Abstract: An experimental platform is constructed to test the stagnant air mass in the top bend section (like an inverted siphon bend) of a pressurized pipeline, and water flow velocity in this bend section is measured using a particle image velocimetry (PIV) system, focusing on the influence of flow velocity and the local pipeline slopes on the motion characteristics of residual air mass. To simulate numerically the flow details in the bend section, we construct a local gas-liquid two-phase fluid dynamics model equipped with a RSM turbulence model and a VOF multiphase flow model, and use it to analyze the influence of water velocity, local pipeline slopes, and air mass volume. The results show that with the increasing water flow velocity, the residual air mass presents three flow regimes: no bubble generated, air bubble generated but discharged partially, and air mass discharged at one time. If other conditions are the same, the threshold water flow rate required for the third regime is relative low when the trapped air volume is greater than a certain value; the threshold water flow velocity for the third regime become greater at larger local slopes of the top bend section.

Key words: pressurized hydraulic pipeline, residual air mass, PIV measurement technology, motion characteristics