## 3 Experimental Results
### Frictional Pressure Gradients  
This section presents the results regarding frictional pressure drops. The following figures present the frictional pressure gradients, obtained by subtracting the momentum term from the total measured pressure drop, dividing the result for the distance between the pressure ports, which is equal to 0.8 m.  
Figure 6 shows the frictional pressure gradient as a function of the mean vapor quality. It reports data for R450A, and the trend was similar for R515B and R12345 ze(E) anyway. The results are reported for a heat flux of 50 kW m\({}^{-2}\) at \(G\) = 100, 150, 200, and 300 kg m\({}^{-2}\) s\({}^{-1}\), whereas all the three heat fluxes are reported for a mass velocity of 400 kg m\({}^{-2}\) s\({}^{-1}\). The frictional pressure gradients at \(G\) = 100 kg m\({}^{-2}\) s\({}^{-1}\) were those that were less affected by the mean vapor quality; indeed, as previously reported, most of the data at this mass velocity were in the stratified flow regime. The classical two-phase pressure drop behavior was confirmed: at constant vapor quality, the frictional pressure gradient increased as mass velocity increased. At constant mass velocity, the frictional pressure gradient increased as vapor quality increased, reaching a maximum value at high vapor quality, after which it decreased as vapor quality further increased. The decrease after the peak is due to liquid entrainment: some liquid droplets were entrained in the liquid  
Figure 5: Heat transfer coefficient vs. mean vapor quality at _t_sat = 30 °C for R450A, R515B, and R1234 ze (E) at _HF_ = 10 kW m\({}^{-2}\) (**a**) and _HF_ = 50 kW m\({}^{-2}\) (**b**). G50 = 50 kg m\({}^{-2}\) s\({}^{-1}\), G100 = 100 kg m\({}^{-2}\) s\({}^{-1}\), and G400 = 400 kg m\({}^{-2}\) s\({}^{-1}\).  
phase at high vapor quality during shear-stress-dominated flow regimes, and this is similar to the effect of higher reduced pressure. Moreover, the liquid interface roughness decreased at high vapor qualities. The figure also shows the effect of heat flux. Only the case at \(G\) = 400 kg m\({}^{-2}\) s\({}^{-1}\) is reported in the figure, but it was similar for the other mass velocities. The heat flux had no effect when the frictional pressure gradient linearly increased with vapor quality. It slightly affected the frictional pressure gradient in the high-vapor-quality region, near the peak of the curve, mainly due to the different vapor quality difference in the test section, at the three heat fluxes, associated with the reported mean vapor quality.  
The two-phase trend was similar among the three tested refrigerants but with different values. A comparison among the hydraulic behaviors of R450A, R515B, and R1234 ze(E) is proposed in Figure 7. The figure shows data with mass velocities of 200 and 400 kg m\({}^{-2}\) s\({}^{-1}\), with an imposed heat flux of 10 kW m\({}^{-2}\). First, the flow regimes were almost the same for the three refrigerants under the same working conditions, so the majority of the data at \(G\) = 200 kg m\({}^{-2}\) s\({}^{-1}\), and even more at \(G\) = 400 kg m\({}^{-2}\) s\({}^{-1}\), were in annular flow regime. R515B and R1234 ze(E) showed similar values of the frictional pressure gradient, whereas the values of R450A were approximately 7% lower at \(G\) = 400 kg m\({}^{-2}\) s\({}^{-1}\). Indeed, the vapor density of R450A was the highest among the three tested fluids (see Table 3): it was about 10% and 13% higher than that of R515B and R1234 ze(E), respectively. Thus, at constant mass velocity, R450A has a lower vapor velocity, which leads to lower frictional pressure gradients.