An investigation into the transient process of water depletion in surge chambers

ABSTRACT This study investigates the transient hydraulic behavior of water depletion in surge chambers connected to pumped storage stations. A mathematical model is developed based on the one-dimensional transient flow theory and the method of characteristics to capture the dynamics of gas–liquid two-phase flow and the entrapped air pocket. The model integrates the discrete free-gas cavity model and incorporates the thermodynamic properties of air. It simulates the gas–liquid two-phase flow and the evolution of the entrapped air pocket. The numerical simulation results demonstrate that a direct water hammer occurs at the point of air discharge, causing a sharp pressure rise. The diameter of the connecting pipe plays a critical role: for larger diameters, bottom pressure decreases with the increasing diameter, while smaller diameters result in the formation of an entrapped air pocket and the air cushion effect, amplifying pressure. The numerical simulation result is consistent with the previous experimental studies on the gas–liquid two-phase flow. The model of the entrapped air pocket interacting with the rigid water body reveals that the short-term pressure extremum induced by the entrapped air pocket can exceed that of standard water depletion conditions. To prevent water depletion in surge chambers, several mitigation strategies that can be implemented in engineering applications are proposed.