Experimental results on, free convection in a vertical annulus filled with a , saturated porous medium, composed of horizontal sub layers with different permeabilities and horizontally divided into fluid and porous regions are reported for height to gap ratio of 1 and radius ratio of4.16. The experiments have been conducted for a vertical annulus whose inner cylinder was maintained at a constant surface heat flux, while the outer cylinder was maintained at constant temperature. The top and bottom walls are kept thermally insulated. The objective of the present work is to study the effects of Rayleigh number(Ra), Darcy-Rayleigh number(Ra*), grain size(d), permeability ratios(KI1K.2) and the dimensionless thickness of horizontal porous medium(Z) on the natural convection and to obtain imperical correlations between average Nusselt number (Nui) and independent variables (Ra*,K1/K2, Ra, Z). Water and sand grains of 1.6mm, 4.2mm and 12mm average diameters were used as the porous medium. The Darcy-Rayleigh number ranged from 4 to 8864 and the Rayleigh number ranged from 5x107 to 5x108 • The permeability ratio ranged from
0.067 to 26.4 and the dimensionless thickness ofhorizontal porous medium ranged from 0 to 1. Results for heat transfer rates in terms of the average Nusselt number, NUb were obtained. Good agreement between the present and previously published work was found.