Numerical study of natural ventilation in urban shallow tunnels: Impact of shaft cross section

Natural ventilation with vertical shafts uses less energy and occupies less space in a tunnel, compared to mechanical ventilation. Therefore, this natural ventilation mode has been adopted in more and more urban tunnels. This study analyzed the influence of cross-sectional area and aspect ratio (ratio of length to width) of the shaft on natural smoke exhaust in a tunnel fire, and 33 cases were simulated using the Fire Dynamics Simulator software. The temperature distribution, velocity vector field, and mass flow rate of gas exhausted by the shaft were investigated in detail. The results show that when the aspect ratio is less than 0.75, the mass flow rate of exhausted gas decreases slightly with the increase of aspect ratio, and when the aspect ratio is greater than 0.75, the mass flow rate of gas does not change obviously. With the increase of aspect ratio, the plug-holing becomes more obvious. With the increase of cross-sectional area, there is initially no plug-holing or slight plug-holing inside the shaft, but an obvious plug-holing is developed gradually, and the air penetration phenomenon with some fresh air being exhausted directly occurs finally. The mass flow rate per cross-sectional area of gas exhausted by the shaft decreases with the increase of cross-sectional area.