In this study, the thermal performance and entropy generation behavior of a heat sink equipped with multilayer phase change materials and copper pin fins under constant heat flux are numerically investigated. Five separator materials with different thermal conductivities (copper, aluminum, stainless steel, alumina, and epoxy) are examined in 25 configurations, including symmetric arrangements with identical separator materials (A1–A5) and asymmetric configurations with different materials used for the lower and upper separators (B1–B20). A three-dimensional transient enthalpy-porosity model is used to analyze melting dynamics, temperature distribution, and the evolution of thermal and frictional entropies. The results show that separator conductivity has little effect on melting time or the overall temperature trend but strongly influences both the magnitude and pattern of entropy generation. In symmetric configurations, changing separator conductivity alters only the entropy magnitude. In asymmetric arrangements, the sequence of conductive and nonconductive layers plays a dominant role. A conductive bottom separator combined with a moderately conductive top layer yields the lowest entropy generation, whereas a nonconductive bottom separator increases thermal entropy by up to 35% and frictional entropy by more than 50%. Local contours further reveal that conductivity discontinuities intensify temperature and velocity gradients, forming concentrated irreversible regions.
