## 5 Comparisons  
### Comparison against an Equivalent Microfin Tube  
The thermal and hydraulic performances, i.e., heat transfer coefficient and frictional pressure gradient, of the 2.5 mm ID smooth tube are compared against those of a microfin tube under the same working conditions. The microfin tube has an inner diameter at the fin tip of 2.4 mm, the fin height is 0.12 mm (i.e., the fin root diameter is 2.64 mm), it has 40 fins with an apex angle of 43\({}^{\circ}\), and the helix angle is 7\({}^{\circ}\). The microfin tube is heated by Joule effect from the bottom by means of a nickel chrome wire resistance connected to a DC power supplier, similarly to the 2.5 mm ID smooth tube. Experimental data for the microfin tube, which are needed for the comparison, are borrowed by [PERSON] et al. [21]. In order to match the experimental working conditions of the microfin tube, additional tests were run during R1234 ze(E) flow boiling inside the smooth tube at mass velocities of 375, 565, and 755 kg m\({}^{-2}\) s\({}^{-1}\), allowing a direct comparison between the performances of the two tubes. Figure 7 reports the comparison between the thermal performances of the two tubes. The effect of mass velocity on the different characteristics of the two tubes is reported in Figure 7a, which shows the heat transfer coefficient plotted versus the mean vapor quality for a saturation temperature at the inlet of the test section of 30 \({}^{\circ}\)C with an imposed heat flux of 30 kW m\({}^{-2}\). At first sight, the heat transfer coefficients of the microfin tube get higher and higher compared to the smooth tube as mean vapor quality increases, meaning that the microfin tube exhibits a higher contribution of convective boiling in the phase change process. The smooth tube shows almost no effect of vapor quality for mass velocity 375 and 565 kg m\({}^{-2}\) s\({}^{-1}\), whereas a weak effect can be seen with mass velocity of 755 kg m\({}^{-2}\) s\({}^{-1}\), i.e., convective boiling weakly affects the flow boiling mechanism only at high mass velocity in the case of the smooth tube at this heat flux. On the contrary, in case of the microfin tube, heat transfer coefficient always increases as vapor quality increases. The effect of mass velocity on the heat transfer coefficient is different among the two tubes. In the case of the smooth tube, the highest heat transfer coefficients are shown at the highest mass velocity of 755 kg m\({}^{-2}\) s\({}^{-1}\), whereas, in the case of the microfin tube, the highest heat transfer coefficients are shown at the lowest mass velocity of 375 kg m\({}^{-2}\) s\({}^{-1}\), probably  
Figure 6: Frictional pressure gradient versus mean vapor quality. (**a**) Data at \(t_{sat}\) = 30 \({}^{\circ}\)C and \(H\)F = 30 kW m\({}^{-2}\) at different mass velocity; (**b**) data at \(H\)F = 12 kW m\({}^{-2}\) and \(G\) = 600 kg m\({}^{-2}\) s\({}^{-1}\) at different saturation temperature. due to a drying effect at the fin tips, which is more emphasized as mass velocity, i.e., shear stress, increases. The effect of the heat flux is reported in Figure 7b, which shows the heat transfer coefficients plotted against the mean vapor quality for a saturation temperature at the inlet of the test section of 30 \({}^{\circ}\)C at a fixed mass velocity of 565 kg m\({}^{-2}\) s\({}^{-1}\). The heat transfer coefficients of the microfin tube always increase with vapor quality, even at the highest tested heat flux, meaning that convective boiling, besides nucleate boiling, plays its role in the flow boiling mechanism at 60 kW m\({}^{-2}\). This consideration is no longer valid for the smooth tube, for which the heat transfer coefficient increases with vapor quality at \(HF\) = 12 kW m\({}^{-2}\), it is almost not affected by vapor quality at \(HF\) = 30 kW m\({}^{-2}\), and it decreases with vapor quality at \(HF\) = 60 kW m\({}^{-2}\) due to the drying-rewetting process, i.e., the effect of convective boiling tends to be weakened as heat flux increases.