A numerical investigation of the steady-state, laminar, axi-symmetric, mixed convection heat transfer in the annulus between two concentric vertical cylinders using porous inserts is carried out. The inner cylinder is subjected to constant heat flux and the outer cylinder is insulated. A finite volume code is used to numerically solve the sets of governing equations. The Darcy-Brinkman-Forchheimer model along with Boussinesq approximation is used to solve the flow in the porous region. The Navier-Stokes equation is used to describe the flow in the clear flow region. The dependence of the average Nusselt number on several flow and geometric parameters is investigated. These include: convective parameter, λ, Darcy number, Da, thermal conductivity ratio, K r , and porous-insert thickness to gap ratio (H/D). It is found that, in general, the heat transfer enhances by the presence of porous layers of high thermal conductivity ratios. It is also found that there is a critical thermal conductivity ratio on which if the values of Kr are higher than the critical value the average Nusselt number starts to decrease. Also, it found that at low thermal conductivity ratio (K r ≈ 1) and for all values of λ the porous material acts as thermal insulation.Keywords Mixed convection · Porous media · Darcy-Brinkman-Forchheimer model · Heat transfer · Critical thermal conductivity ratio Nomenclature C F Forchheimer coefficient , C F = 1.75/ 150φ 5 C p Specific heat (J kg −1 K −1 ) D Gap between the two concentric cylinders (r o − r i ) [m] Da Darcy number (K /D 2 )