Turbulence in multiphase models for aeration bubble plumes.

摘要

Bubbly flows encompass a vast domain of natural and artificial flow conditions. They are encountered, for instance, in the surroundings of surface ships hulls, and in nuclear-engineering devices, such as outputs from vessel cores and pipes passing through cooling systems. Bubble plumes constitute a special case of bubbly flows in which the flow is driven by buoyancy.; Bubble plumes have received considerable attention in the last five decades due to its large range of applications at different spatial scales. At the small scales, they are found in metallurgy in gas stirring of ladles, and in chemical reactors. At extremely-large scales, they take place in induced events of CO2 sequestration; by which this compound is injected into deep seas. At the environmental scales, the application range is vast: bubble plumes have been used as barriers to contain density intrusions or oil spills, as breakwaters, as silt curtains, and for destratification purposes in lakes. In sanitary engineering, bubble plumes are usually employed for aeration purposes in water and wastewater treatment plants. Additionally, arrays of bubble diffusers are used in reservoirs aimed at storing combined sewer overflows, in order to avoid the occurrence of anaerobic conditions. Bubble plumes for these last two applications have been generically dubbed as "aeration" bubble plumes.; This thesis is devoted to the detailed modeling of turbulence in aeration bubble plumes. The theory of multicomponent fluids, blended with computational-fluid-dynamics (CFD) techniques, is used.; A new theoretical formulation for bubble plumes is presented, which serves for the definition of a sophisticated three-dimensional (3D) numerical model. The theoretical/numerical model includes a treatment of turbulence via a k - epsilon model and a Large-Eddy Simulation (LES) approach. It is based on a double averaging procedure that accounts for: (a) the presence of a two-phase flow, and (b) its turbulent nature. The model also allows for the global simulation of phenomena of break-up and coalescence. This theory is employed in the derivation and justification of existing one-dimensional (1D), "integral" models. The approximations involved are clearly explicited and quantified. In this framework, 1D models appear for the first time as a special case of this broader theory. This fact is finally used in extending existing 1D models, now including turbulence and phenomena of break-up and coalescence. (Abstract shortened by UMI.)