Relationship between two-phase flow in bottom outlet and air-core vortices at intake

Model experiments were carried out into the hydraulic characteristics of air−water two-phase flow inside the bottom outlet of shaft spillways as a result of natural air suction from air-core vortices at the pipe intake. Experiments were performed in a cylindrical reservoir 2 m in diameter and 1 m high, under a swirling flow around the intake, investigating the most severe two-phase flow regimes. Vertical pipe intakes with three different diameters and heights, attached to the floor of the reservoir at its centre, were examined for different flow discharges by establishing different vortex types. The results including tangential velocity profiles inside the main reservoir, head−discharge relationships, average internal pressure inside the outlet pipe and the relation between swirling flow at the pipe intake and the two-phase flow inside the bottom outlet are discussed in this paper. Results indicate that the void fraction decreases by changing the type of swirling flow. For a constant submergence depth, reducing the intake pipe diameter or increasing its height lead to an increase in void fraction and two-phase slug flow wave Froude number, velocity, length and frequency. However, by the increase in Kolf number, the frequency of slug waves reduced and the average relative two-phase flow pressure inside the bottom outlet was increased. The relationships between two-phase flow hydraulic characteristics and regimes inside the bottom outlet and types of vortices at the intakes will be of the interest of both the scientific research and design engineers, helping them increase the efficiency and safety of pressurised waterway systems.