where N0 is an arbitrary user-specified flux expression (SI unit: mol/(m
2·s)). For example,
N0 can represent a flux from or into a much larger surrounding environment, a phase change, or a flux due to chemical reactions.
N0 can also be a function of the concentration and the electric potential (if the mass transport includes migration of ionic species).
where kc is a mass transfer coefficient (SI unit: m/s), and
cb is the concentration (SI unit: mol/m
3) in the surroundings of the modeled system (the bulk concentration). The mass transfer coefficient (to be specified) is often given by boundary-layer theory.
