Experimental and modeling studies of wavy films in annular gas-liquid flows under normal and microgravity conditions

Thin film flows appear in many industrial applications such as falling film reactors, wetted-wall absorbers, condensers and vertical evaporators, and transport of oil and gas mixtures in pipelines. Experimental observations show that the presence of waves on the film surface enhances the heat and mass transfer rates substantially across the liquid-gas and liquid-solid interfaces in these processes. A new simplified model is developed for describing the characteristics of free falling wavy liquid films. The model consists of a set of three partial differential equations (in x and t) for the local film thickness h, volumetric flow rate q, and wall shear stress τ. It is shown that the new model is a substantial improvement over all existing simplified models of wavy films such as the Long Wave equation, the Nakaya model (extended third-order Long Wave equation), the Shkadov model, and the Kapitza boundary layer model. These prior models predict non-physical negative wall shear stress when the wave amplitude is large and cannot explain the experimentally observed relationship between the wave amplitude and the Reynolds (Re) and Kapitza (Ka) numbers. In contrast, the present model yields physically meaningful results and quantitative predictions of large amplitude waves. The consistency and accuracy of the model is verified by comparing the linear stability results with the Orr-Sommerfeld studies of the two-dimensional Navier-Stokes equations. Local bifurcation theory is used to analyze the model for small Re, and analytical relations are obtained for predicting the velocity (Ce) and the maximum amplitude of solitary waves.; Experimental studies of free falling viscous films were conducted using water-glycerin solutions and the conductance probe technique. Comparison of the experimental data on wave amplitude and velocity with analytical correlations shows excellent agreement. Numerical simulations of the wave profiles generated from the simplified model also match closely with the experimentally observed wave profiles. Experimental results for two-phase gas-liquid flows under reduced gravity conditions are also presented.