Heat-Transfer Performance of Medium–Low Temperature Molten Salt Phase-Change Heat Exchanger

The charging rate of molten-salt phase-change thermal storage systems is primarily constrained by the low thermal conductivity of phase-change materials. To address this issue, this study numerically investigates a medium–low temperature molten-salt shell-and-tube heat exchanger using the enthalpy–porosity method. The effects of annular fins, fin number, fin thickness, and multi-tube coupling on melting behavior and heat-storage performance were systematically analyzed. Compared with the smooth single-tube configuration, the introduction of annular fins reduced the complete melting time from 1392 s to 1105 s (20.6%). In addition, the melting time of the final 10% decreased by 92 s, indicating reduced thermal resistance in the late stage. Increasing the fin number from four to nine further shortened the melting time by 148 s, although the enhancement diminished due to restricted natural convection. A fin thickness of 2 mm provided optimal performance. When combined with a multi-tube design, the melting time was further reduced to 928 s (33.3%), demonstrating that the integration of distributed tubes and annular fins significantly enhances heat storage performance. These results demonstrate that annular fins and multi-tube coupling effectively enhance heat transfer, improve temperature-field uniformity, and accelerate molten-salt thermal storage.