Author(s): Han Songlin; Hou Xiaoxia; Wang Zhixin; Jiang Zhibing; Chen Duan
Linked Author(s): Duan Chen
Keywords: Water conveyance tunnel; Filling process; Water-air two-phase flow; Flow pattern; Air pockets
Abstract: The filling process is essential and critical to pressurized water conveyance tunnels. This process usually involves rather complicated transients and transitions of water-air two-phase flow, especially for large-scale and long-distance water diversion projects with multiple branches and undulating terrains. To avoid severe water-air interactions, low flow rate filling is normally adopted, although inefficient and time-consuming. Based on the River Diversion Project from the Yangtze River to the Han River, the present study investigates, by means of model tests, the transient water-air two-phase flow during the filling process with a large flow rate. Transitions of flow regimes are analyzed. Distinct flow patterns are revealed when compressed air pockets are released through the lateral ventilation tunnels. It is observed that bubble flow, stratified flow, and plug flow co-exist in the tunnel during the filling process. According to different flow characteristics, the filling process can be divided into four stages: i) pressure flow is formed at the tunnel entrance; ii) pressure flow advances downstream while air is not compressed in the empty tunnel; iii) pressure flow is formed throughout the tunnel while air releases intermittently at lateral ventilation tunnels; and iv) the entire tunnel is fully filled with water while remaining air releases intermittently via the vent shaft at the tunnel exit. Intense water-air interactions are observed during the latter two stages. It is found that the formation of air pockets has an impact on the flow rate along the tunnel. The flow rate upstream of the air pockets decreases, while the flow rate downstream increases for a short time. The experiment results could provide guidelines for the design of filling procedures for large-scale and long-distance water conveyance projects.
Year: 2024