The critical ventilation velocity for transverse double fires in tunnel

The critical velocity of longitudinal ventilation is one of the most important parameters in tunnel fires. Most previous studies simulated fire scenarios in which only one fire source exists in the tunnel. However, the critical velocity will change under the condition of transverse double fires. In this study, the critical ventilation velocity under the transverse double-fire condition compared with the single fire source was analyzed by using the Fire Dynamics Simulator (FDS). The results show that the smoke movement for transverse double fires in the tunnel is affected by both the buoyant force and the shear stress of the sidewalls. As the distance between the double fires increases, the critical velocity decreases first, then increases and eventually decreases. When the double fire sources are both near the side walls, the critical velocity is approximately equal to the critical velocity for adjacent double fires at the center. Finally, relations between the influence coefficient of distance and the dimensionless transverse distance as well as correlations between the critical ventilation velocity with and without distances for double fires were developed. The presented correlation can provide reference value for smoke control and personnel evacuation in case of tunnel fires.