Experimental and theoretical investigation of air-steam condensation in a vertical tube at low inlet steam fractions

Experimental and theoretical studies were conducted to study the water vapor condensation in the presence of air, simulated for flue gas from natural gas fired boiler system. A vertical annulus tube of 2 m length was used for the study. The inlet water vapor mass fraction ranged from 3 to 12% and the gas mixture Reynolds number was between 4600 and 14,000. A theoretical model was developed from heat and mass analogy to study the heat and mass transfer characteristics when water vapor condensed in the presence of air at low inlet water vapor mass fraction. The predicted results compared to the experimental observations showed that the gas mixture at high Reynolds number could cause waviness at the interface, and the suction effect from vapor condensation can enhance heat and mass transfer rate even at low rate of condensation. The sensible and condensation heat transfer coefficient both influence the overall heat transfer coefficient at low (inlet) water vapor mass fraction, but the condensation heat transfer coefficient is more influenced when Reynolds number and inlet water vapor mass fraction are increased. A correlation of condensation heat transfer coefficient has been proposed from the analogy model.