Research On The Diffusion Of Pollutant Around The Double Road Tunnel Portal

In recent years, the longitudinal ventilation is widely used in road tunnel, as aresult of the limitation of construction land and the relatively lower construction costs.But there are two factors in longitudinal ventilation that promote double tunnelone-way traffic gradually to become the mainstream of development. Firstly, thesingle tunnel two-way traffic cannot make the most of traffic wind. Secondly, trafficaccident rate in the single tunnel two-way traffic is far higher than that of doubletunnel one-way. With the development of double tunnel one-way traffic,“secondarypollution” problem becomes increasingly acute. As a result, ventilation effect is notideal and energy consumption of ventilation greatly increases. High concentrationpollutants discharged from the portal will spread in a large space, so that a negativeeffect will be made on the surrounding environment and personnel.For double road tunnel of longitudinal ventilation, this paper analyzes thepollutant diffusion and the “secondary pollution” under several major factors bynumerical simulation. The results show that the secondary pollution rate increaseswith wind speed increase of the tunnel inlet when the wind speed of tunnel outletremains constant. When the tunnel inlet wind speed is constant, the secondarypollution rate decreases with the increase of the outlet wind speed. Withoutenvironmental wind, there is a proper ventilation speed to make the biggest value ofsecondary pollution under any distance of double tunnel, and the proper ventilationspeed increases with the distance increase, while the maximum value of the secondarypollution decreases, from44.46%to10.08%in the range of0m~32m distance. Thecalculation model is concluded using the MATLAB regress function, and it analysesthe influence of tunnel spacing and ventilation speed on secondary pollution rate. Thesecondary pollution should be considered when double tunnel spacing is less than8m.The diffusion distance will increase with tunnel distance increase for a given windspeed case. The secondary pollution rate firstly increases, then decreases when angleis increasing, from0degrees to180degrees. The secondary pollution rate is highestwhen angle is90degrees. Making90degrees as the reference angle, the secondarypollution is not same when the same angle is increased or decreased, and the value ofthe former is bigger. Pollutant diffusion is restricted by wind direction at a great extent. Pollutant diffusion is under the wind outside of the tunnel portal, which maylead to excessive concentration of pollutants within a certain distance. Given tunneldistance, there is one most adverse environmental wind speed to make the secondarypollution rate is the highest, and the wind speed increases as the increase of tunneldistance. The calculation model of secondary pollution rate is carried out by theregress function, which analyses the influence of tunnel spacing and environmentalwind speed. In this paper, two effective engineering measures were proposed toreduce the secondary pollution. One is to set a wall between the double tunnel portal.The wall of length longer and height lower is set to reduce initial investment whenwall height is enough. The other is to stagger a certain length between the doubletunnel portal.