The current study reports a numerical analysis of free convection of air in an isothermal horizontal cylinder,cooled and heated at different wall locations.Three heater sizes are discussed in this study.The first heated zone is spread across one-quarter of the sidewall;the second is uniformly distributed over the half of the wall and the third active wall covers three-quarters of the cylinder.Five various locations are considered and examined for each active zone of the sidewall.The computation is carried out for Rayleigh number ranging from 102 to 106.Numerical results characterizing heat transfer and flow features are achieved using an iterative model developed in COMSOL Multiphysics.The effect of Rayleigh number on heat transfer and fluid flow characteristics within the cavity are investigated.Particular attention is paid to the influence of heater location and heater size on energy efficiency.It is found that the mean Nusselt number and dimensionless velocity increase when increasing the Rayleigh number.Moreover,the optimal level of energy efficiency is achieved if the heating zone is centered at the upper part of the cylinder,regardless of the heater size.It is also shown that the optimal configuration providing higher energy efficiency is obtained when three-quarters of the sidewall are locally heated,and more precisely,if the active zone is centered at the top of the cylinder.