Study on the combination effect of tunnel slope and longitudinal fire location on the asymmetric flow fields in a naturally ventilated tunnel

CFD simulation tests were carried out to study the asymmetric flow phenomenon caused by the combination effect of tunnel slope and longitudinal fire location in a naturally ventilated tunnel. The result shows that the tunnel slope and the longitudinal fire location can both create the induced longitudinal flow solely. The longitudinal flow within tunnel induced by the stack effect caused by the tunnel slope is normally unidirectional, flowing uphill and the speed increases gradually with the tunnel slope. While the longitudinal flow induced by the thermal pressure difference caused by the uneven distribution of upstream and downstream smoke transportation can be bidirectional, which depends on the longitudinal fire location (also known as the downstream and upstream tunnel length difference, Delta L = L-down - L-up). Therefore, the induced longitudinal flow under the combination effect of tunnel slope and longitudinal fire location is very complex. For tunnels going uphill from left (upstream) to the right (downstream) portals, the two effects are positively added when the fire is located at the upstream tunnel (Delta L > 0), while the two effects are counteracted when the fire is located at the downstream (Delta L < 0). To quantify the strength of asymmetric flow caused by the two effects, an empirical equation of mass flow rate of induced longitudinal flow is proposed. Meanwhile, a model to predict the smoke back-layering length under the two effects is also proposed and validated by former experimental data. A prediction model on the upper critical fire position where the two effects are completely canceled out is proposed. Another critical position (the lower critical fire position) where the smoke flow can achieve unidirectional transportation in an inclined tunnel under natural ventilation conditions is deduced as well.