MICROMIXING CHARACTERISTIC TIME AND CLOSURE MODELS FOR PREMIXED TURBULENT COMBUSTION AT MODERATE DAMKOHLER NUMBERS

The modelling of micromixing in premixed turbulent combustion is revie wed for a moderately fast chemistry and existing probabilistic closure s 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 compari sons between closure models using the probability density function and the direct numerical simulation for turbulent combustion were perform ed to assess the sensibility and prediction capabilities of these mode ls; in particular, the binomial model of Dopazo and Valino, compared t o other tested models, yields a good quantitative as well as qualitati ve behavior. More interestingly, the study has shown that the classica l assumption that the micromixing time scale is proportional to the tu rbulence integral time scale was not at all convenient in the investig ated case. This suggest that a precise calculation of the micromixing time scale (i.e., the scalar dissipation time scale) can be more impor tant than the micromixing model itself. For our case, the transient ev olution of this time scale has been found more important than the infl uence of chemical reaction.