The purpose of the present paper is to analyse numerically the flow and heat transfer by transient natural convection in a vertical cylinder filled with a fluid saturated porous medium. The cylinder is opened at both ends and heated with a constant wall heat flux density. This study is carried out using the Forchheimer-extended Darcy flow model and a two-temperature model. The velocity, temperature and pressure drop distributions, the dimensionless flow rate q(v)* and heat flux exchanged at the cylinder exit Phi* are obtained under various conditions. The results presented in this work provide also the validity of Darcy flow model and thermal boundary layer approximations.