## 3 Experimental Results
### Heat Transfer Coefficients
The experimental results showed a weaker effect of the heat flux for R1234 ze(E) compared to R450A: at a mean vapor quality of 0.3, the heat transfer coefficients at \(HF\) = 30 and 50 kW m\({}^{-2}\) were approximately 5% and 20% higher, respectively, than those at \(HF\) = 10 kW m\({}^{-2}\). The higher convective boiling contribution could be particularly seen from the trend of the heat transfer coefficient at \(HF\) = 10 kW m\({}^{-2}\): the heat transfer coefficient at \(x_{mean}\) = 0.8 was approximately 75% higher than that at \(x_{mean}\) = 0.3. The trend of R515B has not been reported in the figure because it is similar  
Figure 4: Heat transfer coefficient vs. mean vapor quality at \(t_{sat}\) = 30 \({}^{\circ}\)C at \(G\) = 150 kg m\({}^{-2}\) s\({}^{-1}\) for R450A (**a**) and R1234 ze(E) (**b**). HF10 = 10 kW m\({}^{-2}\), HF30 = 30 kW m\({}^{-2}\), and HF50 = 50 kW m\({}^{-2}\). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **R450A** & **R515B** & **R1234 ze(E)** \\ \hline \(p\) [bar] & 6.77 & 5.75 & 5.78 \\ \(p_{red}\) [-] & 0.177 & 0.160 & 0.159 \\ \(\rho_{\rm L}\) [kg m\({}^{-3}\)] & 1161 & 1164 & 1146 \\ \(\rho_{V}\) [kg m\({}^{-3}\)] & 34.6 & 31.3 & 30.5 \\ \(\lambda_{L}\) [W m\({}^{-1}\) K\({}^{-1}\)] & 0.075 & 0.072 & 0.073 \\ \(\lambda_{V}\) [W m\({}^{-1}\) K\({}^{-1}\)] & 0.014 & 0.014 & 0.014 \\ \(\mu_{L}\) [Pa s] & \(178\times 10^{-6}\) & \(181\times 10^{-6}\) & \(179\times 10^{-6}\) \\ \(\mu_{V}\) [Pa s] & \(12.2\times 10^{-6}\) & \(12.5\times 10^{-6}\) & \(12.5\times 10^{-6}\) \\ \(\sigma\) [N m\({}^{-1}\)] & \(7.8\times 10^{-3}\) & \(8.1\times 10^{-3}\) & \(8.2\times 10^{-3}\) \\ \(r\) [] kg\({}^{-1}\)] & 166,340 & 158,240 & 163,060 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Thermophysical properties of R450A, R515B, and R1234 ze(E) at a saturation temperature of 30 \({}^{\circ}\)C.  
to that of R1234 ze(E). Furthermore, the higher surface tension of R1234 ze(E), compared to R450A, facilitates the retention of liquid in the throat between two adjacent microfins, enhancing the heat transfer mechanism, especially at _HF_ = 10 kW m\({}^{-2}\), i.e., when dryout effects are small. A direct comparison between R450A, R515B, and R1234 ze(E) is shown in Figure 5, which reports data at _HF_ = 10 kW m\({}^{-2}\) (Figure 5a) and at _HF_ = 50 kW m\({}^{-2}\) (Figure 5b), considering the lowest and highest mass velocities tested for each heat flux. R515B and R1234 ze(E) showed similar values of the heat transfer coefficient at both _HF_ = 10 and 50 kW m\({}^{-2}\); indeed, these two fluids have similar thermophysical properties. At _HF_ = 10 kW m\({}^{-2}\) and \(G\) = 400 kg m\({}^{-2}\) s\({}^{-1}\), which is a condition where convective boiling highly affects the phase change process, R515B and R1234 ze(E) had considerable higher heat transfer coefficients compared to R450A, due to its weaker convective properties. The situation changed at _HF_ = 50 kW m\({}^{-2}\), in particular at \(G\) = 100 kg m\({}^{-2}\) s\({}^{-1}\), which is a condition where nucleate boiling plays a strong role in the flow boiling process: The properties more favorable to nucleate boiling of R450A seem to mitigate its properties less favorable to convective boiling, and as a result, the heat transfer coefficients of R450A are close to those of R515B and R1234 ze(E).