Evaporation and crystallization process for sessile saline droplets during depressurization
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THE EUROPEAN PHYSICAL JOURNAL E
Regular Article
Evaporation and crystallization process for sessile saline droplets during depressurization Lu Liua , Kaiqi Zhang, Shuxian Kong, Xilin Wang, Libo Yong, and Menglong Mi Department of Power Engineering, North China Electric Power University, No. 619 Yonghuabei Street, Baoding 071003, China Received 30 January 2020 and Received in final form 20 April 2020 Published online: 10 June 2020 c EDP Sciences / Societ` a Italiana di Fisica / Springer-Verlag GmbH Germany, part of Springer Nature, 2020 Abstract. The evaporation and crystallization process for sessile saline droplets during depressurization is experimentally studied. The relationship between ambient pressure and the crystallization pattern is primarily discussed. When the ambient pressure is low, salt particles are easily formed at the droplet contact line. In contrast, when the ambient pressure is similar to atmospheric pressure, it is more likely for cubic crystals to be formed inside the droplet. By analysing the contact angle fluctuation during crystallization, the experimental results show that the growth of a cubic salt crystal under high ambient pressure or low salt concentration leads to a greater deformation of the liquid-gas interface and a larger contact angle fluctuation. Finally, the Peclet number P e is introduced to reflect the ratio of the rate of ion advection to the rate of diffusion. The P e number is larger at lower ambient pressure, which means that the external mass transfer and convection effect is more significant under low pressure, with salt particles easily formed at the droplet contact line. The effect of concentration diffusion inside the droplet increases at higher ambient pressure, thereby, making it easy for cubic crystals to be formed inside the droplet.
1 Introduction Saline droplet evaporation during depressurization is a coupled heat and mass transfer process accompanied by a phase change, which is involved in numerous applications, including flash distillation [1,2] and seawater desalination [3–5]. Many researchers have investigated pendant or moving saline droplet evaporation during depressurization. Liu et al. [6,7] experimentally investigated the temperature variation of a saline droplet (with NaCl salinity of 0 wt.%, 10 wt.% and 26 wt.%) during depressurized evaporation, and the result showed that the droplet evaporation rate changed with varying salt concentration. Muthunayagam et al. [8] developed a vapour diffusion model to estimate the variation of droplet temperature and diameter during the evaporation process of a saline droplet at reduced pressure. Fathinia et al. [9] experimentally investigated the temperature distribution of saline spray, with the effects of salinity, superheat degree and flow rate on the temperature distribution and evaporation rate analysed. The above studies focus on pendant or moving saline droplet evaporation at reduced pressure, without considering the evaporation process after droplet impact onto a surface. However, to the best of our knowledge, there
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