## 3 Results and Discussion  
### The Effects of Power Distribution on the Threshold Critical Power of the Heat Pipe Systems  
Figure 3 shows that the threshold critical power of the heat pipe systems was experimentally investigated under different power distributions, including heat load A and heat load B. In the experiment section, heat load B was introduced as an auxiliary heat source to make the threshold critical power of the heat pipe systems adjustable. Therefore, the additional heat load B is similar to a thermal switch. As shown in Figure 3a,b, the surface temperature of two heat pipe systems increased steadily with heat load A. Additionally, there was little difference in the threshold critical power of them without heat load B. For example, the threshold critical power of TSHP was 55 W, while TGHP was 50 W. It could be known that the same length of heat pipe might have almost the same threshold critical power, even though they had different structures. For example, TSHP had a necking section with a length of 30 mm and a diameter of 4 mm. TGHP was only a long straight grooved tube without a necking section. There were some interesting things that occurred that the surface temperature difference of the heat pipes showed obvious distinguish. Nine points for measuring the temperature of TSHP had good consistency, which meant that its thermal performance was pretty good. However, TGHP had good temperature consistency under the low operating power, while it obtained a large temperature deviation under the high operating power. Figure 3c,d demonstrated the threshold critical power of two heat pipe systems under different heat load B inputs in detail, respectively. Additionally, two kinds of the filling working liquid mass for two heat pipe systems were also experimentally investigated, respectively. In this study, the filling working liquid mass was not the key research factor. Therefore only two kinds of the filling working liquid mass for two heat pipe systems were chosen to show how it influenced the thermal performance of the heat pipe systems. Then, the heat pipes with better thermal performance were selected to push ahead with the following experiments. Two kinds of the filling working liquid mass (1.05 g and 0.95 g) for TSHP under different heat loads B were demonstrated as the following Figure 3c. For convenience, TSHP, with the filling working liquid mass of 0.95 g, was named TSHP-1, while the other was named TSHP-2. As mentioned previously, heat load B played a significant role in the threshold critical power adjustment of the heat pipe system. TSHP showed almost the same change with heat load B. Their threshold critical power increased rapidly with low heat load B and increased slowly or even kept constant with high heat load B. For example, both TSHP-1 and TSHP-2 had a large growth of 10 W with heat load B input less than 4 W. It was almost a 20% increase in threshold critical power for TSHP-1 and TSHP-2 (from 55 W to 65 W for TSHP-1; from 50 W to 60 W for TSHP-2). When heat load B increased from 4 W to 12 W, both TSHP-1 and TSHP-2 increased 5 W (from 65 W to 70 W for TSHP-1; from 60 W to 65 W for TSHP-2). Moreover, TSHP-1 could be speculated better than TSHP-2 in the threshold critical power with heat load B increase. As a result, TSHP-1 was chosen to carry on the following experiments. Two kinds of the filling working liquid mass (1.5 g and 1.1 g) for TGHP under different heat loads B were demonstrated as the following Figure 3d. For convenience, TGHP with the filling working liquid mass of 1.5 g was named TGHP-1, while the other was named TGHP-2. Their threshold critical power increased slowly or even kept constant with low heat load B and increased rapidly with high heat load B, which was in contrast with TSHPs. For example, both TGHP-1 and TGHP-2 had a small growth of only 5 W with heat load B input less than 4 W. It was just less than a 10% increase in threshold critical power for TGHP-1 and TGHP-2 (from 50 W to 55 W for TGHP-1; from 45 W to 50 W for TGHP-2).