## 4 Results and Discussion  
Figure 4 illustrates how the liquid fraction changed over time at different velocities of the heat transfer fluid (HTF) at certain time intervals (100, 600, and 1200 s). At first, regardless of the heat transfer fluid's velocity, the melting process's beginning was seen around the domain's boundaries. During the first 100 s, the phase change material (PCM) stayed mostly solid, except for a thin layer near the walls where melting began. Significantly, when the velocity of the heat transfer fluid (HTF) was 0.01 m/s, there was a progressive increase in the size of the liquid phase while the solid phase decreased accordingly. The upward movement of the liquid PCM within the domain was observed, which can be explained by the differences in density between the solid and liquid phases. The direction of molten PCM flow was primarily influenced by convective currents within the PCM domain, driven by factors such as buoyancy forces, thermal gradients, and mixing effects rather than strictly by density differences. When fresh liquid PCM moves upward to a fixed space, the species moving downward could be denser, cooler liquid PCM that sinks due to its lower temperature. The density of solid PCM impacts the flow of liquid PCM because as solid PCM melts, it creates density variations that lead to natural convection. Less dense liquid PCM rises due to increased temperature, while denser, cooler liquid sinks, resulting in intricate flow patterns within the domain. Additionally, an interaction between heat transmission and fluid dynamics characterizes PCM melting. Heat initially transfers from the source via the solid PCM, causing a thin layer near the hot surface to melt. Melting leads to density variations, initiating natural convection. Understanding this complex relationship between thermal and fluidic processes is crucial for enhancing PCM performance in various applications. After 1200 s, a significant amount of the PCM had melted, accounting for approximately 83% of the overall mass. The remaining solid phase (17%) was mainly located at the bottom of the area. An observable pattern arose where a rise in heat transfer fluid (HTF) speed was linked to a faster melting process. This can be attributed to increased heat transfer between the phase change material (PCM) and the HTF. Furthermore, at the 1200 s point, there was a noticeable decrease in the remaining solid material, which became more apparent as the velocity of the HTF increased. This occurrence shows the importance of HTF velocity in affecting the speed of phase transition within a system. Higher velocities lead to faster melting rates and a decrease in the remaining solid phase. These data highlight the complex relationship between fluid dynamics, heat transfer processes, and phase change kinetics, providing a clearer understanding of the mechanisms that drive the observed patterns. Figure 3: Distinction of the melting fraction versus operating time for this study against the work by [PERSON] and [PERSON] (2019). Figure 5 illustrates the temperature distribution contours at different velocities of the heat transfer fluid (HTF) for specific time intervals (100, 600, and 1200 s). Initially, the temperature increased along the walls in each HTF velocity scenario due to heat absorption. During the first 100 s, the entire phase change material (PCM) maintained a relatively low temperature of 310.18 K, except for a narrow area along the walls where heat transfer occurred. At an HTF velocity of 0.01 m/s, heat gradually spread throughout the PCM, causing an increase in temperature in specific areas, particularly the liquid phase zone. Moreover, the areas with higher temperatures in the phase change material (PCM) tended to gather towards the upper part of the region, driven by differences in density between the solid and liquid phases of the PCM.