It has been demonstrated there exists an interfacial nanolayer at the nanoparticles/fluid interface, and the nanoparticles in the base fluid aggregate easily and form clusters. As a result, both the interfacial nanolayer and the mutual interaction between nanoparticles become important. To account for the interfacial nanolayers, we first generalize differential effective dipole approximation to obtain the equivalent thermal conductivity of the coated nanoparticles with graded nanolayers. Then we employ multiple image method to investigate the effect of mutual interaction between nanoparticles on the thermal conductivity of nanofluids. Analytical formulae allow us to check the effect of nanolayer thickness, particle size, and mutual interaction. Numerical results show that when nanoparticles close up and even get touched, the effective thermal conductivity is slightly enhanced in comparison with that predicted from Maxwell–Garnett theory. The nanolayer properties also play a more important role in the conductivity enhancement. In addition, theoretical results on the effective thermal conductivity of nanofluids are in good agreement with experimental data.

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