Numerical analysis of smoke dispersion against the wind in a tunnel fire

In a tunnel fire, the dispersion of smoke in the opposite direction to the longitudinal imposed air flow remains the principal factors prejudicial to users which are blocked upstream of the fire. This phenomenon, usually referred to as “backlayering”, is a key parameter that one must better understand to ensure proper evacuation of users and emergency intervention. This paper carried out full-scale numerical simulation using Fire Dynamic simulator (FDS) to analyze the behavior of backlayering flow in a longitudinally ventilated tunnel fire. FDS predicted backlayering lengths are compared to calculated values using the Hu model. A reasonably good agreement has been obtained. Furthermore, the predicted maximum smoke temperatures are compared to those given by the Kurioka model. CFD results show that the backlayering flow takes CO towards the tunnel entry, which can be deadly to users immediately even at low concentrations. The inertia and buoyancy forces produced by ventilation and fire, respectively, affect the interface height. The backlayering arrival time increases with longitudinal ventilation velocity while it decreases with heat release rate of the fire. An equation is developed to predict the backlayering front arrival time against the distance to fire.