Abstract:
A hydro turbine with a runner unit allows adjusting the gap clearance downstream and between the runner end tip portion and the inner edge of the blades. This gap clearance may depend on the angular position of the runner blades.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments of the present invention generally relate to a hydro turbine, and more particularly, to a runner unit of the hydro turbine. Embodiments may also apply to a tidal turbine. 
       BACKGROUND 
       [0002]    As well known, hydro power plants are arranged to convert into electricity the potential and kinematic energies of rivers and/or lakes as well as sea tides. The turbine housing may include a Kaplan or a bulb runner unit including a plurality of blades fit thereon which are moved by the flow of water. The Kaplan or the bulb runner is integral to a rotating shaft which cooperates with an electricity generator. 
         [0003]    Double regulated turbines conventionally have a flat performance hill chart in flow, nevertheless a significant drop of efficiency can be observed at full load due to a poor performance of the draft tube. 
       BRIEF DESCRIPTION 
       [0004]    It is an object of the present invention to solve the aforementioned technical problem by providing a runner unit for a hydro turbine. 
         [0005]    It is a further object of the present invention to provide a hydro turbine which includes a runner body as described herein. 
         [0006]    According to an aspect of the invention, an increase of the gap clearance between the blades and the runner hub at the downstream of the blade may be an efficient solution to solve the above-mentioned technical problem. Increasing the gap clearance can be performed by increasing the maximum blade angle and consequently the blade cutback to an amount that appears to be necessary to reach the targeted full load operating point. By doing this, the gap clearance between the inner edge blade and the hub is increased downstream and upstream to the blade axis. This is known as “over-tilting”. The gap clearance downstream and upstream may be managed differently according to achievable performance of the turbine. 
         [0007]    By doing so, the hydraulic efficiency may be improved, mainly at full load when utilizing a more divergent runner tip. However, at the optimum load, it could appear necessary to reduce such gap clearance in order at least to keep operations at high efficiency. 
         [0008]    The runner unit according to embodiments of the present invention, as it will be clear from the detailed description of an exemplary and non-limiting embodiment, allows for adjusting the gap clearance downstream and between the runner end tip portion and the inner edge of the blades. This gap clearance depends on the angular position of the runner blades. 
         [0009]    At high load, the runner end tip portion of a double regulated turbine is in a raised, that is retracted, position such to allow a diverging runner tip and thereby a better feeding in flow at the central part of the draft tube cone, below the runner, and at the draft tube itself. 
         [0010]    The runner end tip portion in this position also shows a similar hydraulic behaviour to the one obtained by over-tilting the blades of a conventional runner hub. 
         [0011]    For a lower load which requires a lower blade angle value, the runner end tip portion is moved downstream (according to the axial flow direction), relative to the runner body, to reach an extracted position. Such operative configuration allows to have a similar configuration to the one obtained by means of a non-over tilting of the blade with a conventional runner hub. 
         [0012]    In an embodiment, an over-tilted set of blades allows for reaching a higher output due to a higher blade angle value. 
         [0013]    As a consequence, it also allows shifting power saturation which one can observe on some runner designs due to higher flow. 
         [0014]    According to an aspect of the invention, a runner unit for a hydro turbine is provided, which includes a runner body, at least one blade rotatably mounted thereon and an end tip portion, wherein the end tip portion is movable relative to the runner body between an extended operative position and a retracted operative position. 
         [0015]    According to an aspect of the invention, the runner unit includes a mechanism interposed between the rotatable blade and the end tip portion, the mechanism being configured to adjust the position of the end tip portion based on an angle of rotation of the blade. 
         [0016]    According to an aspect of the invention, the mechanism includes a piston adapted to reciprocate along a shaft of the runner unit and has an upstream portion coupled to the blade to cause its rotation and a downstream portion integral to the runner end tip. 
         [0017]    According to an aspect of the invention, the end tip portion includes an annular seal member coupled to the downstream portion of the piston. 
         [0018]    According to an aspect of the invention, the annular seal member is located inside the runner body when the end tip portion is in the retracted operative position, and protrudes out of the runner body when the end tip portion is in the extended operative position. 
         [0019]    According to an aspect of the invention, the blade is rotatably fitted on the runner body by means of a trunnion portion having an annular lever coupled to the piston. 
         [0020]    According to an aspect of the invention, the annular lever includes a pin eccentric with respect to an axis of rotation of the annular lever, the pin being hosted in a link connected to the piston. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The objects, advantages and other features of the present invention will become more apparent upon reading of the following non-restrictive description of embodiments thereof, given for the purpose of exemplification only, with reference to the accompany drawing, through which similar reference numerals may be used to refer to similar elements, and in which: 
           [0022]      FIG. 1  shows a front sectional view of a runner unit according to the prior art; 
           [0023]      FIG. 2  shows a front sectional view of a runner unit according to embodiments of the present invention, wherein the runner tip is designed according to two different positions as described previously in this document; 
           [0024]      FIG. 3  shows a front sectional view of the runner unit according to embodiments of the present invention when the end tip portion is in the retracted operative position; and 
           [0025]      FIG. 4  shows a front sectional view of the runner unit according to embodiments of the present invention when the end tip portion is in the extracted operative position. 
       
    
    
       [0026]    An exemplary embodiment will be now described with reference to the aforementioned drawings. 
       DETAILED DESCRIPTION 
       [0027]    With reference to  FIG. 1 , a runner unit according to the prior art is shown, generally indicated with numeral reference  100 . Runner unit  100  includes a runner body  2 , where a plurality of blades  3  is rotatably fitted. In the example herewith described rotor body  2  includes four blades, but it will be appreciated that rotor body  2  may include only one blade. Runner body  2  ends with a tip portion  21 . 
         [0028]    Runner body  2  is hollow inside and includes an internal mechanism, generally indicated with numeral  40 , which acts on the blades  3  to cause rotation thereof about an axis R, arranged substantially perpendicular to an external surface of the runner body  2  where the blade  3  is fitted. However, it will be appreciated that axis R may have any inclinations with respect to the external surface of the runner body  2  where the blade is fitted, depending on the particular design selected. The pitch of the blades  3  is regulated by mechanism  40  which depends on the operative conditions to maximise efficiency of the energy conversion process. 
         [0029]    For the runner unit  100 , a gap existing between an inner edge  31  of the blade  3  and the end tip portion  21  is defined and depends on the chosen geometry of runner unit  100 . 
         [0030]    Making reference to  FIG. 2 , a front sectional view of a runner unit  1  according to embodiments of the present invention is shown. Runner unit  1  includes a runner body  2  where a plurality of blades  3  is rotatably fitted and rotatable about the axis R. 
         [0031]    As for the preceding case, runner body  2  in the present non-limiting example includes four blades, but it will be appreciated that it could include one or several blades. 
         [0032]    Runner body  2  terminates with an end tip portion  21 . 
         [0033]    The flow of water along the runner unit  1  causes the runner body  2 , which is integral, usually by bolting, to a shaft  11 , to rotate about a central axis Z for the production of electricity. 
         [0034]    According to embodiments of the present invention, the end tip portion  21  is movable, relative to the rotor body  2  and the shaft  11 , between an extended, or downwards, operative position and a retracted, or inwards, operative position. In particular,  FIG. 2  shows on the same section the two different positions: on the right side the end tip portion  21  is extracted and protrudes downwards, whilst on the left side it is shown the runner unit  1  having the end tip portion  21  in the fully retracted position. 
         [0035]    With the runner unit  1  according to embodiments of the present invention, the existing gap between the inner edge  31  of the blade  3  and the end tip portion  21  may be adjusted, depending on the operative flow condition, to maximise efficiency. 
         [0036]    According to a non-limiting embodiment, runner unit  1  includes an internal mechanism  4  which is interposed between the rotatable blade  3  and the end tip portion  21 . More in particular, mechanism  4  is configured such to act on the blade and cause its rotation about the axis R. Preferably, mechanism  4  may also be associated to the end tip portion  21 , as it will be described below, and configured to adjust the position of the latter based on an angle of rotation of blade  3 . 
         [0037]    Turning next to  FIG. 3 , the runner unit  1  is shown in the retracted operative position. 
         [0038]    Blade  3  is now shown in a frontal view for clarity reasons. 
         [0039]    Mechanism  4  includes a piston  41  (visible in  FIG. 2 ) which is adapted to reciprocate along the shaft  11  of the runner unit  1 . The movement of the piston  41  may be enabled by means of an oil-type drive (not shown). Piston  41  includes an upstream portion  411  which is coupled to the blade  3  to cause its rotation, and a downstream portion  412  which is integral to the end tip portion  21 . 
         [0040]    Blade  3  is rotatably fitted on the runner body  2  by means of a trunnion portion which includes a lever  32 . Lever  32  includes a pin  321  arranged eccentric with respect to the blade axis of rotation R (which in this embodiment is substantially perpendicular to the section plane of the Figure and thus not visible), which is in turn connected to the piston  41  by means of a link member  322 . The movement of the piston  41  along the shaft  11  causes the lever  32  to rotate about blade axis R, and thus such rotation is transferred to blade  3 . 
         [0041]    The end tip portion  21  includes an annular seal member  211  which is coupled to the downstream portion  412  of the piston  41 . When the end tip portion  21  is in the retracted operative position, as shown in  FIG. 3 , the annular seal member  211  is located inside the runner body  2 . In fact, in this configuration, the pitch of the blade  3  is such that the piston  41  is in an upward location. 
         [0042]    Making reference to  FIG. 4 , the runner unit  1  is shown with the end tip portion  21  in the extracted operative position. The extracted end tip portion  21  is associated to a rotation of the blade in a clockwise direction. As the piston  41  moves downwards, the blade  3  rotates clockwise and the annular seal member  211  protrudes out of the runner body  2 . In this way, the end tip portion  21  is movable relative to the runner body  2  such to adjust the gap clearance between end tip portion  21  and the inner edge of the blade  3 . 
         [0043]    In the exemplary embodiment herein described reference was made to the mechanism  4  associated to the blade  3 , but it will be appreciated that the mechanism  4  is also associated in a similar way to all the blades fitted to the rotor body  2 . Therefore the movement of piston  41  will cause synchronous rotation of all the blades. 
         [0044]    It will also be appreciated that movement of the end tip portion  21  may be achieved with a separated servomotor, not directly linked to mechanism associated to the rotation of the blades. 
         [0045]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.