Experimental study on flame characteristics in a branched tunnel fire with asymmetric mainline slopes

Flame length and flame tilt angle are key parameters for evaluating the tunnel fire hazards and radiative damage. Previous study on flame geometry have mainly focused on the ordinary tunnels, whereas fires in branched tunnels have received little attention. This study conducts a series of small-scale experiments to investigate the flame morphology in a branched tunnel with asymmetric mainline slopes on the two sides of the fire source in the mainline. Three heat release rates including 1.12kW, 1.64kW and 2.8kW, five upstream slopes and downstream slopes of mainline are systematically varied. Results show that the mainline slope significantly affects both flame length and flame tilt angle. The flame inclines toward the higher portal side due to the greater stack effect in that direction. The flame tilt angle and flame length increase monotonically with the tunnel slope as it approaches a unidirectional slope. The induced airflow velocity in sloped mainline causes the flame tilt angle to exceed 35°. Under a relatively large upstream slope, a larger downstream slope weakens both the flame length and the flame tilt angle. A new dimensionless airflow velocity uin