Patent Publication Number: US-11378089-B2

Title: Centrifugal compressor and turbocharger

Description:
TECHNICAL FIELD 
     The present disclosure relates to a centrifugal compressor and a turbocharger. 
     BACKGROUND ART 
     A centrifugal compressor for a turbocharger may include a bypass valve (also called a ‘blow off valve’ or ‘recirculation valve’) at the outlet of the centrifugal compressor, in order to avoid an excessive increase of the discharge pressure of the compressor. In such a configuration, the bypass valve opens when the discharge pressure of the compressor becomes excessively high, so as to return the discharged air of the compressor to the inlet side of the compressor. 
     On the other hand, providing such a bypass flow passage may lead to an increase of pressure loss. As depicted in  FIG. 24 , while a circulation flow is formed in the bypass flow passage due to a shear force from the main flow, there is substantially no pressure loss if there is substantially no inflow to the bypass flow passage from the main flow. However, in a case where a high-rate flow enters the bypass flow passage from the main flow as depicted in  FIGS. 25 and 26 , the flow having flown in to the bypass flow passage forms a swirl, which may flow out again to the main flow. In this case, the outflowing swirl flow interferes with the main flow, and generates a significant pressure loss, as depicted in  FIG. 25 . Simultaneously, the compressor efficiency may also deteriorate considerably (sometimes 5% or more). 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP2012-241558A 
     SUMMARY 
     Problems to be Solved 
     To address such a pressure loss increase, Patent Document 1 proposes forming the surface of a valve body of a bypass valve into a shape that follows along the inner wall of the scroll flow passage of the compressor. With such a structure, it is possible to suppress a pressure loss increase caused by inflow of a flow to the bypass flow passage. 
     However, valves are usually general-purpose products, and thus it is necessary to prepare custom-made valves to realize a valve-body surface that has a special shape formed along the inner wall of a tube, which may increase costs. 
     At least one embodiment of the present invention was made in view of the above typical problem. An object of at least one embodiment of the present invention is to provide a centrifugal compressor and a turbocharger capable of suppressing a pressure loss increase while suppressing complication of the valve body shape of the bypass valve. 
     Solution to the Problems 
     (1) According to at least one embodiment of the present invention, a controller includes: an impeller; a compressor inlet tube configured to guide air to the impeller; a scroll flow passage disposed on a radially outer side of the impeller; a bypass flow passage branching from the scroll flow passage via a branch port, the bypass flow passage connecting to the compressor inlet tube not via the impeller; and a bypass valve capable of opening and closing a valve port disposed in the bypass flow passage. The branch port has a non-circular shape when viewed along a normal N1 of the branch port passing through a center of the branch port. 
     With the above configuration (1), by using the branch port having a non-circular shape when viewed along the normal of the branch port, it is possible to prevent formation of a swirl by a flow flowing into the bypass flow passage, compared to the typical configuration where a branch port having a circular shape is used. Accordingly, it is possible to suppress a pressure loss increase that accompanies outflow of a swirl flow from the inside of the bypass flow passage to the scroll flow passage. 
     Furthermore, it is possible to suppress a pressure loss increase without forming the surface of the valve body of the bypass valve along the inner wall of the tube as in the configuration described in Patent Document 1. Thus, it is possible to suppress a pressure loss increase while suppressing complication of the shape of the valve body of the bypass valve and suppressing a cost increase. 
     Furthermore, with the configuration described in Patent Document 1, when the valve body of the bypass valve is disposed along the inner wall of the scroll flow passage, it is necessary to provide a space for installing the valve body and a space for the valve body to move at a position proximate to the scroll flow passage inside the bypass flow passage, which is likely to limit the layout of the bypass flow passage that is required to be connected to the inlet of the compressor. 
     In contrast to this, with the configuration according to the above (1), it is possible to suppress a pressure loss increase without providing the valve body of the bypass valve along the inner wall of the scroll flow passage, and thus it is not necessary to provide a space for the valve body to move at a position proximate to the scroll flow passage inside the bypass flow passage, which makes it possible to improve the flexibility of the layout of the bypass flow passage to be connected to the inlet of the compressor. 
     (2) In some embodiments, in the controller according to the above (1), when G is a flow-passage cross section including the center of the branch port in the scroll flow passage, T is a dimension of the branch port in a flow direction F orthogonal to the flow-passage cross section G, and L is a dimension of the branch port in a flow direction H orthogonal to each of the flow direction F and the normal N1, T is smaller than L. 
     With the controller according to the above (2), with the dimension T being smaller than the dimension L, the distance required for the flow of the scroll flow passage to pass the branch port becomes shorter, and thus it is possible to reduce intrusion of the flow into the bypass flow passage. Furthermore, it is possible to effectively hinder formation of swirls by the flow entering the bypass flow passage. 
     (3) In some embodiments, in the controller according to the above (1) or (2), the branch port has a length larger than a diameter of the valve port, the branch port having a width smaller than the diameter of the valve port. 
     With the controller described in the above (3), it is possible to ensure an appropriate bypass flow rate when opening the bypass valve to bypass the flow, while effectively hindering formation of swirls by the flow entering the bypass flow passage. 
     (4) In some embodiments, in the controller according to any one of the above (1) to (3), when S1 is an opening area of the valve port and S2 is an opening area of the branch port, an expression 0.8S1≤S2≤1.2S1 is satisfied. 
     While it is preferable to reduce the opening area of the branch port in order to minimize pressure loss that accompanies provision of the bypass flow passage, making the opening area of the branch port too small may make it difficult to ensure a sufficient bypass flow rate when opening the bypass valve to bypass the flow. In contrast to this, as described in the above (4), when the opening area S2 of the branch port is close to the opening area S1 of the valve port so that an expression 0.8S1≤S2≤1.2S1 is satisfied, it is possible to suppress generation of swirls inside the bypass flow passage while ensuring the necessary bypass flow rate. 
     (5) In some embodiments, in the controller according to any one of the above (1) to (4), when Te is a width of the branch port at an end portion of the branch port in a radial direction of the impeller and Tc is a width of the branch port at a center portion of the branch port in the radial direction of the impeller, Te is smaller than Tc. 
     With the controller according to the above (5), the diffuser outlet flow having flown out to the scroll flow passage from the diffuser of the centrifugal compressor is likely to flow along the inner wall surface at the outer side, in the radial direction, of the impeller, of the inner wall surface of the scroll flow passage. At this time, the diffuser outlet flow is likely to flow into the branch port at the end portion at the outer side, in the radial direction, of the impeller, and it is desirable to reduce the width Te of the end portion of the branch port in order to suppress inflow of the diffuser outlet flow to the branch port. Meanwhile, it is necessary to connect the bypass flow passage to the circular shape of the valve port smoothly in the end, and thus the width Tc of the center portion of the branch port needs to be large to some extent. Thus, with the width Te of the end portion at the outer side being smaller than the width Tc of the center portion, it is possible to connect the bypass flow passage to the valve port smoothly while suppressing inflow of the diffuser outlet flow to the branch port. 
     (6) In some embodiments, in the controller according to any one of the above (1) to (5), the center of the branch port is shifted inward with respect to a center of the valve port in a radial direction of the impeller. 
     As described above, the diffuser outlet flow is likely to flow into the branch port at the end portion at the outer side, in the radial direction, of the impeller. Thus, with the center of the branch port shifted inward in the radial direction of the impeller from the center of the valve port as described in the above (6), the diffuser outlet flow flows along the inner wall surface of the scroll flow passage and is less likely to enter the bypass flow passage from the branch port, and thus it is possible to suppress a pressure loss increase. 
     (7) In some embodiments, in the controller according to any one of the above (1) to (6), a length direction of the branch port is orthogonal to a flow direction which is orthogonal to a flow-passage cross section of the scroll flow passage. 
     With the controller according to the above (7), the distance required for the flow of the scroll flow passage to pass the branch port becomes smaller, and thus it is possible to reduce intrusion of the flow into the bypass flow passage. Furthermore, it is possible to effectively prevent formation of swirls by the flow entering the bypass flow passage. 
     (8) In some embodiments, in the controller according to any one of the above (1) to (7), when P is a vector indicating a center position of the branch port with respect to a center position of a flow-passage cross section G including the center of the branch port in the scroll flow passage, Q is a vector indicating a flow direction orthogonal to the flow-passage cross section G, R is a cross product of the vector P and the vector Q (=P×Q), and V is a vector parallel to a length direction of the branch port, one of an inner product V·R of the vector V and the vector R or an inner product V·Q of the vector V and the vector Q is positive and the other is negative. 
     With the controller according to the above (8), compared to a case where both of the inner product V·E and the inner product V·Q are positive or both of the inner product V·E and the inner product V·Q are negative, it is possible to make the angle formed between the flow direction of the swirl flow of the scroll flow passage and the length direction of the branch port at the position of the branch port larger, and thus it is possible to suppress inflow of the swirl flow at the branch port and the scroll flow passage into the branch port effectively. 
     (9) According to at least one embodiment of the present invention, a turbocharger includes: a centrifugal compressor according to any one of the above (1) to (8); and a turbine sharing a rotational shaft with an impeller of the centrifugal compressor. 
     With the controller according to the above (9), by providing the centrifugal compressor according to any one of the above (1) to (8), it is possible to suppress a pressure loss increase while suppressing complication of the shape of the valve body of the bypass valve and suppressing a cost increase. 
     Advantageous Effects 
     According to at least one embodiment of the present invention, it is possible to provide a centrifugal compressor and a turbocharger capable of suppressing a pressure loss increase while suppressing complication of the valve body shape of the bypass valve. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a partial cross-sectional diagram showing the schematic configuration of a turbocharger  2  according to an embodiment. 
         FIG. 2  is a partial enlarged view of the centrifugal compressor  4  depicted in  FIG. 1 . 
         FIG. 3A  is a perspective view schematically showing the shape of a branch port  20  according to an embodiment. 
         FIG. 3B  is a diagram showing the shape of the branch port  20  and the shape of a valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  in  FIG. 3A . 
         FIG. 3C  is a diagram for describing the flow direction F of the scroll flow passage  14 . 
         FIG. 4A  is a perspective view schematically showing the shape of a branch port  20   c  according to a conventional example. 
         FIG. 4B  is a diagram showing the shape of the branch port  20   c  and the shape of a valve port  22  viewed along the normal N1 of the branch port  20   c  passing through the center O1 of the branch port  20   c  in  FIG. 4A . 
         FIG. 5  is a diagram for describing the shape of the branch port  20  depicted in  FIGS. 3A and 3B , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 6  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 7  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 8  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 9  is a diagram for describing the diffuser outlet flow D. 
         FIG. 10  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 11  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 12  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 13  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 14  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 15  is a diagram for describing the effect of shifting the center O1 of the branch port  20  with respect to the center O2 of the valve port  22  inward in the radial direction I of the impeller. 
         FIG. 16  is a diagram for describing the definitions of vectors used in description of some embodiments. 
         FIG. 17  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 18  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 19  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 20  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 21  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 22  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 23  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
         FIG. 24  is a diagram showing the circulation flow inside a bypass flow passage accompanying inflow of a flow from the scroll flow passage to the bypass flow passage. 
         FIG. 25  is a diagram for describing generation of pressure loss due to interference between the main flow and a swirl flow flowing out from the bypass flow passage. 
         FIG. 26  is a diagram for describing generation of pressure loss due to interference between the main flow and a swirl flow flowing out from the bypass flow passage. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described in detail 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 in 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 cylindrical 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, an expression such as “comprise”, “include”, “have”, “contain” and “constitute” are not intended to be exclusive of other components. 
       FIG. 1  is a partial cross-sectional diagram showing the schematic configuration of a turbocharger  2  according to an embodiment.  FIG. 2  is a partial enlarged view of the centrifugal compressor  4  shown in  FIG. 1 . 
     As depicted in  FIG. 1 , the turbocharger  2  includes a centrifugal compressor  4 , and a turbine  12  including a turbine rotor  10  which shares a rotational shaft  8  with an impeller  6  of the centrifugal compressor  4 . 
     The centrifugal compressor  4  includes the impeller  6 , a compressor inlet tube  40  that guides air to the impeller  6 , a scroll flow passage  14  disposed on a radially outer side of the impeller  6 , a bypass flow passage  16  branching from an outlet tube  38  of the scroll flow passage  14  via a branch port  20  and connecting to the compressor inlet tube  40  not via the impeller  6 , and a bypass valve  18  capable of opening and closing the valve port  22  disposed in the bypass flow passage  16 . The bypass valve  18  is controlled to open and close by an actuator  19 , and opens when the discharge pressure of the centrifugal compressor  4  becomes excessively high, so as to return a part of the compressed air flowing through the scroll flow passage  14  to the compressor inlet tube  40 . The valve port  22  refers to the opening on a valve seat surface  25  which is to be in direct contact with the valve body  24  of the bypass valve  18 . 
       FIG. 3A  is a perspective view schematically showing the shape of a branch port  20  according to an embodiment.  FIG. 3B  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  in  FIG. 3A .  FIG. 3C  is a diagram for describing the flow direction F of the scroll flow passage  14 .  FIG. 4A  is a perspective view schematically showing the shape of a branch port  20   c  according to a conventional example.  FIG. 4B  is a diagram showing the shape of the branch port  20   c  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20   c  passing through the center O1 of the branch port  20   c  in  FIG. 4A . While the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  coincides with the normal N2 of the branch port  20  passing through the center O2 of the valve port  22  in the depicted illustrative embodiments, the normal N1 and the normal N2 may not necessarily coincide in another embodiment. Furthermore, the center O1 of the branch port  20  refers to the center of a figure, that is, the center of gravity, of the branch port  20 . The center O2 of the valve port  22  refers to the center of a figure, that is, the center of gravity, of the valve port  22  (the opening on the valve seat surface  25  to be in direct contact with the valve body  24  of the bypass valve  18 ). 
     In some embodiments, as depicted in  FIG. 3B  for instance, the branch port  20  has a non-circular shape which is different from a circular shape, when viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20 . 
     As described above, by using the branch port  20  having a non-circular shape when viewed along the normal N1 of the branch port  20 , it is possible to prevent formation of swirls by a flow flowing into the bypass flow passage  16 , compared to the typical configuration (see  FIGS. 4A and 4B ) using the branch port  20   c  having a circular shape. Accordingly, it is possible to address the problem described above with reference to  FIG. 23 . In other words, it is possible to suppress a pressure loss increase that accompanies outflow of a swirl flow from the inside of the bypass flow passage  16  to the scroll flow passage  14 . 
     Furthermore, with the configuration described in Patent Document 1, when the valve body of the bypass valve is disposed along the inner wall of the scroll flow passage, it is necessary to provide a space for installing the valve body and a space for the valve body to move at a position proximate to the scroll flow passage inside the bypass flow passage, which is likely to limit the layout of the bypass valve to be connected to the inlet of the compressor. 
     In contrast to this, with the configuration according to the above embodiment, it is possible to suppress a pressure loss increase without providing the valve body  24  of the bypass valve  18  along the inner wall of the scroll flow passage  14 , and thus it is not necessary to provide a space for installing the valve body  24  and a space for the valve body  24  to move at a position proximate to the scroll flow passage  14  inside the bypass flow passage  16 , which makes it possible to improve the flexibility of the layout of the bypass flow passage  16  to be connected to the inlet of the compressor  4 . 
       FIG. 5  is a diagram for describing the shape of the branch port  20  depicted in  FIGS. 3A and 3B , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment.  FIG. 5  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment.  FIG. 6  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment.  FIG. 7  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment.  FIG. 8  is a diagram showing another shape example of the branch port  20 , showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
     In some embodiments, as depicted in  FIGS. 5 to 8  for instance, the dimension T of the branch port  20  in the flow direction F of the scroll flow passage  14  is of a lateral shape that is smaller than the dimension L of the branch port  20  in the direction H orthogonal to each of the flow direction F and the normal N1. Herein, the flow direction F of the scroll flow passage  14  refers to the flow direction F orthogonal to the flow-passage cross section G including the center O1 of the branch port  20  in the scroll flow passage  14 , as depicted in  FIG. 3C . The shape of the branch port  20  may be, as depicted in  FIGS. 5 to 7  for instance, an oval shape when viewed in the direction of the normal N1, or a rectangular shape as depicted in  FIG. 8 . The shape of the branch port  20  depicted in  FIGS. 5 and 6  is a slit shape when viewed in the direction of the normal N1. The shape of the branch port  20  depicted in  FIG. 5  has a rounded rectangular shape (formed of two semi-circular shapes and two parallel lines of equal lengths) when viewed in the direction of the normal N1. The shape of the branch port  20  depicted in  FIG. 6  is an ellipse shape when viewed in the direction of the normal N1. The shape of the branch port  20  depicted in  FIG. 7  is a rounded rhombic shape when viewed in the direction of the normal N1. 
     With the dimension T being smaller than the dimension L, the distance required for the flow of the scroll flow passage  14  to pass the branch port  20  becomes smaller, and thus it is possible to reduce intrusion of the flow into the bypass flow passage  16 . Furthermore, it is possible to effectively prevent formation of swirls by the flow entering the bypass flow passage  16 . 
     In some embodiments, as depicted in  FIGS. 5 to 8  for instance, the length of the branch port  20  (the dimension L in the direction H in the depicted embodiment) is larger than the diameter R of the valve port  22 , and the width of the branch port  20  (the dimension T in the direction F in the depicted embodiment) is smaller than the diameter R. 
     Accordingly, it is possible to ensure an appropriate bypass flow rate when opening the bypass valve  18  to bypass the flow, while effectively preventing formation of swirls by the flow entering the bypass flow passage  16 . 
     In some embodiments, as depicted in  FIG. 3A  for instance, when S1 is the opening area of the valve port  22  and S2 is the opening area of the branch port  20 , an expression 0.8S1≤S2≤1.2S1 is satisfied. 
     While it is preferable to reduce the opening area of the branch port  20  in order to minimize pressure loss that accompanies provision of the bypass flow passage  16 , making the opening area of the branch port  20  too small may make it difficult to ensure a sufficient bypass flow rate when opening the bypass valve  18  to bypass the flow. In contrast to this, when the opening area S2 of the branch port  20  is close to the opening area S1 of the valve port  22  so that an expression 0.8S1≤S2≤1.2S1 is satisfied, it is possible to suppress generation of swirls inside the bypass flow passage  16  while ensuring the necessary bypass flow rate. 
     In some embodiments, as depicted in  FIGS. 5 to 7  for instance, the width Te of the end portion  26  of the branch port  20  at the outer side, in the radial direction I of the impeller  6 , is smaller than the width Tc of the center portion  28  of the branch port  20 . 
     As depicted in  FIG. 9 , the diffuser outlet flow D having flown out to the scroll flow passage  14  from the diffuser  30  of the centrifugal compressor  4  is likely to flow along the inner wall surface  32  at the outer side, in the radial direction I of the impeller  6 , of the inner wall surface of the scroll flow passage  14 . At this time, the diffuser outlet flow D is likely to flow into the end portion  26  of the branch port  20  at the outer side, in the radial direction I of the impeller  6 , and it is desirable to reduce the width Te of the end portion  26  in order to suppress inflow of the diffuser outlet flow D to the branch port  20 . Meanwhile, it is necessary to connect the bypass flow passage  16  to the circular shape of the valve port  22  smoothly in the end, and thus the width Tc of the center portion  28  of the branch port  20  needs to be large to some extent. Thus, with the width Te of the end portion at the radially outer side being smaller than the width Tc of the center portion  28 , it is possible to connect the bypass flow passage  16  to the valve port  22  smoothly while suppressing inflow of the diffuser outlet flow D to the branch port  20 . 
     In some embodiments, as depicted in  FIG. 8  for instance, the width T of the branch port  20  is constant from one end side to the other end side in the length direction of the branch port  20 . That is, in the embodiment depicted in  FIG. 8 , the shape of the branch port  20  has a rectangular shape when viewed in the direction of the normal N1. 
     With the above configuration, it is possible to suppress a pressure loss increase that accompanies provision of the bypass flow passage  16  thanks to the branch port  20  having a simple configuration. 
     In some embodiments, as depicted in  FIGS. 5 to 8 , the length direction of the branch port  20  is orthogonal to the flow direction F of the scroll flow passage  14  at the center position O1 of the branch port  20 . 
     With the above configuration, the distance required for the flow of the scroll flow passage  14  to pass the branch port  20  becomes smaller, and thus it is possible to reduce intrusion of the flow into the bypass flow passage  16 . Furthermore, it is possible to effectively prevent formation of swirls by the flow entering the bypass flow passage  16 . 
     In the embodiments depicted in  FIGS. 5 to 8 , the center O1 of the branch port  20  coincides with the center O2 of the valve port  22  when viewed in the direction of the normal N1. Nevertheless, the center O1 of the branch port  20  and the center O2 of the valve port  22  may not necessarily coincide. 
     In some embodiments, as depicted in  FIGS. 10 to 14  for instance, the center O1 of the branch port  20  is disposed at the inner side, in the radial direction I, of the impeller, with respect to the center O2 of the valve port  22 . With such a configuration, the center O1 of the branch port  20  is shifted downstream in the circumferential direction (diffuser outlet flow D) in the flow-passage cross section of the scroll flow passage  14 , from the center O2 of the valve port  22 . Furthermore, with the above configuration, as depicted in  FIGS. 10 to 14 , the distance L 1  between the outer end  34  of the branch port  20  and the center O2 of the valve port  22  in the radial direction of the impeller  6  is smaller than the distance L 2  between the inner end  36  of the branch port  20  and the center O2 of the valve port  22  in the radial direction of the impeller  6 . 
     The shape of the branch port  20  in  FIG. 10  is a rounded rectangular shape similar to the branch port  20  depicted in  FIG. 5 . The shape of the branch port  20  in  FIG. 11  is an ellipse shape similar to the branch port  20  depicted in  FIG. 6 . The shape of the branch port  20  in  FIG. 12  is a rounded rhombic shape similar to the branch port  20  depicted in  FIG. 7 . The shape of the branch port  20  in  FIG. 13  is a rectangular shape similar to the branch port  20  depicted in  FIG. 8 . The shape of the branch port  20  depicted in  FIG. 14  is a rounded asymmetric rhombic shape, whose inner two sides, in the radial direction I of the impeller, are longer than the outer two sides. 
     As described with reference to  FIG. 9 , the diffuser outlet flow D is likely to flow into the end portion  26  of the branch port  20  at the outer side, in the radial direction I, of the impeller  6 . Thus, with the center O1 of the branch port  20  shifted inward in the radial direction I of the impeller  6  from the center O2 of the valve port  22 , the diffuser outlet flow D flows along the inner wall surface  32  of the scroll flow passage  14  and is less likely to enter the bypass flow passage  16  from the branch port  20 , and thus it is possible to suppress a pressure loss increase. 
     Next, some other embodiments will be described. The actual flow flowing through the scroll flow passage  14  is a swirl flow that follows a helical trajectory including a component orthogonal to the flow-passage cross section of the scroll flow passage  14  and a swirl component in the flow-passage cross section of the scroll flow passage  14 . In the embodiment described below, the branch port  20  has an oblique angle to effectively suppress inflow of the swirl flow of the scroll flow passage  14  to the bypass flow passage  16  through the branch port  20 . 
       FIG. 16  is a diagram for describing the definitions of vectors used in description of the following respective embodiments. First, as depicted in  FIG. 16 , in the flow-passage cross section G including the center O1 of the branch port  20  in the scroll flow passage  14 , P is the vector indicating the position of the center O1 of the branch port  20  with respect to the position of the center O3 of the flow-passage cross section G, Q is the vector indicating the flow direction orthogonal to the flow-passage cross section G (flow direction F of the scroll flow passage  14 ), and E is the cross product of the vector P and the vector Q (=P×Q). When J is the vector indicating the swirl flow of the scroll flow passage  14  at the position of the center O1 of the branch port  20 , J can be expressed by an expression J=aQ+bE. Next, some embodiments will be described on the basis of the definitions of the above vectors. 
       FIG. 17  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment.  FIG. 18  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment.  FIG. 19  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment.  FIG. 20  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment.  FIG. 21  is a diagram showing the shape of the branch port  20  and the shape of the valve port  22  viewed along the normal N1 of the branch port  20  passing through the center O1 of the branch port  20  according to an embodiment. 
     In some embodiments, as depicted in  FIGS. 17 to 21 , when the origin point is the center O2 of the valve port  22 , x-axis direction is the direction indicated by the vector Q, and y-axis is the direction indicated by the vector E, the branch port  20  extends from the fourth quadrant A4 toward the second quadrant A2. That is, when V is a vector parallel to the length direction of the branch port  20 , one of the inner product V·E of the vector V and the vector E or the inner product V·Q of the vector V and the vector Q is positive and the other is negative. In the embodiment depicted in  FIGS. 17 to 21 , the angle θ1 formed between the length direction of the branch port  20  and the vector E is 0°&lt;θ1&lt;90°, and preferably 30°&lt;θ1&lt;60°. For example, θ1=45°. 
     With the above configuration, compared to a case where the branch port  20  extends from the third quadrant A3 to the first quadrant A1 (when both of the inner product V·E and the inner product V·Q are positive, or when both of the inner product V·E and the inner product V·Q are negative), it is possible to bring the angle θ2 closer to a right angle, where the angle θ2 is an angle formed between the flow direction of the swirl flow of the scroll flow passage at the position of the branch port  20  (the direction indicated by the vector J) and the length direction of the branch port  20 . Thus, it is possible to suppress inflow of the swirl flow of the branch port  20  and the scroll flow passage  14  into the branch port  20  effectively. 
     As described in the above, also in an embodiment where the branch port  20  has an oblique angle, the shape of the branch port  20  may be, as depicted in  FIGS. 17 to 20  for instance, an oval shape when viewed in the direction of the normal N1, or a rectangular shape when viewed in the direction of the normal N1 as depicted in  FIG. 21 . The shape of the branch port  20  depicted in  FIGS. 17 and 18  is a slit shape when viewed in the direction of the normal N1. The shape of the branch port  20  depicted in  FIG. 17  is a rounded rectangular shape when viewed in the direction of the normal N1. The shape of the branch port  20  depicted in  FIG. 18  is an ellipse shape when viewed in the direction of the normal N1. The shape of the branch port  20  depicted in  FIG. 19  is a rounded rhombic shape when viewed in the direction of the normal N1. The shape of the branch port  20  depicted in  FIG. 20  is a rounded asymmetric rhombic shape when viewed in the direction of the normal N1. 
     In the embodiments depicted in  FIGS. 17 to 21 , the center O1 of the branch port  20  is shifted inward in the radial direction I of the impeller from the center O2 of the valve port  22 . Also in a case where the branch port  20  has an oblique angle, the center O1 of the branch port  20  and the center O2 of the valve port  22  may coincide when viewed in the direction of the normal N1. 
     Embodiments of the present invention were described in detail above, but the present invention is not limited thereto, and various amendments and modifications may be implemented. 
     For instance, the shape of the branch port  20  is not limited to the above described shape, and may be a bend shape obtained by bending a straight line shape as depicted in  FIG. 22 , or a curved shape obtained by curving a straight line shape as depicted in  FIG. 23 , when viewed along the normal N1 of the branch port  20 . 
     REFERENCE SIGNS LIST 
     
         
           2  Turbocharger 
           4  Centrifugal compressor 
           6  Impeller 
           8  Rotational shaft 
           10  Turbine rotor 
           12  Turbine 
           14  Scroll flow passage 
           16  Bypass flow passage 
           18  Bypass valve 
           19  Actuator 
           20  Branch port 
           22  Valve port 
           24  Valve body 
           25  Valve seat surface 
           26  End portion 
           28  Center portion 
           30  Diffuser 
           32  Inner wall surface 
           34  Outer end 
           36  Inner end