Numerical simulation of laminar film condensation in a horizontal minitube with and without non-condensable gas by the VOF method

Based on the volume of fluid (VOF) method, a steady three-dimensional numerical simulation of laminar film condensation of water vapor in a horizontal minitube, with and without non-condensable gas, has been conducted. A user-defined function defining the phase change is interpreted and the interface temperature is correspondingly assumed to be the saturation temperature. An annular flow pattern is to be expected according to a generally accepted flow regime map. The heat-transfer coefficient increases with higher saturation temperature and a smaller temperature difference between the saturation and wall temperatures, but varies little with different mass flux and degree of superheat. The existence of a non-condensable gas will lead to the generation of a gas layer between vapor and liquid, resulting in a lower mass-transfer rate near the interface and higher vapor quality at the outlet. In consequence, the heat-transfer coefficient of condensation with a non-condensable gas drops sharply compared with that of pure vapor condensation. Meanwhile, the non-condensable gas with a smaller thermal conductivity would cause a stronger negative effect on heat flux as a result of a higher thermal resistance of heat conduction in the non-condensable gas layer.