CFD simulation of stratified fuel-air explosions

The analysis of consequences of gas explosions is often performed by considering "worst case" conditions, i.e. when the flammable gas forms a homogenous stoichiometric mixture. Actually, following the initial dispersion, it is very likely that a stratified mixture is formed where large part of the fuel forms rich or lean mixtures, within or outside the flammability limits. The few models presented in the literature were based on simple gas-dynamic considerations, aiming to furnish the explanation for the observed larger speed of the flame front with respect to the nominal burning velocity. None of them take into account either the different combustion regimes behind the flame front or the concentration gradient itself. These aspects could be predicted by more advanced numerical tools as Computational Fluid Dynamics. However the adoption of very large cell size is mandatory for the description of the explosion behavior in medium and large-scale environment, with the need to introduce a sub-grid model for the combustion. A "front capturing" model for the premixed laminar combustion front, propagating in a layered mixture, has been then developed. The model is based on an in-cell reconstruction procedure from the knowledge of the cell average values. Simulation results compare satisfactory with experimental data obtained for a medium scale combustion chamber. The ability of the code to reproduce the flame speed, by varying the filling ratio along the height, has been proven even adopting very coarse grid.