Air-core characteristics in a swirling tunnel flow

A cost-effective technique to dissipate the energy in hydropower systems is the formation of a swirling flow within a tunnel. In such flows, an air core with a significant cross section usually occurs. To reveal the air-core features in the horizontal tunnel of a high-head shaft spillway, laboratory tests, numerical modeling, and prototype observations are performed, to examine issues such as the formation of the air core, the interjacent air motion, the air-carrying capacity, and the scale effects. It is shown that the shape of the air core varies greatly in the axial and radial directions along the tunnel and that the center of the core deviates from the axis of the tunnel. The motion of the air within the core is caused by the combined action of the water entrainment on the inner surface of the swirling flow and the axial pressure difference in the air core. The aeration process can be divided into five processes with respect to the changes of the gate openings. A theoretical expression is established for the air-carrying capacity of the swirling flow. The vacuum degree is the similarity condition of the air-carrying capacity of the swirling flow between the model and prototype tests based on the Froude law of the similitude, and this similarity condition is verified by both the model and prototype results. This work provides a reference for the application of the swirling flows in horizontal hydropower tunnels.