*4.3. Heat Transfer Performance*

The temperature change in the middle of the condenser section was most obvious in a single boiling. The wall temperature was greatly affected by the cooling water when the vapor just entered the condenser section. Because the vapor temperature was higher before boiling, the wall temperature change was small at the entrance of condenser section. When the vapor arrived the end of the condenser section, most of the heat had been taken away by the cooling water, the cooling water temperature was also increased and the cooling capacity decreased, so the temperature change of the heat pipe was also small at end of the condenser section.

Figures 9 and 10 show the heat transfer quantity, the equivalent heat transfer coefficient, the equivalent thermal resistance, and the surface heat transfer coefficient of evaporator section and condenser section of heat pipe during working process according to Equations (1)–(5). With the increase of heating power, the heat transfer performance

of heat pipe was improved. Because the average temperature of the evaporator section changed little, it can be further proved that the mass flow of working fluid increases with the heating power in single boiling. The heat transferred by the heat pipe was only about half of the input of the heating controller, and a lot of heat was lost by the heating furnace. In this study, the minimum thermal resistance was about 0.6 K/W, and equivalent heat transfer coefficient could reach about 5500 W/(m2· ◦C).

**Figure 9.** Heat transfer quantity and equivalent thermal resistance of heat pipe under different heating power.

**Figure 10.** Heat transfer coefficient of heat pipe under different heating power.

Compared with Figures 6, 9 and 10, although there was a large temperature gradient along the heat pipe under 800 W to 1400 W heating, the increased of heating power could improve the evaporation rate and promoted the heat transfer of the evaporator section. Due to a large amount of vapor entered the condenser section, the heat transfer of the condenser section had been promoted and the vapor flow rate was increased, so the heat transfer coefficients increased linearly.

Table 3 presents the operating parameters of Na-K heat pipe under different heating power. From the Table 3, the effective heat transfer of heat pipe increased with the heating power, but the maximum temperature of heat pipe and the average temperature of evaporation section (*T*e) change little, while the fluctuation cycle of geyser boiling decreased and the average temperature of condensation section (*T*c) increased. Therefor, the geyser boiling mainly increases heat transfer by increasing boiling frequency rather than increasing the temperature of Na-K heat pipe.


**Table 3.** Operating parameters of Na-K heat pipe under different heating power.

Although geyser boiling will cause temperature fluctuation and shell vibration. Geyser boiling can make Na-K alloy heat pipe working at lower temperature. Table 3 presents the maximum temperature of heat pipe under geyser boiling was only about 700 ◦C when the effective heat transfer quantity of heat pipe reached 800 W. Compared with our previous work of Guo et al. [24], under the natural convection cooling, the effective heat transfer quantity can reach 800 W only when the maximum temperature is above 800 ◦C. Therefore, geyser boiling reduced the operating temperature of the Na-K alloy heat pipe by 100 ◦C.

Geyser boiling also can make Na-K alloy heat pipe have higher heat transfer quantity at lower temperature. Figure 6 and Table 3 present that when the maximum average temperature of the evaporator section was 600 ◦C, the effective heat transfer quantity under geyser boiling reached 800 W. While the Na-K alloy heat pipe has not start at the same temperature under natural convection cooling conditions, and the effective heat transfer was less than 50 W [24,25].

#### **5. Conclusions**

Due to geyser boiling can occur before continuous flow is formed in the condenser section under forced convection cooling, the start-up capacity and heat transfer capacity of a Na-K alloy heat pipe under forced convection cooling were experimentally studied at different heating power (800 W, 1000 W, 1200 W and 1400 W). The Na-K alloy heat pipe can work in geyser boiling mode, and transfer a lot of heat quantity at lower temperature than natural convection cooling. The main conclusions were as follows:


**Author Contributions:** Conceptualization, H.Z. and H.G.; Data curation, H.Z.; Formal analysis, H.Z.; Funding acquisition, F.Y. and X.Y.; Investigation, H.Z.; Methodology, H.Z.; Project administration, F.Y. and X.Y.; Resources, F.Y. and X.Y.; Supervision, H.G. and F.Y.; Visualization, H.Z.; Writing-original draft, H.Z.; Writing-review and editing, H.G. and F.Y. All authors listed have made a substantial, direct, and intellectual contribution to the work. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the National Key R&D Program of China [grant number 2017YFF0205901].

**Data Availability Statement:** The supporting data will be made available on request.

**Conflicts of Interest:** The authors declare that they have no conflict of interest.
