Hydraulic Characteristic Analysis of Changing Top-Altitude Tail Tunnel and its Application

For some hydropower stations with a proper length tail tunnel, if its tailrace level is varied within a certain range, changing top-altitude tail tunnel instead of tail surge tank with large area is a reasonable choice to improve the min. pressure at draft tube inlet section. After the basic differential equations were deduced based on the characteristic implicit model for the transient flow in the changing top-altitude tail tunnel and some complex boundary conditions were built, a unified algorithm tracing the separation point was given combined with characteristic method in pressurized pipeline, improved slot model in channel and mathematical method in state equations. For a given case, the hydraulic characteristic of changing top-altitude tail tunnel was analyzed by this unified algorithm and further comparison was performed with the tail surge tank plan. The results indicate that, for the changing top-altitude tail tunnel, the computational results of its large fluctuation transient process are satisfied with the requirement of regulation guaranteed computation effectively; From the viewpoint of the effect of hydraulic disturb and small fluctuation on operation unit’s stability and regulation performance, it is superior to the tail surge tank plan.