Fan: Pollution Ventilation in a Subway

Pollution Ventilation in a Subway — Description

This project simulates pollution ventilation in a subway station using ANSYS Fluent, with a focus on the station's air-conditioning system. A subway station is one of the busiest of public spaces and is therefore readily exposed to pollution. In this model, a polluting gas is defined with a molecular weight of 77.5 kg/kmol and a specific heat capacity of 1100 J/kg·K.

The subway doors are treated as the sources of pollutant, since they are the points where passengers gather. The air-conditioning system is positioned on the ceiling of the station and uses its suction power to draw pollutants out into the surrounding environment. The pollutant gas enters the station at a velocity of 0.1 m/s and is carried outside by the vacuum pressure.

The Species Transport model is used for the simulation, with two species defined — the air already present inside the station and the pollutant gas that enters it. The model solves the transport equations for each species. The suction at the ceiling outlet panels is defined using the Exhaust Fan boundary condition, in which a pressure jump draws the pollutant gas out of the environment. This pressure jump is set to 1,000,000 Pa.

Geometry & Mesh

The geometry was drawn in Design Modeler and represents a subway station comprising a subway line with rails.

Setup & Solution

Viscous model — RNG k-epsilon with standard wall functions

Species — Species Transport with 2 volumetric species (air and pollutant gas)

Energy — enabled

Boundary conditions — Inlet-Doors: velocity inlet at 0.1 m/s, 300 K, pollutant mass fraction 1; Inlet-Subway: pressure inlet at 0 Pa gauge, 300 K, pollutant mass fraction 0; Outlet-Panels: exhaust fan at 0 Pa gauge with a 1,000,000 Pa pressure jump; Walls: stationary, with zero heat flux

Methods — SIMPLE pressure-velocity coupling; second-order discretization for pressure; first-order upwind for momentum, turbulent kinetic energy, turbulent dissipation rate, pollutant, and energy

Initialization — standard method, with an initial velocity of 0 m/s, temperature of 300 K, and pollutant mass fraction of 0

Conclusion

On completion of the solution, 3D contours of pressure, velocity, and pollutant mass fraction were obtained. The pollutant contour indicates that the station's air-conditioning system performs effectively: although pollutants enter through the subway doors, they do not spread into the station space. The powerful ceiling-mounted fans of the air-conditioning system draw the contaminated air out to the external environment, keeping the station interior clear — demonstrating the role of the exhaust fan in maintaining air quality within the station.