Evaluating the impact of tunnel slope on critical velocity and confinement velocity in metro tunnel carriage fires

Smoke control within metro tunnels stands as a pivotal measure in ensuring both the orderly evacuation and safety of passengers. This study investigated the critical velocity and confinement velocity in inclined metro tunnel fires via model-scale experiments. The tunnel slope varied from 2 % ∼ 8 % and four fire heat release rates of 2.29 kW, 3.44 kW, 4.59 kW and 5.74 kW. Results show that the confinement velocity and critical velocity monotonically increase with the heat release rate and tunnel slope. The confinement velocity V_conf was notably affected by the equivalent fire source Q_u. The natural exponential function could effectively express the relationship between Q* and Q_u*, with the correction factor of tunnel slope utilized to adjust the expression. A new model for critical velocity and confinement velocity was characterized by considering the tunnel slope and heat release rate. Comparative results indicated that the optimizing confinement velocity was significantly lower than that defined in previous studies, while the proposed critical velocity ratio aligns well with findings from prior research. This study provides valuable insights into the design of smoke control systems customized for fires occurring in trains immobilized within inclined underground tunnels.