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Definition of species within the Nernst-Planck application in Chem Eng module
Posted Oct 15, 2009, 11:07 p.m. EDT Version 3.5a 1 Reply
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When using the Nernst-Planck (with electro-neutrality) option in the Chemical Engineering module, ensure the first species (which has its concentration calculated by COMSOL based on electro-neutrality) is one with a higher initial concentration, e.g. either Na+ or Cl- ions in a seawater droplet, rather than the H+ or OH- ions that are present in trace amounts.
Otherwise, rounding off errors will result in the calculated concentrations of dilute species (e.g. H+ or OH-) to reach un-physical values, including negative concentrations, even if the relative and absolute errors are defined to be extremely small. In a time-marching scheme, these errors progressively get blown out.
This thread also is related to Solution No. 952 in the knowledge base related to negative concentrations .
Hello Dayalan Gunasegaram
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Do you know by any chance why in this same module the defined species have in the subdomain settings different equations? You can see that the nomenclature is not the same.
And why the second specie has only the option of artificial diffusion?
Thanks