## 3 Results and Discussion
### The Effects of Power Distribution on the Threshold Critical Power of the Heat Pipe Systems
When heat load B increased from 4 W to 10 W, TGHP-1 increased rapidly (From 55 W to 80 W for TGHP-1), while TGHP-2 still increased slowly (from 50 W to 55 W for TGHP-2). It was almost a 50% increase in threshold critical power for TGHP-1 when heat load B increased from 4 W to 10 W. Moreover, TGHP-1 could be found to be better than TGHP-2 in the threshold critical power with heat load B  
increase, especially with high heat load B input. Therefore, TGHP-1 was chosen to carry on the following experiments. In conclusion, the addition of heat load B served as a thermal switch in both TSHP and TGHP, which could greatly increase the threshold critical power of a heat pipe system. The distribution of steam pressure inside the heat pipes is demonstrated in Figure 4. In this work, the addition of heat load B was introduced into the heat pipe systems, including TSHP-1 and TGHP-2. Therefore, it was quite different from the heat pipe system with only one single heat load, especially in the distribution of steam pressure inside the heat pipes. As shown in Figure 4a, the steam pressure of P1 increased when the heating block was under normal working, and then the steam flowed along the heat pipe to the condensation section. Finally, the steam was condensed into water. The condensed water reflowed to the evaporation section through the capillary structure. It also meant that the steam pressure of P2 was low when the heat pipe was working normally. Moreover, the steam pressures of P1 and P2 were under dynamic equilibrium, but the condensed water was blocked in the condensation section when the heat pipe was under working overload status. It meant that the heat pipe could not be carried on heat dissipation in time due to large heat input, which led to the dry-out status of the evaporation section. As a result, the steam pressure of P1 was much higher than that of P2. Figure 4b shows the distribution of steam pressure inside the heat pipe with two heat loads when the heat pipe was under normal and overload working status, especially for necking heat pipes such as TSHP-1. Before the heat pipe with two heat loads came into working overload status and the additional heat block was out of work, the distribution of steam pressure inside the heat pipe was the same as shown in Figure 4a. When TSHP-1 was under overload working status without heat  
Figure 3: The threshold critical power of the heat pipe systems under different power distribution: (**a**) the surface temperature distribution of TSHP-1 with different heat load A; (**b**) the surface temperature distribution of TGHP-1 with different heat load A; (**c**) the threshold critical power of TSHP-1 with different heat load B. (**d**) the threshold critical power of TGHP-1 with different heat load B.  
load B working, the condensed water was blocked in the condensation section. However, the steam pressure of P4 and P5 rapidly grew up after heat load B was on working due to the small volume of the necking section. Therefore, the blocked working liquid reflowed because the steam pressure of P4 and P5 was higher than that of P3. As a result, TSHP-1 came into normal working under higher heat load A input. Then, the steam pressures of P3, P4, and P5 returned to dynamic equilibrium. In summary, the addition of heat load B served as a thermal switch could improve the threshold critical power of a heat pipe system by increasing the reflow flux of the blocked working liquid inside the condensation section.