CFD analysis of smoke backlayering dispersion in tunnel fires with longitudinal ventilation

In a longitudinally ventilated tunnel fire, the backlayering flow propagated in the opposite direction to the air current is the most fatal contaminations to users which are blocked upstream of the fire. In the present paper, numerical simulations were conducted using Fire Dynamic Simulator, which is based on large eddy simulations to estimate the backlayering arrival time in a longitudinally ventilated tunnel fire. The effect of a vehicle obstruction on the backlayering arrival time will be also investigated. For this, a vehicle model occupying about 31% of the tunnel cross section is simulated upstream of the fire source with its location relative to the tunnel floor is varied. The numerical investigation shows that the inertia and the buoyancy forces produced by ventilation and fire, respectively, affect the backlayering spread. The backlayering arrival time increases with the longitudinal ventilation velocity while it decreases with the fire heat release rate. When a vehicle obstruction existed within the tunnel, the numerical results show an increase of backlayering arrival time. This increase is significantly more important with the fire distance when the vehicle obstruction approaches the tunnel floor. Two correlations are developed, with and without obstruction in the tunnel, to predict the backlayering arrival time against the distance to fire. Copyright (c) 2016 John Wiley & Sons, Ltd.