Thermal performance and structural stability of gyroid heat exchanger for supercritical CO₂ cycle

The supercritical CO₂ Brayton cycle is widely recognized for its high efficiency, compact size, and moderate operating temperatures. To achieve optimal cycle efficiency, it is crucial to integrate compact high-performance heat exchangers in the cycle. Gyroid-based triply periodic minimal surface (TPMS) structures, known for their exceptional surface-area-to-volume ratio, are particularly promising for such heat exchangers. However, the influence of varying periodicity and wall thickness of gyroid structures on both heat transfer characteristics and structural deformation has yet to be systematically explored. By filling this gap, the novelty and impact of this study stem from its comprehensive and systematic analysis of gyroid-based TPMS heat exchangers for supercritical CO₂ cycles, addressing heat transfer performance, pressure drop, and structural robustness to optimize their design. The approach combines fluid flow and heat transfer analysis using ANSYS CFX with structural deformation evaluation using ANSYS Mechanical. A counter-flow configuration is employed in all cases, with a fixed mass flow rate of 0.01 kg/s in a compact heat exchanger of 100 mm in length, yielding Reynolds numbers between Re ≈ 31,000 and 34,000. Key design parameters of the gyroid heat exchanger, namely the wall thickness (t = 0.5, 1, 1.5, 2, and 3 mm) and the axial periodicity of the unit cell (3π, 5π, 7π, and 9π), are systematically varied. The results show that the heat transfer coefficient increases progressively from 5678 W/m²K to 7312 W/m²K, reflecting a 29 % enhancement with higher thickness and periodicity. However, the pressure drop rises sharply from 20.3 kPa/m to 183.4 kPa/m, marking an almost ninefold increase. Significant structural deformation, which would lead to rupture, is observed in configurations with smaller frame thicknesses and higher periodicity. To quantify, assuming a hypothetical infinite yield stress to visualize deformations without material rupture, the largest deformation of approximately 35 mm can occur when the wall thickness is t = 0.5 mm (5π) for a mean pressure differential of 6 MPa between the two fluid streams. These parametric numerical studies provide valuable quantitative insights and guidelines for optimizing the structural and operational parameters of gyroid-based heat exchangers, aiming for superior heat transfer, minimal pressure drop, and robust structural integrity.