Hi! Who can explain me what is the general difference between the 'Transport' and the 'Internal' solution methods? Thank you in advance, Julie

Transport solves the conservation equations for an active scalar. Internal means that the amount of that scalar can be determined just by knowing how much of the other scalars are present. In most cases, when you set up a reaction the distinction are set within Prostar and generally do not need to be changed. It certainly would be time consuming to solve all scalars as transport equations.