*2.1. Test Apparatus*

A schematic diagram of the test apparatus employed in this condensation and vaporization study is given in Figure 2a. It consisted of three fluid loops: (i) a working fluid loop containing the test section; (ii) a water circuit used for adjusting the heat input, used to regulate the vapor quality of the test section; (iii) a sub-cooled ethyl alcohol loop, used to cool the saturated two-phase refrigerant to a sub-cooled liquid at a fixed temperature.

**Figure 2.** Schematic diagram of (**a**) experimental apparatus, and (**b**) test section.

The refrigerant loop consisted of the following components: a tube-in-tube heat exchanger (test section), condenser, reservoir, digital gear pump, flow regulator, Coriolis mass flow meter, electric preheater, sight glasses, and valves. The test section was manufactured as a straight, horizontal counter-flow tube heat exchanger (length of 2 m). Platinum RTDs (resistance temperature detectors) were installed at the inlet and outlet of both the water side and the refrigerant side; they were able to measure the temperature with a calibrated uncertainty of ±0.1 K. Refrigerant saturation pressure was determined by a pressure transducer (Rosemount 3051) with a range of 0–5 MPa, installed at the entrance of the test section; additionally, a differential pressure transducer was employed to measure the overall pressure drop for refrigerant flows. After leaving the test section, the refrigerant flowed into a low-temperature alcohol thermostat bath; then, the two-phase refrigerant was condensed, sub-cooled, and recirculated back into the reservoir. The sub-cooled refrigerant liquid in the reservoir was then pumped into the preheater by a gear pump; the mass flux meter (between the gear pump and the electric preheater) was utilized to ensure that the actual mass flux of the refrigerant loop was within an uncertainty of ±0.2% of the reading. The sub-cooled refrigerant was heated (in the preheater) by an electric heater; the electric heater was powered by a low-voltage and high-current direct current (DC) regulated power supply. The electric current varied in the range of 0–100 A with a fixed voltage of 50 V, and a maximum power of 5 kW could be supplied to heat the refrigerant in the preheater. Both the voltage and the current were collected to calculate the heating power. Therefore, the outlet vapor quality at the preheater could be controlled by the current. The enthalpy of the inlet refrigerant for the preheater was determined from the temperature and pressure, which were measured using the Platinum 100 RTD and the pressure transducer. Inlet vapor quality was calculated from the heat balance of the preheater; the electric preheater was well insulated (electrically and thermally) with a heat loss fraction lower than 5%, in order to ensure the accuracy of measurements. The water loop contained the tube side of the test section, a centrifugal pump, a thermostat bath, a magnetic flow meter, and a regulator valve. The heat input of the test section varied with the temperature of the thermostat bath and the water flow rates; in order to achieve the desired vapor quality, the refrigerant was heated or cooled in the test section. Additional details of the test apparatus can be obtained from Li et al. [21].

Figure 2b illustrates the test section employed in this work; the test section was a double-pipe heat exchanger with an effective heat length of 2 m. In this study, tubes with an inner diameter of 25 mm and 26 mm were employed as the outer tube in order to investigate the effect of annulus width; the tested tubes were used as the inner tube of the heat exchanger. In addition, the whole test section was well insulated, covered by a 40-mm-thick layer of polyurethane (thermal insulation), ensuring the accuracy of the experimental results. Before the evaporation and condensation experiments, the thermal insulation of the test section was verified using single-phase flow tests; the heat balance of the water side and refrigerant side in the test section and the preheater is given in Figure 3. The deviation in the heat balance between the water side and refrigerant side fell in an error band of ±5%; from this analysis, it is reasonable to conclude that the test section and the preheater were well insulated.

**Figure 3.** Heat balance measurements for single-phase flow in the smooth tube.
