Experimental And Theoretical Study Of Sessile Droplet Evaporation During Depressurization

The evaporation processes of sessile water droplets and brine droplets under depressurization were experimentally studied in this paper.The evaporation characteristics of water drops under a reduced pressure environment can be widely used in food freezing,aerospace,medical and other fields,while the evaporation characteristics of brine droplets under a reduced pressure environment can be applied to seawater desalination,wastewater treatment and other fields.The mechanism of phase change heat and mass transfer is involved in the process of depressurized evaporation of water droplets and brine droplets.At the same time,it also relates to the impact of environmental pressure change on droplet evaporation.And the evaporation process of brine droplets is more complicated due to the continuous change of component concentration.In terms of experiments,deionized water droplets and brine(NaCl solution)droplets were used.For deinoized water droplets,the evaporation process of water droplets on copper,glass slide,and polytetrafluoroethylene(PTFE)substrates under the final environmental pressure of 0.99～6.5k Pa was studied.The results showed that the evaporation process of deionized water droplets followed the constant contact line stage and the contact line contraction stage.And the lower the environmental pressure,the faster the evaporation rate.The evaporation process of water droplets on the cooper surface was similar to that on the slide surface.When the environmental pressure was higher(p>4.17k Pa),the droplet height changed linearly with time in most of the evaporation time,while when the environmental pressure was lower,the droplet height changed faster with time during the later stage of evaporation.And when the ambient pressure was in the range of 1.76～5k Pa,bubbles appeared on the surface of the PTFE,which caused the dimensionless height of the droplets to increase,and the contact angle of the droplet increased.For saline(NaCl solution)droplets,the variations of droplets contact line and contact angles of droplets on three substrates,as well as the final deposition morphology on the three substrates were related to environmental pressure and droplet concentration.When the mass concentration of NaCl droplet was 15%and the environmental pressure p≥3.34k Pa,the contact angle of the brine droplets on the surface of the glass slide increased.The increment of the contact angleΔθincreased with the increase of the environmental pressure.With the increase of environmental pressure,the value of k_θdecreasd first and then increased.On the surface of PTFE,the contact angle of droplets fluctuated up and down due to bubbles generated by vaporization.The change of the contact angle was related to the environmental pressure and the droplet concentration.When the final environmental pressure p=2.65k Pa,the contact angle differenceΔθand the angle growth rate k_θdecreased with the increasing of NaCl concentration.Finally,the influence of Pe number on the crystallization pattern was analyzed.The results showed that the Pe number on the surface of smooth copper was the smallest during the entire depressurized evaporation process,while the Pe number on the PTFE surface was the largest during the depressurization stage.The larger the Pe number was,the more significant the effect of external mass transfer and diffusion was,and the crystallization was more likely to form at the contact line.On the contrary,the smaller the Pe number was,the stronger the concentration diffusion effect inside the droplet,and the tetragonal crystal was easier to form inside the droplet.