Numerical investigation of droplet-droplet collisions in a water and milk spray with coupled heat and mass transfer

Abstract

Large scale simulation models can aid in improving the design of spray dryers. In this work an Eulerian-Lagrangian model with coupled gas phase and droplet heat and mass transfer balances is used to study airflow dynamics, temperature and humidity profiles at different positions in the spray. The turbulent gas flow is solved using large eddy simulation (LES). A turbulent dispersion model accounts for the stochastic subgrid fluid velocity fluctuations along the droplet trajectory. The dispersed phase is treated with Lagrangian transport of droplets, and collisions between droplets which are detected with a stochastic Direct Simulation Monte Carlo (DSMC) method. The outcome of a binary collision is described by collision boundary models for water and milk concentrates. The drying of droplets is modeled by the reaction engineering approach (REA). The effect of the inlet air conditions and of droplet viscosity on the temperature and humidity distributions are analyzed. Most of the heat and mass transfer occurs in the first 10-20 cm from the nozzle where the slip velocities and temperature and humidity driving forces are higher. The droplets size increases, both in the axial and radial direction, because of the dominance of coalescence over separation in the droplet spray studied here. Because the spray domain considered in this work is relatively small, the droplet residence time is small, and consequently the amount of evaporation is still low. The droplet size distributions of milk concentrates are affected by the predominance of coalescence over separation events. The coalescence dominated regime increases when the droplet viscosity is higher.

Unraveling Particle Formation: From Single Droplet Drying to Spray Drying and Electrospraying

“…It is worth mentioning that the boundaries between these regimes are influenced by the properties of the feed solution (e.g., concentration and hence viscosity) and the drying gas environment. Studies that investigate the effect of feed viscosity on collision outcomes are nicely reviewed by Finotello et al [ 164 ]. At low number, coalescence of two individual droplets into one larger droplet takes place.…”

“…When increasing the feed rate, more energy is required for solvent evaporation and hence the outlet temperature in the spray drying process will decrease [ 5 ]. According to Finotello et al, the liquid flow rate can be considered as the most important parameter for the number of droplet collisions during spray drying [ 164 ]. Environmental Conditions …”