Abstract:
The invention relates to a machine through which a main flow (E) of water passes, including a turbine wheel ( 4 ), with at least a turbulent zone, reduced-pressure zone or cavitation zone ( 8, 10, 33 ) being formed close to said wheel. The wheel includes blades ( 6 ) disposed between a ceiling ( 28 ) and a belt ( 30 ). The machine also includes means for injecting a flow (E 2 ) drawn from the main flow into the aforementioned zone ( 8, 10, 33 ) such as to alter the main flow (E) locally or increase the pressure in said zone ( 8, 10, 33 ). Said injection means inject the drawn flow (E 2 ) from the ceiling ( 28 ) or from the belt ( 30 ) through holes ( 31, 32 ) provided in the ceiling ( 28 ) or belt ( 30 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a Francis type hydraulic machine of the type traversed by a main flow of water, comprising a wheel of a turbine in the vicinity of which is formed at least one eddy zone or a reduced-pressure zone or a cavitation zone, the wheel comprising blades arranged between a ceiling and a belt, the machine comprising means of injecting a flow tapped from said main flow, not modified relative to the main flow, into said eddy or reduced-pressure or cavitation zone so as to locally modify the main flow or increase the pressure in this zone. 
     2. Brief Description of the Related Art 
     Such a machine is used, for example, in a plant for producing hydroelectricity. The machine is installed in the path of the current or is supplied with water from a reservoir into which one or more water courses are discharged. 
     In these hydraulic machines, there are zones in which the main flow traversing the machine is disturbed and forms eddies or exhibits a reduced pressure or cavitation zones, because of the configuration of the machine. Such zones disrupt the general performance of the hydraulic machine because they reduce the efficiency of action of the main flow in the hydraulic machine or cause problems of operation of the hydraulic machine. 
     The document U.S. Pat. No. 1,942,995 describes a hydraulic machine of the abovementioned type, making it possible to inject a flow tapped from the main flow into the cavitation zone being formed along the blades of the wheel of the turbine. 
     However, such a machine does not make it possible to effectively increase the pressure in the spaces extending between the blades where reduced-pressure zones are formed or eliminate the eddy zones that are also formed between the blades. 
     SUMMARY OF THE INVENTION 
     One of the objects of the invention is to alleviate these drawbacks by proposing a hydraulic machine making it possible to eradicate the eddy, reduced-pressure and cavitation zones in a simple manner at the same time. 
     Accordingly, the invention relates to a hydraulic machine of the aforementioned type, in which said means injecting taps flow from the ceiling or from the belt by means of openings formed in the ceiling or in the belt. 
     The injection of a flow tapped from the ceiling or from the belt makes it possible to effectively make good the lack of performance in the action of the main flow in these zones, which improves the performance and behaviour of the hydraulic machine, and to choose precisely the places where the tapped flow must be injected to eliminate the cavitation zones and the reduced-pressure zones or the eddy zones. 
     According to other features of the hydraulic machine:
         the injection means comprise at least one duct comprising an inlet tapping off a flow from the main flow upstream of the rotor and an outlet opening into the eddy or reduced-pressure or cavitation zone,   the injection means comprise a valve placed in the path of the tapped flow, the said valve being able to be moved between an open position in which it allows the tapped flow to pass from the main flow and a closed position in which it prevents the passage of the tapped flow,   the movement of the valve is controlled by control means,   the openings made in the ceiling or in the belt lead into the spaces between the blades,   the tapped flow passes through openings made in the ceiling or the belt opposite the blades of the rotor and is injected onto the profile of the said blades via orifices arranged in a side wall of the said blades in the vicinity of the upstream end and/or of the downstream end of the said blades,   the tapped flow passes through openings made in the ceiling or the belt opposite the blades of the rotor and is injected into the downstream end of the blades.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects and advantages of the invention will appear during the following description, given as an example and made with reference to the appended drawings in which: 
         FIG. 1  is a partial schematic representation in section of a Francis turbine according to the invention, 
         FIG. 2  is a schematic representation seen from above of the Francis turbine rotor of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention described below applies to hydraulic machines of the Francis turbine type. Since this machine is known, it is not described in detail in the present description. The invention also applies to other types of hydraulic machines in which problems of the formation of eddy, reduced-pressure or cavitation zones occur. 
     In the description, the terms “upstream” and “downstream” are defined with respect to the direction of flow of the main flow E traversing the hydraulic machine. 
       FIG. 1  shows a Francis turbine  2  comprising a wheel  4  comprising blades  6  arranged between a ceiling  28  and a belt  30 . 
     In the case of a blade  6 , there is a problem of the creation of cavitations on the profile of the blades  6  of the rotor  4  in a zone in the vicinity of the inlet edges or upstream end  8  and/or of the outlet edges or downstream end  10  of the blades. In order to alleviate this drawback, the blade  6 , shown in  FIG. 1 , comprises ducts (not shown) extending inside the blade between an inlet opening  22  and an outlet opening  24 ,  26 . The inlet openings  22  of the ducts are placed in the vicinity of the upstream end  8  of the blade  6  so as to tap off a flow from the main flow E upstream of the blade. The outlet openings  24 ,  26  of the ducts are arranged to inject the tapped flow on the side walls of the blades  6  in the vicinity of the upstream end  8  and/or of the downstream end  10  of the blade  6 . The effect of the tapped and injected flow is to locally modify the main flow E and thereby to prevent the phenomena of forming cavitation on the profile of the blades. Certain ducts therefore comprise an outlet opening  24  leading into a side wall of the blade  6  in the vicinity of the upstream end  8  in order to prevent the phenomena of forming cavitation on the blades in the vicinity of the upstream end  8 . Other ducts comprise an outlet opening  26  opening into a side wall of the blade  6  in the vicinity of the downstream end  10  in order to prevent the phenomena of forming cavitation on the blades in the vicinity of the downstream end  10 . 
     According to various embodiments, the inlet and outlet openings may be placed in series along the upstream end  8  and the downstream end  10  of the blade  6  in a direction which may be perpendicular to the direction of the main flow E, as shown by the outlet openings  24  of  FIG. 1 . 
     According to a particularly advantageous embodiment, outlet openings are arranged so as to open into the downstream end  10  of the blade  6  in the direction of the main flow E. The injection of the tapped flow into the downstream end makes it possible to eliminate the eddy zone which is formed in the trail of the blades  6 . The tapped flow is, for example, injected into the base of the downstream end  10  of the blade  6 . 
     The blades  6  of the rotor  4  are placed between a ceiling  28  and a belt  30 . 
     According to the invention, the phenomena of cavitation on the blades are prevented by openings  31  made in the ceiling  28  opposite the blades  6 , as shown in  FIG. 2 . These openings  31  communicate with the outlet openings  24  and  26  and with the outlet openings opening into the downstream end  10  by means of channels not shown. The flow E 2  is tapped off from the main flow E supplying the Francis turbine  2  upstream of the blades  6 . The tapped flow E 2  passing between the fixed part and the moving blades of the turbine  2  then entering an annular space situated above the ceiling  28  may, for example, be conveyed by means of ducts not shown. This flow E 2  enters the openings  31  and is then guided to the outlet openings  24 ,  26 . 
     In addition to the cavitation phenomena on the blades, there may also be phenomena of forming a vortex in the space  33  between the blades  6 . These phenomena may be alleviated by means of ducts, the inlet and outlet orifices of which are placed between the upstream and downstream ends of the blades and open into the space  33  between the blades. According to one embodiment, the problem of forming a vortex between the blades  6  is solved by means of orifices  32  made in the ceiling  28 , as shown in  FIG. 1 . 
     In this embodiment, the flow E 2  tapped off in the annular space  34  travels into the openings  32  and supplies the spaces  33  between the blades  6 , as shown in  FIGS. 1 and 2 . The openings  32  are distributed in the ceiling  28  facing the spaces  33  separating the blades  6 . Therefore, the tapped flow E 2  is injected between the blades  6  and modifies the properties of the flow E in order to prevent the phenomena of forming a vortex between the blades  6 . 
     As a variant, instead of or in addition to travelling via the ceiling  28 , the tapped flow E 2  can travel through the belt  30  by means of openings (not shown) made in the latter. 
     The openings  31 ,  32  formed in the ceiling  28  and/or the belt  30  thus make it possible to overcome, in a simple manner and altogether, the problems of formation of cavitation zones on the blades, of vortex-formation zones between the blades and of eddies downstream of the blades. 
     According to an embodiment that can be applied to all the injection means described above, the injection means comprise a valve  72  placed in the path of the tapped flow, as shown in  FIG. 1 . The valve  72  can be moved between an open position in which it allows the tapped flow to pass and a closed position in which it prevents the passage of the tapped flow. The valve  72  is for example placed in the vicinity of each inlet opening of the injection means and makes it possible manually or automatically to control the injection of the tapped flow. In the case of the Francis turbine, the valve  72  is provided in the vicinity of each opening  32  arranged in the ceiling  28 . 
     The movement of the valve  72  is controlled by control means (not shown) which are mechanical or electric in a manner known per se. Therefore, during operating conditions of the hydraulic machine causing the formation of eddy or reduced-pressure or cavitation zones, an automatic system or an operator of the machine switches the valve(s) to the open position which makes it possible to inject the tapped flow in the said zones and to prevent the formation of these zones, as described above. 
     It should be noted that the tapped flow is not modified relative to the main flow E, that is to say that the water does not sustain any operation to modify its composition during the tapped flow.