A nanofluid is the suspension of nanoparticles in a base fluid. Nanofluids are promising fluids for heat transfer enhancement due to their anomalously high thermal conductivity. At present, there is significant discrepancy in nanofluid thermal conductivity and viscosity data in the literature. On the other hand, thermal conductivity enhancement mechanisms of nanofluids have not been fully understood yet. In the first part of this study, a detailed literature review about thermo-physical properties of nanofluids is performed. Experimental studies are discussed in terms of the effects of some parameters such as particle volume fraction, particle size, and temperature on the thermal conductivity and viscosity of nanofluids in literatures. In the second part of the present study an empirical expression for the effective thermal conductivity of nanofluids based on interfacial shells and Brownian motion is derived. Comparing with conventional models, the expression is not only depended on the thermal conductivity of the solid and liquid and their volume fraction, but also depended on the particle size, temperature and interfacial properties. A new empirical correlations for predicting dynamic viscosity of nanofluids, based on the effects of temperature, particle size and volume fraction are proposed, too. The results of current models on the effective thermal conductivity and viscosity of CuO/water, Al 2 O 3 /water and TiO 2 /water nanofluids are in good agreement with the experimental data. Research about the natural convection of nanofluids is important for the practical application of nanofluids in heat transfer devices. The objective of the present study is to investigate numerically the effects of variable properties on natural convection heat transfer of aqueous nanofluids. On this regard thermophoresis of nanoparticles and the Dufour effects on natural convective heat transfer in nanofluids are investigated simultaneously. By using double diffusive formulation the combined effects of Brownian Motion, thermophoresis and the Dufour on the variation of heat transfer and Nusselt number of natural convection in rectangular enclosures under constant wall temperature is computed. In order to numerically solve the governing equations, a control volume approach is used. Central differences are used to approximate the advection-diffusion terms, i.e. The scheme is Implicite and second-order accurate in space and the SIMPLE algorithm is employed to solve the equations. Numerical results are compared with experimental and numerical data in the literature and good agreement is observed especially with experimental data. The agreement can be considered as an indication of the validity of the new double diffusive model for explaining nanofluid heat transfer. Keywords: Nanofluids, Natural convection, Brownian motion, Thermophoresis, Dufour effect
