Numerical study of the effect of blockage on critical velocity and backlayering length in longitudinally ventilated tunnel fires

In a tunnel fire, one of the protective strategies of users and vehicles which are blocked upstream of fire is provided by longitudinal ventilation systems which serve to limit the rise of smoke flow (backlayering). The presence of these vehicles creates an obstruction which affects the plume fire behavior and the smoke movement. The longitudinal ventilation velocity from which the backlayering disappears, usually designated by “critical ventilation velocity”, is a key parameter to ensure proper evacuation of users and emergency intervention. This study performs numerical simulation using Fire Dynamic Simulator (FDS) to estimate the effect of an obstacle blockage according to its location relative to the tunnel floor on the backlayering flow behavior and the critical velocity. An obstacle occupying about 31% of the tunnel cross section is placed symmetrically upstream of fires by changing its location relative to the tunnel floor. The validity of the FDS Numerical results is firstly demonstrated through a comparison, in terms of critical velocity, with experimental results available for public in the literature. Results which are based on CFD modeling show that the effect of obstacle blockage brings about a decrease of the critical velocity compared than to those obtained with an empty tunnel. This decrease depends on the obstacle location relative to tunnel floor. It is slightly greater when the distance between the bottom of obstacle and the tunnel floor increases. Further, when the obstacle exists in tunnel, the backlayering length become much smaller compared to those predicted in an empty tunnel.