Investigation of the effect of mid-tunnel shaft geometry on dispersion of the pollutant in environment

In recent years, increased pollution and traffic in urban has caused the development of underground transport. One conventional approach for urban tunnel ventilation is construction mid-tunnel shafts. These shafts are usually located in high-density urban areas and emissions from them can be harmful for the residents of adjacent buildings. The geometry of these shafts is determined so far based on taste and only for the purpose of compliance standards criteria for tunnel indoor air. In this paper, pollutant dispersion from different and conventional geometries of mid-tunnel shafts with the assumption of a taller downstream building was investigated for the first time. The results can help to reach a better design of these shafts and surrounding buildings to have healthier air for residents of buildings. For this purpose, simulations were done by OpenFOAM. Reynolds-Averaged Navier-Stokes equations method and standard k-ε model were used in simulations. The results showed for the same exhaust velocity, the effect of rectangular and square configuration on front wall depends on dimension of the side which is perpendicular to wind direction. For the exhaust velocity less than 6 m/s, the downstream building prevents pollutants to reach higher altitude and the amount of pollutants will be increased around 100 percent and for exhaust velocity of more than 6m/s, increasing velocity will cause to less changes in pollutant concentration at lower level. In addition, the results showed that exhaust velocity has more effect than geometry configuration on pollutant dispersion and this influence will be decreased by increasing the velocity.