A generalised non-Darcian porous medium model for natural convective flow has been developed taking into account linear and non-linear matrix drag components as well as the inertial and viscous forces within the fluid. The results of the general model have been Validated with the help of experimental data and compared with the various non-Darcy porous media model predictions reported in literature. It has been observed that the wall Nusselt number is significantly affected by the combination of dimensionless parameters such as Rayleigh number, Darcy number and porosity in the non-Darcy flow regime. A detailed parametric study has been presented for natural convective flow inside a rectangular enclosure filled with saturated porous medium of constant or variable porosity. It is observed that the thickness of the porous layer and the nature of variation in porosity significantly affect the natural convective flow pattern as well as the heat transfer features. The present model is also able to predict the channeling effect and associated heat transfer in forced flow through packed beds.