A proof for insignificant effect of Brownian motion-induced micro-convection on thermal conductivity of nanofluids by utilizing molecular dynamics simulations

It has been recently demonstrated through experiments that the observed high enhancements in thermal conductivity of nanofluids are due to aggregation of nanoparticles rather than the previously stated mechanism of the Brownian motion-induced micro-convection. In this paper, we use equilibrium molecular dynamics simulations to investigate the role of micro-convection on the thermal conductivity of well-dispersed nanofluids. We show that while the individual terms in the heat current autocorrelation function associated with nanoparticle diffusion achieve significant values, these terms essentially cancel each other if correctly defined average enthalpy expressions are subtracted. Otherwise, erroneous thermal conductivity enhancements will be predicted, which are attributed to Brownian motion-induced micro-convection. Consequently, micro-convection does not contribute noticeably to the thermal conductivity and the predicted thermal conductivity enhancements are consistent with the effective medium theory. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4791705]