## 3 Results and Discussion
### System Performance According to Quality Variation
However, during wet compression, despite an increase in the suction temperature (Figure 7), and a  
Figure 6: Pressure ratio as a function of quality for different EWT of OD HEX. Figure 7: Suction temperature and superheat as a function of quality for different EWT of OD HEX. decrease in the pressure ratio (Figure 6), with a decrease in the quality of the two-phase mixtures, the discharge temperature consistently dropped, because the droplets of liquid were vaporized by the internal cooling heat transfer process during compression [3,6]. At a high pressure ratio in severe climatic conditions, such as in cold regions, the discharge temperature may exceed the allowed compressor discharge temperature, resulting in the chemical degeneration of the lubricating oil, causing excessive wear and reducing the lifetime of a compressor [17,18]. Thus, the reduction of quality at the compressor inlet can lead to a sufficient decrease in the discharge temperature, which is considerably attractive for a heat pump system operating at a high compression ratio. Figure 9 shows the measured mass flow rate at the condenser outlet as a function of quality for different EWT of OD HEX. When the EWT of OD HEX is decreased, the mass flow rate decreased due to the decrease in the refrigerant density. The mass flow rate linearly increased with decreasing quality up to specific values for all EWT of OD HEX, with an increasing refrigerant inflow to the compressor. An increase of a maximum 70%, 80%, and 88% in the mass flow rate was observed for EWT of OD HEX at 10 \({}^{\circ}\)C, 5 \({}^{\circ}\)C, and 0 \({}^{\circ}\)C, respectively. Under the quality of 0.91, 0.88, and 0.84 for EWT of OD HEX at 10 \({}^{\circ}\)C, 5 \({}^{\circ}\)C, and 0 \({}^{\circ}\)C, respectively, the mass flow rate dramatically decreased, presumably due to the presence of overfull liquid droplets in the two-phase mixture. Similar results were reported by [PERSON] et al. [5] and [PERSON] et al. [18]. These studies reported that the excessive liquid inflow in the compressor was not fully evaporated during compression. Thus, the rest of the liquid refrigerant accumulated in the compressor chamber. This could flow back into the suction chamber which, in turn, decreased the suction mass flow rate. Figure 10 shows the suction (a) and discharge (b) pressure of the compressor as a function of quality for different EWT of OD HEX. When the EWT of OD HEX decreased, the suction pressure decreased by a drop in the evaporating temperature, leading to a decrease in the refrigerant density. This resulted in a decrease in the discharge pressure. Both the suction and discharge pressure increased on decreasing quality, by the rising refrigerant at the compressor inlet. In earlier studies [19,20] focusing on an injection circuit, the discharge and suction pressure were unchanged, due to the cooling effect of the compressor head and fixed superheat; while, in the present study with no injection circuit, the discharge and suction pressure of the compressor increased. This leads to a decreasing pressure ratio as the increase in the suction pressure is larger than that of the discharge pressure. This is also related to an increase in the mass flow rate at the compressor suction, yielding a positive effect on the heating capacity of the heat pump system. This could lead to a decrease in the performance of the heat pump in the cooling condition, as the increase in the suction pressure leads to a decrease in  
Figure 8: Discharge temperature as a function of quality for different EWT of OD HEX. the average temperature difference between the working fluid (refrigerant) and the secondary fluid (water) in the evaporator, although it increases the mass flow rate. Figure 11 shows the variation in the heating capacity as a function of quality for different EWT of OD HEX. When the EWT of OD HEX increased, the heating capacity increased as the refrigerant mass flow rate increased. It increased with respect to the decreasing quality, due to the increasing mass flow rate.