This paper investigates experimentally the flow and heat transfer characteristics in horizontal annuli filled with porous media. With air as the working fluid, the effects of the annulus radius ratio, flow velocity and particle thermal conductivity are examined for a wide range of thermal conductivity (from 61 W/m K for carbon steel to 0.16 W/m K for polyvinyl chloride), radius ratio (inner/outer) ranging from 0.200 to 0.625 and a range of Reynolds number from 5200 to 12000. The results indicate that, in the presence of particles, the measured average Nusselt number was up to four times higher than that for an annulus without spherical particles. It was found that higher heat transfer coefficients are obtained with packing particles of higher thermal conductivity. Also, the average Nusselt number always increases as the radius ratio increases. The present experimental data are compared against those available in the literature and good correspondence is noticed. Furthermore, an empirical correlation for the average Nusselt number with the governing dimensionless parameters was developed.