Experimental study on temperature profile and smoke movement in a model-branched tunnel fire under longitudinal ventilation

Reduced scale experiments in a branched tunnel were carried out to study the temperature profile and critical ventilation velocity under different longitudinal ventilation conditions. Branch orientation was upstream and the branch angle was 15 degrees. The results show that when dimensionless ventilation velocity is lower than 0.19, the maximum temperature is lower than that of traditional single tube tunnel because hot smoke could propagate into the branch. When dimensionless ventilation velocity is higher than 0.19, the maximum temperature is close to that of single tube tunnel. Because more heat is blown downstream, which is similar to ventilated single tube tunnel. A new model predicting the maximum temperature beneath the ceiling is developed. With increasing ventilation velocity, the upstream and branch temperature decay factor increase while the downstream temperature factor decrease. Critical ventilation velocity for the branched tunnel is higher than that of single tube tunnel. A correlation of critical ventilation velocity is proposed. The 1/3 power law still holds.