Experimental study on smoke movement characteristics and control in branched tunnel fire under longitudinal ventilation

Reduced-scale experiments were conducted using 1/10 branched tunnel model. The temperature field of hot smoke in the tunnel was measured under different heat release rates and different longitudinal ventilation velocities. The mechanism of air entrainment of hot smoke in the tunnel fire was qualitatively analyzed. The results show that the ultimate motivation of air entrainment is the baroclinicity and velocity shear due to viscosity of hot smoke. The air entrainment directly affects the maximum temperature beneath the ceiling. When the dimensionless ventilation velocity is less than 0.19, fire plume in branched tunnel fire could entrain more ambient air, which reduces the maximum temperature. When ventilation velocity is higher than 0.19, most heat are blown downstream. The allocation of heat is similar to that of single-tube tunnel. In such circumstances, the maximum temperature is close to that of single-tube tunnel. With increase of ventilation velocity, the air entrainment strengthens, and smoke stratification is destroyed gradually. Since a part of the hot smoke in the branched tunnel can enter the branch tunnel, the air entrainment is less, so the critical ventilation velocity is higher than that of the single tunnel. According to the experimental data, a critical ventilation velocity model was proposed for branched tunnel concerned in this paper. It was found that the dimensionless critical velocity and dimensionless heat release rate still have power relations similar to that of single tunnel. © 2022, Central South University Press. All rights reserved.