Flow characteristics and circulatory motion in wavy falling films with and without counter-current gas flow

Abstract

A laser-induced photochromic tracer technique was used to investigate the flow structure in a thin, wavy falling liquid film in a vertical tube both with and without interfacial shear induced by a counter-current flow of gas. Instantaneous velocity profile and film thickness were measured simultaneously to better understand the turbulence characteristics and the enhancement effect of interfacial waves and shear on the heat and mass transfer rates. While surface ripples were found to exert almost no influence on the near-wall hydrodynamics, the large wave effects could easily be sensed close to the solid wall, altering the turbulence intensity profiles. Flow visualization experiments revealed the occurrence of circulatory motions with significant velocities normal to the tube wall under the large waves, which may be the dominant mechanism for wall-to-liquid and interfacial transport phenomena for wavy films.