Heat and Mass Exchange of a Drop of a Solution Subjected to a Combined Energy Action Under Conditions of Deepening of the Evaporation Zone

Abstract

A model of the heat and mass exchange in the evaporation of drops moving in the carrying gas phase of a solution subjected to the combined energy action of a convective heat fl ow and electromagnetic infrared and microwave radiations under conditions where the size of the drops changes at the low-temperature stage of their evaporation and the evaporation zone deepens at the high-temperature stage of the evaporation process has been formulated. Kinetic dependences and features of the movement of the evaporation boundary in the process of dehydration of the indicated drops have been determined. Keywords: heat and moisture transfer, kinetics and dynamics of mass transfer, mass exchange of a drop of a solution.Introduction. The heat and mass transfer taking place in disperse systems gas-liquid particles, used in different industries, determines the effi ciency of many production processes, including the dispersion dehydration of solutions [1-3]. However, the interrelation of the structural, mechanical, physicochemical, heat transfer, and mass transfer effects in the indicated processes is not completely understood.The high-temperature dehydration of dispersed solutions can be simulated on the basis of the equations of transfer in each phase of a solution with regard for its thermophysical characteristics and the coeffi cient of heat and mass transfer in it. However, since the heat and mass exchange in a dispersed solution infl uences the formation of the structure of dry particles in it, the dehydration of drops of this solution is fairly diffi cult to simulate from the physical and mathematical standpoints. A characteristic feature of the drying of drops of a solution or a suspension is the formation, at a defi nite stage of the process, of a solid-phase structure or a crust on their surface [2]. Moisture is transferred though this crust to the surface of a drop, with the result that its porous structure is deformed. The low-temperature stage of dehydration of a drop is completed when the moisture content in it decreases and the dry structure of its solid phase is formed. The structure of a dehydrated particle and its shape and size are determined by the physical properties of the material of the particle, the conditions of its dispersion, and the heat and mass transfer in it. The processes of heat and mass exchange of drops of solutions subjected to the spray drying under the action of a convective heat fl ow are well understood. As already noted, the formation of a crust on the surface of a drop of a solution has been detected and, in [2], this physical phenomenon was described mathematically.The heat and mass transfer in drops of solutions and suspensions and the structure formation in them under the combined energy action of a convective heat fl ow, an infrared (IR) radiation, and a microwave (MW) radiation are less understood. It is known that, in the case of convective heat supply to such a drop, heat is transferred from the surface of the drop into its inside due to the heat conduction of the material of the ...