A B S T R A C TIn order to investigate the effect of nanoparticle volume fraction, nanoparticle size and temperature on the thermal conductivity of glycerol based alumina (α-Al 2 O 3 ) nanofluids, a set of experiments were carried out for temperature ranging from 20°C to 45°C. The nanofluids contained α-Al 2 O 3 nanoparticles of three different sizes (31 nm, 55 nm and 134 nm) were prepared by two-step method at volume fractions ranging from 0.5% to 4%. The experimental results show that α-Al 2 O 3 -glycerol nanofluids have substantially higher thermal conductivity than the base fluid and the maximum enhancement of the relative thermal conductivity was 19.5% for the case of 31 nm at 4% volume fraction. The data analyses indicated that the volume fraction and size of the nanoparticles have significant effects on the thermal conductivity ratio (TCR) of Al 2 O 3 -glycerol nanofluids, while the temperature has almost no significant effect on the data for range of this study. At room temperature, the effective thermal conductivity remains almost constant for 50 h at 4% volume fractions. The comparison of the obtained experimental data and predictions from some existing theoretical and empirical models reveals that the thermal conductivity ratio and its trend could not be accurately explained by the models in open literature. Consequently, a new empirical correlation based on the experimental data has been developed in this study.

There is a lack of reported research on comprehensive hybrid models for the effective thermal conductivity of nanofluids that takes into consideration all major mechanisms and parameters. The major mechanisms are the nanolayer, Brownian motion and clustering. The recognized important parameters can be the volume fraction of the nanoparticles, temperature, particle size, thermal conductivity of the nanolayer, thermal conductivity of the base fluid, PH of the nanofluid, and the thermal conductivity of the nanoparticle. Therefore, in this work, a parametric analysis of effective thermal conductivity models for nanofluids was done. The impact of the measurable parameters, like volume fraction of the nanoparticles, temperature and the particle size for the more sited models, were analyzed by using alumina-water nanofluid. The result of this investigation identifies the lack of a hybrid equation for the effective thermal conductivity of nanofluids and, consequently, more research is required in this field.

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Viscosity is an important consideration in the application of nanofluids as heat transfer fluids. Various models have been developed to predict the viscosity of nanofluids. The accuracy of these models is of important benefit in determining the rheological performance of nanofluids, particularly in conditions which vary continuously. In this paper, a parametric analysis is undertaken to investigate the degree of variability between empirical data and model predictions. It was found that there is high variability in the compared results, which suggests that a wide range of constitutive factors need to be incorporated into the models in order to account adequately for the rheological behaviour of nanofluids.

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