[0001] This invention relates to hydraulic turbines, such as the Francis- type hydraulic turbine. The invention particularly relates to reducing pressure pulsations in a hydraulic turbine by optimizing the shape of the flow passage through the turbine.

[0002] A hydraulic turbine typically includes a rotating runner in stationary housing. As water, or another liquid, flows through a flow passage in the housing the flow produces mechanical work by turning the runner. The shaft of the runner may drive an electrical generator or other rotary device. As water flows through the runner, pressure pulsations generally result from a cavitating vortex rope formed in the flow immediately downstream of the runner. These pulsations act on the runner and propagate downstream into the draft tube that receives the water discharged from the runner.

[0003] The pressure pulsations may arise while the turbine is in a part- load operating range or in a full load or overloaded operating range. The pressure pulsations generated can become severe and damage the turbine and its runner, and the draft tube. In particular, the pulsations may cause the turbine and housing to vibrate and cause noise. The vibrations may fatigue the components of the turbine and housing. The vibrations may also cause the turbine to be unstable at certain operational conditions. The operation of the turbine may be limited to avoid conditions that cause severe pressure pulsations.

[0004] The vortex rope is typically a cavitating vortex (referred to as a "rope") downstream of the cap on the runner. For example, a helicoidal vortex (rope) may form downstream of the cap during a part-load condition and rotate at about one-third the rotational speed of the runner. A symmetric vortex (rope) may form downstream of the cap while the turbine is operating at or above its full-load operating power.

[0005] A conventional approach to reducing vibration in a hydraulic turbine is to inject air into the flow passing through the turbine. The air dampens the pressure pulsations and increases water pressure downstream the runner to avoid cavitation. Injecting air tends to reduce the hydraulic efficiency of the turbine and often requires a costly air compressor and air conduits to feed pressurized air to the turbine. In view of the decrease in efficiency and the cost of injecting air, there is a long felt need for techniques to reduce the severity of pressure pulsations in a hydraulic turbine.

BRIEF DESCRIPTION OF THE INVENTION

[0006] A cap that extends the runner crown or hub in a hydraulic turbine has been conceived that reduces the pressure pulsations in the draft tube. The cap expands in its cross-sectional area in a downstream direction. The expanding cap has been shown to significantly lower the pressure pulsations in a draft tube and to have no significant impact on the overall hydraulic efficiency of the turbine.

[0007] A hydroelectric turbine has been conceived that includes: a crown or hub configured to connect to a shaft; an annular array of runner blades connected to an outer surface of the crown or hub; a trailing edge of each of the runner blade; a cap extending downstream from the crown or hub, wherein the diameter of the cap expands in a downstream direction and an outer surface of the cap exposed to a flow passage through the turbine has a diameter that is entirely substantially greater than a diameter of the downstream end of the crown or hub. The downstream end of the cap may be flat and in a plane perpendicular to a rotating axis of the shaft. Alternatively, the downstream end of the cap may be tapered such as in a hemispherical or conical shape.

[0008] A cap has been conceived that is configured for a runner in a hydroelectric turbine, wherein the runner includes a crown, and an annular array of runner blades connected to an outer surface of the crown, and the cap comprises: an upstream rim configured to seat on an outer surface of the crown, an outer surface configured to be generally symmetrical to an axis of the crown, wherein the diameter of the outer surface increases in a downstream direction and the diameter of the outer surface that is entirely substantially greater than a diameter of the crown where the cap seats on the crown.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIGURE 1 is perspective view of the bottom and side of a runner for a hydroelectric turbine having an expanding cap on the crown of the runner.

[0010] FIGURE 2 is a side view of the runner with portion cut away to show the cap on the crown.

[001 1] FIGURE 3 is a cross sectional view of the runner having the expanding cap on the crown.

[0012] FIGURE 4 is a cross sectional view of an alternative runner having an expanding and contracting cap on the crown. DETAILED DESCRIPTION OF THE INVENTION

[0013] FIGURES 1 , 2 and 3 illustrate an exemplary runner 10 for hydroelectric turbine 12. The runner 10 has at its center a crown or hub (collectively referred to as the "crown") 14. An expanding cap 16 is fixed to the downstream end of the crown.

[0014] The runner 10 includes blades 18 arranged in an annular array around the crown 4. The inner edges of the blades are attached to the outer surface of the crown. The outer edges of the blades are fixed to a band or shroud (collectively referred to as a "band") 20.

[0015] The runner is mounted in a housing (not shown) that has a water inlet that receives water through an inlet passage 22 oriented generally perpendicular to a rotating axis 24 of the runner. The water flows through and is turned by the runner to a flow direction general parallel to the axis 24. The water is discharged from the runner into a draft tube 26 which is adjacent the downstream rim of the runner. The water flow through the blades turns the runner and drives a shaft 28 that may be coupled to an electrical generator or other mechanical load.

[0016] As the water flows over the end of the cap 16, a flow vortex 30 may form. The cap extends the crown downstream and moves downstream the point in the turbine where cavitating flow vortices form due to the crown and cap assembly.

[0017] The flow vortex, along with other flow conditions such as cavitation, may generate pressure pulsations in the draft tube and runner. It is believed that the expanding shape of the cap 16 suppress the pressure pulsations in the flow, such as by reducing the vortex and minimizing cavitation on the side surface of the cap.

[0018] The cap 16 expands in its cross-sectional area in a downstream direction. The narrowest region of the cap is at or near the annular junction 32 between the crown and cap. This junction may be substantially smooth as the upstream rim seats on the outer surface of the crown. The diameter of the crown at the transition junction 32 may be substantially equal to the smallest diameter of the cap. The upstream rim of the cap may be proximate to the downstream corners 34 of the blades where the trailing edges 36 of the blades join the crown.

[0019] The cap 16 expands radially outwardly from the crown 14 in a downstream direction. The cap 16 may have a shape that is generally frustoconical, conical or bulbous. A cylindrical region 38 of the cap may be at the upstream end of the cap. As an alternative to the cylindrical region, the cylindrical region of the cap may expand or include a slight constriction. A slight constriction may cause the smallest cross-section of the cap to have an area of at least ninety percent (90%) of the cross- sectional area of the upstream rim of the cap.

[0020] The cylindrical region 38 of the cap, if it is present, may be short compared to the overall length (L) of the cap, such as being twenty percent to zero percent of the length (L). Downstream of the upstream region, the cap has an expanding portion 40 that may comprise the entire length (L) of the cap or the length of the cap except for the cylindrical region 38.

[0021] A downstream rim 42 of the cap may have the largest diameter of the cap. The downstream rim 42 of the cap may be at or near a plane that includes the downstream edge 44 of the band 20 of the turbine. The downstream rim 42 of the cap may be axially offset from the downstream edge 44 of the band by twenty percent or less of the diameter (D) of the runner. In other embodiments the downstream rim 42 of the cap may be substantially upstream, e.g., by a distance of greater than 20 percent of D, or substantially downstream of a plane that includes the downstream edge 44 of the band. The selection of the position of the downstream rim 42 of the cap relative to the axial position of the downstream edge 44 of the band may depend on whether the turbine is intended to operate at part load or full (or greater) load conditions.

[0022] The expanding portion 40 of the cap 14 may have an outer surface 48 with a slope with respect to the vertical axis 24 of the shaft and runner. The slope may be at an angle (φ) in a range of one degree to thirty-five degrees. The slope may be linear, or may curve such as in an exponential shape. The slope may include abrupt transitions, e.g., discontinuities, such as shown in the transition 50 between the cylindrical portion 38 and the expanding portion 40 of the cap. The surface texture of the cap may be substantially similar to the surface texture of the blades, crown and band or shroud of the turbine.

[0023] The cap 14 may be used to extend the crown in hydraulic turbine, such as Francis turbines, Kaplan turbines, diagonal or mixed flow turbines, pump-turbines and propeller turbines. The cap 14 may be attached to or integral with the crown. If integral with the crown, the demarcation between the crown and cap may be a narrow region of the crown proximate to the downstream corners 34 of the blades where they join the crown. The cap 14 may be formed of the same material as the crown, or may be formed of another material, e.g., a metal, selected to withstand the environment in the turbine. The cap may be hollow or solid, and may have connection devices, e.g., brackets and bolts, to secure the cap to the crown.

[0024] Testing of models of the cap 14 in a hydraulic laboratory has shown the cap to have a considerable effect on the pressure pulsations during part-load operation while a cavitating rope was formed downstream of the cap. The testing also indicated that an expanding cap actively suppresses the formation of cavitation on the side of the cap during full-load operation. The testing showed that the expanding cap did not significantly affect the overall hydraulic efficiency of the turbine.

[0025] FIGURE 4 is a cross sectional view of an alternative cap 60 on the crown 14 of a runner 10. The runner and crown shown in Figure 4 are similar to the runner and crown shown in Figures 1 to 3 and thus common reference numbers are used for the crown and runner in all figures. The alternative cap 60 may be similar to the cap 16 shown in Figures 1 to 3, except that the alternative cap 60 includes a downstream region 62 that tapers radially inward. The downstream region may be conical or bulbous. The alternative cap may be used, for example, in a pump-turbine. During pumping operation of a pump turbine, the water flow is up through the draft tube and into the runner. This water flow during pumping is opposite to the water flow direction during power generation operation of the pump-turbine. The tapered downstream region of the cap assists in directing the water flow into the runner.

[0026] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.