Experimental analysis of thermo-hydraulic performance of quadratic duct with artificial roughness on a single broad heated surface of the absorber plate of solar air heater

The thermo-hydraulic performance of an S-shaped ribbed absorber plate in a solar air heater (SAH) was investigated experimentally. The present work examines a roughened surface characterised by a roughness pitch (P = 10 mm), relative roughness (p/e = 8.33), rib diameter (1.2 mm), duct aspect ratio (8), and relative roughness height (e/D h = 0.027). Experiments were conducted across a Reynolds number range of 3,000 to 14,000, with the arc angle of the roughness element varied between 30° and 75°. Results show that as the Reynolds number increases, the Nusselt number also rises for the single S-shaped rib configuration. Although surface roughness leads to increased frictional losses, the rate of heat transfer enhancement outweighs the friction penalty. The highest observed improvement in heat transfer was 2.475 times, while the friction factor increased by up to 4.78 times compared to a smooth duct. Additionally, ribs with a 10 mm gap at varying arc angles resulted in a 20.31% increase in thermal efficiency. The thermo-hydraulic performance parameter initially increased and then declined with higher Reynolds numbers, indicating an optimal operating range. These findings reinforce the role of optimised surface roughness in improving solar air heater performance, aligning with previous research trends.