## 5 Conclusions  
The paper presented the results of an experimental campaign aimed at studying R450A, R515B, and R1234 ze(E) during flow boiling inside a microfin tube with an inner diameter at the fin tip of 6.14 mm. To understand how each working parameter affects flow boiling behavior, the mass velocity was varied from 50 to 400 kg m\({}^{-2}\) s\({}^{-1}\), and the heat flux from 10 to 50 kW m\({}^{-2}\). The main outcomes can be summarized as follows:  
* At the lowest tested heat flux in the range of 75-300 kg m\({}^{-2}\) s\({}^{-1}\), the higher the mass velocity and vapor quality, the higher the heat transfer coefficient, that is, convective boiling is the main phase change process. At \(G\) = 400 kg m\({}^{-2}\) s\({}^{-1}\), the heat transfer coefficients in the high-vapor-quality region are lower than those at \(G\) = 300 kg m\({}^{-2}\) s\({}^{-1}\). A possible explanation is that the tips of the fins may be partly dried due to the high shear stress, deteriorating the heat transfer process. Figure 10: Calculated vs. experimental frictional pressure gradient. Model developed by [PERSON] et al. [39]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Model** & **Fluid** & \(dev_{rel}\) & \(dev_{abs}\) \\ \hline \multirow{2}{*}{[PERSON] and [PERSON] [43]} & R450A & \(-16.5\%\) & 22.3\% \\  & R515B & \(-12.7\%\) & 20.4\% \\  & R1234 ze(E) & \(-7.3\%\) & 18.9\% \\ \hline \multirow{2}{*}{[PERSON] et al. [39]} & R450A & \(-10.4\%\) & 13.5\% \\  & R515B & \(-5.6\%\) & 10.3\% \\  & R1234 ze(E) & 1.7\% & 9.4\% \\ \hline \multirow{2}{*}{[PERSON] and [PERSON] [40]} & R450A & \(-13.1\%\) & 22.1\% \\  & R515B & \(-9.0\%\) & 21.6\% \\ \multirow{2}{*}{[40]} & R1234 ze(E) & \(-2.9\%\) & 17.8\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Accuracy of implemented correlations for pressure gradient. * At the highest tested heat flux, the effect of mass velocity in the range of 150-400 kg m\({}^{-2}\) s\({}^{-1}\) is weak, as well as the effect of vapor quality: under these conditions, nucleate boiling plays an important role in the phase change mechanism. * R51 SB and R1234 ze(E) show values of heat transfer coefficients and pressure drops close to each other under all the investigated operative conditions. R450A shows considerably lower heat transfer coefficients at \(G\) = 400 kg m\({}^{-2}\) s\({}^{-1}\) and \(HF\) = 10 kW m\({}^{-2}\), i.e., at low heat flux and high mass velocity, whereas the gap of the thermal performance between the fluids reduces at high heat flux. * The values of the frictional pressure gradient for R515B and R1234 ze(E) are similar, whereas the values shown by R450A are approximately 7% lower than those shown by R515B and R1234 ze(E). * The experimental heat transfer coefficients and pressure drops were compared against models available in the literature. The model of [PERSON] et al. [39] satisfactorily predicts experimental heat transfer coefficients, with a relative and absolute deviation of \(-\)5.5% and 8.9%, respectively, and frictional pressure gradients, with a relative and absolute deviation of \(-\)4.6% and 11.1%, respectively. Moreover, the correction factor \(F_{C}\), needed to account for the temperature glide in the estimation of the nucleate boiling contribution, can be neglected for R450A. * Considering the model suggested by [PERSON] et al. [39], a detailed analysis about nucleate and convective boiling was presented. This analysis showed how the contribution of nucleate boiling and convective boiling varies under the different working conditions of mass velocity, heat flux, and vapor quality. Conceptualization, A.D.; validation, Y.L. and A.D.; investigation, Y.L. and A.D.; writing--original draft preparation, A.D.; writing--review and editing, Y.L. and L.R. ; supervision, L.R. ; funding acquisition, L.R. All authors have read and agreed to the published version of the manuscript. This research was funded by the Italian Ministry of Education, Universities and Research-MIUR, PRIN Project number 2017F7 KZWS. Not applicable. Data will be made available on request.