Heat and Mass Transfer from Air with High Water Vapor Content : Latent Heat Recovery from Flue Gas

Abstract

Experimental measurements are compared with theoretical predictions for heat and mass transfer from hot and highly humid air (130°C, 0-15 wt% of water vapor content) to horizontal cooling tubes (30°C). The Reynolds number ranges from 2800 to 9000. Heat and mass transfer coefficients are measured under conditions with and without condensation by using a newly developed condensate collector. It is found that the convective heat transfer coefficient and mass transfer coefficient are enhanced by 68% and 28%, respectively, by increasing the water vapor content in the air flow from 0 to 15 wt%. The mass transfer data fits well with Rose's equation and the heat transfer data are shown to give 20-30% higher values than the prediction. This means that the analogy between heat and mass transfer does not hold under condensation conditions. This phenomenon is explained quantitatively by considering that the water vapor content is reducing faster than the temperature in the separated flow region on the rear side of the tubes, due to the value of Pγ/Sc being greater than unity.