Patent Publication Number: US-2022228602-A1

Title: Scroll structure of centrifugal compressor and centrifugal compressor

Description:
TECHNICAL FIELD 
     The present disclosure relates to a scroll structure of a centrifugal compressor and the centrifugal compressor. 
     BACKGROUND 
     A centrifugal compressor used in a compressor part or the like of a turbocharger for an automobile or a ship imparts kinetic energy to a fluid through rotation of an impeller and discharges the fluid radially outward, thereby achieving a pressure increase by utilizing the centrifugal force. 
     Such a centrifugal compressor is required of the high pressure ratio and high efficiency in a broad operational range. 
     The centrifugal compressor is provided with a scroll flow passage formed into a scroll shape. The scroll flow passage includes a flow passage connection section where a scroll start portion and a scroll end portion intersect. 
     At a high-flow operating point, an accelerating flow is formed from the scroll start portion to the scroll end portion of the scroll, and a pressure in the scroll start portion is higher than a pressure in the scroll end portion, rarely causing a recirculation flow from the scroll end portion to the scroll start portion in the flow passage connection section. 
     However, at a low-flow operating point, a decelerating flow is formed from the scroll start portion to the scroll end portion of the scroll, and the pressure in the scroll start portion is lower than the pressure in the scroll end portion, causing the recirculation flow from the scroll end portion to the scroll start portion in the flow passage connection section. The above phenomenon causes a separation loss or the like in the scroll. 
     That is, a fluid flow direction is changed in the flow passage connection section when the recirculation flow flows into the scroll start portion from the scroll end portion, causing the loss if the fluid separates from a wall surface forming the scroll flow passage in the scroll start portion. 
     Thus, for example, in a scroll structure of a centrifugal compressor described in Patent Document 1, the above-described loss is suppressed by changing the cross-sectional shape of the flow passage connection section (see Patent Document 1). 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Document 1: JP5479316B 
       
    
     SUMMARY 
     Technical Problem 
     For example, in the scroll structure of the centrifugal compressor described in Patent Document 1, a recirculation flow is suppressed by decreasing a cross-sectional area of the flow passage connection section, thereby suppressing the above-described loss. However, for example, in the scroll structure of the centrifugal compressor described in Patent Document 1, even though the loss due to the separation can be suppressed, the flow passage cross-sectional area in the scroll start portion is decreased, which may result in an excessive flow velocity and an increase in loss. 
     In view of the above, an object of at least one embodiment of the present invention is to provide a scroll structure of a centrifugal compressor where efficiency increases in a broad operational range, and the centrifugal compressor. 
     Solution to Problem 
     (1) A scroll structure of a centrifugal compressor according to at least one embodiment of the present invention is a scroll structure of a centrifugal compressor provided with a scroll flow passage formed into a scroll shape, which includes, of a flow passage connection section where a scroll start portion and a scroll end portion of the scroll flow passage intersect, a connection region where a first inner circumferential surface of the scroll end portion in the centrifugal compressor and a second inner circumferential surface of the scroll start portion in the centrifugal compressor are connected. The connection region includes a turning start point where a direction starts to change from the first inner circumferential surface toward the second inner circumferential surface, and a turning end point where the change in direction from the first inner circumferential surface toward the second inner circumferential surface comes to an end. Where a cross-section orthogonal to an extension direction of a center line of the scroll flow passage in the connection region is a first cross-section, the turning start point on the first cross-section is a first turning start point, a turning end point on the first cross-section is a first turning end point, and a tangent line to the first inner circumferential surface passing through the first turning start point on the first cross-section is a first direction, the first turning start point exists at a position away from the first turning end point along the first direction by a distance not less than 30% of a height dimension along an axial direction of the centrifugal compressor at a minimum cross-sectional area position of the scroll flow passage. 
     In the above-described connection region in the flow passage connection section, an extension direction of the inner circumferential surface of the scroll flow passage changes relatively largely from the first inner circumferential surface of the scroll end portion in the centrifugal compressor to the second inner circumferential surface of the scroll start portion in the centrifugal compressor. Thus, the fluid flowing along the first inner circumferential surface is likely to separate from the second inner circumferential surface when flowing into the scroll start portion as the recirculation flow. 
     To cope therewith, in the above configuration (1), the first turning start point exists at the position away from the first turning end point along the first direction by the distance not less than 30% of the height dimension along the axial direction at the minimum cross-sectional area position of the scroll flow passage. Thus, the change in direction of the inner circumferential surface of the scroll flow passage changing from the first inner circumferential surface to the second inner circumferential surface becomes slow, the fluid flowing along the first inner circumferential surface is unlikely to separate from the second inner circumferential surface when flowing into the scroll start portion as the recirculation flow, making it possible to suppress the loss associated with the separation. Therefore, in the centrifugal compressor, it is possible to increase efficiency in a broad operational range. 
     (2) In some embodiments, in the above configuration (1), at least at an intermediate position between the first turning start point and the first turning end point, the connection region exists at the same position as a virtual inscribed circle or at a position on a center side of the virtual inscribed circle relative to the position in question, the virtual inscribed circle being in contact with the first inner circumferential surface at the first turning start point and in contact with a virtual line obtained by extending the second inner circumferential surface at the first turning end point along an extension direction of the scroll flow passage. 
     Causing the connection region to have the above configuration (2), the change in direction of the inner circumferential surface of the scroll flow passage changing from the first inner circumferential surface to the second inner circumferential surface becomes slow, the fluid flowing along the first inner circumferential surface is unlikely to separate from the second inner circumferential surface when flowing into the scroll start portion as the recirculation flow, making it possible to suppress the loss associated with the separation. 
     (3) In some embodiments, in the above configuration (1) or (2), the first turning end point is located on a downstream side of the scroll flow passage relative to a position where a virtual inscribed circle contacts a virtual line obtained by extending the second inner circumferential surface at the first turning end point along an extension direction of the scroll flow passage, the virtual inscribed circle being in contact with the first inner circumferential surface at the first turning start point and in contact with the virtual line. 
     With the above configuration (3), as compared with a case in which the first turning end point is set at the position where the above-described virtual inscribed circle contacts the above-described virtual line, it is possible to set the position of the first turning end point on the downstream side of the scroll flow passage. Thus, the change in direction of the inner circumferential surface of the scroll flow passage changing from the first inner circumferential surface to the second inner circumferential surface becomes slower. Thus, the fluid flowing along the first inner circumferential surface is more unlikely to separate from the second inner circumferential surface when flowing into the scroll start portion as the recirculation flow, making it possible to further suppress the loss associated with the separation. 
     (4) In some embodiments, in any one of the above configurations (1) to (3), the connection region may include a curved portion ranging from the first turning start point to the first turning end point. 
     (5) In some embodiments, in the above configuration (4), the curved portion has a curvature radius gradually increasing from the first turning start point toward the first turning end point. 
     With the above configuration (5), the change in direction of the inner circumferential surface of the scroll flow passage changing from the first inner circumferential surface to the second inner circumferential surface becomes slow toward the second inner circumferential surface. Thus, the fluid flowing along the first inner circumferential surface is more unlikely to separate from the second inner circumferential surface when flowing into the scroll start portion as the recirculation flow, making it possible to further suppress the loss associated with the separation. 
     (6) In some embodiments, in any one of the above configurations (1) to (3), the connection region may include a straight line portion in at least a partial area ranging from the first turning start point to the first turning end point. 
     (7) In some embodiments, in any one of the above configurations (1) to (6), the connection region includes an area where a ratio (a2/a1) of a distance a2 from a straight line L to a farthest position on the connection region to a distance a1 of the straight line L decreases from a downstream side toward an upstream side along the extension direction of the center line of the scroll flow passage, the straight line L joining the first turning start point and the first turning end point. 
     The above-described connection region extends along the extension direction of the center line of the scroll flow passage in the scroll end portion (first inner circumferential surface), as the scroll end portion is viewed from the radially outer side of the centrifugal compressor. 
     As a result of intensive researches by the present inventors, it was found that the separation is more likely to occur in the fluid flowing into the scroll start portion from the upstream area of the connection region along the extension direction than in the fluid flowing into the scroll start portion from the downstream area of the connection region along the extension direction. 
     With the above configuration (7), since the connection region includes the area where the above-described ratio (a2/a1) decreases from the downstream side toward the upstream side along the extension direction of the center line of the scroll flow passage, an area exists where the change in direction of the inner circumferential surface of the scroll flow passage changing from the first inner circumferential surface to the second inner circumferential surface becomes slow from the downstream side toward the upstream side along the extension direction. Therefore, with the above configuration (7), it is possible to effectively suppress occurrence of the separation. 
     (8) In some embodiments, in the above configuration (7), the ratio (a2/a1) takes a minimum value in an area, of the connection region, on the upstream side of the scroll flow passage relative to a position of a tongue portion. 
     As described above, the above-described separation is more likely to occur in the fluid flowing into the scroll start portion from the upstream area of the connection region along the extension direction of the center line of the scroll flow passage than in the fluid flowing into the scroll start portion from the downstream area of the connection region along the extension direction. 
     With the above configuration (8), since the above-described ratio (a2/a1) takes the minimum value in the area, of the connection region, on the upstream side of the scroll flow passage relative to the position of the tongue portion, the change in direction of the inner circumferential surface of the scroll flow passage changing from the first inner circumferential surface to the second inner circumferential surface becomes slow in the region on the upstream side. 
     Therefore, with the above configuration (8), it is possible to effectively suppress occurrence of the separation. 
     (9) In some embodiments, in the above configuration (7) or (8), the ratio (a2/a1) takes a minimum value in an area, of the connection region, on the upstream side of the scroll flow passage relative to a position on a most upstream side in the axial direction. 
     The above-described connection region first heads for the axially upstream side of the centrifugal compressor to reach the position on the most upstream side in the axial direction, and then extends toward the axially downstream side, as the connection region moves from the most downstream side toward the upstream side along the extension direction of the center line of the scroll flow passage. 
     Further, as described above, the above-described separation is more likely to occur in the fluid flowing into the scroll start portion from the area of the connection region on the upstream side along the extension direction than in the fluid flowing into the scroll start portion from the area of the connection region on the downstream side along the extension direction. However, an area of the scroll start portion suffering the most from the loss due to the separation in the scroll flow passage is an area reached by the fluid passing through the connection region at the position on the upstream side along the extension direction of the center line of the flow passage relative to the above-described “position on the most upstream side in the axial direction”. 
     Therefore, disposing the connection region to have the above configuration (9), the change in direction of the inner circumferential surface of the scroll flow passage changing from the first inner circumferential surface to the second inner circumferential surface can be made much slower in an area of the connection region which is passed by the fluid flowing into the area where the loss due to the separation is relatively large. Thus, it is possible to effectively suppress occurrence of the separation. 
     (10) A centrifugal compressor according to at least one embodiment of the present invention includes the scroll structure of the centrifugal compressor according to any one of the above configurations (1) to (9), making it possible to increase efficiency in a broad operational range. 
     Advantageous Effects 
     According to at least one embodiment of the present invention, it is possible to increase efficiency in a broad operational range in a centrifugal compressor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional schematic view of a centrifugal compressor according to some embodiments. 
         FIG. 2  is a view schematically showing a cross-section of a casing in the centrifugal compressor cut off at a cross-section orthogonal to the axis direction of a rotational shaft in the centrifugal compressor according to some embodiments. 
         FIG. 3  is an arrow view of a cross-section along line A-A in  FIG. 2 . 
         FIG. 4  is an enlarged view of the vicinity of a flow passage connection section in  FIG. 3 . 
         FIG. 5  is a view corresponding to the enlarged view of the vicinity of the flow passage connection section in  FIG. 3 . 
         FIG. 6  is a view corresponding to the enlarged view of the vicinity of the flow passage connection section in  FIG. 3 . 
         FIG. 7  is a view corresponding to the enlarged view of the vicinity of the flow passage connection section in  FIG. 3 . 
         FIG. 8  is an arrow view of a cross-section along line B-B in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Some embodiments of the present invention will be described below with reference to the accompanying drawings. It is intended, however, that unless particularly identified, dimensions, materials, shapes, relative positions and the like of components described or shown in the drawings as the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention. 
     For instance, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function. 
     For instance, an expression of an equal state such as “same”, “equal”, and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function. 
     Further, for instance, an expression of a shape such as a rectangular shape or a tubular shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved. 
     On the other hand, the expressions “comprising”, “including”, “having”, “containing”, and “constituting” one constituent component are not exclusive expressions that exclude the presence of other constituent components. 
       FIG. 1  is a cross-sectional schematic view of a centrifugal compressor  1  according to some embodiments. The centrifugal compressor  1  according to some embodiments is the centrifugal compressor  1  applied to a turbocharger. In the centrifugal compressor  1  according to some embodiments, a compressor wheel  8  and a turbine wheel of a turbine (not shown) are coupled by a rotational shaft  3 . The compressor wheel  8  includes a plurality of compressor blades  7  erecting on the surface of a hub  5 . The compressor wheel  8  is covered with a compressor housing (casing)  9  on the outer side of the compressor blades  7 . In the centrifugal compressor  1  according to some embodiments, a diffuser  11  is formed on the outer peripheral side of the compressor blades  7 , and in addition, a scroll flow passage  13  formed into a scroll shape is disposed in the periphery of the diffuser  11 . 
       FIG. 2  is a view schematically showing a cross-section of the casing  9  in the centrifugal compressor  1  cut off at a cross-section orthogonal to the direction of an axis X of the rotational shaft  3  in the centrifugal compressor  1  according to some embodiments. The casing  9  includes the scroll flow passage  13 , and an outlet flow passage  15  connected to the downstream side of the scroll flow passage  13 . The scroll flow passage  13  includes a scroll start portion  17  and a scroll end portion  19  of the scroll flow passage. The scroll flow passage  13  is formed such that a flow passage cross-sectional area thereof increases as the scroll flow passage  13  moves clockwise as shown in  FIG. 2  from the scroll start portion  17 . 
     In  FIG. 2 , an arrow R indicates the rotational direction of the compressor wheel  8 . In the centrifugal compressor  1  according to some embodiments, the compressor wheel  8  rotates clockwise in  FIG. 2 . 
     A fluid flow in the scroll flow passage  13  involves a main flow  91  (see  FIG. 2 ) of a circumferential flow from the scroll start portion  17  to the scroll end portion  19 , and a swirl flow  93  (see  FIG. 4  to be described later) flowing while swirling in the scroll flow passage  13  along the main flow. 
     In the following description, the direction of the axis X of the rotational shaft  3  of the centrifugal compressor  1  may be referred to as the axial direction of the centrifugal compressor  1  or may simply be referred to as the axial direction. Of the axial direction, an upstream side along the flow of the fluid flowing into the centrifugal compressor  1  is an axially upstream side, and a side opposite thereto is an axially downstream side. Further, in the following description, the radial direction of the compressor wheel  8  of the centrifugal compressor  1  may be referred to as the radial direction of the centrifugal compressor  1  or may simply be referred to as the radial direction. Of the radial direction, a direction close to the axis X of the rotational shaft  3  is a radially inner side, and a direction away from the axis X of the rotational shaft  3  is a radially outer side. 
     Further, in the scroll flow passage  13  and the outlet flow passage  15 , of an extension direction of the flow passages, an upstream side of the main flow of the fluid will be referred to as an upstream side of the scroll flow passage  13  and an upstream side of the outlet flow passage  15 , and a downstream side of the main flow of the fluid will be referred to as a downstream side of the scroll flow passage  13  and a downstream side of the outlet flow passage  15 . The upstream side of the scroll flow passage  13  and the upstream side of the outlet flow passage  15  may each be referred to as a flow passage upstream side or simply be referred to as the upstream side, and the downstream side of the scroll flow passage  13  and the downstream side of the outlet flow passage  15  may each be referred to as a flow passage downstream side or simply be referred to as the downstream side In the scroll flow passage  13 , the extension direction of the scroll flow passage  13  is substantially the same direction as the circumferential direction of the centrifugal compressor  1 . 
     In a scroll structure  10  of the centrifugal compressor  1  according to some embodiments, the casing  9  forms a flow passage connection section  20  where the scroll start portion  17  and the scroll end portion  19  of the scroll flow passage  13  intersect. In the flow passage connection section  20 , in the scroll end portion  19  of an inner circumferential surface  13   a  of the scroll flow passage  13 , an opening portion  21  communicating with the scroll start portion  17  is formed. Of an opening forming portion  23  enclosing the opening portion  21 , at a position on the most downstream side of the scroll flow passage  13 , a tongue portion  25  is formed which separates the scroll flow passage  13  and the outlet flow passage  15  from each other. 
       FIG. 3  is an arrow view of a cross-section along line A-A in  FIG. 2 . That is,  FIG. 3  is a schematic cross-sectional view of the casing  9  when the casing  9  is cut off at a cross-section extending in a direction orthogonal to the extension direction of the scroll end portion  19  at a position including the flow passage connection section  20 .  FIG. 3  and  FIGS. 4 to 7  to be described later each represent a first cross-section  9   c  which is a cross-section orthogonal to an extension direction of a center line AX of the scroll flow passage  13  in a connection region  30  to be described later.  FIG. 3  is also a view where the inside of the scroll flow passage  13  in the scroll end portion  19  is viewed on the upstream side from the downstream side of the outlet flow passage  15 .  FIG. 3  omits the illustration of the diffuser  11 . 
       FIG. 4  is an enlarged view of the vicinity of the flow passage connection section  20  in  FIG. 3 , and is a view showing an embodiment of the connection region  30  to be described later. 
       FIG. 5  is a view corresponding to the enlarged view of the vicinity of the flow passage connection section  20  in  FIG. 3 , and is a view showing another embodiment of the connection region  30 . 
       FIG. 6  is a view corresponding to the enlarged view of the vicinity of the flow passage connection section  20  in  FIG. 3 , and is a view showing still another embodiment of the connection region  30 . 
       FIG. 7  is a view corresponding to the enlarged view of the vicinity of the flow passage connection section  20  in  FIG. 3 , and is a view showing yet another embodiment of the connection region  30 . 
       FIG. 8  is an arrow view of a cross-section along line B-B in  FIG. 2 . 
     For example, as shown in  FIGS. 3 and 8 , in some embodiments, the flow passage connection section  20  includes the connection region  30  where, of the flow passage connection section  20 , a first inner circumferential surface  19   a  of the scroll end portion  19  in the centrifugal compressor  1  and a second inner circumferential surface  17   a  of the scroll start portion  17  in the centrifugal compressor  1  are connected. Hereinafter, the connection region  30  according to some embodiments will be described in detail. 
     At a high-flow operating point, an accelerating flow is formed from the scroll start portion  17  to the scroll end portion  19 , and a pressure in the scroll start portion  17  is higher than a pressure in the scroll end portion  19 , rarely causing a recirculation flow  95  (see  FIG. 4 ) from the scroll end portion  19  to the scroll start portion  17  in the flow passage connection section  20 . 
     However, at a low-flow operating point, a decelerating flow is formed from the scroll start portion  17  to the scroll end portion  19 , and the pressure in the scroll start portion  17  is lower than the pressure in the scroll end portion  19 , causing the recirculation flow  95  from the scroll end portion  19  to the scroll start portion  17  in the flow passage connection section  20 . The above phenomenon causes a separation loss or the like in the scroll flow passage  13 . 
     That is, a fluid flow direction is changed in the flow passage connection section  20  when the recirculation flow  95  flows into the scroll start portion  17  from the scroll end portion  19 , causing the loss if the fluid separates from a wall surface (second inner circumferential surface  17   a ) forming the scroll flow passage  13  in the scroll start portion  17 . 
     Thus, in some embodiments, the above-described separation is suppressed by setting the form of the connection region  30  to a form to be described below. 
     In some embodiments shown in  FIGS. 3 to 7 , the connection region  30  includes a turning start point  71  where a direction starts to change from the first inner circumferential surface  19   a  toward the second inner circumferential surface  17   a , and a turning end point  73  where the change in direction from the first inner circumferential surface  19   a  toward the second inner circumferential surface  17   a  comes to an end. The turning start point  71  on the first cross-section  9   c  is a first turning start point  71   a , and the turning end point  73  on the first cross-section  9   c  is a first turning end point  73   a . Further, for example, as shown in  FIG. 4 , an extension direction of a tangent line L 1  (tangent direction) to the first inner circumferential surface  19   a  passing through the first turning start point  71   a  on the first cross-section  9   c  will be referred to as a first direction Dr1. 
     For example, in the case of some embodiments shown in  FIGS. 3 to 6 , the position of the turning start point  71  may be an intersection of the first inner circumferential surface  19   a  and an arc of a virtual inscribed circle, a virtual inscribed ellipse, a virtual circle, or a virtual ellipse to be described later, or a position where the direction starts to change from the first inner circumferential surface  19   a  toward the arc to be connected to the arc. Likewise, in the case of some embodiments shown in  FIGS. 3 to 6 , the position of the turning end point  73  may be an intersection of the second inner circumferential surface  17   a  and the arc, or a position where the direction starts to change from the second inner circumferential surface  17   a  toward the arc to be connected to the arc. 
     Further, for example, in the case of another embodiment shown in  FIG. 7 , the position of the turning start point  71  may be an intersection of the first inner circumferential surface  19   a  and a straight line  87  to be described later, or a position where the direction starts to change from the first inner circumferential surface  19   a  toward the straight line  87  to be connected to the straight line  87 . Likewise, in the case of still another embodiment shown in  FIG. 7 , the position of the turning end point  73  may be an intersection of the second inner circumferential surface  17   a  and the straight line  87 , or a position where the direction starts to change from the second inner circumferential surface  17   a  toward the straight line  87  to be connected to the straight line  87 . 
     Further, in some embodiments shown in  FIGS. 3 to 7 , the first turning start point  71   a  exists at a position away from the first turning end point  73   a  along the above-described first direction Dr 1  by a distance h not less than 30% of a height dimension Ha along the axial direction of the centrifugal compressor  1  at a minimum cross-sectional area position  13   b  (see  FIG. 3 ) of the scroll flow passage  13 . In other words, in some embodiments, in at least a part of the connection region  30 , the positional relationship between the first turning start point  71   a  and the first turning end point  73   a  is preferably the above-described relationship. In some embodiments shown in  FIGS. 3 to 7 , it is more preferable that the turning start point  71  exists at a position away from the first turning end point  73   a  along the first direction Dr1 by the distance h not less than 50% of the above-described height dimension Ha. 
     In the connection region  30  in the flow passage connection section  20 , an extension direction of the inner circumferential surface  13   a  of the scroll flow passage  13  changes relatively largely from the first inner circumferential surface  19   a  of the scroll end portion  19  to the second inner circumferential surface  17   a  of the scroll start portion  17 . Thus, the fluid flowing along the first inner circumferential surface  19   a  is likely to separate from the second inner circumferential surface  17   a  when flowing into the scroll start portion  17  as the recirculation flow  95 . 
     To cope therewith, in some embodiments shown in  FIGS. 3 to 7 , the first turning start point  71   a  exists at the position away from the first turning end point  73   a  along the first direction Drl by the distance h not less than 30% of the height dimension Ha along the axial direction at the minimum cross-sectional area position  13   b  of the scroll flow passage  13 . Thus, the change in direction of the inner circumferential surface  13   a  of the scroll flow passage  13  changing from the first inner circumferential surface  19   a  to the second inner circumferential surface  17   a  becomes slow, the fluid flowing along the first inner circumferential surface  19   a  is unlikely to separate from the second inner circumferential surface  17   a  when flowing into the scroll start portion  17  as the recirculation flow  95 , making it possible to suppress the loss associated with the separation. Therefore, in the centrifugal compressor  1  according to some embodiments, it is possible to increase efficiency in a broad operational range. 
     In the connection region  30  according to an embodiment shown in  FIG. 3, 4 , the first inner circumferential surface  19   a  and the second inner circumferential surface  17   a  are connected by an arc  81   a  of a virtual inscribed circle  81  in contact with the first inner circumferential surface  19   a  at the first turning start point  71   a  and in contact with the second inner circumferential surface  17   a  at the first turning end point  73   a . The virtual inscribed circle  81  is a true circle. 
     That is, a connection surface  31  which is the inner circumferential surface  13   a  of the scroll flow passage  13  in the connection region  30  according to an embodiment shown in  FIG. 3, 4  coincides with a part of the arc  81   a  of the virtual inscribed circle  81  in the first cross-section  9   c.    
     In the following description, a position through which the center, that is, the center line AX of the scroll flow passage  13  passes is the center of gravity (centroid) of the scroll flow passage  13  in the above-described virtual cut surface. 
     In the connection region  30  according to another embodiment shown in  FIG. 5 , the first inner circumferential surface  19   a  and the second inner circumferential surface  17   a  are connected by an arc  83   a  of a virtual inscribed ellipse  83  in contact with the first inner circumferential surface  19   a  at the first turning start point  71   a  and in contact with the second inner circumferential surface  17   a  at the first turning end point  73   a . In the connection region  30  according to another embodiment shown in  FIG. 5 , the virtual inscribed ellipse  83  has a major axis  83   b  oriented in the radial direction of the centrifugal compressor  1 , and a minor axis  83   c  oriented in the axial direction of the centrifugal compressor  1 . 
     That is, the connection surface  31  of the connection region  30  according to another embodiment shown in  FIG. 5  coincides with a part of the arc  83   a  of the virtual inscribed ellipse  83  in the first cross-section  9   c.    
     In the connection region  30  according to still another embodiment shown in  FIG. 6 , the center of a curvature exists on the axially inner side of the first turning start point  71   a , and the first turning start point  71   a  and the first turning end point  73   a  are connected by an arc  85   a  of the virtual circle  85  whose curvature radius is larger than that of the above-described virtual inscribed circle  81 . 
     That is, the connection surface  31  of the connection region  30  according to still another embodiment shown in  FIG. 6  coincides with a part of the arc  85   a  of the virtual circle  85  in the first cross-section  9   c.    
     In the connection region  30  according to still another embodiment shown in  FIG. 6 , the virtual circle  85  is a true circle. However, the virtual circle  85  may be an ellipse (virtual ellipse). If the virtual circle  85  is the ellipse (virtual ellipse), the virtual ellipse preferably has a major axis oriented in the radial direction of the centrifugal compressor  1 , and a minor axis oriented in the axial direction of the centrifugal compressor  1 . 
     Unlike the connection region  30  according to still another embodiment shown in  FIG. 6  and the connection region  30  according to yet another embodiment shown in  FIG. 7  to be described later, the connection surface  31  may not necessarily internally contact the first inner circumferential surface  19   a  and the second inner circumferential surface  17   a . The connection surface  31  may internally contact one of the first inner circumferential surface  19   a  or the second inner circumferential surface  17   a  and may not internally contact the other, or may not internally contact neither the first inner circumferential surface  19   a  nor the second inner circumferential surface  17   a.    
     In the connection region  30  according to yet another embodiment shown in  FIG. 7 , the first inner circumferential surface  19   a  and the second inner circumferential surface  17   a  are connected by a straight line joining the first turning start point  71   a  and the first turning end point  73   a.    
     That is, the connection surface  31  of the connection region  30  according to yet another embodiment shown in  FIG. 7  coincides with the straight line  87  ranging from the first turning start point  71   a  to the first turning end point  73   a  in the first cross-section  9   c . The connection surface  31  of the connection region  30  according to yet another embodiment shown in  FIG. 7  will also be referred to as a straight line portion  39 . 
     In the connection region  30  according to an embodiment shown in  FIG. 3, 4 , as described above, the connection surface  31  coincides with a part of the arc  81   a  of the virtual inscribed circle  81  in contact with the first inner circumferential surface  19   a  at the first turning start point  71   a  and in contact with the second inner circumferential surface  17   a  at the first turning end point  73   a.    
     Further, in the connection region  30  according to another embodiment shown in  FIG. 5 , the connection surface  31  exists at a position on the side of a center O of the virtual inscribed circle  81  relative to the position of the above-described virtual inscribed circle  81 . The virtual inscribed circle  81  is a virtual inscribed circle which is in contact with the first inner circumferential surface  19   a  at the first turning start point  71   a  and in contact with a virtual line  89  obtained by extending the second inner circumferential surface  17   a  at the first turning end point  73   a  along the extension direction of the scroll flow passage  13 . 
     In the connection region  30  according to still another embodiment shown in  FIG. 6  and yet another embodiment shown in  FIG. 7 , the connection surface  31  exists at the position on the side of the center O of the virtual inscribed circle  81  relative to the position of the above-described virtual inscribed circle  81 . 
     That is, in some embodiments shown in  FIGS. 3 to 7 , at least at an intermediate position between the first turning start point  71   a  and the first turning end point  73   a , the connection region  30  exists at the same position as the virtual inscribed circle  81  or at the position on the side of the center O of the virtual inscribed circle  81  relative to the position in question. The virtual inscribed circle  81  is in contact with the first inner circumferential surface  19   a  at the first turning start point  71   a  and in contact with the virtual line  89  obtained by extending the second inner circumferential surface  17   a  at the first turning end point  73   a  along the extension direction of the scroll flow passage  13 . 
     Thus, since the change in direction of the inner circumferential surface  13   a  of the scroll flow passage  13  changing from the first inner circumferential surface  19   a  to the second inner circumferential surface  17   a  becomes slow, the fluid flowing along the first inner circumferential surface  19   a  is unlikely to separate from the second inner circumferential surface  17   a  when flowing into the scroll start portion  17  as the recirculation flow  95 , making it possible to suppress the loss associated with the separation. 
     For example, as in another embodiment shown in  FIG. 5 , the first turning end point  73   a  is located on the downstream side of the scroll flow passage  13  (scroll start portion  17 ) relative to a position (contact position)  75  where the above-described virtual inscribed circle  81  contacts the above-described virtual line  89 . 
     Thus, as compared with a case in which the first turning end point  73   a  is set at the contact position  75  where the above-described virtual inscribed circle  81  contacts the above-described virtual line  89 , it is possible to set the position of the first turning end point  73   a  on the downstream side of the scroll flow passage  13  (scroll start portion  17 ). Thus, the change in direction of the inner circumferential surface  13   a  of the scroll flow passage  13  changing from the first inner circumferential surface  19   a  to the second inner circumferential surface  17   a  becomes slower. Therefore, the fluid flowing along the first inner circumferential surface  19   a  is more unlikely to separate from the second inner circumferential surface  17   a  when flowing into the scroll start portion  17  as the recirculation flow  95 , making it possible to further suppress the loss associated with the separation. 
     The first turning end point  73   a  may be shifted to the downstream side of the scroll flow passage  13  (scroll start portion  17 ) relative to the contact position  75  by shifting the position of the arc  85   a  of the virtual circle  85  according to still another embodiment shown in  FIG. 6 , by changing the oblateness of the virtual circle  85 , or by changing the curvature radius of the virtual circle  85 . 
     Alternatively, the first turning end point  73   a  may be shifted to the downstream side of the scroll flow passage  13  (scroll start portion  17 ) relative to the contact position  75  by changing an inclination angle of the straight line portion  39  according to yet another embodiment shown in  FIG. 7 . 
     For example, as in some embodiments shown in  FIGS. 3 to 6 , the connection region  30  may have a curved portion  33  ranging from the first turning start point  71   a  to the first turning end point  73   a.    
     Connecting the first turning start point  71   a  and the first turning end point  73   a  by the curved portion  33 , it is possible to suppress the loss of the fluid passing along the connection region  30 . 
     If the connection region  30  has the curved portion  33 , for example, as in some embodiments shown in  FIGS. 3 to 6 , the curvature radius of the curved portion  33  may gradually increase from the first turning start point  71   a  toward the first turning end point  73   a . For example, in another embodiment shown in  FIG. 5 , the first inner circumferential surface  19   a  and the second inner circumferential surface  17   a  are connected by the arc  83   a  of the virtual inscribed ellipse  83 . In this case, as shown in  FIG. 5 , if an intersection P 1  of the minor axis  73   c  and the arc  83   a  on the axially downstream side of a center O 1  of the virtual inscribed ellipse  83  is located on the downstream side of the scroll flow passage  13  (scroll start portion  17 ) relative to the first turning end point  73   a , the arc  83   a  of the virtual inscribed ellipse  83  gradually increases in curvature radius from the first turning start point  71   a  toward the first turning end point  73   a.    
     Thus, the change in direction of the inner circumferential surface  13   a  of the scroll flow passage  13  changing from the first inner circumferential surface  19   a  to the second inner circumferential surface  17   a  becomes slow toward the second inner circumferential surface  17   a.    
     Thus, the fluid flowing along the first inner circumferential surface  19   a  is more unlikely to separate from the second inner circumferential surface  17   a  when flowing into the scroll start portion  17  as the recirculation flow  95 , making it possible to further suppress the loss associated with the separation. 
     For example, as in yet another embodiment shown in  FIG. 7 , the connection region  30  may have the straight line portion  39  in at least a partial area ranging from the first turning start point  71   a  to the first turning end point  73   a.    
     Connecting the at least partial area between the first turning start point  71   a  and the first turning end point  73   a  by the straight line portion  39 , it is possible to shorten a distance (creepage distance) along the connection surface  31  between the first turning start point  71   a  and the first turning end point  73   a , and to suppress the loss of the fluid passing along the connection region  30 . 
     In some embodiments described above, in the first cross-section  9   c , that is, a cross-section appearing on the drawings of  FIGS. 3 to 6 , the curvature of the curved portion  33  may be different in curvature radius depending on the position between the first turning start point  71   a  and the first turning end point  73   a  to have a different curved line from the arc  83   a  of the virtual inscribed ellipse  83 . That is, the shape of the curved portion  33  appearing in the first cross-section  9   c  may be the shape of the curved line represented by an exponential function, and the curvature radius may increase/decrease from the first turning start point  71   a  toward the first turning end point  73   a.    
     Further, in some embodiments described above, in the first cross-section  9   c , that is, the cross-section appearing on the drawing of  FIG. 7 , the straight line portion  39  may have not less than two connected straight lines different in extension direction, and may have a bending point between the first turning start point  71   a  and the first turning end point  73   a.    
     Further, in yet another embodiment shown in  FIG. 7 , the first inner circumferential surface  19   a  and the straight line portion  39  may be connected by a curved line, such as an arc, at the first turning start point  71   a . Likewise, in yet another embodiment shown in  FIG. 7 , the straight line portion  39  and the second inner circumferential surface  17   a  may be connected by a curved line, such as an arc, at the first turning end point  73   a.    
     Hereinafter, the connection region  30  according to some embodiments will further be described with reference to  FIG. 8  as well.  FIG. 8  is an arrow view of a cross-section along line B-B in  FIG. 2 , that is, a schematic cross-sectional view of the casing  9  when the casing  9  is cut off at a cross-section extending in substantially the same direction as the extension direction of the scroll end portion  19  and extending in the axial direction of the centrifugal compressor  1 .  FIG. 8  is also a view where the inside of the scroll flow passage  13  in the scroll end portion  19  is viewed from the radially outer side of the centrifugal compressor  1 . 
     As shown in  FIG. 8 , in the flow passage connection section  20  according to some embodiments, the opening portion  21  is disposed in a partial section along the extension direction (circumferential direction) of the scroll flow passage  13 . In the flow passage connection section  20  according to some embodiments, the connection region  30  exists in the opening forming portion  23  enclosing the opening portion  21 . In the flow passage connection section  20  according to some embodiments, the connection region  30  is configured such that, as the scroll end portion  19  (first inner circumferential surface  19   a ) is viewed from the radially outer side of the centrifugal compressor  1 , an area on the axially upstream side and the axially downstream side of the tongue portion  25  exists along the extension direction of the center line AX of the scroll flow passage  13  in the scroll end portion  19 . 
     Further, in the flow passage connection section  20  according to some embodiments, the connection region  30  first heads for the axially upstream side of the centrifugal compressor  1  to reach a position P 3  on the most axially upstream side, and then extends toward the axially downstream side, as the connection region  30  moves from the most downstream side toward the upstream side (the upstream side of the flow passage) along the extension direction of the center line AX of the scroll flow passage  13 , on the axially upstream side of the tongue portion  25  and on the upstream side of the flow passage. 
     For example, as shown in  FIG. 4 , reference character al denotes a distance of a straight line L joining the first turning start point  71   a  and the first turning end point  73   a  in the first cross-section  9   c , and reference character a2 denotes a distance from the straight line L to a farthest position P 5  on the connection region. The connection region  30  according to some embodiments includes an area where a ratio (a2/a1) of the distance a2 to the distance a1 decreases from the downstream side toward the upstream side along the extension direction of the center line AX. 
     As described above, the connection region  30  extends along the extension direction of the center line AX of the scroll flow passage  13  in the scroll end portion  19 , as the scroll end portion  19  is viewed from the radially outer side of the centrifugal compressor  1 . 
     As a result of intensive researches by the present inventors, it was found that the separation is more likely to occur in the fluid flowing into the scroll start portion  17  from the upstream area of the connection region  30  along the extension direction than in the fluid flowing into the scroll start portion  17  from the downstream area of the connection region  30  along the extension direction. 
     According to some embodiments described above, since the connection region  30  includes the area where the above-described ratio (a2/a1) decreases from the downstream side toward the upstream side along the extension direction of the center line AX of the scroll flow passage  13 , an area exists where the change in direction of the inner circumferential surface  13   a  of the scroll flow passage  13  changing from the first inner circumferential surface  19   a  to the second inner circumferential surface  17   a  becomes slow from the downstream side toward the upstream side along the extension direction. 
     Therefore, according to some embodiments described above, it is possible to effectively suppress occurrence of the separation. 
     Further, in the flow passage connection section  20  according to some embodiments, the above-described ratio (a2/a1) takes a minimum value in an area REa, of the connection region  30 , on the upstream side of the scroll flow passage  13  relative to the position of the tongue portion  25 . 
     As described above, the above-described separation is more likely to occur in the fluid flowing into the scroll start portion  17  from the upstream area of the connection region  30  along the extension direction of the center line AX of the scroll flow passage  13  than in the fluid flowing into the scroll start portion  17  from the downstream area of the connection region  30  along the extension direction. 
     According to some embodiments described above, since the above-described ratio (a2/a1) takes the minimum value in the above-described area REa, the change in direction of the inner circumferential surface  13   a  of the scroll flow passage  13  changing from the first inner circumferential surface  19   a  to the second inner circumferential surface  17   a  becomes slow. 
     Therefore, according to some embodiments described above, it is possible to effectively suppress occurrence of the separation. 
     Further, in the flow passage connection section  20  according to some embodiments, the above-described ratio (a2/a1) takes the minimum value in an area REu, of the connection region  30 , on the upstream side of the flow passage relative to the position P 3  on the most axially upstream side. In some embodiments, the area REu is an area on the upstream side of the flow passage relative to the above-described position P 3  of the area located on the axially upstream side of the opening portion  21 , of the opening forming portion  23 . 
     As described above, the connection region  30  according to some embodiments first heads for the axially upstream side of the centrifugal compressor  1  to reach the position P 3  on the most axially upstream side, and then extends toward the axially downstream side, as the connection region  30  moves from the tongue portion  25  toward the upstream side of the flow passage. 
     Further, as described above, the above-described separation is more likely to occur in the fluid flowing into the scroll start portion  17  from the area of the connection region  30  on the upstream side of the flow passage than in the fluid flowing into the scroll start portion  17  from the area of the connection region  30  on the downstream side of the flow passage. 
     However, an area of the scroll start portion  17  suffering the most from the loss due to the separation in the scroll flow passage  13  is an area reached by the fluid passing through the connection region  30  at the position on the upstream side of the flow passage relative to the above-described position P 3 , that is, the fluid passing through the area REu. Therefore, disposing the connection region  30  such that the above-described ratio (a2/a1) takes the minimum value in the above-described area REu, the change in direction of the inner circumferential surface  13   a  of the scroll flow passage  13  changing from the first inner circumferential surface  19   a  to the second inner circumferential surface  17   a  can be made much slower in an area (area REu) of the connection region  30  which is passed by the fluid flowing into the area where the loss due to the separation is relatively large. Thus, it is possible to effectively suppress occurrence of the separation. 
     The present invention is not limited to the above-described embodiments, and also includes an embodiment obtained by modifying the above-described embodiments and an embodiment obtained by combining these embodiments as appropriate. 
     REFERENCE SIGNS LIST 
     
         
           1  Centrifugal compressor 
           9  Compressor housing (casing) 
           13  Scroll flow passage 
           15  Outlet flow passage 
           17  Scroll start portion 
           17   a  Second inner circumferential surface 
           19  Scroll end portion 
           19   a  First inner circumferential surface 
           20  Flow passage connection section 
           25  Tongue portion 
           30  Connection region 
           31  Connection surface 
           71  Turning start point 
           73  Turning end point