Study on the Production and Transport Properties of Nanofluids
thesisposted on 21.06.2016 by Gi-Hwan Lee
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A new engineering medium, called nanofluid, has attracted wide attention due to its relevance to many cooling processes in engineering applications. Nanofluids have been shown to possess unique thermal transport properties and superior performance that cannot be attained by traditional heat transfer fluids or conventional particle fluid suspensions. These novel materials show great promise as next-generation heat transfer fluids for innovative applications in many industrial fields. A conventional ultrasonic bath was used in this work to examine the feasibility of forming aqueous suspensions of spherical gold nanoparticles (GNPs). The effects of ultrasonic energy on the size and morphology of GNPs were investigated. Gold nanofluids with highly monodispersed spherical GNPs were successfully synthesized by sodium citrate reduction in a conventional ultrasonic bath, without additional heating or magnetic stirring, as evidenced by ultraviolet–visible spectra and transmission electron microscopy. Ultrasonic energy was shown to be a key parameter for producing gold nanofluids with spherical GNPs of tunable sizes. A proposed scheme for understanding the role of ultrasonic energy in the formation and growth of GNPs has been discussed. The single-step method demonstrated in this study offers new opportunities in the production of aqueous suspensions of monodispersed spherical GNPs. Centrifugation was also used with the two-step nanofluid production method in the present study. The research employed three different alumina nanofluids, each with different nanoparticle size, which can be produced from the same original nanoparticle dispersion by means of centrifugation without dispersants or surfactants. Measurements of thermal conductivity of alumina nanofluids were conducted over a broad range of temperatures from 10 to 80 °C. The experimental results were compared to the effective medium theory models and the calculated Brownian velocity. The experimental results show the size- and temperature-dependency of the thermal conductivity enhancement. Moreover, it is found that the temperature effect depends on the particle size. In other words, there is a coupling dependency between the nanoparticle size and temperature in heat conduction enhancement of alumina nanofluids. Finally, it is concluded that the Brownian velocity is the key factor responsible for the temperature- and size-dependency of the thermal conductivity of alumina nanofluids.