## 1 Introduction
Its thermal and hydraulic performance during two-phase heat transfer is almost unexplored in the literature. Just a few studies can be found regarding R515B. [PERSON] et al. [14] presented an energy, exergy, and environmental study of a compound ejector-heat pump system for simultaneous data center cooling and district heating; the analysis included 12 refrigerants; among those, R515B showed the most pronounced cooling coefficient of performance and the lowest exergy destruction, viz., the highest exergy efficiency. [PERSON] et al. [15] presented an overall comparison of different refrigerants, including R515B, for low-temperature organic Rankine cycle (ORC) systems, considering both the thermodynamic performance and the economic cost; the proposed evaluation method provided a reference for preliminary evaluation of working fluids for ORC applications. [PERSON] et al. [16] measured the heat transfer coefficients and pressure drops of R1234 ze(E), R450A, and R515B during condensation inside two smooth channels with an inner diameter of 3.38 mm and 0.96 mm. Furthermore, flow pattern visualizations were recorded. In addition to the use of new lower-GWP working fluids, more efficient heat exchangers are to be developed in order to reduce the energy consumption of air-conditioning and refrigeration equipment. In this context, microfin tubes can play a role in the enhancement of the heat transfer process: compared to a smooth tube with the same inner diameter, the heat transfer augmentation led by microfins can be higher than the pressure drop penalization, as reported in [PERSON] et al. [17] and in [PERSON] et al. [18]. However, while some studies are available in the literature regarding flow boiling of HFCs inside microfin tubes [19; 20; 21; 22], more data are needed for the validation of empirical correlations able to estimate the thermal and hydraulic behavior of the new low-GWP refrigerants during two-phase heat transfer inside microfin tubes. In this scenario, this paper presents experimental results obtained during flow boiling of R515B and R450A, as well as R1234 ze(E), inside a microfin tube with an inner diameter at the fin tip of 6.14 mm. During the experimental flow boiling campaign, the mass velocity was varied between 50 and 400 kg m\({}^{-2}\) s\({}^{-1}\); the heat flux was set at 10, 30, and 50 kW m\({}^{-2}\); and the vapor quality was investigated from 0.1 to 0.99, with a fixed inlet saturation temperature of 30 \({}^{\circ}\)C. The flow boiling of refrigerants at this high saturation temperature is used, for instance, in heat pump systems for clothes dryers, in industrial equipment that produces hot water by using waste heat as a source for the evaporator, in organic Rankine cycle systems, and in the cooling management of electronic equipment. The low tested mass velocities, which are not much investigated in the open literature, are useful to predict the flow boiling behavior of evaporators when the heat pump system, having the compressor connected to an inverter, works under reduced loads. The comparison among the three fluids is given in terms of the heat transfer coefficient and the frictional pressure gradient. The experimental values of the heat transfer coefficient and the pressure drop are then compared against the values predicted by some empirical models available in the literature in order to verify their suitability for the tested new mixtures evaporating inside the microfin tube.