Smoke blocking effect of air curtain: longitudinal ventilation system based on branch tunnel

To investigate the effectiveness of an air curtain installed at the entrance of a "T"-shaped bifurcation tunnel under longitudinal ventilation in blocking smoke, a full-scale numerical model of the Bazi Ridge Tunnel was established using Fire Dynamics Simulator (FDS) software. The Bazi Ridge Tunnel is located in Changyang and Badong counties in Hubei Province. By varying the jet velocity and jet angle of the air curtain at the entrance of the bifurcation tunnel, an optimal set of values for controlling smoke in the ramp was determined. Additionally, the reasons behind the different phenomena observed at various jet velocities and angles were analyzed using velocity flow field slices at the entrance of the bifurcation tunnel.The results show that when the jet velocity of the air curtain is equal to or greater than 20 m/s, or the jet angle is equal to or greater than 15°, a phenomenon of smoke reversal occurs during the spread of smoke in the ramp. Increasing the jet angle initially strengthens the smoke control effect of the air curtain, but then weakens it. On the other hand, increasing the jet velocity enhances the smoke control effect of the air curtain. The smoke reversal phenomenon is caused by the formation of vortices due to the interaction between the airflow blown out by the air curtain device and the longitudinal wind in the main tunnel. These vortices are influenced by both the inflow and outflow airflow, and the overall effect depends on which airflow's capability is more significant.