Experimental Assessment of Heat Transfer Enhancement in an Air Duct Using Removable Helical Rib Inserts with Pitch Ratios

Background: Passive inserts can improve convective heat transfer in ducts, but the resulting increase in flow resistance must also be assessed. Objective: This study experimentally investigated heat transfer and thermo-hydraulic behaviour in a circular air duct equipped with removable helical rib inserts at different diameter and pitch ratios. Methods: Air with a Prandtl number of approximately 0.7 was passed through a brass test section with an internal diameter of 17 mm and a length of 1.5 m. The Reynolds number ranged from 984 to 9100. Four insert geometries were investigated, representing diameter ratios (d/D) of 0.05 and 0.08 and pitch ratios (p/D) of 1 and 2, and were compared with a smooth duct. Nusselt numbers, friction factors, and the thermo-hydraulic performance criterion were evaluated using measured temperatures and pressure differences. Results: The configurations fitted with helical ribs produced higher Nusselt numbers than the smooth duct, indicating improved convective heat transfer. The Nusselt number increased with Reynolds number, while the friction factor declined with increasing Reynolds number for the investigated configurations. The ribbed ducts nevertheless showed greater frictional resistance than the smooth duct. The configuration with a diameter ratio of 0.08 and pitch ratio of 1 gave the strongest heat transfer response and the most favourable overall thermo-hydraulic performance. Conclusion: The findings demonstrate a heat transfer benefit from removable helical rib inserts under the tested conditions, together with a measurable hydraulic penalty. Optimisation should therefore consider heat transfer and pressure loss jointly.