Influence of mixed free-surface and pressurized flow on transients process in a tailrace tunnel of a tailrace surge chamber

Abstract In the calculation of hydraulic transients in a hydropower station, the hydraulic problems caused by free-surface and pressurized flow are relatively complex. In consideration of this phenomenon, numerical simulation is used in this paper to study the effect of the transient free-surface, pressurized flow in a tailrace tunnel on the minimum pressure at inlet of the draft tube and the lining segment of the tailrace tunnel and the effect on the primary frequency regulation of the unit. The results show that the existence of tailrace surge tank can reduce the disturbance of the mixture flow on the units. Meanwhile, free-surface and pressurized flow can speed up the attenuation of the surge in the surge tank. Reasonable arrangement can mitigate transients in the tailrace system, which may provide a reference for similar projects.