Study on the double effect of the tunnel slope on the fire induced smoke back-layering distance in naturally ventilated inclined tunnels

This study focuses on the smoke back-layering distance (BLD), a crucial feature during a tunnel fire, in naturally ventilated inclined tunnels. It is explained, using a theoretical analysis as well as CFD simulations, that the tunnel slope has a double impact on the smoke BLD, through the fire-induced buoyancy: it naturally induces inlet airflow, but it also induces natural resistance against smoke downward flow. It is illustrated that, for a fire in a tunnel with inclination, this 'buoyancy resistance' results in a smoke BLD that is largely independent of the fire heat release rate. In horizontal or downward inclined tunnels, the natural buoyancy does not result in a limited smoke BLD. The effect of the inlet airflow can be characterized by a Froude number and the buoyancy resistance effect results in a newly defined dimensionless smoke BLD, based on the tunnel height, length and slope. A novel expression is proposed for the smoke BLD in inclined naturally ventilated tunnels, based on the CFD simulation results. It is validated by comparison to experimental data.