We carry out extensive experimental studies of turbulent convective heat transfer of several water-based Al2O3, SiO2, and MgO nanofluids with a nanoparticle volume fraction up to 4%. The experimental setup consists of an annular tube, where sub-atmospheric condensing steam is used to establish a constant wall temperature boundary condition, with nanofluid forced through the inner tube. To unravel the influence of particle shape and size to heat transfer we also present a detailed characterization of the nanofluids using Dynamic Light Scattering and Transmission Electron Microscopy techniques in situ. In agreement with previous studies, we find that the average convective heat transfer coefficients of nanofluids are typically enhanced by up to 40% when compared to the base fluid on the basis of constant Reynolds number in the turbulent regime, where Re = 3000-10,000. However, the increase of the dynamic viscosity of nanofluids leads to significant pressure losses as compared to the base fluids. To account for this, the convective heat transfer efficiency eta is determined by comparing the enhanced heat transfer performance to the increased pumping power requirement. When this has been properly taken into account, only the SiO2 based nanofluid with smooth spherical particles (of average size 6.5 +/- 1.8 nm) shows noticeable improvement in heat transfer with a particle volume fraction of 0.5-2%. Increasing the nanoparticle volume fraction beyond 2% enhances the heat transfer coefficient but at the same time lowers heat transfer efficiency eta due to pressure losses, which result from the increased fluid density and viscosity. Through our nanoparticle size and shape analysis we find that in general small, spherical and smooth particles (less than 10 nm in size) are best in enhancing heat transfer and keeping the increase of pressure losses moderate. Our results show that the nanoscale properties of the particle phase must be carefully considered in heat transfer experiments. (C) 2013 Elsevier Ltd. All rights reserved.