Experimental investigation of the thermal back-layering length in a branched tunnel fire under longitudinal ventilation

The smoke propagation in branched tunnel is more complicated because of the special bifurcation structure that has not been clarified, despite the fact that the scenario exists in practice. In this paper, the thermal smoke movement behavior in branched tunnel fire were experimentally investigated under the combined effect of bifurcation angle and ventilation velocity. The theoretical analysis was conducted to develop the smoke back-layering length equation based on the applied force of smoke propagation upstream of the fire source, and the smoke back-layering length under various longitudinal ventilation velocities was quantified. The results indicate that the smoke back-layering length decreases with the increasing of longitudinal ventilation velocity for a given heat release rate. The stronger heat release rate results in the longer smoke backflow distance under same ventilation velocity. The smoke back-layering length varied with bifurcation angle due to the bifurcation structure weakens the inertial force of longitudinal ventilation, but the affected degree varies with bifurcation angle. The experimental smoke back-layering length can be well correlated using the deduced logarithmic function. The empirical model taking the bifurcation angle into consideration is proposed to predict the smoke back-layering length in branched tunnel fire. Finally, the correlation for critical ventilation velocity of branched tunnel fire under varied bifurcation angles was also proposed based on the boundary condition of none smoke back-layering.