## 3 Result and Discussion
### The Operation Characteristics of the PHP  
#### 3.2.1 The Flow Characteristics of the Liquid Slugs in the PHP  
The PHP with the adiabatic section length of 180 mm was taken as an example to illustrate the flow characteristics during the operation. Figure 5 shows the temperature fluctuation and the momentum of the liquid slug at the heat input of 20 W. E1 and E2 are the temperature monitoring points in the evaporation section, and C1 and C2 are  
Figure 4: Comparison of the numerical results and the previous experimental results [21; 31]. the temperature monitoring points in the condensation section. The location of temperature monitoring points E1, E2, C1, and C2 in Figure 5a are indicated in Figure 2a. The momentum (\(p\)) of the liquid slug was calculated by:  
\[p=m_{1}v \tag{45}\]  
where \(m_{1}\) and \(v\) represent the mass and the velocity of the liquid slug, respectively. When \(p>0\), the flow direction of the liquid slug was consistent with the positive direction of the \(x\)-axis. When \(p<0\), the liquid slug flowed along the negative direction of the \(x\)-axis. From Figure 5a,b, it can be seen that the evaporation temperature continuously increased before the liquid slug started to move. At time \(t_{0}\), the absolute value of the momentum increased; that is, the liquid slug started to move. Then, the evaporation temperature decreased and the condensation temperature increased because the heat was transferred from the evaporation section to the condensation section by the movement of the liquid slug. Therefore, it can be concluded from Figure 5 that the PHP started at \(t_{0}\). After \(t_{0}\), the PHP achieved a stable operation state. During the steady operation of the PHP, the liquid slug were driven by the pressure differences between the vapor plugs. Figure 6a-f shows the distribution of the liquid slugs and the vapor plugs in the PHP with the time interval of 0.05 s, and the flow direction of the liquid slugs is indicated by red arrows. The length of the adiabatic section was 180 mm and the heat input was 20 W. The expansion-shrink and generation-collapse processes of the vapor plugs which drove the movements of the liquid slugs were clearly shown in the results of this numerical simulation. As shown in Figure 6a,b, the third vapor plug expanded and pushed the adjacent liquid slugs. Then, the second liquid slug in Figure 6b reached the saturated temperature, new vapor plugs generated in the liquid slug; the liquid and vapor plugs are renumbered as shown in Figure 6c. The newly generated vapor plugs expanded and pushed the adjacent liquid slugs. Then, due to the different flow velocity, the second and third liquid slugs in Figure 6e merged with each other and a new second liquid slug was formed, as shown in Figure 6f. Therefore, during the operation of the PHP, the expansion-shrink and generation-collapse processes of the vapor plugs have a significant influence on the movements of the liquid slugs. Figure 5: Operation characteristics of the PHP at 20 W (La = 180 mm): (**a**) temperature fluctuation of the evaporation section and the condensation section; (**b**) momentum of the liquid slugs. To investigate the flow characteristics in the PHP under different heat input, the momentum of the liquid slug in the PHP with 180 mm adiabatic length is shown in Figure 7. The sign of the momentum represents the flow direction of the liquid slugs; that is, when the momentum is larger than zero, the flow direction is the positive direction of the \(x\)-axis. Figure 7 shows that the absolute value of the momentum of the liquid slug increased with the increase in the heat input. Furthermore, the oscillation frequency of the momentum obviously increased with the increase in the heat input. The main reason for this was that the bubble generation can be more intense at higher heat input and the driving force for the liquid plugs was higher at larger heat input. In addition, Figure 7 also shows that the oscillating frequency was much lower at the low heat inputs.