## 2 Experimental Facility and Test Section  
The experimental flow boiling tests were run in a facility that was previously built in the Laboratory of Heat Transfer in Microgeometry located in the Department of Industrial Engineering of the University of Padova. The facility was developed with the aim of studying two phase heat transfer (flow boiling and/or condensation) of refrigerants in a wide range of operating conditions. The maximum allowable pressure is 30 bar with a maximum mass flow rate of 72 kg h\({}^{-1}\). The facility is used in the present research in flow boiling configuration. The facility, a schematic of which is shown in Figure 1, is composed of the main refrigerant loop whose heat exchangers are connected to auxiliary loops. It is a liquid pumped facility, where a magnetic coupled gear pump is aimed at circulating the refrigerant through the different heat exchangers. After passing through a Coriolis effect mass flow meter (accuracy \(\pm\)0.1% of the reading), the refrigerant passes from subcooled liquid conditions to superheated vapor conditions in the heat exchanger called evaporator. This heat exchanger is a counter-current brazed plate heat exchanger, where the refrigerant is evaporated and superheated by hot water, supplied in the hot water loop by means of an electric boiler. Superheated conditions are guaranteed by a proportional integral derivative (PID) controller, which controls the electric power of the resistances of the electric boiler and sets the water temperature at the inlet of the heat exchanger at a fixed value of 60 \({}^{\circ}\)C.  
The superheated refrigerant is then de-superheated and partially condensed in the heat exchanger called pre-condenser, which is connected to a cold water auxiliary loop. This heat exchanger is a double-passage tube-in-tube heat exchanger, where the refrigerant flows in the inner tube, and cold water flows counter-currently in the annulus between inner and outer tube. A stabilized water-cooled chiller, located in the cold water loop, permits to set the water temperature at the inlet of the pre-condenser, and a manually controlled valve permits to vary the water flow rate. Therefore, the heat flow rate in the pre-condenser can be adjusted to set the vapor quality conditions at the outlet of this heat exchanger, which coincide with the inlet of the test section. A magnetic volumetric flow meter (accuracy of \(\pm\)0.25% of the reading) measures the water flow rate, and a T-type  
Figure 1: Schematic of the implemented facility. thermopile (accuracy of \(\pm\)0.03 K) measures the temperature difference at the pre-condenser on the water side. In the test section the refrigerant partially evaporates, and it is then condensed and subcooled in the heat exchanger called post-condenser, before being pumped by the pump. The brazed plate post-condenser is connected to another auxiliary cold water loop where a mixture of water-glycol (0.66/0.34 by mass) is supplied by a stabilized air-cooled chiller. A picture of the test facility, as well as a picture of the test section, are shown in Figure 2a,b, respectively. Saturation conditions are controlled by means of a damper, which permits to vary the volume of a cylinder connected to the refrigerant loop, and so to vary the pressure of the refrigerant. The damper is connected to a compressed air line. Temperatures and pressures of the refrigerant are measured in several locations throughout the facility: pressures are measured by mean of absolute pressure transducers (accuracy \(\pm\)1950 Pa), and temperatures by means of T-type calibrated thermocouples (accuracy \(\pm\)0.05 K) connected to an ice point reference (K170-75C, Kaye Instruments). The test section is composed of the smooth tube soldered inside a guide milled on the top surface of a copper plate. The copper plate is 240 mm long, 20 mm high, and 10 mm wide.