Numerical simulation of natural convection heat transfer in nanofluids

The goal of this paper is to model the behavior of the nanofluids so that their performance can be evaluated analytically and computationally. We consider analytical models that describe molecular viscosity ∝, thermal conductivity k, density ρ, specific heat c_p and the coefficient of thermal expansion β for a nanofluid in terms of volume fraction φ{symbol} of nanoparticles, size of the nanoparticles (e.g radius of the nanoparticle, r_p), size of the base fluid molecule (e.g. radius of the liquid molecule, r_f) and the temperature T. In order to validate these analytical models, we study numerically the natural convection heat transfer in a closed pipe using the commercially available CFD software FLUENT 6.0, since the experimental data is available for this configuration. In particular, we study the natural convection flow field in two configurations of L/D=0.5 and L/D=1.0, where L is the length of the pipe and D is the diameter. For nanofluids, we consider the suspensions of and CuO particles in water. Three cases with volume fraction φ{symbol} = 0, 1% and 4% for both Al₂O₃ and CuO are considered. It is assumed that the nanoparticles of Al₂O₃ or CuO are uniformly suspended in water; there is no aggregation of nanoparticles in the fluid medium. It is shown that the use of experimentally measured values of k, or the kinetic model of k, gives better correlation with experimental data for heat transfer compared to the Maxwell model of k.