Effects of microscale physics on macroscale thermal properties in nanofluids

Abstract

In an attempt to determine how the presence of nanoparticles affects the heat conduction at the macroscale and isolate the mechanisms responsible for the reported significant enhancement of thermal conductivity, a macroscale heat-conduction model in nanofluids has been recently developed by using the volume-averaging method with help of multiscale theorems for scaling-up. It shows that the microscale physics (structures, properties and activities) in nanofluids manifest themselves as heat diffusion and thermal waves at the macroscale through four effective coefficients. We numerically examined how these effective coefficients response to particle-fluid conductivity ratio, particle shape, volume fraction, distribution and size-uniformity for nanofluids consisting of two-dimensional circular, square, and hollow particles. For the simulated nanofluids, the heat conduction is diffusion-dominant so that the mixture rule is applicable for predicting their effective thermal conductivity with the effect of particle-fluid structures and conductivity ratio incorporated into its empirical parameter. For the same particle-fluid conductivity ratio (>1) and volume fraction, size-uniform particles with larger surface-to-volume ratio can result in higher thermal conductivity of nanofluids than those with smaller surface-to-volume ratio or size-nonuniform particles. Particle aggregation can also benefit the enhancement of thermal conductivity of nanofluids when particle-fluid conductivity ratio is larger than 1. Moreover, aggregates with larger effective radius give rise to higher thermal conductivity of nanofluids than those with smaller effective radius. When particles touch each other to form a network (maximum effective radius), the impact of nanoparticles on the thermal conductivity enhancement of nanofluids is much stronger than that of separated particles for the same particle volume fraction. This may provide an explanation for the reported significant enhancement of thermal conductivity in nanotube-suspension because suspended nanotubes are more likely to form a network in the base fluid.