## 3 Phase Change Processes
### Melting Process  
In the melting process, heat conduction is dominant only in the initial stages [58]. In any case, convective heat transfer influences most of the melting process [60], as can be detected in the morphology of the solid phase. Regarding rectangular cavities with heat transfer in one side, the liquid phase close to the hot wall flows upwards due to buoyancy forces induced by density reduction in the function of temperature increase. The convective motions heated by the hot wall cause a higher melting rate in the upper region, as the solid assumes a tapered shape that becomes more evident over time, as shown in Figure 3 about experimental results outlined in work [61]. Regarding the shape of the solid phase, vertical cylinders with an adiabatic base and heat transfer on the cylindrical wall present results similar to rectangular cavities with heat transfer on one side. Vertical cylinders have symmetry around the vertical axis, which induces the solid phase to assume a conical shape, as shown in Figure 4, concerning experimental results obtained by [PERSON] et al. [62]. Besides the shape of a solid PCM, the adiabatic bottom surface is an additional standard feature regarding PCMs melting in rectangular cavities and vertical cylinders with a one side heated surface. The absence of heat transfer through the bottom surface maintains the position of the remaining solid phase throughout the process. However, most PCM containers present heat transfer through the bottom surface, which is more evident in horizontal cylinders and spheres but can occur as well in rectangular cavities and vertical cylinders. As different densities characterize solid and liquid PCMs, the portions with lower density tend to flow upwards. The density of the PCM is higher for the solid  
Figure 4: Melting process of a PCM inside a vertical cylinder, adapted from [PERSON] et al. [62]. Figure 3: Melting process in a rectangular cavity with heat transfer on one side, adapted from work [61]: the dark region corresponds to liquid and the light region to solid. than for the liquid. Thus, in melting processes with heat transfer through the lower surface, the liquid portion flows upwards and the solid is located close to the heated lower surface. For materials with higher density in the liquid phase, such as water, the opposite occurs. Melting processes in which the solid material moves towards the heated surface or vice-versa are known as \"close contact melting\" [63] or \"unconstrained melting\" [58]. The main characteristic of this process is the formation of a thin liquid layer between the solid and the heated surface. As more material melts, the liquid portion formed in this layer flows continuously towards the side edge of the solid portion, out of the region between the solid and the heated surface [64]. An example of close contact melting in a rectangular cavity is shown in Figure 5, adapted from work [65]. The shape of the solid (bright color) remains similar to the cavity shape. In comparison with the case of the rectangular cavity highlighted in Figure 3, the influence of the thermal configuration in the shape of the solid portion proves to be evident, even with the different aspect ratios of the cavities. The differences in the phase morphology when comparing close contact melting and fixed solid melting are also observable in horizontal cylinders as well as in spheres. However, other results, such as melting rate, heat flux, or total melting time, are different due to geometric characteristics. When the solid is fixed, the melting process starts symmetrically. However, over time, a wavy profile is formed in the lower region of the solid, due to convective motions of the liquid at the bottom of the cavity [58]. On the other hand, close contact melting presents shapes that are more symmetrical in the solid. Figure 6 shows the shapes of the solid PCM in the melting process in the horizontal cylinder analyzed by [PERSON] and [PERSON] [66].