Multi-field coupling mechanism of internal fin radial length on two-phase flow in gravity-assisted heat pipe

Aiming at the unclear regulatory mechanism of internal fin parameters in the evaporation section of gravity-assisted heat pipes (GAHPs) and the insufficient systematic interpretation of the physical nature behind non-monotonic heat transfer behavior in industrial waste heat recovery, this paper establishes a three-dimensional numerical simulation framework coupled with the VOF-Lee-CSF model. Validated by bench-scale experiments, it quantitatively analyzes how circumferential internal fins with radial lengths of 1–8 mm affect in-tube gas-liquid two-phase flow and phase-change heat transfer. The results show that fin radial length exerts bidirectional nonlinear regulation on heat pipe performance through the synergy of vortex evolution and nucleate boiling modulation, with 6 mm as the optimal radial dimension. Under this condition, the heat transfer coefficients of the evaporation and condensation sections rise by 27.27% and 24.67% respectively compared with the 1 mm baseline, and the relative deviation between simulation and experiment stays within 12%–20%. These findings supplement the mechanistic explanation of fin-enhanced heat transfer in gravity-driven two-phase systems and provide quantitative reference for refined heat pipe structural design.