Experimental investigation and theoretical analysis on the effects of nanolayers on nanofluids' thermo-physical properties

Abstract

Nanofluids, which are suspensions of nanoparticles in conventional heat transfer fluids, attracted research studies on different heat transfer applications, while they enhance thermal transport properties in comparison with conventional base fluids. Recently, the use of these new fluids has been growing increasingly. However, the ambiguities of their thermo-physical properties cause them to function inefficiently in industrial design. The recognised important parameters that affect the properties of nanofluids include the volume fraction of the nanoparticles, temperature, nanoparticle size, nanolayer, thermal conductivity of the base fluid, pH of the nanofluid and the thermal conductivity of the nanoparticles. However, there is a distinct lack of investigation and reported research on the nanolayer and its properties. In this study, the effect of uncertainty of the nanolayer properties on the effective thermal conductivity and viscosity of nanofluids, and heat transfer are discussed in detail. The results show that the uncertainties can cause 20% error in the calculation of the Nusselt number and 24% for the Reynolds number. Therefore, more research needs to be conducted on nanolayer properties in order to identify them accurately. The density of some nanofluids, such as SiO2-water, SiOx-EG-water, CuO-glycerol and MgO-glycerol, has also been investigated experimentally. Therefore, the effects of nanolayer thickness and density on nanofluid properties are discussed in detail. The results show that nanolayer density and thickness have a significant effect on nanofluid density, and nanolayer density is found to be between void and base fluid density. Consequently, by analysing experimental results and performing a theoretical analysis, a model has been derived to calculate the density of nanofluids. Specific heat capacity is the other nanofluid property that is discussed in this study. Experimental data from literature, available formulae and the presented model for nanofluid density have been used to identify nanofluid-specific heat capacity, while nanofluid density is one of the parameters in calculating specific heat capacity. This investigation was performed using a model ? used by different authors ? that also considers the nanolayer. The specific heat capacity of nanofluids that resulted from two methods of calculation has been compared with available experimental data. This investigation shows that the proposed model for the density of nanofluids provides better agreement for specific heat capacity in comparison to experimental data.