## 2 Experimental Test Rig and Test Section  
The experimental facility, which is located in the Heat Transfer in MicroGeometries Lab of the University of Padua, is a liquid pumped circuit, which allows setting the desired conditions at the inlet of the test section by adjusting the heat flow rate exchanged in a pre-condenser, which is fed by superheated vapor coming from an evaporator/superheater. The pre-condenser is a tube-in-tube heat exchanger: the refrigerant flows in the inner tube, whilst cold water, supplied by a water-cooled chiller, flows in the annulus. By controlling both the water flow rate and the temperature, it is possible to set the vapor quality at the inlet of the test section. The refrigerant mass flow rate is measured using a Coriolis effect mass flow meter, with an accuracy of \(\pm\)0.1% of the reading. More details concerning the experimental facility can be found in [PERSON] et al. [23]. A schematic of the experimental test rig is represented in Figure 1.  
A water-heated test section, made by the microfin tube under investigation and a smooth tube where hot water flows, was developed and created. Hot water was supplied by a thermostatic bath. A schematic of the test section is reported in Figure 2. The microfin tube had an outer diameter of 7.0 mm and an inner diameter (\(D_{l}\)) at the fin tip of 6.14 mm, it had 50 fins, and the helix angle was 18\({}^{\circ}\). Each fin was 0.18 mm high. A smooth tube, with an inner diameter of 1.9 mm, was wrapped around the microfin tube. Hot water, supplied by a thermostatic bath and whose volumetric flow rate was measured using a magnetic flowmeter having an accuracy of \(\pm\)0.50% of the reading, flowed inside the wrapped smooth tube. An aluminum housing contained the test section, and an alloy of tin and lead was melted inside; once solidified, it filled the gap between the microfin  
Figure 1: Schematic of the experimental test rig.  
tube and the wrapped tube. The heated length (\(L_{HT}\)) of the test section was 236 mm. The top side of two curves, one at the inlet and one at the outlet, hosted T-type thermocouples (denoted as \(T_{water,IN}\) and \(T_{water,OUT}\) in Figure 2) for the measurements of the water inlet and outlet temperatures. Six T-type thermocouples (denoted as \(T_{r1,top}\), \(T_{r1,bottom}\), \(T_{r2,top}\), \(T_{r2,bottom}\), \(T_{r3,top}\), and \(T_{r3,bottom}\) in Figure 2) were attached on the external wall of the microfin tube to measure the wall temperature. All the implemented thermocouples presented an accuracy of \(\pm\)0.05 K, and they were previously calibrated using a Pt100 thermo-resistance as the reference temperature (accuracy of \(\pm\)0.03 K).  
Pressure ports were soldered to the microfin tube, down- and upstream of the thermal section. The distance between the two pressure ports was 0.8 m. The upstream pressure port was connected to an absolute pressure transducer (accuracy \(\pm\)1950 Pa), whereas both up- and downstream pressure ports were connected to a differential pressure transducer to measure the total pressure drop across the test section (accuracy of \(\pm\)25 Pa). The entire test section was insulated with Armaflex@in order to minimize the heat losses through the ambient. This means that the two segments between the pressure ports and the part of the test section where the flow boiling occurs, i.e., where the wrapped tube is located, were considered adiabatic. Table 1 shows the accuracy of the implemented sensors for the measurements of the physical quantities.