A note on heat transfer modelling of Newtonian nanofluids in laminar free convection

The natural convection heat transfer of Newtonian nanofluids in a laminar external boundary-layer is investigated from the integral formalism approach. In particular, this study deals with gamma-Al2O3/water nanofluids whose Newtonian behaviour was experimentally confirmed for particle volume fractions less than 4% [N. Putra, W. Roetzel, S.K. Das, Natural convection of nano-fluids, Heat and Mass Transfer 39 (2003) 775-784; S.Z. Heris, S.Gh. Etemad, M.N. Esfahany, Experimental investigation of oxide nanofluids laminar flow convective heat transfer, Int. Comm. Heat Mass Transfer 33 (2006) 529-535; R. Prasher, D. Song, J. Wang, Measurements of nanofluid viscosity and its implications for thermal applications, Appl. Phys. Lett. 89 (2006) 133108]. Based on a macroscopic modelling and under the assumption of constant thermophysical nanofluid properties, it is shown that special care has to be exercised in drawing generalized conclusions about the heat transfer enhancement with the use of nanofluids. It has been found that natural convection heat transfer is not solely characterized by the nanofluid effective thermal conductivity and that the sensitivity to the viscosity model used seems undeniable and plays a key role in the heat transfer behaviour. (c) 2006 Elsevier Masson SAS. All rights reserved.