## 3 Experimental Results
### Preliminary Considerations
\(h_{L}\) and \(h_{V}\) are the saturated liquid and vapor specific enthalpies, respectively, which are calculated using the knowledge of the inlet pressure (in the case of \(x_{in}\)) or outlet pressure (in the case of \(x_{out}\)). Following the procedure suggested by [PERSON] and [PERSON] [26], the mean uncertainty on the mean vapor quality was \(\pm\)0.028, whereas the minimum and maximum values were \(\pm\)0.021 and \(\pm\)0.039, respectively. Experimental tests were carried out at different mass velocities and heat fluxes by varying the inlet vapor quality. Only data in saturated conditions were considered, that is, the maximum tested outlet vapor quality was around 0.99. The mass velocity is defined as the refrigerant mass flow rate divided by the cross-sectional area calculated with the fin tip diameter:  
\[G=\frac{4\cdot\dot{m}_{ref}}{\pi\cdot D_{L}^{2}} \tag{6}\]  
It is worth remembering that the accuracy of the Coriolis flowmeter, which measures the mass flow rate, is \(\pm\)0.1% of the reading. The heat flux is the heat flow rate exchanged in the test section divided by \(A_{D}\), previously defined:  
\[HF=\frac{q_{TS}}{A_{D}} \tag{7}\]  
The mass velocity \(G\) was varied from 50 to 400 kg m\({}^{-2}\) s\({}^{-1}\). The heat flux \(HF\) was set at 10, 30, and 50 kW m\({}^{-2}\). Following the procedure suggested by [PERSON] and [PERSON] [26], the mean uncertainty of the heat flux was \(\pm\)2.2%, whereas the minimum and maximum values were \(\pm\)1.3% and \(\pm\)4.1%, respectively. About 480 data points were obtained during the experimental campaign. Every single data point derived from calculations over 100 readings taken at a frequency of 1 Hz in steady-state conditions. This means that every single datum was obtained by averaging 100 readings for each working parameter under steady-state conditions, that is, when the standard deviation of each working parameter was below the experimental uncertainty. Only some data during dryout conditions presented a standard deviation on the wall temperatures and outlet water temperature higher than the experimental uncertainty of the calibrated thermocouples, due to the intrinsic unsteady behavior of the dryout itself. Furthermore, the data set at \(G\) = 100 kg m\({}^{-2}\) s\({}^{-1}\) and \(HF\) = 30 kW m\({}^{-2}\) with the refrigerant R1234 ze(E) was carried out twice as a repeatability test, and the relative experimental results among the two sets were within the experimental uncertainties. The differential pressure transducer allows measuring the total pressure drop \(\Delta p_{\rm TOT}\), which is equal to the sum of three contributions: friction (\(\Delta p_{t}\)), momentum (\(\Delta p_{\rm m}\)), and gravity (\(\Delta p_{\rm g}\)):  
\[\Delta p_{TOT}=\Delta p_{f}+\Delta p_{m}+\Delta p_{g} \tag{8}\]  
The tube is horizontally located, so the gravity contribution is null (\(\Delta p_{\rm g}=0\)). Applying the separated flow model, the momentum contribution, which considers the acceleration effects due to the variation of the vapor quality along the thermal part of the test section, can be calculated as follows:  
\[\Delta p_{m}=G^{2}\cdot\left\{\left[\frac{x^{2}}{\rho_{V}\times\varepsilon}+ \frac{(1-x)^{2}}{\rho_{L}\times(1-\varepsilon)}\right]_{out}-\left[\frac{x^{2} }{\rho_{V}\cdot\varepsilon}+\frac{(1-x)^{2}}{\rho_{L}\times(1-\varepsilon)} \right]_{in}\right\} \tag{9}\]  
where \(\varepsilon\) is the void fraction. \(\rho_{V}\) and \(\rho_{L}\) are the saturated vapor and liquid densities, respectively, and \(x\) is the vapor quality. The model of [PERSON] and [PERSON] [27] was implemented to evaluate the void fraction, needed in Equation (9). The frictional contribution was thus calculated as the difference between the total measured pressure drop and the calculated momentum pressure drop.