High Ttemperture Experimental Study And Numerical Simulation Of Bubble Dynamics In Aluminum Electrolytic Process

The anodic process in aluminum electrolysis is a complex process,which integrates gas generation on the anode-electrolyte interface that forming a gas-liquid-solid electrochemical system and involves electric,magnetic and gas-liquid flow fields that coupling a thermal-electric-magnetic-flow field.The detailed behaivor of anodic bubbles is a difficulty and hot spot in aluminum smelting.A better understanding of anodic bubbels would fundamentally develop technologies and reduce energy costs and greenhouse gas emissions of aluminium electrolytic industry.This dispertation reviewed the studies of anodic bubbles in previous work,developed high temperature electrolytic model(bottom-view transparent electrolytic cell)and numerical models(Volume of Fluid(VOF)model and Volume of Fluid-Magneto Hydro Dynamics(VOF-MHD)model)to investigate the behaivor of anodic bubbles during aluminum electrolysis.The main work includes:Using the bottom-view transparent electrolytic cell,the bubble dynamics beneath anode were directly observed for the first time from bottom view in a similar industrial CO2-cryolite system.The bubble generation,growth,coalescence and releasing process at different current densities were studied.The corresponding cell voltage was measured simultaneously for quantitatively investigating its relevance to bubble dynamics.It was found that the cell voltage was consistent with bubble coverage,indicating the bubble evolution is a key factor affecting cell voltage fluctuation;the maximum and average coverage decreased with the increase of current density;the magnitude of voltage fluctuation increased with current density firstly and decreased when the current density was further increased.The transparent electrolytic cell was further used to study the effects of anode slots on bubble behavior.The results showed that the slot with a width of 4 mm was able to slightly reduce the bubble thickness,but it significantly reduced the bubble size,bubble coverage and cell voltage fluctuation.Aimed on the widely applied substitute for real CO2-cyroltie system,the air-water system(water models),this paper established three dimensional(3D)VOF models to numerically compare the similarities and differences in bubble dynamics between the two systems.In micro-scale single bubble simulation,it was found that the bubble sliding under an anode in a CO2-cryolite system had a larger covering area,a smaller bubble thickness and a higher sliding velocity than those in the air-water system.Dimensionless analysis and numerical simulations illustrated that the wettability was the dominant factor producing these differences;the effects of kinematic viscosity,surface tension and density were very small.In macro-sacle bath flow driven by gas bubbles,although the bubbles behaved slightly differently,the quasi-stable state flow characteristics(flow pattern,turbulent parameters and gas volume fraction)showed a remarkable agreement between the two systems in terms of distribution and magnitude.The results demonstrated that the water models are reliable substitutes for real aluminum reduction cells in the study of bubble driven flow and its associated turbulence.The bubble induced turbulence kinetic energy(K)and turbulence energy dissipation rate(?)were also quantitatively addressed by the VOF model.The turbulence was closely related with the trajectories of gas movement and the strong turbulent area mainly distributed near the anode sidewall and bath surface.The detailed information of bubble induced K and? would build constitutive correlations to improve the accuracy of macro-scale modelling.By coupling with the MHD module in ANSYS/Fluent,the VOF model was further extended to an electric-magnetic-flow model.The influence of Lorenz force on bubble behavior was assessed.It was found that the Lorenz force generated by industrial measured magnetic field had little effect on the bubble movement,in which the Lorenz force ranges from 0 to 50N/m3.