## 1 Introduction
[14] analyzed a multiport tube with a hydraulic diameter of 1.45 mm for mass velocity in the range 50-400 kg m\({}^{-2}\) s\({}^{-1}\), heat flux in the range 3-107 kW m\({}^{-2}\), for saturation temperature from 30 \({}^{\circ}\)C to 70 \({}^{\circ}\)C. The experimental results showed that the heat transfer coefficient increased as saturation temperature, heat flux, and mass velocity increased. R1234 ze(E) showed higher values of heat transfer coefficient compared to R245 fa for all of the tested working conditions. Another multiport microchannel with a hydraulic diameter of 0.643 mm was studied by [PERSON] and [PERSON] [15] during flow boiling of R1234 ze(E) at mass velocity from 100 to 200 kg m\({}^{-2}\) s\({}^{-1}\), heat flux from 2 to 6 kW m\({}^{-2}\), and saturation temperature from 20 \({}^{\circ}\)C to 40 \({}^{\circ}\)C. Compared to R134a, R1234 ze(E) showed similar heat transfer coefficients and slightly higher pressure drops. Flow boiling of R1234 ze(E) inside larger diameter tubes were studied by [PERSON] et al. [16], who considered a horizontal smooth tube with an inner diameter of 6 mm for mass velocity from 130 to 258 kg m\({}^{-2}\) s\({}^{-1}\) and heat flux from 10.6 to 74.8 kW m\({}^{-2}\). A comparison with R600a revealed that R1234 ze(E) had lower heat transfer coefficients and pressure drops. A commercial tube with an internal diameter of 4 mm was analyzed by [PERSON] et al. [17], for mass flux from about 200 to 600 kg m\({}^{-2}\) s\({}^{-1}\), heat flux in the range 10-30 kW m\({}^{-2}\), and saturation temperature from 10 \({}^{\circ}\)C to 20 \({}^{\circ}\)C. In the tested working conditions, R1234 ze(E) showed heat transfer coefficients similar to those of R134a and frictional pressure drop 10-25% higher than those of R134a. As it appears from this brief introduction, most of the works related to R1234 ze(E) flow boiling are related either to microchannels or to macrochannels, whereas there is a lack of experimental data in the transition range of diameters. Regarding the transition criterion between microscale and macroscale, [PERSON] and [PERSON] [18] introduced threshold values based on the confinement number, Co: microscale flow for values of Co higher than 1.0, whilst macroscale flow for values of Co lower than 0.3-0.4. The region in between is referred as mesoscale. This paper is aimed at presenting some experimental data during R1234 ze(E) flow boiling inside a smooth tube with an internal diameter of 2.5 mm, in a wide range of working conditions: mass velocity from 200 to 600 kg m\({}^{-2}\) s\({}^{-1}\), heat flux from 12 to 60 kW m\({}^{-2}\), and saturation temperature from 25 \({}^{\circ}\)C to 35 \({}^{\circ}\)C, which is suitable for high temperature heat pumps or for electronics cooling applications. These working conditions, coupled with the tube diameter, correspond to a confinement number equal to 0.34, which falls into the mesoscale flow as suggested by [PERSON] and [PERSON] [18]. These experimental data will permit to validate empirical correlations from the literature for the estimation of heat transfer coefficient and frictional pressure drop. Furthermore, additional experimental tests at different mass velocities have permitted a comparison among the smooth tube and a microfin tube with an inner diameter at the fin tip of 2.4 mm, whose experimental tests were previously carried out, in order to highlight the effect of the internal fins on R1234 ze(E) flow boiling mechanism.