Measurement of shrinkage void and identification of solid-liquid phases in phase change materials: Ultrasound-based approach and simulated predictions

Phase change materials in thermal energy storage systems undergo solidification and melting during cyclic operations. During solidification, internal shrinkage voids are formed randomly in phase change materials due to density differences in the solid and liquid phases. These shrinkage voids can severely affect the heat transfer performance and storage capacity of thermal energy storage systems. Additionally, the containers in which phase change materials are kept are usually metallic, making them opaque. Therefore, identification of the solid-liquid phase by direct visualisation during the phase change is limited. The present work aims to develop a non-destructive ultrasonic scanning approach to detect and measure internal shrinkage voids in solid phase change materials and the solid-liquid phases during melting and perform multi-phase numerical simulations that simultaneously capture solidification/melting and the associated shrinkage/expansion. The most commonly used phase change materials, n-eicosane, and n-octadecane, are used in this study. The effectiveness of the ultrasonic technique is evaluated by measuring cavities of known sizes. Further, three cases are considered for examining and measuring the shrinkage void. Case A involves two-dimensional solidification with a surface void, while Case B and C involve three-dimensional solidification with an internal shrinkage void. The numerically obtained shrinkage cavity profile shows a maximum deviation of 7.8% with experimental results for case C. Furthermore, the developed approach is implemented to uniquely detect and identify the solid and liquid phases during melting in an opaque container.