Effects of nanoparticles transport mechanisms on Al2O3-water nanofluid natural convection in a square enclosure

In this numerical study, effects of nanoparticles transport in natural convection of Al₂O₃–water nanofluid on flow field and heat transfer in a square cavity have been investigated and comparisons between predictions of newly developed transport model and the homogeneous model have been made. In the transport model, transport mechanisms including Brownian and thermophoresis diffusions, which cause non-homogeneity, have been considered. The governing equations have been discretized using the control volume method. Variable properties of nanofluid have been assumed functions of temperature and volume fraction of nanoparticles. Since constant Rayleigh number could not be used, simulations have been performed for various physical conditions such as temperature differences between the hot and cold walls from 2 to 10 °C, bulk volume fractions of nanoparticles from 0 to 0.04 and nanoparticles sizes from 25 nm to 105 nm. Comparisons revealed better agreement with experimental results considering the transport model instead of the homogeneous model. Moreover, it was shown that Dufour effect on heat transfer is negligible. Both models suggested heat transfer reduction by increasing the bulk volume fraction of nanoparticles, but transport model predicted a greater reduction. Transport model predicted heat transfer decrease with increased nanoparticles size, but homogeneous model acted vice versa.