## 4 Experimental Results and Discussion  
### Frictional Pressure Drop Results and Discussion  
The adiabatic pressure drop of R134a at saturation temperatures of 20 and 30 \({}^{\circ}\)C, mass fluxes ranging from 50 to 200 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 of the frictional pressure drop were analyzed. Figure 5a,b shows the effects of mass flux and vapor quality on the frictional pressure drop, in smooth and microfin tubes for mass fluxes of 50, 75 and 100 kg m\({}^{-2}\)s\({}^{-1}\) at a saturation temperature of 20 \({}^{\circ}\)C. As shown in Figure 5a,b, the frictional pressure drop increased as the mass flux increased. For a given mass flux, the pressure drop increased slightly at the low vapor quality (\(x\) < 0.3) region for both tubes. However, the rate of pressure drop increase was not same for the two tubes with the same experimental conditions: when mass flux increased from 50 to 100 kg m\({}^{-2}\)s\({}^{-1}\) for smooth tube,  
Figure 4: Repeatability of experiment: (**a**) heat transfer; and (**b**) pressure drop. pressure drop increased about 2.3-4.5 times, while, for microfin tube, pressure drop increased about 2.4-6 times. The frictional pressure drop of microfin tube was approximately 1.5 times higher than that of smooth tube at \(G\) = 50 kg m\({}^{-2}\)s\({}^{-1}\). Therefore, the frictional pressure drop increased with fin due to the flow loss increase and turbulence generated by the fins. The range of pressure drop penalty factor was 1.01-2.11. Figure 6 shows the effect of the saturation temperature on the frictional pressure drop at saturation temperatures of and 30 \({}^{\circ}\)C for mass fluxes of 100 and 200 kg m\({}^{-2}\)s\({}^{-1}\). According to the experimental results, when saturation temperature increased, the pressure drop decreased significantly because the saturation temperature changed the refrigerant properties, such as viscosity and density. Table 2 shows that, at 20 \({}^{\circ}\)C, the liquid/gas density (\(\rho_{l}\)/\(\rho_{v}\)) and viscosity (\(\mu_{l}\)/\(\mu_{v}\)) ratio are 44.34 and 18.09, respectively, while, at 30 \({}^{\circ}\)C, they are 31.79 and 15.41, respectively. Similar experimental results were also observed by [PERSON] et al. [9] and [PERSON] and [PERSON] [21]. Experimental data were collected for R134a with different diameter test tubes. Figure 7 shows the frictional pressure drop of R134a in microfin tube of 2.17 mm (this work), 2.68 mm [22] and 3.18 mm [8] at the mass flux and saturation temperature of 50 kg m\({}^{-2}\)s\({}^{-1}\) and 20 \({}^{\circ}\)C, respectively. The results for different tubes show that the frictional pressure drop of microfin tube (\(d_{e}\) = 3.18 mm) was lower than that of other microfin tubes. The second lowest was that of microfin tube (\(d_{e}\) = 2.68 mm), and the highest was that of microfin tube (\(d_{e}\) = 2.17 mm). The frictional pressure drop of \(d_{e}\) = 2.17 mm tube was on average approximately 1.2 times higher than that of the \(d_{e}\) = 3.18 mm tube. Thus, decreasing the tube diameter could increase the pressure drop. Similar experimental trends were also observed by [PERSON] and [PERSON] [23] and [PERSON] et al. [24]. Figure 5: Effects of mass flux and vapor quality on frictional pressure drop: (**a**) smooth tube; and (**b**) microfin tube. Comparison with the Pressure Drop Correlations  
To validate the experimental procedure, the experimental frictional pressure drop data of the microfin and smooth tube were compared with the four pressure drop correlations proposed by [PERSON] et al. [2], [PERSON] [25], [PERSON] et al. [26] and [PERSON] et al. [27], as shown in Figure 8. The compared frictional pressure drop correlations are listed in Table 3. The average deviation (AD) and mean deviation (MD) are listed in Table 4 and calculated by Equations (11) and (12), respectively. All correlations were well predicted for smooth tube data, but the microfin tube data were not predicted well.