Environ Model Assess (2012) 17:555–564 DOI 10.1007/s10666-012-9308-4 Numerical Simulation of Air Pollutant Distribution in Urban Tunnels Khosro Ashrafi · Majid Shafie-pour · Mostafa Kalhor · Vahid Esfahanian Received: 24 May 2010 / Accepted: 1 February 2012 / Published online: 24 February 2012 © Springer Science+Business Media B.V. 2012 Abstract Numerical simulation of air pollution disper- sion inside tunnels is a suitable method for studying air pollutants’ spatial distribution and for evaluating the tunnel’s ventilation efficiency. In the present work, Fluent 6.2, a computational fluid dynamic software, has been used for full-scale numerical simulation of air flows and carbon monoxide (CO) concentrations inside the Resalat Tunnel of Tehran. Fans and vehicles are simulated with source of momentum and porous jump boundary conditions, respectively. Also, source of air pollutants, i.e., vehicle emissions, is simulated as uniform area source in the tunnel floor. Modeling results for concentrations of CO are validated by mea- surement data in 24 points adjacent to the fans inside the tunnel. Calibration of model indicated that the moving wall porous jump method for simulation of vehicle geometry and their effects, momentum source for fans modeling, standard k − ǫ scheme for turbu- lence modeling, and hexahedral mesh type are proper choices for the developed model. The results show a good correlation ( R = 0.9) between modeling and measurement data. Five different scenarios (namely 1–vehicles to be stopped, 2–fans off, 3–two vertical K. Ashrafi (B ) · M. Shafie-pour · M. Kalhor Faculty of Environment, University of Tehran, Ghods St. Enghelab Av., Tehran, Iran e-mail: khashrafi@ut.ac.ir V. Esfahanian Faculty of Mechanical Engineering, School of Engineering, University of Tehran, North Kargar Av., Tehran, Iran ventilation ducts considered in the tunnel, 4–vehicles with Euro-IV emissions standard instead of Euro-II, and 5–blowing power of fans increased to twice the present) are examined for CO concentrations inside the tunnel. The numerical simulations for these scenarios are modified using a relation between measurement data and modeling results. Following the modification, results show that at the last measurement point (near the end of the tunnel), concentrations of CO is 59, 77, 9, 23, and 14 ppm for the five mentioned scenarios, respectively, and it is 32 ppm for normal condition. Hence, appropriate measures may be undertaken by the city authorities for air quality improvements in urban tunnels. Keywords Numerical simulation · Ventilation · Tunnel · Air pollution 1 Introduction Inside the road tunnels, concentration of air pollutants emitted from vehicles may exceed the standard levels and can be unhealthy for drivers and passengers. The air flow inside such tunnels is induced by piston effect of vehicles and axial jet fans (the latter is only used in long tunnels). Numerical simulation can be used to assess and study the effects and efficiency of the these mechanisms. Recently, computational fluid dynamics (CFD) is used to predict and study many environmental phenomenon [1–6]. The study and prediction of pollu- tant distributions in environments such as ambient air, indoor air, and other places are becoming thoughtful fields in CFD applications [7–12].