A numerical study on smoke back-layering length and inlet air velocity of fires in an inclined tunnel under natural ventilation with a vertical shaft

When a fire occurs in an inclined tunnel, the tunnel inlet air velocity induced by the stack effect will help to prevent the smoke backflow. This work presents numerical studies on the smoke flow behaviors induced by fires in inclined tunnels under natural ventilation with a vertical shaft. The tunnel slope, heat release rate (HRR) and distance from the fire source to the downhill inlet of tunnel were varied. The effect of tunnel slope on temperature distributions in the shaft and tunnel, smoke back-layering length and tunnel inlet air velocity were specifically focused. Results showed that the maximum ceiling gas temperatures are presented in the downstream of fire axis for all slopes (5%-25%). The plug-holing phenomenon occurs for small slopes and disappears for large slopes. Increasing the slope can help to diminish the plug-holing while it decreases the smoke temperature in the inner region of shaft as well. The established correlation shows that the ratio of the back-layering length to the hydraulic tunnel height (which is defined as the 4 times the tunnel cross-sectional area to the tunnel perimeter) is proportional to -1.57 power of the slope, while it is independent on the HRR and fire source location. The tunnel inlet air velocity increases with increasing the tunnel slope and HRR while it decreases as the fire source placed far away from the downhill inlet of tunnel. Based on dimensional analysis and using the hydraulic tunnel height as the characteristic length, the final equation suggests that the normalized tunnel inlet air velocity is proportional to 0.42 power of the normalized HRR and -0.57 power of the normalized fire source location and it increases exponentially with the tunnel slope.