## 4 Results and Discussion
### Comparison of Steam-CO\({}_{2}\) Mixture Condensation Features on Horizontal and Vertical Plates  
Due to the different inlet velocity settings and driving forces of the liquid film, the boundary layer models of the steam-CO\({}_{2}\) mixture condensation on the surfaces of the vertical and horizontal plates are not the same.  
Figure 9 shows a comparison of the condensation \(\Delta\)_HTC_ values at CO\({}_{2}\) inlet fractions of 2% and 10% on horizontal and vertical plates at 10 MPa. The result shows that the \(\Delta\)_HTC_ caused by the solubility of CO\({}_{2}\) on a vertical plate is larger than that on a horizontal plate under the same \(\alpha_{\infty}\). Thus, the effect of CO\({}_{2}\) solubility on improvements in average HTC values on the vertical plate is larger than those on the horizontal plate. On the vertical plate, the gas phase accounts for the main thermal resistance. However, it becomes smaller when CO\({}_{2}\) dissolves in the liquid phase.  
Figure 10 shows a comparison of some other calculation results on the condensation on horizontal and vertical plate surfaces. Figure 10a shows the calculation results of the average HTC values at the inlet CO\({}_{2}\) mass fractions of 2% and 10% under total pressure conditions of 1 atm and 10 MPa. The average HTC condensation value on the vertical plate is higher than that on the horizontal plate under a very small subcooling regime. This shows the advantages of using gravity to drain the condensate compared with using gas flow.  
Figure 9: Comparison of condensation \(\Delta\)_HTC_ at CO\({}_{2}\) inlet fractions of 2% and 10% on horizontal and vertical plates (\(U_{\infty}=1\) m/s; \(L=0.2\) m).  
Figure 10b shows the heat transfer reduction (\(q/q_{0}\)) curves of the steam-CO\({}_{2}\) mixture condensation, and the label is same as Figure 10a. Condensation on the horizontal plate surface is significantly less affected by CO\({}_{2}\) than that on the vertical surface at the same CO\({}_{2}\) mass fraction. Due to the influence of gravity, the thickness of the liquid film accumulated in the horizontal plate is thicker than the vertical plate, which can be confirmed by the calculation of the thickness of the local liquid film in Figure 10c. With an increase in the CO\({}_{2}\) mass fraction, the condensation heat transfer is reduced and the accumulation of liquid film decreases, so the thickness of the liquid film becomes thinner for larger \(\alpha_{\infty}\) conditions in Figure 10c. In addition, with the increase in pressure, an improvement is shown in the reduction in condensation heat transfer, and the condensate quality also increases.  
Through the above analysis, at medium and low concentrations, the concentration of condensate is the main factor determining the thermal resistance to condensation on the horizontal plate. As shown in Figure 10c, compared with the vertical plate, the heat transfer reduction in the horizontal plate is not sensitive to changes in CO\({}_{2}\) concentration. As the thickness of the liquid film covered by the vertical plate is thinner, the thermal resistance in the liquid phase is smaller than that in the gas phase. Figure 10d shows the ratio of the temperature difference in the gas boundary layer and the liquid film, which represents the relative thermal resistance of both the gas phase and the liquid phase. Figure 10d shows that the thermal resistance in the gas phase is higher than that in the liquid phase on the vertical plate. Thus, a small mass fraction of CO\({}_{2}\) will have a significant impact.  
Figure 10: Comparison of steam–CO\({}_{2}\) mixture condensation on the horizontal and vertical surfaces. (a) Average HTC; (b) \(q/q_{0}\), (c) \(\delta\); (d) Ratio of temperature differences in gas and liquid-phases.