Performance enhancement of concentric annular heat exchangers by adopting constructal T-Y-fins: Numerical studies via composite-function, ANN and NSGA-Ⅱ

Small concentric annular heat exchangers are widely used in compact thermal systems, but their performance is limited by weak convective mixing in the annulus. Constructal T-Y-fins have improved heat removal in flat and single-side geometries; however, their behavior in a conjugate annular configuration, together with the thermal-hydraulic trade-off, remains unclear. This study introduces constructal T-Y-fins into a small concentric annular heat exchanger and evaluates their thermal-hydraulic performance under parallel- and counter-flow conditions. With fixed total volume and fin material, two-degree-of-freedom constructal design is performed, and multi-objective optimization is carried out using an artificial neural network, the NSGA-II algorithm and three decision-making approaches. Under parallel-flow, the optimal composite function value is 0.941, which is 5.9% lower than the reference and 25.2% lower than the model without fins. Under counter-flow, the corresponding value is 0.920 (8% lower than the reference and 27.1% lower than the model without fins), indicating superior performance. The Shannon Entropy DMA yields the smallest deviation indices (0.3816 for parallel-flow, 0.3920 for counter-flow). The maximum thermal driving force concentrates at the branched-fin tips, where the temperature gradient increases by order of magnitude, indicating that flow-field redirection and boundary-layer thinning, rather than surface extension alone, dominate the enhancement.