Experimental characterization of viscous film flows over complex surfaces

Abstract

Ordered packings are made of a variety of materials such as ceramics, metal and plastics, and shaped to a wide spectrum of macro- and micro-structures. Macro-structures, usually in the form of corrugations, sustain good liquid-vapor contact with low overall pressure drop. Micro-structures in the way of surface treatment, metal gauze and surface indentations, help to increase the stability of the liquid film by preventing breakups and dry patches. An experimental research program was carried out in order to characterize the mechanics of viscous flows over model complex surfaces. Film thickness profiles, streamline patterns and free-surface velocities were measured for a variety of surface shapes and fluids. These results are important in the understanding of the interaction of capillary, viscous and gravity forces on the shaping of the film free surface. The position of the liquid film interface was found, in general, to have the same period as the wavy solid but amplitude and phase-shift vary with flow parameters. Free-surface profiles are patterned around the shape of the solid surfaces but their amplitude decreased with increasing Nusselt film thickness. Free-surface velocity measurements show peaks of velocity larger than the maximum predicted for vertical films. These peaks are associated to a minimum in film thickness and are detected in regions where there is an inflexion point in free surface curvature. Three parameters, Nusselt film thickness, Reynolds number and Capillary number, are necessary to characterize accurately these liquid film flows.