Thermal performance of a packed bed spherical alumina ball regenerative heat exchanger under varying porosity and mass flow rate

Regenerative heat exchangers are extensively employed in industrial energy recovery systems for their ability to store and reutilise thermal energy, thereby enhancing overall process efficiency. Among various configurations, fixed-bed and packed-bed regenerators offer advantages including high surface-area-to-volume ratio, mechanical simplicity, and operational flexibility. Despite considerable research on metallic packing materials such as stainless steel and wire mesh, the use of alumina (Al 2 O 3 ) spheres as heat storage media in fixed-bed regenerators remains relatively unexplored. In the present work, a compact fixed-bed regenerative heat exchanger with housing dimensions of 60 mm × 40 mm × 40 mm, packed with spherical alumina balls, is designed, fabricated, and experimentally investigated to evaluate its thermal and hydraulic performance under cyclic heating and cooling conditions. The bed porosity is systematically varied by employing three packing configurations — 6 mm spheres (ε = 0.62), 10 mm spheres (ε = 0.56), and a mixed arrangement of 6 mm and 10 mm spheres (ε = 0.52), and experiments are conducted at three air mass flow rates ranging from 0.0025 to 0.005 kg/s. The Nusselt number, thermal effectiveness, pressure drop, friction factor, and NTU are evaluated as functions of porosity and mass flow rate. Results show that the Nusselt number increases with Reynolds number and peaks at ε = 0.56, while effectiveness decreases with increasing mass flow rate due to reduced residence time. Experimental pressure drop values agree with Ergun equation predictions within 15–30%, and an empirical Nusselt number correlation is proposed. The findings confirm alumina balls as a viable, cost-effective alternative to metallic packings for fixed-bed regenerators.