Large Eddy Simulation of effectiveness of solid screen on improving natural ventilation performance in urban tunnels

Abstract

To improve natural ventilation performance in urban tunnels, a method of adding solid screens below shafts, was proposed to accumulate fire-induced smoke and weaken negative influence of plug-holing. A series of simulations were carried out to analyze the reliability of this method based on Large Eddy Simulation. Thermal and spreading characteristics of smoke were studied in detail, with systematically varying the angle of solid screen and the longitudinal wind speed in the tunnel. Results show that without wind in the tunnel, all smoke is blocked from spreading downstream after adding the solid screen, thus the smoke layer thickness downstream of the fire increases, and the plug-holing is prevented. The stack effect is enhanced due to the thicker and higher-temperature smoke layer. With wind in the tunnel, the smoke spreads across the solid screen and continues spreading downstream, but the solid screen with different angles can still increase the effective mass flow rate of exhausted smoke greatly. The solid screen with an acute angle performs better than that with an obtuse angle in exhausting smoke. Meanwhile, the practical engineering application of solid screens in tunnels with natural ventilation was demonstrated in detail.