## 3 Experimental Results
### Heat Transfer Coefficients
The main difference among the three tested fluids is related to the effect of vapor quality on the heat transfer coefficients at low heat flux and high vapor quality, where convective boiling is the main heat transfer mechanism. R515B and R1234 ze(E) seem to be fluids with better convective features, compared to R450A, i.e., they presented a higher value of vapor velocity at the same mass velocity, compared to R450A, due to their lower vapor densities. As a result, they presented higher values of the heat transfer coefficient at high vapor quality and high mass flux. Furthermore, it is interesting to highlight that even though the three tested fluids had slightly different thermophysical properties, as shown later, the vapor quality differences between the inlet and the outlet of the test section were quite similar for the three refrigerants. This is due to the similar latent heats of vaporization, which were about 166 kJ kg\({}^{-1}\), 158 kJ kg\({}^{-1}\), and 163 kJ kg\({}^{-1}\), at a saturation temperature of 30 \({}^{\circ}\)C, for R450A, R515B, and R1234 ze(E), respectively. Table 2 shows the vapor quality variations for the data reported in Figure 3.  
A more detailed comparison between R450A, R515B, and R1234 ze(E) can start from the analysis of Table 3, which reports the main thermophysical properties of the three refrigerants at a saturation temperature of 30 \({}^{\circ}\)C. Being an azeotropic mixture made of 91.1% of R1234 ze(E) by mass, R515B has properties close to those of R1234 ze(E), and this was later reflected also in the experimental results of the heat transfer coefficient and the frictional pressure drop. R450A is a refrigerant that, among the three fluids tested here, has the most diverse properties. R450A has a higher vapor density, with a consequent lower vapor velocity at the same mass velocity, which is reflected in the convective behavior. R450A has a higher reduced pressure, which implies a higher nucleate boiling contribution to the phase change mechanism. \begin{table}
\begin{tabular}{c c c c c} \hline
**HF** & **G** & **\(\Delta\)x (R450A)** & **\(\Delta\)x (R515B)** & **\(\Delta\)x (R1234 ze(E))** \\
**[kW m\({}^{-2}\)]** & **[kg m\({}^{-2}\) s\({}^{-1}\)]** & **[-]** & **[-]** & **[-]** \\ \hline
10 & 50 & 0.185 & 0.194 & 0.189 \\
10 & 75 & 0.123 & 0.130 & 0.126 \\
10 & 100 & 0.092 & 0.097 & 0.094 \\
10 & 150 & 0.062 & 0.065 & 0.063 \\
10 & 200 & 0.046 & 0.049 & 0.047 \\
10 & 300 & 0.031 & 0.032 & 0.031 \\
10 & 400 & 0.023 & 0.024 & 0.024 \\
50 & 100 & 0.462 & 0.486 & 0.471 \\
50 & 150 & 0.308 & 0.324 & 0.314 \\
50 & 200 & 0.231 & 0.243 & 0.236 \\
50 & 300 & 0.154 & 0.162 & 0.157 \\
50 & 400 & 0.116 & 0.121 & 0.118 \\ \hline \end{tabular}
\end{table} 
Table 2: Vapor quality differences for the data reported in Figure 3 at \(t_{sat}\) = 30 \({}^{\circ}\)C.  
The different effects of the heat flux and mean vapor quality of R450A and R1234 ze(E) on the heat transfer coefficient are shown in Figure 4, which reports the heat transfer coefficient plotted versus the mean vapor quality for a mass flux of 150 kg m\({}^{-2}\) s\({}^{-1}\) at three different heat fluxes for R450A (Figure 4a) and R1234 ze(E) (Figure 4b). As already stated in the analysis of the thermophysical properties reported in Table 3, R450A has properties that enhance the nucleate boiling effects compared to R1234 ze(E) and R515B, whereas R1234 ze(E) and R515B have properties that enhance the convective boiling contribution. Indeed, there is a strong effect of the heat flux on the heat transfer coefficients collected for R450A until a mean vapor quality of about 0.8: the higher the heat flux, the higher the heat transfer coefficient. At a mean vapor quality of 0.3, the heat transfer coefficients at \(HF\) = 30 and 50 kW m\({}^{-2}\) were approximately 37% and 59% higher, respectively, than those at \(HF\) = 10 kW m\({}^{-2}\). At a heat flux of 10 kW m\({}^{-2}\), the heat transfer coefficient at \(x_{mean}\) = 0.8 was approximately 63% higher than that at \(x_{mean}\) = 0.3. The behavior is different for R1234 ze(E).