CFD Comparison of Al2O3 and ZnO Ceramic Coatings on Non-Insulated Copper Heat Exchangers

Non-insulated heat exchangers in gas-to-gas service lose substantial energy to the surroundings. This study evaluates Al2O3 and ZnO ceramic coatings (200 μm) as passive thermal retention layers on the inner surface of the outer tube in a copper double-pipe heat exchanger, using 3D CFD simulations verified for internal consistency against Log Mean Heat Transfer Rate analytical solutions. Six cases were modelled: three coating conditions across parallel-flow and counter-flow configurations under laminar conditions (Rei≈525, Reo≈192) with air as the working fluid. The coating elevates the outer tube inner wall temperature T3, increasing the convective driving force to the cold fluid while suppressing ambient dissipation. In parallel flow, Al2O3 increases the net inter-fluid heat transfer rate by 35.7% and reduces ambient losses by 81.4%; ZnO achieves 30.9% and 70.4%, respectively. In counter-flow, Al2O3 yields a 26.6% enhancement and 73.2% loss reduction. The coated parallel-flow configuration outperforms the uncoated counter-flow baseline. Thermal circuit analysis shows that Al2O3 superiority arises from its higher conductivity (40 vs. 19 W m−1 K−1), which sustains a higher equilibrium T3 and a heat partition ratio of 11.84 versus 7.17 for ZnO. These results show that a single ceramic coating layer can recover a large fraction of the thermal energy lost through non-insulated walls, offering a low-cost, retrofit-compatible pathway to improve the energy efficiency of gas-to-gas heat exchangers in HVAC, building energy recovery, and industrial process heat applications.