Numerical Prediction and Experimental Verification of Non-Linear Phase Change Phenomena and Supercooling in Geometry-Dependent Sub-Zero Latent Storage Units

Abstract

This study evaluated the effect of macro-encapsulation geometry on the sub-zero freezing behaviour of a 5% aqueous sodium chloride solution used as a phase change material in latent thermal energy storage units. Four containment geometries, namely cylindrical, square, triangular and rectangular profiles, were tested with an identical filling volume of 148 mL. Core temperature was recorded using thermocouples during cooling in a controlled freezer environment maintained at −18.0 °C. A numerical model was developed to describe the main thermal stages observed during freezing, including sensible cooling, supercooling, nucleation rebound and solid-state sub-cooling. The experimental temperature profiles showed that geometry influenced both the depth of supercooling and the subsequent cooling behaviour. The cylindrical cell exhibited the deepest supercooling, reaching −5.4 °C before nucleation, while the square, triangular and rectangular cells reached −3.5 °C, −3.7 °C and −3.1 °C, respectively. Model validation using experimental core-temperature data over a 33,500 s cooling period showed overall root mean square error values of 0.671 °C for the cylindrical cell, 0.221 °C for the square cell, 0.273 °C for the triangular cell and 0.312 °C for the rectangular cell. Heat transfer evaluation indicated that the rectangular geometry produced the highest steady-state energy extraction rate of 0.31 J/s, followed by the triangular, cylindrical and square geometries, with extraction rates of 0.27 J/s, 0.23 J/s and 0.21 J/s, respectively. The results indicate that container geometry governs freezing progression, model fit parameters, localised supercooling behaviour and steady-state heat extraction rates under sub-zero operating conditions.