Gas-Phase Mixing in Droplet Arrays

Droplet evaporation is usually modelled as a subgrid process and induces local inhomogeneities in the mixture fraction probability density function (PDF) and its scalar dissipation. These inhomogeneities are usually neglected, however, they can be significant and determine the combustion regime. In the present work, Direct Numerical Simulations (DNS) of fully resolved evaporating methanol droplets are analysed, assessing fuel vapour mixing in laminar and turbulent flows. The results show that scalar probability distributions and scalar dissipation vary greatly depending on the position relative to the droplet position, on droplet loading and on flow conditions. The beta-PDF seems to capture the global behaviour for laminar flows around droplet arrays with low droplet density, however, mixing characteristics for higher droplet densities in stagnant and turbulent flows cannot be approximated by a beta-PDF, and modelling approaches based on cell mean values will lead to erroneous results.