## 3 Results and Discussion  
### Influence of Gas Cooler Air Volume Flow Rate on Key Temperatures and Pressures  
The effect of the gas cooler air volume flow rate on the COP, cooling capacity, compressor power input, key temperatures and pressures as well as on the compressor isentropic \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Evaporator Side** & **Gas Cooler Side** \\ \hline Ambient temperature and humidity & 27 \({}^{\circ}\)C, 50\% & 30 \({}^{\circ}\)C, 50\% \\ \hline Air volume flow rate & 480 m\({}^{3}\cdot\)h\({}^{-1}\) & 500/1000/1500/2000/2500/3000/3500 m\({}^{3}\cdot\)h\({}^{-1}\) \\ \hline Refrigerant charge & 700 g [24] \\ \hline Compressor speed & 3500/5000/6500 rpm \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Detailed experimental conditions. and volumetric efficiencies under the optimal discharge pressure was evaluated at the ambient temperature and compressor speed of 30 \({}^{\circ}\)C and 3500 rpm, respectively. Figure 2 shows the variation characteristics of the evaporator (\(T_{\text{eva}}\)), suction (\(T_{\text{suc}}\)), discharge (\(T_{\text{dis}}\)) and CO\({}_{2}\) gas cooler outlet (\(T_{\text{gasc\_o}}\)) temperatures with the air volume flow rate on the gas cooler side. It could be seen that all the aforementioned temperatures decreased rapidly and then gradually became constant with the rise in gas cooler air volume flow rate. As the gas cooler air volume flow rate increased from 500 m\({}^{3}\cdot\)h\({}^{-1}\) to 2500 m\({}^{3}\cdot\)h\({}^{-1}\), the discharge temperature dropped by 20.88 K. Compared with the compressor discharge, the suction temperatures and the CO\({}_{2}\) gas cooler outlet temperature showed less marked changes, which were equal to 8.00 K and 8.03 K, respectively. Additionally, the change in evaporation temperature was the smallest (i.e., 4.28 K). Therefore, the discharge temperature was the most suitable parameter for evaluating whether the gas cooler air volume flow rate was optimal. However, considering that the discharge temperature could not exceed 135 \({}^{\circ}\)C (i.e., the safety limit), another parameter for assessing the optimal gas cooler air volume flow rate was necessary. The CO\({}_{2}\) gas cooler outlet temperature was found to be more affected by the air volume flow rate in the gas cooler than the evaporator and compressor suction temperatures. Moreover, it was the most directly influencing parameter of the air flow rate on the gas cooler side. Therefore, the CO\({}_{2}\) gas cooler outlet temperature was taken as the second evaluation parameter. Figure 3 illustrates the change characteristics of the evaporator (\(P_{\text{eva}}\)), compressor discharge (\(P_{\text{dis}}\)) and suction (\(P_{\text{suc}}\)) pressures as well as the pressure upstream of the expansion valve (\(P_{\text{PBE}}\)) with the gas cooler air volume flow rate. The discharge pressure and the pressure upstream the EXV were the high pressures, whereas the evaporation pressure and suction pressure were the low pressures. It could be concluded that the pressures decreased rapidly at first and then gradually stabilized as the gas cooler air volume flow rate increased. Moreover, it was found that the high pressures of the system changed more significantly with the gas cooler air volume flow rate than the low pressures. As the gas cooler air volume flow rate increased from 500 m\({}^{3}\cdot\)h\({}^{-1}\) to 2500 m\({}^{3}\cdot\)h\({}^{-1}\), the discharge pressure and PBE decreased by 1.34 MPa and 1.42 MPa, respectively. However, the suction pressure and the evaporation pressure decreased only by 0.48 MPa and 0.47 MPa, respectively. Therefore, it could be stated that the discharge pressure and the PBE were more suitable as parameters for assessing whether the gas cooler air volume flow rate was optimal.