Abstract:
To provide a mixed flow turbine in which supply of working fluid is designed to be supplied at a hub and a shroud, and the shape of the inlet-side edges of the blades is made to work effectively, thereby reducing incidence loss. A mixed flow turbine including blades whose leading edges are formed in a convex shape toward the upstream side; and a scroll that is a space formed upstream of the blades by a casing having a shroud that covers the radially external edges of the blades, the space being for supplying working fluid toward the leading edges of the blades, wherein the scroll is separated by a scroll partition wall into a shroud-side space and a hub-side space; and a shroud-side partition wall surface and a hub-side partition wall surface at the rear edge side of the scroll partition wall form a shroud-side inlet channel in which the working fluid flows substantially radially and a hub-side inlet channel in which the working fluid flows substantially in the same direction as the inclination direction of the hub side at the inlet of the blades between the partition wall surfaces and portions facing the partition wall surfaces, respectively.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a mixed flow turbine for use in compact gas turbines, turbochargers, expanders and so on. 
         [0003]    2. Description of the Invention 
         [0004]    There are constant demands to increase the efficiency of this type of mixed flow turbine. 
         [0005]    The efficiency of a turbine is expressed relative to a theoretical speed ratio (=U/CO) that is the ratio of the circumferential speed U of a blade inlet to a maximum flow rate, that is, a theoretical speed CO, at which working fluid (gas) is accelerated in accordance with the temperature and the pressure ratio of a turbine inlet. 
         [0006]    One factor that reduces the efficiency is incidence loss. This is caused by the incidence, which is the difference between the flow angle β of gas flowing to the leading edge of a blade and the blade angle βk of the leading edge. That is, when the incidence increases, inflow gas is separated by the leading edge, which increases incidence loss. 
         [0007]    Since the whole cross section of a blade of a mixed flow turbine, when cut along a given radius from a rotation axis, including the vicinity of the leading edge is formed in a curve (like a parabola), mixed flow turbines can be designed so that the flow angle β and the blade angle βk come close to each other. Although this allows the incidence of, for example, a hub surface, to come close to zero, incidence occurs between the hub and the shroud. 
         [0008]    That is, since the cross section of the blade of the mixed flow turbine, when cut along a radius from a rotation axis, is formed in a curve (like a parabola), the distribution  109  of the flow angle β changes linearly between the hub and the shroud, whereas the distribution  110  of the blade angle βk changes like a parabola, as shown in  FIG. 20 , so that the incidence at the central area of the blade height increases to the maximum. 
         [0009]    Incidence loss due to an increase in incidence caused by the difference between the distribution of the flow angle β and the distribution of the blade angle βk conversely causes an increase in the loss of the mixed flow turbine, decreasing the efficiency thereof. 
         [0010]    One example that reduces the decrease in efficiency is proposed in Japanese Unexamined Patent Application, Publication No. 2004-92498. 
         [0011]    As shown in  FIG. 18  or  19 , it is configured such that a leading edge  103  of a moving blade  101  is formed in a convex shape toward the upstream side in the working-fluid flowing direction. Thus, as shown in  FIG. 20 , a flow angle distribution  115  forms a downwardly convex curve. 
         [0012]    As a result, the blade angle distribution  110  of the blade angle βk and the flow angle distribution  115  of the flow angle β come close to each other, particularly at the center between the hub and the shroud, and the incidence becomes Ina. That is, since the incidence decreases by ΔIn, the incidence loss is decreased by a corresponding amount. 
         [0013]    One example of a mixed flow turbine is that disclosed in Japanese Unexamined Utility Model Application, Publication No. Sho-62-79938, in which the scroll is separated into two, and working fluid is supplied from one or both of them at the same time so that the supply of the working fluid can be varied. 
         [0014]    As shown in  FIG. 21 , it is configured such that a scroll  120  is separated into a first inlet channel  122  for supplying gas to the hub side of a leading edge  125  of a moving blade  124  and a second inlet channel  123  for supplying gas to the shroud side by means of a partition wall  121 . When the quantity of gas is small, for example, it is supplied only through the first inlet channel  122 , and when the quantity increases, it is supplied through the first inlet channel  122  and the second inlet channel  123 . 
         [0015]    Although the one disclosed in Japanese Unexamined Patent Application, Publication No. 2004-92498 exhibits the flow angle distribution  115  shown in  FIG. 20  when gas is supplied to the leading edge  103  of the moving blade  101  in an ideal state, actual products cannot be expected to supply gas in such an ideal state. 
         [0016]    That is, in general, a flow channel from a scroll  105  to the moving blade  101  extends radially, as shown in  FIG. 18 , or this flow channel is inclined in such a manner as to extend on the slopes of the shroud surface and the hub surface, as shown in  FIG. 19 . 
         [0017]    Furthermore, a nozzle  107  having a vane-shaped blade is sometimes provided directly upstream of the moving blade  101 . 
         [0018]    In the case where gas is supplied radially to the moving blade  101 , as shown in  FIG. 18 , the inclination angle δ of the hub-side flow becomes small, which reduces the effect of the upstream convex shape of the leading edge  103  by half and increases the flow angle β of the hub side, as shown in a flow angle distribution  102  in  FIG. 20 . 
         [0019]    This increases the incidence, which is the difference between the blade angle βk and the flow angle β, thus posing the problem of increased loss on the hub side. 
         [0020]    As shown in  FIG. 19 , in the case where gas is supplied at an angle to the moving blade  101 , the deflection angle of the flow from the scroll  105  to the shroud at a meridian plane increases because the inclination angle of the shroud is large. Thus, when the deflection angle at the meridian plane increases, the boundary layer on the shroud surface expands at the leading edge of the moving blade  101 , so that the flow angle β at the shroud side decreases. That is, the incidence increases in the opposite direction, as shown by the flow angle distribution  104  in  FIG. 20 , thus posing the problem of increased loss on the shroud side. 
         [0021]    On the other hand, the one disclosed in Japanese Unexamined Utility Model Application, Publication No. Sho-62-79938 is configured such that the partition wall  121  is present, as a plate having a limited thickness, upstream from the leading edge  125  of the moving blade  124 , which develops a wake  128  downstream from the trailing edge of the partition wall  121 , thus increasing loss. 
         [0022]    Since the gas from the first inlet channel  122  and the second inlet channel  123  is supplied at an angle to the leading edge  125  of the moving blade  124 , a boundary layer  127  develops at the shroud-side wall and the hub-side wall of the leading edge  125 , thus increasing loss. 
       BRIEF SUMMARY OF THE INVENTION 
       [0023]    In consideration of the above problems, an object of the present invention is to provide a mixed flow turbine in which working fluid is designed to be supplied at the hub side and the shroud side, and the shape of the inlet-side edges of the blades is used effectively, thereby reducing incidence loss. 
         [0024]    To solve the above problems, the present invention adopts the following solutions. 
         [0025]    According to an aspect of the present invention, there is provided a mixed flow turbine including: mixed flow turbine moving blades whose outline of inlet-side edges located at an upstream side is formed in a convex shape toward the upstream side; a scroll that is a space formed upstream of the mixed flow turbine moving blades by a casing having a shroud that covers the radially external edges of the mixed flow turbine moving blades, the space being for supplying working fluid toward the inlet-side edges of the mixed flow turbine moving blades, wherein the scroll is separated by a scroll partition wall into a shroud-side space and a hub-side space; and wherein a shroud-side partition wall surface and a hub-side partition wall surface at the rear edge side of the scroll partition wall form a shroud-side inlet channel in which the working fluid flows substantially radially and a hub-side inlet channel in which the working fluid flows substantially in the same direction as the inclination direction of a hub circumferential surface at the inlet of the mixed flow turbine moving blades between the partition wall surfaces and portions facing the partition wall surfaces, respectively. 
         [0026]    In the mixed flow turbine moving blades of the mixed flow turbine, regarding the line connecting the hub side and the shroud side of the inlet-side edge, the shroud side is located radially outward. 
         [0027]    According to this aspect, the inlet-side edges of the mixed flow turbine moving blades are formed in a convex shape toward the upstream side. In other words, the intermediate portion extends to the upstream side of the line connecting the hub side and the shroud side of the inlet-side edge, so that the shroud-side portion of the inlet-side edge is disposed along substantially the same radial position. 
         [0028]    Since working fluid that is supplied from the shroud-side space through the shroud-side inlet channel flows substantially radially, it flows parallel to the shroud-side wall surface and in the direction substantially perpendicular to the inlet-side edges of the moving blades. Thus, this can prevent a wall-surface boundary layer from increasing in size at the shroud portion of the inlets of the mixed flow turbine moving blades. 
         [0029]    Since working fluid that is supplied from the hub-side space through the hub-side inlet channel flows in substantially the same direction as the inclination direction of the hub circumferential surface at the inlet of the mixed flow turbine moving blades, it flows parallel to the hub circumferential surface. Thus, this can prevent a wall-surface boundary layer from increasing in size on the hub circumferential surface. 
         [0030]    Since the working fluid flows substantially radially in the shroud-side inlet channel, whereas in the hub-side inlet channel, it flows substantially in the same direction as the direction of inclination of the hub side of the inlet of the mixed flow turbine moving blades, the working fluid that has passed through both the inlet channels flows in an intersecting manner to the inlet-side edges of the mixed flow turbine moving blades. 
         [0031]    In other words, there is no object that stops the flow of the working fluid at the upstream side, as viewed from the inlet-side edges of the mixed flow turbine moving blades. 
         [0032]    Thus, this can prevent development of a wake due to the scroll partition wall. 
         [0033]    In this case, the shroud-side partition wall surface and the hub-side partition wall surface at the trailing edge side of the scroll partition wall form a shroud-side inlet channel and a hub-side inlet channel that intersect each other between the partition wall surfaces and portions facing the partition wall surfaces, respectively; therefore, the shroud-side surface and the hub-side surface are extended so as to intersect at the trailing edge side. Thus, if the thickness of the scroll partition wall is eliminated at the trailing edge, occurrence of a wake can be prevented. 
         [0034]    Thus, since this can prevent an increase in the size of the wall-surface boundary layer on the hub circumferential surface and the shroud at the inlet-side edges of the mixed flow turbine moving blades and can eliminate the influence of a wake due to the scroll partition wall in the center, an incidence reducing effect of the mixed flow turbine moving blades whose inlet-side edges are formed in a convex shape toward the upstream side can be reliably achieved, and thus incidence loss can be reduced. 
         [0035]    In the above aspect, a blade-shaped nozzle whose blade surfaces are formed substantially parallel to a rotation axis may be provided between the scroll partition wall and the mixed flow turbine moving blades. 
         [0036]    Provision of this blade-shaped nozzle can increase the circumferential speed of the flow, thus increasing the efficiency of the mixed flow turbine. 
         [0037]    Since the blade surfaces of the blade-shaped nozzle are formed substantially parallel to the rotation axis, the nozzle can easily be manufactured by casting. 
         [0038]    In the above aspect, the hub-side inlet channel may be provided with a hub-side blade-shaped nozzle whose blade surfaces are formed substantially parallel to a rotation axis. 
         [0039]    Since the hub-side inlet channel inclines with respect to the radial direction, it is longer than the shroud-side inlet channel along the radius. Furthermore, since the hub side decreases in radius, the flow rate of the hub side is high in accordance with the conservation of angular momentum. Therefore, the working fluid has increased frictional loss on the hub-side wall surface, and therefore the boundary layer may expand in the vicinity of the inlet of the hub circumferential surface. 
         [0040]    Since this configuration is provided with the hub-side blade-shaped nozzle, the circumferential speed of the flow in the hub-side inlet channel can be increased. This can prevent expansion of the boundary layer and improve the efficiency of the mixed flow turbine. 
         [0041]    Since the hub-side blade-shaped nozzle is formed such that the blade surfaces are substantially parallel to the rotation axis, the nozzle can easily be manufactured by casting. 
         [0042]    In the above configuration, it is preferable that the hub-side blade-shaped nozzle be provided with a guide plate, which is a substantially extended portion of the hub-side blade-shaped nozzle, extended to a position close to the inlet-side edges of the mixed flow turbine moving blades. 
         [0043]    Since the hub-side inlet channel inclines with respect to the radial direction, and the hub-side blade-shaped nozzle is formed substantially parallel to the rotation axis, a long space is formed between the trailing edge of the hub-side blade-shaped nozzle and the inlet-side edges of the mixed flow turbine moving blades. Furthermore, since the hub side decreases in radius, the flow rate at the hub side increases in accordance with the conservation of angular momentum, frictional loss on the hub-side wall surface increases, and therefore the boundary layer may expand in the vicinity of the hub circumferential surface. In other words, when a radially inward spiral flow runs on an inclined surface, the flow tends to run backward and radially outward because of the action of centrifugal force due to the spiral, and thus a boundary layer may expand more than a boundary layer on a flat plate. 
         [0044]    Therefore, a guide plate, which is a substantially extended portion of the hub-side blade-shaped nozzle, is provided from the trailing edge of the hub-side blade-shaped nozzle to a position close to the inlet-side edges of the mixed flow turbine moving blades, which can prevent the tendency for the flow to run backward and radially outward, thereby preventing expansion of a boundary layer. 
         [0045]    In the above configuration, it is preferable that a shroud-side blade-shaped nozzle whose blade surfaces are formed substantially parallel to the rotation axis be provided in the shroud-side inlet channel, and that the throat width of the shroud-side blade-shaped nozzle be set larger than the throat width of the hub-side blade-shaped nozzle. 
         [0046]    The blade angle of the inlet-side edges of the mixed flow turbine moving blades is as large as, for example, 40 degrees, at the hub side, and at the shroud-side, it is as small as, for example, the blade angle of the moving blades of a mixed flow turbine. In other words, the turbine characteristics that the flow of the shroud side exhibits are reaction turbine characteristics, and the turbine characteristics that the flow of the hub side exhibits are impact turbine characteristics. 
         [0047]    Since the throat width of the shroud-side blade-shaped nozzle is set larger than the throat width of the hub-side blade-shaped nozzle, the angle of the blades of the shroud-side blade-shaped nozzle relative to the circumference is larger than that of the hub-side blade-shaped nozzle. 
         [0048]    Accordingly, this allows the hub-side blade-shaped nozzle to have a nozzle blade angle suitable for the hub-side area having impact turbine characteristics and allows the shroud-side blade-shaped nozzle to have a nozzle blade angle suitable for the shroud-side area having reaction turbine characteristics. 
         [0049]    In the above aspect, the inlet of the scroll may be provided with an inlet partition wall that separates the inlet into a shroud-side flow channel communicating with the shroud-side space and a hub-side flow channel communicating with the hub-side space; the inlet partition wall may be mounted at a position at which the cross-sectional area of the shroud-side flow channel is larger than the cross-sectional area of the hub-side flow channel; and the upstream side of the inlet partition wall may be provided with a control member that allows at least the shroud-side flow channel to be fully closed and controls the working-fluid inflow ratio of the shroud-side flow channel to the hub-side flow channel. 
         [0050]    With this configuration, working fluid is separated by the inlet partition wall into the shroud-side flow channel and the hub-side flow channel. 
         [0051]    The working fluid that has flowed into the shroud-side flow channel is supplied to the shroud side of the inlet-side edges of the mixed flow turbine moving blades through the shroud-side space and the shroud-side inlet channel. 
         [0052]    On the other hand, the working fluid that has flowed into the hub-side flow channel is supplied to the hub side of the inlet-side edges of the mixed flow turbine moving blades through the hub-side space and the hub-side inlet channel. 
         [0053]    At that time, since the shroud-side flow channel can be fully closed, the working fluid is allowed to flow only into the hub-side flow channel. 
         [0054]    If the shroud-side flow channel is not fully closed, the working fluid is allowed to flow into the shroud-side flow channel and the hub-side flow channel at a flow rate according to a flow ratio adjusted by the control member. 
         [0055]    For example, assuming that the condition of working fluid flowing to the hub side of the mixed flow turbine moving blades is substantially fixed, the flow ratio in the fully closed state at which working fluid flows only through the hub-side flow channel to the flow rate when working fluid flows through the shroud-side flow channel and the hub-side flow channel in the unchanged valve state differ. That is, the flow rate of working fluid depends on the flow-channel cross-sectional area of the hub-side flow channel in the former closed state, and in the latter state, depends on the sum of the flow-channel cross-sectional area of the shroud-side flow channel and the flow-channel cross-sectional area of the hub-side flow channel, and thus this ratio indicates a change in flow rate. 
         [0056]    Since the inlet partition wall is mounted at a position at which the flow-channel cross-sectional area of the shroud-side flow channel is larger than the flow-channel cross-sectional area of the hub-side flow channel, the change in flow rate can be increased. 
         [0057]    This allows coping with large fluctuations in the supply of working fluid, thus improving the controllability of a mixed flow turbine. 
         [0058]    The turbine characteristics exhibited by the flow at the shroud side of the mixed flow turbine moving blades are reaction turbine characteristics, and the turbine characteristics exhibited by the flow at the hub side are impact turbine characteristics. 
         [0059]    The reaction turbine is highly efficient when U/CO is small, and the impact turbine is highly efficient when U/CO is large. 
         [0060]    Since the control member allows the shroud-side flow channel to be fully closed, and can control the working-fluid inflow ratio of the shroud-side flow channel to the hub-side flow channel, the mixed flow turbine can be used in a high efficient state by controlling the working-fluid inflow ratio of the shroud-side flow channel to the hub-side flow channel. 
         [0061]    For example, when the U/CO is small, the shroud-side flow channel is fully closed by the control member to allow the working fluid to flow only into the hub-side flow channel, thereby supplying the working fluid to the hub side of the inlet-side edges of the mixed flow turbine moving blades. This allows high-efficiency operation by the high-efficiency hub-side flow at a low U/CO. 
         [0062]    In the above configuration, it is preferable that the control member be provided with a plate member that can pivot about a axial center which is mounted to the upstream end of the inlet partition wall. 
         [0063]    This facilitates controlling the working-fluid flow ratio by pivoting the plate member. 
         [0064]    To ensure control, it is preferable that the plate member be of a shape along the flow-channel cross-sectional form of a portion where it is disposed. 
         [0065]    In the above configuration, it is preferable that the height, along the axial center, of the flow-channel cross section of the inlet of the scroll at which at least the plate member pivots be set substantially constant, and that the length, along the axial center, of the plate member be set substantially the same as the height. 
         [0066]    This can substantially prevent leakage of working fluid from the ends of the plate member orthogonal to the axial center, which can increase the accuracy of control, thereby preventing a decrease in efficiency. 
         [0067]    It is preferable that the shape of the flow-channel cross section and the plate member be, for example, a rectangle and an oblong figure having a linear portion. 
         [0068]    According to the present invention, the scroll is separated by a scroll partition wall into a shroud-side space and a hub-side space; and a shroud-side partition wall surface and a hub-side partition wall surface at the rear edge side of the scroll partition wall form a shroud-side inlet channel in which the working fluid flows substantially radially and a hub-side inlet channel in which the working fluid flows substantially in the same direction as the inclination direction of a hub circumferential surface at the inlet of the mixed flow turbine moving blades between the partition wall surfaces and portions facing the partition wall surfaces, respectively; therefore, this can prevent an increase in the size of a wall-surface boundary layer on the hub circumferential surface and the shroud at the inlet-side edges of the moving blades and can eliminate the influence of a wake due to the scroll partition wall in the center. This allows an incidence reducing effect of the moving blades whose inlet-side edges are formed in a convex shape toward the upstream side to be reliably achieved, thus reducing incidence loss. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0069]      FIG. 1  is a front view of a casing of a mixed flow turbine according to a first embodiment of the present invention; 
           [0070]      FIG. 2  is an X-X sectional view of  FIG. 1 ; 
           [0071]      FIG. 3  is a fragmentary sectional view showing part of  FIG. 2  in an enlarged scale; 
           [0072]      FIG. 4  is a fragmentary sectional view showing part of another embodiment, similar to the mixed flow turbine according to the first embodiment of the present invention shown in  FIG. 3 ; 
           [0073]      FIG. 5  is a fragmentary longitudinal sectional view of part of the main body of a mixed flow turbine according to a second embodiment of the present invention; 
           [0074]      FIG. 6  is a schematic diagram showing the shape of blades and guide plates of a hub-side nozzle according to the second embodiment of the present invention; 
           [0075]      FIG. 7  is a fragmentary longitudinal sectional view of part of the main body of a mixed flow turbine according to a third embodiment of the present invention; 
           [0076]      FIG. 8  is a Y-Y cross section of  FIG. 7 ; 
           [0077]      FIG. 9  is a longitudinal sectional view of a mixed flow turbine according to a fourth embodiment of the present invention; 
           [0078]      FIG. 10  is a diagram as viewed from Z in  FIG. 9 ; 
           [0079]      FIG. 11  is a diagram as viewed from W in  FIG. 9 ; 
           [0080]      FIG. 12  is a longitudinal sectional view of a blade  7  and a hub according to the fourth embodiment of the present invention; 
           [0081]      FIG. 13  is a graph showing changes in a hub-side flow channel area and a shroud-side flow channel area at various positions of a valve according to the fourth embodiment of the present invention; 
           [0082]      FIG. 14  is a graph showing an incidence distribution according to the fourth embodiment of the present invention; 
           [0083]      FIG. 15  is a diagram showing the relationship between the hub-side blade-shaped nozzle and the hub-side shape of the blades according to the fourth embodiment of the present invention; 
           [0084]      FIG. 16  is a diagram showing the relationship between the shroud-side blade-shaped nozzle and the shroud-side shape of the blades according to the fourth embodiment of the present invention; 
           [0085]      FIG. 17  is a diagram for describing the dimensionless characteristics of the mixed flow turbine according to the fourth embodiment of the present invention; 
           [0086]      FIG. 18  is a fragmentary longitudinal sectional view of the main body of a conventional mixed flow turbine; 
           [0087]      FIG. 19  is a fragmentary longitudinal sectional view of the main body of a conventional mixed flow turbine; 
           [0088]      FIG. 20  is a graph showing the incidence distribution of the conventional mixed flow turbine; and 
           [0089]      FIG. 21  is a fragmentary longitudinal sectional view of the main body of another conventional mixed flow turbine. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0090]    Embodiments of the present invention will be described below with reference to the drawings. 
       First Embodiment  
       [0091]    A mixed flow turbine  1  according to a first embodiment of the present invention will be described below with reference to  FIGS. 1 to 3 . The mixed flow turbine  1  is used in a turbocharger for an automobile diesel engine. 
         [0092]      FIG. 1  is a front view of a casing  3  of the mixed flow turbine  1  of this embodiment.  FIG. 2  is an X-X sectional view of  FIG. 1 .  FIG. 3  is a fragmentary sectional view showing part of  FIG. 2  in an enlarged scale. 
         [0093]    The mixed flow turbine  1  is provided with the casing  3 , a hub  5 , and a plurality of blades (mixed flow turbine moving blades)  7  provided at substantially equal intervals around a circumferential surface  6  of the hub  5 . 
         [0094]    A rotation shaft  9  is fixed to the axial central portion of the hub  5 . The rotation shaft  9  is freely rotatably supported by a bearing (not shown) attached to a bearing housing  11 , and the other end is connected to a turbocompressor (not shown). 
         [0095]    The casing  3  is provided with a substantially hollow cylindrical main body  13 , a substantially hollow cylindrical inlet section (inlet of the scroll)  15  connected to the main body  13  in the direction of the tangential line, and a substantially hollow cylindrical discharge section  17  protruding from one surface of the main body  13  so as to have a substantially coaxial center C. 
         [0096]    The inlet section  15  is connected to an exhaust section of a diesel engine (not shown), and the discharge section  17  is connected to an exhaust section of an automobile (not shown). 
         [0097]    The hollow portions of the main body  13 , the inlet section  15 , and the discharge section  17  communicate with one another, and exhaust gas (working fluid) exhausted from the diesel engine and introduced through an inlet  19  of the inlet section  15  acts on the blades  7  in the main body  13  to rotate the hub  5  and is then exhausted from an outlet  21  of the discharge section  17 . 
         [0098]    The rotation of the hub  5  is transmitted to a turbocompressor (not shown), so that the turbocompressor is rotated. The rotation of the turbocompressor causes air to be compressed and the air is supplied to the diesel engine. 
         [0099]    The inner space of the main body  13  constitutes a scroll  23  that accelerates the exhaust gas and supplies it to the blades  7 . A shroud  27  that covers outside-diameter edges  25  of the blades  7  is formed inside the main body  13 , as shown in  FIG. 3 . 
         [0100]    A scroll partition wall  29  that projects from the outside toward the inside in the radial direction is provided inside the main body  13 . The scroll  23  is separated by the scroll partition wall  29  into a shroud-side space  31  and a hub-side space  33 . 
         [0101]    The hub side at the inner-periphery (trailing edge) side of the scroll partition wall  29  forms a hub-side partition wall surface  35  inclined so as to be tapered toward the shroud side. The shroud side at the inner-periphery side of the scroll partition wall  29  forms a shroud-side partition wall surface  37  extending substantially radially. 
         [0102]    A hub-side wall surface  39  opposing the hub-side partition wall surface  35  at the hub side of the casing  3  is configured to be substantially parallel to the hub-side partition wall surface  35  and forms a hub-side inlet channel  41  between it and the hub-side partition wall surface  35 . 
         [0103]    The hub-side inlet channel  41  is inclined in substantially the same inclination direction as the inclination direction of the upstream end of the circumferential surface  6  of the hub  5 . 
         [0104]    A shroud-side wall surface  43  opposing the shroud-side partition wall surface  37  on the shroud side of the casing  3  is configured to be substantially parallel to the shroud-side partition wall surface  37  and forms a shroud-side inlet channel  45  between it and the shroud-side partition wall surface  37 . 
         [0105]    Since the shroud-side partition wall surface  37  extends substantially radially, the shroud-side inlet channel  45  extends substantially radially. 
         [0106]    The blades  7  are plate-like members formed vertically erected from the circumferential surface  6  of the hub  5  in such a manner that their surfaces extend in the axial direction. 
         [0107]    The intersection of a leading edge  47  and the outside-diameter edge  25  is located farther outside, in the radial direction, with respect to the intersection of the hub  5  and the leading edge  47 . 
         [0108]    The blades  7  are each provided with the leading edge (inlet-side edge)  47  located at the upstream side in the exhaust-gas flowing direction. The leading edge  47  is formed of a curve that expands smoothly like a convex over the entire region thereof to the upstream side, as shown in  FIG. 3 . 
         [0109]    The shroud-side portion of the leading edge  47  is shaped so as to extend along substantially the same radial position, in other words, so as to be substantially orthogonal to the radial direction. 
         [0110]    The blade  7  has a parabolic shape that forms, when projected at a given radius, a convexity in the rotating direction from the leading edge  47  toward the downstream side. 
         [0111]    The blade angle βk of the leading edge  47  is configured to decrease parabolically from the hub  5  side to the outside-diameter edge  25  side, like the conventional one shown in  FIG. 20 , for example. The blade angle βk at the leading edge  47  is set, for example, at 40 degrees at the hub  5  side and at 0 degree at the outside-diameter edge  25  side. 
         [0112]    The hub  5  and the blades  7  are integrally formed by casting or cutting. Alternatively, the hub  5  and the blades  7  may be separate objects that are then securely fixed together by welding or the like. 
         [0113]    The action of the mixed flow turbine  1  according to this embodiment, described above, will be described. 
         [0114]    The exhaust gas from the diesel engine flows through the inlet  19  into the inlet section  15  and is supplied to the main body  13 . The exhaust gas that has flowed into the main body  13  is separated by the scroll partition wall  29  and flows into the shroud-side space  31  and the hub-side space  33 , respectively. 
         [0115]    The exhaust gas that has flowed into the shroud-side space  31  is supplied to the leading edges  47  of the blades  7  through the shroud-side inlet channel  45 . 
         [0116]    At that time, the exhaust gas flows substantially radially since the shroud-side inlet channel  45  extends substantially radially. Since the shroud-side portions of the leading edges  47  are substantially orthogonal to the radial direction, the exhaust gas flows so as to be substantially orthogonal thereto. Thus, this can prevent a wall-surface boundary layer from increasing in size in the vicinity of the leading edges  47  at the shroud  27 . 
         [0117]    On the other hand, the exhaust gas that has flowed into the hub-side space  33  is supplied to the leading edges  47  of the blades  7  through the hub-side inlet channel  41 . 
         [0118]    At that time, the exhaust gas that is supplied to the leading edges  47  through the hub-side inlet channel  41  flows parallel to the hub circumferential surface  6  since the hub-side inlet channel  41  is formed in substantially the same inclination direction as the inclination direction of the upstream end of the circumferential surface  6  of the hub  5 . Accordingly, this can prevent a wall-surface boundary layer from increasing in size in the vicinity of the leading edges  47  at the circumferential surface  6 . 
         [0119]    The exhaust gas flows substantially radially in the shroud-side inlet channel  45 , whereas it flows in substantially the same direction as the inclination direction of the circumferential surface  6  at the leading edges  47 . Since the shroud-side partition wall surface  37  and the hub-side partition wall surface  35  join at the trailing edge of the scroll partition wall  29 , the exhaust gas that flows in the shroud-side inlet channel  45  and the hub-side inlet channel  41  join at the trailing edge of the scroll partition wall  29 . This can prevent development of a wake that is generated at the trailing edge of the scroll partition wall  29 . 
         [0120]    This results in the absence of an object that stops the flow of exhaust gas at the upstream side, as viewed from the leading edges  47 . 
         [0121]    Thus, since this can prevent an increase in the size of the wall-surface boundary layer on the circumferential surface  6  of the hub  5  and the shroud  27  at the leading edges  47  of the blades  7  and eliminates the influence of a wake due to the scroll partition wall  29  in the center, an incidence reducing effect of the blades  7 , whose leading edges  47  are formed in a convex shape toward the upstream side, can be reliably achieved, and thus incidence loss can be reduced. 
         [0122]    As shown in  FIG. 4 , a blade-shaped nozzle  49  whose blade surface is formed substantially parallel to the rotation axis C may be provided between the scroll partition wall  29  and the blades  7 . 
         [0123]    Provision of this blade-shaped nozzle  49  can increase the circumferential speed of the flow, thus increasing the efficiency of the mixed flow turbine  1 . 
         [0124]    Since the blade-shaped nozzle  49  is formed such that the blade surfaces are substantially parallel to the rotation axis C, it can easily be manufactured by casting. 
         [0125]    The exhaust gas that has flowed to the blades  7  passes between the blades  7 . At that time, the exhaust gas pushes the pressure surfaces of the blades  7  to move the blades  7  in the rotating direction. 
         [0126]    Thus, the hub  5  integrated with the blades  7  rotates in the rotating direction. The rotating force of the hub  5  rotates the turbocompressor via the rotation shaft  9 . The turbocompressor compresses air and supplies it as compressed air to the diesel engine. 
       Second Embodiment  
       [0127]    Next, a second embodiment of the present invention will be described with reference to  FIGS. 5 and 6 . 
         [0128]    A mixed flow turbine  1  of this embodiment differs from that of the above-described first embodiment in the configuration of the hub-side inlet channel  41 . Since the other components are the same as those of the above-described first embodiment, duplicated descriptions of the components will be omitted herein. 
         [0129]    The same components as those of the above-described first embodiment are given the same reference numerals. 
         [0130]      FIG. 5  is a fragmentary longitudinal sectional view of part of the main body  13 .  FIG. 6  is a schematic diagram showing the shape of blades  53  and guide plates  55  of a hub-side blade-shaped nozzle  51 . 
         [0131]    In this embodiment, the hub-side inlet channel  41  is provided with the hub-side blade-shaped nozzle  51  constituted by the plurality of blades  53  whose blade surfaces are formed substantially parallel to the rotation axis C. 
         [0132]    As shown in  FIG. 6 , the blades  53  of the hub-side blade-shaped nozzle  51  are mounted in such a manner as to incline at a predetermined angle relative to the circumference. 
         [0133]    The blades  53  are mounted between a nozzle inlet radius NI and a nozzle outlet radius NO. 
         [0134]    Since the hub-side blade-shaped nozzle  51  is formed such that the blade surfaces are substantially parallel to the rotation axis C, it can easily be manufactured by casting. 
         [0135]    The guide plates  55  are mounted to the downstream side of the hub-side blade-shaped nozzle  51  in correspondence with the individual blades  53 . 
         [0136]    The guide plates  55  have a logarithmic spiral cross section and are mounted so as to be substantially extending portions of the blades  53 . 
         [0137]    The downstream ends G of the guide plate  55  extend near to the leading edges  47  to form a shape substantially along the hub-side shape of the leading edges  47 . 
         [0138]    Since the action of the mixed flow turbine  1  with such a configuration according to this embodiment is basically the same as that of the above-described first embodiment, duplicated descriptions will be omitted and differences will be described. 
         [0139]    Since the hub-side inlet channel  41  inclines with respect to the radial direction, it is longer than the shroud-side inlet channel  45  along the radius. Furthermore, since the hub side decreases in radius, the flow rate of the hub side is high in accordance with the conservation of angular momentum. Therefore, the exhaust gas has increased frictional loss on the hub-side wall surface  39 , and therefore the boundary layer may expand in the vicinity of the leading edges  47  on the circumferential surface  6  of the hub  5 . 
         [0140]    Since this embodiment is provided with the hub-side blade-shaped nozzle  51  in the hub-side inlet channel  41 , the circumferential speed of the flow in the hub-side inlet channel  41  can be increased. 
         [0141]    This can prevent expansion of the boundary layer and improve the efficiency of the mixed flow turbine  1 . 
         [0142]    Since the hub-side inlet channel  41  inclines with respect to the radial direction, and the hub-side blade-shaped nozzle  51  is formed substantially parallel to the rotation axis C, a long space is formed between the trailing edge of the hub-side blade-shaped nozzle  51  and the leading edges  47  of the blades  7 . Furthermore, since the hub side decreases in radius, the flow rate at the hub side increases in accordance with the conservation of angular momentum, frictional loss on the hub-side wall surface  39  increases, and therefore the boundary layer may expand in the vicinity of the circumferential surface  6  of the hub  5 . In other words, when a radially inward spiral flow runs on an inclined surface, the flow tends to run backward radially outward because of the action of centrifugal force due to the spiral, and thus a boundary layer may expand more than a boundary layer on a flat plate. 
         [0143]    In this embodiment, the flow that has left the blades  53  of the hub-side blade-shaped nozzle  51  is guided to the vicinity of the leading edges  47  by the guide plates  55 . 
         [0144]    Since the flow that has left the hub-side blade-shaped nozzle  51  flows in accordance with the conservation of angular momentum, it flows ideally in a logarithmic spiral shape. 
         [0145]    Since the guide plates  55  are shaped in a logarithmic spiral cross section, this ideal flow can be maintained. 
         [0146]    The guide plates  55  may be omitted depending on the conditions because they are used as necessary. 
       Third Embodiment  
       [0147]    Next, a third embodiment of the present invention will be described with reference to  FIGS. 7 and 8 . 
         [0148]    A mixed flow turbine  1  of this embodiment differs from that of the above-described second embodiment in the configuration of the shroud-side inlet channel  45 . Since the other components are the same as those of the above-described second embodiment (and the first embodiment), duplicated descriptions of the components will be omitted herein. 
         [0149]    The same components as those of the above-described first embodiment and second embodiment are given the same reference numerals. 
         [0150]      FIG. 7  is a fragmentary longitudinal sectional view of part of the main body  13 .  FIG. 8  is a Y-Y cross section of  FIG. 7 . 
         [0151]    In this embodiment, the shroud-side inlet channel  45  is provided with a shroud-side blade-shaped nozzle  59  constituted by a plurality of blades  57  whose blade surfaces are formed substantially parallel to the rotation axis C. Since the shroud-side blade-shaped nozzle  59  is formed such that the blade surfaces are substantially parallel to the rotation axis C, it can easily be manufactured by casting. 
         [0152]    As shown in  FIG. 8 , the blades  57  are mounted in such a manner as to incline at a predetermined angle (blade angle) relative to the circumference. 
         [0153]    As shown in  FIG. 8 , the blade angles of the blades  57  are set larger than the blade angles of the blades  53 . Thus, the throat width δs (the width at a portion where the flow is reduced most) of the shroud-side blade-shaped nozzle  59  is larger than the throat width δh of the hub-side blade-shaped nozzle  51 . Thus, the flow rate at shroud-side blade-shaped nozzle  59  is larger than that at the hub-side blade-shaped nozzle  51 . 
         [0154]    Since the action of the mixed flow turbine  1  with such a configuration according to this embodiment is basically the same as those of the above-described first embodiment and second embodiment, duplicated descriptions will be omitted and differences will be described. 
         [0155]    The blade angle of the leading edges  47  of the blades  7  is as large as, for example, 40 degrees, at the hub side, and at the shroud-side, it is as small as, for example, the blade angle of the moving blades of a mixed flow turbine. In other words, the turbine characteristics exhibited by the flow at the shroud side are reaction turbine characteristics, and the turbine characteristics exhibited by the flow at the hub side are impact turbine characteristics. 
         [0156]    The exhaust gas flowing through the shroud-side inlet channel  45  is guided by the relatively small blade angle of the shroud-side blade-shaped nozzle  59  and flows smoothly to the shroud side of the leading edges  47  having a relatively small blade angle. On the other hand, the exhaust gas flowing through the hub-side inlet channel  41  is guided by the relatively large blade angle of the hub-side blade-shaped nozzle  51  and flows smoothly to the hub side of the leading edges  47  having a relatively large blade angle. 
         [0157]    Thus, setting the throat width δs of the shroud-side blade-shaped nozzle  59  larger than the throat width δh of the hub-side blade-shaped nozzle  51  allows the hub-side blade-shaped nozzle  51  to have a nozzle blade angle suitable for the hub-side area having impact turbine characteristics and allows the shroud-side blade-shaped nozzle  59  to have a nozzle blade angle suitable for the shroud-side area having reaction turbine characteristics. 
         [0158]    The guide plates  55  may be omitted depending on the conditions because they are used as necessary. 
       Fourth Embodiment  
       [0159]    Next, a fourth embodiment of the present invention will be described with reference to  FIGS. 9 to 17 . 
         [0160]    A mixed flow turbine  1  of this embodiment is the same as the above-described second embodiment in basic configuration and differs in the configuration of the inlet section  15 . Accordingly, this difference will be mainly described, and duplicated descriptions of the other components will be omitted herein. 
         [0161]    The same components as those of the above-described first and second embodiments are given the same reference numerals. 
         [0162]      FIG. 9  is a longitudinal sectional view of the mixed flow turbine  1 .  FIG. 10  is a diagram as viewed from Z in  FIG. 9 .  FIG. 11  is a diagram as viewed from W in  FIG. 9 .  FIG. 12  is a longitudinal sectional view of the blade  7  and the hub  5 . 
         [0163]    The mixed flow turbine  1  according to this embodiment is of a variable displacement type whose capacity can be changed in a broad range. 
         [0164]    In the case where the pressure ratio of the inlet of the turbine to the outlet is constant, when a turbocharger that incorporates the mixed flow turbine  1  is mounted in a passenger car, a truck, or the like, their engine characteristics require that the flow ratio of the minimum flow rate of the mixed flow turbine  1  to the maximum flow rate vary in the range from 1:3 to 1:5. 
         [0165]    The inlet section  15  is substantially rectangular in cross section. The inner space of the inlet section  15  is partitioned by an inlet partition wall  61  into a hub-side flow channel  63  that communicates with the hub-side space  33  and a shroud-side flow channel  65  that communicates with the shroud-side space  31 . 
         [0166]    The inlet partition wall  61  is disposed substantially parallel to one side of the rectangular inner space. The inlet partition wall  61  is disposed in a position displaced from the center line CL of a side substantially perpendicular to the one side so that the cross-sectional area of the hub-side flow channel  63  is smaller than the cross-sectional area of the shroud-side flow channel  65 . 
         [0167]    Although the amount of displacement is set as appropriate, here it is set so that the cross-sectional area of the hub-side flow channel  63  is substantially half of the cross-sectional area of the shroud-side flow channel  65 . 
         [0168]    That is, since the widths are almost the same, the height AH of the hub-side flow channel  63  is set at substantially half of the height As of the shroud-side flow channel  65 , in other words, Ah:As≈1:2. 
         [0169]    The upstream end, which is the upstream portion of the inlet partition wall  61 , is provided with a flow-rate adjusting valve (control member)  67 . 
         [0170]    The flow-rate adjusting valve  67  is provided with a pivot shaft  69  that is rotatably mounted to the upstream end of the inlet section  15  and the inlet partition wall  61 , a rectangular valve element (plate member)  71  one side of which is fixed to the pivot shaft  69 , and a hydraulic cylinder  73  that rotates the pivot shaft  69  around the axial center. 
         [0171]    The height, along the pivot shaft  69 , of the inner space of the rectangular inlet section  15  is substantially fixed, and the length of the valve element  71  along the pivot shaft  69  is set substantially the same as the height. 
         [0172]    This allows the gap between the inner wall surface of the inlet section  15  and the valve element  71  to be maintained constant at all degrees of opening when the valve element  71  moves. This also allows a flow at the shroud side of the valve element  71  to run into the shroud-side flow channel  65  substantially uniformly in the height direction and allows a flow at the hub side of the valve element  71  to run into the scroll substantially uniformly in the height direction. 
         [0173]    The cross-sectional shape of the inner space of the inlet section  15  and the shape of the valve element  71  are not limited to rectangular and may be any shape. 
         [0174]    In this case, it is preferable that the cross section of the flow channel of the inner space of the inlet section  15  where the valve element  71  pivots be substantially fixed in height along the pivot shaft  69 , and that the length, along the pivot shaft  69 , of the valve element  71  be substantially the same as the height. The shape may be, for example, an oblong figure having a linear portion. 
         [0175]    This can substantially prevent leakage of working fluid from the ends of the valve element  71  orthogonal to the pivot shaft  69 , which can increase the accuracy of control, thereby preventing a decrease in efficiency. 
         [0176]    The minimum length of the valve element  71  and the range of expansion and contraction of the hydraulic cylinder  73  are set so that the valve element  71  can fully close the shroud-side flow channel  65 . 
         [0177]    The range of expansion and contraction of the hydraulic cylinder  73  is limited so that, when the valve angle of the valve element  71  when the valve element  71  is set so as to narrow the hub-side flow channel  63  is defined to be minus, the maximum angle on the minus side is from −20 to −30 degrees. 
         [0178]    The maximum length of the valve element  71  is set so that, when it has reached the maximum angle on the minus side, the ratio of the area of the end portion to the area of the hub-side flow channel  63  becomes 1:2.5 or less. This is because, since the flow channel constituted by the valve element  71  and the scroll wall serves as a diffuser, pressure loss increases rapidly if the area ratio exceeds 2.5. 
         [0179]      FIG. 12  shows the shape of the leading edge  47  of the blade  7 . Let the straight line at the middle angle between a radial line K and a perpendicular line V perpendicular to the circumferential surface of the hub  5  be a straight line H, and a straight line at a fixed radius close to the shroud be a straight line S. The leading edge  47  is shaped such that the hub side follows the straight line H, the shroud side substantially follows the straight line S, and the intermediate portion is shaped in arc connecting the two straight lines. 
         [0180]    This shape allows the distance between the trailing edge of the shroud-side blade-shaped nozzle  59  and the leading edges  47  and the distance between the trailing edges of the guide plates  55  provided downstream of the hub-side blade-shaped nozzle  51  and the leading edges  47  to be held substantially constant. 
         [0181]    The relationship between the representative length bh of the hub side of the leading edge  47  and the representative length bs of the shroud side is set at bh:bs≈2:3. 
         [0182]    The relationship between the throat width δh of the hub-side blade-shaped nozzle  51  and the throat width δs of the shroud-side blade-shaped nozzle  59  is set at δh:δs≈3:4. 
         [0183]    Thus, the relationship between the throat area Sh of the hub-side blade-shaped nozzle  51  and the throat area St of the shroud-side blade-shaped nozzle  59  becomes 2×3:3×4≈1:2. 
         [0184]    That is, the area ratio of the hub-side flow channel  63  to the shroud-side flow channel  65 , serving as the inlet, and the area ratio of the throats serving as the outlet are set substantially the same. 
         [0185]    Since the action of the mixed flow turbine  1  with such a configuration according to this embodiment is basically the same as those of the above-described first embodiment, second embodiment, and third embodiment, duplicated descriptions will be omitted and differences will be described. 
         [0186]    According to this embodiment, the exhaust gas that flows into the inlet section  15  is separated by the inlet partition wall  61  into the shroud-side flow channel  65  and the hub-side flow channel  63 . 
         [0187]    The exhaust gas that has flowed into the shroud-side flow channel  65  is supplied to the shroud side of the leading edges  47  of the blades  7  through the shroud-side space  31  and the shroud-side inlet channel  45 . 
         [0188]    On the other hand, the exhaust gas that has flowed into the hub-side flow channel  63  is supplied to the hub side of the leading edges  47  of the blades  7  through the hub-side space  33  and the hub-side inlet channel  41 . 
         [0189]    At that time, of the shroud-side flow channel  65  and the hub-side flow channel  63 , the shroud-side flow channel  65  can be fully closed, so that the working fluid is allowed to flow only into the hub-side flow channel  63 . 
         [0190]    If the shroud-side flow channel  65  is not fully closed, the exhaust gas is allowed to flow into the shroud-side flow channel  65  and the hub-side flow channel  63  at a flow rate according to a flow ratio adjusted by the valve element  71 . 
         [0191]      FIG. 13  shows the relationship between a hub-side flow channel area AhS and a shroud-side flow channel area AsS at various flow channel positions. 
         [0192]    The flow channel positions at various positions are the position SI of the inlet  19  which is the inlet of the scroll, the distal end BI of the valve element  71 , the upstream end DI of the inlet partition wall  61 , a nozzle inlet radius NI position, and a nozzle outlet radius NO position. 
         [0193]    When the valve element  71  is located at a position of the extension of the inlet partition wall  61 , that is, at a fully open state, the hub-side flow channel area AhS and the shroud-side flow channel area AsS at position SI, position BI, and position DI are constant, as indicated by line Zos and line Zoh. 
         [0194]    When the valve element  71  fully closes the shroud-side flow channel  65 , that is, at a fully closed state, the shroud-side flow channel area AsS is zero at position BI, as indicated by line Zss, and increases toward position DI so as to reach the flow-channel area in the fully open state. On the other hand, the hub-side flow channel area AhS exhibits the whole flow channel area of the inlet section  15  at position BI, as indicated by line Zsh, and decreases toward position DI so as to reach the flow-channel area in the fully open state. 
         [0195]    When the valve element  71  is located at a minus angle, the shroud-side flow channel area AsS increases at position BI by an area corresponding to the upstream end position of the valve element  71 , as indicated by line Mks, and decreases toward the position DI so as to reach the flow-channel area in the fully open state. On the other hand, the hub-side flow channel area AhS decreases at position BI by an area corresponding to an increase in the shroud-side flow channel area AsS, as indicated by line Mkh, and increases toward position DI so as to reach the flow-channel area in the fully open state. 
         [0196]    Since the hub-side flow channel  63 , the hub-side space  33 , and the hub-side inlet channel  41  gradually decrease in area, the hub-side flow channel area gradually decreases from position DI to position NO. 
         [0197]    Since the shroud-side flow channel  65 , the shroud-side space  31 , and the shroud-side inlet channel  45  gradually decrease in area, the shroud-side flow channel area gradually decreases from position DI to position NO. 
         [0198]    At the fully closed state, the exhaust gas flows only through the hub-side flow channel  63 , and at the fully open state, it flows through the hub-side flow channel  63  and the shroud-side flow channel  65 . 
         [0199]    Since the cross-sectional area, that is, the flow channel area, of the hub-side flow channel  63  is set at substantially half of the cross-sectional area, that is, the flow channel area, of the shroud-side flow channel  65 , the flow-channel area ratio in the fully closed state to that in the fully open state becomes 1:3. 
         [0200]    For example, assuming that the conditions of working fluid flowing to the hub side of the leading edge  47  are substantially constant, the flow ratio in the fully closed state to the flow rate in the fully open state becomes 1:3. 
         [0201]    In this way, since the inlet partition wall  61  is mounted at a position at which the flow-channel cross-sectional area of the shroud-side flow channel  65  is twice the flow-channel cross-sectional area of the hub-side flow channel  63 , the flow ratio of the minimum flow rate to the maximum flow rate can be increased. This can improve the controllability of the mixed flow turbine  1  and the ability to respond to engine demands. 
         [0202]    Furthermore, since, in the fully open state, exhaust gas of a quantity corresponding to the area ratio flows through the hub-side flow channel  63  and the shroud-side flow channel  65 , the flow rates of the exhaust gas flowing through the hub-side flow channel  63  and the shroud-side flow channel  65  become substantially equal, which can make the pressure loss of the hub-side flow channel  63  and the shroud-side flow channel  65  substantially equal and the minimum. 
         [0203]    The exhaust gas that has flowed into the shroud-side flow channel  65  and the hub-side flow channel  63  flows into the shroud-side space  31  and the hub-side space  33 , respectively. 
         [0204]    The exhaust gas that has flowed into the shroud-side space  31  is supplied to the leading edges  47  of the blades  7  through the shroud-side inlet channel  45 . The exhaust gas that has flowed into the hub-side space  33  is supplied to the leading edges  47  of the blades  7  through the hub-side inlet channel  41 . 
         [0205]    At that time, as described in the first embodiment, since an increase in the size of the wall-surface boundary layer on the circumferential surface  6  of the hub  5  and the shroud  27  at the leading edges  47  of the blades  7  can be prevented, and the influence of a wake at the intermediate part due to the scroll partition wall  29  can be eliminated, the incidence reducing effect of the blades  7  whose leading edges  47  are formed in a convex shape toward the upstream side can be reliably achieved, and thus incidence loss can be reduced. 
         [0206]    That is, as shown in  FIG. 14 , with a conventional mixed flow turbine not having the features of Japanese Unexamined Patent Application, Publication No. 2004-92498, the flow angle β 71  of the leading edges  47  of the blades  7  changes linearly between the hub  5  and the shroud  27 . On the other hand, the blade angle βk  73  changes parabolically, and therefore the incidence at the center of the blade height increases to the maximum. 
         [0207]    In this embodiment, since the leading edges  47  of the blades  7  are formed in a convex shape toward the upstream side, the flow angle β 75  decreases and changes like a curve, as shown in  FIG. 14 . 
         [0208]    As a result, the distribution of the blade angle βk  73  and the distribution of the flow angle β 75  come close to each other at the center between the hub  5  and the shroud  27  to cause incidence Ina. That is, since the incidence decreases by ΔIn, incidence loss is reduced correspondingly. 
         [0209]      FIG. 15  is a diagram showing the relationship between the hub-side blade-shaped nozzle  51  and the hub-side shape of the blade  7 . The throat width δh of the hub-side blade-shaped nozzle  51  is narrow, and the inclination angles of the blades  53  are small. On the other hand, the blade angle βkh of the leading edge  47  of the blade  7  is set large. Turbines of this shape are called “impact turbines”, which are highly efficient when U/CO is small. 
         [0210]      FIG. 16  is a diagram showing the relationship between the shroud-side blade-shaped nozzle  59  and the shroud-side shape of the blade  7 . The throat width δs of the shroud-side blade-shaped nozzle  59  is wide, and the inclination angles of the blades  57  are small. On the other hand, the blade angle βks of the leading edge  47  of the blade  7  is set small. Turbines of this shape are called “reaction turbines”, which are highly efficient when U/CO is large. 
         [0211]    In the case where the mixed flow turbine  1  is used as a variable displacement turbocharger of passenger cars and trucks, U/CO decreases to 0.5 to 0.6 during acceleration, whereas U/CO increases to 0.7 to 0.8 at the maximum power output. A usable area UA is used when U/CO is in the range from 0.6 to 0.7. 
         [0212]      FIG. 17  is a diagram for describing the dimensionless characteristics of the mixed flow turbine  1 . 
         [0213]    When the valve element  71  is fully closed, exhaust gas flows only through the hub-side flow channel  63 , and is supplied to the hub side of the leading edges  47  through the hub-side inlet channel  41 , so that an efficient fully-closed characteristic  75  is exhibited at low U/CO. 
         [0214]    On the other hand, in the fully open state, the exhaust gas flows through the hub-side flow channel  63  and the shroud-side flow channel  65 , and is supplied to the hub side and the shroud side of the leading edges  47 . At that time, since the quantity of exhaust gas supplied to the shroud side becomes substantially twice as much as the quantity of exhaust gas supplied to the hub side, the characteristic of the shroud-side flow predominates. Accordingly, an efficient fully-open characteristic  77  is exhibited at high U/CO. 
         [0215]    When the valve element  71  comes to an intermediate position between the fully open state and the fully closed state, an intermediate-opening characteristic  79  between the fully-closed characteristic  75  and the fully-open characteristic  77  is exhibited. 
         [0216]    The fully-closed characteristic  75  exhibits the maximum efficiency around an acceleration-time working point A, whereas the fully-open characteristic  77  exhibits the maximum efficiency around a maximum-power-output-time working point M. The intermediate opening characteristic  79  sometimes exhibits high efficiency at the usable area UA. 
         [0217]    Thus, adjusting the opening of the valve element  71  allows continuous high-efficiency operation from the acceleration time to the maximum power output time. 
         [0218]    When a higher U/CO characteristic is required depending on the operating point of the engine, the ratio of the shroud-side characteristic to the hub-side characteristic can be controlled so that the shroud-side characteristic predominates by controlling the angle of the valve element  71  to the minus side. 
         [0219]    Although this embodiment uses the shroud-side blade-shaped nozzle  59 , the hub-side blade-shaped nozzle  51 , and the guide plates  55 , they are used when needed, and may be omitted depending on the conditions. That is, the configuration of the inlet section  15  according to this embodiment may be combined with the configuration of the first embodiment or the second embodiment. 
         [0220]    It is to be understood that the present invention is not limited to the above-described embodiments and that various modifications may be made as appropriate without departing from the spirit and scope of the present invention.