We numerically investigate the phase change melting enhancement in a latent heat thermal energy storage (LHTES) unit by arranging the internal double-fin length. The schemes for double-fin setting in unequal length are proposed. A fin length ratio is defined by the upper-fin length over the lower-fin length. RT42, used as the phase change material (PCM), is stored in a rectangular container to absorb thermal energy. Heat is added from the vertical side of the enclosure by a constant temperature input of 70 degrees C. A transient numerical model is developed and validated to simulate the phase change process with the natural convection involved. The melting enhancement by fin length arrangement is firstly characterized by the evolution of melt fraction with time. It reveals that in the premise of total fins length invariant, the schemes with short upper fin and long lower fin can positively enhance the PCM melting rate. With the length ratio at 0.25, the total melting time can save about 25% based on the equal length scheme. Such fin length arrangement scheme also performs better on the storage rate of sensible and latent heat energy. However, the schemes with long upper fin and short lower fin show a negative effect. To explore the mechanism of heat transfer enhancement by the double-fin length arrangement, the contours of liquid fractions are successfully predicted, as well as the liquid PCM flow motions. It is found that, when the length ratio is lower than 1, the natural convection currents are intensified significantly, with the clear chaotic flow structures appearing. However, the flow motions driven by natural convection become weak with the length ratio higher than 1. Later, the maximum velocity recorded during the entire melting process is also compared in detail. The length of solid-liquid PCM interface is proposed to evaluate the heat transfer for the first time. The scheme of fin length ratio at 0.25 performs a longer length of solid-liquid interface than the others, which is another important factor for the PCM melting enhancement. The heat transfer mechanism is detailed analyzed by the surface-averaged Nusselt number into three stages. The effects of fin length arrangement are reported mainly taking at the strong convection stage. In the later optimization studies, it is observed that when the length ratio is further decreased to 0.11, the total melting time is minimum for the current LHTES unit. Comparing with the equal length scheme, about 40.5% melting time is reduced. When the temperature input decreases from 70 degrees C to 50 degrees C, the time saving ratio can increase to 45%. (C) 2018 Elsevier Ltd. All rights reserved.