Investigation On The Coupling Characteristics Of Stable Evaporation Of Sessile Droplets And Its Induced Thermal Convection In Different Environments

Sessile droplet evaporation is widely present in practical processes such as spray printing,spray cooling,pesticide deposition,and disease diagnosis.The evaporation dynamics contained in it has an important influence on the actual process.In recent years,the evaporation process of sessile droplet has received increasing attention,and the coupling relationship between the evaporation and its induced thermal convection has gradually become a research hotspot.Most of the existing researches focus on the evaporation process of sessile droplets in a normal temperature and pressure environment,and rarely involve the evaporation process in pure vapor environment at a low-pressure pure.In the pure vapor environment with low-pressure,the evaporation driving force is greater,the evaporation is stronger,and the evaporation-induced thermal convection process is also more complicated.At present,little is known about the thermal convection flow pattern and its evolution process induced by the sessile droplet evaporation in pure vapor environment at low-pressure.Moreover,the understanding of the coupling relationship between thermal convection and evaporation rate is almost blank.Therefore,this thesis intends to use a combination of steady-state evaporation experiment and three-dimensional numerical simulation to systematically study the coupling characteristics of sessile droplet evaporation and the thermal flow induced by itself in different environments.The effects of liquid type,substrate properties,gas pressure and droplet size on evaporation rate and thermal convection are analyzed.The coupling mechanism of evaporation rate and thermal convection is revealed.The main research contents and results are as follows:(1)The evaporation process of sessile droplets in normal temperature and pressure environment is studied by using steady-state evaporation experiment.The results show that when ethanol and isopropanol droplets evaporate on a copper substrate,the surface temperature distribution will present the uneven temperature fluctuations near the droplet apex,hydrothermal waves(HTWs),and Bénard-Marangoni(B-M)cells in sequence with the decrease of the droplet height.Compared with copper substrate,when isopropanol evaporating on aluminum substrate,there are only uneven temperature fluctuations and convection cells on the evaporation interface.When water droplet evaporates on copper substrate,it is difficult to observe the thermal patterns caused by thermocapillary flow.Only when the substrate temperature and the droplet height are all high enough,a ring-shaped temperature distribution caused by buoyancy flow is observed on the interface The increase of substrate temperature or thermal conductivity will promote the evaporation rate.For ethanol droplet,the evaporation rate is affected by the evaporation area,surface temperature and thermal flow.As the height decreases,the evaporation rate decreases first and then increases.During the evaporation process,the upward thermal flow will promote the evaporation rate.For non-volatile liquid as water,there is almost no obvious thermal patterns during the evaporation process.Thus,the evaporation rate of water droplet is less affected by the evaporation area and surface temperature.Therefore,its evaporation rate does not change much with the droplet height.(2)The evaporation process of ethanol and water droplets in pure vapor at low-pressure are studied by using the steady-state evaporation experiment.The results show that the strong evaporative cooling effect makes the droplet surface temperature distribution more uniform in the pure vapor environment at low-pressure than in the normal temperature and pressure environment.When the pressure is relatively high,no matter what kind of liquid,the temperature distribution in most interface seems uniform.However,when the pressure is relatively small,temperature fluctuations will appear on the droplet surface.For water droplet,as the height decreases,a ring-shaped temperature distribution appears on the surface.For ethanol droplet,when the substrate temperature is low,a steady-state gear-like thermal pattern appears near the three-phase contact line.With the increase of the substrate temperature,the steady-state gear-like thermal pattern becomes unstable,and thermal cells will appear near the droplet apex.As the pressure decreases,the evaporation rate of water and ethanol increases.In pure vapor at low-pressure,as the droplet height decreases,the evaporation rates of both water and ethanol decrease first and then increase.(3)A three-dimensional numerical simulation was used to study the ethanol evaporation process on a copper substrate with a radius of 2.5 mm.The results show that the buoyancy has little effect on the evolution of surface temperature when the radius is 2.5 mm.However,it is more consistent with the experimental results when considering the buoyancy.From the internal flow and temperature fields of the droplet,it can be seen that the thermal cells caused by the thermal flow from the bottom to the surface appear on droplet surface when the pressure ratio is 0.6.When the pressure ratio is increased to 0.9 and the droplet is high enough,there is a central high temperature area on the droplet surface caused by buoyancy convection.When the pressure ratio is0.6,regardless of whether the buoyancy effect is considered,the evaporation rate decreases first and then increases with the decrease of the droplet height decrease.When the pressure ratio is 0.9,with the decrease of the droplet height,the evaporation rate keeps almost constant first,and then gradually increases.The droplet evaporation area and the average internal thermal resistance are the key factors affecting the variation of the evaporation rate.The production of thermal cells will promote the evaporation rate.With the evolution of the thermal cells,the evaporation rate will change accordingly.(4)Based on the three-dimensional numerical simulation of the evaporation process of sessile droplets on different substrates,the following conclusions can be drawn.Compared with the results on copper substrate,the droplet surface temperature is more uniform when ethanol droplet evaporates on a PTFE substrate with a radius of2.5 mm in the pure vapor environment at low-pressure.When the pressure ratio is 0.6,the height of the first appearance of thermal cell on the PTFE substrate is higher than that on copper substrate.When the pressure ratio is 0.9,the central high temperature area on droplet surface is more obvious than that on copper substrate.The evaporation rate of droplet on the PTFE substrate is significantly lower than that on the copper substrate,and this difference becomes more obvious as the droplet height decreases.According to the changes in the surface temperature distribution,internal flow field,temperature field and evaporation rate of ethanol droplets on a copper substrate with a radius of 1.5 mm,it can be seen that the buoyancy is negligible in pure vapor at low-pressure pure when the droplet radius is smaller than the capillary length.