Effect of Brownian Motion on Thermal Conductivity of Nanofluids

Nanofluids, i.e., liquids containing nanometer sized metallic or nonmetallic solid particles, show an increase in thermal conductivity compared to that of the pure liquid. In this paper, a simple model for predicting thermal conductivity of nanofluids based on Brownian motion of nanoparticles in the liquid is developed. A general expression for the effective thermal conductivity of a colloidal suspension is derived by using ensemble averaging under the assumption of small departures from equilibrium and the presence of pairwise additive interaction potential between the nanoparticles. The resulting expression for thermal conductivity enhancement is applied to the nanofluids with a polar base fluid, such as water or ethylene glycol, by assuming an effective double layer repulsive potential between pairs of nanoparticles. It is shown that the model predicts a particle size and temperature dependent thermal conductivity enhancement. The results of the calculation are compared with the experimental data for various nanofluids containing metallic and nonmetallic nanoparticles.