## 2 Experimental Setup and Procedure  
A schematic of the experimental setup is shown in Figure 1, a vapor compression heat pump system consisting of a refrigerant loop and four heat-source water loops. The refrigerant loop consists of a compressor, oil separator, pre-condenser, test section, rear condenser, liquid receiver, mass flow meter, expansion valve, and evaporator. Each heat-source water loop consists of a water tank, temperature control unit, water pump, and flow meters. The refrigerant flow rate is adjusted by the expansion valve, bypass valve, and rotational speed of the compressor. Details of the pre-condenser and test section are shown in Figure 2 for measurements using R1234 ze(E), which is a double-tube heat exchanger consisting of three subsections with a length of 200 mm. In each subsection, a test tube is cooled by water passing through the annulus, while the refrigerant flows within the inner tube. Measurement lengths for the pressure drop and heat transfer are 800 mm and 200 mm, respectively. Therefore, the test section for the measurements using R32, R410A, and R152a, was different from that of R1234 ze(E), and it was shown in a published paper [13]. The device measurement accuracies were shown in Table 1, and the reliability of the experimental loop confirmed that heat balance is consistent, with a 10% difference between the refrigerant and heat-source water side. Figure 1: Schematic of the experimental setup. The test copper tube used a smooth tube, with an outer diameter of 4.02 mm and an inner diameter of 3.48 mm. In the experimental setup, the pressure and the thermal equilibrium vapor quality of inlet No.1 subsection are changed to steady values by the pre-condenser, and the condensation process of refrigerant progresses from the No.1 to the No.3 subsection. Under experimental conditions, the refrigerant mass velocity varied from 100 to 400 kg m\({}^{-2}\)s\({}^{-1}\), and the saturation temperature at the inlet test section was maintained at 35 \({}^{\circ}\)C. The amount of quality change, which depended on the refrigerant, mass velocity, and heat flux, in each subsection, was approximately 0.2 on average, with a maximum of 0.35 for R410A in mass velocity of 100 kg m\({}^{-2}\)s\({}^{-1}\). The physical properties of used refrigerants in this study are shown in Table 2. \begin{table}
\begin{tabular}{c c c} \hline
**Instrument** & **Full Scale** & **Uncertainty** \\ \hline T-type thermocouple & - & 0.05 K \\ Coriolis mass flow meter & 43 kg h\({}^{-1}\) & 0.2\% \\ Absolute pressure transducer & 3 MPa & \(\pm\)0.3 kPa \\ Differential pressure transducer & 100 kPa & \(\pm\)0.2 kPa \\ Differential pressure transducer & 20 kPa & \(\pm\)0.04 kPa \\ Pt-resistance thermometer & - & \(\pm\)0.04 K \\ Volumetric flow meter & 500 L h\({}^{-1}\) & \(\pm\)0.01 L h\({}^{-1}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Measurement accuracies of the measuring devices. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **R410A** & **R32** & **R152a** & **R1234 ze(E)** & **R404A** & **R290** \\ \hline Global Warming Potential* & 1924 & 677 & 138 & \textless{}1 & 3942 & 3 \\ Pressure (MPa) & 2.14 & 2.19 & 0.79 & 0.67 & 1.61 & 1.22 \\ Dew point temp. (\({}^{\circ}\)C) & 35.0 & & & & 35.0 & \\ Boiling point temp. (\({}^{\circ}\)C) & 34.9 & 35.0 & 35.0 & 35.0 & 34.6 & 35 \\ Vapor density (kg m\({}^{-3}\)) & 88.9 & 63.3 & 24.6 & 35.3 & 87.5 & 26.6 \\ Liquid density (kg m\({}^{-3}\)) & 1005 & 917.1 & 873.4 & 1129 & 995.0 & 476.1 \\ Vapor viscosity (\(\mu\)Pa s) & 14.5 & 13.5 & 10.5 & 12.7 & 12.5 & 8.7 \\ Liquid viscosity (\(\mu\)Pa s) & 103.2 & 101.0 & 145.3 & 177.2 & 119.4 & 87.4 \\ Vapor thermal cond. (mW m\({}^{-1}\) K\({}^{-1}\)) & 17.7 & 17.2 & 16.1 & 14.5 & 17.7 & 20.5 \\ Liquid thermal cond. (mW m\({}^{-1}\) K\({}^{-1}\)) & 83.6 & 118.3 & 93.8 & 70.9 & 60.0 & 89.1 \\ Latent heat (kJ kg\({}^{-1}\)) & 169 & 249 & 267 & 159 & 128 & 317 \\ \hline \end{tabular}
\end{table}