Flow and heat transfer performance analysis and multi-objective optimization of supercritical CO2 in a vertical double-helical U-tube

High-performance compact heat exchangers are essential for improving the efficiency and operational safety of supercritical carbon dioxide energy systems. However, severe thermophysical-property variations near the pseudo-critical temperature may weaken heat transfer and cause excessive wall temperatures. This work proposes a novel compact vertical double-helical U-tube heat exchanger. A three-dimensional numerical model incorporating a modified turbulent model was developed and validated against available experimental data. The effects of helix diameter, pitch, tube diameter, heated height, heat flux, mass flux, and pressure were systematically investigated. Compared with a conventional vertical U-tube under identical operating conditions, the proposed configuration increased the average heat transfer coefficient by 193%, while the pressure drop per unit length increased by 1738%. Enhanced heat transfer resulted from sustained centrifugal-force-induced secondary circulation, which enhanced radial mixing and weakened the thermal boundary layer. Tube diameter exerted the greatest influence on thermal-hydraulic performance. Moreover, an artificial neural network (ANN) surrogate model combined with a non-dominated sorting genetic algorithm (NSGA-II) was subsequently employed for multi-objective geometric optimization. The optimal compromise solution corresponded to a helix diameter of 47.69 mm, a pitch of 28.36 mm, a tube diameter of 2.01 mm, and a heated height of 232.91 mm. These results demonstrate that the proposed configuration achieves significant heat transfer enhancement while mitigating heat transfer deterioration, but its significant hydraulic penalty limits its application to compact systems prioritizing thermal safety and heat-transfer performance over pumping-power consumption.