## 4 Experimental Results
### Experimental Heat Transfer Coefficients and Frictional Pressure Drops  
Figure 3 shows the collected R1234 ze(E) flow boiling heat transfer coefficients plotted against the mean vapor quality with a constant saturation temperature at the inlet of the test section of 30 \({}^{\circ}\)C for two different imposed heat fluxes: 12 kW m\({}^{-2}\) (Figure 3a) and 60 kW m\({}^{-2}\) (Figure 3b). These figures permit to highlight the effect of both vapor quality and mass velocity. At the lowest heat flux, there is a strong effect of mass velocity. Mass velocity \(G\) = 200 kg m\({}^{-2}\) s\({}^{-1}\) shows almost no effect of vapor quality, with a constant heat transfer coefficient at a value of around 6000 W m\({}^{-2}\) K\({}^{-1}\), until dryout conditions are reached at a mean vapor quality of around 0.8. At a mass velocity \(G\) = 300 kg m\({}^{-2}\) s\({}^{-1}\), the heat transfer coefficients overlap with those at \(G\) = 200 kg m\({}^{-2}\) s\({}^{-1}\) at low vapor quality, but the heat transfer coefficient slightly increases as vapor quality increases, meaning that convective boiling starts to play its role in the flow boiling mechanism. As mass velocity further increases, there is a clear effect of mass velocity and of vapor quality on the heat transfer coefficient: the higher the vapor quality and mass velocity, the higher the heat transfer coefficient, meaning that convective boiling is affecting the phase change mechanism. Furthermore, the highest tested mass velocity shows the highest dependence on vapor quality, i.e., the slope of the heat transfer coefficients respect to vapor quality is the highest among the tested mass velocities. Figure 3: Heat transfer coefficient versus mean vapor quality at \(t_{sat}\) = 30 \({}^{\circ}\)C for different mass velocities. (**a**) Data at \(HF\)=12 kW m\({}^{-2}\); (**b**) data at \(HF\) = 60 kW m\({}^{-2}\). \(G\) expressed in (kg m\({}^{-2}\) s\({}^{-1}\)). The mean thermodynamic vapor quality where the heat transfer coefficient starts to decrease should not be intended as the local vapor quality at the onset of dryout, because data reported in the figure represent average conditions between inlet and outlet of the test section, not local conditions. The thermodynamic vapor quality difference between inlet and outlet of the test section varies from about 0.05 at \(G\) = 600 kg m\({}^{-2}\) s\({}^{-1}\) up to about 0.14 at \(G\) = 200 kg m\({}^{-2}\) s\({}^{-1}\) at \(HF\) = 12 kW m\({}^{-2}\). It is also interesting to highlight that the dryout at \(G\) = 200 kg m\({}^{-2}\) s\({}^{-1}\) at \(HF\) = 12 kW m\({}^{-2}\) leads to a temperature increase, along the test section, of about 1.5 K, with an almost negligible saturation temperature drop (lower than 0.2 K) due to pressure drop. Therefore, the heat transfer coefficient at \(G\) = 200 kg m\({}^{-2}\) s\({}^{-1}\) decreases at high mean thermodynamic vapor qualities. On the contrary, at high mass velocities, the saturation temperature drop is not negligible. Considering the mass velocity \(G\) = 600 kg m\({}^{-2}\) s\({}^{-1}\), at high vapor quality, the saturation temperature drop due to pressure drop is about 1.5 K, which tends to balance the temperature increase due to dryout. Therefore, dryout, intended as a drop of the heat transfer coefficient's trend, is not shown at high mass velocities at \(HF\) = 12 kW m\({}^{-2}\). When heat flux is increased to \(HF\) = 60 kW m\({}^{-2}\) (Figure 3b), the situation completely changes. First of all, the heat transfer coefficients in the low vapor quality region are higher than those at \(HF\) = 12 kW m\({}^{-2}\), meaning that a higher contribution of nucleate boiling affects the flow boiling mechanism, due to the higher heat flux. Another evidence of the higher nucleate boiling contribution, with almost negligible effect of convective boiling, is the absence of the effect of mass velocity, indeed the heat transfer coefficients are not affected by mass velocity, and the values almost overlap at different mass velocities.