## 3 Data Reduction  
The local condensation heat transfer coefficient, \(\alpha\), is calculated by Equation (1). \[\alpha\ =\ \frac{Q_{c}}{\pi d_{eq}L(T_{R}-T_{wi})} \tag{1}\]  
where \(Q_{c}\) is the heat transfer amount of cooling water side, in each sub-section. \(L\) is the effective heat transfer length of the test section, and \(d_{eq}\) is the equivalent diameter of the test microfin tubes. When the smooth tube \(d_{eq}\) was used in the experiment, it was defined by the inner diameter \(d_{i}\). \(T_{R}\) is the refrigerant temperature and it was estimated from the measured refrigerant pressure. \(T_{wi}\) is the inner wall temperature and it was calculated by the one-dimensional equation of heat conduction. \[T_{wi}\ =\ T_{wo}+Q_{c}\ln\left(\frac{d_{o}}{d_{eq}}\right)/(2\pi\lambda_{c}L) \tag{2}\]  
where \(T_{wo}\) is test tube outside wall surface temperature, \(d_{o}\) is the outside diameter of the test tube, and \(\lambda_{c}\) is the thermal conductivity of the copper test tube. The total two-phase adiabatic pressure drop \(\Delta P_{T}\) is expressed as the sum of frictional pressure drop \(\Delta P_{F}\), pressure drop due to the abrupt contraction \(\Delta P_{c}\) and expansion \(\Delta P_{e}\) at the inlet and outlet of the test section, respectively. \[\Delta P_{T}=\Delta P_{F}+\Delta P_{C}+\Delta P_{e} \tag{3}\]  
The pressure drop because of the abrupt contraction and expansion were estimated by Equations (4) and (5), respectively, according to the Collier and Thome method [18]. \[\Delta P_{c}=\frac{G^{2}v_{l}}{2}\left[\left(\frac{1}{C_{c}}-1\right)^{2}+ \left(1-\frac{1}{\beta^{2}}\right)\right]\left[1+\left(\frac{\Delta v_{v}}{v_ {l}}\right)x_{a}\right] \tag{4}\]  
\[\Delta P_{e}=G^{2}\beta(1-\beta)v_{l}\left[\frac{(1-x_{a})^{2}}{(1-\xi)}+ \left(\frac{v_{v}}{v_{l}}\right)\frac{x_{a}{}^{2}}{\xi}\right] \tag{5}\]  
where \(\beta\) is area ratio and the coefficient of contraction \(C_{c}\) is a function of \(\beta\). The void function, \(\xi\), is calculated by [PERSON]'s separated flow model [19]. The pressure drop because of the abrupt contraction and expansion was less than 2% of the total pressure drop. The vapor quality at the inlet of the test section is calculated as Equation (6). \[h_{o}=h_{in}+\left(\frac{Q}{G}\right) \tag{6}\]  
\[x=\frac{h_{o}-h^{\prime}}{h^{\prime\prime}-h^{\prime}} \tag{7}\]  
where \(h_{o}\) and \(h_{in}\) are the outlet and inlet enthalpies of refrigerant. The inlet enthalpy of refrigerant just before the first preheater is estimated with refrigerant's bulk temperature and pressure. \(Q\) is the amount of heat which is supplied by the electricity into the preheaters in direct heating method. \(G\) is the mass flux of refrigerant. \(h^{\prime}\) and \(h\"\) are the saturated liquid and vapor enthalpies, respectively. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \(T_{sat}\) & \(\rho_{I}\) & \(\rho_{P}\) & \(\rho_{I}\) & \(\mu_{I}\) & \(\mu_{o}\) & \(\mu_{i}\) & \(\sigma\) \\ [\({}^{\circ}\)C] & [kg m\({}^{-3}\)] & [kg m\({}^{-3}\)] & [kg m\({}^{-3}\)] & [\(\mu\)Pa-s] & [uPa-s] & \(\mu_{o}\) & [mN m\({}^{-1}\)] \\ \hline
20 & 1225.90 & 27.65 & 44.34 & 207.75 & 11.48 & 18.09 & 8.71 \\
25 & 1207.30 & 32.20 & 37.50 & 195.25 & 11.68 & 16.71 & 8.05 \\
30 & 1188.00 & 37.37 & 31.79 & 183.47 & 11.90 & 15.41 & 7.40 \\ \hline \end{tabular}
\end{table} 
Table 2: Thermophysical properties of R134a [17]. The vapor quality \(x\) does not change enough along the test tube because no further heat is added to the refrigerant flowing in the test section. The uncertainty of the condensation heat transfer coefficient and adiabatic pressure drop were calculated following the method by JCGM 100 [20]. The uncertainty analysis of the condensation heat transfer coefficient and frictional pressure drop can be carried out based on the basic equation of condensation heat transfer and frictional pressure drop as mentioned in Equations (1) and (3), respectively.