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

In the field of ice storage technology, the problems of most phase changematerials(PCMs) which have large supercooling degree and difficulty to happen phasechange can be solved by nanofluids. It has attracted many scholars to study thesupercooling characteristics of nanofluids, but these studies were operated in thecontainer. Since the fluids have fluidity, its shape is affected by the container. The errorof the research on thermal properties of the fluid was caused by the effect of solid-liquidinterface. For the study of cold experiment, fluid in the container could causeheterogeneous nucleation and effect of transformation of nanofluids, so could not getthe real cold characteristics of nanofluids. Therefore, in order to eliminates the influenceof container wall contact, in this paper the acoustic levitation containerless processingmethod was used to study the supercooling degree of graphene oxide nanofluids. Themain research contents and results of this paper are fllowing:（1）Experiments were conducted to test the supercooling degree of five nanofluidscontaining different concentrations of graphene oxide. The results indicate that thesupercooling degree of nanofluids was greatly suppressed, and the supercooling degreecan be reduced by62.9%at most, furthermore the nucleation was started in advance.The effect of different refrigerant temperature on supercooling degree of nanofluids isresearched, it suggests that supercooling degree increases with the increase of coolingrate.（2）Asingle layer and n layers physical model of graphene oxide was built, and thenucleation condition of nanofluids was calculated based on the theory of heterogeneous nucleation,and it was verified by the results of nanosheets size distribution and supercooling degree. The resultsconfirme that graphene oxide nanosheets can be used as nucleating agent for reducing thesupercooling degree of PCMs. In addition, the surface energy of nanofluids is calculatedbase on the equation of T.Young and the theory of Good-Girifalco, the result shows thatnanoparticle belongs to the high surface energy solid, so the surface heterogeneityrequires less nucleation energy. The excellent surface activity and wetting properties ofgraphene oxide also help to reduce the nucleation energy.（3）The supercooling degree of deionized water was investigated under acousticlevitation, the results indicate that the average supercooling degree increases with theincrease of cooling rate. Through contrasted with supercooling degree of deionizedwater in the container, it suggests that the method of acoustic levitation could effectively avoid the influence of contacting with container wall, but the ultrasonic fieldcould induce surface nucleation and inhibit the supercooling degree.（4）The supercooling degree of graphene oxide nanofluids was researched underacoustic levitation, the results indicate that the average supercooling degree increaseswith the increase of cooling rate, when the electric power value was24W, the averagesupercooling degree was greatly suppressed and was reduced by62%at most. In orderto analyze the common effect of ultrasonic field and nanoparticles on nucleation andcrystallization, the following work was done:①the temperature and pressure of bubbletransient collapose were calculated. In the level of109Pa high pressure, the surfaceenergy and wettability of nanosheets could be increased by the effect of strong shockwave or high speed jet.②Supercooling degree under local high pressure was estimatedby the Clausius-Clapeyron equation, the result shows that it is far greater thanexperimental measurement of supercooling degree, equivalent to increase the drivingforce for nucleation and crystallization.③The cavitation nuclei could be formed by airgap on nanoparticles or clusters, then increase the number of cavitation nuclei andstrengthen the cavitation. Also the additional nucleation energy is provided by highenergy particles. It indicates that these are beneficial to reducing the supercoolingdegree.（5）The effect of sound pressure amplitude on supercooling degree of nanofluidsdroplet was investigated. The results indicate that the supercooling degree reduces withthe increase of sound pressure amplitude, and deformation of droplet is more obvious.Under the same conditions contrasted with supercooling degree of nanofluids in thecontainer, it shows that supercooling degree of suspended droplet is less thansupercooling degree of nanofluids in the container only when acoustic field reaches acertain strength. According to the theory of Crum, it suggests that cavitation of bubblenumbers increase with the increase of sound pressure amplitude, in other words,heterogeneous nucleation is enhancing.The results of this paper confirmed that graphene oxide nanosheets can a used asnucleating agent for reducing the supercooling degree of PCMs,，and the supercoolingdegree of PCMs could be reduced by acoustic field. It will be useful in ice storage andrapid ice making applications.