Experimental study on evaporation of aqueous NaCl solution droplet heating by thermal irradiation
In order to achieve efficient and low-cost desalination of saline wastewater using solar energy,this study designed and constructed a single droplet experimental system with thermal radiation heating.NaCl water solution was used as the working fluid and experimental research on the evaporation process of single droplets under thermal radiation heating was conducted.A laser with wavelengths of 1450 and 1930 nm was used as the thermal radiation source,with a power density between 1.4×105 and 2.1×105 W·m-2,the initial mass fraction of the droplets ranged from 0.13 to 0.26 and the initial volume ranged from 1.0 to 5.0 μl.The results indicate:(1)The evaporation process of NaCl water solution droplets can be divided into four stages:heating,crystallization,thermal expansion,and cooling stage,with crystallization and thermal expansion being the main stages of droplet evaporation.(2)Increasing external radiation power or using high absorption coefficient bands can simultaneously shorten the duration of the heating,crystallization,and thermal expansion stages;increase the average surface evaporation rate.(3)Increasing the initial mass fraction or initial volume of the droplets can simultaneously increase the crystallization temperature,the maximum temperature during the thermal expansion stage and the average surface evaporation rate;the duration of the crystallization stage decreases with an increase in the initial mass fraction,but it is independent of the initial volume of the droplets.(4)Within the scope of this study,the average surface evaporation rate of NaCl water solution droplets ranged from 0.7×10-8 to 7.4×10-8 kg·s-1 and the droplet temperature remains the main factor affecting the surface evaporation rate.Based on the experimental results,an experimental correlation formula for the average surface evaporation rate of NaCl water solution droplets under thermal radiation heating was provided,with the main error between calculated and experimental values within±20％.The results of this study can provide reference for the design and operation of desalination systems for saline wastewater under thermal radiation drive.
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