Study On Thermophysics And Phase Change Properties Of Graphene Oxide-Al₂O₃ Hybrid Nanofluids

Nanofluids have attracted many researchers over the past 20 years to design new thermal systems for different engineering applications due to their good heat transfer characteristics.Single-molecule nanofluids prepared from a single nanoparticle have certain special advantages depending on the nature of the suspended nanoparticles in the nanofluid.However,in order to further improve the properties of nanofluids,researchers have developed a new type of heat transfer fluid cal ed hybrid nanofluids.In this paper,the preparation,stability,thermal conductivity and phase transformation characteristics of water-based grapHene oxide-Al₂O₃hybrid nanofluids were studied experimentally,and the results were analyzed and summarized to provide a reference for the study of hybrid nanofluids.The main research work is as follows:The water-based grapHene oxide-Al₂O₃ hybrid nanofluid was prepared by"two-step method".The stability of the hybrid nanofluid was studied by pHysical sedimentation method and absorbance test method.It was found that pH,dispersant dosage,ultrasonic frequency and ultrasonic time are important factors affecting the stability of hybrid nanofluids.At pH=11,grapHene oxide:Al₂O₃:SDS=1:1:1,ultrasonic frequency is 375W,the stability of the hybrid nanofluid is best when the ultrasonic time is 120 min.The thermal cycle has a certain decrease in the absorbance of the hybrid nanofluid,but the decrease is smal,and gradually stabilizes after 6 cycles.The thermal conductivity of grapHene oxide-Al₂O₃ hybrid nanofluids was measured by transient hot line method.It was found that the thermal conductivity of hybrid nanofluids increased with the increase of nanoparticle mass fraction and temperature.At 20℃-70℃,0.025%-0.2 wt%,the thermal conductivity of the hybrid nanofluid increased by 4.71%compared to the thermal conductivity of the base fluid.Comparing the experimental values of the thermal conductivity of the hybrid nanofluid with the model values obtained by the modified Maxwell model,it is found that the experimental values are larger than the model values,and it is known that the Brownian motion and the intermolecular interaction of the nanoparticles are important for the thermal conductivity of the nanofluid.At the same time,a new mathematical model for the effective thermal conductivity of graphene oxide-Al₂O₃ hybrid nanofluids is proposed,which can predict the thermal conductivity of hybrid nanofluids with different parameters.By measuring and analyzing the supercooling degree of graphene oxide-Al₂O₃ hybrid nanofluids,it was found that the addition of graphene oxide-Al₂O₃ hybrid nanoparticles greatly reduced the supercooling of the base fluid.Under the condition of 0.05%mass fraction,the supercooling degree of the hybrid nanofluid decreased firstly and then increased with the increase of ultrasonic time in the range of 30min-150min,which was 87.5%lower than the supercooling degree of the base liquid;Under the ultrasonic time of 60 min,in the range of0.05wt%-0.2wt%,the degree of supercooling of the hybrid nanofluid decreases first and then increases with the increase of the mass fraction of the hybrid nanoparticle,compared with the base solution,the maximum subcooling degree decreased by 77.8%.The supercooling degree of hybrid nanofluids decreases with the increase of initial cooling temperature.The supercooling degree of hybrid nanofluids at the initial cooling temperature of-5℃is 30.23%lower than that at the initial cooling temperature of-15℃.Comparing the effects of hybrid nanofluids and graphene oxide nanofluids on the supercooling of the base fluid,it was found that the hybrid nanofluids can better reduce the supercooling of the base fluid.Under the condition of 0.05%mass fraction,the ultrasonic range is 30min-150min.Hybrid nanofluids have a maximum reduction of 83.92%in supercooling compared to graphene oxide.By measuring and analyzing the specific heat capacity and latent heat of the hybrid nano-fluid of grapHene oxide-Al₂O₃,it is found that the specific heat capacity of the hybrid nano-fluid decreases with the increase of the mass fraction of the nano-particle,and increases with the increase of the fluid temperature.At 20℃-70℃,0.05 wt%-0.15 wt%of hybrid nanofluids heat capacity than the heat capacity of the base fluid biggest was reduced by 7.01%.At the same time,the addition of nanoparticles reduced the latent heat of phase transition of the base solution.The latent heat of phase change of graphene oxide-Al₂O₃ hybrid nanofluids decreased with the increase of the mass fraction of nanoparticles,and the latent heat of melting and crystal ization of hybrid nanofluids decreased by 7.1%and 16.12%respectively within the range of 0.05wt% to 0.2wt%.