Studies of mass and momentum transfer in free-falling wavy films.

摘要

A wavy liquid film flowing downward under the influence of gravity exposed to still air at the surface is the simplest form of two phase annular flow. This configuration has important industrial applications, but the transport of heat and mass in the flow is poorly understood. Large increases in these rates are attributable to the presence of interfacial waves. This study explores the hydrodynamic behavior of wavy films with particular emphasis on the large wave structure and its role in the transport of mass.;An algorithm for solving the Navier-Stokes equations inside large waves was developed and applied to a variety of wave shapes and sizes. Computed flow fields showed significant streamwise inertial forces and normal velocities throughout the wave as well as the presence of recirculation beneath the wave peaks when viewed in a travelling coordinate system. The hydrodynamics were shown to be extremely sensitive to the wave shape and evolution. All results suggest the commonly used parabolic streamwise velocity profile is inadequate to describe the flow within the wavy film.;Mass transport in large waves was studied numerically and experimentally. Numerical study was restricted to the large waves whose hydrodynamics were determined using the previous algorithm. Mass transfer rates computed for transport through both the free surface and from the wall agree well with published data and suggest the large waves control the overall transfer rate. These results show that even for laminar flow the wavy free surface causes large increases in the transport rate and that the presence of turbulent behavior is not necessarily a significant factor. An experimental method combining optical and conductance techniques was developed and employed to measure instantaneous film thickness and local average concentration in falling wavy films. Measurements showed that average concentration values within large waves were significantly larger than predicted if parallel streamlines existed in the wave. Comparison between measured and predicted average concentration profiles showed reasonable agreement.;A simplified model of wavy film flow was developed to generate a wavy interfacial profile in conjunction with the flow field. The behavior of travelling waves was studied numerically and suggests the highly random appearance of the wavy falling film may be a manifestation of deterministic chaos. Agreement between statistical characteristics of the wave profiles predicted by the model and determined experimentally was satisfactory at large Reynolds numbers.