Experimental investigation on maximum gas temperature beneath the ceiling in a branched tunnel fire

Abstract

The effect of bifurcation structure in a road branched tunnel on ceiling gas temperature needs to be clarified. However, the previous studies mainly focused on the temperature distribution in ordinary single tunnel fires, it is insufficient in branched tunnels. Therefore, this paper carried out a series of reduced scale experiments in a branched tunnel to investigate the maximum excess temperature beneath the tunnel ceiling. The reduced scale experiments with heat release rates varied from 1.72 kW to 6.04 kW were conducted under natural ventilation and forced ventilation. Three bifurcation angles of 5°, 10°, and 15° were conducted. The predicted model by dividing dimensionless ventilation velocity into two parts was established on the bases of theoretical analysis and experiments. Results show that the bifurcation structure has a significant effect on the flame plume. For a given heat release rate, the maximum ceiling temperature decreases with the increasing of ventilation velocity because of the more significant cooling effect compared with oxygen supply effect. The maximum ceiling temperature varies as 2/5 power of the heat release rate when the dimensionless ventilation velocity is lower than 0.19. The affection of bifurcation angle on the maximum temperature could be ignored under natural ventilation, but this affection is reinforced under forced ventilation. The predictions by proposed model are compared with experiments and they are in good agreement.