Study on the smoke flow characteristics in inclined tunnel with an upstream shaft under natural ventilation

The rapid expansion of urban highway tunnels has led to a significant increase in the risk of fire incidents. Moreover, the urban highway tunnels often have inclinations, which further exacerbates the complexity of the smoke flow. In contrast to the extensive research on natural ventilation in horizontally tunnels, there is still a lack of studies on the natural ventilation of shaft in inclined tunnels. Therefore, this study employed numerical simulations and theoretical analysis to investigate the length of smoke back-layering and flow patterns in inclined tunnel fires with an upstream shaft. By comparing the induced air inflow velocity and smoke back-layering length in inclined tunnels, both with and without an upstream shaft, three types of smoke flow states were identified. State 1: smoke spreading to the upstream shaft, State 2: smoke being limited between the shaft and the fire source, State 3: no smoke backflow in the inclined tunnel. In State 1, the shaft discharges smoke outward. The downstream length and the distance between the shaft and the fire source determined smoke back-layering length. In States 2 and 3, the shaft fills the air inward. The shaft has no obvious effect on the smoke back-layering length. Furthermore, the effects of HRR, fire source position and shaft position on smoke flow characteristics are analyzed. As a result of the analysis, a segmented prediction model for smoke back-layering length and a quantitative method for predicting smoke flow state are proposed. By fine-tuning the tunnel slope, two critical values between the three smoke flow states are obtained. The research results can provide a useful reference for ventilation and evacuation design of inclined tunnels.