SUMMARY
Diversion tunnels are often used as tailrace tunnels in underground hydropower stations. The special layout results in complex flow regimes, including air-water two-phase flow. A set of experiments is conducted based on the model of a hydropower station which combines partial diversion tunnels with tailrace tunnels to investigate the interactions between the air and water phases in the combined diversion tunnels. Interactions between the air and water phases observed in the combined diversion tunnel significantly alter flow dynamics, and are classified into four types according to the initial tail water level. There is a range of initial tail water levels in which the interaction between the air and water phases cannot be neglected, and the range becomes greater when the change in flow rate increases. Such interactions may cause a pressure surge and the pressure surge reaches the maximum when the initial tail water level is approximately equal to the crown of the tunnel. The surge pressures do harm to the safety and stability of hydropower stations, so the condition should be considered and controlled.