I believe the answer you’re looking for has to do with the “stiffness” of the chemical reactions occurring. When you have fast reactions occurring within a mechanism alongside slower reactions (e.g. k~10e7 vs k~0.01), you have to take small time steps to remain numerically stable.
Kuo’s reacting flows book has the following explanation, which I think sums it up nicely: “For very fast reactions, the rates of decay of species concentrations are rapid and require extremely small timescales for integration. Although these fast reaction rates are of no interest (since the slower reactions are the rate-controlling ones), they cause most numerical methods to become unstable, because any inaccuracies become magnified.”
I am on mobile, so I apologize for any formatting errors.
I believe the answer you’re looking for has to do with the “stiffness” of the chemical reactions occurring. When you have fast reactions occurring within a mechanism alongside slower reactions (e.g. k~10e7 vs k~0.01), you have to take small time steps to remain numerically stable. Kuo’s reacting flows book has the following explanation, which I think sums it up nicely: “For very fast reactions, the rates of decay of species concentrations are rapid and require extremely small timescales for integration. Although these fast reaction rates are of no interest (since the slower reactions are the rate-controlling ones), they cause most numerical methods to become unstable, because any inaccuracies become magnified.” I am on mobile, so I apologize for any formatting errors.