SUMMARY

The main goal of this study is to investigate in detail the flow physics and transport phenomena associated with falling liquid film evaporation based on numerical solutions of the governing mass, momentum, energy and species conservation equations. The study focuses on falling films of Newtonian and non-Newtonian (black liquor) liquids. Conventional black liquor evaporation is characterized by high rates of soluble scaling resulting from crystallization of dissolved salts on heat transfer surface. Soluble scaling is a concentration and temperature dependent process. Due to its short residence time and high heat transfer rates, falling film evaporation has been identified as an attractive alternative for black liquor concentration. Thus, this study also investigates the effect of falling film fluid mechanics and heat transfer on crystal formation on the surface of evaporators.
Surface waves on falling liquid films enhance heat transfer between the liquid film and heating surface. Early studies attribute this transport enhancement to circulation motion within the core of large amplitude interfacial waves. More recently, results from numerical and experimental studies show that separation vortices formed at the capillary wave region preceding large amplitude falling film solitary waves induce relatively high crosswise velocities that enhance film transport. Preliminary studies indicate that these separation vortices exhibit varying dynamical behavior depending on fluid properties, flow conditions and wavelength of flow disturbance. Detailed understanding and characterization of the formation and evolution of these separation vortices suggests a more established theory describing transport enhancement in falling liquid films not dominated by turbulent mixing. Specifically, this research aims to:
• investigate the dynamics and effects of separation vortices for growing and saturated disturbances in the sinusoidal and solitary wave regimes of Newtonian laminar falling liquid films
• characterize in detail the flow structure and surface waves dynamics for non-Newtonian black liquor falling films
• analyze for an evaporating falling film the thermal boundary layer development, functional relationship between streamwise variation in film thickness, surface temperature, heat transfer coefficient and mass rate of evaporation
• investigate the influence of film hydrodynamics and heat transfer on crystallization for black liquor evaporation