## 5 Comparisons
### Comparison against Empirical Correlations  
In this paragraph, the experimental heat transfer coefficients and frictional pressure drops for the smooth tube are compared against values estimated by empirical correlations available in the open literature. Figure 9 shows a comparison between calculated and experimental heat transfer coefficients. The following empirical models were considered: [PERSON] and [PERSON] [22], [PERSON] and [PERSON] [23], and [PERSON] et al. [24]. Only data prior to the onset of dryout were chosen for the comparison, since the correlations predict values of heat transfer coefficient in the pre-dryout region. The correlation of [PERSON] and [PERSON] [22] was developed from experimental data of a large data bank comprising data including the following fluids: water, R11, R12, R113, R114, R22 and ethylene glycol. The correlation of [PERSON] and [PERSON] [23] was validated with experimental data of refrigerants, water, CO\({}_{2}\), and FC72 flow boiling for mass velocities between 19 and 1608 kg m\({}^{-2}\) s\({}^{-1}\) and reduced pressures between 0.005 and 0.69. The correlation of [PERSON] et al. [24] was validated from experimental measurements carried out during flow boiling of the pure refrigerants R22, R123, R134a, R114, and R12 for mass velocities from 100 to 361 kg m\({}^{-2}\) s\({}^{-1}\) for reduced pressures in the range 0.06-0.22. The experimental data of the present research cover reduced pressures from 0.14 to 0.18. The correlation, which shows the best prediction of the experimental heat transfer coefficients, is the one developed by [PERSON] et al. [24], with a relative, absolute and standard deviation of \(-16.6\%\), \(19.9\%\), and \(15.6\%\), respectively. The model developed by [PERSON] et al. [24] takes into account the surface roughness of the tube in a parameter called nucleation cavity size: the areal roughness parameter \(S_{a}\) is considered for the calculation as nucleation cavity size. The model of [PERSON] and [PERSON] [22] shows a relative, absolute, and standard deviation of \(-28.7\%\), \(28.8\%\), and \(12.3\%\), respectively, whereas the model of [PERSON] and [PERSON] [23] of \(-24.0\%\), \(25.7\%\), and \(16.8\%\), respectively. Figure 8: Comparison between frictional pressure gradient of smooth tube and microfin tube. Data at \(t_{sat}=30\)\({}^{\circ}\)C and \(HF=30\) kW m\({}^{-2}\) at different mass velocity. \(G\) expressed in (kg m\({}^{-2}\) s\({}^{-1}\)). Data for the microfin tube are borrowed by [PERSON] et al. [21]. A further analysis on the correlation developed by [PERSON] et al. [24], which is the correlation that better estimates the experimental heat transfer coefficients, is given in Figure 10, which reports the ratio between the calculated heat transfer coefficient and the experimental counterpart, plotted against mass velocity (Figure 10a), mean vapor quality (Figure 10b), and heat flux (Figure 10c). The underestimation of the correlation of [PERSON] et al. [24] is mainly due to a not perfect effect of vapor quality on the calculated heat transfer coefficient: the correlation tends to underestimate the experimental heat transfer coefficient especially as vapor quality decreases. The comparison between calculated and experimental frictional pressure drops is reported in Figure 11. The models of [PERSON] et al. [25] and of [PERSON] [26] were considered. The correlation of [PERSON] et al. [25] was validated against data of halogenated refrigerants with reduced pressures between 0.1 and 0.6 and mass velocities above 150 kg m\({}^{-2}\) s\({}^{-1}\). The correlation of [PERSON] and [PERSON] [26] was developed from data including different fluids, such as refrigerants, water, argon, nitrogen, for mass velocities generally higher than 120 kg m\({}^{-2}\) s\({}^{-1}\). Figure 10: Ratio between calculated and experimental heat transfer coefficient versus: (**a**) mass velocity; (**b**) mean vapor quality; (**c**) heat flux. Model by [PERSON] et al. [24]. Figure 9: Comparison between calculated and experimental heat transfer coefficients.