# Flow Condensation of Low-GWP Zeotropic Mixtures Inside 5 mm OD Micro-Finned Tube
## 2 Methodology  
The overall outlook of the thermodynamic cycle is presented in Figure 1. Pressure and temperature for both water and refrigerant cycles are duly recorded throughout the means of acquisition devices whose detailed information is provided in Table 1. The mass flux of the refrigerant cycle at the test section is simply set by modifying the variable-speed pump. After reaching the super-heated state by passing the evaporator that operates using an electrically heated hot water supply, the refrigerant proceeds to enter the pre-condenser, where saturated conditions are acquired. The process through which the vapor quality is set at the inlet of the test section can be grasped in the upcoming section of the methodology. The refrigerant then proceeds to be subcooled in a post-condenser whose water supply is obtained using a chiller. Refer to Figure 1 for a clear grasp of the positioning of the measurement devices, and to [PERSON] et al. [26] for a more detailed description of the experimental apparatus. The test section is comprised of the aforementioned micro-finned tube that exchanges heat with a helically wrapped smooth tube with water flowing at temperatures below saturation temperature. To guarantee optimal thermal contact, the space between the helical tube and the test section is filled with an alloy, and the entire test section is enshrouded by an aluminum case. It is important to note that the entire test section is covered by insulating material to avoid the exchange of heat with the surroundings. Figure 2 illustratesthe abovementioned remarks, where \"REF\" stands for \"refrigerant\". Furthermore, to better grasp the geometrical parameters of the micro-finned tube, a circumferential pictorial demonstration of the tube is displayed in Figure 3. The inner diameter at the fin tip for the tested micro-finned tube is 4.28 mm, with an outer diameter of 5.0 mm. There are 54 fins, each one is 0.15 mm high. The helix angle is 30\({}^{\circ}\). The micro-finned tube is made of copper, which is a common material for tubes implemented in heat exchangers involved in air conditioning and refrigeration equipment, and suitable for a large variety of refrigerants. Two pressure ports are devised for the test section at the upper and lower stream. A differential pressure transducer is implemented for the purpose of recording pressure drops. Six T-type (copper/constant) thermocouples are installed at the tube wall to capture the mean wall temperature (\(T_{wall,mean}\)). These thermocouples have been previously calibrated in the temperature range of 10-50 \({}^{\circ}\)C, which is suitable for the present experimental tests. A thermo-resistance Pt100 is used as a reference temperature during the calibration procedure. \begin{table}
\begin{tabular}{c c} \hline \hline
**Instrument** & **Accuracy** \\ \hline Coriolis mass flow meter (refrigerant side) & \(\pm\)0.10\% of the reading \\ Magnetic volumetric flow meter (water at pre-condenser) & \(\pm\)0.25\% of the reading \\ Magnetic volumetric flow meter (water at test section) & \(\pm\)0.50\% of the reading \\ T-type thermocouples & \(\pm\)0.05 K \\ T-type thermopiles & \(\pm\)0.03 K \\ Absolute pressure transducer & \(\pm\)1950 Pa \\ Differential pressure transducer & \(\pm\)25 Pa \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Accuracy of measurement devices. Figure 2: Test section. The length of the test section in which heat exchange takes place is around 0.217 m and the distance between pressure drop ports is around 0.5 m.  
The data analysis in the test section for both HTC and FPD is delineated by expressing the evaluation steps as follows:  
1. Evaluation of inlet vapor quality: \[h_{ref,test,in}=h_{ref,Pre-cond,in}-\frac{q_{pre-cond}}{\dot{m}_{ref}}\] (1) \[x_{in}=\frac{h_{ref,in,test}-h_{L}}{h_{V}-h_{L}}\] (2)
2. Evaluation of output vapor quality: \[h_{ref,test,out}=h_{ref,in,test}-\frac{q_{test}}{m_{ref}}.\] (3) \[x_{out}=\frac{h_{ref,out,test}-h_{L}}{h_{V}-h_{L}}.\] (4)
3.