Study On The Supercooling Inhibition And Nucleation Characteristics Of Graphene Oxide Nanofluids Under Acoustic Levitation

Ice storage technology can play an effective role peak-load shifting, balance network load, and promote power generation efficiency. But water don't behave a high thermal conductivity in the solidification process and has a large supercooling degree, which decreases the operating efficiency of ice-storage system. Nanofluids, as a new type of phase change materials for cool storage, can improve cool storage system in heat transfer performance. Moreover, heterogeneous nucleation effect caused by nanoparticles is helpful to reduce supercooling degree and raise the evaporation temperature of refrigerating unit. Compared with base fluid, nanofluids has a smaller supercooling degree is verified by many researchers, however, almost all kinds of nanofluids are measured in container, and few literature has reported the mechanism of nanoparticles suppress supercooling degree. In order to eliminate the interference of container wall on nucleation process, this article adopts acoustic levitation technology to research the supercooling and nucleation characteristics of graphene oxide nanofluids. The main contents and results are summarized as follows:(1)Graphene oxide nanofluids with mass concentration of 0.03% are prepared by utilizing ultrasonic dispersion technology and deionized water is used as base fluid. The consequences of static observation and particle size and zeta potential measurement indicate that the prepared nanofluids have a good suspension stability. Constructing an acoustic levitation cooling experimental system, debugging and running results show that the constructed experimental system can make droplet stable suspension, cool and freeze, measure and record temperature in real time.(2)Analyzing the factors that influence on the droplet suspension stability The calculations indicate that ultrasonic transmitter and reflector will lose resonance when medium temperature declines a lot, acoustic levitation force decreases rapidly. On the basis of calculation and analysis results, this article puts forward a method to keep suspension droplet stable. The supercooling characteristics of deionized water and graphene oxide nanofluids are studied by comparative experiments. The results confirm that acoustic levitation technology can remove the container wall which may induce heterogeneous nucleation and contribute to achieve large supercooling degree.(3)Nucleation experiments of deionized water droplet with a volume of 30?L are conducted when ultrasonic generator power is set as 20 W and 32 W respectively, and phase change supercooling degree distributions are acquired through conducting five hundred nucleation experiments under each power value. The nucleation rate expressions under the two kinds of power condition are worked out by using statistical nucleation theory and classic nucleation theory to deal with supercooling degree distributions. Further analysis shows that augment acoustical power can enlarge effective nucleation area and decrease critical nucleation work, consequently, increasing the nucleation rate of droplet. Based on experimental observation and Seeley-Seidler statistical approach, nucleation characteristics of deionized water droplet is researched, which reveals that nucleation mainly occurs on the surface area of droplet, in addition, the mechanism of surface nucleation is elucidated.(4)The same experimental conditions and methods are adopted to measure the supercooling degree of graphene oxide nanofluids with a volume of 30?L and mass concentration of 0.03%. Nucleation rate of nanofluids droplet is worked out by using statistical analysis and fitting calculation. Analyzing the influence of acoustical power on nucleation area, critical nucleation work, and nucleation rate. Based on volume nucleation and surface nucleation hypothesis respectively, droplet nucleation characteristics is analyzed and it turns out that graphene oxide nanofluids droplet occurs volume nucleation and surface nucleation meanwhile. Triangle cylinder model of single graphene oxide nanoparticle is established on basis of AFM and STEM images. Applying heterogeneous nucleation theory to study the mechanism of nanoparticles contributes to nucleation and the results show that thermodynamic conditions of surface nucleation and volume nucleation are much the same.(5)Supercooling degree, critical nucleation work, nucleation rate of deionized water and graphene oxide nanofluids droplets are compared and analyzed. The results indicate that adding nanoparticles can decline supercooling degree, when power is 20 W and 32 W, the average supercooling degree of nanofluids reduces by 2.7K and 2.3K, critical nucleation work decreases by sixty and fifty five percent. In addition, nanofluids has a larger nucleation rate and a higher nucleation rate growth than deionized water in the same condtion. The main reason is that nanoparticles has a hydrophilic behavior and heterogeneous nucleation effect, which can lessen critical nucleation work and make crystal nucleus easy to form.