This paper presents the numerical study of internal free convection in a vertical annulus. To predict the characteristics of nanofluids in heat transfer regime, thermal conductivity, density, and viscosity of clear fluid need to be modified. Numerous models have been disclosed to calculate these properties but recent models for viscosity and thermal conductivity which are in excellent agreement with experiments have been used. These models are thermal conductivity model of Jang and Choi and viscosity correlation of Nguyen et al. for 36nm Al2O3 particles. Inner and outer vertical walls are in constant temperature while horizontal walls are adiabatic. The continuity, Navier-Stokes and energy equation were solved numerically. Effect of nanofluids on buoyancy-driven heat transfer is investigated as a function of geometrical and physical parameters and various particle concentrations for aspect ratio of 0.2<H/L<5, Grashof number of 103<Gr<105 and concentration of 0<φ<0.12. Finally, one correlation is developed to demonstrate the effect of using atom-size particles on Nusselt number.

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