Investigation of nanoparticle aggregation effect on thermal properties of nanofluid by a combined equilibrium and non-equilibrium molecular dynamics simulation

Abstract

Many theoretical and experimental studies on heat transfer and flow behavior of nanofluids have been done and the results show that nanofluids significantly increase heat transfer. Nevertheless, there is no accurate understanding from the effect of different mechanisms on nanofluid heat transfer. Computer simulations are a suitable tool for description of physical mechanisms in many processes. In this study, molecular dynamics simulation was used to investigate the effect of nanoparticle aggregation on thermal properties of water-silicon dioxide nanofluid, specifically its thermal conductivity. For calculating nanofluid thermal conductivity a combination of two equilibrium and non-equilibrium molecular dynamics simulations was performed to calculate the specific heat and thermal diffusivity of the nanofluid, respectively. Simulations were performed in NVT ensemble and spherical coordinate. The model was validated by comparison of thermal properties of water base fluid with experimental data in four various temperatures. Results also were compared with theoretical models such as HC model for nanofluid. To investigate the effect of nanoparticle aggregation, two cases of constant and variable volume fractions (i.e. 1.5, 3 and 4.5%) at temperature of 308K were considered. The results showed that when the aggregation occurs with increasing nanoparticle concentrations, there are an increase in the thermal conductivity and thermal diffusivity of the nanofluid and a decrease in its specific heat. Moreover, when aggregation takes place at constant nanoparticle concentration, the specific heat of nanofluid with suspended nanoparticles did not change with respect to nanofluid with aggregated nanoparticles, but its diffusivity and thermal conductivity increase.