Study On The Nucleation And Liquid-solid Phase Change Characteristics Of Graphene Oxide Nanofluids Under Acoustic Levitation

Cool storage technology is an effective measure for power demand side management to achieve the electrical peak-modulating of electric power load. However, the supercooling degree of most phase change materials(PCMs) is so high that the energy efficiency ratio of refrigeration unit is too low. Nanofluids as new type of PCMs received extensive attention because of their ability of reducing supercooling degree during the process of solid-liquid phase change. But phase change experiments were usually accomplished in containers such as test tubes and beakers on which confusing the contribution of nanoparticles, because of existing rough wall surface and adhering foreign substance. In this paper, the acoustic levitation containerless technology is a promising way to study the solid-liquid phase change process of graphene oxide nanofluids. It can efficiently eliminate heterogeneous nucleation induced by container wall. The main contents and results are summarized as follows:①Graphene oxide nanofluids was prepared through supersonic oscillating. The results of static observation, measurement of particle size and zeta potential, and transmission electron microscope experiment show that graphene oxide nanofluids with good suspension stability.②Solid-liquid phase change process of deionized water droplets were studied under acoustic levitation. The results show that supercooling degree of deionized water droplets increases with the addition of cooling rate. Meanwhile, we analyzed the impact of ultrasonic field over the solid-liquid phase change process of deionized water droplets. The result indicates that droplet rotation, morphological oscillation and cavitation effect caused by acoustic field can reduce supercooling degree.③Solid-liquid phase change process of graphene oxide nanofluids droplets were studied under acoustic levitation. The results show that, with the increases of cooling rate, the beginning temperature and the finishing temperature of phase change of nanofluids are both decrease, and supercooling degree increases obviously. Meanwhile, we compared liquid-solid phase change process of nanofluids and deionized water. The result indicates that graphene oxide nanoparticles can reduce supercooling degree.④The homogeneous nucleation rate of deionized water and the heterogeneous nucleation rate of graphene oxide nanofluids were calculated theoretically. The results show that, deionized water freezing is difficult in the case of without any external influence, and the heterogeneous nucleation rate of graphene oxide nanofluids is far greater than the homogeneous nucleation rate of deionized water, and the heterogeneous nucleation rate of nanofluids gradually increases with the increase of concentration and supercooling degree.⑤Through the analysis,we found that inhibition mechanism of supercooling degree of graphene oxide nanofluids droplets under acoustic levitation are as follows: Firstly, Graphene oxide nanosheets have the promotional effects on heterogeneous nucleation of nanofluids droplets. The effective nucleation conditions of graphene oxide nanosheets were calculated based on heterogeneous nucleation theory, and graphene oxide nanosheets were verified can effectively promote heterogeneous nucleation of nanofluids droplets. Secondly, the rotation and vibration of nanofluids droplets caused by acoustic field can inhibit supercooling degree. The result shows that the rotation and vibration of nanofluids droplets which will introduce external energy that can overcome the nucleation barrier and make nanofluids droplets into the nucleation stage in advance. Thirdly, Cavitation effect contributes to nucleation and crystallization of nanofluids droplets. Microbubble in nanofluids will rupture causing nanofluids under instantaneous high pressure which can make nanofluids with local supercoling degree at acoustic pressure. And local supercooling degree was calculated according to the Clausius-Clapeyron equation. The results show that local supercooling degree was benefit to promote the crystallization nucleation. In addition, the thin layer of ice caused by cavitation effect have the effect of promoting nucleation and crystallization,because it can be used as the substrate of heterogeneous nucleation. Furthermore, the acoustic radiation pressure also has great influence on supercooling degree of nanofluids droplet. The results show that cavitation bubbles and the probability of nucleation of nanofluids are increase with the increase of acoustic radiation pressure.The research results of this paper have important academic value on insight into graphene oxide nanofluids as the new type of PCMs and will be of important guiding significance to development of high efficiency and saving energy of PCMs.