Influence of cross-sectional area and aspect ratio of shaft on natural ventilation in urban road tunnel

This paper has analysed the influence of cross-sectional area and aspect ratio of shaft on natural ventilation in urban road tunnel fires by Large Eddy Simulation (LES). Results have shown that the performance of smoke exhausting process displays different smoke extraction modes with the increasing of the cross-sectional aspect ratio. When the cross-sectional aspect ratio increases to a certain value, the hot smoke will be adhered to the upstream wall of the shaft and be exhausted through the upstream region. In this case, the cold air from tunnel bottom penetrates the vertical shaft leading to the smoke exhausting process inefficiently. The 2-D line plume in near region could be used to predict the process of hot smoke exhausting by shaft upstream after penetration and the predicted values give a good agreement with the simulated values. It comes to conclusion that the shaft with larger cross-sectional aspect ratio should be divided into several smaller shafts in order to exhaust as much smoke as possible and avoid overmuch entrainment of fresh air.