- Abstract Melting of a phase change material (PCM) is studied in a heat sink with vertical internal fins and a horizontal base to which a constant heat flux is applied. The phase change material is stored between the fins. A detailed parametric investigation explores various fin height and thickness, PCM layer thickness, and applied heat flux. Transient numerical simulations are performed using the Fluent 6 software. The results show how the transient phase change process depends on the heat flux from the base, heat capacity of the PCM, and fin dimensions. Dimensional analysis of the results is performed, and the generalized results are presented in terms of the melt fractions and Nusselt numbers vs. the Fourier, Stefan and Rayleigh numbers.