Influence of the Size of Spherical Nanoparticles on the Effective Thermal Conductivity of Nanofluids

Motivated by contradictory statements in the literature, the present study provides an experimental and theoretical investigation on the influence of the size of spherical nanoparticles on the effective thermal conductivity λeff of nanofluids. Using dispersions of silicon dioxide or titanium dioxide in water or glycerol with particle diameters dp from (10 to 400) nm, λeff was determined experimentally at temperatures between (283 and 358) K up to particle volume fractions φp of 0.31. For unimodal nanofluids containing nanoparticles with one size, the experimental results for λeff normalized to the thermal conductivity of the base fluid λbf were found to increase with increasing dp, until they reach a plateau above about 100 nm. This behavior is represented well by a prediction model that combines two approaches proposed in the literature. Here, a semiempirical model allows for representing λeff by the geometric mean of λbf and an effective thermal conductivity of the particles. The latter considers the particle thermal conductivity λp and a size-dependent Kapitza resistance at the particle/liquid interface. This new prediction model describes 301 measurement results for λeff·λbf–1 of 15 different nanofluids at varying dp, φp, and T obtained from the present work and previous studies with an average absolute relative deviation of 2.2%. The same model could also be successfully applied to represent the experimental data for λeff·λbf–1 of bimodal nanofluids containing two nanoparticle fractions with different sizes.