Measurement of mass and momentum transport in wavy-laminar falling liquid films

Abstract

Experiments were performed to assess the role of large waves in the transport of mass and momentum in falling liquid films. Simultaneous two component velocity and film thickness measurements were made in wavy laminar films falling on the exterior of a vertical column. Solutions of water and propylene glycol, used as the working fluid in order to create relatively thick viscous films, yielded wavy laminar films over a Reynolds number range of 209 to 414. Examination of these conditions revealed that large waves behave as lumps of liquid sliding over a continuous substrate. Velocity fields within the substrate displayed reduced sensitivity to the large waves with increases in substrate thickness or reductions in dynamic viscosity. Furthermore, time traces of steam-wise velocity in the substrate were found to loosely resemble traces of film thickness, with fluctuations in velocity lagging fluctuations in thickness. By decomposing the stream-wise and radial velocity components into mean, wave induced velocity fluctuations, and turbulent fluctuations, a determination could be made as to whether these films were truly wavy laminar or turbulent. It is shown that, in wavy laminar films, a large portion of the flow, 40–70%, is transported by large waves.