Identifying a Controlling Parameter Alongside the Arrangement Effect on HTF Temperature-Fluctuation Mitigation in Cylindrical PCM Arrays

This study numerically investigates the capability of cylindrical phase change material (PCM) encapsulations to attenuate inlet-temperature fluctuations in water as the heat transfer fluid (HTF). A sinusoidal inlet profile with a 20 K amplitude is imposed, and melting and solidification are modeled using the enthalpy–porosity method. The analysis begins with a single encapsulation, which reduces the outlet temperature amplitude by 45.06%, and extends to three, nine, and 15 cylinders in aligned and staggered arrangements. Increasing the cylinder count enhances fluctuation reduction but with diminishing returns, as the HTF thermal energy reaching downstream cylinders decreases. Spatial arrangement is equally important: a staggered array of nine encapsulations achieves a 65.91% reduction, surpassing a 15-cylinder aligned configuration (65.80%), while the highest reduction, 73.01%, is obtained with a 15-cylinder staggered configuration. Across all configurations, the outlet fluctuation reduction follows a single near-linear relationship with the total melted PCM mass (R2 = 0.96), across cylinder count and arrangement, identifying melted mass as a controlling parameter for fluctuation mitigation rather than the cylinder-averaged liquid fraction. These results indicate that the total melted PCM mass is a practical criterion for comparing PCM encapsulation configurations, while the arrangement remains a distinct factor.