## 3 Experimental Results
### Heat Transfer Coefficients  
The effects of mass velocity and vapor quality on the flow boiling heat transfer coefficient are depicted in Figure 3 for the three tested fluids. Figure 3a reports the case for R450A with an imposed heat flux of 10 kW m\({}^{-2}\). The heat transfer coefficients at \(G\) = 50 kg m\({}^{-2}\) s\({}^{-1}\) were almost not affected by the mean vapor quality, showing a value of around 3500 W m\({}^{-2}\) K\({}^{-1}\). As the mean vapor quality increased, the vapor velocity increased, but at this low value of mass velocity, the convective mechanisms, which depend on the vapor velocity and are typical of annular flow regimes, are not involved in the phase change mechanism. As suggested by [PERSON] et al. 's flow patterns map [32], most of the data points at mass velocities of 50, 75, 100 kg m\({}^{-2}\) s\({}^{-1}\) were in the wavy-stratified flow. This was confirmed by the flow pattern map developed by [PERSON] et al. [33] suggested for microfin tubes by [PERSON] et al. [34]. However, for higher mass velocities, the heat transfer coefficients increased as vapor quality increased, highlighting an increasingly stronger contribution of convective boiling as mass velocity increased. At \(HF\) = 10 kW m\({}^{-2}\), the heat transfer coefficient increased as mass velocity increased from 50 to 300 kg m\({}^{-2}\) s\({}^{-1}\), but a further increase in mass velocity does not imply anymore a higher heat transfer coefficient in the high mean vapor quality region. The flattening of the heat transfer coefficient's curve at \(G\) = 400 kg m\({}^{-2}\) s\({}^{-1}\) can be attributed to presence of the fins, which, due to the combined effect of heat flux and high shear stress at high vapor qualities, may be partly dried, deteriorating the heat transfer performance. The effects of vapor quality and mass velocity were different when the applied heat flux was set at 50 kW m\({}^{-2}\) (Figure 3b). First, the heat transfer coefficients in the low vapor quality region were higher than those at \(HF\) = 10 kW m\({}^{-2}\). For instance, at \(G\) = 400 kg m\({}^{-2}\) s\({}^{-1}\) and \(x_{mean}\) = 0.2, the heat transfer coefficient was approximately 50% higher than the correspondent value at \(HF\) = 10 kW m\({}^{-2}\). This is attributed to the higher contribution of nucleate boiling because of the higher heat flux. The stronger effect of nucleate boiling seems to mitigate the effect of convective boiling; indeed, the heat transfer coefficient is almost not affected by vapor quality, and a weak effect of mass velocity can be appreciated when it increases from 150 to 400 kg m\({}^{-2}\) s\({}^{-1}\). On the contrary, the effect of mass velocity was highlighted when mass velocity increased from 100 to 150 kg m\({}^{-2}\) s\({}^{-1}\), mainly due to an early fully dryout at \(G\) = 100 kg m\({}^{-2}\) s\({}^{-1}\). The fact that the heat transfer coefficient does not dramatically decrease at high vapor quality does not imply that dryout does not occur: the reported heat transfer coefficients are an average value along the test section, and only the last part of the test section may experience dryout conditions. Furthermore, it is proven from the open literature [35; 36; 37; 38] and from our resultsthat microfin tubes tend to make the onset of dryout happen at higher vapor qualities compared to a smooth tube under the same working conditions. Figure 3c-f report the experimental heat transfer coefficients for R515B and R1234 ze(E). The trend was similar at \(HF\) = 50 kW m\({}^{-2}\), i.e., the heat transfer coefficients were weakly affected by vapor quality, since it is a condition where nucleate boiling plays an important role; the effect of mass velocity was weak in the mass velocity range 150-400 kg m\({}^{-2}\) s\({}^{-1}\), whereas the heat transfer coefficients were lower at \(G\) = 100 kg m\({}^{-2}\) s\({}^{-1}\).