A numerical study of transient buoyancy-driven convective heat transfer of water-based nanofluids inside a bottom-heated horizontal isosceles triangular cylinder is presented. Nano-sized copper oxide (CuO) particles suspended in water with two different volume fractions are considered. The thermophysical properties of water in the presence of nanoparticles are predicted using existing models, in which the effects of the Brownian motion of nanoparticles are taken into account. It is shown that pitchfork bifurcation appears for relatively high Grashof numbers and the critical Grashof number is found to be 5.60 x 10(4). The predicted development of convective flow of nanofluids is presented by means of the average Nusselt number over the bottom. Additionally, the flow development time towards a steady/quasi-steady state and the time-averaged Nusselt number are scaled with Grashof number. It is also shown that at constant Grashof numbers the time-averaged Nusselt number is lowered as more nanoparticles are added to the base liquid and will be overestimated if the Brownian motion effects are not considered. (C) 2010 Elsevier Ltd. All rights reserved.