Bubble movement under a solid surface

点击翻译按钮获取中文摘要

In aluminium electrolysis, gas bubbles are generated under the anode. These bubbles form a bubble layer and their movement enhances the transport processes, improving the homogeneity of the temperature and concentration fields. On the other hand, the gas bubbles partially block the conducting section of the passage of electric current, increasing the electrical resistance and energy consumption of the cell, ultimately leading to instabilities. Furthermore, the nucleation, movement and coalescence of the bubbles make the cell voltage fluctuating. Despite of the recent developments in mathematical modeling of single and multiphase flows, the quantitative description of bubble behaviour under a solid surface is not satisfactory. The objective of the proposed research is to characterize the size and shape evolution of gas bubbles moving under a solid surface, to determine the drag coefficients of the bubbles at different stages of their evolution and to describe quantitatively the perturbation of the flow field in the liquid phase by the passage of the individual bubbles. Phenomena like the hydrodynamics in the liquid separation layer between the solid and the moving bubbles or the formation of a "head" during the movement of big bubbles play a decisive role in establishing the terminal velocity of bubbles and ultimately in the transfer of momentum between the liquid and gas. The methodology involves a combination of experiments and mathematical modeling. The shape evolution and kinematics of the bubbles will be studied in transparent laboratory models, using high-speed videography and image analysis. The influence of the material properties will be analyzed by using different liquids and solids. The drag coefficients will be extracted from the measurement of the velocity of the bubbles. Mathematical modeling will be used to determine the flow pattern and rate of dissipation in the measured liquid separation layer as well as to clarify the liquid-bubble interaction around the large bubbles. The expected results will help both the improvement of the design of the aluminium electrolysis cells and their safer and more economical operation.