Transport processes associated with melting of a phase change material (PCM) placed inside a vertical rectangular enclosure with a free-moving ceiling are simulated numerically. The enclosure is side-heated isothermally. Buoyancy-induced flow in the melt region is modeled utilizing the Boussinesq approximation. The effect of solid-liquid density change upon melting is accounted that liquid density of the PCM is assumed to be lower than the solid density and as a result, the melt bulk expands while melting, transmitting a linear motion of the free-moving ceiling of the enclosure. Numerical simulations for the twodimensional melting process using a finite-difference method on a fixed grid are undertaken to delineate the motion characteristics of the ceiling related to the melting process of solid PCM inside the enclosure with the relevant dimensionless parameters in the following ranges: the enclosure aspect ratio Asp = 2-12; the Stefan number Ste = 0.1, 03, and 0.5; the Rayleigh number Re = 10(3)-10(6); and the Prandtl number Pr = 41.7. Simulation results clearly reveal the influence of the Rayleigh number on the displacement of movable ceiling of the enclosure and the average Nusselt number over the isothermally heated wall of the enclosure tend to degrade greatly with the increase of the aspect ratio. (C) 2014 Elsevier Ltd. All rights reserved.