A Molecular dynamics study on viscous and thermal properties of Nanofluids

Abstract

The aim of this study is to understand the microscopic behavior of heat and momentum transfer in nanofluids. With nanofluids reporting enhanced thermal conductivities (𝜅) and viscosities (𝜂), a microscopic understanding is essential for engineering nanofluids to be practical in heat transfer applications. Therefore, to study the microscopic transport behavior, copper-argon nanofluids simulated by classical molecular dynamics are employed. The Applicability of the Green-Kubo (GK) method in nanofluid 𝜅 evaluation is questioned as the calculated thermal conductivities through the GK method are considerably higher than the direct method in Non-EquilibriumMolecular-Dynamics

(NEMD). Green-Kubo calculations are found to be very sensitive to the ill-defined partial enthalpy computation, resulting in an overestimation of the 𝜅. However, the Green-Kubo and the direct method viscosity calculations demonstrate a reasonable agreement. Following the more reliable method, the NEMD direct approach, 𝜅 of the nanofluids consisting of spherical nanoparticles with different diameters are investigated. The computational results are compared with the classical effective medium theories and no anomalous 𝜅 enhancements are observed in the nanofluids having fully dispersed spherical particles. Various microscopic mechanisms such as liquid layering and micro-convection are found to be ineffective for 𝜅 enhancements in nanofluids. However, greatly enhanced 𝜅 are achieved, a maximum of 63% relative to pure argon, in nanofluids consisting of chain-like particle arrangements. This demonstrates the potential origin of anomalous 𝜅 enhancements in experimental measurements and the capability of nanofluids with extended nanostructures to deliver better 𝜅 enhancements. Further investigating the capability of extended nanostructures in nanofluid thermal transport, 𝜅 enhancements of nanofluids consisting of nanowires with different lengths and diameters are evaluated. It is shown that the heat conduction in the parallelly arranged liquid and the nanowires exhibit a coupled thermal behavior owing to the interface thermal resistance (R b ). This contradicts with the predictions of the classical parallel heat conduction model and therefore, a novel model is proposed taking this coupled behavior into account. New heat transfer characteristics at the nanoscale are identified including the R b -driven coupled heat conduction, the reduced 𝜅 of suspended nanowires, and the solid-like liquid layering. Using the new model, the importance of these microscopic thermal characteristics in accurately predicting the effective 𝜅 is shown. The sole contribution from the solid-like liquid layer to the 𝜅 enhancement is found to be in between 20-30% for the nanofluids considered. Extending the investigation of heat transfer phenomena in nanofluids based on spherical nanoparticles, 𝜂 of nanofluids with different nanoparticle sizes, concentrations, and arrangements are evaluated. Both the Green-Kubo and the direct methods are employed and unlike the 𝜅, both methods show a reasonable agreement with one another. Viscosity is observed to decrease as the particle diameter increases in fully dispersed nanofluids. The ratio 𝐶 ⁄ shows a decreasing trend indicating better heat transfer performance in nanofluids with large particles. Nanofluid 𝜂 is 𝜂 𝐶 𝜅 observed to increase rapidly as the concentration increase. This makes 𝐶 ⁄ to increase as well indicating the diminished heat transfer performance in nanofluids with high particle concentrations. As the particles in the nanofluid arrange into chain-like structures, 𝜂 remains unaffected. This makes 𝐶 ⁄ to decrease rapidly indicating the greater heat transfer performance in nanofluids with chain-like nanoparticle arrangements or in general, extended nanostructures.