The thermal control in many systems is widely accomplished applying natural convection process due to its low cost, reliability and easy maintenance. Typical applications include the heat exchangers, cooling of electronic equipment and nuclear reactors, solar chimneys and Trombe walls in building industry, etc. Numerical and experimental studies of natural convective flows in two dimensional channels, opened to ambient conditions at both end sections, are extensively reported in the literature but most of them are treated severely idealized situations. CFD code of natural convection with variable properties and slip condition are presented in the present work. The 2D, laminar simulations are obtained by solving the governing equations using a Fluent 6.2.16. It is considered that the temperature variations are not so high and the Boussinesq approximation is applied. The latter leads to the simplification of the system of equations. The computed results for Nusselt number, velocity and temperature profiles and heat transfer rate are directly compared with those proposed in the bibliography letting therefore the validation of the employed numerical procedure.