The present numerical investigation deals with the natural convection in a vertical cylindrical enclosure with a water-based Al_{2}O_{3} nanofluids. The enclosure with an aspect ratio of 1 (= diameter / height) is filled with Al_{2}O_{3}-water nanofluids or pure water. These fluids are isothermally heated from below and cooled from above, while the sidewall is thermally insulated. Both fluids are assumed to be Newtonian and incompressible, with laminar flows. In addition, Al_{2}O_{3}-water nanofluids are assumed to be a single-phase (homogeneous) fluid, due to the extremely tiny particles and very low volume fraction of the suspended nanoparticles, even though nanofluids are a mixture in which nanoparticles are dispersed in a base fluid. Thermophysical properties of Al_{2}O_{3}-water nanofluids are estimated by the experimental correlation equations reported by Khanafer and Vafai. The main objective of this numerical investigation is to clarify the influences of the reference temperature, the nanoparticle diameter, the volume fraction of Al_{2}O_{3} nanoparticles, and the Rayleigh number on the convective heat transfer of the one-sided natural convection of Al_{2}O_{3}-water nanofluids induced in the enclosure. Transient three-dimensional numerical results are presented over a wide range of reference temperatures (θ_{0} = 293.15, 303.15, 313.15 K), nanoparticle diameters (d_{p} = 25, 50, 100 nm) , volume fractions of Al_{2}O_{3} nanoparticles ( φ_{p} = 0 - 0.04), and Rayleigh numbers (Ra = 10^{4}, 10^{5}). Furthermore, the convective heat transfer characteristics of the unique one-sided natural convections of Al_{2}O_{3}-water nanofluids and water are presented by the average Nusselt number, isotherms, and particle paths. Comparison between the one-sided natural convection of Al_{2}O_{3}-water nanofluids and that of water shows that the deterioration of the natural convective heat transfer of Al_{2}O_{3}-water nanofluids is observed with the decrease of the nanoparticle diameter and with the increase of the volume fraction of Al_{2}O_{3} nanoparticles under three different reference temperatures and two different Rayleigh numbers. However, the degree of deterioration depends on the reference temperature and the Rayleigh number.
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