## 4 Results and Discussion
### Variable Condensation Temperature  
First of all, it can be seen from Figure 13b that the mass flow rate was calculated by the evaporator model and was thus less susceptible to variations in the condensation temperature. It increases slightly as the condensation temperature rose and its average value was 3.36 kg/s, which was essentially stable.  
The curve of evaporator-phase zone ratio, varying with the condensation temperature, is presented in Figure 11. The increase in the condensation temperature makes the inlet temperature of the evaporator increase, which leads to the supercooled zone decreasing. The tiny increase in the two-phase zone and superheated zone mainly occurs because the mass flow rate slightly increases. From the figure, the ratio of the two-phase zone increased from 74.91% to 75.81% and the superheated zone increased from 3.28% to 3.32%. The ratio of the supercooled zone decreased from 21.80% to 20.87%.  
Figure 11: The curves of the evaporator-phase zone ratios varying with \(T_{c}\).  
Figure 10: The influence of \(P_{e}\) on the system and heat exchangers. (**a**) The utilization ratios of the heat exchangers. (**b**) Parameters of the system.  
The data above show that the mass flow rate varied relatively little, indicating that the heat flow rate had the greatest impact on the evaporator heat transfer coefficient. As the condensation temperature increased, the evaporator inlet specific enthalpy and temperature increased, and the inlet and outlet specific enthalpy difference and heat transfer temperature difference decreased, resulting in a decrease in the evaporator heat load. However, the increase in the temperature of the working fluid side helped to increase the generation rate of bubbles, thus raising the heat transfer coefficient. As shown in Figure 12, when the heat flux increased from 52.83 kW/m\({}^{2}\) to 63.94 kW/m\({}^{2}\), the heat transfer coefficient increased from 1721.31 W/(m\({}^{2}\cdot\)K) to 2374.77 W/(m\({}^{2}\cdot\)K).  
It can be seen from Figure 13a that, as the condensation temperature increased, the evaporator utilization ratio dropped from 70.56% to 51.91%, and the condenser utilization ratio dropped from 95.94% to 45.71%. This is because the heat transfer coefficient of the evaporator decreased, so the heat exchange process was suppressed. Therefore, the actual heat exchange area of the evaporator decreased; phase change heat can bring greater heat transfer benefits. The two-phase process of the condenser accounted for the largest proportion, which was reduced from 90.7% to 80.5%. The decrease in the proportion of the two-phase zone weakened the phase heat transfer process inside the condenser, which made its actual heat exchange area be reduced, resulting in a decrease in the utilization ratio.  
The parameters of the system are presented in Figure 13b. The system efficiency dropped from 13.64% to 10.43%, and the expander output power dropped from 116.95 kW to 80.60 kW. As the condensation temperature increases, the specific enthalpy at the outlet of the expander increases, and the specific enthalpy difference between the inlet and outlet  
Figure 12: The linear relationship between the \(K_{e}\), \(m_{r}\), and heat flux in the evaporator.  
Figure 13: The influence of the condensation temperature on the system and heat exchangers. (**a**) The utilization ratios of the heat exchangers. (**b**) Parameters of the system.  
of the expander decreases. In addition, the mass flow rate decreases. Therefore, the output power showed a downward trend. From the above analysis, it can be seen that although the heat exchange amount of the evaporator decreased, the decrease in the output power was much greater than the decrease in the heat exchange amount in the evaporator, resulting in a continuous decline in system efficiency. Based on analysis from the perspective of system physics principles, the increase in the condensation temperature caused the total heat exchange temperature difference in the system to decrease, resulting in a continuous decline in system efficiency.