A numerical simulation of the steady-state, laminar, two-dimensional, natural convection heat transfer in an open-ended channel partially filled with an isotropic porous medium is presented. The Darcy-Brinkman-Forchheimer model along with Boussinesq approximation is used to describe the fluid flow in the porous region. Meanwhile, the Navier-Stokes equation along with Boussinesq approximation is used to describe the flow in the clear flow region. The dependence of the average Nusselt number on Rayleigh number, inclination angle, Darcy number, inertia coefficient, Prandtl number, porous width to channel width ratio, the ratio of the porous effective conductivity to fluid conductivity, and channel width to length ratio is investigated. The numerical results obtained indicate that air gap presence may reduce the average flow in the porous substrate to zero. This leads to the presence of an optimum average Nusselt number at low and high values of the effective thermal conductivity ratios.