# Flow Condensation of Low-GWP Zeotropic Mixtures Inside 5 mm OD Micro-Finned Tube
## 3 Results and Discussions
### Effect of Vapor Quality and Mass Flux  
A preface is provided before the assessment of experimental results in the current and upcoming sections. To state the pellucid reasoning behind the discrepancies between the results of two refrigerants, five factual points concerning their thermophysical properties are hereby listed and will be cited in the upcoming sections (a complete list of such properties can be found in Table 2):  
1. Vapor density of 450A is 41% lower than R454B (average increment between 30 and 40 \({}^{\circ}\)C as saturation temperature). 2. Surface tension of 450A is 52% higher than R454B (average increment between 30 and 40 \({}^{\circ}\)C as saturation temperature). 3. Liquid viscosity of R450A is considerably higher than R454B. 4. Higher saturation temperature implies higher vapor density. 5. Higher saturation temperature implies lower liquid viscosity. 6. Higher temperature glide of R454B. The measured values of HTC are reported in Figure 4 as a function of vapor quality for different mass fluxes. The overall trend of the HTC values is identical for both refrigerants as the HTC elevates for higher vapor qualities. Regarding different mass fluxes, however, there is a large discrepancy in terms of the slope by which the trend varies at different vapor qualities. For instance, at low mass fluxes, the trend of HTC does not demonstrate a significant slope, while the trend at high mass fluxes conspicuously displays a large slope. The causality for this occurrence is quite deductive from the identification of a two-phase flow pattern whose role in deciding thermal performance is salient. It is noteworthy to refer to the flow pattern map of [PERSON] et al. (2018) that is constructed for condensation inside micro-finned tubes based on a dimensionless Martinelli parameter (\(X_{tt}\)) and non-dimensional gas velocity (\(J_{V}\)), whose expressions are provided in Equations (11) and (12). \[X_{tt}=\left(\frac{1-x}{x}\right)^{0.9}\times\left(\frac{\rho_{V}}{\rho_{L}} \right)^{0.5}\times\left(\frac{\mu_{L}}{\mu_{V}}\right)^{0.1} \tag{11}\]  
\[J_{V}=\frac{G\cdot x}{\sqrt{D\times\rho_{V}\times(\rho_{L}-\rho_{V})\times g}}. \tag{12}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Property** & **R450A@30 \({}^{\circ}\)C** & **R454B@30 \({}^{\circ}\)C** & **R450A@40 \({}^{\circ}\)C** & **R454B@40 \({}^{\circ}\)C** \\ \hline \(\Delta t_{glide}\) [K] & 0.64 & 1.33 & 0.64 & 1.33 \\ \(p_{sat}\) [bar] & 6.77 & 17.6 & 9.01 & 22.98 \\ \(\rho_{L}\) [kg m\({}^{-3}\)] & 1161 & 966 & 1121 & 914 \\ \(\rho_{V}\) [kg m\({}^{-3}\)] & 34.6 & 59.8 & 46.2 & 77.6 \\ \(\lambda_{L}\) [mW m\({}^{-1}\) K\({}^{-1}\)] & 74.8 & 102.8 & 70.9 & 96.1 \\ \(\lambda_{V}\) [mW m\({}^{-1}\) K\({}^{-1}\)] & 14.4 & 16.1 & 15.3 & 18.5 \\ \(\mu_{L}\) [uPa s] & 178 & 108 & 156.7 & 95.1 \\ \(\mu_{V}\) [uPa s] & 12.2 & 13.2 & 12.6 & 13.9 \\ \(\sigma_{L}\) [mN m\({}^{-1}\)] & 7.76 & 5.46 & 6.43 & 3.95 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Thermophysical properties. Based on flow visualizations and experimentally collected HTCs, authors suggested a transition line that was given the name of critical non-dimensional gas velocity (\(\dot{J}^{*}\
u\)), whose expression can be found in Equation (13). \[J^{*}_{V}=0.6\bigg{\{}\left[7.5/\left(4.3X_{tt}^{1,111}+1\right)\right]^{-3}+2.5 ^{-3}\bigg{\}}^{-1/3} \tag{13}\]  
The transition line clearly demarcates the gravitationally controlled flow patterns (stratified and intermittent) from the ones controlled by shear stress (annular flow). Plotting the current data points on the abovementioned map (Figure 5) further ascertained that the slope of the HTC trend is directly related to the flow patterns. For the cases under study, the annular flow patterns are never observed below a mass flux of 100 kg m\({}^{-2}\) s\({}^{-1}\). Since there exists a large discrepancy between wall liquid thickness at gravitationally controlled regimes as opposed to annular flow regimes, the HTC values never undergo a large alteration at low mass fluxes, regardless of the changes in vapor quality.