## 5 Conclusions  
We carried out the theoretical modeling and simulation of the boundary layer for the condensation of the steam-CO\({}_{2}\) mixture on the surface of horizontal and vertical plates. (i) Based on the boundary layer theory, CO\({}_{2}\) is regarded as an NCG, and the solubility of CO\({}_{2}\) also matters. The boundary layer model is modified to obtain a new CG model. At the same time, a new iterative calculation method is used in the iterative calculation to extend the calculation range of the NG model, including extending the NG model to calculate the steam-air mixed condensation to calculate the steam-CO\({}_{2}\) mixed condensation and expanding the calculation range of \(\alpha_{\infty}\) and pressure. (ii) The influence of CO\({}_{2}\) solubility on the heat transfer of mixed gas condensation is clarified, which adds a consideration factor for the next research study on steam-CO\({}_{2}\) condensation heat transfer. Comparing the differences between steam-CO\({}_{2}\) and steam-air, besides the difference in gas physical properties, solubility was also the factor that caused these differences. (iii) The effect of CO\({}_{2}\) solubility on the condensation heat transfer of mixed gas on the horizontal plate and vertical plate was compared. The main conclusions are as follows:  
Due to the slight solubility of CO\({}_{2}\), part of CO\({}_{2}\) can pass through the gas-liquid interface and enter the liquid film, and the CO\({}_{2}\) accumulation at the surface of the liquid film decreases, which reduces the condensation heat transfer. However, this effect is only manifested under high-pressure conditions, where the solubility of CO\({}_{2}\) is non-negligible. In this paper, the data show that when the total pressure of the steam-CO\({}_{2}\) mixture is higher than 1 MPa, the influence of the slightly soluble CO\({}_{2}\) on the condensation of the mixed gas should be considered. The same mass fraction condition was selected to compare the condensation features with different mixture gases. The differences in condensation heat transfer of steam-CO\({}_{2}\) and steam-air mixtures under the same conditions are caused by different physical properties of different components at low pressures and low concentrations. CO\({}_{2}\) reduces condensation less compared to air. When the total pressure or concentration is high, the slight solubility of CO\({}_{2}\) can also be regarded as a physical property, which has an effect on condensation heat transfer. Within the calculation range, the effect of the CO\({}_{2}\) concentration on the condensation of the horizontal plate surface is smaller than that of the vertical plate surface. However, the effect of CO\({}_{2}\) solubility in terms of improvements in the heat transfer coefficient of the vertical plate is better than that of the horizontal plate. For the NG and CG models of the developed boundary layer theory, the inclination angle \(\beta\) of the plate was introduced to unify one set of models of the horizontal plate and vertical plate to determine the influence of geometric position on condensation heat transfer. Based on the limitations of the model, CFD was used in the following calculation to simulate a new scheme containing a high mass fraction of CO\({}_{2}\) in steam-CO\({}_{2}\) condensation, and the influence of solubility was further compared. The rationality of simulation calculation is to show the difference between the two calculation schemes. Conceptualization, Y.C. and J.W. ; Methodology, J.W. ; Investigation, Y.J. and H.G. ; Writing--original draft, B.J. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China (grant number: 51976033). Not applicable. Some or all of the data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.