Abstract:
The invention relates to a wheel (R) of the Francis type that comprises a ring ( 1 ) with revolution symmetry about the rotation axis (Z) of the wheel (R), and curved blades ( 21, 22 ) connected to the ring ( 1 ) and each having an outer peripheral edge ( 212, 222 ) and an inner central edge ( 211, 221 ). The connection points (B 21 , B 22 ) between the ring ( 1 ) and the inner central edges ( 211, 221 ) of the blades ( 21, 22 ) are located on a same circle (C 20 ) centered on the axis (Z). The connection points (A 21 , A 22 ) between the ring ( 1 ) and the outer peripheral edges ( 212, 222 ) of the blades ( 21, 22 ) are located on at least two distinct circles (C 21 , C 22 ) (D 21 , D 22 ) centered on the axis (Z).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wheel of the Francis type for a hydraulic machine, which wheel is designed to pass a forced flow of water. Such a flow drives the wheel in rotation when the machine is a turbine. Such a flow results from said rotation when the machine is a pump. The present invention also relates to a hydraulic machine including such a wheel. The present invention also relates to an energy conversion installation equipped with such a hydraulic machine. 
     2. Brief Description of the Related Art 
     Document JP-A-2005 48608 describes a wheel or “runner” for a hydraulic machine of the pump-turbine type. That wheel has an axisymmetric band and a plurality of curved blades that are secured to or integral with the band and each of which has an outside peripheral leading edge and an inside central trailing edge. The connections via which the band is connected to the inside central edge of every other blade are situated on a first circle of large radius, whereas the connections via which the band is connected to the inside central edge of each blade adjacent to said every other blade are situated on a second circle that is of smaller radius. In the outside peripheral region of the wheel, the connections via which the band is connected to the blades having their inside central edges lying on the first circle are situated on the periphery of the band, whereas the connections via which the band is connected to the blades adjacent to those blades and having their central edges lying on the second circle of small radius are situated set back from the periphery of the band. That particular shape for the wheel of JP-A-2005 48 608 is designed to reduce the cavitation arising at the inlet of the turbine, i.e. at the periphery of the wheel, while the hydraulic machine is operating at steady speeds, so as to increase its hydraulic efficiency at steady speeds. 
     In a prior art hydraulic machine, at transient speeds that occur after starting, or before stopping in turbine mode, the wheel is subjected to rotating and asynchronous radial forces that tend to “skew” it. At transient speeds, those radial forces exerted on the wheel give rise to mechanical stress levels that can be very high and that therefore determine the dimensioning of the components of the hydraulic machine, such as the shaft or the bearings thereof. The cost of those components is thus necessarily increased due to such radial forces. 
     SUMMARY OF THE INVENTION 
     A particular object of the present invention is to remedy that drawback, by proposing a wheel of a shape that attenuates, or indeed avoids, the generation of rotating radial forces at transient speeds. 
     To this end, the invention provides a wheel of the Francis type for a hydraulic machine designed to pass a forced flow of water, said wheel comprising:
         a band that is circularly symmetrical about the axis of rotation of the wheel; and   a plurality of curved blades that are secured to or integral with the band and each of which has an outside peripheral edge and an inside central edge;       

     said wheel being characterized in that the connection points at which the band is connected to the inside central edges of the blades are substantially situated on a common circle that is centered on said axis and in that the connection points at which the band is connected to the outside peripheral edges of the blades are situated on at least two distinct circles that are centered on said axis. 
     Such a shape for the wheel imparts azimuthal asymmetry, thereby significantly reducing the radial forces to which the wheel is subjected at transient speeds. 
     According to other advantageous but optional characteristics of the invention, taken in isolation or in any technically feasible combination:
         the connection points at which the band is connected to the outside peripheral edges of the blades are situated on two distinct circles centered on said axis, “long” blades having connection points situated on a first circle, and “short” blades having connection points situated on a second circle of diameter smaller than the diameter of the first circle;   the wheel further comprises a crown that is circularly symmetrical about said axis and that is situated facing said band, the blades being secured to or integral with the crown, the connection points at which the crown is connected to the inside central edges of the blades are situated on a common circle centered on said axis, and the connection points at which the crown is connected to the outside peripheral edges of the blades are situated on at least two distinct circles that are centered on said axis;   the connection points at which the crown is connected to the outside peripheral edges of the blades are situated on two distinct circles that are centered on said axis;   the short blades are distributed uniformly about said axis and the long blades are distributed uniformly about said axis;   the wheel has nine blades, including six long blades and three short blades, the short blades being disposed on the band with angular spacing of 120°;   the wheel has an even number of blades and every other blade is a short blade; and   each short blade has a developed length lying in the range 50% of the developed length of a long blade to 99% of the developed length of a long blade, and preferably in the range 70% of said developed length of a long blade to 85% of said developed length of a long blade.       

     The invention also provides a hydraulic machine, characterized in that it includes a wheel as defined above. 
     The components of a hydraulic machine of the invention may therefore be dimensioned for lower mechanical stresses, and thus for lower cost, while retaining equal performance. 
     The invention also provides an energy conversion installation that is equipped with at least one hydraulic machine as defined above. 
     An energy-conversion installation of the invention is thus less costly than prior art installations while offering equal performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood and other advantages of the invention also appear from the following description, given by way of non-limiting example and with reference to the accompanying drawings, in which: 
         FIG. 1  is a section view showing the principle of an energy conversion installation of the invention, equipped with a hydraulic machine of the invention including a wheel of the invention; 
         FIG. 2  is a diagrammatic section view of the wheel on its own on the broken line II in  FIG. 1 ; 
         FIG. 3  is a diagrammatic section view of the wheel on the broken line III of  FIG. 1 ; 
         FIG. 4  is a view in partial perspective of the wheel of  FIG. 2 ; and 
         FIG. 5  is a diagrammatic section view of the wheel showing an even number of blades and wherein every other blade is a short blade. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The installation i shown in  FIG. 1  includes a reversible hydraulic machine M that is a Francis-type pump turbine whose wheel or “runner” R is fed with water from a casing  3  into which a forced-flow duct  4  opens out. In operation, the wheel R rotates about an axis of rotation Z that is vertical. In order to generate electricity in turbine mode, the machine M is coupled to an alternator  5  via a shaft  50  that rotates about the axis Z. Between the casing  3  and the wheel R there are disposed static stay vane blades  6  and steerable wicket gates  7  whose function is to guide a flow and to regulate the flow-rate of water E that is coming from the duct  4  and that is to pass through the wheel R towards a discharge conduit  8 . 
       FIG. 2  shows a portion of the wheel R that includes a band  1  that is circularly symmetrical about the axis Z. The band  1  has an outside peripheral edge  10  of diameter D 10 .  FIG. 3  shows a portion of the wheel R that includes a crown  12  that defines a central hole  11 . The crown  12  is also circularly symmetrical about the axis Z and it is situated facing the band  1 . The diameter of the outside peripheral edge of the crown  12  is referenced D 12 . The outside peripheral edge  10  and the central hole  11  are centered on the axis Z. As shown in  FIGS. 1 and 4 , the surfaces of the band  1  and of the crown  12  have skew shapes, generated by rotating a curve segment about and distant from the axis Z. Any shape for a band and for a crown of a conventional wheel of the Francis-type is adapted to define the respective surfaces of the band and of the crown of a wheel of the invention. 
     The wheel R also includes nine blades  21  and  22  secured to or integral with the band  1  and secured to or integral with the crown  12  and that extend between the band  1  and the crown  12  about the axis Z, as shown by  FIG. 4 . Each blade  21  or  22  has a curved shape and is substantially in the shape of a portion of a spiral. Any conventional blade curvature can be suitable for defining the curvature of a blade  21  or  22 . Each blade  21  or  22  has a peripheral edge, respectively  212  and  222 , situated at the periphery of the band  1  and a central edge, respectively  211  and  221 , facing towards the axis Z. In this example, an object is said to be “central” when it is situated in the vicinity of the axis Z, in opposition to the adjective “peripheral” that designates an object that is remote from the axis Z. 
     When the wheel R operates in turbine mode, the outside peripheral edge  212  or  222  forms a leading edge of a blade  21  or  22 , and the inside central edge  211  or  221  forms its trailing edge. The description given below for the turbine mode uses these expressions “leading edge” and “trailing edge”; it can be transposed to when the wheel operates in pump mode by interchanging these expressions. 
     Of the nine blades  21  and  22  of the wheel R, three blades  22  are of developed length less than the developed length of the other six blades  21 . The blades  22  are thus said to be “short”, while the blades  21  are said to be “long” 
     In a central region of the band  1 , the points at which the band  1  is connected to the trailing edges  211  and  221  of the respective blades  21  and  22  are respectively referenced B 21  and B 22 . The points B 21  and B 22  are situated on a common central circle of diameter D 20  centered on the axis Z. Thus, all of the blades  21  and  22  extend to the central circle C 20  of diameter D 20 . 
     In a peripheral region of the band  1 , the points at which the band  1  is connected to the leading edges  212  of the long blades  21  are referenced A 21 . The points A 21  are situated on a first circle C 21  of diameter D 21  centered on the axis Z. Similarly, the points at which the band  1  is connected to the leading edges  222  of the short blades  22  are referenced A 22 . The points A 22  are situated on a second circle C 22  that is centered on the axis Z and that is of diameter D 22  less than the diameter D 21  of the first circle. The points A 21  are thus closer to the edge  10  of the band  1  than are the points A 22 . The respective diameters D 21  and D 22  of the first and second circles C 21  and C 22 , and the diameter D 20  of the central circle C 20  are less than the diameter D 10  of the edge  10 . 
     In other words, the short blades  22  do not extend as far towards the edge  10  of the band  1  as the long blades  21  do. That is why the developed length of the short blades  22  is less than the developed length of the long blades  21 . Thus, the respective points A 21  and A 22  of the blades  21  and  22  are situated on two distinct circles C 21  and C 22  that are centered on the axis Z and that are of different respective diameters D 21  and D 22 . 
     In a variant of the invention that is not shown, the points at which the band is connected to the leading edges may be situated on more than two distinct circles, e.g. on three circles. 
       FIG. 3  shows the locations of the blades  21  and  22  on the crown  12  of the wheel R. Analogously to the connection of the band  1  to the blades  21  and  22 , in the central region of the crown  12 , the points at which the crown  12  is connected to the trailing edges  211  and  221  of the respective blades  21  and  22  are referenced respectively G 21  and G 22 . The points G 21  and G 22  are situated on a common central circle C 40  that is of diameter D 40  and that is centered on the axis Z. Thus, all of the blades  21  and  22  extend to a common central circle C 40  of diameter D 40 . 
     In a peripheral region of the crown  12 , the points at which the crown  12  is connected to the leading edges  212  of each long blade  21  are referenced F 21 . The points F 21  are situated on a third circle C 41  that is of diameter D 41  and that is centered on the axis Z. Similarly, the points at which the crown  12  is connected to the leading edges  222  of each short blade  22  are referenced F 22 . The points F 22  are situated on a fourth circle C 42  centered on the axis Z and of diameter D 42  less than the diameter D 41  of the third circle. 
     In practice, depending on the desired power for the hydraulic machine equipped with the wheel R, the diameter D 10  of the edge  10  of the band  1  may lie in the range 0.4 m to 10 m. Depending on the dimensions selected for the band  1 , the diameter D 21  of the first circle may represent in the range 90% of the diameter D 10  to 100% thereof, and the diameter D 22  of the second circle may represent in the range 70% of the diameter D 21  to 99% thereof, and preferably in the range 80% of the diameter D 21  to 95% thereof. Depending on the dimensions selected for the crown  12 , the diameter D 41  of the third circle may represent in the range 90% of the diameter D 12  to 100% thereof, and the diameter D 42  of the fourth circle may represent in the range 70% of the diameter D 41 , to 99% thereof, and preferably, in the range 80% of the diameter D 41  to 95% thereof. 
     Depending on the characteristics of the wheel R, in particular its dimensions and the frequency of rotation at which it is designed to rotate at steady speeds, each short blade  22  may have a developed length lying in the range 50% of the developed length of a long blade  21  to 99% of the developed length of a long blade. The developed length of a long blade  21  can be measured between the points A 21  and B 21  or between the points F 21  and G 21 . Similarly, the developed length of a short blade  22  can be measured between the points A 22  and B 22  or between the points F 22  and G 22 . Preferably, the developed length of a short blade  22  lies in the range 70% of the developed length of a long blade  21  to 85% of the length of a long blade. Thus, if a long blade  21  has a developed length of 1 m, a short blade  22  can, for example, have a developed length of 0.8 m. 
     In order to guarantee good dynamic balancing of the wheel R, both the short blades  22  and the long blades  21  are distributed uniformly about the axis of rotation Z of the wheel R. In this example, the three short blades  22  are disposed on the band  1  with angular spacing α 22  of 120° between them. Two long blades  21  are disposed at regular intervals between two consecutive short blades  22 , with angular spacing of 40° for two adjacent long blades  21  and angular spacing of 80° for two long blades  21  between which a short blade  22  is disposed. 
     With reference to  FIG. 5 , other distributions of the short and long blades are possible. It is thus possible to provide an even number of blades, e.g. ten blades, with every other blade being a short blade and the blades between them being long blades, both the short and the long blades being distributed uniformly about the axis of rotation of the wheel. In  FIG. 5 , eight blades are shown with every other blade being a short blade. 
     A wheel of the invention makes it possible to attenuate or even to avoid the radial forces generated on wheels having conventional blading, in particular at transient speeds when starting or stopping in turbine mode. It has been observed that, surprisingly, a wheel of the invention makes it possible to reduce the magnitude of the radial forces to which the wheel is subjected at transient speeds. By optimizing the shape of the blades of the wheel as a function of the use to be made thereof, it is possible to reduce that magnitude even further. 
     A hydraulic machine of the invention thus presents radial deflection of its wheel and of its shaft that is low or even zero. This thus reduces the costs of the hydraulic machine since it can be of small dimensions. 
     The invention is described herein with reference to a reversible hydraulic machine of the pump turbine type. However, the invention also applies to “simple” hydraulic machines of the turbine type.