# Melting of Paraffin Waves Embedded in a Porous Matrix Made by Additive Manufacturing
## 5 Experimental Results
### Common Characteristics of the Melting Process  
Figure 5 shows the temperature profiles during the heating process for the case with the paraffin RT42 with an imposed heat flux of 10 kW m\({}^{-2}\). The red line represents the temperature of the plate in contact with the heater, i.e., it represents the temperature of the heated side. Since the temperatures recorded by the six thermocouples located in the heated side are within \(\pm\)1.2 K at each time during the heating process, the average value of temperature is considered in the figure. After just a few seconds, during which the electric power is set to 100 W, the temperature of the heated side increases almost linearly when only sensible heat is involved in the heating process. As soon as the temperature of the heating side approaches the low value of the melting temperature range, the paraffin close to the heated plate starts to melt; thus, the slope of the curve of the temperature of the heated side decreases, i.e., latent heat is also involved. The green, blue and gray lines represent the temperatures of the phase change material in the centerline at different heights: 25, 50, and 75 mm from the top side, respectively. As it can be noticed from Figure 5, these three lines almost overlap, meaning that there is no temperature gradient along the height of the paraffin. This may indicate that the melting front is moving quite parallel to the heated surface. The curves related to the temperatures of the PCM present two changes of slope: the first one is in correspondence with the lower value of the melting temperature range, whereas the second one is in correspondence with the upper value of the melting temperature range. The temperature of the paraffin during the phase change process from solid to liquid state is not constant since the melting occurs in a temperature interval (38-43 \({}^{\circ}\)C in the case of RT42). Similar conclusions can be depicted for the other combinations of heat flux and paraffin: each experimental test will have different temperatures of the heated side at the end of the heating process as well as different times needed to completely melt the paraffin, as it can be seen in Table 3. The table reports the values of \(T_{i}\), \(T_{f_{i}}\) and \(t_{melt}\), which are, respectively, the initial temperature of the heated side, the temperature of the heated side at the end of the melting process, and the time needed to completely melt the paraffin. An overview of the melting front is given in Figure 6, which reports pictures at different times taken from the glass window in front of the test module for the paraffin RT42 with an imposed heat flux of 10 kW m\({}^{-2}\). The figure demonstrates that the melting front propagates almost vertically, i.e., the tested porous matrix tends to limit the natural convection of the PCM embedded inside. Due to volume variation during the phase transition from solid to liquid conditions, the melted liquid paraffin coming from the heated side tends to overcome the still solid paraffin on the top part, and this phenomenon is responsible for the slight inclination of the melting front as time goes by. Similar conclusions can be drawn for the other paraffin waves and heat fluxes. subsectionEffect of the Heat Flux on the Heating Process  
The effect of the heat flux on the heating process is highlighted in Figure 7. The figure proposes the case for the paraffin with a melting temperature of 42 \({}^{\circ}\)C, but similar conclusions can be drawn for the other paraffins, whose data can be extrapolated from Table 3. Since experimental tests at different heat fluxes were carried out on different days, with consequently different ambient temperatures, the difference between the temperature of the heated side (average value between the six values recorded by the six thermocouples) and the ambient temperature is considered in the \(y\)-axis instead of the temperature itself, allowing a better comparison for different initial temperatures.