A molecular dynamics study on thermal conductivity enhancement mechanism of nanofluids – Effect of nanoparticle aggregation

Aggregation of nanoparticles is crucial in enhancing the thermal conductivity of nanofluids, but the underlying mechanism remains unclear so far. This paper used the non-equilibrium molecular dynamics (NEMD) simulation method to compare the thermal energy transfer characteristics of Ar-Cu nanofluids containing non-aggregated and aggregated nanoparticles. Simulations of thermal conductivity and its component calculations were performed to elucidate how the energy transport terms of molecular motions and intermolecular interactions are related to the nanoparticle aggregation state. The simulation results illustrated that the interaction between Ar atoms dominates the thermal conductivity, and its contribution increases with the volume fraction of nanoparticles, which is mainly caused by the solid-liquid interaction at the interface. The thermal conductivity of nanofluids is closely related to the aggregation morphology. Radial distribution function (RDF) analysis shows that when the nanoparticles change from a fully dispersed state to a compact aggregation state, the density of the nanolayer near the nanoparticles decreases, thus reducing the contribution of Ar-Ar interaction to heat transfer. Nanoparticle aggregates with chain-like structures maximize the thermal conductivity of the nanofluids due to the increased interaction between the copper atoms providing a more efficient heat transfer. Our results will provide essential insights into understanding the effects of nanoparticle aggregation on the microscopic heat transfer of nanofluids.