Abstract:
A plurality of vaned diffusers is disposed on a concentric plate at intervals in a circumferential direction thereof, and each of the diffusers is a curvilinear element three-dimensional diffuser having blades which are extended from a hub side of a impeller to a shroud side thereof. The blades are formed in a form in which a blade serving as a reference is stacked in a direction of the height of the blade, which is a direction of a gap between the hub and the shroud. A dihedral distribution in which moving in a direction perpendicular to a chord direction linking a leading edge of the blade as the reference with a tailing edge thereof, is set as a positive movement is non-uniform from an end portion on the hub side to an intermediate portion of the height of the blade.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a centrifugal turbomachine including a centrifugal impeller, such as a centrifugal compressor, centrifugal blower, centrifugal fan or centrifugal pump. 
       BACKGROUND ART 
       [0002]    The multistage centrifugal compressor, a kind of centrifugal turbomachine, has a number of impellers mounted on the same shaft. A diffuser and a return guide vane are installed side by side downstream of each of the impellers. The impeller, the diffuser, and the return guide vane constitute a stage. Here, a vaneless diffuser, a vaned diffuser, a low solidity diffuser that is a kind of the vaned diffuser, or the like, is used as the diffuser, depending on the purpose and intended use. 
         [0003]    Among these diffusers, the low solidity diffuser has the property of being able to increase the choke margin that is the operating range on a high flow side, because it does not have a geometrical throat. Also, the low solidity diffuser has the advantage of being able to sufficiently ensure the surge margin that is the operating range on a low flow side, because separation on the blade surface in a low flow area is suppressed by the effect of a secondary flow which sweeps the boundary layer on the blade surface. For this reason, the low solidity diffuser is frequently used. 
         [0004]    A 2D blade with the same blade profiles stacked in a blade height direction is commonly used for a vaned diffuser for centrifugal turbomachines as typified by the low solidity diffuser. However, in response to the demands for a further performance improvement, attempts are also being made to use a 3D blade. For example, in a centrifugal compressor disclosed in Patent Literature 1, the stagger angle of a diffuser blade section is gradually varied in the blade height direction of the diffuser to form the 3D blade, thereby realizing a collisionless flow for an unevenly distributed inflow and achieving both of an improvement in efficiency and an increase in operating range. 
         [0005]    Furthermore, in a centrifugal compressor disclosed in Patent Literature 2, a diffuser inlet diameter is changed by bending downstream a heightwise central portion of a blade at a leading edge portion of a diffuser. Thus, a collision-free flow for the unevenly distributed inflow is realized and both of an improvement in efficiency and an increase in operating range are achieved. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: JP-A No, 2009-504974 
         Patent Literature 2: JP-A No. 2004-92482 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    In the airfoil diffuser for the centrifugal compressor disclosed in the above-described Patent Literature 1, a 3D diffuser blade is formed by stacking divided diffuser blades virtually in an axial direction (direction from a hub plane to a shroud plane). At this time, there is also suggested a bow diffuser blade in which a lean angle is varied along a diffuser blade span, the lean angle meaning the angle which the stacking direction of the blades makes with the direction perpendicular to the hub or shroud plane. 
         [0009]    However, the authors consider that the curvilinear element blade does not necessarily lead to sufficient improvements because many options are available. That is, if lean distribution is applied to the blade, the secondary flow might be increased by its application, resulting in performance deterioration. Therefore, there is a need for clarification of the stacking pattern of divided blades which leads to performance improvement. 
         [0010]    Furthermore, in the centrifugal compressor disclosed in the Patent Literature 2, lean is applied to a local portion, namely, the diffuser leading edge to achieve inflow angle matching. However, any construction of the curvilinear element diffuser is not adopted, and no consideration is given to control of the secondary flow in a flow path between diffuser blades which becomes more conspicuous when the curvilinear element diffuser is employed. 
         [0011]    The present invention has been made in view of problems of the related art described above, and an object of the present invention is to provide effectively suppress a secondary flow between blades and improve performance when a curvilinear element diffuser is used to increase efficiency in a vaned diffuser for use in a centrifugal turbomachine. Another object of the present invention is to obtain a stacking pattern of divided blades which leads to performance improvement, in the curvilinear element diffuser for use in a centrifugal turbomachine. 
       Solution to Problem 
       [0012]    Firstly, referring to  FIGS. 1 and 2 , some terms used in this specification will be defined as follows.  FIG. 1  is a plan view of one diffuser blade for explaining the movement of a blade profile.  FIG. 2  is a perspective view of one blade taken from a vaned diffuser, showing the state in which basic blade profiles are stacked in a Z direction. A coordinate system is a cylindrical coordinate system (R, θ, Z), in which the radial direction of an impeller is denoted by R, the direction of rotation of the impeller is denoted by θ, and the axial direction of a rotating shaft is denoted by Z. The Z direction from a shroud  102  to a hub  101  is set as positive. 
         [0013]    Chord (C): line that connects a leading edge  208  and a trailing edge  209  of a blade profile  104  serving as a basis of a diffuser blade  103 . 
         [0014]    Lean: degree of tilt of the diffuser blade  103  relative to the surface of the hub  101 , and it can be regarded as a combination of sweep and dihedral to be described below. 
         [0015]    Stagger Angle (θ SG ): angle (tan θ SG =dC/dR) which the chord C forms with the radial direction (R direction). 
         [0016]    Sweep (Δσ): as indicated by alternate long and short dashed lines in  FIG. 1 , to move the blade profile  104  of the diffuser blade  103  parallel to the direction of the chord C. The movement in a downstream direction is set as positive. 
         [0017]    Dihedral (Δδ): as indicated by dashed lines in  FIG. 1 , to move the blade profile  104  of the diffuser blade  103  in the direction perpendicular to the chord C. The movement in the opposite direction of rotation of the impeller is set as positive. 
         [0018]    Blade Height (h): height of the diffuser blade, the height being measured from the hub surface. If the hub and shroud surfaces are parallel walls normal to the axis, the blade height is the height in the negative Z direction. If at least one of the hub and shroud surfaces includes a tilted surface, the blade height is the height measured from a line that connects the leading edge and the trailing edge on the hub side of the diffuser blade. The height of an intermediate point in a flow direction between the leading edge and the trailing edge is determined with reference to a line that connects the leading edges on the hub and shroud sides of the diffuser blade and a line that connects the trailing edges on the hub and shroud sides of the diffuser blade. The total height of the blades is represented by H. 
         [0019]    Using these definitions, in order to address the above-described problems, the present invention provides a centrifugal turbomachine including: at least one or more impellers attached to an identical rotating shaft and composed of a hub, a shroud, and a plurality of circumferentially spaced apart blades between the hub and the shroud; and a vaned diffuser disposed downstream of at least one of the impellers, wherein: the vaned diffuser includes a plurality of circumferentially spaced apart blades in a flow passage that is formed downstream of the impeller, each of the blades being formed with basic blade profiles stacked in a blade height direction that corresponds to an axial direction of the rotating shaft; and dihedral distribution in which movement in a direction perpendicular to a chord direction connecting leading and trailing edges of each of the basic blade profiles and in an opposite direction of rotation of the impeller is set as positive is made uneven from a hub-side end to an intermediate portion in the blade height direction on a hub wall surface side. 
         [0020]    Also in this feature, preferably, the dihedral distribution of each of the diffuser blades is increased from the hub-side end to the intermediate portion in the blade height direction, and, in each of the diffuser blades, an angle between a plane virtually formed at a leading edge portion on the hub-side end and a suction surface of the diffuser blade is an obtuse angle. 
         [0021]    Furthermore, preferably, the dihedral distribution increases from a shroud-side end to the intermediate portion in the blade height direction, and, in each of the diffuser blades, an angle between a plane virtually formed at the leading edge portion on the shroud-side end and the suction surface of the diffuser blade is an obtuse angle. 
         [0022]    In the above-described feature, the arrangement may be such that the dihedral distribution of each of the diffuser blades decreases from the hub-side end to the intermediate portion in the blade height direction, and sweep distribution in which movement in a direction parallel to the chord direction of the basic blade profiles and in a downstream direction is set as positive is decreased from the hub-side end to the intermediate portion in the blade height direction. 
         [0023]    It should be noted that, in any of the above-described features, preferably, at least one of the dihedral distribution and the sweep distribution is applied to at least a first half portion in a flow direction of the diffuser blades. 
       Advantageous Effects of Invention 
       [0024]    According to the present invention, in the vaned diffuser for use in the centrifugal turbomachine, the 3D curvilinear-element blade is applied to the diffuser blade, and the sweep and dihedral distributions are given, thereby reducing the loss due to the collision of the flow with the diffuser blade. Furthermore, because the flow at the intermediate portion of the blade can be controlled, the secondary flow between the blades is effectively suppressed and the diffuser performance and the compressor performance can be improved. Moreover, in the present invention, in this curvilinear-element diffuser for use in the centrifugal compressor, a stacking pattern of divided blades which leads to performance improvement can be provided. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  is a view illustrating tilting in a vaned diffuser. 
           [0026]      FIG. 2  is a view illustrating a 3D blade included in the vaned diffuser. 
           [0027]      FIG. 3  is a longitudinal sectional view of one embodiment of a centrifugal turbomachine according to the present invention. 
           [0028]      FIG. 4  is a view illustrating the classification of vaned diffusers. 
           [0029]      FIG. 5  is a graph illustrating dihedral distribution, according to one embodiment, of the diffuser included in a compressor shown in  FIG. 3 . 
           [0030]      FIG. 6  is a perspective view of the diffuser having the dihedral distribution shown in  FIG. 5 , and a partially-enlarged view thereof. 
           [0031]      FIG. 7  is a graph illustrating dihedral distribution, according to another embodiment, of the diffuser included in the compressor shown in  FIG. 3 . 
           [0032]      FIG. 8  is a perspective view of the diffuser having the dihedral distribution shown in  FIG. 7 , and a partially-enlarged view thereof. 
           [0033]      FIG. 9  is a graph illustrating dihedral and sweep distributions, according to still another embodiment, of the diffuser included in the compressor shown in  FIG. 3 . 
           [0034]      FIG. 10  is a perspective view of the diffuser having the dihedral and sweep distributions shown in  FIG. 9 , and a partially-enlarged view thereof. 
           [0035]      FIG. 11  is an exemplary performance diagram of the centrifugal compressor including the diffuser according to the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0036]    Hereinafter, several embodiments of the present invention will be described by using the accompanying drawings. Firstly, a multistage centrifugal compressor  300  serving as an example of a centrifugal turbomachine will be described by using a longitudinal sectional view of  FIG. 3 . The multistage centrifugal compressor  300  is a two-stage centrifugal compressor. It should be noted that the subject of the present invention is not particularly limited to the two-stage centrifugal compressor, but also can include single-stage or multistage centrifugal turbomachines. 
         [0037]    The multistage centrifugal compressor  300  shown in  FIG. 3  is the two-stage centrifugal compressor that is composed of a first stage  301  and a second stage  302 . A first-stage impeller  308  and a second stage impeller  311  are attached to an identical rotating shaft  303  to constitute a rotating body. The rotating shaft  303  is rotatably supported by a journal bearing  304  and a thrust bearing  305  that are attached to a compressor casing  306  for storing the rotating shaft  303  and the impellers  308  and  311 . 
         [0038]    Downstream of the first-stage impeller  308 , there are disposed a diffuser  309  that recovers the pressure of working gas compressed by the impeller  308  and forms a radially outwardly directed flow, and a return guide vane  310  that directs radially inwardly the radially outward flow of working gas caused by the diffuser  309  and guides it to the second-stage impeller  311 . Downstream of the second-stage impeller  311 , a diffuser  312  is similarly disposed, and recovery means  313 , called a collector or scroll, for gathering and sending out the working gas subjected to pressure rise by the second-stage diffuser  312  is disposed. 
         [0039]    The first- and second-stage impellers  308  and  311  have hub-side plates  308   a  and  311   a , shroud-side plates  308   b  and  311   b , and a plurality of blades  308   c  and  311   c  arranged circumferentially with almost equal spacing between the core plate  308   a  and the side plate  308   b  and between the core plate  311   a  and the side plate  311   b , respectively. Labyrinth seals  315  are disposed at outer peripheral portions of the shroud-side plates  308   b  and  311   b  on the entrance sides of the impellers  308  and  311 . Also, shaft seals  316  and  317  are disposed at the rear of the hub-side plates  308   a  and  311   a . Working gas flowing from a suction nozzle  307  passes in order through the first-stage impeller  308 , the vaned diffuser  309 , the return guide vane  310 , the second-stage impeller  311 , and the vaned diffuser  312 , and is guided without leakage to the recovery means  313  such as the collector or scroll. 
         [0040]    The diffusers  309  and  312  for use in the centrifugal compressor  300  as constructed in this manner will be described in detail below. It should be noted that the diffuser  309  is attached to a diaphragm constituting a portion of the compressor casing  306  and has a hub  309   a  with a passage plane located at almost the same axial position as that of the impeller  308  and a plurality of circumferentially-spaced-apart blades  309   c  provided in a standing manner on the surface of the hub  309   a . Furthermore, the wall surface of an inner casing constituting a portion of the compressor casing  306  forms a flow passage as a shroud surface. Although not descried here, the diffuser  312  has the same construction. It should be noted that, although the above construction is described in this embodiment, the construction of the diffuser is not limited thereto. The present invention, of course, also includes the construction being such that the diffuser is separate from the diaphragm. 
         [0041]    In  FIG. 4 , vaned diffusers  400  to be used in the following description are classified and shown.  FIG. 4(   a ) is a cross-sectional view of the diffuser  400 . A plurality of diffuser blades  420   a  arranged circumferentially with almost equal spacing are provided in a standing manner on a hub plate  410   a . The flow from the impeller, which is not shown, is guided so as to flow along the diffuser blades  420   a  from the inner periphery as indicated by arrow FL in the drawing. At this time, the impeller, not shown, rotates in the direction of arrow R N . 
         [0042]    The shapes of the diffuser are classified into: a 2D diffuser which has conventionally been employed ( FIG. 4(   b )); a 3D straight-line element diffuser having a lean ( FIG. 4  ( c )); and a 3D curvilinear-element diffuser also having a lean and represented by a set of curvilinear elements ( FIG. 4(   d )). Here, diffuser blades  420   b  to  420   d  are represented as a shape with linear elements  423   b  to  423   d  connecting the contours of hub-plate-side sections  421   b  to  421   d  and shroud-side sections  422   b  to  422   d . The same flow is discharged from the impeller to the diffuser blades  420   b  to  420   d  to form a diffuser entry flow  402 . 
         [0043]    The 2D straight-line element diffuser blade  420   b  shown in  FIG. 4  ( b ) is a 2D diffuser that is formed of the straight-line element  423   b , not tilted, with the same blade profiles stacked straight in a height direction of the blade  420   b . That is, the straight-line element  423   b  is perpendicular to the hub plate  410   a . In the diffuser having this blade  420   b , it is impossible to prevent the flow from colliding with the blade  420   b  in all positions in the height direction (h direction) of a leading edge of the blade  420   b  when the inlet flow  402  is distributed, and there is a limit to the improvement in performance. 
         [0044]    In the 3D straight-line element diffuser shown  FIG. 4  ( c ), a twist is added to the diffuser blade  420   c  by varying the stagger angle (θ SG ). This allows the flow from the impeller to flow into the diffuser blade  420   c  without colliding with the diffuser blade  420   c . That is, even if an uneven flow is discharged from the impeller, the shape at a leading edge portion of the diffuser blade  420   c  can be changed according to the inlet flow  402 . 
         [0045]    In this 3D straight-line element diffuser blade  420   c , the linear element  423   c  connecting the contours of the hub-plate-side section  421   c  and the shroud-side section  422   c  is a straight line, and the lean distribution in the height direction (h direction) of the blade  420   c  also has a linear design. However, the linear element  423   c  is not necessarily perpendicular to the hub plate  410   a . After entry of the flow between the blades  420   c  and  420   c , the lean angle cannot be changed to a value corresponding to a flow angle because the blade  420   c  is formed, for example, in a basic NACA airfoil shape. Therefore, although a greater improvement in efficiency than the 2D diffuser can be expected, sufficient flow control is difficult. 
         [0046]    In the 3D curvilinear-element diffuser shown  FIG. 4  ( d ), the blade profiles are stacked along the optional curvilinear element  423   d . In other words, the curvilinear element  423   d  connecting the contours of the hub-plate-side section  421   d  and the shroud-side section  422   d  is a curve line. In this diffuser, the lean angle is varied, rather than being constant, in the height direction (h direction) of the blade  420   d . Thus, with the 3D curvilinear-element diffuser, it is possible to not merely realize a collision-free inflow at a leading edge portion of the blade  420   d  but also change the direction of action of blade force by bending a passage plane of the blade  420   d.    
         [0047]    Therefore, the flow in a flow passage between the blades  420   d  and  420   d  can be controlled. Therefore, in the present invention, as shown in  FIG. 3 , the diffusers  309  and  312  that recover the dynamic pressure at exits of the impellers  308  and  311  as static pressure are made 3D curvilinear-element diffusers. 
         [0048]    Meanwhile, although there are various methods for making the diffuser being three dimensional, the diffuser can be systematically made three dimensional by using the above-described dihedral and sweep. Therefore, a specific example of the 3D curvilinear-element diffuser represented using the dihedral and sweep will be described by using  FIGS. 5 to 11 . In the following description, the first stage diffuser  309  is used as an example. However, the second and subsequent stage diffusers are also used in the same manner. 
         [0049]    One embodiment of the 3D curvilinear-element diffuser will be described by using  FIGS. 5 and 6 . Only the dihedral distribution is shown.  FIG. 5  is a graph illustrating dihedral distribution in a blade height direction (h direction) of a blade  620 , in which the amount of dihedral (Δδ) is made dimensionless with the chord length (C) and the blade height is made dimensionless with the total height H.  FIG. 6  is a perspective view of a diffuser  600  having the dihedral distribution of  FIG. 5 , in which  FIG. 6(   a ) is a general perspective view;  FIG. 6(   b ) is a detail view of portion C in  FIG. 6(   a ); and  FIG. 6(   c ) is a detail view of portion D in  FIG. 6(   a ). A diffuser plate  610  is attached to the hub side of the impeller. 
         [0050]    As shown in  FIG. 5 , in this embodiment, the dihedral increases in the blade height direction in the vicinity of a hub-side end face (h=0) (see a portion  501  surrounded by a circle). That is, a suction surface  601  of the diffuser blade  620  forms an obtuse angle with a hub surface  603 . It should be noted that the suction surface  601  of the diffuser blade  620  corresponds to the blade surface that is located to the rear with respect to the direction of rotation of the impeller. 
         [0051]    Studies by the inventors of the present invention showed that, in the dihedral distribution shown in  FIG. 5 , the influence of the dihedral or sweep distribution on performance was generally small in a portion other than the portion  501  surrounded by a circle, that is, the portion  501  in the vicinity of the hub-side end face. Therefore, the dihedral and sweep distributions can be set in the portion other than the portion  501  in the vicinity of the hub-side end face in consideration of the workability or handleability of the blades  309   c.    
         [0052]    As shown in  FIG. 6(   b ), in the diffuser  600  of this embodiment, a blade force component  602  is generated in the blade height direction. The blade force component  602  has the effect of forcing back the secondary flow because a boundary layer on the hub surface  603  is located in the opposite direction of the secondary flow that tends to migrate toward the hub-side suction surface  601 . Thus, according to this embodiment, the secondary flow is suppressed, leading uniform distribution of the flow between the blades and an improvement in diffuser performance. 
         [0053]    Another embodiment of the present invention will be described by using  FIGS. 7 and 8 . These drawings are the same as those of the above-described embodiment.  FIG. 7  is a graph of dihedral distribution, and  FIG. 8  is a perspective view of a diffuser  800  having the dihedral distribution shown in  FIG. 7 .  FIG. 8(   a ) is a general perspective view of the diffuser  800 ;  FIG. 8(   b ) is a detail view of portion E in  FIG. 8(   a ); and  FIG. 8(   c ) is a detail view of portion F in  FIG. 8(   a ). Also in the diffuser  800 , a diffuser plate  810  is attached to the hub side of the impeller. This embodiment differs from the above-described embodiment in that the dihedral is reduced in the blade height direction in the vicinity of a shroud-side end face (a portion  702  surrounded by a circle). 
         [0054]    Although in the above-described embodiment, the influence of the dihedral distribution is greater on the hub-surface side, it has turned out that the dihedral distribution on the shroud-surface side also exerts an influence upon the diffuser according to the flow from the impeller. It should be noted that, even in this case, the dihedral distribution on the shroud side should be the same as the above-described embodiment. A specific example thereof will be described below. 
         [0055]    On the hub-side end face, the amount of dihedral (Δδ) is increased in the blade height direction (h direction) in the same manner as the above-described embodiment (see a portion  701  surrounded by a circle). Also in this embodiment, the influence of the dihedral or sweep distribution on performance is small in a center region in the blade height direction other than the two regions in the vicinity of the hub-side end face and the shroud-side end face. That is, in the vicinity of the hub-side and shroud-side end faces, the angle that suction surfaces  801  and  802  of a diffuser blade  820  form with the hub and shroud end faces is an obtuse angle. Therefore, the secondary flow can be suppressed by the same working effects as the above-described embodiment. 
         [0056]    It should be noted that the distribution shown in  FIG. 7  is preferably used if the flow at the exit of the impeller is relatively uniform, while the distribution shown in  FIG. 5  is preferably used if the nonuniformity is high. This is because the diffuser blade  820  is affected by the uniformity or nonuniformity of the flow at the exit of the impeller. That is, if the nonuniformity of the flow at the exit of the impeller is high, a high-energy portion of the flow is controlled by focusing on the flow control on the hub-surface side on where the mainstream exists, and consequently the overall flow can be effectively controlled. 
         [0057]    Still another embodiment of the present invention will be described by using  FIGS. 9 and 10 .  FIG. 9(   a ) is a graph of dihedral distribution, and  FIG. 9(   b ) is a graph of sweep distribution which is made dimensionless with the chord length.  FIG. 10  is a perspective view of the diffuser  309  having the distributions shown in  FIG. 9 , in which  FIG. 10(   a ) is a general view of the diffuser;  FIG. 10(   b ) is a detail view of portion G in  FIG. 9(   a ); and  FIG. 10(   c ) is a detail view of portion H in  FIG. 9(   a ). In the same manner as the above-described embodiments, a diffuser plate  1010  is attached to the hub side of the impeller. 
         [0058]    In the above-described two embodiments, the dihedral distribution on the hub side is important, and the increase in dihedral in the blade height direction is effective from the viewpoint of flow control. However, it has turned out that the combination of dihedral and sweep provides benefits even when the dihedral is reduced in the blade height direction. A specific example thereof will be described below. 
         [0059]    As shown in  FIG. 9 , in this embodiment, the dihedral is reduced in the blade height direction in the vicinity of the hub-side end face (see a portion  901  surrounded by a circle), and furthermore, the sweep is reduced similarly in the vicinity of the hub-side end face (see a portion  902  surrounded by a circle). That is, the diffuser has a lean with the dihedral and sweep combined and is a diffuser  1000  in which the 3D curvilinear-element is used. Because the effects on performance are small in a region other than the vicinity of the hub-side end face, both dihedral and sweep can be arbitrarily set, as long as an extreme change is not caused. 
         [0060]    In this embodiment, the direction of the dihedral on the hub-side end face is the reverse of those of the above-described embodiments. As a result, the angle formed by a diffuser suction surface  1001  and the surface of a hub plate  1010  is an acute angle, and a blade force opposite in direction to the blade force component  602  shown in  FIG. 6  is generated. This reversed blade force appears to increase the secondary flow, but actually serves to suppress the secondary flow. The reason is as follows. 
         [0061]    In this embodiment, a diffuser blade  1020  is composed of a combination of dihedral and sweep. Because the diffuser blade  1020  has a sweep  1002 , a notch-shaped gap  1003  is formed between a leading edge  1005  of the diffuser blade  1020  and the surface of the hub plate  1010 . In the notch-shaped gap  1003 , a flow that tends to migrate from the pressure surface to the suction surface of the diffuser blade  1020  occurs, thereby generating a longitudinal vortex  1004 . Vorticity  1006  to suppress the secondary flow is generated in a corner formed by the suction surface of the diffuser blade  1020  and the surface of the hub plate  1010 . At the same time, separation on the blade surface in the diffuser blade  1020  is suppressed by the promotion of agitation with the surrounding fluid or the negative pressure effect of the vortex center. In this manner, the secondary flow is suppressed by the action of the longitudinal vortex, and the flow field is made uniform, thereby improving the performance of the 3D curvilinear-element diffuser. 
         [0062]      FIG. 11  shows a state in which the compressor performance is improved when the 3D curvilinear-element diffuser shown in this embodiment is used in place of the 2D straight-line element diffuser in a compressor. The horizontal axis of the graph represents flow rate Q made dimensionless with design point flow rate Qdes, and the vertical axis represents: adiabatic efficiency η of the compressor stage made dimensionless with adiabatic efficiency η 2DIM  in the 2D diffuser; and pressure coefficient ψ made dimensionless with pressure coefficient ψ 2DIM  in the 2D diffuser. 
         [0063]    The adiabatic efficiency η and the pressure coefficient ψ are improved over a wide flow range, not to mention the design flow rate. Also, the vaned diffuser according to the present invention is superior in performance at an off design point (Q≠ 1.0) because the amount of performance improvement increases with distance from the design point flow rate (Q=1.0). That is, the compressor operating range is improved. 
         [0064]    In the above-described embodiments, the diffuser blade has at least one of sweep distribution and dihedral distribution, thereby realizing the 3D curvilinear-element diffuser. Furthermore, the secondary flow in the vicinity of a hub wall surface and a shroud wall surface of the diffuser and the impinging flow near the leading edge of the diffuser blade are controlled by inclining the diffuser blades. As a result, the diffuser performance can be improved. It should be noted that the sweep and dihedral distributions shown in the above-described embodiments are just an example and, also in the region that is not limited in shape because the influence on performance is small, the sweep and dihedral distributions are illustrative only. 
         [0065]    Furthermore, although preferably, the entire blades have the shape feature shown in the embodiments, the advantages of the present invention can be obtained even if the blades have the above-described shape especially only in a first half portion in the flow direction of the diffuser because the shapes in the first half portion (upstream) of the diffuser blades have a relatively great influence on performance. Therefore, the 2D straight-line element diffuser or the like, which has been conventionally frequently employed, may be used for a latter half portion in the flow direction. 
         [0066]    Although in the above-described embodiments, the diffuser blades are provided on the hub plate, the diffuser blades may be of course provided on the surface thereof facing the hub plate, that is, the plate on the shroud-surface side. In any case, the diffuser blades are mounted on the hub or shroud side for ease of assembly, etc. Further, there is no need for the multistage compressor to be entirely provided with vaned diffusers. Even if a vaned diffuser is provided on at least one stage of the compressor and the present invention is applied to the diffuser, the advantages of the present invention can be obtained. 
       REFERENCE SINGS LIST 
       [0000]    
       
           101  Hub 
           102  Shroud 
           103  Diffuser blade 
           104  Blade profile 
           105  Diffuser plate 
           208  Leading edge 
           209  Trailing edge 
           300  Centrifugal turbomachine (multistage centrifugal compressor) 
           301  First stage 
           302  Second stage 
           303  Rotating shaft 
           304  Journal bearing 
           305  Thrust bearing 
           306  Compressor casing 
           307  Suction nozzle 
           308  First-stage impeller 
           308   a  Hub-side plate 
           308   b  Shroud-side plate 
           308   c  Blade 
           309  Vaned diffuser 
           309   a  Hub 
           309   c  Blade 
           310  Return guide vane 
           311  Second-stage impeller 
           311   a  Hub-side plate 
           311   b  Shroud-side plate 
           311   c  Blade 
           312  Vaned diffuser 
           313  Recovery means (scroll or collector) 
           315  Labyrinth seal 
           316 ,  317  Shaft seal 
           400  Vaned diffuser 
           401  Straight-line element 
           402  Inlet flow 
           403  Hub-side blade section 
           404  Shroud-side blade section 
           405  Straight-line element 
           407  Hub-side blade section 
           408  Shroud-side blade section 
           409  Curvilinear element 
           410  Hub plate 
           411  Hub-side blade section 
           412  Shroud-side blade section 
           420   a  to  420   d  Diffuser blade 
           421   b  to  421   d  Hub surface 
           422   b  to  422   d  Shroud surface 
           423   b  to  423   d  Linear element 
           501  Dihedral distribution 
           600  Vaned diffuser 
           601  Hub-side suction surface 
           602  Blade force component 
           603  Hub surface 
           610  Hub plate 
           620  Blade 
           701  Dihedral distribution 
           702  Dihedral distribution 
           800  Vaned diffuser 
           801  Hub-side suction surface 
           802  Shroud-side suction surface 
           810  Hub surface 
           820  Blade 
           901  Dihedral distribution 
           902  Sweep distribution 
           1000  Vaned diffuser 
           1001  Hub-side suction surface 
           1002  Sweep 
           1003  Notch 
           1004  Longitudinal vortex 
           1005  Diffuser leading edge 
           1006  Vorticity 
           1010  Hub plate 
           1020  Blade 
         C Chord 
         FL Inlet flow 
         h Height of diffuser blade 
         H Total height of diffuser blades 
         R N  Direction of rotation of impeller 
         Δδ Amount of dihedral 
         Δσ Amount of sweep 
         Q Flow rate 
         Qdes Design point flow rate 
         η Adiabatic efficiency 
         η 2DIM  Efficiency of 2D blade diffuser 
         ψ Pressure coefficient 
         ψ 2DIM  Pressure coefficient of 2D blade diffuser