Establishment of thermal conductivity model and analysis of enhancement mechanism in nanofluids: A molecular dynamics study

Abstract

The requirements for high efficient heat exchange technology in many fields are gradually increasing. Nanofluids have attracted extensive attention due to their enhanced thermal conductivity. Revealing the mechanism of thermal conductivity enhancement is key to the preparation of high-efficiency nanofluids. In molecular dynamics (MD), the accuracy of simulation models is crucial to predict the thermal conductivity of nanofluids. Here, systems with various volume fractions were established by utilizing three different approaches. These approaches, denoted as Model 1, Model 2, and Model 3, involved changing the number of nanoparticles (NPs), the size of NPs, and the size of the simulation box as well as number of water molecules, respectively. The thermal conductivity values obtained from the simulations were validated using experimental data. The mechanism of thermal conductivity enhancement was explained by microscopic parameters. The results show that Model 1 was best able to capture the trends in thermal conductivity seen in the experimental data. Further, the thickness of the interfacial layer, estimated from the RDF and number density, was not affected by volume fraction and temperature, which instead determine the interactive forces between the NPs and the base fluid. The mechanism of enhancement of thermal conductivity may be attributed to the fact that the L-J potential energy of the Cu-O pair (−1.21 Kcal.mol−1) is stronger than that of the O-O pair (−0.15 Kcal.mol−1), thereby bringing water molecules into closer contact with the Cu NPs. As the volume fraction and temperature increase, the heat exchange between water molecules inside and outside the interfacial layer also increases, which accelerates the attainment of thermal equilibrium in the nanofluidic system.