Assessment of the parcel model in evaporating turbulent diluted sprays within a Large-Eddy-Simulation approach

The parcel concept has been exploited in the Eulerian-Lagrangian framework to alleviate the computational cost of tracking an enormous number of particles/droplets in the perspective of dispersed two-phase flows. In this work, with fully systematic analysis, we present a numerical investigation of the reliability of this parcel approach in turbulent flows bearing evaporating droplets. To fulfil this task, we address Large-eddy Simulations (LESs) of a turbulent diluted acetone jet-spray in conjunction with two filtering widths and various numbers of parcels, in which the latter is achieved by reducing the ensemble of all physical droplets with a coarsening factor varying from 0(1) to 0(1000), i.e. parcel ratio (PR). By comparing different realizations of parcel in LESs against that with PR = 1, as well as a fully resolved Direct Numerical Simulation (DNS), our numerical results demonstrate the robustness of the parcel concept only when the ratio between the computational and physical droplets is carefully implemented, depending on the grid spacing. In particular, a significant underestimation of the evaporation process is anticipatable if the parcel ratio exceeds a specific threshold which is much larger in the case of a coarse mesh than a fine one. We propose that the appropriate parcel ratio should be of the order of the filtering width and the dissipative length ratio, thus of the same order as the coarsening factor of the Eulerian mesh spacing with respect to the reference DNS.