A finite-volume approach is developed to numerically predict natural convection heat transfer in enclosures with isolated solid islands in the flow field. The computations were performed for the natural convection of air in a cylindrical envelope with an internal, concentric, slotted hollow cylinder for the range of Ra = - 103– 106and dimensionless slot width S = 0-0.5. The computed average equivalent thermal conductivities and the predicted flow patterns are compared with the experimental results available in the literature. It is found that for a fixed Rayleigh number there exists a critical slot width at which the total heat transfer rate reaches maximum.