Experimental and numerical study on critical ventilation velocity for confining fire smoke in metro connected tunnel

•The critical velocity for confining smoke in metro connected tunnel was studied.•Reduced-scale experiments combined with numerical simulations were carried out.•The connected tunnel slope and longitudinal airflow in main tunnel were considered.•Prediction model of critical velocity in metro connected tunnel was developed.•The model achieved an improvement in connected tunnel compared with previous ones. Extensive studies have been conducted on critical velocity in ordinary single tunnels while still lacking the direct guidance for smoke confinement in connected metro tunnels, which is noteworthy as metro networks have expanded in recent years. Focusing on this issue, this study aimed to predict the critical velocity for confining fire smoke in connected metro tunnels. Model experiments and numerical simulations were carried out while considering the inclination of connected tunnels and the incorporation of longitudinal ventilation in a main tunnel, which represent the typical structure and ventilation characteristics of metro connected tunnels. The results show that, under a node-area fire scenario, the critical velocity for confining upstream smoke in a connected tunnel was lower than that for an ordinary single tunnel, caused by the discrepancy in smoke density and heat allocation. Additionally, due to the stack effect driving and hindering smoke diffusion in upward and downward connected tunnels, the dimensionless critical velocity increased and decreased with the increase in upward and downward slopes, respectively. Meanwhile, smoke diffusion was reduced by incorporated longitudinal ventilation in the main tunnel, resulting from a decrease in smoke temperature. A prediction model of the critical velocity for confining smoke in metro connected tunnels was developed, which achieved an improvement on previous model and could support the smoke control design in such metro systems.