Wave-augmented mass transfer in a liquid film falling inside a vertical tube

Uniformly distributed water films were formed inside vertical tubes, and partial disintegrations of hump-like surface waves (or large waves) into clusters of dimples were observed on the films at the Reynolds number Re∼40 and larger, associated with marked deceleration of mass-transfer augmentation. In about 1 m or taller films with the uniform distribution, two empirical correlations between the Sherwood number Sh and Re at the ranges of Re=20–40 and 40–400 were constructed. The transition to turbulent flow occurs in the films with decelerating inlet flow at the range of Re=400–700 where Sh sharply rises, suggesting that the laminar developing entry region rapidly shortens to nearly disappear, though the region had been observed only in film flow measurements. Periodic perturbations imposed on the inlet flow trigger tall humps close to the inlet, causing the laminar developing region to vanish and the critical Re to reduce from 400 to about 300. Waves covering a whole film of 0.4–1.0 m height increase in Sh up to 2.2–2.7 times the theoretical prediction for a smooth film at the laminar-flow range. The amount of increase is larger in a taller film. The decrease in Sh due to the tube inclination from the vertical is more serious for a taller film with laminar flow, and a 0.2° inclination causes a 5% decrease in a 0.7 m tall film.