Modeling of the Denebulization of Warm Fogs by Hygroscopic Seeding: Effect of Various Operating Conditions and of the Turbulence Intensity

This study addresses the modeling of the denebulization (i.e., the removal of droplets) of warm fogs (T >= 0 degrees C) by hygroscopic salt microparticles from the initial seeding at the top of the fog layer to the fall of the rain droplets on the ground. Two main phenomena can occur: condensation of water vapor on salted droplets and the concomitant evaporation of fog droplets, and coalescence between the salted droplets and the fog droplets. Three salts have been investigated: NaCl, CaCl2, and KCl. Based on the conservation equations, the modeling approach (1D) considers the hygroscopicity of the salts through the water activity in the aqueous solution and the coalescence induced by gravity and turbulence. From this study, NaCl is the most efficient salt in the tested operating conditions. Actually, this result can be explained by the strong hygroscopicity of this salt in very dilute solutions. From the calculations, 15 kg of NaCl particles of 6.7-mm diameter can dissipate a typical fog layer of 40m in height within less than 17 min over 0.25 km(2). According to the calculations, a fog layer of 100m in height can be denebulized within 45 min. The contribution of the coalescence induced by gravity and by turbulence seems to have a negligible effect on the final horizontal visibility, the condensation/evaporation phenomena being preponderant for these operating conditions.