A dispersion model of enhanced mass diffusion in nanofluids

An explanation for the enhancement of mass diffusion in nanofluids is presented using arguments based on dispersion in diluted fixed beds. Starting from the generalized Langevin equation, it is shown that the velocity field established around a Brownian nanoparticle is similar to the velocity field predicted by Brinkman equations leading to the analogy between dispersion in diluted fixed beds and dispersion in nanofluids. The proposed model predicts the order of magnitude of mass diffusion enhancement we observed recently (10-fold enhancement of rhodamine 6G mass diffusivity under the optimum conditions for a suspension of 10-nm alumina nanoparticles in deionized water). Contrarily to other Brownian motion-based models of diffusion in nanofluids, the proposed model samples the whole Maxwell-Boltzmann distribution of particle velocities rather than taking the non-representative root mean square velocity. The model also shows a strong dependence on the mass transfer Peclet number and, consequently, justifies the order of magnitude differences between the mass diffusivity and thermal conductivity enhancements reported in the literature. (C) 2011 Elsevier Ltd. All rights reserved.