Numerical investigation of airflow patterns and pollutant dispersions induced by a fleet of vehicles inside road tunnels using dynamic mesh Part Ⅱ: Pollutant dispersion and exposure levels

Abstract

Following our simulation results in Part I (Song and Zhao, 2019), in this study we continued to analyze pollutant dispersion patterns and exposure levels when a fleet of vehicles passed through a 450-m long tunnel by employing the Realizable k–ε model and pollutant transport equation together with the dynamic mesh technique. The same 450-m-long tunnel model with a 450-m extension from the outlet in the driveway direction in Part I was developed; it contained 36 gasoline-powered vehicles in nine rows with four vehicles per row. The transient pollutant dispersion profiles were validated by comparing the simulation results to field-measurement data in a real tunnel. The simulation results show that volume-averaged CO and NOx concentrations inside the tunnel were less than the exposure limit in China (150 ppm for CO and 1 ppm for NOx) at 60 km/h and 40 km/h, which is in accordance with the study in Part I. However, in the breathing zone (0.5–1.5 m in height), accumulated pollutants and averaged CO and NOx concentrations were 0.85–2.44 times the exposure limit in China, even at 60 km/h and 40 km/h. The personal intake fraction (P_IF) CO and NOx values inside the tunnel in this study were at least an order of magnitude higher than those outdoors, including in Beijing, Hong Kong, and the US urban areas. The results indicate that occupant exposure risk is relatively high inside the tunnel and ventilation systems should continue to work to decrease the exposure levels in the breathing zone, even when vehicles move at medium or high speeds inside the tunnel.