Numerical and theoretical evaluations of the propagation of smoke and fire in a full-scale tunnel

Most CFD tunnel fire simulations have so far focused on the thermal field and the critical velocity for suppression of the hot backlayering flow. However, there is a great need in understanding the characteristics of a real-scale tunnel fire in terms of the flame propagation and the toxic gas generation. In this study, an extension of the eddy dissipation concept incorporating two chemical reaction steps is integrated into an internationally recognised CFD fire simulation code. A full-scale over-ventilated tunnel fire is simulated with the model and the measured temperature profiles are correctly reproduced. The model is then used to investigate the characteristics of a tunnel fire in three aspects: the length of backlayering, the flame length and the effects of an object within the tunnel. An inverse dependence of the backlayering length with the wind velocity, and the levelling-off of a maximum of the backlayering length for a ventilation velocity of 2 m/s are predicted. Both the CFD prediction and an analytical correlation indicate that an increase of the heat generation rate enhances the flame length, and while, a growth of the ventilation velocity shortens the flame length. The effects of blockage in a tunnel on the propagation of smoke and fire are numerically investigated with the model. The results of this study have shown that the model is promising in qualitatively examining the correlation of the propagation of smoke, the production and the transport of carbon monoxide and soot with the ventilation rate and the fire heat release rate.

► An increase of the flame length occurs with an increase of heat release rate.
► A growth of the wind velocity results in a reduction in the flame length.
► A large recirculation behind a blockage leads to a reduction in the critical ventilation velocity.
► The flame propagation is strongly enhanced when an object is located at the upstream of the fire source.
► A decrease of the ventilation velocity results in an increase of soot and CO productions.