Experimental study on fire characteristics of the bifurcated tunnel under multi-directional ventilation

With the increasing complexity of tunnel structures, the fire characteristics of traditional single-tube tunnels are not suitable for those with complex structures. Multi-directional ventilation is mostly used in underground interchange tunnels to prevent fire smoke spread to other tunnel sections. In this paper, by means of experimental analysis, the maximum temperature rise distribution beneath the ceiling and the longitudinal distribution of heat flux at the same height position with the fire source in the bifurcated tunnel under multi-directional ventilation are studied. The results show that when the longitudinal ventilation is only added to the main tunnel, the maximum temperature position beneath the ceiling shifts downstream along the longitudinal direction with the increase of the wind speed in the main tunnel. Under the multi-directional ventilation of the main tunnel and the branch tunnel, the maximum temperature position beneath the ceiling shifts along the transverse direction with the increase of the wind speed in the branch tunnel. Based on the dimensionless analysis, a model is proposed to predict the maximum temperature rise beneath the ceiling in the bifurcated tunnel under multi-directional ventilation. The heat flux at the same height position with the fire source first increases and then keeps relatively stable or decreases slightly as the wind speed of the main tunnel increases. In addition, for the smaller wind speed of the main tunnel, the heat flux increases with the wind speed of the branch tunnel. For the larger wind speed of the main tunnel, the heat flux no longer shows regular variations with the wind speed of the branch tunnel.