Figure 5 illustrates the time-lapse of melting temperature. Observing the temperature contours over time, it’s evident that the temperature distribution closely mirrors the behavior of the liquid fraction. In the initial stage, the temperature in the boundary layer gradually rises until it reaches the PCM’s liquidous temperature, signifying the onset of PCM melting. Subsequently, the temperature experiences a gradual increase over time.
In the second stage, the temperature of the melting PCM steadily rose until it entered the third stage, marked by a notable increase until it reached the heat transfer fluid (H.T.F.) temperature. During this temperature increase, the primary mode of heat transfer to the PCM was sensible heat in the upper half, with a minor contribution of latent heat in the lower half. As the upper half reached the H.T.F. temperature, natural convection reached its minimal level. Subsequently, heat transfer predominantly occurred through conduction as latent heat to the lower solid PCM.
These stages appeared in three models, although at different times, as illustrated in figures 3 and 5. Whereas model A-2 melts faster than model A-3, model A-1 melts the slowest. The merging of regions (1, 2, and 3) is slower in models A-2 and A-3 than in model A-1 because the side fins (fins 2 and 5) are shorter, and those fins contributed to generation region (2 and 3), so the merging will be delayed with region (1), as discussed in heat transfer rate graphs. While region (4) is created faster in models A-2 and A-3 and contains more melted PCM, this is due to longer lower fins (fin number 3 and 4) that contribute to region (4) generation.
Figure (6) displays the results of the volume-averaged temperature of the PCM over time. The temperature behavior over time reveals distinct patterns among the three models. Model A-2 and Model A-3 maintain a consistent temperature over time, whereas Model A-1 exhibits a higher overall temperature. The temperature graph exhibits three distinct regions for all three models. The first region corresponds to stage (1), marked by a rapid temperature increase over time. The second region aligns with stage (2), where the temperature increase slows due to the influence of latent heat as opposed to sensible heat. In the third region, a temporary, rapid temperature increase over time is observed. This increase results from the rising temperature of the upper half due to natural convection until it reaches the temperature of the heat transfer fluid. At this point, the heat transfer rate initiates the melting of the solid PCM in the lower half, maintaining a constant temperature. The fresh melted PCM gradually heats to match the temperature of the upper half, contributing to a gradual increase in the volume-averaged temperature over time.