Numerical investigation of heat transfer mechanism and characteristics of liquid alkali metal two-phase closed thermosyphon

A refined dynamic Lee model incorporating an optimized condensation mass transfer time relaxation parameter (β_c) is applied to numerically assess the performance of a liquid alkali metal two-phase closed thermosyphon. The analysis delves into the complex flow features and heat transfer characteristics, integrating vapor‒liquid two-phase volume fraction contours, average heat transfer coefficients, total thermal resistances, and Jacob numbers. The results reveal that at low filling ratios, no flow pattern transition occurs, regardless of the heating power. Conversely, at higher filling ratios, there's a discernible progression from bubble to complex bubble and slug flows, with synchronized transition points when filling ratios are 100 % and 150 %. The heat transfer efficiency is enhanced with increased filling ratios, concurrently diminishing thermal resistance, and the Jacob number. As the heating power escalates from 100 W to 1000 W, the transition times for two-phase flow patterns decrease, exerting negligible influence on the thermal resistance and heat transfer coefficients within the evaporation section. The model, with its β_c adjustment, provides insights into the geyser boiling phenomenon. The bursting of slug bubbles boosts the saturation pressure, causing a temporary surge in the evaporation rate over condensation. This triggers a negative feedback loop that elevates the β_c, thereby reinforcing condensation and suppressing boiling until the emergence of the next slug bubble, which initiates the cycle anew and culminates in the distinctive eruption characteristic of geyser boiling.