The present numerical work explores the fluid flow and heat transfer characteristics due to combined forced and natural convection in a tall lid-driven cavity containing uniformly heated equilateral triangular block. The flow is assumed to be steady, laminar, and two dimensional with incompressible non-Newtonian power-law fluids. The study is carried out to illustrate the convection behavior in tall cavities of different aspect ratios (AR = length / height = 0.25 , 0.5 , 0.75) at different Richardson numbers (Ri=0.1,1and10), non-Newtonian power-law index (0.6 ≤ n≤ 1.4) and Prandtl number (1 ≤ Pr ≤ 100) for a constant Grashof number of 104. The flow circulation is observed to be limited near the region of the moving wall irrespective of Richardson number for lower Pr. With the increase in the AR of the cavity, fluid–structure becomes more dispersed within the cavity. Higher heat transfer rates are obtained for the low AR (AR = 0.5). © 2020, Akadémiai Kiadó, Budapest, Hungary.