Effects of stagnant air on condensation heat transfer over a horizontal tube

Abstract The effects of stagnant air on the condensation heat transfer of vapor over a horizontal tube were investigated by experiments. The condensation wall temperatures along the circumference and the cooling water temperature were measured at various cooling water and vapor mass fluxes as well as vapor pressures. The heat transfer coefficients and Nusselt number were calculated, and the effects of stagnant air on several parameters, including the wall temperature of the condensation tube, Nusselt number, heat transfer coefficient, and flow pattern transition point of liquid film, were analyzed. Results showed that stagnant air was a main factor that affected the heat and mass transfer uniformity of vapor. Unlike in pure vapor condensation, the temperature difference between the top and bottom parts of the condensation tube increased when the mole fraction of stagnant air was less than 0.17, whereas it decreased when the mole fraction of stagnant air was larger than 0.47. The increment in the mole fraction of stagnant air reduced the temperature difference along the circumference of the condensation tube wall. As the mole fraction of stagnant air increased, the heat transfer coefficient and Nusselt number decreased. However, Nusselt number was insensitive to the effect of air when the mole fraction of stagnant air was larger than 0.25. As the circumferential angle of horizontal condensation tube increased from 0° to 180°, the Reynolds number of liquid film transformed from the laminar state to the turbulence state and then changed into the laminar state. Reynolds number reached a maximum value within the circumferential angles of 45°–90°. As the circumferential angle changed from 90° to 180°, a large amount of condensate fell, and the liquid film attached to the condensation tube wall became thinner, which resulted in an intensive reduction in the Reynolds number of the liquid film. As the mole fraction of stagnant air increased, a transition point of liquid film moved to a larger circumferential angle, which delayed the transition of liquid film from laminar flow to turbulence. The present experimental results were compared with the data from Sherkriladze and Gomelauri (1966), Rose (1984), Fujii et al. (1972), and Nusselt (1916).