Effects of thermodiffusion and nanoparticles on convective instabilities in binary nanofluids

In this article, the effects of thermodiffusion of nanoparticles and solute in binary nanofluids and nanoparticles on the convective instabilities of a binary nanofluid is theoretically investigated. Thermodiffusion implies that mass diffusion is induced by thermal gradient, which is the so-called Soret effect. In order to analyze the convective instabilities of a binary nanofluid, a new stability criterion is obtained based on the linear stability theory and new factors g and f are proposed. The results show that the Soret effect of solute makes the binary nanofluids unstable significantly and the convective motion in a binary nanofluid sets in easily as the ratio of Soret coefficient of nanofluid to that of binary basefluid δ₄ increases for δ₄ > -1. It is also found that with an increase of the volume fraction of nanoparticles, the nanofluid becomes stable, but at or near ψ_bf = - 0.3 the state of nanofluid changes from stable to unstable. The results from the addition factor analysis show that an asymptotic point of ψ_bf where the maximum value of g diverges infinitely exists in the range of - 1.2 < ψ_bf < - 1.1 with given conditions. The binary addition factor g is always higher than the normal addition factor f, which means that the heat transfer enhancement by the Soret effect in binary nanofluids is more significant than that in normal nanofluids.