Study on the disruption of water flow in the system of tailrace tunnel combined with diversion tunnel

Abstract

In order to solve the space-intensive problems of large-scale underground hydropower stations, diversion tunnel is usually used as the end part of the tailrace tunnel and connected with the front one by an abrupt slope. However, shallow water depth and large velocity happened at the slope. And the disruption of water flow even probably happened after the unit load rejection. According to CFD numerical calculations of the transition process, the mechanism of water flow disruption was analysed in this paper and the main conclusion is drawn on that Froude number Fr at the slope, especially the end section of the slope, is smaller than 1 at first, increased to 1 and then greater in the transient process. The formation of wave breaking blocked the supply of the reverse flow. In this way, Fr>1 can be regarded as the internal cause of disruption. Furthermore, the water depth at the combined point was lowered by the increasing water level of the surge tank. Inevitably, the disruption would occur when the water level of the surge tank is rising.