Numerical Simulation of Crystallization in Flash and Mixing Evaporation(FME)of Quasi-saturated Aqueous NaCl Solution
The combination of spray flash and mixing evaporation(FME)was one of the most effective way to process wastewater with high aqueous salt concentration.In this paper,aqueous NaCl solution was selected as working fluid,a comprehensive calculation model for flow field of FME,including movement,evaporation and crystallization of droplets was set up on basis of previous experimental results.Numerical simulation was carried out with initial diameter of droplets between 20 and 200 μm,initial temperature between 100 and 120℃,initial mass fraction 0.26,initial speed 20 m-s-1,and air speed of 15 m-s-1,air temperature between 100～300℃,and spray angle between 0 and 90° Results suggested that,during the increasing of spray angle from 0 to 90° the main location of crystallization moved from the spray axis to the top of spray plume,making crystal easier to be separated.Besides,with the increasing of spray angle,superheat or air temperature,the average mass fraction of crystallization increased,but the average particle size and crystallization distance decreased.In order to measure the effect of crystallization within a given distance,complete crystallization efficiency was defined as the ratio of the mass flow rate of crystal salt of droplets to the mass flow rate of dissolved salt at inlet.Results suggested that this efficiency could be improved by increasing spray angle or air temperature.A semi-empirical formula for complete crystallization efficiency was proposed,and the main error between its calculated value and the simulated value was in±35％.Above conclusion could provide technical support for design and operation of industrial desalination system.
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