Effect of Longitudinal Mini-Grooves on Flow Stability and Wave Characteristics of Falling Liquid Films

Falling liquid films are used in many industrial apparatuses. In many cases the film flow along a wall with topography is considered advantageous for intensification of the transport processes. We use the shadow method and the chromatic white light sensor (CHR) method to study the wavy structure of falling films on flat walls and on walls with longitudinal grooves. We show that the wavy pattern substantially changes on walls with topography. The wave frequency, the wave propagation velocity and the area of the liquid-gas interface decrease on grooved walls. The linear stability of the film has been analyzed using the long-wave theory, which relies on the assumption that the average film thickness is much smaller than the scale of the film thickness variation. The linear stability analysis predicts that the disturbance growth rate, the frequency of the fastest growing disturbance mode and the wave propagation speed decrease on a tube with longitudinal mini-grooves in comparison with a smooth tube. These results agree well with the experimental findings.