Synergistic thermo-hydraulic performance enhancement in a coaxial heat exchanger using hybrid Al2O3–ZnO nanofluids and turbulator inserts

The present study investigates the thermo-hydraulic enhancement of a coaxial heat exchanger through the combined use of an Al 2 O 3 –ZnO/water hybrid nanofluid and optimized internal turbulator geometries under turbulent flow conditions. A hybrid nanofluid containing equal proportions of Al 2 O 3 and ZnO nanoparticles dispersed in water was numerically investigated for nanoparticle volume fractions ranging from 1 to 3% over a Reynolds number range of 5 × 10 4 to 10 5 . The results show that increasing the nanoparticle concentration significantly enhances convective heat transfer, with the 3% volume fraction providing the best overall thermo-hydraulic performance among the investigated cases. The introduction of turbulator inserts further promotes fluid mixing and thermal boundary-layer disruption, resulting in a maximum heat transfer enhancement of approximately 115% compared with the smooth tube using pure water. Although the inserts increase the friction factor, the optimal configuration maintains a favorable balance between heat transfer enhancement and hydraulic losses, achieving a maximum thermal enhancement factor (TEF) of 1.73. Among the investigated geometries, Model 5 exhibits the best thermo-hydraulic performance. These findings demonstrate that combining an optimized Al 2 O 3 –ZnO/water hybrid nanofluid with an appropriate turbulator geometry is an effective passive strategy for improving the efficiency of compact coaxial heat exchangers.