Experimental and numerical study on crystallization paths and crystal morphology of water-NaCl droplets evaporation

The interaction between crystallization and internal concentration profile leads to diverse crystallization paths and crystal morphologies during the brine droplet evaporation. It can not rely solely on experimental observation or numerical simulation to explain the differences between crystallization paths and crystal morphologies when brine droplets evaporate under different conditions. Here, a numerical model was developed based on the Population Balance Model to provide the detailed internal concentration profile, serving as a complementary microcosmic perspective to droplet evaporation experiments. A new five-stage partition for the evolution of the dimensionless temperature was proposed to describe the evaporation and crystallization of the brine droplets. The crystallization start time is summarized as a function of the initial droplet concentration and the theoretical evaporation time of pure water droplets at identical conditions. The nucleation mechanism was investigated, explaining the self-regulation mechanism of critical supersaturation S_c = 1.8, at which crystallization begins, by classical nucleation theory. And explanations for the formation of single-crystal and multi-crystal morphologies were provided. Four distinct crystallization paths and their associated morphological patterns were summarized. The different paths were distinguished by the Peclet number and the droplet initial concentration. The relevant results have implications for the understanding and design of the brine spray evaporation process.