The present invention relates generally to hydroelectric turbine or pump installations.
As is well known in prior art, the damaging effects of cavitation in hydraulic machines can be reduced by the introduction of air. For example, U.S. Pat. No. 2,803,428 to W. R. Garnett discloses a hydraulic machine with wicket gates whereas air is introduced through the wicket gates into the fluid passage ahead of the runner. Another positive effect of the introduction of air is the reduction of vibration and noise, which can occur especially at low flows.
The objective of the present invention is to increase the amount of air introduced through a wicket gate over the amount of air achieved by a state of the art wicket gate for a given air input source.
The present invention provides a wicket gate of a hydraulic turbine or pump which is capable of introducing high amounts of air.
In some embodiments the invention will even function with an air input source at atmospheric pressure, avoiding the expense of an air compressor.
The problem is solved by a wicket gate according to claim 1. Other favorable implementations of the invention are disclosed in the depended claims.
The inventors have recognized that the problem can be solved by altering the geometry near the trailing edge of the wicket gate to create a local drop in pressure on the trailing edge surface.
The invention will hereinafter be described in conjunction with the appended drawings:
FIG. 1 is a cross-sectional view of a hydraulic machine comprising a wicket gate according the present invention;
FIG. 2 is a cross-sectional view of a hydraulic machine comprising a wicket gate according the present invention;
FIG. 3 is a cross-sectional view of a wicket gate blade according to a first embodiment of the present invention;
FIG. 4 is a cross-sectional view of a wicket gate blade according to a second embodiment of the present invention;
FIG. 5 is a cross-sectional view of a wicket gate blade according to a third embodiment of the present invention.
FIG. 1 displays schematically a cross-sectional view of a hydraulic machine. The hydraulic machine comprises wicket gates. FIG. 1 displays two of them designated as 1. Each wicket gate 1 comprises a blade 2, a first trunnion 3 and a second trunnion 4. The blade 2 comprises a leading edge 5 and a trailing edge 6. The wicket gate 1 on the right side of FIG. 1 comprises an air passage 7, an air inlet aperture 8 and two air outlet apertures each designated as 9. The air inlet aperture 8 is located at first trunnion 3 and the air outlet apertures are located on the trailing edge 6 of the wicket gate blade 2. The distance between leading edge 5 and trailing edge 6 is known as the wicket gate length (L).
During the operation of the hydraulic machine, air enters the air inlet aperture 8 flows through the air passage 7, passes the air outlet aperture 9, and enters thus the working fluid of the hydraulic machine.
FIG. 2 displays schematically a cross-sectional view of a hydraulic machine. The only difference to FIG. 1 is that the air inlet aperture 8 is located at the second trunnion 4.
In FIGS. 1 and 2, one of the air outlet apertures 9 is located in the region of the upper end of the wicket gate blade 2, viz. in the region of the wicket gate blade 2 adjoining the first trunnion 3. Air admission in this region is favorable for mitigation of vibration and noise, which are caused primarily by vortices that initiate near the runner crown, which is designated by 10. The other air outlet aperture 9 is located in the region of the lower end of the wicket gate blade 2, viz. in the region of the wicket gate blade 2 adjoining the second trunnion 4. Air admission in this region is favorable for mitigation of cavitation. The number and location of air outlet apertures 9 is of course not restricted to the number shown in embodiments in FIGS. 1 and 2. Of course, the present invention will also work with only one air outlet aperture 9.
FIGS. 1 and 2 display a hydraulic machine having a vertical axis of rotation. Accordingly, the wicket gates are orientated vertically too. Of course, the invention is not limited to wicket gates for hydraulic machines having a vertical axis of rotation.
FIG. 3 is a cross-sectional view of a wicket gate blade according to a first embodiment of the present invention. FIG. 3 displays the portion of the wicket gate blade near the trailing edge in a region where an air outlet aperture 9 is located. The pressure side surface of the wicket gate blade is designated as 13 and the suction side surface of the wicket gate blade is designated as 14. The point designated as P1 is located on the suction side surface 14 of the trailing edge 4. The point designated as P2 is located on the suction side surface 14 of the wicket gate blade. The distance between point P1 and point P2 is less than 10% of the wicket gate length L measured in the direction perpendicular to the trailing edge. The arrows are indicating the normal vectors on the suction side surface 14 of the wicket gate blade, whereas the normal vector designated as N1 is located at point P1 and the normal vector designated as N2 is located at point P2. According to the prior art the profile of the suction side surface 14 between the points P1 and P2 is straight or slightly convex meaning that the orientations of the normal vectors N1 and N2 are the same (or differing only very little) or are pointing away from each other. According to the present invention, the profile of the suction side surface 14 between the points P1 and P2 is concave.
During operation of the wicket gate, the working fluid of the hydraulic machine is flowing from the leading edge towards the trailing edge. Therefore point P2 is located upstream of point P1.
This special geometry near the wicket gate trailing edge has not been applied in the hydro industry since it would normally result in higher dynamic loading on the wicket blades due to the resulting increased strength of von Kárman vortices. The inventors have realized that this problem can however be overcome since the flow of gas through the openings at the trailing edge is mitigating the formation of the vortices.
FIG. 4 shows a second embodiment of the present invention. The difference to the embodiment of FIG. 3 is that the pressure side surface 13 near the trailing edge is not straight as in FIG. 3 but concave.
FIG. 5 shows third embodiment of the present invention. The difference to the embodiments of FIGS. 3 and 4 is that the pressure side surface 13 near the trailing edge is convex.
The inventors have realized that the positive effect of the invention increases, if the profile of the suction side surface 14 between the points P1 and P2 is concave and the angle between the normal vectors N1 and N2 is at least 2 degrees. Because the profile between P1 and P2 is concave, it is clear that the vectors N1 and N2 are pointing towards each other.
The above-described embodiments of the present invention can be modified in a way that the pressure side 13 of the wicket gate blade extends further than the suction side 14 in the direction to the trailing edge of the blade.
Patent Application
20210088022
