Micromixing CharacteristicTime and Closure Models for Premixed Turbulent Combustion at Moderate Damköhler Numbers

Abstract

The modelling of micromixing in premixed turbulent combustion is reviewed for a moderately fast chemistry and existing probabilistic closures for small scale mixing term are presented, discussed and tested. The numerical study is made on a statistically homogeneous and isotropic decaying turbulent premixed medium where, at initial time, pockets of burnt gases are randomly placed, to initiate the reaction. The comparisons between closure models using the probability density function and the direct numerical simulation for turbulent combustion were performed to assess the sensibility and prediction capabilities of these models; in particular, the binomial model of Dopazo and Valin¯o, compared to other tested models, yields a good quantitative as well as qualitative behavior. More interestingly, the study has shown that the classical assumption that the micromixing time scale is proportional to the turbulence integral time scale was not at all convenient in the investigated case. This suggest that a precise calculation of the micromixing time scale (i.e.. the scalar dissipation time scale) can be more important than the micromixing model itself. For our case, the transient evolution of this time scale has been found more important than the influence of chemical reaction.