Evaporation and heat transfer of aqueous salt solutions during crystallization

Abstract

Evaporation, heat transfer and surface crystallization of thin layers of aqueous solutions of CaCl2, LiBr and LiCl salts have been studied experimentally. The control of crystallization rate is extremely important for a wide range of modern technologies, associated with both slow cooling rates and extremely rapid droplet and film crystallization (for example, in nanocoatings). Usually, when calculating the crystallization, the heat transfer coefficient and the evaporation rate are assumed quasi-constant, and the effect of evaporation rate of solution on the rate of crystallization is not taken into account. In this paper, it is shown that even without crystallization the heat transfer coefficient may vary several times, when the height of the film rapidly decreases. This study has shown that the character of crystallization curve and crystallization rate strongly depend on the rate of solution evaporation before the onset of phase transition. Abnormally high crystallization rates 30–50 mm/s are achieved at extremely low supersaturations of solutions (the salt solution is in the quasi-equilibrium region), and this is associated with the rapid growth of dendritic forms. With the growth of the surface crystalline film, the evaporation rate of the solution in metastable state increases significantly and has a non-stationary “oscillation” character. The coefficient of heat transfer before the onset of crystallization also changes significantly. The formation of dendrites is possible if crystallization is realized in thin supersaturated layers with high gradients of temperature and concentration. In this case, very high crystallization rates will be provided by dendritic forms.