An Enhanced Multi-Shaft Model for Transient Mixed Flows in Large-Scale Pipeline Filling Applications

Long-distance water conveyance systems often experience free-surface-pressurized flow transitions and air pocket entrapment during filling, which may trigger hazardous phenomena such as air explosions and geysering. Existing models typically lack sufficient predictive accuracy due to oversimplified descriptions of dynamic air exchange and multi-shaft ventilation coupling mechanisms. To resolve this limitation, we propose an enhanced AirSWMM model integrated with a comprehensive ventilation calculation module. The model adopts a unified air pocket formulation and simulates real-time air exchange via predefined ventilation areas along the pipeline. Experimental validation confirms its reliability in predicting key hydraulic parameters, including filling duration, pressure variation, and flow rates. When applied to a prototype project, the model classifies the filling process into four distinct phases based on gas release characteristics and air–water interface movement: initial pressurization, advancing pressurized flow with free venting, system-wide pressurized flow with intermittent venting, and full-pipe flow with terminal intermittent venting. This study provides a robust numerical tool for the safety-oriented management of filling operations in multi-shaft water conveyance systems, delivering practical insights for engineering design and operational optimization.