(665i) Monte Carlo Simulations of Nanofluids

Adding nanoparticles to a fluid results in considerable improvement of the fluid thermal properties but the magnitude of the enhancement and the physical mechanism by which this enhancement occurs are open to debate. Maxwell's mean field theory, the standard model for these systems, predicts two limiting bounds for this enhancement depending on whether the dispersed phase (nanoparticles) has higher or lower conductivity than the continuous phase (fluid). This also suggests that the enhancement depends on the configuration and degree of aggregation of the particles, though such effects are not captured by Maxwell's theory. In our past work we demonstrated that aggregated nanoparticles have higher conductivity compared to fully dispersed particles at the same volume fraction. Here we employ a computational model to explain the experimental results and quantify the effect of aggregation on thermal conductivity.

We generate model clusters in a base fluid and evaluate the thermal conductivity of the system using a Monte Carlo algorithm. Briefly, we discretize the aggregate and embed a large number of random walkers whose displacement is biased by the thermal conductivity in the vicinity of their location. The thermal diffusivity and heat conduction are then obtained from the average mean-squared displacement.We vary the degree of aggregation in a systematic way and find the conductivity increases up to 30% above the lower Maxwell bound. We attribute the increase the formation of a neck between neighboring particles and show that a pair of partially merged particles has 15% higher conductivity than either the full dispersed or fully merged particles. Further increases in conductivity are achieved by allowing the particles to be hollow such that fluid is contained in the core. Clusters formed by hollow particles exhibit even higher enhancements, though in all cases these remain below the upper Maxwell bound. The results suggest that manipulation of the microstructure of particle-fluid systems has the potential to produce enhanced thermal properties and provides an explanation for the "anomalous'' enhancements often reported by experimentalists who do not control the state of aggregation in the suspension.