Evaluation of different numerical approaches for a ventilated tunnel fire

The objective of this study is to examine the feasibility of two combustion models combined with a large-eddy simulation for a full-scale ventilated tunnel fire. The numerical model solves 3D, time-dependent Navier-Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. Turbulent combustion process is modeled by an eddy dissipation concept (EDC) and mixture fraction (MF) approach by using two chemical reaction steps to CO prediction. The predicted temperature in the smoke and flame shape was compared through a Blind method to the experimental data of Apte et al. [Apte VB, Green AR and Kent JH. Pool fire plume flow in a large-scale wind tunnel. Proceedings of the Third International Symposium on Fire Safety Science, July 8-12, Edinburgh, UK: Elsevier; 1991, pp. 425-434] and Fletcher et al. [Fletcher DF, Kent JH, Apte VB and Green AR. Numerical simulations of smoke movement from a pool fire in a ventilated tunnel. Fire Safety J 1994; 23(4): 305-325]. The predicted results of Fletcher et al. [Fletcher DF, Kent JH, Apte VB and Green AR. Numerical simulations of smoke movement from a pool fire in a ventilated tunnel. Fire Safety J 1994; 23(4): 305-325] with k-epsilon model and of Gao et al. [Gao PZ, Liu SL, Chow WK and Fong NK. Large eddy simulations for studying tunnel smoke ventilation. Tunn Undergr Sp Tech 2004; 19(6): 577-586] with large-eddy simulation, both based on a single chemical reaction, are also included. Two boundary conditions, such as adiabatic wall and heat loss inside wall, were applied on the tunnel structure. Both the EDC and MF approaches are shown to possess the ability to predict the general behavior of the experimentally determined temperature field as the heat loss inside the tunnel walls is taken into account. While, it is felt that EDC is more convenient for simulation of a large-scale under-ventilated tunnel fire where the CO and soot yields required by using MF approach, vary significantly with the air to fuel ratio by mass.