Solução do Campo Hidrodinâmico em Células Eletroquímicas pelo Método de Elementos Finitos

• rotating disk flow
• mass transport
• electrochemical cell
• Navier-Stokes equation
• finite element

Master thesis — Universidade Federal do Rio de Janeiro — UFRJ

ABSTRACT:

Current instabilities in electrochemical cells is a research subject of the Applied Electrochemistry Group in the Metallurgy and Materials Program 􏰈 COPPE/UFRJ. Electrochemical cells containing a 1M sulfuric acid electrolyte and an iron rotating disk electrode are used by the group and present this instability. A possible reason for the phenomena is a reduction of the stability of the 􏰋ow, induced by the exis- tence of a thin mass concentration boundary layer, produced by the dissolution of the electrode. The group has addressed the problem by performing linear stability analysis of the 􏰋ow for electrolytes with a viscosity pro􏰊le depending on the axial coordinate z only and, subsequently, by coupling the hydrodynamic and the mass concentration 􏰊eld produced by the dissolution of the electrode, through the 􏰋uid viscosity. The results suggest a signi􏰊cant reduction of the stability of the 􏰋ow, due to the coupling of 􏰊elds. The purpose of this work is to develop a computational platform for further studying the hydrodynamic 􏰊eld close to the rotating disk electrode, through a 3D Direct Numerical Simulation using the Finite Element Method. Viscous and pressure terms are discretized through the Galerkin method and the convective term is treated through a semi-Lagrangean method. Time derivatives are discretized by a first-order backward Euler implicit scheme. Velocity and pressure are decoupled through a projection method based on LU decomposition. The resulting linear system is solved by the pre-conditioned conjugate gradient method. A hydrodynamic 􏰊eld, very close to the generalized von Kármán solution for the rotating disk 􏰋ow was obtained, validating the code. The code, developed within the object oriented paradigm, constitutes a platform for the study of 3D perturbations in electrochemical cells in the linear regimen and in the saturation and the modes interaction in the non-linear regimen.

download