This study numerically investigates the effect of fin arrangement on entropy generation and thermal performance in a heat sink subjected to a constant heat flux of 20 kW/m². Five different configurations are examined under laminar flow conditions within a Reynolds number (Re) range of 200–500. The analysis focuses on evaluating the contributions of fluid friction and heat transfer to the total entropy generation using a second-law approach. The results indicate that fin geometry and distribution significantly influence the thermodynamic behavior of the system. Increasing the Re intensifies viscous effects, leading to higher entropy generation due to fluid friction, while enhanced convection improves heat removal from the heated surface. Among the investigated configurations, Configuration 4 demonstrates the best overall performance. The larger number of fins in this configuration increases the effective contact area between the coolant and the solid surfaces, which enhances heat transfer and reduces overall entropy generation compared with the other cases.