*Article* **A Study on the Evacuation Spacing of Undersea Tunnels in Different Ventilation Velocity Conditions**

**Wei Na \* and Chen Chen**

School of Environment and Energy Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, China; chenchen@stu.bucea.edu.cn

**\*** Correspondence: nawei@bucea.edu.cn

**Abstract:** Ventilation velocity conditions may affect the smoke diffusion and evacuation environment in a tunnel fire, which should be fully considered in evacuation spacing designs of undersea tunnels. This study focuses on reasonable evacuation spacing under various possible velocity conditions of an undersea tunnel, providing a design method reference for calculating safe evacuation spacing. Fire Dynamic Simulator and Pathfinder software were used for numerical simulations of a 50 MW fire and evacuation process in a full-scale undersea tunnel with traffic congestion. The simulation cases contained velocity modes from zero to satisfying the critical velocity and evacuation spacings from 30 m to 80 m. The calculated distributions of the available safe escape time indicated that a low ventilation velocity, such as 1.0 m/s, is beneficial to the downstream evacuation, but turning off mechanical ventilation increased risk near the fire source. The required safe escape time is shortened with a reduction in slide spacings, but the shortened rate slowed down after spacing was less than 60 m. In addition, the slow evacuating areas from 100 m to 300 m from the fire source independent of spacing are identified. Ultimately, the reasonable evacuation spacings of 60 m, 50 m, and 30 m, corresponding to three possible ventilation velocity modes of low, medium, and high, are proposed through the comparisons of the distributions of available safe escape time and required safe escape time.

**Keywords:** undersea tunnel fire; evacuation spacing; longitudinal ventilation velocity; numerical simulation; ASET; RSET
