The influence of the radiative heat transfer and the effects of air variable properties on the natural convection flows established in square cavities with thermal passive device geometry are numerically investigated. Two-dimensional, laminar, transitional and turbulent simulations are obtained, considering both uniform heat flux and uniform wall temperature heating conditions. The average Nusselt number and the dimensionless mass -flow rate have been obtained for a wide range of the Rayleigh number varying from 10(3) to 10(16). The results obtained for different heating intensities are analyzed and compared. In addition, the influence of considering surface radiative effects on the differences reached for the Nusselt number and the mass flow rate obtained with several heating intensities is studied. The obtained results show that the effects of thermal radiation on the appearance of the burnout phenomenon are particularly relevant under given circumstances. The influence of the wall-to-wall spacing of the vertical channel inside the cavity is also analyzed. The changes produced in the flow patterns into the cavity when the radiative heat transfer and the effects of variation of properties are relevant, are also shown. (C) 2017 Elsevier Ltd. All rights reserved.