Tunnel-scale design of a floor-mounted electrostatic precipitation system: Piston-wind-assisted ultrafine particle control in subway tunnels

Ultrafine particles (UFPs) generated by wheel-rail interaction can accumulate in subway tunnels and are strongly affected by train-induced unsteady airflow. This study proposes a floor-mounted static two-stage electrostatic precipitation system for tunnel-scale particle control by utilizing piston wind during subway operation. A full-scale numerical model coupling train-induced airflow, UFPs transport, and electrostatic removal is developed to compare piston-wind-driven and fan-driven configurations and to optimize key layout parameters, including module spacing, inlet velocity, and ESP height. The results show that UFPs preferentially accumulate in the near-ground region beneath the train, supporting the feasibility of floor-mounted particle interception. The piston-wind-driven system provides passive purification without additional ventilation demand, whereas the fan-driven system improves removal only when the imposed inlet velocity exceeds the local piston-wind velocity of approximately 6 m/s. Reducing module spacing and increasing ESP height improve tunnel-scale removal by expanding spatial coverage and increasing the inlet airflow rate. The optimized configuration reduces the total amount of UFPs in the tunnel by up to 37.38 %. These findings provide practical guidance for tunnel-scale deployment of passive electrostatic purification systems in subway tunnel environments.