At this stage of nanofluids development, their thermal conductivity it is not yet known precisely and the judgment of their true potential is difficult. This fact was illustrated by analyzing their heat transfer performance for laminar fully developed forced convection in a tube with two zones: one adiabatic and one with uniform wall heat flux. Forced convective of a nanofluid that consists of water and Al2O3 in horizontal tubes has been studied numerically. Three different models from the literature are used to express the thermal conductivity in terms of particle loading and they led to different qualitative and quantitative results in a classical problem of replacement of a simple fluid (water) by a nanofluid in a given situation. In particular, the heat transfer coefficient of water-based Al2O3 nanofluids is increased by 3.4-27.8% under fixed Reynolds number compared with that of pure water. Also, the enhancement of heat transfer coefficient is larger than that of the effective thermal conductivity at the same volume concentration. Moreover, the effect of uncertainties in modeling nanofluids properties was noticed. (C) 2013 Elsevier Ltd. All rights reserved.