Experimental investigation of shell-side heat transfer deterioration and dynamic behavior in a horizontal kettle-type molten salt steam generator

Increasing penetration of renewable power requires greater operational flexibility from coal-fired units, and molten salt thermal storage is an effective option for improving such flexibility. In these systems, the molten salt steam generator acts as a key interface for converting stored thermal energy into deliverable steam. However, existing studies have mainly focused on steady-state performance and dynamic simulation, while direct experimental evidence under variable-load conditions remains limited. This study establishes a horizontal kettle-type molten salt steam generation experimental platform and conducts steady-state and variable-load experiments to investigate the thermohydraulic behavior of the steam generator. The results show that, as the load decreases from 100% to 40%, the overall heat-transfer coefficient continuously declines while the logarithmic mean temperature difference increases significantly, indicating that the deterioration cannot be explained by tube-side flow reduction alone. Comparisons with theoretical reference baseline and reference correlations further suggest that shell-side boiling progressively departs from the behavior expected for effective nucleate boiling under low-heat-flux conditions. Dynamic tests show a marked increase in response time in the 70%–80% load range, with low-load response times about 2–3 times those in the high-load region. Clear path-dependent thermal states are also observed, especially at 80%–100% load, where the molten salt outlet temperature differs markedly among operating paths at the same load. These findings indicate that low-load deterioration and dynamic behavior are closely associated with shell-side boiling-state variation and history dependence, providing experimental support for controlling molten salt steam generation systems in flexible thermal energy storage applications.