Optimization Research On Ventilation Effect For City Road Tunnel With Upper Shafts

In recent years, the scale of tunnel construction has been increasing with the development of China’s transport. Tunnel ventilation systems must guarantee that tunnel internal and external environment meet standard requirements. Longitudinal ventilation system becomes the mainstream of tunnel ventilation model. Long city road tunnel with longitudinal ventilation system frequently leads to total pollutant of tunnel exit beyond standard limit. City road tunnel with shafts can exhaust pollutants to alleviate the pollution of tunnel exit, so the ventilation mode is applied widely.According to the ventilation mode of city road tunnel with shafts, this paper analyses ventilation effect of shafts when the traffic is blocked in tunnel. The result shows that shaft ventilation which only relies on traffic wind pressure as power source is unstable, and has the less flow of tunnel airstream. The hybrid ventilation that combines jet fans and shafts can control the ventilation effect of shafts, and make shafts be in stable ventilation state. For this mode of ventilation, the paper studies vehicle percentage of blockage and vehicle speed as two main influence factors of shafts ventilation effect by the methods of model experiment and numerical simulation. The greater vehicle percentage of blockage, the longer ventilation cycle time is. The relationship between vehicle speed and ventilation cycle time can be described by quadratic equations. To enhance shafts ventilation effect, its structural type is further optimized. Shafts exhaust effect can be enhanced through changing its inclination angle and setting baffles in the bottom of shafts. It is found that 45°inclined shaft whose bottom installs the single 45°baffle has the best exhaust state and divided sewage rate, when the length of the baffle is equal to the width of city road tunnel. Shafts ventilation effect is closely related to the location of shafts. For the purpose of increasing shafts air intake volume and reducing pollutants concentration of tunnel, shafts should be centralized setting near the tunnel entrance. For the purpose of exhausting more polluted air from shafts and alleviating the pollution of environment in the outlet, shafts should be centralized setting near the tunnel exit.