Numerical Assessment of Smoke Spread and Evacuation Safety for a Battery Electric Vehicle Fire in a Single-Lane Road Tunnel

This study investigates the smoke spread characteristics and occupant evacuation safety of a 15 MW battery electric vehicle (BEV) fire in a 1.5 km small-bore single-lane road tunnel. Owing to its minimized cross-section (a single 3.0 m lane within a 3.7 m inner diameter), no fire protection or evacuation facilities can be accommodated, and its fire safety therefore requires performance-based verification. Fire Dynamics Simulator (FDS) was used to simulate the fire under different longitudinal ventilation velocities, and the smoke temperature, visibility and CO concentration were examined. Based on these indices, the available safe egress time (ASET) was determined, while the required safe egress time (RSET) was obtained from Pathfinder evacuation simulations for a worst-case congestion scenario involving 420 occupants, so as to evaluate whether the evacuation distance of the 1.5 km tunnel is adequate. The results show that under natural ventilation, the upstream section 645–750 m violates the RSET < ASET criterion; under 1 m/s longitudinal ventilation, the failure zone shrinks to 740–750 m; at 2 m/s and 3 m/s, the upstream region remains tenable throughout the 900 s simulation and all occupants can evacuate safely. A longitudinal ventilation velocity of at least 2 m/s is therefore required to ensure safe upstream evacuation in this tunnel. The conclusions are drawn for the present worst-case configuration and are intended for the pre-construction performance-based verification of similar single-lane tunnels without evacuation facilities.