Numerical simulation of natural ventilation in city road tunnel with upper vents

Abstract

City road tunnels can use longitudinal ventilation to keep driving workers safe, which also ensures air quality to accord with code requirement. If the length of tunnel is too long, pollutant concentration and total emissions of the tunnel exit can not demand city environment evaluation. To solve the problem, some city road tunnels are set up a few vents on the roof of tunnels, which can exhaust polluted air and get fresh air through the upper vents along the tunnel. The environment pollution in the outlet can be alleviated. According to the ventilation mode of city road tunnels, the paper discusses and analyzes the effect of traffic wind force and jet fans on the natural ventilation of city road tunnel with a few upper vents. The research method adopts CFD numerical simulation and model experiment. The results show that the ventilation situation of vents presents cyclical change in air intake and exhaust, when vehicles keep continuous movement in the tunnel, and air exhaust time and air intake time partly depend on the distance between the natural vents and the tunnel exit. The moving vehicles in tunnels can generate a small amount of ventilation volume, which can not demand required airflow of long road tunnels. We can adopt the coupling effect between traffic wind and jet fans to ventilate for tunnels. The position change of the natural vents has little effect on the ventilation volume of tunnels which is only generated by traffic wind. With the vents' static pressure adjustment by jet fans, natural vents can present steady air exhaust state. And it is linear between air exhaust volume and the distance which is between the vents and jet fans. In practice, we can make use of jet fans to increase air exhaust volume of natural vents when the arrangement mode and space of jet fans are determined reasonably. Natural vents combined with jet fans to ventilate can decrease total pollutants emission amount in the tunnel exit, and reduce pollutant concentration in tunnels.