Theoretical study on forced convection filmwise condensation inside converging channels in the presence of a non-condensable gas

Abstract

In this paper, a theoretical study is performed on forced convection filmwise condensation inside a vertical converging channel in the presence of a non-condensable gas. Two solution schemes are developed with consideration of the turbulence effect in gas mixture and a laminar condensate film flow. Various heat transfer mechanisms are revealed by examining condensation heat transfer inside converging channels. In the present study, steam and air are taken as the vapor and non-condensable gas, respectively. It is found that the film velocity transits from a linear profile to a parabolic profile for a straight channel along the channel axial direction under the combined influence of gravitational force and interface shear stress. However, for a converging channel, the transition process from linear to parabolic velocity profiles is substantially delayed along the axial direction due to the enhanced interfacial shear stress in the latter condensing section. The film thickness decreases along the channel axial direction at the latter condensing section for both natural and forced convection condensation due to the substantially decreased local condensation rate and/or the enhanced interfacial shear stress in a converging channel. The combined influence of the curvature effect and enhanced interfacial shear stress leads the local heat transfer enhancement to increase substantially at the latter condensing region for forced convection condensation inside a converging channel. A uniform enhancement on the normalized local heat transfer coefficient in the inlet condensing section was found for both natural and forced convection condensation. However, the reduced condensation area has led the normalized local heat transfer coefficient to decrease substantially along the axial direction for converging channel as compared to straight channel in the case of natural convection condensation. Comparatively, the increased interfacial shear stress results in the normalized heat transfer coefficient along the entire channel to be enhanced. The average heat transfer coefficient increases as the converging angle or the Reynolds number increase. The turbulence effect can enhance the condensation heat transfer. However, the presence of air can cause a significant decrease in the condensation heat transfer. For the case of pure vapor condensation, the enhancement ratio increases as the Reynolds number increases. When the air is present in the system, an insufficient vapor supply is observed when the Reynolds number is low. Therefore, the enhancement ratio increases substantially as the converging angle increases. When the air mass fraction is high (W=0.4), the enhancement ratio appears to be insensitive to the Reynolds number in the laminar and turbulent regime, respectively. In addition, the enhancement ratio generally increases as the air mass fraction increases.