A numerical and experimental study of pollutant dispersion in a traffic tunnel.

Three-dimensional turbulent flow and dispersion of gaseous pollutants carbon monoxide (CO) and nitrogen oxides (NOx) in a road tunnel was modeled using the standard k-epsilon turbulence model and solved numerically using the finite volume method. Vehicle emissions were estimated from the measured traffic flow rates and modeled as banded line sources along the tunnel floor. The effects of fan ventilation and piston effect of moving vehicles on the airflow and pollutant dilution were examined. The numerical results reveal that a peak velocity exists near the tunnel floor due to the piston effect of vehicles. The cross-sectional concentrations of air pollutants are non-uniformly distributed and concentrations rise with downstream distance. The piston effect of vehicles can alone provide 25%-34% dilution of air pollutants in the tunnel, compounded 43%-70% dilution effect according to the ventilation condition.