Experimental study of close-contact melting in a cylindrical enclosure

In this study, an investigation of close contact melting (CCM) in a phase change material (PCM) is conducted. The motivation is that while undergoing a phase change from one state to another, e.g. solid to liquid, a considerable amount of energy can be stored via the latent heat. A new experimental system was designed and built in order to study CCM, which is a process in which the solid phase is kept at a small distance from the hot surface, thus ensuring that the active heat transfer mechanism is conduction and not convection. Thanks to the small thickness of the liquid layer, the heat transfer rate to the solid phase remains high through the entire process. The way to keep the solid phase near the hot surface is by making use of the density difference between the solid and liquid phase. The key aspects of the system are the ability to see through the melting chamber, as this is the most accurate way to estimate the melting process, and the use of a cylindrical body, to avoid any negative effects of undesired three-dimensionality. Thus, the system is built of two concentric Perspex tubes of different sizes: the small one acts as the melting chamber with a paraffin (eicosane) in it, while the large one allows water to flow around the melting chamber in order to heat the wall and achieve melting also from the side. The water temperature is controlled by a thermo-static bath. At the bottom of the chamber, melting is achieved by using an electrical heater that is attached to an aluminum plate in order to obtain a uniform temperature at the base. The first steps of an experimental study include learning of how to capture the melting chamber and receive a clear image of the solid phase, which is not transparent. During the experiments, it is essential to prevent air dissolution in the PCM, because it affects the solid phase density and thus its behavior. The experiments are performed for different heating rates at the bottom and different temperatures of the side wall of the melting chamber. The images obtained characterize various instants of the melting under different conditions. The entire melting process is presented and analyzed in detail.