## 3 Results and Discussion
### Influence of Gas Cooler Air Volume Flow Rate on Optimal Discharge Pressure  
In this subsection, the effect of the air volume flow rate in the gas cooler on the optimal discharge pressure (P\({}_{\rm opt}\)) was investigated at the ambient temperature and the air temperature at the evaporator inlet of 35 \({}^{\circ}\)C and 27 \({}^{\circ}\)C, respectively. The tests were carried out at the compressor speed of 5000 rpm and air volume flow rate in the gas cooler side was varied between 1000 m\({}^{3}\cdot\)h\({}^{-1}\) and 3500 m\({}^{3}\cdot\)h\({}^{-1}\).  
Figure 7 shows that the COP increased first and then decreased with the rise in discharge pressure as the air flow rate was higher than 2000 m\({}^{3}\cdot\)h\({}^{-1}\). As the air flow rate was 1000 m\({}^{3}\cdot\)h\({}^{-1}\), the COP value kept increasing with the rise in discharge pressure and there was no COP peak value. This was due to the fact that the compressor discharge temperature had reached the safety limit, as the discharge pressure was 9.47 MPa. The optimal discharge pressure and the maximum COP were 9.6 MPa and 1.64 at the air flow rate of 1500 m\({}^{3}\cdot\)h\({}^{-1}\), 9.4 MPa and 1.67 at 2000 m\({}^{3}\cdot\)h\({}^{-1}\) and 9.2 MPa and 1.69 at 2500 m\({}^{3}\cdot\)h\({}^{-1}\), respectively. It could be concluded that the optimal discharge pressure gradually decreased as the air flow rate increased, whereas the COP gradually increased. As the air flow rate continued to increase, the optimal discharge pressure and COP value did not change significantly. As the air flow rate increased, the heat exchange performance of the gas cooler gradually improved. The temperature difference between the refrigerant at the outlet of the gas cooler and the air at the inlet gradually decreased, resulting in a gradual reduction in optimal discharge pressure value and a gradual increase in COP value. However, as the air flow rate was greater than 2500 m\({}^{3}\cdot\)h\({}^{-1}\), the heat exchange between the refrigerant in the gas cooler and the air was full. As the air flow rate increased, the change in temperature difference between the refrigerant and the air through the gas cooler decreased gradually. Therefore, the influence of air flow rate on the optimal discharge pressure and COP of the air conditioning system was gradually reduced.  
Figure 6: Change rate of evaluation parameters and performance parameters with air volume flow rate in the gas cooler.  
Figure 8 illustrates the p-h diagrams of the investigated cases under different air flow rates. The temperature before the EXV gradually decreased as the air flow rate increased, resulting in a reduction in optimal discharge pressure. The decrease in the temperature upstream of the EXV led the enthalpy difference between the inlet and outlet of the evaporator to increase. The growth in cooling capacity reduced the HVAC supply air temperature, leading to a gradual decrease in evaporation temperature. However, with the further increase in the air flow rate, the p-h diagrams of the investigated cases almost overlapped. This suggests that the air flow rate was almost sufficient, as the effect of further increasing the air flow rate on the system was minimal.  
Figure 8: P–h diagrams of the investigated air conditioning system at gas cooler air volume flow rates between 1000 m\({}^{3}\).h\({}^{-1}\) and 3500 m\({}^{3}\).h\({}^{-1}\).  
Figure 7: Influence of discharge pressure on COP at gas cooler air volume flow rates between 1000 m\({}^{3}\).h\({}^{-1}\) and 3500 m\({}^{3}\).h\({}^{-1}\).