| In recent years, the energy shortage problem is becoming increasingly prominent. As the most commonly used cooling medium, water leads to lower COP of the system because of the limit of its physical properties. Therefore, the new medium is urgently needed to replace it for phase-changing cool-storage. At the same time, the inorganic nanoparticles and sub-micron latex are mixed into a new intelligent coating. In the process of its drying, the particle migration will affect coating structure, and lead to uneven distribution of the particles in the thickness direction, which is also needed to be solved urgently. Now the good heat transfer performance of nanofluid has been verified by many researchers since its appearance, so it is hopeful to be applied to the field of phase-changing cool-storage and become a new generation of cold storage engineering. This project aims to study the process of Brownian motion and the freezing points of nanofluid in its temperature reducing process, which contributes to the in-depth analysis of the characteristics of its phase transition and the realization of this beautiful vision. So it has important practical significance to exploration. At the same time, the study of Brownian motion of nanofluids will also help to further precise the interpretation of nanoparticles migration phenomenon, and promote the technology of coating manufacture and application.A cryomicroscopy was used to analyze the particles motion in Al2O3-H2 O nanofluids of different particle sizes(50, 100, 500 nm) and different concentrations(0.01%, 0.05%, 0.1%),and the changes of the surface and diameters of nanoparticles aggregates, single nanoparticles aggregates trajectory, speeds with time and temperature are summarized during the process of cooling. Thus the dispersion was assessed in the different time and temperature. By regression we can get various parameters’ approximate function relationship in the process of variation, and the diffusion coefficient and the theoretical diffusion coefficients were compared.And we carried experiments on the Al2O3-H2 O nanofluids’ cooling and phase transition process with the size of 500 nm, the mass fraction of 0.01%, 0.05%, 0.1%, and the cooling rate for 1, 5, 10 ℃ / min, and explored the influence of cooling rates and Al2O3 concentrations on the phase transition temperature of nanofluid.The conclusions are as follows:(1)During the cooling process, the variation of the diameter and area of the nanoparticles in the whole field of the visual of the microscope is approximately subject to the sine distribution.(2)When the cooling rate is 1 ℃ / min and the nanoparticles size are 50, 100, 500 nm, the moments and temperature of the best dispersion are found to be different with themass fractions changing for the Al2O3-H2 O nanofluids of the same size. When the mass fraction is 0.01%, the time and temperature where nanofluids of different sizes(50, 100, 500 nm) present the best dispersion are t = 366 s, T = 8.7 ℃; t = 428 s, T = 5.7 ℃; t = 497 s, T = 6.7 ℃; When the mass fraction is 0.05%, the moments and temperature of the best dispersion are: t = 624 s, T = 5.2 ℃; t = 1002 s, T = 1.6 ℃; t = 361.5 s, T = 9.6 ℃;(3)By comparing the experimental diffusion coefficient and the theoretical diffusion coefficient, we can find the experimental values of the diffusion coefficient are less than the theoretical value.(4)The probabilities of freezing during the experiment Al2O3-H2 O nanofluids distribution fitting shows that its freezing points are approximately Gaussian distribution.(5) When the nanoparticles size is 500 nm,the mass fraction is 0.1%, and the cooling rate is 1, 5 and 10 ℃ / min respectively, the freezing points increase first and then down with the increase of cooling rate.(6) When the nanoparticles size is 500 nm, the mass fraction is 0.05% and 0.01%, and the cooling rate is 1, 5, 10 ℃ / min respectively, the freezing points decrease with the cooling rate increasing.(7) When the particle size is 500 nm and the mass fractions are 0.01%, 0.05% and 0.1%, the rules of the freezing points changing with the mass fractions are different with the cooling rates variation. When the cooling rate is 1 ℃ / min, the freezing points decrease with mass fraction increasing; When the cooling rate is 5 ℃ / min, the freezing points decrease first and then rises along with the increase of mass fractions; When the cooling rate is 10 ℃ / min, the freezing points increase with the increase of mass fraction.In summary, the parameters of the nanoparticles in nanofluid present sine law in the Brownian motion process; and the nanoparticles in nanofluid can lead to lower freezing points with the increase of cooling rate and the freezing point probabilities distribution approximate to Gaussian distribution. Therefore the study of nanofluid freezing heat transfer process and Brownian motion is of great significance for its application in heat transfer enhancement and solving coating particle migration problems. |
PDF Full Text Request