## 3 Results and Discussion
### Thermal Management Experiment
#### 3.2.1 Heating Run  
From DSC analysis, the lowest solidus temperature exhibited by the PCMs is 10 \({}^{\circ}\)C (milk fat). All the heating runs therefore started from the lowest solidification temperature (10 \({}^{\circ}\)C) to assure all PCMs being fully solid at start. Figure 4 shows the transient temperature profile for each tested case at a heat load of 4 W.  
In the case of Ref-NV shown in Figure 4A, the \(T_{b}\) rapidly rises above the 60 \({}^{\circ}\)C threshold, the safety temperature limit for most of the electronics [37]. The inclusion of PCMs (represented by Salt-NV, Milk fat-NV and Paraffin-NV) induces a drop and time lag in the said temperature rise. The temperature drop happened due to increased thermal mass by virtue of PCM's latent heat of fusion and increase in specific heat capacity. It can be seen in Figure 4A that milk fat maintained lower temperature for first few minutes compared to salt hydrate and paraffin wax. However, over a longer time period, the salt hydrate outperformed both the paraffin wax and milk fat. The salt performs better because of higher density, higher heat of fusion (\(L\)) and higher thermal conductivity (\(K\)) compared to the other two PCMs. Paraffin wax and milk fat performed similarly. It is worthy to highlight that milk fat starts melting at 10 \({}^{\circ}\)C (below room temperature), while the electronic devices operate normally at room temperature (25 \({}^{\circ}\)C). It indicates that milk fat may remain partially melted in electronic devices even prior to the start of the device operation. Figure 4B compares the temperature regulation performance of PCMs under FV for the same (6-10 W) heat loads. It is evident that the FV with PCM achieved lower temperature compared to NV with PCM due to increased convection heat losses [18; 25]. Under FV, milk fat produced the highest cooling for 20 min. mainly due to lower melting start (10 \({}^{\circ}\)C) indicating that a lower melting point PCM can better perform with FV. On the other hand, paraffin does not show a clear phase transition under FV because its melting point is very close to the highest steady state temperature. To understand the effectiveness of each PCM for different operation times, \(T_{b}\) is compared for short (5 min), average (15 min) and longer (60 min) duty times, as shown in Figure 5A-E for heat loads (4-10 W) covering intensities ranging from 4027 W/m\({}^{2}\) to 9748 W/m\({}^{2}\). Figure 5A shows that for short time (5 min) the HS with PCMs (Paraffin-NV, Salt-NV and Milk fat-NV) maintained lower temperatures by 6-10 \({}^{\circ}\)C compared to the HS only (Ref-HS). Note that HS only restricted the temperature below  
Figure 4: Time-temperature profile on the HS base surface for the complete heating run for three PCMs subjected to: (**A**) Natural ventilation (NV); and (**B**) Forced Ventilation (FV) at heat load of 4 W.  
60 \({}^{\circ}\)C at lower heat loads (4-6 W) while at higher heat loads (8-10 W), the device temperature rises above 60 \({}^{\circ}\)C and would need additional cooling for safe operation. It shows that inclusion of PCM under NV can maintain the device temperature below or near 60 \({}^{\circ}\)C up to 10 W. Comparing between PCMs, milk fat (Milk fat-NV) maintained the lowest temperature up to 8 W.  
Figure 5B shows that for a moderate time (15 min) the HS only under NV cannot restrict the device temperature under safety limit (60 \({}^{\circ}\)C) and would not be advisable to use. However, when the PCMs are included in HS, the device can be maintained below safety limit only at lower heat loads (4-6 W). Figure 5B shows that salt hydrate maintained the lowest temperature while milk fat reached the highest temperature. Furthermore milk fat becomes least effective at prolonged operation time mainly due to lower \(L\) and \(\rho\) compared to salt hydrate. Figure 5C shows that for substantially longer time (60 min) neither HS only nor the HS with PCMs under NV would restrict the device temperature below safety thus the device would always need the FV.