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Limiting current from Rotating Disk Electrode - Simulation doesn't match theory

Posted 19 août 2015, 17:37 UTC−4 Computational Fluid Dynamics (CFD), Electrochemistry, Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers Version 5.1 3 Replies

Aleksander Skuratovsky
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I am having an problem getting a value for the current at an electrode, that is being rotated in solution, that would be expected from theory.

The Levich equation ( en.wikipedia.org/wiki/Levich_equation ) models the diffusion and solution flow conditions around a Rotating disk electrode (RDE). The Levich equation gives the height of the sigmoidal wave observed in rotating disk voltammetry, or the limiting current. The limiting current can be approached, for example, by increasing the electric potential or decreasing the rate of mass transfer to the electrode. The RDE reduces the rate of mass transfer through induced flux and is equivalent to a silent solution at steady state.

I began with model library. I found the microdisk volatmmetry model. This model shows the limiting current as the applied potential is changed. I added swirl flow and got rid of the infinite boundary domain because the laminar flow physics did not allow me to select it. I am integrating the local current density along the electrode and revolving around the z axis (elan.iloc_er1*r*pi*2). this does not match the expected solution from the Levich equation. Also the current applied will result in the maximum current for this system.

What am I doing wrong? I've been at this for a while and have had no luck at all.
3 Replies Last Post 25 août 2015, 01:53 UTC−4
Lasse Murtomäki
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Posted: 9 years ago 20 août 2015, 03:29 UTC−4
Hi

I looked at your model. Your elctrode surface has a sliding wall condition (why -omega*r, why not +) but its extension in r direction has a non-slip condition. How could that be?

BR
Lasse
Hi I looked at your model. Your elctrode surface has a sliding wall condition (why -omega*r, why not +) but its extension in r direction has a non-slip condition. How could that be? BR Lasse
Aleksander Skuratovsky
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Posted: 9 years ago 24 août 2015, 15:14 UTC−4
Thank you for catching that error. I have fixed it. The solution for the current is still off by a few orders of magnitude.
I have attached an updated file with the fixed boundary conditions.
Thank you for catching that error. I have fixed it. The solution for the current is still off by a few orders of magnitude. I have attached an updated file with the fixed boundary conditions.
Lasse Murtomäki
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Posted: 9 years ago 25 août 2015, 01:53 UTC−4
You still have -r*omega, but in my opinion - and following a tutorial - the sliding wall condition should be in U_w component, not in the phi component. Also your simulation domain is far too small and meshing too coarse in the vicinity of the electrode. I suggest to mesh the electrode surface first with the Edge --> Distribution pathway. The symmetry axis must also be meshed densely towards the electrode because current is proportional to the gradient normal to the surface. Use again the Edge ---> Distribution. I prefer geometrical distribution with the element ratio 10. But try different options and look what happens.

BR
Lasse
You still have -r*omega, but in my opinion - and following a tutorial - the sliding wall condition should be in U_w component, not in the phi component. Also your simulation domain is far too small and meshing too coarse in the vicinity of the electrode. I suggest to mesh the electrode surface first with the Edge --> Distribution pathway. The symmetry axis must also be meshed densely towards the electrode because current is proportional to the gradient normal to the surface. Use again the Edge ---> Distribution. I prefer geometrical distribution with the element ratio 10. But try different options and look what happens. BR Lasse

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