Thermal–hydraulic optimization of airfoil fins in an air–molten salt printed circuit heat exchangers for Carnot battery thermal energy storage

Abstract Printed circuit heat exchangers show considerable potential for high-temperature air–molten salt heat transfer and thermal storage in Carnot batteries, but weak air-side heat-transfer capability and the flow resistance induced by enhancement structures limit their thermal–hydraulic performance. To address this issue, a three-dimensional conjugate heat-transfer model of a representative periodic air–molten salt print circuit heat exchanger segment was developed, with the local boundary conditions mapped from a nominal 89.3 MW Carnot-battery heat-transfer duty. The shear stress transport k–ω turbulence model was used to examine the effects of air-side fin geometry. Triangular, diamond, rectangular, and NACA0025 airfoil fins were first compared under identical chord length and cross-sectional area. The diamond fin produced the highest Nusselt number (Nu) but caused a large flow-resistance penalty, whereas the NACA0025 airfoil fin achieved the lowest Fanning friction factor and the highest heat-transfer-to-flow-power ratio, making it a suitable baseline for low-pressure-drop air-side enhancement. The effects of maximum fin thickness, maximum-thickness location, and leading-edge radius were then analyzed. Based on 100 computational fluid mechanics samples, a back-propagation neural network-assisted two-objective optimization framework was developed to maximize Nu and minimize f under baseline non-deterioration constraints. Entropy-weight TOPSIS (technique for order of preference by similarity to ideal solution) identified Wf=0.934 mm, Df=0.804 mm, and Rf=0.260 mm as the recommended design. Computational fluid mechanics verification yielded Nu=61.07, f=0.00830, and ηQ/P=122.92. Compared with the baseline NACA0025 fin, the recommended structure increased Nu by 1.03%, reduced f by 3.48%, and increased ηQ/P by 4.48%. A controlled local air-side sensitivity analysis showed that the recommended fin retained its relative thermal–hydraulic advantages over the baseline structure under the investigated air-side inlet-temperature and mass-flow-rate ranges, while the molten-salt-side boundary conditions were fixed. This study provides local fin-level design guidance for low-pressure-drop air-side enhancement of air–molten salt printed circuit heat exchangers under operating conditions relevant to Carnot-battery thermal energy storage.