Transcritical transpiration cooling mechanism and entropy analysis within triply periodic minimal surface topological porous structure

• Hybrid thermal MRT-LBM is applied to methane transcritical transpiration cooling. • Delayed supercritical transition position enhances thermal protection. • Optimal cooling is achieved by matching structures to thermodynamic regimes. • Entropy generation analysis quantifies irreversibility of transpiration cooling. Transpiration cooling technology in liquid rocket engines holds great potential, while its application faces challenges due to drastic thermophysical property variations of coolants near critical region. Based on hybrid thermal lattice Boltzmann method and entropy generation analysis, transcritical transpiration cooling mechanism of methane are investigated in triply periodic minimal surface (TPMS) topological porous structures. Focusing on Primitive (P), Gyroid (G), and Diamond (D) structures, the effects of various subcritical and supercritical inlet temperatures on flow and heat transfer characteristics, as well as irreversible losses, are evaluated under supercritical pressures. Additionally, the study investigates the mechanism governing entropy generation and its inherent connectivity with heat and mass transfer. The results reveal that different TPMS topological structures influence phase transition dynamics behavior, thereby affecting heat and mass transfer characteristics. Notably, a delay in the supercritical state transformation position could improve thermal protection performance, where the D structure’s phase transition advances by 25 μm compared to P structure, causing heat transfer degradation by 15.38%. P structure provides the best cooling effect in subcritical conditions, while G performs better in supercritical regimes. Supercritical fluids as coolants should be decided with caution due to thermophysical property variations and Joule-Thomson effect. In addition, heat transfer contributes much more entropy than that of friction, highlighting the dominate role of temperature gradient. For three structure types, the largest irreversible loss is derived from heat transfer between the fluid and solid. This research provides theoretical guidance for porous structure selection and optimization in next-generation liquid rocket engine injectors.