Supercooling And Surface Tension Of Graphene Oxide Nanofluids Under Acoustic Levitation

Phase change cold storage technology has been widely used in air conditioning systems,refrigerators,refrigerated transport and building energy conservation,and has broad application prospects.At present,water is often used as a cold storage material in a phase change cold storage system,but there are two problems to be solved urgently: high supercooling degree and low thermal conductivity.Due to its ability to effectively reduce supercooling and rapid condensation,nanofluids have become a new type of phase change cold storage material that has attracted the attention of researchers.Supercooling is common in phase change materials.The supercooling of the nanofluid in the vessel is susceptible to interference from the wall of the vessel.Therefore,in this paper,the supercooling characteristics of graphene oxide nanofluids were studied by using acoustic levatation technology to realize the containerless treatment.The surface tension of graphene oxide nanofluids under supercooled state was also measured.The main works and results are summarized as follows:(1)Graphene oxide nanofluids with different concentrations of 0.03wt% and 0.05wt% were prepared by ultrasonic oscillation using deionized water as the base liquid.The particle size and Zeta potential of graphene oxide nanofluids at different concentrations were measured by Laser Size and Zeta Potential Analyzer(Nano ZS90).The average particle size of all nanofluids was less than 100 nm,and the Zeta potential was more than 30 mV.According to Zeta potential,the prepared nanofluids have good stability.The prepared nanofluids were placed in closed containers for 30 days without aggregation and precipitation,indicating that the prepared nanofluids were stable.(2)The supercooling degree of deionized water and 0.03 wt% nanofluid at a cooling temperature of-21? was measured and the step-cooling curves were drawn.By comparing the step-cooling curves of graphene oxide nanofluids and deionized water,nanofluids are cooled faster than deionized water during the droplet cooling phase.It can be seen that the heat transfer of nanofluids is better than that of deionized water.Comparing the nucleation temperature distribution of the two fluids,the median nucleation temperature of the graphene oxide nanofluid is-8.9? higher than-10.4? of deionized water,and the nucleation temperature of the graphene oxide nanofluid is more concentrated.It is more conducive to the selection and operation adjustment of the evaporation temperature of the cold storage system.(3)Based on the heterogeneous nucleation theory,the surface nucleation mechanism of nanoparticles was analyzed.According to the statistical nucleation theory based on classical nucleation theory,the supercooling was studied,and the heterogeneous nucleation factors were calculated.Then the nucleation process of nanofluids was analyzed according to the heterogeneous nucleation factor.The calculations show that the contact angle between graphene oxide nanoparticles and ice is 18.18°,and the heterogeneous nucleation factor of graphene oxide nanofluids is far less than 1.The heterogeneous nucleation factor 0.001838 of graphene oxide nanofluids is less than 0.00339 of deionized water.The minimum diameter required for nucleation was calculated by heterogeneous nucleation factor.The results show that heterogeneous nucleation in graphene oxide nanofluid droplets occurs on the upper and lower surfaces of nanosheets.(4)The perturbation theory is applied to approximate the control equation of acoustic levitation.The levitation force of ultrasonic standing wave and the stable levitation position in the acoustic field of standing wave were briefly deduced.The influence of medium temperature variation on the stability of droplet suspension in acoustic levitation chamber were analyzed.The medium temperature in the chamber can be cooled first,and then the distance between emitting and reflector can be adjusted to make the acoustic radiation force reach the local maximum,thus achieving droplet stability.(5)After stable suspension,the surface tension of the supercooled graphene oxide nanofluids at-7 °C,-4 °C,-2 °C,1 °C,7 °C,12 °C and 18 °C was 79.47 mN/m,78.60 mN/m,78.31 mN/m,78.02 mN/m,77.45 mN/m,76.65 mN/m and 76.02 mN/m,respectively,using the droplet oscillation method.The results show that the surface tension of the graphene oxide nanofluid gradually increases with the decrease of temperature.The supercooled graphene oxide nanofluid is consistent with its tendency to change in the normal temperature region.In this paper,the supercooling properties of graphene oxide nanofluids are studied.Graphene oxide nanofluids effectively solve the problem of large supercooling of phase change materials,and can be used as an efficient cold storage material.At the same time,the surface tension measurement of graphene oxide nanofluids during supercooling completed the thermal property data of graphene oxide nanofluids,which is of great significance for the practical application of graphene oxide nanofluids in cold storage systems.