Effect of the blockage ratio on the smoke extraction efficiency in tunnel fires with natural ventilation

Natural ventilation by shafts is widely applied in urban shallow tunnels due to its simple structure, steady smoke extraction efficiency and environment-friendly means. In order to investigate the influence of vehicle blockage on the smoke extraction efficiency of the shaft in tunnel fires, a series of n-heptane fire experiments were conducted in a 1:15 small-scale tunnel model with seven blockage ratios (0, 0.16, 0.24, 0.32, 0.40, 0.48, 0.56). The smoke extraction efficiency is calculated by the measured smoke temperature and CO volume fraction in the shaft and in the tunnel near the exhaust vent. Results show that the smoke extraction efficiency of the tunnel with shafts decreases firstly, then increases and drops lastly with the increasing blockage ratio. The smoke temperature in the shaft and at the downstream tunnel change similarly, which indicates that the vehicle blockage can affect the thermal energy discharged by a shaft. The main influence factors are the blocking effect, the necking effect caused by the blockage and the stack effect of the shaft, resulting in the variability of the smoke extraction efficiency. The smoke extraction efficiency reaches the maximum around the blockage 0.48 in the present tunnel, which can be referred to design the tunnel cross section in engineering applications.