Experimental comparison on the smoke movement in horizontal and downhill tunnel fires under subcritical ventilation

Abstract

In longitudinally ventilated tunnel fires, subcritical ventilation is widely used in the early stage of accidents to maintain smoke stratification. Due to the stack effect, the smoke movement in inclined tunnels is different from that in horizontal tunnels. In this study, a series of brine-water experiments are carried out to compare the characteristics of buoyant contaminants flow in horizontal and downhill tunnels under subcritical ventilation, which has not been experimentally investigated. The results suggest that subcritical ventilation is an effective longitudinal ventilation strategy in the early stage of horizontal tunnel fires, as it can retain the smoke stratification and an acceptable back-layering flow. Moreover, the critical ventilation condition can be resumed after the ventilation pressure increases to the critical value. However, if subcritical ventilation is first adopted in downhill tunnel fires, no stable back-layering flow will be observed, the buoyant contaminants consistently flow out from the higher exit, ultimately leading to the establishment of a “stack-dominated flow regime”. Under this circumstance, even if the ventilation pressure increases to the critical value or slightly higher, the buoyant contaminants upstream of the source still cannot be pushed back. Moreover, the reduced gravity of the smoke in the tunnel increases as the ventilation pressure rises. Therefore, subcritical ventilation is not suitable for downhill tunnel fires. The study is helpful for understanding the smoke movement in horizontal and inclined tunnel fires under subcritical ventilation.