In this study, two-dimensional steady-state simulations of laminar natural convection in rectangular enclosures with differentially heated side walls have been conducted for a range of different aspect ratios AR (=H/L where H is the enclosure height and L is the enclosure width). The rectangular enclosures are considered to be completely filled with a yield-stress fluid obeying the Bingham model. Yield stress effects on heat and momentum transport are investigated for nominal values of Rayleigh number (Ra) in the range 10(4)-10(6) and the aspect ratio range 1/8 to 8 for a single Prandtl number (Pr = 7). Iris found that the mean Nusselt number (Nu) over bar increases with increasing values of Rayleigh number for both Newtonian and Bingham fluids. However, (Nu) over bar values obtained for Bingham fluids are smaller than that obtained in the case of Newtonian fluids with the same nominal value of Rayleigh number Ra due to weakening of convective transport. The mean Nusselt number (Nu) over bar in the case of Bingham fluids is found to decrease with increasing Bingham number, and, for large values of Bingham number Bn, the value of (Nu) over bar settles to unity (i.e. (Nu) over bar = 1.0) as heat transfer takes place principally due to thermal conduction. The effects of aspect ratio AR have also been investigated in detail and it has been found the effects of thermal convection (diffusion) strengthens (weakens) with increasing aspect ratio and vice versa, for a given set of nominal values of Rayleigh number Ra and Prandtl number Pr. It is found that the aspect ratio AR(max) at which the maximum mean Nusselt number (Nu) over bar occurs is found to decrease with increasing Rayleigh number. However, the value of AR(max) is shown to increase with increasing Bingham number Bn for a given set of values of Ra and Pr. Detailed physical explanations are provided for the observed phenomena. New correlations are proposed for the mean Nusselt number (Nu) over bar for Bingham fluids, which are shown to satisfactorily capture the correct qualitative and quantitative behaviour of (Nu) over bar in response to changes in Ra. AR and Bn. (C) 2010 Elsevier B.V. All rights reserved.