This study analyses thermodynamically consistent melting behaviour of a high-temperature phase change medium (PCM), (sodium nitrate, NaNO₃) using variable domain methodology. A rectangular cavity of aspect ratio (AR) one containing NaNO₃ is treated as latent heat storage unit (LHSU). A two-dimensional analysis enables examining the influence of combined convection and diffusion during charging of domain. The present model simultaneously considers the more accurate variable domain numerical formulation, automatic remeshing and buoyancy driven natural convection to observe the phase change phenomenon. A sharp melting interface is reproduced for pure NaNO₃ using the present methodology which is virtually impossible to achieve in fixed domain method. Temporal evolution of the sharp melting front is represented inside the rectangular domain having Prandtl no. (Pr) = 7.42 and Rayleigh no. (Ra) = 1.1×10⁸. Moreover, temporal temperature distribution in both phases, velocity distribution in liquid phase, shape, position and velocity of moving interface are evaluated and represented using COMSOL Multiphysics. The thermodynamic consistency of the results is verified by performing Stefan's heat balance at the moving interface.