Patent Publication Number: US-11384766-B2

Title: Diffuser vane geometry for a centrifugal compressor and turbocharger

Description:
TECHNICAL FIELD 
     The present disclosure relates to a centrifugal compressor and a turbocharger. 
     BACKGROUND 
     As a centrifugal compressor applied to a turbocharger or the like, a centrifugal compressor may be used, which is provided with a diffuser vane for decreasing the velocity and increasing the pressure of a fluid downstream of an impeller for applying a centrifugal force to the fluid. 
     For example, Patent Document 1 discloses a centrifugal gas compressor which includes a plurality of diffuser vanes each configured to convert the flow velocity of a fluid from an impeller into a pressure and a scroll for guiding the flow of the fluid from the diffuser vanes to the outside. In the centrifugal gas compressor, in order to improve efficiency of a diffuser, the plurality of diffuser vanes are arranged as an asymmetrical pattern in the circumferential direction of the fluid in the scroll in consideration of a pressure distribution in the circumferential direction. That is, the shapes, the orientations, or the positions of the plurality of diffuser vanes arranged in the circumferential direction are not uniform. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP2013-519036A (translation of a PCT application) 
     SUMMARY 
     Technical Problem 
     Meanwhile, in a centrifugal compressor with a diffuser vane, the shape of a flow passage changes from a spiral shape to a linear shape in the vicinity of the outlet of a scroll flow passage, and thus a circumferential component of a flow velocity decreases in an angular range in the vicinity of the outlet of the scroll flow passage in the circumferential direction as compared with another angular range. Accordingly, the flow stalls on a pressure surface of the diffuser vane (negative stall), which may cause separation. 
     In this regard, in the centrifugal gas compressor described in Patent Document 1, although the plurality of diffuser vanes are arranged as the asymmetrical pattern in consideration of the pressure distribution in the circumferential direction, Patent Document 1 does not disclose a specific configuration for suppressing separation of the flow in the diffuser vanes in the vicinity of the outlet of the scroll flow passage. 
     In view of the above, an object of at least one embodiment of the present disclosure is to provide a centrifugal compressor and a turbocharger including the same. The centrifugal compressor can suppress separation of a flow in diffuser vanes in an angular range in the vicinity of the outlet of a scroll flow passage. 
     Solution to Problem 
     (1) A centrifugal compressor according to at least one embodiment of the present invention includes an impeller, a plurality of diffuser vanes arranged in a circumferential direction on a radially outer side of the impeller, and a housing which includes a scroll portion forming a scroll flow passage positioned on a radially outer side of the plurality of diffuser vanes. The plurality of diffuser vanes include at least one first diffuser vane positioned at least partially in an angular range between a tongue section of the scroll portion and a scroll end of the scroll portion in the circumferential direction, and a second diffuser vane positioned outside the angular range. A vane outlet angle formed by a tangent line at a trailing edge to a pressure surface of each of the plurality of diffuser vanes satisfies β 1 &lt;β 2 , where β 1  is the vane outlet angle of the first diffuser vane, and β 2  is the vane outlet angle of the second diffuser vane. 
     As described above, in the angular range between the tongue section of the scroll portion and the scroll end of the scroll portion in the circumferential direction (that is, an angular range in the vicinity of the outlet of the scroll flow passage), a flow stalls on the pressure surface of each of the diffuser vanes (negative stall), which may cause separation. This is considered because in the angular range in the vicinity of the outlet of the scroll flow passage, the flow direction of a fluid is turned, and a circumferential component of a flow velocity is decreased as compared with another angular range, and thus an effect of pressing a flow in the vicinity of each of the diffuser vanes against the pressure surface thereof is small. 
     In this regard, with the above configuration (1), since the vane outlet angle β 1  of the first diffuser vane positioned in the angular range in the vicinity of the outlet of the scroll flow passage where the circumferential component of the flow velocity decreases is smaller than the vane outlet angle β 2  of the second diffuser vane positioned outside the angular range, the pressure surface in the vicinity of the trailing edge of the first diffuser vane is positioned upstream in the rotational direction of the impeller as compared with the second diffuser vane, making it possible to suppress separation on the side of the pressure surface of the first diffuser vane. 
     (2) In some embodiments, in the above configuration (1), on a linear vane-arrangement mapping of the plurality of diffuser vanes, a camber angle α 1  of the first diffuser vane and a camber angle α 2  of the second diffuser vane satisfy α 1 &gt;α 2 . 
     A camber angle of each of the diffuser vanes is an angle between a tangent line at the leading edge and a tangent line at the trailing edge of a camber line of each of the diffuser vanes. 
     With the above configuration (2), since the camber angle α 1  of the first diffuser vane is larger than the camber angle α 2  of the second diffuser vane, the pressure surface of the first diffuser vane deviates upstream in an impeller rotational direction with reference to the leading edge, as compared with the second diffuser vane. Thus, it is possible to achieve the above configuration (1). 
     (3) In some embodiments, in the above configuration (1) or (2), a vane thickness t 1  at the trailing edge of the first diffuser vane and a vane thickness t 2  at the trailing edge of the second diffuser vane satisfy t 1 &gt;t 2 . 
     With the above configuration (3), since the vane thickness t 1  at the trailing edge of the first diffuser vane is larger than the vane thickness t 2  at the trailing edge of the second diffuser vane, it is possible to deviate the pressure surface of the first diffuser vane upstream in the impeller rotational direction without greatly changing the position of the suction surface of the first diffuser vane, as compared with the second diffuser vane. Thus, it is possible to achieve the above configuration (1). 
     (4) In some embodiments, in any one of the above configurations (1) to (3), a stagger angle formed by a chordwise direction of each of the plurality of diffuser vanes with respect to the radial direction satisfies γ 1 &lt;γ 2 , where γ 1  is the stagger angle of the first diffuser vane, and γ 2  is the stagger angle of the second diffuser vane. 
     The above-described stagger angle may be a stagger angle at the leading edge or the trailing edge of each of the diffuser vanes. 
     With the above configuration (4), since the stagger angle γ 1  of the first diffuser vane is smaller than the stagger angle γ 2  of the second diffuser vane, the pressure surface of the first diffuser vane deviates upstream in an impeller rotational direction with reference to the leading edge, as compared with the second diffuser vane. Thus, it is possible to achieve the above configuration (1). 
     (5) In some embodiments, in the above configuration (4), in a cross section orthogonal to an axial direction, the first diffuser vane has the same cross-sectional shape as the second diffuser vane. 
     Satisfying the magnitude relationship between the stagger angles γ 1  and γ 2  described in the above configuration (4), it is possible to achieve the above configuration (1) even if the first diffuser vane having a common cross-sectional shape with the second diffuser vane is adopted. 
     (6) A turbocharger according to at least one embodiment of the present invention includes the centrifugal compressor according to any one of the above configurations (1) to (5). 
     With the above configuration (6), since the vane outlet angle β 1  of the first diffuser vane positioned in the angular range in the vicinity of the outlet of the scroll flow passage where the circumferential component of the flow velocity decreases is smaller than the vane outlet angle β 2  of the second diffuser vane positioned outside the angular range, the pressure surface in the vicinity of the trailing edge of the first diffuser vane is positioned upstream in the rotational direction of the impeller as compared with the second diffuser vane, making it possible to suppress separation on the side of the pressure surface of the first diffuser vane. 
     Advantageous Effects 
     According to at least one embodiment of the present invention, a centrifugal compressor and a turbocharger including the same are provided. The centrifugal compressor can suppress separation of a flow in diffuser vanes in an angular range in the vicinity of the outlet of a scroll flow passage. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a centrifugal compressor along the axial direction according to an embodiment. 
         FIG. 2A  is a view showing the interior of the centrifugal compressor shown in  FIG. 1 . 
         FIG. 2B  is a partial enlarged view of  FIG. 2A . 
         FIG. 3  is a view showing the configuration of diffuser vanes in the centrifugal compressor according to an embodiment. 
         FIG. 4  is a view showing the configuration of the diffuser vanes in the centrifugal compressor according to an embodiment. 
         FIG. 5  is a view showing the configuration of the diffuser vanes in the centrifugal compressor according to an embodiment. 
         FIG. 6  is a schematic view showing the configuration of a typical centrifugal compressor  100 . 
     
    
    
     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. 
     A centrifugal compressor according to embodiments to be described below is applicable to, for example, a turbocharger. However, the application of the centrifugal compressor is not limited to the turbocharger. 
       FIG. 1  is a schematic cross-sectional view of the centrifugal compressor along the axial direction according to an embodiment.  FIGS. 2A and 2B  are views for explaining the arrangement of components of the centrifugal compressor shown in  FIG. 1 .  FIG. 2A  is a view showing the interior of the centrifugal compressor shown in  FIG. 1  as viewed from the axial direction.  FIG. 2B  is a partial enlarged view of  FIG. 2A . In order to clarify the positional relationship among the respective components, each of the components is indicated by a solid line in  FIG. 2A . 
     As shown in  FIGS. 1 and 2A , a centrifugal compressor  1  according to an embodiment includes an impeller  4  and a housing  6 . The impeller  4  includes a plurality of rotor blades  5  and can rotate about a rotational axis O with a rotational shaft  2 . The housing  6  houses the impeller  4  and a plurality of diffuser vanes  10  to be described later. 
     On the outer side of the impeller  4  in the radial direction of the centrifugal compressor  1  (to be simply referred to as the “radial direction” hereinafter), a scroll flow passage  7  formed by a scroll portion  8  of the housing  6  is provided. As shown in  FIG. 2A , the scroll flow passage  7  has a flow-passage cross-sectional area from a scroll start  8   a  to a scroll end  8   b  of the scroll portion  8 , which gradually increases from upstream toward downstream in a rotational direction of the impeller  4  (that is, from upstream toward downstream in a flow direction of a fluid). 
     The scroll flow passage  7  communicates with an outlet flow passage  17  formed by an outlet portion  16  of the housing  6 . In the housing  6 , the scroll portion  8  and the outlet portion  16  are connected to each other, and a tongue section  22  is formed by a part of the scroll start  8   a  of the scroll portion  8  and the outlet portion  16  connected to the part of the scroll start  8   a.    
     On the radially outer side of the impeller  4  and the radially inner side of the scroll flow passage  7 , a diffuser passage  9  is formed by a hub-side wall surface  18  and a shroud-side wall surface  20  of the housing  6 . In the diffuser passage  9 , the plurality of diffuser vanes  10  are arranged in the circumferential direction of the centrifugal compressor  1  (to be simply referred to as the “circumferential direction” hereinafter). That is, the scroll flow passage  7  is positioned on the radially outer side of the diffuser passage  9  and the plurality of diffuser vanes  10 . 
     Each of the plurality of diffuser vanes  10  has a leading edge  24 , a trailing edge  26  positioned on the radially outer side of the leading edge  24 , and a pressure surface  28  and a suction surface  30  extending between the leading edge  24  and the trailing edge  26 . 
     The diffuser vanes  10  are installed in the above-described diffuser passage  9  while being fixed to the surface of a disc-shaped mounting plate  14 . The diffuser vanes  10  may be joined to the mounting plate  14  by welding. Alternatively, the diffuser vanes  10  and the mounting plate  14  may integrally be formed by, for example, cutting work or the like. 
     In an illustrated example, the mounting plate  14  is installed on the shroud-side wall surface  20  forming the diffuser passage  9 . However, in other embodiments, the mounting plate  14  may be installed on the hub-side wall surface  18 . 
     In the centrifugal compressor  1 , a fluid (such as a gas) flowing into the impeller  4  in the axial direction of the centrifugal compressor  1  (to be simply referred to as the “axial direction” hereinafter) is accelerated and pushed out in the circumferential direction and the radial direction due to the rotation of the impeller  4 . The fluid accelerated by the impeller  4  passes through the diffuser vanes  10  disposed in the diffuser passage  9 . At this time, kinetic energy of a fluid flow is converted into pressure energy (that is, the fluid is decreased in velocity and increased in pressure). Then, the flow passing through the diffuser vanes  10  and including a velocity component in the radial direction flows into the scroll flow passage  7  and is guided to the outlet flow passage  17  downstream thereof. The centrifugal compressor  1  thus generates a high-pressure fluid. 
     In the centrifugal compressor  1  according to some embodiments, the plurality of diffuser vanes  10  include first diffuser vanes  11  and second diffuser vanes  12  having different vane outlet angles β. 
       FIG. 2B  is a view showing the diffuser vanes  10  in the vicinity of the outlet of the scroll flow passage  7  of the centrifugal compressor  1  shown in  FIG. 2A . Each of the vane outlet angles β of a corresponding one of the diffuser vanes  10  is an angle formed by a tangent line LT at the trailing edge  26  to the pressure surface  28  of the diffuser vane  10  with respect to the radial direction (0°≤β≤90°) (that is, an angle formed by the aforementioned tangent line LT with respect to a radial straight line LR TE  passing through the trailing edge  26 ). 
     More specifically, as shown in  FIGS. 2A and 2B , the plurality of diffuser vanes  10  include at least one first diffuser vane  11  positioned at least partially in an angular range A 1  (see  FIG. 2A ) between the tongue section  22  of the scroll portion  8  and the scroll end  8   b  of the scroll portion  8 , and the second diffuser vanes  12  positioned in an angular range other than the angular range A 1 . 
     Then, a vane outlet angle β 1  of the first diffuser vane  11  (see  FIG. 2B ) and a vane outlet angle β 2  of the second diffuser vane  12  (see  FIG. 2B ) satisfy the relation of β 1 &lt;β 2 . 
       FIG. 6  is a schematic view showing the configuration of a typical centrifugal compressor  100 , and is a view showing a linear vane-arrangement mapping of the diffuser vanes  10 , of the plurality of diffuser vanes  10 , positioned in the above-described angular range A 1  (that is, the angular range between the tongue section  22  and the scroll end  8   b  of the scroll portion  8 ) and in the vicinity thereof, and the scroll flow passage  7  and the outlet flow passage  17  corresponding to the linear vane-arrangement mapping. 
     In the typical centrifugal compressor  100  shown in  FIG. 6 , the plurality of diffuser vanes  10  each have the same shape and are uniformly arranged at intervals in the circumferential direction. That is, the plurality of diffuser vanes  10  respectively have the above-described vane outlet angles β and angles (stagger angles) γ, which are identical to each other. Each of the angles (stagger angles) γ is formed by a chordwise direction with respect to the radial direction. 
     In the angular range A 1  in the vicinity of the outlet of the scroll flow passage  7  in the circumferential direction, the flow stalls on the pressure surface  28  of the diffuser vane  10  (in a region  32  of  FIG. 6 ) (negative stall), which is likely to cause separation as compared with another angular range. 
     This is considered for the following reason. That is, as shown in  FIG. 6 , the fluid accelerated by the impeller (not shown in  FIG. 6 ) flows into the diffuser passage  9  at an incident angle I, passes through the diffuser vanes  10 , and flows into the scroll flow passage  7 . A flow velocity vector V 1  in the scroll flow passage  7  is basically a circumferential vector. In the angular range A 1  in the vicinity of the outlet of the scroll flow passage  7 , a fluid flow is guided from the scroll flow passage  7  to the outlet flow passage  17 , turning the direction of the fluid flow and decreasing a circumferential component Vc of the flow velocity as compared with the other angular range. Therefore, in the angular range A 1  in the vicinity of the outlet of the scroll flow passage, an effect of pressing the flow in the vicinity of the diffuser vanes  10  against the pressure surface  28  by the circumferential flow in the scroll flow passage  7  is small as compared with the other angular range, which is likely to cause separation of the flow on the pressure surface  28 . 
     In this regard, in the above-described embodiment, since the vane outlet angle β 1  of the first diffuser vane  11  positioned in the angular range A 1  in the vicinity of the outlet of the scroll flow passage  7  is smaller than the vane outlet angle β 2  of the second diffuser vane  12  positioned outside the angular range A 1 , the pressure surface  28  in the vicinity of the trailing edge  26  of the first diffuser vane  11  is positioned upstream in the rotational direction of the impeller  4  as compared with the second diffuser vane  12  (see a second diffuser vane  12 ′ indicated by a dashed line in  FIG. 2B ). Therefore, it is possible to suppress separation on the side of the pressure surface  28  of the first diffuser vane  11 . 
     The second diffuser vane  12 ′ shown in  FIG. 2B  is a virtual diffuser vane illustrated for comparison of the shape and the like with the first diffuser vane  11 , and is shown by rotary-moving about the rotational axis O of the centrifugal compressor  1  so that the position of the leading edge  24  of the second diffuser vane  12  positioned outside the angular range A 1  overlaps the first diffuser vane  11 . 
     If the plurality of diffuser vanes  10  positioned at least partially in the above-described angular range A 1  exist, only some of the diffuser vanes  10  may be the first diffuser vanes  11  (that is, the diffuser vanes each having the vane outlet angle β 1  satisfying the above-described relation of β 1 &lt;β 2 ). 
     Some embodiments of the centrifugal compressor in which the vane outlet angle β 1  of the first diffuser vane  11  and the vane outlet angle β 2  of the second diffuser vane  12  satisfy the relation of β 1 &lt;β 2  will be described below in more detail. 
     Each of  FIGS. 3 to 5  is a view showing the configuration of the diffuser vanes  10  in the centrifugal compressor according to an embodiment.  FIG. 3  is a view showing the linear vane-arrangement mapping of the diffuser vanes  10 , of the plurality of diffuser vanes  10  (including the first diffuser vanes  11  and the second diffuser vanes  12 ) of the centrifugal compressor  100 , positioned in the above-described angular range A 1  (that is, the angular range between the tongue section  22  and the scroll end  8   b  of the scroll portion  8 ) and in the vicinity thereof according to an embodiment. Each of  FIGS. 4 and 5  is a view of the diffuser vanes  10  positioned in the above-described angular range A 1  and in the vicinity thereof in the centrifugal compressor as viewed from the axial direction according to an embodiment. 
     In  FIGS. 3 and 5 , the components other than the diffuser vanes  10  and the mounting plate  14  are not shown. Moreover, each second diffuser vane  12 ′ shown in  FIGS. 3 to 5  is the virtual diffuser vane illustrated for comparison of the shape and the like with the first diffuser vane  11 , and is shown by rotary-moving about the rotational axis O so that the position of the leading edge  24  of the second diffuser vane  12  positioned outside the angular range A 1  overlaps the first diffuser vane  11 . 
     In an embodiment, for example, as shown in  FIG. 3 , on the linear vane-arrangement mapping of the plurality of diffuser vanes  10 , a camber angle α 1  of the first diffuser vane  11  positioned at least partially in the angular range A 1  and a camber angle α 2  of the second diffuser vane  12  positioned outside the angular range A 1  satisfy α 1 &gt;α 2 . 
     A camber angle α of each of the diffuser vanes  10  is an angle formed between a tangent line LG at the leading edge  24  and a tangent line LH at the trailing edge  26  of a camber line LF of each of the diffuser vanes  10 . Provided that P 1  is an intersection point between the tangent line LG at the leading edge  24  and the tangent line LH at the trailing edge  26  described above, the camber angle α is an angle between a vector in a direction from the leading edge  24  toward the intersection point P 1  and a vector in a direction from the intersection point P 1  toward the trailing edge  26  (0°≤α≤180°) (see  FIG. 3 ). 
     Thus, since the camber angle α 1  of the first diffuser vane  11  is larger than the camber angle α 2  of the second diffuser vane  12 , the pressure surface  28  of the first diffuser vane  11  deviates upstream in the impeller rotational direction with reference to the leading edge  24 , as compared with the second diffuser vane  12  (see the second diffuser vanes  12 ′ each indicated by the dashed line in  FIG. 3 ). Thus, it is possible to achieve the configuration in which the vane outlet angle β 1  of the first diffuser vane  11  and the vane outlet angle β 2  of the second diffuser vane  12  satisfy the relation of β 1 &lt;β 2 . 
       FIG. 3  shows a vane outlet angle β 1 ′ of the first diffuser vane  11  and a vane outlet angle β 2 ′ of the second diffuser vane  12  in the linear vane-arrangement mapping. The magnitude relationship between the vane outlet angle β 1 ′ and the vane outlet angle β 2 ′ in the linear vane-arrangement mapping is the same as the magnitude relationship between the vane outlet angle β 1  and the vane outlet angle β 2 . That is, in the linear vane-arrangement mapping of the diffuser vanes, the relation of β 1 &lt;β 2  is also satisfied as long as β 1 ′&lt;β 2 ′ holds. 
     In an embodiment, for example, as shown in  FIG. 4 , a vane thickness t 1  at the trailing edge  26  of the first diffuser vane  11  and a vane thickness t 2  at the trailing edge  26  of the second diffuser vane  12  satisfy t 1 &gt;t 2 . 
     In the exemplary embodiment shown in  FIG. 4 , while the suction surface  30  of the first diffuser vane  11  has the same shape as the suction surface  30  of the second diffuser vane  12 , the pressure surface  28  of the first diffuser vane  11  deviates upstream in the impeller rotational direction as compared with the second diffuser vane  12 . That is, a distance (vane thickness t) between the pressure surface  28  and the suction surface  30  of the first diffuser vane  11  has a special vane thickness distribution increasing from the side of the leading edge  24  toward the side of the trailing edge  26 . 
     Since the vane thickness t 1  at the trailing edge  26  of the first diffuser vane  11  is thus larger than the vane thickness t 2  at the trailing edge  26  of the second diffuser vane  12 , it is possible to deviate the pressure surface  28  of the first diffuser vane  11  upstream in the impeller rotational direction without greatly changing the position of the suction surface  30  of the first diffuser vane  11 , as compared with the second diffuser vane  12  (see the second diffuser vane  12 ′ indicated by the dashed line in  FIG. 4 ). Thus, it is possible to achieve the configuration in which the vane outlet angle β 1  of the first diffuser vane  11  and the vane outlet angle β 2  of the second diffuser vane  12  satisfy the relation of β 1 &lt;β 2 . 
     In an embodiment, for example, as shown in  FIG. 5 , the stagger angle γ formed by the chordwise direction of each of the plurality of diffuser vanes  10  with respect to the radial direction satisfies γ 1 &gt;γ 2 , where γ 1  is a stagger angle of the first diffuser vane  11 , and γ 2  is a stagger angle of the second diffuser vane  12 . 
     The stagger angle γ is an angle formed by the chordwise direction (a direction of a straight line passing through the leading edge  24  and the trailing edge  26 ) of each of the diffuser vanes  10  with respect to the radial direction (0°≤γ≤90°). 
     The above-described stagger angle γ may be a stagger angle γ A  with reference to the leading edge  24  or a stagger angle γ B  with reference to the trailing edge  26  of each of the diffuser vanes  10 . The stagger angle γ A  with reference to the leading edge  24  of each of the diffuser vanes  10  is an angle between a straight line Lc in the chordwise direction of each of the diffuser vanes  10  and a radial straight line passing through the leading edge  24  of each of the diffuser vanes  10  (see  FIG. 5 ). Moreover, the stagger angle γ B  with reference to the trailing edge  26  of each of the diffuser vanes  10  is an angle between a straight line Lc in the chordwise direction of each of the diffuser vanes  10  and a radial straight line passing through the trailing edge  26  of each of the diffuser vanes  10  (see  FIG. 5 ). 
     In the exemplary embodiment shown in  FIG. 5 , a stager angle γ A   1  with reference to the leading edge  24  of the first diffuser vane  11  is smaller than a stagger angle γ A   2  with reference to the leading edge  24  of the second diffuser vane  12  (that is, γ A   1 &lt;γ A   2  is satisfied). 
     Moreover, in the exemplary embodiment shown in  FIG. 5 , a stager angle γ B   1  with reference to the trailing edge  26  of the first diffuser vane  11  is smaller than a stagger angle γ B   2  with reference to the trailing edge  26  of the second diffuser vane  12  (that is, γ B   1 &lt;γ B   2  is satisfied). 
     Thus, since the stagger angle γ 1  (γ A   1  or γ B   1 ) of the first diffuser vane  11  is smaller than the stagger angle γ 2  (γ A   2  or γ B   2 ) of the second diffuser vane  12 , the pressure surface  28  of the first diffuser vane  11  deviates upstream in the impeller rotational direction with reference to the leading edge  24 , as compared with the second diffuser vane  12  (see the second diffuser vanes  12 ′ indicated by the dashed line in  FIG. 5 ). Thus, it is possible to achieve the configuration in which the vane outlet angle β 1  of the first diffuser vane  11  and the vane outlet angle β 2  of the second diffuser vane  12  satisfy the relation of β 1 &lt;β 2 . 
     Furthermore, in the exemplary embodiment shown in  FIG. 5 , in a cross section orthogonal to the axial direction, the cross-sectional shape of the first diffuser vane  11  is the same as the cross-sectional shape of the second diffuser vane  12 . 
     Since the stagger angle γ 1  of the first diffuser vane  11  and the stagger angle γ 2  of the second diffuser vane satisfy the relation of γ 1 &lt;γ 2 , it is possible to achieve the configuration in which the vane outlet angle β 1  of the first diffuser vane  11  and the vane outlet angle β 2  of the second diffuser vane  12  satisfy β 1 &lt;β 2 , even if the first diffuser vane having the common cross-sectional shape with the second diffuser vane  12  is adopted as in the exemplary embodiment shown in  FIG. 5 . 
     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. 
     Further, in the present specification, 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. 
     As used herein, the expressions “comprising”, “containing” or “having” one constitutional element is not an exclusive expression that excludes the presence of other constitutional elements. 
     REFERENCE SIGNS LIST 
     
         
           1  Centrifugal compressor 
           2  Rotational shaft 
           4  Impeller 
           5  Rotor blade 
           6  Housing 
           7  Scroll flow passage 
           8  Scroll portion 
           9  Diffuser passage 
           10  Diffuser vane 
           11  First diffuser vane 
           12  Second diffuser vane 
           14  Mounting plate 
           16  Outlet portion 
           17  Outlet flow passage 
           18  Hub-side wall surface 
           20  Shroud-side wall surface 
           22  Tongue section 
           24  Leading edge 
           26  Trailing edge 
           28  Pressure surface 
           30  Suction surface 
           32  Region 
         O Rotational axis 
         t 1 , t 2  Vane thickness at trailing edge 
         α 1 , α 2  Camber angle 
         β 1 , β 1 ′ Vane outlet angle 
         γ 1 , γ 2  Stagger angle 
         γ A   1 , γ A   2  Stager angle with reference to leading edge 
         γ B   1 , γ B   2  Stager angle with reference to trailing edge