Experimental investigation on the smoke back-layering length in a branched tunnel fire considering different longitudinal ventilations and fire locations

Most previous works concentrated on the smoke back-layering length in traditional single tunnel fires, whereas very few studies have been performed related to the smoke back-layering length(L) in a branched tunnel fire. In the present paper, a series of fire tests were performed in a 1/10 scale branched tunnel to investigate L under various longitudinal ventilation velocities (v), heat source locations(d) (the distance between the heat source centerline and junction) and heat release rate (HRR). The smoke back-layering length and smoke temperature for various conditions are obtained and analyzed. The results are summarized as follows: With longitudinal ventilation velocity increasing to large enough, the smoke back-layering phenomenon will be observed and the maximum smoke temperature tends to deflect downstream. The larger the v and the farther the heat source is to the intersection, the smaller the L is. The dimensionless smoke back-layering length L/H is proportional to gHQ˙/ρ0cpT0v3A, which is consistent with Thomas's work. Whereas, due to that the smoke will split in intersection of branched tunnel, the proportional coefficient increases from 0.467 to 0.754, which is less than that in traditional single tunnels in previous work. Moreover, a revised relation is proposed to describe the smoke back-layering length in branched tunnel fires considering the longitudinal ventilation and fire location.