1. Field of the Invention
The present invention relates to a turbine wheel and to a Pelton-type turbine equipped with such a wheel.
2. Brief Description of the Related Art
Pelton turbines are conventionally used for transforming the kinetic energy of a fluid, such as the water of a waterfall, into mechanical energy. Such transformation is effected by rotating the turbine wheel under the effect of a tangential effort exerted, on buckets disposed on the periphery of a wheel rim, by a jet of water emerging from one or more injector nozzles distributed around the wheel.
The effort created on each bucket is transmitted to the wheel rim through an area of attachment of the bucket on the wheel rim. This area of attachment, defined by the geometrical intersection of the bucket with the rim, is subjected to considerable mechanical stresses. In effect, each bucket may be mechanically modelized as a beam fixed on one side only, with the result that it undergoes a considerable bending strain due to the effort of the jet. In addition, high vibratory stresses are applied to the bucket and there is a danger of resonance of the bucket or the rim.
In known devices, particularly of EP-A-0 346 681 or of EP-A-0 522 336, attempts have been made to overcome these drawbacks by reinforcing the area of attachment of the buckets on the wheel rim. This leads to structures which are complicated and therefore expensive, and of reduced reliability. In addition, each bucket remains attached to the rim by one side, with the result that the nature of the problems encountered is not fundamentally changed.
FR-A-704 875 teaches making the buckets of a turbine abut on one another by means of rings mounted on their periphery. The distribution of the efforts is not optimum and arms must be interposed between adjacent buckets, being mounted with the greatest care, which results in a non-homogeneous distribution of the efforts.
It is a particular object of the present invention to overcome these problems by proposing a turbine wheel of Pelton type in which the buckets are connected to the wheel rim so that the mechanical stresses are not concentrated in an area of attachment of the bucket to the rim and the dangers of resonance of the wheel are substantially reduced thanks to an optimalized distribution of efforts.
In that spirit, the invention relates to a wheel of the above type, which comprises at least one annular disk fast with the wheel rim and provided with housings for receiving the buckets. This disk is adapted to absorb at least part of the efforts transmitted by the buckets to the rim and forms a peripheral band at the level of the outer radial areas of the housings.
Thanks to the invention, the disk which is on one piece and surrounds the buckets, makes it possible to support them, near the rim, in an intermediate area and in an outer radial area, with the result that each bucket is maintained efficiently in its inner, outer and central radial areas and that the effort to which it is subjected, due to the existence of the jet of fluid driving the turbine, has for its effect to apply it on the disk. This effort is in that case not concentrated solely in one area of attachment of the bucket to the wheel rim, but distributed over the whole of the disk. In other words, the tangential effort created on a bucket is transmitted to the disk. The bucket and that part of the disk supporting the bucket may be mechanically modelized as a beam on two bearings, and even a beam bearing on its length, the first bearing being towards the wheel rim, while the second bearing is constituted by the peripheral band of the disk. Such a distribution of the efforts induces a reduction of the stresses exerted on the mechanical components, which makes it possible to envisage pieces of lesser thickness.
Taking the foregoing into account, a turbine wheel according to the invention may be made of metal, composite material or be a mixed structure. For example, certain pieces, such as the wheel rim or the disk, are made of metal, while other pieces, such as the buckets, are made of composite synthetic material. A wheel rim or a disk made of composite plastics material may also be associated with metal buckets.
In addition, the presence of the outer peripheral band of the disk, which creates a continuity of matter outside the wheel, makes it possible substantially to reduce the vibratory stresses exerted on the buckets. In effect, the peripheral band contributes to distributing these stresses on the different buckets, with the result that the equivalent inherent mode of each bucket is no longer that of a beam fitted at one end only, as in the devices of of the prior art, but that of a beam fitted on one side and bearing on or fitted on the other side.
Finally, it has been surprizingly found that the presence of the disk makes it possible very substantially to improve the aerodynamic performances of the wheel of the invention. In effect, the disk makes it possible to limit the so-called xe2x80x9cventilatorxe2x80x9d effect of the wheel during rotation, i.e. the recirculation of the air between the outside of the wheel and the spaces between two consecutive buckets, due to the movement of rotation. Such reduction of the transverse air movements with respect to the buckets was able to be quantified, by laboratory tests, at about 20% of the braking force due to the ventilation of the wheel in the absence of disk. In this way, the invention also enables the yield of a Pelton turbine wheel to be substantially improved.
According to a first advantageous aspect of the invention, the wheel comprises two disks disposed on either side of one or more injector nozzles of the turbine. This aspect of the invention makes it possible to dispose the disks opposite each bowl or scoop of the buckets when the buckets are double, as is conventionally the case in Pelton turbines. In that case, the disks may be provided to be substantially aligned on the median axes of the scoops of the buckets.
According to another advantageous aspect of the invention, the disk or disks is/are provided with spokes each adapted to substantially fill the space separating two adjacent buckets of the wheel. Thanks to this aspect of the invention, the spokes present a sufficient cross-section to absorb the mechanical stresses transmitted by the buckets. Moreover, the fact that they substantially fill the space separating two buckets makes it possible to optimalize the reduction of the xe2x80x9cventilatorxe2x80x9d effect.
According to a first embodiment of the invention, the disk or disks is/are in the form of a perforated ring, means for removably mounting the or each disk on the wheel rim being provided near an inner edge of the or each disk.
According to a second embodiment of the invention, the disk or disks is/are formed in one piece with at least part of the wheel rim. Thanks to this aspect of the invention, transfer of the efforts of the disk towards the wheel rim is optimalized. In addition, assembly of the turbine wheel may be facilitated. In that case, the wheel rim may be provided to be formed by two half-rims, each half-rim extending radially towards the outside by a disk.
According to another advantageous aspect of the invention, applicable whatever the embodiment, the buckets are removably fixed on the disk or disks and/or the wheel rim. This aspect of the invention guarantees a good dimensional stability of the wheel during use and an efficient transfer of the efforts from the buckets towards the wheel rim. In addition, the operations of maintenance on the buckets or the disks are facilitated.
According to another aspect, the bucket may be permanently fixed on the disk or disks and/or the wheel rim.
The invention also relates to a Pelton-type turbine comprising a wheel as described hereinbefore. Such a turbine may be manufactured more economically than the prior art ones since it is possible to optimalize the dimensioning of the pieces by taking into account the fact that the mechanical stresses are better distributed over the wheel. In addition, the yield of this turbine is improved by reduction of the ventilator effect.
The invention will be more readily understood and other advantages thereof will appear more clearly from the following description of two embodiments of a turbine wheel in accordance with its principle, given solely by way of example and made with reference to the accompanying drawings, in which: