The flexibility of COMSOL means that a reaction can result in nearly any reaction rate contribution Ric to a mass fraction ωi. But a reactant, ωr, can physically only be consumed if ωr > 0 and a product, ωk, can only be formed if ωk < 1. The Reaction feature in Transport of Concentrated Species Interface contains a way to regularize make the rate expression contributions comply to these restrictions.

When selected the “core”, or unregularized reaction rate contribution, Ric, to a mass fraction ωi is replaced by

The first term on the right hand side of Equation 3-87 is active if Ric < 0, that is if ωi is a reactant. The reaction rate contribution, Ri, is equal to the “core” reaction rate, Ric, as long as ωi > ωidl. As ωi approaches zero, the regularization damps out negative Ric and for ωi < 0, Ri for reactant ωi is equal to zero.

The second term on the right hand side of Equation 3-87 is active if Ric > 0, that is if ωi is a product. The reaction rate contribution, Ri, is equal to the “core” reaction rate, Ric, as long as ωi < 1 − ωidl. As ωi approaches one, the regularization damps out positive Ric and for ωi > 1, Ri for product ωi is equal to zero.

The damping limits, ωidl, should be in an order of magnitude that can be considered numerical noise for species i. The damping limits are per default set to 1e−6, which is appropriate for most applications. It can be advantageous to lower some limits when working with for example catalytic trace species and the limits can sometimes be raised to gain additional robustness.