Criterion for the idling failure state of the upstream exhaust vent in inclined tunnel fires with ceiling central smoke extraction

Compared with horizontal tunnels, the upstream exhaust vent in inclined tunnels under a centralized ceiling smoke exhaust system is more likely to operate in an idling failure state (IFS), where due to the smoke layer failing to reach beneath the vent, it thereby extracts only fresh air while capturing no smoke, which significantly reduces system performance. Accordingly, this study systematically investigated the smoke back-layering length and the operational status of the upstream exhaust vent in inclined tunnels with a centralized ceiling exhaust system, through theoretical analysis, numerical simulation, and small-scale experiments. The results show that a larger tunnel slope or a greater distance from the downstream exhaust vent to the tunnel portal leads to a shorter back-layering length, which can gradually cause the upstream exhaust vent to enter an IFS. Similarly, a greater distance between the fire source and the upstream vent results in a longer back-layering length, but may also eventually lead to an IFS. Based on the analysis of influencing factors, a predictive correlation for the dimensionless upstream smoke back-layering length was established, incorporating the aforementioned three key parameters. Furthermore, a criterion for identifying the IFS of the upstream vent was proposed. Both the predictions from the correlation and the determinations from the criterion were validated against simulation results, showing that all predictions fall within an acceptable range.