Effect of Interfacial Velocity Fluctuations on the Enhancement of the Mass-Transfer Process in Falling-Film Flow

To investigate the relationship between surface instability and mass-transport behavior, a laser Doppler anemometer (LDA) system was used to measure the liquid velocity components in a falling film, even very close to the liquid surface. The experimental results show that the presence of countercurrent gas flow tends to reduce and flatten the liquid velocity in the surface region and that the maximum liquid velocity might occur at some distance from the interface. The fluctuating liquid velocity field shows a maximum at a position of 0.8-1.0 in the dimensionless liquid film thickness, and a fluctuation frequency is obtained. Interfacial mass transport in a falling film is strongly dependent on interfacial small-scale convection. The experimental mass-transfer rates are larger than those predicted by penetration theory. The enhanced liquid Sherwood number, ShL, was calculated, and the predicted values were found to be in agreement with the experimental data.