In my case, the inlet is not very far from the outlet. Therefore, some contaminant comes into domain through inlet again. I simulate the concentration of outlet for each time step, how could I add this value for each time step to inlet? Thank you very much for any suggestions.

1/ Store the values at each time step into a spare variable 2/ Set these stored values as those entering the domain through the inlet.

Note ground coding required for this, but it is not too difficult to do.