The present invention relates to a Francis-type runner, and, more particularly, to the contour of a runner blade at the runner entrance.
In general, a Francis-type runner comprises a crown ring, a shroud ring and a plurality of runner blades interconnected therebetween. Pump-turbines having conventional Francis-type runners have dual characteristics of operation in which two efficiencies correspond with a given rotational speed of the runner within a predetermined range during the operation of the turbine when the opening of the runner blade remains constant.
The reason for the dual characteristics of operation is that flowing water is separated from the runner blades on the side of the crown ring at the runner entrance. That is, if consideration is given to the case where the rotational speed of the runner and the angle of the absolute velocity of water entering the runner are maintained constant and the head of water is reduced, the direction of the relative velocity of water flow at the runner entrance forms a large negative angle relative to the working surface of the runner blade because the flow rate of the entering water is decreased. Consequently, flowing water does not flow smoothly at the working surface of the runner blade and is separated therefrom.
As described above, when the phenomenon of the separation of flowing water occurs, the flow rate of flowing water or the work done on the working surface of the runner blade is changed, thereby increasing or decreasing the efficiency of the pump-turbine.