Numerical Simulation of Wall Condensation from a Superheated Steam and Air Mixture in a Vertical Pipe

The objective of this study was to evaluate the effects of superheat on wall condensation from a steam and air mixture. We previously measured the radial and axial temperature profiles of a superheated steam-air mixture in a vertical pipe with a diameter of 49.5 mm and a cooling height of 610 mm. In this study, we carried out a numerical simulation for the previous measurements by using the computational fluid dynamics (CFD) code FLUENT, and evaluated the profiles of the mixture temperature T_g and steam mass fraction X_s. The profiles of T_g and the saturated temperature T_s obtained from X_s agreed well with those measured with superheated and saturated conditions, respectively. The validity of the correlation to evaluate a condensation heat flux q_c (which was based on the gradient of X_s) was confirmed. Profiles of the dimensionless velocity u⁺, temperature T⁺, and steam mass fraction Y_s⁺ were obtained, and they were compared with wall functions (i.e., the linear function for a viscous sublayer and the logarithmic law for a turbulent layer). The computed profile agreed with the wall function for u⁺, agreed relatively well with the wall function for T⁺, and agreed well with the correlation for Y_s⁺ obtained from data measured with saturated steam-air conditions in the region of the turbulent layer.