Modelling and experimental investigation of critical velocity and driving force for preventing smoke backlayering in a branched tunnel fire

•Experiments are conducted in a branched tunnel under the cases of different bifurcation angles.•The critical velocity are investigated during branched tunnel fires on the basis of pressure balance.•The mathematical model for critical velocity together with driving force for preventing smoke backlayering are proposed. The previous researchs of the critical velocity are mainly for single-point tunnel and limited for branched tunnel. The critical velocity together with the driving force for preventing smoke backflow in branched tunnel cannot be precisely predicted by former correlations. Experiments are conducted in this work to investigate the fire-induced smoke movement in branched tunnel under the cases of critical ventilation velocity condition. The pressure change in branched tunnel is analyzed on the basis of one dimensional theory, and the fire source and bifurcation angle was taken into account. Results show that the bifurcation angle has a significant influence on the critical velocity and the driving force for preventing smoke backlayering due to the local resistance. The pressure difference due to the plume blockage created by the fire are expressed on the basis of experimental data. Based on the theoretical analysis of pressure change, new relatively correlations considering the bifurcation angle and heat release rate are proposed to predict the critical velocity together with the required pressure rise for preventing smoke backflow in branched tunnel. The predicted critical velocity by proposed model are found to comply well with the experimental data.