Investigation on smoke propagation behavior and smoke back-layering length of fires in an inclined tunnel under natural ventilation

This study investigates the smoke flow behaviors in an inclined tunnel under natural ventilation. Numerical simulations are conducted considering different tunnel slopes, heat release rates (HRR), tunnel lengths, and tunnel widths. The focus is on the effects of tunnel slope on smoke propagation, maximum smoke temperature beneath the tunnel ceiling, tunnel inlet air velocity, and smoke back-layering length. The findings indicate that in an inclined tunnel, the dispersion of upstream smoke is influenced by downstream smoke plume propagation. The upstream smoke plume exhibits a reflow towards the fire source after reaching its maximum travel distance, ultimately achieving a quasi-steady state. The maximum smoke temperature beneath the ceiling decreases with increasing slope or decreasing tunnel width, but increases with increasing HRR. The wind speed at the tunnel entrance increases with the increase of slope and HRR, and decreases with the increase of tunnel width. Based on dimensional analysis, using the hydraulic tunnel height as a characteristic length, a simple model is developed to predict the tunnel inlet air velocity, considering the effects of tunnel slope, HRR, tunnel length, and tunnel width. The dimensionless induced airflow velocity is directly proportional to the 0.26 power of the dimensionless heat release rate, the -0.88 power of the dimensionless tunnel width, the 0.47 power of the dimensionless downstream tunnel length and the 0.59 power of the tunnel slope. On this basis, a simplified model for predicting the smoke back-layering length is established using the Richardson number, and its accuracy and feasibility are further validated by comparing with experimental data at different scales.