Research On Heat And Mass Transfer Mechanisms And Optical Character Of Nanofluids

In1995, the concept of nanofluids was first proposed by Choi who was worked at Argonne National Laboratory of the United States. Nanofluids refer to a new class of heat transfer fluids by suspending nanoscaled metallic or nonmetallic particles in base fluids. Since the born of nanofluids, it had attracted the attention of researchers. As a new type of functional fluids, nanofluids have some special characters. The heat and mass transfer characters and the optical character are three of the most important characters of nanofluids. Reported experimental studies have shown that nanofluids have a good prospect of application in heat and mass transfer and optical engineering. So far, our knowledge about nanofluids is not deep enough to explain the special characters of nanofluids. Further studies about the characters of nanofluids are necessary. The research work of this paper focuses on the heat and mass transfer characters and the optical character of nanofluids. The mechanisms of heat conduction, mass transfer and the optical absorption of nanofluids are investigated to provide theoretical guidance for the application of nanofluids. The main research work of this paper includes the following aspects.1Research on the mechanism of heat conduction of nanofluidsSince the concept of nanofluids have been proposed, many attentions have been paid to the heat transfer character of nanofluids. A lot of experimental researches about the thermal conductivity of nanofluids have been reported. Experimental results showed that the thermal conductivity of nanofluids is remarkably higher than that of base fluid. Considering various factors, researchers had proposed many theoretic models to explain the mechanism of the thermal conductivity of nanofluids. But there is not a perfect model, the mechanism is still unclear. In order to make the heat conduction mechanism of nanofluids clear, further research is needed.Based on analyzing the reported researches about the thermal conductivity of nanofluids, the factors which affect the thermal conductivity of nanofluids were summarized. The thermal conductivity of the nanofluids can be regarded as the sum of the static thermal conductivity and the dynamic thermal conductivity. In this paper, by considering the various factors which affect the thermal conductivity of nanofluids, the static thermal conductivity model and the dynamic thermal conductivity model of nanofluids are proposed. The heat conduction mechanism of nanofluids is studied by the proposed thermal conductivity model and the main factors which affect the thermal conductivity of nanofluids are found. The thermal conductivity of magnetic nanofluids is anisotropic in the presence of an external magnetic field because of the magnetic nanoparticles. At present, few theoretical studies about this have been reported. In this paper, combining the dynamics simulation of magnetic nanofluids’ microstructure and the static thermal conductivity model of nanofluids, the anisotropic heat conduction mechanism of magnetic nanofluids is studied. First, the force models and motion equations of magnetic particles are established by considering the various forces acting on the magnetic particles. On this basis, the microstructures of magnetic nanofluids in the presence of different magnetic field are obtained by dynamics simulations. Then, the anisotropic thermal conductivity of magnetic nanofluids is calculated by the static thermal conductivity of nanofluids. The results show that in the presence of an external magnetic field, the magnetic particles form chain-like structures along the direction of the magnetic field. The chain-like structures provide an effective heat transfer pathway in the fluid so that the thermal conductivity of magnetic nanofluids along the chain direction is bigger than that perpendicular to the chain direction. The anisotropic feature of the thermal conductivity becomes more evident with increasing the external magnetic field strength.2Research on the enhanced mass transfer of nanofluidsThe study of mass transfer of nanofluids is in the initial stage, few researches about this have been reported. This paper studies the mechanism of mass transfer of nanofluids from both theoretical and experimental aspects.(1) Theoretical researchThe effect of Brownian motion of the suspended nanoparticles on the mass transfer of nanofluids is analyzed. Based on the similarity theory of heat and mass transfer, the criterion equation of effective mass diffusivity of nanofluids is proposed. By the theoretical analysis of mass transfer in nanofluids, the main factors which affect the mass diffusivity of nanofluids are found.(2) Experimental researchAccording to the idea of the Taylor dispersion method, an experimental system for measuring the mass diffusivity of nanofluids is devised. In order to investigate the effect of particle volume fraction, temperature and the base fluid properties on the mass diffusivity of nanofluids, we measured the mass diffusivities of Rhodamine B in both Cu-water and Cu-ethylene glycol nanofluids at different temperatures (15℃,20℃,25℃). The particle volume fraction of the nanofluids is from0.1%to0.5%. The experimental results show that the mass transfer in fluid is strengthened by the suspended nanoparticles. The mass diffusivity of Rhodamine B in nanofluids is larger than that in base fluid and the mass diffusivity increases with increasing the particle volume fraction. For a given particle volume fraction, the mass diffusivity increases with the increasing the temperature. The mass diffusivity of Rhodamine B in Cu-water nanofluids is greater than that in Cu-glycol nanofluids.The exploratory work in this paper on the mass transfer of nanofluids has figured out the key factors that affect the mass transfer of nanofluids. This work plays a guiding role in promoting the application of nanofluids in engineering.3Research on the optical character of nanofluidsThe extinction coefficient of nanofluids is an important parameter which affects the optical character of nanofluids. In this paper, the extinction coefficient of nanofluids is studied from both experimental and theoretical aspects. Because of the special magneto-optical effects, the magnetic nanofluids are more applicable in optical engineering than the ordinary nanofluids. In this paper, the optical anisotropy mechanism of magnetic nanofluids is researched by the new theoretical model.(1) Experimental researchBased on the film transmission principle, the experimental method for measuring the extinction coefficient of nanofluids is established. The extinction coefficients of the water at different wavelengths are measured and compared to the literature value. The results indicate that the present method is suitable for measuring the extinction coefficient of the nanofluids and has high accuracy. Then the extinction coefficients of Fe3O4-water nanofluid with different particle volume fractions are measured for providing experimental data for the next theoretical research.(2) Theoretical researchBased on T-matrix method and considering the particle aggregation microstructure and the multiple scattering between nanoparticles, a theoretical method for the extinction coefficient of magnetic nanofluids is proposed. By using the proposed method, the influence of particle diameter, particle volume fraction and external magnetic field on the extinction coefficient of magnetic fluid is studied and the mechanism of the anisotropic optical character of magnetic fluid is explored. The results show that the extinction coefficients of magnetic fluid increase linearly with the increase of particle volume fraction. For a given particle volume fraction, the lager the particle diameter, the bigger the extinction coefficient is. In the presence of an external magnetic field, the microstructure of magnetic fluid presents anisotropic feature which causes the optical property of magnetic fluid presents anisotropic feature. The extinction coefficient of magnetic fluid along the magnetic field direction is smaller than that without external magnetic field and the extinction coefficient decreases with the increase of the magnetic field strength. For the extinction coefficient of magnetic fluid perpendicular to the magnetic field direction, there are two different results. When the polarization direction of the incident light is parallel to the magnetic field, the extinction coefficient of magnetic fluid is bigger than that without external magnetic field and the extinction coefficients increase with the increase of the magnetic field strength. But when the polarization direction of the incident light is perpendicular to the magnetic field, the extinction coefficient of magnetic fluid is smaller than that without external magnetic field and the extinction coefficients decrease with the increase of the magnetic field strength.