From Reduced- to Full-Scale Validation of a Numerical Model of Air Entry in a Penstock Pipe During Safety Valve Closing

In penstock pipes, upstream of hydropower stations, a safety valve is located at the top in order to cut the flow rate before the downward slope if necessary. During its closing, the pressure downstream reduces whereas the upstream one remains constant. To avoid low pressures in the penstock pipe, air-venting valves are present and open when the pressure inside the pipe is lower than outside. Once opened, the air from the atmosphere flows into the penstock pipe and counters the pressure decrease. The correct design of air entry systems is of primary interest for hydraulic engineering. The present work is focused on the numerical simulation with neptune_cfd of air entry in a penstock pipe during the safety valve closing. To close or open valves, the CFD model uses a discrete forcing method (Immersed Boundary) to allow solid motion without any need for dynamic re-meshing. For the air-venting valve, its motion depends directly on the pressure forces on its two sides (the atmosphere outside, the penstock pressure inside). The two-phase air–water flow is modeled with an Eulerian-Eulerian approach with a multi-regime model solving large gas structures and modeling bubbles. A dedicated reduced-scale closed-loop experiment is used to characterize the flow in terms of pattern and to record pressure evolution in the pipe. The numerical model is validated for different flow rates and for different number of air-venting valves (1–3 with different diameters). Moreover, operating condition data are used to validate the model at full-scale. The numerical model is in a satisfactory agreement with measurements and show its ability to increase our knowledge on the phenomenon of air entry in such configuration.