Flow-thermal-mechanical coupling analysis of molten salt phase change unit during cold start melting

The molten salt phase change unit is widely used in the high-temperature energy storage. Nevertheless, the volume expansion of the molten salt significantly increases the unit internal pressure during the cold start melting with large temperature differences. The resulting thermal-pressure loads can lead to the damage or cracks to the unit shell, yet it has rarely been analyzed. The objective of this study is to investigate the coupling flow-thermal-mechanical characteristics of the NaNO₃ phase change unit during the cold start melting by the finite volume method and finite element method. On account of the volume expansion and sophisticated energy variations of NaNO₃, the volume of fluid method, effective heat capacity method, and enthalpy method are innovatively combined to simulate cold start melting processes under three large temperature differences (296 K, 316 K, and 336 K). Based on the sequential coupling method, the thermal-mechanical responses are accurately calculated under the temperature and pressure loads. In the flow-thermal analysis, it is found that the great volume expansion of NaNO₃ leads to two distinctive phenomena: liquid landslide and expanding convection. The liquid landslide has the potential to induce fluctuant behaviors to the melting process. The expanding convection can enhance heat transfer during the early melting stage. As the reserved gas is compressed, the unit internal pressure can be increased by about 10 times. In the thermal-mechanical analysis, the deformation of the unit shell is primarily along the radial direction, and its peak zones appear at the upper and lower of the hoop wall. The stress concentration is located at the rounded corners of the unit shell. The largest equivalent stress is greater than the yield strength, which indicates the unit shell could be subjected to the plastic deformation. As the temperature difference rises (from 296 to 336 K), the largest deformation is increased (from 0.042 to 0.048 mm), while the largest stress is decreased (from 300 to 296 MPa). The results can be utilized as the guidance for the flow-thermal optimization and mechanical safety design of the phase change unit.