Numerical Modelling for the Phase Change of the Sodium Sulfate Decahydrate Based on the Phase Field Theory

Sodium sulfate decahydrate (Na2SO4·10H2O) can be utilized as a type of latent-heat energy storage material with satisfied heat storage capacity as much as 250 kJ/kg. In this study, a new model for the phase change of the sodium sulfate decahydrate was proposed based on the phase field theory, in which the phase field equation is coupled with the concentration field as well as the mass and energy conservations. With this model, the heterogeneous crystal growth can be described, and the subcooling effect can also be incorporated. The proposed model was then calculated by the finite differencing method and validated by the experimental data. Numerical investigations were carried out into the characteristics of the phase change of the sodium sulfate decahydrate, with the validated model. The results show that the growth rate of the dendrites decreases with the increase of the source temperature, and thus the time required for the solidification increases accordingly, and the crystal morphology show difference for different source temperature; and that the solidification becomes faster at larger degree of subcooling due to the larger driving force; and that with the increase of the number of nuclei, the initial crystallization rate can be increased gradually to a certain extent, since more nuclei cause more solidification, but further increase in nuclei will slower down the solidification because of the interactive crystallization.