Study of slope effect on smoke back-layering length and ceiling temperature in tunnel fires under natural ventilation

In this paper, experiments focus on the fire smoke flow characteristics under different slopes in a vaulted tunnel and no-mechanical ventilation conditions. Explored the smoke back-layering length Lb and ceiling temperature, with the main research factors tunnel slope and heat release rate (HRR). Research showed that the influences of HRR on the length Lb decreases with increasing slope (especially after 10%) for all slopes where the lengths Lb are present (4%–35%). The slope will also intensify the flow of smoke in the downstream ceiling and the heat exchange between the smoke-wall, resulting in a decrease in the ceiling temperature, while the decay trend remains consistent at the three heat release rates. Combined with the dimensional analysis, the respective dimensionless expressions were established. The results indicated dimensionless smoke back-layering length (Lb*) and the 0.12 power of the normalized HRR decrease with the slope as a power function, while dimensionless ceiling temperature rise and the 3/5 power of the dimensionless HRR decrease exponentially with the slope. These conclusions can provide useful suggestions for engineering practice.