Numerical Simulation Of The Evaporation Process Of A Sessile Droplet In Its Low-pressure Vapor

The evaporation process of sessile droplets is widely existed in many fields such as agriculture,industry,aerospace and medicine.It is a complex physical process involving fluid flow,heat transfer,phase change and contact line dynamics.Efficient heat and mass transport characteristics in the process of sessile droplet evaporation have great application prospects in the fields of science and technology,such as ink-jet printing,disease diagnose,spray cooling,stretching and mapping of DNA molecules,etc.To the best of our knowledge,there are many basic researches on the process of sessile droplet evaporation.However,most of them focus on the evaporation process of the sessile droplets in the open air,but the evaporation of the sessile droplets in the low-pressure pure vapor environment is rarely involved.Many problems about evaporation of sessile droplets in vapor remain unsolved.For this reason,the numerical simulations on the evaporation process of sessile water droplets and ethanol droplets in pure vapor are carried out,and the evolution of the physical field as well as the change of evaporation intensity of the droplets are obtained.The main conclusions are as follows:(1)For the evaporation of the sessile water droplet in its vapor,it is found that the increase of substrate temperature will lead to the increase of droplet surface evaporation rate and the strength of Marangoni convection.With the increase of substrate temperature,the internal flow in the droplet changes from double vortex to four vortex,and the surface temperature distribution transmits from a symmetrical ring shape to a petal shape and a zigzag shape.The Marangoni flow in the sessile water droplet is enhanced with the increase of the contact angle,but the surface evaporation rate shows a trend of increasing first and then decreasing.The surface evaporation rate of the droplet gradually increases with the decrease of the vapor pressure,while the Marangoni flow tend to be stronger.When the vapor pressure is reduced,the flow changes due to the instability of Marangoni flow,and the surface temperature transits from a ring distribution to a triple-cell distribution.(2)For the evaporation process of the sessile ethanol droplet,the surface temperature distribution and flow in the droplet undergo a change from a ring-shaped distribution to a petal-like distribution with the increase of the substrate temperature.The internal flow becomes unstable when the substrate temperature is furtherly increased,and forms a periodic flow.The surface evaporation rate of ethanol droplets increases with the increase of contact angle when the substrate temperature is low,but increases first and then decreases when the substrate temperature is high.The relationship between the evaporation rate and the vapor pressure is the same as that of the water droplet.As the vapor pressure decreases,the evaporation rate of the ethanol droplet gradually increases and the Marangoni flow is strengthened.Besides,when the pressure of ethanol droplet is lower than the saturated vapor pressure,the flow will be unstable and forms a periodic flow.The oscillation period is shortened but the amplitude of the oscillation is increased with the decrease of pressure.