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 1.2–۱٫۵ 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 (a) over the conductive heat transfer (k) is observed for the layer height d over 1.8–۲٫۰ mm. For pure water, convective and conductive components are comparable even for d = 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.

Introduction

Evaporation of aqueous salt solutions is widely observed in nature, biology and medicine. Evaporation of highly concentrated salt solutions is accompanied by appearance of a new crystalline phase in the form of crystalline hydrates and salt crystals [1,2]. Hightemperature evaporation of LiBr salt solutions results in formation of salt crystalline hydrates and may lead to crystalline plugs. Evaporation and absorption of water in aqueous salt solutions of LiBr; CaCl2; LiCl are used in desorbers and absorbers of heat pumps [3]. The evaporation rate of solutions depends on many factors: the temperature of the free surface of the liquid, the concentration of the components, the thermophysical properties of the wall heater [4], wettability [5], the pressure of the external medium [6], as well as on the convection in the gas and liquid phase [7]. Thermophysical features of convection in gas and evaporation were presented in [8]. At evaporation of a thin layer on a heated wall, the important role is played by gas convection and external turbulence, which accelerates drying and intensifies heat and mass transfer [9]. A structural wall influences evaporation and convection in a liquid layer [5]. Stability and break-up of a thin liquid film on structured surface is discussed in [10]. The effect of wettability on evaporation and crystallization was considered in [11–۱۳]. The behavior of aqueous salt solutions significantly differs from the behavior of multicomponent volatiles liquids [14,15]. At evaporation of aqueous salt solutions only water evaporates, and salts remain in the solution. The rate of evaporation of water solutions of salts decreases with time due to the increase in salt concentration. In most studies the numerical simulation of heat and mass transfer processes in a thin film of aqueous salt solutions does not take into account convection in the gas and liquid phases [16–۱۸]. Modeling of heat transfer conditions in lubricant emulsions was considered in [19]. The dependence of the physical properties of solutions on concentration of salts is discussed in [20–۲۳].