Theoretical and Numerical Study of Smoke Back-Layering Length for an Inclined Tunnel Under Longitudinal Ventilation

Clarifying the smoke spread and temperature distribution pattern on both sides of the fire source is the key to fire rescue. However, the back-layering length characteristics of tunnel fire smoke under multi-factor influences of tunnel slope and ventilation velocity have still not been revealed. In this paper, the smoke back-layering lengths in uplink and downlink tunnels are presented theoretically from the perspective of smoke dynamics. Meanwhile, a series of tunnel fires simulated by FDS are carried out in a reduced scale model tunnel with the dimensions of 40 m (length) × 0.93 m (width) × 0.93 m (height). Four typical slopes of 0%, 1%, 2% and 3% and five longitudinal ventilation velocities of 0.33 m/s, 0.67 m/s, 1 m/s, 1.33 m/s and 1.67 m/s are considered to investigate the influences of tunnel slope and ventilation velocity on the temperature attenuation on both sides of fire source. The theoretical analysis gives relatively good consistency with numerical results. Finally, the analytical models of non-dimensional smoke back-layering length in the inclined tunnel are obtained based on the theoretical and numerical simulation results. This work provides a reference for the smoke control and fire rescue during the tunnel fire period.