Simulation on PCM melting enhancement with double-fin length arrangements in a rectangular enclosure induced by natural convection

•Double-fin length arrangement is firstly proposed for PCM melting enhancement.•Transient numerical simulation is performed with considering natural convection.•Proper double-fin length allocation scheme can significantly enhance the PCM melting rate.•The mechanism of melting enhancement is revealed with the simulation.•The length of solid-liquid PCM interface can be used to estimate the heat transfer rate.