## 4 Experimental Results and Discussion
### Condensation Heat Transfer Coefficient Results and Discussion  
The condensation heat transfer coefficient of R134a at saturation temperatures of 20, 25 and 30 \({}^{\circ}\)C, mass fluxes ranging from 50 to 300 kg m\({}^{-2}\)s\({}^{-1}\) and vapor qualities from 0 to 1 were investigated. The effects of mass flux, vapor quality, saturation temperature and tube diameter on condensation heat transfer were analyzed. It was expected that the refrigerant's flow pattern would influence the mechanism of heat transfer during condensation process. Therefore, to categorize the condensation flow pattern inside the test tube, the present experimental data were predicted by two widely used flow pattern maps proposed by [PERSON] [28] and [PERSON] et al. [29]. Figure 9 shows the experimental data plotted on the [PERSON] [28] flow pattern map. In this flow pattern map, at a low mass flux of 50 kg m\({}^{-2}\)s\({}^{-1}\) for both tubes, wavy flow occurred in the entire range of quality. At mass fluxes of 100-300 kg m\({}^{-2}\)s\({}^{-1}\), many flow regimes occurred as the quality changed, and maximum of the data fall in annular flow and annular wavy regime. Moreover, few data obtained at the low vapor quality (\(x<0.3\)) points fall into the slug and plug flow region. In the [PERSON] flow pattern map shown in Figure 10, most of the experimental data fall in the annular and semi-annular flow region, although very few data obtained at the low vapor quality (\(x<0.2\)) fall in the slug flow region. Figure 11 shows the effect of mass flux and vapor quality on condensation heat transfer coefficient by varying three mass fluxes. The figure also shows the heat transfer coefficient comparison of smooth and microfin tubes. For both tubes, the heat transfer coefficient increased significantly with an increase in mass flux, especially when the mass flux was larger than 50 kg m\({}^{-2}\)s\({}^{-1}\). At mass flux 300 kg m\({}^{-2}\)s\({}^{-1}\), the heat transfer coefficient was higher than at the two other mass fluxes because, at this mass flux, the data fall in annular flow region (shear dominated flow region), as shown in Figures 9 and 10. Moreover, in shear dominated flow regime, the local heat transfer was primarily affected by the mass flux and vapor quality [30]. However, the heat transfer coefficient was higher at the beginning of condensation for all mass fluxes and decreased as vapor shear stress decreased. A comparison of the results, for microfin and smooth tube, shows that the heat transfer coefficient of microfin tube was higher than that of smooth tube, for all mass flux conditions because the microfin provides additional heat transfer area and thin condensate film. The heat transfer coefficient of the microfin tube was about 2-5 times higher than that of the smooth tube at 100 kg m\({}^{-2}\)s\({}^{-1}\). Figure 8: Comparison between the experimental and predicted frictional pressure drop using correlations by: (**a**) [PERSON] et al. [2]; (**b**) [PERSON] [25]; (**c**) [PERSON] et al. [26]; and (**d**) [PERSON] et al. [27]. Figure 10: Flow pattern prediction by [PERSON] flow pattern map [29]. Figure 9: Flow pattern prediction by [PERSON] flow pattern map [28]. Saturation temperature has an effect on condensation heat transfer coefficient. Experimental studies were conducted by changing the saturation temperature at a fixed mass flux of 100 kg m\({}^{-2}\)s\({}^{-1}\). Figure 12a shows the effect of saturation temperature on the heat transfer coefficient at saturation temperatures of 20 and 30 \({}^{\circ}\)C for the smooth tube. Figure 12b shows the effect of saturation temperature on the heat transfer coefficient at saturation temperatures of 20 and 25 \({}^{\circ}\)C for microfin tube. According to the experimental results, when saturation temperature increased, the heat transfer coefficient decreased. Basically, when saturation temperature increases, system pressure also increases, thus the vapor velocity decreases due to the increase in vapor density.