## 2 Condensation Heat Transfer Model  
Figure 1 shows the condensation model of steam with noncondensable gas on the vertical wall. For the condensation process containing noncondensable gas, when the wall temperature is lower than the saturation temperature corresponding to the partial pressure of steam in the mixture, the saturated steam will condensate on the wall and form a liquid film. Meanwhile, the liquid film flows along the direction of gravity, and its flow state changes from laminar to turbulent as the liquid film thickness increases [1; 14]. Due to the accumulation of noncondensable gas on the surface of the liquid film, when the mixture moves towards the interface, the aggregation of noncondensable gas molecules causes an increase in the gas's partial pressure and forms the driving force of the reverse diffusion of noncondensable gas to the mainstream gas. While the steam condenses and its partial pressure decreases below that of the steam in the mainstream gas at the interface between the phases, this pressure differential promotes the diffusion of steam towards the interface, which is facilitated by the pressure gradient. The diffusion of steam towards the condensing surface and the noncondensable gases towards the mainstream gas maintain a dynamic equilibrium with a constant total pressure [12]. Under the influence of two conditions, the state parameters at the phase interface cannot be determined. The steam in the gas mixture is transferred to the wall by means of mass diffusion, which requires it to pass through the highly concentrated noncondensable gas layer that gathers on the surface of the condensate film, and then condenses on the surface and releases latent heat. The heat transfer resistance of the whole steam condensation is mainly divided into three parts, and the steam transfers the heat to the surface of the liquid film by means of condensation and convection, respectively, that is, there is condensation thermal resistance and convection thermal resistance, and further heat passes through the liquid film thermal resistance and finally reaches the condensing wall. Correspondingly, the total heat transfer coefficient is determined by the thermal resistance of the three aspects above: the liquid film heat transfer coefficient \(h_{\rm film}\) determined by \(T_{\rm i}-T_{\rm w}\), the convective heat transfer coefficient \(h_{\rm conv}\) and the condensation heat transfer coefficient \(h_{\rm cond}\) determined by \(T_{\rm g}-T_{\rm i}\). The energy conservation equation is as follows [29]:  
\[\frac{q}{h_{\rm tot}}=\frac{q}{h_{\rm film}}+\frac{q}{h_{\rm conv}+h_{\rm cond}} \tag{1}\]  
\[h_{\rm tot}\big{(}T_{\rm g}-T_{\rm w}\big{)}=h_{\rm film}(T_{\rm i}-T_{\rm w} )=(h_{\rm conv}+h_{\rm cond})\big{(}T_{\rm g}-T_{\rm i}\big{)} \tag{2}\]  
where: \(q\)--total heat transfer flux;  
\(h_{\rm tot}\)--total heat transfer coefficient;  
\(h_{\rm film}\)--film heat transfer coefficient;  
\(h_{\rm conv}\)--convective heat transfer coefficient;  
\(h_{\rm cond}\)--steam condensation heat transfer coefficient;  
\(T_{\rm g}\)--mixture gas temperature;  
\(T_{\rm i}\)--interface temperature;  
\(T_{\rm w}\)--surface temperature of the plate. As can be seen from Equation (2), the total heat transfer coefficient \(h_{\rm tot}\) is related to the liquid film heat transfer coefficient \(h_{\rm film}\), convective heat transfer coefficient \(h_{\rm conv}\) and steam condensation heat transfer coefficient \(h_{\rm cond}\), as well as the mixture gas temperature, interface  
Figure 1: Condensation model of steam with noncondensable gas on vertical wall. temperature and surface temperature. So, in order to obtain the final total heat transfer coefficient, it is necessary to solve the \(h_{\rm film}\), \(h_{\rm conv}\) and \(h_{\rm cond}\) one by one, and determine the interface temperature \(T_{\rm i}\).