Prediction of soot and carbon monoxide production in a ventilated tunnel fire by using a computer simulation

Numerical study is performed by using a large eddy simulation to give a quantitative description of the toxic products (CO, soot), temperature stratification and heat fluxes from a circular pool fire in a longitudinally ventilated tunnel. The turbulent combustion process is modelled by an eddy break-up concept by using two sequential, semi-global steps to CO prediction. The numerical model solves three-dimensional, time-dependent Navier–Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. The smoke movement is modelled as an unsteady process, from the time of ignition until convergence to a quasi-steady state. The computed, time-averaged temperatures are compared with experimental data from a full-scale ventilated tunnel, and a relatively good agreement is attained. Below a critical ventilation velocity, the back layer flow carries the CO and soot production towards the entry, and an inverse dependence of the backflow length to ventilation velocity is predicted. For the tunnel hydraulic height of 3.3 m considered here, the heat-release rate (HRR) approaches a maximum of approximately 35 MW, beyond which the critical ventilation velocity is unchanged even though the theoretical HRR continues to increase. In the case where HRR reaches to a maximum, the high-temperature level (T>500 °C) in the smoke layer and the strong heat flux (>20 kW/m2) pose significant threat to people or combustible objects exposed downstream the fire source, potentially inducing flashover within tunnel. This study highlights the utility of field modelling for the analysis of fires and smoke (CO and soot) movement in tunnels.