the kinetic energy is converted into pressure energy by a first-stage vaneless diffuser  12  which is provided in a discharge outlet  6 B of the impeller  6 . In addition, the fluid is introduced into the intake port  6 A of the second-stage impeller  6  which is the next-stage compression stage via a return bend  14  and a return vane  15 . 
         [0022]    Similarly, a centrifugal force is also applied to the compressed fluid by the second-stage impeller  6 , kinetic energy is converted into pressure energy by the second-stage vaneless diffuser  12 , and the fluid becomes a compressed fluid having a higher pressure and is discharged to a scroll  16 . In addition, the fluid is sent from the scroll  16  to a discharge pipe (not shown) via a discharge port  17  which is provided in the casing  2 . In addition, a reference numeral  18  in  FIG. 1  is a balance piston which is provided so as to adjust a thrust of the impeller  6 . 
         [0023]    Meanwhile, the vaneless diffuser  12  is provided to communicate with the outlet side of a space in which the impeller  6  rotates, and configures a flow path through which the fluid to which the centrifugal force is applied by the impeller  6  is converted from the kinetic energy into the pressure energy and is sent. In the vaneless diffuser  12 , in a case where the centrifugal compressor  1  is operated in a small flow-rate region, a rotating stall in which flows in the circumferential direction is nonuniform. In order to prevent the occurrence of the rotating stall, the present embodiment adopts the following configuration. 
         [0024]      FIG. 2  is a main portion sectional view showing the configuration in the vicinity of the vaneless diffuser. The vaneless diffuser  12  includes a shroud casing  2 A and a hub casing  2 B which configure the casing  2 . In addition, a fluid circulation flow path (bypass flow path, and simply referred to as a bypass)  21  is provided in the hub casing  2 B, and the fluid in the vaneless diffuser  12  is returned to the vicinity of the discharge outlet (impeller outlet)  6 B of the impeller  6  so as to be circulated through the fluid circulation flow path. Specifically, as shown in  FIG. 2 , the fluid circulation flow path  21  includes an inlet  21 A and an outlet  21 B, the inlet  21 A opens into the first wall section  22  of the hub casing  2 B forming the vaneless diffuser  12  and the outlet  21 B opens into a second wall section  23  of the hub casing  2 B facing a back face of a hub disk  6 C configuring the impeller  6 . 
         [0025]    In the fluid circulation flow path  21 , a portion of the high-pressure compressed fluid via the vaneless diffuser  12  from the inlet  21 A is drawn and blown out from the outlet  21 B, and a portion of the compressed fluid is circulated by the vaneless diffuser  12  and the discharge outlet  6 B of the impeller  6 . The fluid circulation flow path  21  may be a plurality of paths, and the outlet  21 B of the fluid circulation flow path  21  is formed so as to face the back face of the hub disk  6 C in the impeller  6 . Accordingly, a circumferential flow velocity is applied to the fluid blown out from the outlet  21 B by the back face of the hub disk  6 C of the impeller  6  according to the rotation of the impeller  6 . Accordingly, the circumferential flow velocity when the fluid blown out from the outlet  21 B combines with the main flow of the fluid blown out from the impeller  6  can be approximately the circumferential flow velocity as that of the main flow of the fluid. 
         [0026]    Moreover, the first wall section  22  of the hub casing  2 B is formed to further protrude toward a wall section  31  side of the shroud casing  2 A facing the first wall section  22  than the second wall section  23 . Specifically, the first wall section  22  of the hub casing  2 B further protrudes toward the wall section  31  side of the shroud casing  2 A than a hub-side extension line HA which extends to the outside (an arrow A direction in  FIG. 2 ) in a radial direction of the impeller  6  from the end section of the hub casing  2 B side in the discharge outlet  6 B of the impeller  6 . The first wall section  22  and the second wall section  23  are connected to each other by an inclined wall section  24 . 
         [0027]    Meanwhile, in the present embodiment, the wall section  31  of the shroud casing  2 A is formed to protrude toward the first wall section  22  side of the hub casing  2 B than a shroud-side extension line SA which extends from the end section of the shroud casing  2 A side of the discharge outlet  6 B of the impeller  6  to the outside in the radial direction of the impeller  6 . The shroud casing  2 A includes a horizontal wall  30  which faces the discharge outlet  6 B of the impeller  6  and extends in the axis L direction of the rotary shaft  5 , and the horizontal wall  30  and the wall section  31  are connected to each other via a connection wall  32 . The connection wall  32  can adopt various shapes, and may be an R wall which is formed so as to have a predetermined curvature radius or an inclined wall. In a case where the connection wall  32  is provided, the end section  31 A of the wall section  31  on the inside in the radial direction of the impeller  6  becomes a connection portion between the connection wall  32  and the wall section  31 . 
         [0028]    In the present embodiment, the wall section  31  of the shroud casing  2 A and the first wall section  22  of the hub casing  2 B extends in a projection plane of the discharge outlet  6 B of the impeller  6 , and a flow path width D of the vaneless diffuser  12  which is formed of the wall section  31  and the first wall section  22  is formed to be smaller than a width (outlet width) W of the discharge outlet  6 B in the impeller  6 . In this way, the flow path width D of the vaneless diffuser  12  is formed so as to narrow the width W of the discharge outlet  6 B in the impeller  6 . 
         [0029]    In addition, in a case where a length from the hub-side extension line HA to the first wall section  22  in the axis L direction is set to a hub-side protrusion length HD, preferably, the hub-side protrusion length HD is 50% or less of the width W of the discharge outlet  6 B in the impeller  6 . Moreover, like the present embodiment, in the configuration in which the first wall section  22  of the hub casing  2 B and the wall section  31  of the shroud casing  2 A are formed to protrude so as to extend into the projection plane of the discharge outlet  6 B of the impeller  6 , in a case where a distance from the shroud-side extension line SA to the wall section  31  in the axial L direction is set to a shroud-side protrusion length SD, the total of the hub-side protrusion length HD and the shroud-side protrusion length SD is 50% or less of the width W of the discharge outlet  6 B in the impeller  6 . In addition, preferably, the shroud-side protrusion length SD is 20% or less of the width W of the discharge outlet  6 B in the impeller  6 . 
         [0030]    Next, a throttle of the first wall section  22  of the hub casing  2 B will be described. As described above, the first wall section  22  protrudes so as to gradually extend in the projection plane of the discharge outlet  6 B of the impeller  6  by the inclined wall section  24 . The shape of the inclined wall section (protrusion)  24  being important is identified by a test or the like. In the present embodiment, in the inclined wall section  24 , a protruding end  24 A which is connected to the second wall section  23  is provided between the discharge outlet  6 B of the impeller  6  and the inlet (the position of the horizontal wall  30 ) of the vaneless diffuser  12  in the radial direction of the impeller  6 . In addition, a trailing end  24 B of the inclined wall section  24  is provided at a position closer to the outside in the radial direction than the end section  31 A positioned on the inside in the radial direction of the impeller  6  in the wall section  31  of the shroud casing  2 A. Accordingly, the inclined wall section  24  can be formed at a gentle throttle inclination, and it is possible to smoothly combine the circulating flow with the main flow of the fluid without disturbing the flow of the main flow of the fluid blown out from the discharge outlet  6 B of the impeller  6 . In addition, the inclined wall section  24  includes not only a plane wall which is formed at a predetermined angle but also a curved wall in the surface is curved, for example. Next, Example and Comparative Examples will be described. 
       COMPARATIVE EXAMPLE 1 
       [0031]    In Comparative Example 1, the wall section  31  of the shroud casing  2 A further protrudes than the shroud-side extension line SA, and the first wall section  22  of the hub casing  2 B is formed so as to be positioned on the hub-side extension line HA. Each of the shroud casing  2 A and the hub casing  2 B includes an inclined wall section (not shown), and the trailing end of the inclined wall section on the shroud casing  2 A side is formed to be closer to the outside in the radial direction than the trailing end of the inclined wall section of the hub casing  2 B (gently protrudes to SH side). In addition, the shroud-side protrusion length SD of the wall section  31  of the shroud casing  2 A is 40% of the width W of the discharge outlet  6 B in the impeller  6 , and a ratio D/W between the flow path width D of the vaneless diffuser  12  and the width W of discharge outlet  6 B is 0.6. In addition, in Comparative  1 , the fluid circulation flow path  21  is not provided. 
       COMPARATIVE EXAMPLE 2 
       [0032]    In Comparative Example 2, the shroud-side protrusion length SD of the wall section  31  of the shroud casing  2 A is changed, and a corner R portion having a small curvature radius R is only provided between the wall section  31  of the shroud casing  2 A and the horizontal wall (steeply protrudes to SH side). The shroud-side protrusion length SD is 17% of the width W of the discharge outlet  6 B in the impeller  6 , and the ratio D/W between the flow path width D of the vaneless diffuser  12  and the width W of discharge outlet  6 B is 0.83. Other configurations are the same as those of Comparative Example 1. 
       COMPARATIVE EXAMPLE 3 
       [0033]    In Comparative Example 3, a corner R portion having a small curvature radius R is only provided between the wall section  31  of the shroud casing  2 A and the horizontal wall  30  (steeply protrudes to SH side). Other configurations are the same as those of Comparative Example 1. 
       COMPARATIVE EXAMPLE 4 
       [0034]    In Comparative Example 4, the shroud-side protrusion length SD of the wall section  31  of the shroud casing  2 A is changed. The shroud-side protrusion length SD is 60% of the width W of the discharge outlet  6 B in the impeller  6 , and the ratio D/W between the flow path width D of the vaneless diffuser  12  and the width W of discharge outlet  6 B is 0.4. Other configurations are the same as those of Comparative Example 1. 
       COMPARATIVE EXAMPLE 5 
       [0035]    In Comparative Example 5, a corner R portion having a small curvature radius R is only provided between the wall section  31  of the shroud casing  2 A and the horizontal wall  30  (steeply protrudes to SH side). Other configurations are the same as those of Comparative Example 4. 
       COMPARATIVE EXAMPLE 6 
       [0036]    In Comparative Example 6, the fluid circulation flow path  21  is provided in the first wall section  22  and the second wall section  23  in the hub casing  2 B. When the radius of the discharge outlet  6 B of the impeller  6  is set to R 1 , the fluid circulation flow path  21  is open at a position at which a radius R 2  of an opening center of the inlet  21 A of the fluid circulation flow path  21  satisfies 1.1 R 1 ≦R 2 ≦1.4 R 1 . Other configurations are the same as those of Comparative Example 1. 
       COMPARATIVE EXAMPLE 7 
       [0037]    In Comparative Example 7, a corner R portion having a small curvature radius R is only provided between the wall section  31  of the shroud casing  2 A and the horizontal wall  30  (steeply protrudes to SH side). Other configurations are the same as those of Comparative Example 6. 
       COMPARATIVE EXAMPLE 8 
       [0038]    In Comparative Example 8, the shroud-side protrusion length SD of the wall section  31  of the shroud casing  2 A is changed, and a corner R portion having a small curvature radius R is only provided between the wall section  31  of the shroud casing  2 A and the horizontal wall (steeply protrudes to SH side). The shroud-side protrusion length SD is 17% of the width W of the discharge outlet  6 B in the impeller  6 , and the ratio D/W between the flow path width D of the vaneless diffuser  12  and the width W of discharge outlet  6 B is 0.83. Other configurations are the same as those of Comparative Example 6. 
       EXAMPLE 
       [0039]    In Example, the wall section  31  of the shroud casing  2 A further protrudes the shroud-side extension line SA, and the first wall section  22  of the hub casing  2 B also further protrudes than the hub-side extension line HA. The shroud-side protrusion length SD is 17% of the width W of the discharge outlet  6 B in the impeller  6 , and the hub-side protrusion length HD is 23% of the width W of the discharge outlet  6 B of the impeller  6 . Accordingly, the ratio D/W between the flow path width D of the vaneless diffuser  12  and the width W of discharge outlet  6 B is 0.6. In addition, a corner R portion having a small curvature radius R is provided between the wall section  31  of the shroud casing  2 A and the horizontal wall  30  (steeply protrudes to SH side). In addition, in the inclined wall section  24  of the hub casing  2 B, the trailing end  24 B is provided in the vicinity of the inlet  21 A of the fluid circulation flow path  21  and narrows the flow path width D of the vaneless diffuser  12  at a gentle inclination (gently protrudes toward the hub side). Other configurations are the same as those of Comparative Example 6. 
         [0040]      FIG. 3  is a graph showing a relationship among a flow rate coefficient, a pressure coefficient, and efficiency in Comparative Examples 1 to 8, and Example, and  FIG. 4  is a graph showing a relationship between a flow rate coefficient of a rotating stall starting point and efficiency of a design point (a position of a vertical broken line in  FIG. 3 ). In  FIG. 4 , ratios based on Comparative Example 1 are shown. 
         [0041]    As shown in a region which is surrounded by a broken-line ellipse in  FIG. 4 , in the configuration (Comparative Example 1 to Comparative Example 5) in which the fluid circulation flow path is not provided, if approximately the shroud-side protrusion length SD increase, although the efficiency is likely to be decreased, it is possible to shift the rotating stall starting point to a small flow rate side. 
         [0042]    Meanwhile, in the configuration (Comparative Example 6 to Comparative Example 8: bypass is attached) in which the fluid circulation flow path  21  is provided, the shroud-side protrusion length SD increasing is not necessarily preferable. Specifically, as shown in Comparative Example 7 and 8, if the shroud-side protrusion length SD is set to 17% (Comparative Example 8) to 40% (Comparative Example 7) of the width W of the discharge outlet  6 B in the impeller  6 , the flow rate coefficient of the rotating stall starting point increases, and efficiency decreases. It is considered that this is because if a wall (wall section  31  of the shroud casing  2 A) opposite to the wall of the fluid circulation flow path  21  from which the circulating flow is blown out largely protrudes, the wall section  31  and the circulating flow collides and interfere with each other, and a separating flow portion is promoted on the surface of the wall section  31 . Accordingly, in the case where the wall section  31  of the shroud casing  2 A protrudes, preferably, the shroud-side protrusion length SD is 20% or less of the width W of the discharge outlet  6 B of the impeller  6 . 
         [0043]    In Example, as described above, the shroud-side protrusion length SD is 17% of the width W of the discharge outlet  6 B in the impeller  6 , the hub-side protrusion length HD is 23% of the width W of the discharge outlet  6 B in the impeller  6 , and the ratio D/W between the flow path width D of the vaneless diffuser  12  and the width W of the discharge outlet  6 B is 0.6. In addition, as shown in  FIG. 2 , the trailing end  24 B of the inclined wall section  24  of the hub casing  2 B is provided at a position closer to the outside in the radial direction than the end section  31 A positioned on the inside in the radial direction of the impeller  6  in the wall section  31  of the shroud casing  2 A. Accordingly, the inclined wall section  24  can be formed at a gentle throttle inclination, and it is possible to smoothly combine the circulating flow with the main flow of the fluid without disturbing the flow of the main flow of the fluid blown out from the discharge outlet  6 B of the impeller  6 . Accordingly, as shown in  FIG. 4 , it is possible largely shift the rotating stall starting point to a small flow rate side, and it is possible to prevent occurrence of the rotating stall until reaching a surge point. In addition, it is possible to further increase efficiency than that of the configuration which has a rotating stall generation point at which the flow path width D of the vaneless diffuser  12  is largely narrowed in a state where the fluid circulation flow path  21  is not provided. In Example, even when the width ratio D/W is not decreased to 0.4 of Comparative Example 5, it is possible to realize the same rotating stall generation point at 0.6, and it is possible to further increase efficiency than that of Comparative Example 5. 
         [0044]    In addition, in Example, the shroud-side protrusion length SD and the hub-side protrusion length HD are 40% of the width W of the discharge outlet  6 B in the impeller  6 . However, in a case where the protrusion length are more than 50%, since the rotating stall starting point far exceeds the surge point and the efficiency is decreased, significance of the protrusion formation is lowered. Accordingly, preferably, the hub-side protrusion length HD (and the shroud-side protrusion length SD) is set to 50% or less of the width W of the discharge outlet  6 B in the impeller  6 . 
         [0045]    As described above, the centrifugal compressor  1  of the present embodiment includes the vaneless diffuser  12  which is provided on the discharge outlet  6 B side of the impeller  6 , and the fluid circulation flow path  21  which includes the inlet  21 A which opens into the first wall section  22  of the hub casing  2 B forming the vaneless diffuser  12  and the outlet  21 B which opens into the second wall section  23  of the hub casing  2 B facing the back face of the hub disk  6 C in the impeller  6 , in which in the vaneless diffuser  12 , the first wall section  22  protrudes toward the wall section  31  side of the shroud casing  2 A, the flow path width D of the vaneless diffuser  12  is smaller than the width W of the discharge outlet  6 B in the impeller  6 , the hub casing  2 B includes the inclined wall section  24  which has the portion between the discharge outlet  6 B of the impeller  6  and the inlet of the vaneless diffuser  12  as the protruding end  24 A and connects the second wall section  23  and the first wall section  22 . Accordingly, since the circulating flow blown out from the outlet  21 B of the fluid circulation flow path  21  flows along the inclined wall section  24 , the circulating flow can smoothly combine with the main flow of the fluid blown out from the discharge outlet  6 B of the impeller  6 , and disturbance of the fluid in the vaneless diffuser  12  is not generated. In addition, since the flow path width D of the vaneless diffuser  12  is narrowed by the inclined wall section  24 , even in the time of a small flow rate in which the flow rate of the fluid is small, the flow in the vaneless diffuser  12  becomes smooth, and it is possible to further shift the rotating stall generation point to the small flow rate side. 
         [0046]    In addition, according to the present embodiment, in the inclined wall section  24 , since the trailing end  24 B which is continued to the first wall section  22  is disposed to be closer to the outside in a radial direction than the end section  31 A on the inside in the radial direction of the impeller  6  in the wall section  31  of the shroud casing  2 A, the inclination angle of the inclined wall section  24  of the hub casing  2 B can be more gently formed than the shroud casing  2 A side. Accordingly, it is possible to smoothly combine the circulating flow with the main flow of the fluid without disturbing the flow in the main flow of the fluid blown out from the discharge outlet  6 B outlet of the impeller  6 . Therefore, it is possible to smoothly maintain the flow in the vaneless diffuser  12  at the time of the small flow rate, and it is possible to shift the rotating stall generation point to the small flow rate side. 
         [0047]    Moreover, according to the present embodiment, since the first wall section  22  of the hub casing  2 B further protrudes toward the wall section  31  side of the shroud casing  2 A than the hub-side extension line HA which extends to the outside in the radial direction from the discharge outlet  6 B of the impeller  6 , and the hub-side protrusion length HD from the hub-side extension line HA to the first wall section  22  is 50% or less of the width W of the discharge outlet  6 B of the impeller  6 , it is possible to prevent a decrease in efficiency of the centrifugal compressor  1  while shifting the rotating stall generation point to the small flow rate side. 
         [0048]    In addition, according to the present embodiment, since the wall section  31  of the shroud casing  2 A further protrudes toward the first wall section  22  side of the hub casing  2 B than the shroud-side extension line SA which extends to the outside in the radial direction from the discharge outlet  6 B of the impeller  6 , and the shroud-side protrusion length SD from the shroud-side extension line SA to the wall section  31  is 20% or less of the outlet width W of the impeller  6 , effects capable of shifting the rotating stall generation point to the small flow rate side are exerted. 
         [0049]      FIG. 5  is a main portion sectional view showing a configuration in the vicinity of a vaneless diffuser according to another embodiment. In this embodiment, the same reference numerals are assigned to the same configurations as those of the embodiment, and descriptions thereof are omitted. In the above-described embodiment, both of the wall section  31  of the shroud casing  2 A and the first wall section  22  of the hub casing  2 B extends in the projection plane of the discharge outlet  6 B of the impeller  6 . However, the wall section  31  of the shroud casing  2 A may be disposed on the shroud-side extension line SA. That is, if at least the first wall section  22  of the hub casing  2 B further protrudes toward the inside (wall section  31  side) than the hub-side extension line HA, it is possible to exert effects capable of shifting the rotating stall generation point to a small flow rate side. 
         [0050]    Hereinbefore, the embodiments of the present invention are described. However, the present invention is not limited by the above-described contents. For example, in the above-described embodiment, the impeller  6  and the vaneless diffuser  12  are provided in the two-stage centrifugal compressor  1 . However, the impeller  6  and the vaneless diffuser  12  can be applied to a single-stage centrifugal compressor or a multiple-stage centrifugal compressor having three stages or more as long as the compressor includes the impeller and the vaneless diffuser. 
         [0051]    Moreover, in the above-described embodiment, the wall section  31  of the shroud casing  2 A is connected to the horizontal wall  30  via the connection wall  32  which is formed at a predetermined curvature radius. However, the connection wall  32  can adopt various shapes, and may be an inclined wall which extends from the tip of the horizontal wall  30  toward the end section  31 A of the wall section  31 . In a case where this connection wall  32  is provided, the end section  31 A on the inside in the radial direction of the impeller  6  in the wall section  31  becomes the connection portion between the connection wall  32  and the wall section  31 . 
       REFERENCE SIGNS LIST 
       [0052]      1 : centrifugal compressor 
         [0053]      2 : casing 
         [0054]      2 A: shroud casing 
         [0055]      2 B: hub casing 
         [0056]      5 : rotary shaft 
         [0057]      6 : impeller 
         [0058]      6 A: intake port 
         [0059]      6 B: discharge outlet (outlet) 
         [0060]      6 C: hub disk 
         [0061]      10 : suction port 
         [0062]      10 A: suction space 
         [0063]      11 : suction flow path 
         [0064]      12 : vaneless diffuser 
         [0065]      14 : return bend 
         [0066]      15 : return vane 
         [0067]      16 : scroll 
         [0068]      17 : discharge port 
         [0069]      18 : balance piston 
         [0070]      21 : fluid circulation flow path (bypass flow path) 
         [0071]      21 A: inlet (fluid inlet) 
         [0072]      21 B: outlet (fluid outlet) 
         [0073]      22 : first wall section 
         [0074]      23 : second wall section 
         [0075]      24 : inclined wall section 
         [0076]      24 A: protruding end 
         [0077]      24 B: trailing end 
         [0078]      30 : horizontal wall 
         [0079]      31 : wall section 
         [0080]      31 A: end section 
         [0081]      32 : connection wall 
         [0082]    D: flow path width 
         [0083]    HA: hub-side extension line 
         [0084]    HD: hub-side protrusion length 
         [0085]    SA: shroud-side extension line 
         [0086]    SD: shroud-side protrusion length 
         [0087]    W: width (outlet width) 
         [0088]    L: axis