Coupled Simulation of Smoke Propagation and Evacuation in a Metro-Connected Underground Commercial Space Under Different Fuel Scenarios

Metro-connected underground commercial spaces present complex fire safety challenges due to enclosed layouts, dense pedestrian flows, shared evacuation routes, and diverse combustible materials. This study conducts a scenario-based fire evacuation assessment of a typical metro-connected underground commercial space in Nanjing, China, using coupled FDS–Pathfinder simulations. Three fuel scenarios were considered: polyurethane (PU), polyvinyl chloride (PVC), and a combined PU–PVC fuel scenario. Available safe egress time (ASET) was evaluated using visibility, CO concentration, and smoke temperature thresholds, while smoke-affected evacuation performance was assessed using a visibility-based walking speed attenuation model. The results show that visibility reached the critical threshold earlier than CO concentration and smoke temperature in all scenarios. The PU-only scenario produced the fastest smoke spread and the shortest ASET, with Stair 2 reaching the visibility threshold at 244 s. Under smoke-affected conditions, the total evacuation time increased from 364 s to 378 s. More importantly, a local ASET/RSET mismatch was identified at Stair 2, where 62 occupants remained near the node after the local ASET threshold was reached and local clearance was completed at approximately 250 s. By contrast, Stairs 4 and 7 retained safety margins of 133 s and 113 s, respectively. These findings indicate that evacuation safety in the simulated case was controlled not only by smoke-induced walking speed reduction, but also by route convergence, local stair capacity, and uneven exit utilization. The study provides case-based evidence for node-level fire evacuation assessment and evacuation management in metro-connected underground commercial spaces.