دانلود رایگان مقاله انگلیسی انتقال حرارت از لایه های محلول نمکی آب - الزویر 2018

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 (a) over the conductive heat transfer (k) is observed for the layer height d over 1.8–2.0 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.

Conclusion

Evaporation and heat mass transfer of the layers of aqueous salt solutions have been studied experimentally. Experimental data of aqueous salt solutions are compared with the water layer. Aqueous solutions of salts can be divided into two characteristic groups. For water and for the first group of salts (NaCl, CsCl, BaCl2), the evaporation rate j is quasi constant for most part of the evaporation time and heat transfer coefficient increase with time at the end of evaporation. The evaporation rate of salt solutions of CaCl2, LiCl, LiBr and MgCl2 decreases with time. For the second group (NaCl, CsCl, BaCl2), the rate of evaporation falls sharply with increasing salt concentration and with decreasing height of the liquid layer, and the heat transfer coefficient al has a pronounced extremum. The division of salts into two groups can be explained by different values of the equilibrium partial vapor pressure. In order to determine the group of salts to which the aqueous salt solution should be attributed, it is necessary to consider the following properties of salts: molar mass of salt, the heat of salt dilution and the degree of salt solubility (the beginning point of crystallization). This analysis will help to determine the conditions for modeling the evaporation and heat transfer process. For example, it will become clear whether to consider the diffusion equation or limit to solving the energy and momentum transfer equation. The equation for the evaporation rate for the second group of salts can be considered in the quasi-stationary formulation, i.e. assuming that pvs = const. For the first group of salts, pvs can change by an order of magnitude. In this case, the entire evaporation period can be divided into small intervals, and for each Dt interval one should assume pvsi = const. Thus, the nonstationary problem can be reduced to quasi-stationary even for the first group of salts.