## 2 Materials and Methods
### Experimental Setups  
The experimental setup for the thermal performance of TSHP is demonstrated in Figure 2. As shown in Figure 2, the whole setup consisted of three parts, including a DC power supply for heat input, a data receiver, and wireless acquisition unit for data collection, constant temperature adjustable cooling bath for heat dissipation. In this study, there were two heat load inputs located separately at both ends of heat pipes, which were called primary and additional heat sources. For distinction, the heat block on the left side of the heat pipes was called heat load A, while the other side was called heat load B. In this part, heat load A was discussed in detail due to heat loads A and B with the same structure. It should be noted that heat load B played the role of auxiliary heat source, which is similar to a thermal switch to adjust the threshold critical power of a heat pipe system. The DC power supply (MAISHENG MP6010D) was purchased from Dongguan Maihao Electronic Technology Co., Ltd., Dongguan, China. It was used to supply electric power for driving the heat block as the heat source. The heat block of dimensions 20 \(\times\) 25 \(\times\) 30 mm\({}^{3}\) was made of copper with a high thermal conductivity of approximately 400 W/(m\(\cdot\)K) for better heat loading, and the heat block was fixed to the test platform by spring and bolts to ensure test stability. The data acquisition module (HIOKI-LR8510) was purchased from HIOKI FOREST PLAZA CORPORATION, Nagano, Japan, and it was composed of a data receiver and wireless acquisition unit. There were nine micro-thermal couples attached tightly to the surface of TSHP or TGHP to obtain the surface temperature distribution of them. The data of the surface temperature were then transferred to the wireless acquisition unit through WiFi. Finally, data processing was carried out on a personal computer to obtain different thermal performances of the tested samples. The constant temperature adjustable cooling bath (DC-1006) for heat dissipation was purchased from Ningbo Scientz Biotechnology Co., Ltd., Ningbo, China. A flow meter was used for controlling the flow rate of cooling water, and thereby it could adjust the rate of heat dissipation. The cooling block was also made of copper with a water channel inner side, which served as the condensation section during testing, and it also fixed well to the test platform by spring and bolts. Moreover, TSHP or TGHP with heating and cooling blocks was covered by thermal insulation materials. A thin layer of thermal silicone grease was coated between TSHP or TGHP and heating or cooling blocks for lessening thermal contact resistance. All thermal tests were under room temperature and suitable humidity without airflow effects. All the micro-thermal couples were with accurate size and position among TSHP or TGHP. In order to distinguish two experimental heat pipe systems in this study, micro-thermal couples named T1 to T9 were utilized to discuss the thermal performance of TSHP, while t1 to t9 were for TGHP.  
\begin{table}
\begin{tabular}{l l} \hline
**Parameters** & **Value/Material** \\ \hline Tube material & Copper \\ Working fluid & Deionized water \\ Pipe length \(l\) (mm) & 310 \\ Necking length \(l_{o}\) (mm) & 30 \\ Necking diameter \(d_{n}\) (mm) & 4 \\ Pipe outer diameter \(d_{i}\) (mm) & 8 \\ Pipe inner diameter \(d_{o}\) (mm) & 6.9 \\ Groove depth \(h\) (mm) & 0.25 \\ Groove width \(m\) (mm) & 0.17 \\ Number of grooves \(n\) & 68 \\ \hline \end{tabular}
\end{table}
Table 1: The geometric parameters of grooved heat pipes.