Heat transfer of aqueous salt solution layers

Abstract

Heat transfer and evaporation of layers of water and aqueous solutions of salts on a heated horizontal wall were studied experimentally. Aqueous solutions of salts can be divided into two characteristic groups. For the first group of salts, the evaporation rates and heat transfer coefficients increase with time. For the second group, the rate of evaporation falls sharply with increasing salt concentration and with decreasing liquid layer height. This difference in salts’ behavior is determined by the difference in equilibrium curves and in physical and chemical properties of salts. The heat transfer coefficient for water and salt solutions increases when the layer height becomes less than 1.2–1.5 mm. With increasing concentration of salt and when approaching the crystallization point the role of free convection in the liquid phase decreases sharply, and the Nusselt number approaches 1. For salt solutions (LiBr, CaCl2 and LiCl), a significant excess of convection (α) over the conductive heat transfer (λ) is observed for the layer height δ over 1.8–2.0 mm. For pure water, convective and conductive components are comparable even for δ = 3 mm. This difference for salts is associated with substantial intensification of heat transfer, which is probably caused by the concentration flow of Marangoni MaC. Strong influence of MaC on heat and mass transfer in a thin layer and at high temperatures is detected for the first time and is extremely important for accurate modeling in unsteady and non-isothermal processes. Experimental data show a surprising result. The free liquid convection for salt solutions significantly exceeds the convection in the water layer for the most part of the evaporation time.