The Experimental Reserch Of The Beaded Nanofluid Morphological Change During The Evaporation Process

Nanofluid as a new kind of heat transferring mediums, has greater thermal conductivity and better heat transfer performance, so they have broad application prospect and value; Droplets impingement and spray cooling are based on the droplets evaporation and have some advantages such as small demand of working media,uniform and powerful heat dissipation performance,therefore the combination of the latest heat transfer working medium and the best way of heat transfer has a practical significance in cooling technique impovement and optimization. This article adopts the experimental method to study beaded nanofluid droplets evaporation dynamic characteristics deposited on the heated solid substrate with controlled wettability,and analyzes the influential factors of the droplets evaporation rate.This paper uses TiO2 aqueous nanofluid which the volume fraction is 5%, and the particle diameter respectively are 5nm and 20 nm,and adopts the video contact angle measuring instrument to observe and record droplets evaporation characteristics on hydrophilic glass plate. The experiment finds that during the whole evaporation process, the dynamic characteristics of the two kinds of droplets are mainly concentrated in two stages, and for different modes of droplet evaporation, temperature is the most direct influence factor. In the whole temperature ranges nanosuspension with a size of 5nm particle has experienced the process of the pinning in the first stage and depinning in the second stage respectively. With the increasing of temperature,the proportion of pinning phase is rising during the droplet lifecycle,that is to say, the higher the temperature is,the more obvious of the pinning effect of the three-phase contact line.For nanofluid with a size of 20 nm particle,droplet contact diameter and contact angle decrease continuously in the first stage,while in the second stage the contact diameter still keeps the trend of decreasing, but the contact angle starts to increase. With the increasing of the heating plate temperature, the contact angle increasing magnitude maintains upward momentum. Two kinds of droplets evaporation rate are almost the same under the condition of low temperature,but in the high situation, the former’s evaporation rate is ahead of the latter significantly.Considering the hydrophicity and hydrophobicity,the dynamic wetting and dewetting of sessile droplets in the course of evaporation have been investigated on various hydrophilic and different degrees of hydrophobicity surfaces. Three different hydrophilic and hydrophobicity surfaces are used in this work: glass,stainless steel and polytef,and the experiments find that evaporation characteristics of droplets on the polytef surfaces have the same state roughly in the whole temperature ranges and droplets are in the pinning station which the contact diameter remains constant and the contact angle decreases continuously,as well as the duration of the pinning station is 80% of the whole evaporation time.Evaporation characteristics of droplets on the stainless steel surfaces present uncertainty and randomness.Droplets evaporation rate on three kinds of heating surfaces keep to the following rule: droplets evaporation rate on the polytef surfaces is in status of inferior position all the time,and droplet on the glass surfaces is the strongest one and droplet on the stainless steel surfaces is the stronger one in the low temperature range. However, in the high temperature range the situation is reversed.On the basis of experiment this paper deduces the variable relationships between the contact diameter and the contact angle over time following the corresponding evaporation models.When the nanofluid evaporation process of is finished,different sediment patterns are observed on solid surfaces.