Smoke Control in Naturally Ventilated Tunnels

Abstract

Vehicle fires are a relevant emergency in tunnels. In such an event, the most dangerous hazards to tunnel passengers are the inhalation of toxic combustion products (namely, carbon monoxide - CO) and the loss of visibility due to soot, which are generated in the fire. This paper studies the flow of combustion products during fires in naturally ventilated tunnels, comparing the situations without and with wind action. Natural ventilation may be an effective way of guaranteeing the safety of passengers, especially in short tunnels. If the lower layer of the tunnel remains relatively uncontaminated while the combustion products flow in a buoyancy-driven ceiling jet, passengers can escape the tunnel. However, wind opposing to the ceiling jet may reduce its velocity and disturb it, causing the contamination of the lower layer closer to the fire, when compared with the no wind condition. There is a lack of research to predict in detail under which conditions the contamination of the lower layer with combustion products occurs, when affected by the wind action. An open-source Computational Fluid Dynamics (CFD) code, fireFOAM, was used to conduct Large Eddy Simulations (LES) of naturally ventilated tunnel fires. The numerical model was validated by simulating a pool fire from the Memorial Tunnel Fire Test Program and comparing with experimental results, showing reasonable agreement. Several other scenarios were simulated, considering different values of the external wind velocity, heat release rate (HRR) and tunnel slopes. The contamination of the lower region of the tunnel with soot was analysed as a function of the distance to the fire. The influence of outside wind was also analysed by comparing the fields of soot mass fractions in simulations with and without wind. The consequences of the smoke flow to the tunnel user’s safety were examined.