his study numerically investigates the effect of pin-fin cross-sectional geometry on the thermo-hydrodynamic and thermodynamic performance of an ultrasonic-assisted phase change material (PCM)-based heat sink using CFD. To isolate the geometric effect, all simulations are performed under identical conditions, including constant heat flux, fixed ultrasonic configuration, equal fin cross-sectional area, and a simulation time of 0–2000 s. Six pin-fin geometries (circular, square, triangular, elliptical, diamond, and hexagonal) are evaluated in terms of hotspot mitigation, PCM melting, and thermal/frictional entropy generation. The results reveal a clear trade-off between hotspot suppression and melting enhancement. Triangular and elliptical fins accelerate melting due to stronger convection and ultrasonic-induced mixing, whereas circular fins provide superior long-term hotspot control through more uniform thermal redistribution. At 2000 s, the circular geometry yields the lowest maximum base temperature (333.315 K), while elliptical fins produce the highest melting level and mean PCM temperature. Second-law analysis further indicates that enhanced melting does not necessarily reduce thermodynamic irreversibility. Among all configurations, the square geometry exhibits the lowest cumulative thermal and frictional entropy generation. Overall, fin geometry strongly affects the balance between thermal performance, melting efficiency, and thermodynamic losses in ultrasonic-assisted PCM heat sinks.
