Effects of the preferential segregation of droplets on evaporation and turbulent mixing

Abstract

Droplet segregation in isotropic homogeneous turbulence is analysed using a spectral direct numerical simulation solver to describe the evolution of the turbulent carrier phase, whose characteristic properties remain statistically stationary due to a semi-deterministic forcing scheme. Lagrangian dilute spray modelling is employed to describe the discrete-phase evolution. The liquid density is distributed on an Eulerian mesh to analyse the evolution of the spray and its spatial distribution. This gives results in accordance with classical methods for droplet segregation. It also allows a deeper analysis of the spray evolution. In particular, droplet segregation and vapour mass fraction may be analysed jointly. First, droplet segregation phenomena are studied through the analysis of the formation and the geometry of the droplet clusters. Then, the effects of segregation on spray evaporation are investigated from both the dispersed and carrier phase points of view. At equilibrium, droplet dynamics leads to different segregation levels that are associated with characteristic Stokes numbers. It appears that the evaporation process evolves in three different stages in time: single-droplet mode in the early stage, cluster mode in the intermediate stage and a gaseous mode in the late stage. Segregation levels strongly affect the evolution of the mean vapour mixture fraction during the second stage, while the corresponding standard deviation is affected for longer, up to the third stage in our simulations. However, from the evolution of the integral scale and the shape of the energy spectrum, it appears that turbulent mixing eliminates the segregation effects, apart from the first evaporation stage when the droplet segregation determines the vapour distribution.