## 4 Experimental Results
### Experimental Heat Transfer Coefficients and Frictional Pressure Drops
The heat transfer coefficients tend to decrease as vapor quality increases, until fully dryout conditions are reached. This trend can be attributed to the high applied heat flux, which tends to intermittently dry local sections of the tube, and these dried sections are almost instantly rewetted by the liquid phase. As vapor quality increases, these dried sections get larger and larger, with consequent deterioration of the thermal performance of the tube, until dryout conditions are reached, i.e., the liquid phase is no longer able to rewet the surface of the tube. The effect of heat flux can be highlighted from Figure 4, which shows the heat transfer coefficients plotted versus the mean vapor quality for a saturation temperature at the inlet of the test section of 30 \({}^{\circ}\)C with a fixed mass velocity of 400 kg m\({}^{-2}\) s\({}^{-1}\). In the low vapor quality region, there is a clear effect of heat flux on the heat transfer coefficient: the higher the heat flux, the higher the heat transfer coefficient, due to the stronger contribution of nucleate boiling as heat flux increases. As vapor quality increases, the effect of vapor quality depends on the applied heat flux. At the lowest heat flux, as vapor quality increases, the heat transfer coefficient increases, meaning that the effect of convective boiling on the phase change process is gaining importance. When heat flux is increased at a value of 30 kW m\({}^{-2}\), the heat transfer coefficient does not increase anymore as vapor quality increases, so it seems that the higher contribution of nucleate boiling at \(HF\) = 30 kW m\({}^{-2}\), compared to the case at \(HF\) = 12 kW m\({}^{-2}\), mitigates the effect of convective boiling. At the highest tested heat flux of 60 kW m\({}^{-2}\), the heat transfer coefficient always decreases as vapor quality increases, due to the drying-rewetting process that appears on the wall surface of the tube until fully dryout conditions are reached. Similar trends were observed for the other mass velocities. Further experimental tests were run at different saturation temperatures, in order to highlight the effect of this working parameter. Saturation temperatures of 25 \({}^{\circ}\)C and 35 \({}^{\circ}\)C were also investigated, and they correspond to pressures of 4.985 bar and 6.674 bar, respectively. Figure 4 reports the heat transfer coefficient plotted against the mean vapor quality at different working conditions. A higher saturation temperature implies a higher reduced pressure, with a consequent higher effect of nucleate boiling. At the same time, a higher saturation temperature implies a higher vapor density, i.e., a lower vapor velocity at constant mass velocity, with a consequent lower effect of convective boiling. These considerations will help to understand the trends of Figure 5. Data at a mass velocity of 300 kg m\({}^{-2}\) s\({}^{-1}\) with an imposed heat flux of 12 kW m\({}^{-2}\) for three different saturation temperatures are reported in Figure 5a. In this case, as saturation temperature increases, the higher augmentation of nucleate boiling contribution is higher than the reduction of convective boiling due to a low mass velocity, and, as a result, the highest heat transfer coefficients are shown at \(t_{sat}\) = 35 \({}^{\circ}\)C. At a higher mass velocity of 600 kg m\({}^{-2}\) s\({}^{-1}\) at the same imposed heat flux of 12 kW m\({}^{-2}\) (Figure 5b), the trends are different. Due to the higher mass velocity, the effect of convective boiling is higher in the phase change process. In this case, the heat transfer coefficients seem to overlap between each other, and so the augmentation of nucleate boiling at the highest saturation temperature is somehow mitigated by the lower contribution of convective boiling at the highest saturation temperature. The situation changes at the highest heat flux, where the phase change mechanism is mainly dominated by nucleate boiling.