## 2 Experimental Apparatus and Procedure  
A detailed schematic diagram of the experimental apparatus including its different components is presented in Figure 1. The experimental apparatus comprises of a Coriolis mass flow meter, three mixing chambers, two pre-heaters, test sections, a cooler, an accumulator, cooling/heating water loop and data logger. The liquid refrigerant is kept in the refrigerant tank. Then, the refrigerant is pumped by a gear pump. After that, it flows through the filter, mixing chamber, pre-heaters, sight glass, test section, cooler and accumulator. To continue the cycle, the refrigerant is again circulated from the cooler after being collected in an accumulator and returned to the pump. However, to get the desire vapor quality in the test section inlet, the refrigerant quality is controlled and superheated by the first and second preheaters, respectively. The heated/superheated refrigerant passes into the test section and experimental data are obtained in the different vapor quality ranges. To exchange the heat between refrigerant and water, the test section consists of a tube-in-tube heat exchanger. After exchanging heat, refrigerant leaves the test section, flows through the cooler, and is completely subcooled by brine. Brine is reserved at a fixed temperature in the heat source unit. The accumulator is used to control the system pressure of the experimental apparatus. The Coriolis mass flow meter measures the mass flow of refrigerant. Three mixing chambers are fitted in the different locations to measure the bulk temperature of the refrigerant. Two sight glasses at the inlet and outlet of the test section are fitted to observe the refrigerant's quality. The absolute pressure transducer is fitted to measure the pressure of refrigerant. K-type and T-type thermocouples are fitted at different locations to measure the temperature of refrigerant and tube wall, respectively. All signals from the measuring devices are collected by a data logger. Moreover, the total experimental apparatus is well insulated so it is not affected by outside temperature.  
Figure 2 shows details of the test section, which consists of a horizontally installed copper tube, two headers, three water channels and three sub-sections. The total test section length is 852 mm, each sub-section length is 248 mm and effective cooling/heating length is 744 mm. The refrigerant flows inside the test tube, while the counter-current water flows through the water loop, as shown in Figure 2. A differential pressure transducer is fitted in the test section header to measure the pressure difference. Thermocouples are inserted at various locations to measure the temperature (Figure 2). Devices such as thermocouple calibration were measured over the applicable range. The K-type and T-type thermocouples measurement uncertainty was estimated as \(\pm\)0.03\({}^{\circ}\)K. The absolute and differential pressure transducer measurement uncertainty was estimated as \(\pm\)0.1 KPa. The Coriolis mass flow meter measurement uncertainty was estimated as \(\pm\)0.15%.  
The test tube is made of copper. Two types (smooth and microfin) of test tube were used in this study and both tubes have outside diameters of 2.5 mm, as shown in Figure 3. The tube dimensions are listed in Table 1. Refrigerant mass flux varied from 50 to 300 kg m\({}^{-2}\)s\({}^{-1}\) and the saturation temperature varied from 20 to 30 \({}^{\circ}\)C. The usual refrigerant thermophysical properties of R134a used in the experiment are shown in Table 2.  
Figure 1: Schematic diagram of the experimental apparatus.  
Figure 3: Photography and schematic view of the test tube: (**a**) smooth tube; (**b**) microfin tube; and (**c**) schematic view of the microfin tube.  
Figure 2: Details of the test section.