mass flux toward a boundary
 

Hi guys,

i am modeling a chemical surface reaction in Fluent. This reaction is limited by mass transfer and in order to calculate a mass transfer coefficient (by 1 Fick law) i need a mass flux toward my surface there the reaction is happening.
There is no build-in option which i can choose to get a mass flux.
Maybe someone can provide me a suggestion, how can i get the flux (UDF)?

Thank you