<p>This study presents a comprehensive numerical investigation of the heat transfer and flow behaviour in solar air heaters (SAHs) employing Al₂O₃ nanofluids with air and ammonia as base fluids under different ribbed duct geometries. The simulations conducted using the Reynolds-Averaged Navier–Stokes (RANS) formulation with the RNG k–ε turbulence model for improved prediction of turbulence in separated and recirculating ribbed flows to evaluate the Nusselt number, friction factor, and thermo-hydraulic performance over a Reynolds number range of 3800–20,000. Triangular ribbed ducts demonstrate superior heat transfer characteristics compared to rectangular ducts due to enhanced wall shear and geometric convergence effects. Under the adopted effective-property modelling framework, ammonia-based nanofluid cases exhibited comparatively higher thermal performance. For instance, NH₃ with 3% Al₂O₃ in a triangular duct achieved a peak heat transfer coefficient of approximately 750&#xa0;W/m²·K and a Nusselt number of about 135 at Re = 19,000. However, this enhancement is accompanied by elevated frictional losses, especially at lower Reynolds numbers and higher nanoparticle concentrations. A detailed thermo-hydraulic analysis identified 2–3% Al₂O₃ as the optimal concentration range, offering an effective balance between heat transfer gain and flow resistance. The highest thermo-hydraulic performance factor (THPF ≈ 2.0) was recorded for NH₃ + 3% Al₂O₃ at Re = 15,000. These findings highlight the potential of ammonia-based nanofluids combined with triangular geometries for achieving compact and thermally efficient solar air heating systems.</p> Graphical abstract <p></p>