Study on the differences in thermal physical properties of nanofluids formed by metal and metal oxide nanoparticles: A molecular dynamics simulation of Cu-H2O and CuO-H2O nanofluids

Abstract

Nanofluids, known for their high thermal conductivity and excellent thermal physical properties, are widely used in various engineering fields. However, the impact of the type of nanoparticles on the thermal physical properties of nanofluids and the exploration of their microscopic mechanisms remain underexplored. This study employs molecular dynamics methods combined with various techniques (non-equilibrium molecular dynamics (NEMD), reverse non-equilibrium molecular dynamics (RNEMD), radial distribution function (RDF), phonon density of states (PDOS), etc.) to thoroughly investigate the differences in thermal physical properties between nanofluids formed by metals and their oxides in the same type of base fluid, using Cu-H₂O and CuO-H₂O nanofluids as examples. The study aims to provide a microscopic explanation and reveal the underlying mechanisms. The study found that under the consideration of practical factors such as temperature, nanoparticle shape (S/V ratio), and nanoparticle volume fraction in the nanofluid, the thermal conductivity of Cu-H₂O nanofluid is higher than that of CuO-H₂O nanofluid. For example, the thermal conductivity values are 0.83296 W/m·K (Cu-H₂O nanofluid, 2.5 %, 330 K, Cylinder) and 0.8288 W/m·K (CuO-H₂O nanofluid, 2.5 %, 330 K, Cylinder).Additionally, the viscosity of Cu-H₂O nanofluid is lower than that of CuO-H₂O nanofluid, as shown by the values 0.002284123 Pa·S (Cu-H₂O nanofluid, 280 K, 1.5 %, Platelets) and 0.0023976 Pa·S (CuO-H₂O nanofluid, 280 K, 1.5 %, Platelets).Subsequently, by comparing the phonon density of states (PDOS) overlap between nanoparticle atoms and base fluid atoms in both Cu-H₂O and CuO-H₂O nanofluids, the study elucidated the microscopic mechanism behind the higher thermal conductivity of Cu-H₂O nanofluid from the perspective of interfacial thermal conductance. The radial distribution function (RDF) was then used to explain the lower viscosity of Cu-H₂O nanofluid compared to CuO-H₂O nanofluid from the solid-liquid interface angle in the nanofluid, and to further elucidate the internal mechanism behind the higher thermal conductivity of Cu-H₂O nanofluid. This study fills a significant gap in the comprehensive comparative research of the thermal physical properties of nanofluids formed by metals and their oxides. It reveals the internal mechanisms behind the differences in thermal physical properties at a microscopic level, providing important guidance for the engineering application of nanofluids.