A New Model for Thermal Conductivity in Nanofluids

Nanofluids exhibit enhanced thermal conductivity superior to traditional heat transfer fluids. The conventional theoretical models cannot explain the large enhancement of the thermal conductivity of nanofluids. It has been proposed that an interfacial structure formed by liquid molecular layering might play an important role. To date there is no known procedure to properly calculate the nanolayer thickness, and all previous investigators arbitrarily adopt a layer thickness which is consistent with experimental results. Here we investigate the thermal conductivity structure of this interfacial layer and its impact on the effective thermal conductivity and an expression for the thermal conductivity profile in the nanolayer is investigated using matching boundary conditions for the general heat conduction equation. An expression for the thickness of the nanolayer is derived. The assumed profile for the thermal conductivity within the nanolayer, k = k_O (1 - alpha r)^m, is found to be appropriate when m < 15. It is found that the thickness of the nanolayer is approximately 19% and 22% of the radius of a nanoparticle, for Al₂O₃/ethylene glycol and CuO/H₂O nanofluids, respectively. This evaluation of the nanolayer thickness is consistent with the range of values used by several authors in their theoretical models for the thermal conductivity of nanofluids which include a nanolayer.