This paper presents a numerical modelling approach to investigate convective heat transfer in a rectangular cavity filled with a Phase Change Material (PCM), specifically organic paraffin-based Rubitherm RT 35 HC. Natural convection occurs during both the melting and solidification processes due to the changes in the density of a liquid PCM. Therefore, a significant enhancement of the heat transfer in the upper part of the cavity can be observed. The study aims to investigate, both experimentally and numerically, the effect of convective flow on the overall heat transfer performance of a PCM in the cavity. The computational analysis was performed using a two-dimensional explicit finite difference model implemented in MATLAB. To accurately represent the thermal behaviour of the PCM enclosed in a cavity, the proposed modelling approach incorporates the effects of natural convection by considering the effective thermal conductivity of the PCM as a function of both temperature and position within the cavity. By doing so, heat transfer in the upper part of the cavity is enhanced while heat transfer in the bottom part is mitigated, resulting in a more precise representation of the actual thermal behaviour of the PCM. The comparison between the constant thermal conductivity and the effective thermal conductivity with a linear increase has been made, resulting in a significant improvement in the model accuracy. The analysis demonstrated that the effective thermal conductivity approach led to a significant improvement in the accuracy of the numerical model. Specifically, a reduction of 47.5 % was observed in the root mean square error (RMSE) value, decreasing from 4.0 K to 1.9 K.