Thermophysical characteristics of ethylene glycol-based copper nanofluids using nonequilibrium and equilibrium methods

Abstract

Enhancement of thermal conductivity for nanofluids has been demonstrated in numerous experiments and analysis. The present study calculates the thermal conductivity and reveals molecular-level mechanisms for copper nanoparticles suspended in ethylene glycol using molecular dynamic simulations. Computed thermal conductivities of the nanofluids using Green-Kubo formalism and using Nonequilibrium MD Methods are compared. Contributions for possible heat transfer modes in molecular level are quantized, including modes of convection and interaction using Green-Kubo formalism. The simulations not only confirm that the enhancement of thermal conductivity due to the suspending nanoparticle is increased with volume fraction and the size of the nanoparticle but also identify the significant contributions from atom interaction.

Introduction

Nanofluids are prepared by dispersing solid nanoparticles in fluids such as water, ethylene glycol (EG), oils, etc. Abnormal thermal conductivity increases relative to the base fluid have been demonstrated [1], [2], [3], [4], [5], [6]. The mechanism behind the abnormally enhanced thermal conductivity of nanofluids is a hotly debated topic. Although models have been used to describe physical mechanisms for effective thermal conductivity of nanofluids, such as the Brownian motion of particles [7], [8], molecular-level layering of liquid at the liquid/particle interface [9], [10], the nature of heat transport in nanoparticles [7], and the effects of nanoparticle clustering [7], [11], no final conclusions have been made [12] because most of the prediction models were described by macroscale or macroscale with modifications on the molecular level. Molecular Dynamic (MD) Simulations are an ultimate tool to clarify and to identify the major mechanisms because they are based on the basic law of Newton and provide significant insights at the atomic level and have been applied in recent studies.

In 2002, Keblinski et al. [7] applied MD simulations to show the ballistic, rather than the diffusive, nature of heat transport in higher thermal properties of nanofluids. Li et al. [13] further applied MD simulations to study the physical mechanism in molecular layering at liquid–solid interface. Teng et al. [12] analyzed the intermolecular interactions between Cu–Cu atoms, layer structure surrounding nanoparticles, the convection effect induced by the Brownian motion of Cu atoms; and the particle–particle interactions. Sarkar et al. [14] demonstrated the increased movement of liquid atoms in the presence of nanoparticles; while Eapen et al. [15], [16], [17] showed short-ranged attraction in a percolating network of thermal conduction paths between the clusters and fluid atoms, and a microconvection of the fluid medium around randomly moving nanoparticles.

Of importance in the preceding research was that the base fluids for the above-mentioned simulations and analysis were made of monoatomic systems (Argon or Xenon) and these were comprised of low Prandtl number liquids. However, Lee et al. [2] and Xie et al. [18], [19] reported that EG-based nanofluids indicated a larger increase in thermal conductivity than water-based ones in their experiments. Koo and Kleinstreuer [20] also claimed that the addition of 1–4% CuO nanoparticles to high Prandtl number base fluids such as EG and oils, significantly increased the heat transfer performance of micro-heat sinks. Prandtl numbers are ∼5, 100, and 5000 for water, EG, and engine oil, respectively, at room temperature, and Prandtl numbers are 2.3, 34.6, and 546 at T = 350 K. No computations have been made for the nanofluids with base fluids of non-mono-atomic systems, especially of high Prandtl number.

Accordingly, the present study aims to calculate the enhancement of thermal conductivity and identify the major contributions for Cu nanoparticles suspended in an EG base fluid, utilizing MD simulations. Ethylene Glycol (H–O–CH₂–CH₂–O–H, abbreviated as EG) is a simple polar and chain-like molecule and is considered as a water analogue. Both Nonequilibrium MD (NEMD) and Equilibrium MD (EMD) are applied for EG–Cu system so that these two approaches can be compared and provide more information on physical insights.