DROPLET-TURBULENCE INTERACTIONS IN LOW-MACH-NUMBER HOMOGENEOUS SHEAR 2-PHASE FLOWS

Several important issues pertaining to dispersion and polydispersity o f droplets in turbulent flows, are investigated via direct numerical s imulation (DNS). The carrier phase is considered in the Eulerian conte xt, the dispersed phase is tracked in the Lagrangian frame and the int eractions between the phases are taken into account in a realistic two -way (coupled) formulation. The resulting scheme is applied for extens ive DNS of low-Mach-number, homogeneous shear turbulent hows laden wit h droplets. Several cases with one- and two-way couplings are consider ed for both nonevaporating and evaporating droplets. The effects of th e mass loading ratio, the droplet time constant, and thermodynamic par ameters, such as the droplet specific heat, the droplet latent heat of evaporation, and the boiling temperature, on the turbulence and the d roplets are investigated. The effects of the initial droplet temperatu re and the initial vapour mass fraction in the carrier phase are also studied. The gravity effects are not considered as the numerical metho dology is only applicable in the absence of gravity. The evolution of the turbulence kinetic energy and the mean internal energy of both pha ses is studied by analysing various terms in their transport equations . The results for the non-evaporating droplets show that the presence of the droplets decreases the turbulence kinetic energy of the carrier phase while increasing the level of anisotropy of the flow. The dropl et streamwise velocity variance is larger than that of the fluid, and the ratio of the two increases with the increase of the droplet time c onstant. Evaporation increases both the turbulence kinetic energy and the mean internal energy of the carrier phase by mass transfer. In gen eral, evaporation is controlled by the vapour mass fraction gradient a round the droplet when the initial temperature difference between the phases is negligible. In cases with small initial droplet temperature, on the other hand, the convective heat transfer is more important in the evaporation process. At long times, the evaporation rate approache s asymptotic values depending on the values of various parameters. It is shown that the evaporation rate is larger for droplets residing in high-strain-rate regions of the flow, mainly due to larger droplet Rey nolds numbers in these regions. For both the evaporating and the non-e vaporating droplets, the root mean square (r.m.s.) of the temperature fluctuations of both phases becomes independent of the initial droplet temperature at long times. Some issues relevant to modelling of turbu lent flows laden with droplets are also discussed.