## 3 Results and Discussion
### Freezing and Melting Characteristics
#### 3.2.2 Melting Process  
The solidified nanocomposite is sensibly heated up to the melting temperature when placed inside the hot bath. After sensible heating, the nanocomposite begins to melt starting from the outermost surface, which is exposed to the heat-conducting surface. Consequently, the outermost layer starts to melt first. The melting process continues until the midpoint of the nanocomposite melts completely. Then the sensible heating of the liquid sample takes place until it is in thermal equilibrium with the hot bath temperature. The melting experiment is also carried out for the base fluid as well as the nanocomposites. The bath temperatures for the melting experiments were 31 \({}^{\circ}\)C and 36 \({}^{\circ}\)C, respectively. The melting process was carried out until the nanocomposite sample reached 30 \({}^{\circ}\)C.  
Figure 6 shows the comparison of melting curves from \(-\)3 \({}^{\circ}\)C to 30 \({}^{\circ}\)C for the base fluid and nanocomposites, kept at bath temperatures of 31 \({}^{\circ}\)C and 36 \({}^{\circ}\)C. The sensible heating process was faster compared to the latent process. The time taken by the DI water to melt completely was 6 min, whereas the time taken by the 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% vol. fractions of nanocomposites was 5.5 min, 5 min, 4.5 min, 4 min, and 3.5 min, respectively. Thus, the addition of the nanoparticles reduced the melting time by 8.33%, 17.67%, 25%, 33.33%, and 41.67% for the 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% vol. fractions, respectively. Similar trends were observed when the samples were melted from \(-\)3 to 30 \({}^{\circ}\)C when kept in a hot bath at 36 \({}^{\circ}\)C. Under this condition, the time taken by DI water to melt completely was 4 min, whereas for the 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% vol. fractions of nanocomposites, the time taken was 3.5 min, 3 min, 2.5 min, 2 min, and 1.5 min, respectively. Thus, the addition of nanoparticles reduced the melting time by 12.5%, 25%, 37.5%, 50%, and 62.5% for the 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% vol. fractions, respectively. During melting, the solidified fluid near the walls absorbs heat and starts to melt. Conduction is the dominant mechanism in the initial time when the thickness of the liquid layer is so thin. The thickness of the liquid layer increases and buoyancy force is developed relative to time. The difference between the solid and liquid density causes the melted water to sink towards the bottom of the capsule and to consequently push the ice up. It intensifies the convection force and accelerates the melting rate. However, it can be noticed that the thermal conductivity of the liquid is lower than  
Figure 6: Melting curves from \(-\)3 \({}^{\circ}\)C to 30 \({}^{\circ}\)C with a bath temperature at 31 \({}^{\circ}\)C and 36 \({}^{\circ}\)C.  
the solid, and therefore the heat conduction in the liquid is lower than in the solid. Thus, as the thickness of the liquid layer increases, the heat transfer conduction is reduced, and conversely the natural convection is enhanced. Therefore, melting is accelerated because of this natural convection in the liquid [33]. During freezing, the liquid near the walls rejects heat to the surrounding cold bath and starts to solidify. As time passes, the thickness of the solid layer increases. Water is denser than ice, which causes ice to float. Therefore, water gets accumulated at the bottom of the capsule. It can be seen that in the initial period of time, the solidification rate is high and conduction is the dominant mechanism of heat transfer between liquid and cold surfaces. As time progresses, a greater amount of liquid becomes solid and therefore the solid layer near the cold surfaces becomes thicker. Although the thermal conductivity of the solid is higher than the liquid, the solid layer imposes thermal resistance for heat conduction from the cold surface to the warm liquid. Thus, heat conduction decreases by increasing the thickness of the solid layer.