Investigation on thermal performance and flow characteristics of hybrid nanofluids in uniform and non-uniform twisted oval tubes

The pursuit of compact and high-efficiency heat exchangers has driven interest in combining hybrid nanofluids with enhanced heat transfer geometries. This study numerically investigates the thermal-hydraulic performance of Al 2 O 3 -CuO/water hybrid nanofluids within both uniform and non-uniform twisted oval tubes. The effects of nanoparticle mixing ratio (0-100% Al 2 O 3 /CuO), volume fraction (0.25-1.25%), and tube geometry (aspect ratio of 1.43-2.57, twisted pitch ratio of 0.125-1) were systematically examined. Results indicate that increasing the nanoparticle volume fraction enhances heat transfer, with the Nusselt number rising by up to 12.30% compared to deionized water; this improvement, however, comes with a 26.67% increase in pressure drop. An optimal performance evaluation criterion (PEC > 1) was achieved at a Reynolds number of 6000 for a tube with an aspect ratio of 1.43 and a twisted pitch ratio of 0.25. Furthermore, the 100% CuO and 75% CuO/25% Al 2 O 3 nanofluids demonstrated superior performance at a 0.5% volume fraction. The study conclusively shows that twisted oval tubes, particularly non-uniform designs, yield significantly better heat transfer efficiency than straight elliptical tubes, achieving an optimal balance between thermal enhancement and flow resistance. These findings provide valuable guidance for designing advanced heat exchange systems utilizing hybrid nanofluids in geometrically optimized tubes.