Study On Temperature Distribution And Deposition Pattern Of Nanofluid Droplet Evaporation Process

The free evaporation of droplets is common in nature and human life,and humans have been studying liquid evaporation long ago.The nanofluid is a potential high-efficiency heat-transfer fluid which has a wide range of development prospects in the fields of aerospace,electronics,surface coating and so on.A complete understanding and unified knowledge of the mechanism of nanofluid droplet evaporation process is lacked in the current study,and there are huge controversies in some parts.Therefore,targeted research on the internal flow,temperature distribution and deposition patterns of droplets can effectively solve the current controversies and problems,and have a more profound understanding of nanofluid droplet evaporation.In this paper,the evaporation behavior of Al₂O₃ water-based nanofluid droplets on glass slides was recorded by experiments on two different nanoparticle sizes?20nm,50nm?,three different nanofluid mass fractions?0.05%,0.1%,0.2%?and four different slide heating temperature?30°C,47°C,64°C,81°C?.The evaporation kinetic parameters were recorded and the evaporation mode was studied.The temperature field distribution at the gas-liquid interface during evaporation of droplets was recorded using an infrared thermography,the deposition pattern was photographed with a microscope at the end of the droplet evaporation,the evaporation flow pattern was inferred.The effects of particle size,mass fraction of nanofluids,and heating temperature of the substrate on the kinetics of droplet evaporation,temperature field distribution,and deposition patterns were obtained.The main conclusions of the experiment are as follows.1.The effect of particle size on the evaporation pattern of nanofluid droplets is the largest.The influence of two factors?substrate heating temperature and fluid mass fraction?is secondary.The evaporation mode is kept in a single mode,and the contact line has a similar constant contact line stage,the contact angle continuously decreases with evaporation.The total evaporation time of the nanofluid droplets decreases by about 80%with the increase of the substrate heating temperature?from 30?to 81??,and decreases by about 21%with the decrease of the mass fraction of the fluid?from0.2%to 0.05%?.The decrease rate of the contact angle increases significantly with the increase of the substrate heating temperature,up to 9 times,increases by approximately 16%with the decrease of the fluid mass fraction.And the change caused by the particle size is not obvious.2.In the experiment,the distribution of the temperature field at the gas-liquid interface during the evaporation of the droplets obtained under all experimental conditions is consistent.The heat conduction effect in the evaporation process is more significant than the evaporative cooling effect,resulting in the highest temperature at the droplet edge and the lowest temperature at the center of the droplet,and the symmetrical temperature distribution.The temperature difference between the center of the droplet and the edge of the droplet at the initial time of evaporation is the largest,and increases with the increase of the substrate heating temperature and the mass fraction of the nanofluid,with the progress of evaporation,the temperature difference decreases until it disappears.The initial Marangoni value increases with the heating temperature of the glass slide,up to 1.7 times.As the evaporation progresses,the Marangoni number will be suppressed instead.Under the same conditions,the initial Marangoni value will increase slightly with the increase of the nanoparticle size and the nanofluid mass fraction.3.The formation of a"the coffee ring"pattern and a"double-ring"pattern are leaded by the outward capillary compensated flow and the presence of the Marangoni flow driven by temperature gradients and stagnation point.The integrity of the inner ring in the double ring pattern increase with increasing Marangoni number.The heating temperature of the substrate has no significant influence on the width of the inner ring in the deposition pattern,and the width of the outer ring of the coffee ring becomes wider as the substrate heating temperature increases,up to 3 times.After the evaporation of the droplets of the large particle size,the outer ring and the inner ring of the coffee ring having a radial structure are obtained.The width of the outer ring of the coffee ring increases as the mass fraction of the nanofluid increases,up to 2 times,and the particles near the coffee ring shift from an orderly packing to an unordered packing.