Doctoral defense
Turbulent Convective Mass Transfer in Electrochemical Systems
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| Defendant |
Main Advisor |
Extra Advisor |
Date |
| Francois Gurniki |
Said Zahrai |
Fritz Bark |
2000-12-05 |
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| Opponent |
| Hiroshi Kawamura, Science University of Tokyo, Japan
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| Evaluation committee |
Lars Davidsson, CTH
Eduardo Fontes, Comsol AB, Tegnergatan 23, Stockholm
Jesper Oppelstrup, KTH, NADA
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AbstractElectrochemical mass transfer in turbulent flows and binary electrolytes is investigated. The primary objective is to provide information about mass transfer in the near-wall region between a solid boundary and a turbulent fluid flow at high Schmidt number. Natural and forced convections are investigated with two different methods; the turbulence model k-epsilon and large-eddy simulations (LES). The k-epsilon method does not solve the fluctuating part of the flow and assumes isotropic turbulence. LES solve only the large scales of the fluctuations. The computations made with natural convection reveal that the standard wall-functions give acceptable results for the velocity field, but not for the concentration. The Boussinesq approximation for the Reynolds-flux in the mass-transport equation and the wall-function for concentration in the logarithmic layer are shown to fail in the prediction of the turbulent mass transfer. A method for large-eddy simulations is developed to study the Reynolds-flux and mass-transfer in the near-electrode region. In order to make numerical integration of governing equations at high Schmidt number economic, a numerical scheme is developed in which two different meshes are used for hydrodynamic variables and the concentration field. With the help of a fringe technique the finest mesh used for the computation of mass transfer is reduced to the near-wall region only. A study of the electrical distribution along the electrode reveals that the intensity of the current influences the fluctuations of the concentration field but not the mean values in time. Some models for the Reynolds-flux validated for Sc=1 are succesfully tested for Sc=3000. At high Schmidt number, a new model for the Reynolds-flux and a new wall-function for concentration are found. Descriptors: electrolyte, mass transfer, turbulent channel flow, forced convection, natural convection, wall-functions, explicit algebraic modelling, large-eddy simulations.
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